The natural accelerators of the neutrino astrophysicist are also humbling. Consider, for instance, the extraordinary relativistic jets emerging from the supermassive black hole in Messier 87 – an accelerator that stretches for about 5000 light years, or roughly 315 million times the distance from the Earth to the Sun.

Alongside gravitational waves, high-energy neutrinos have opened up a new chapter in astronomy. They point to the most extreme events in the cosmos. They can escape from regions where high-energy photons are attenuated by gas and dust, such as NGC 1068, the first steady neutrino emitter to be discovered (see “The neutrino sky” figure). Their energies can rise orders of magnitude above 1 PeV (10¹⁵ eV), where the universe becomes opaque to photons due to pair production with the cosmic microwave background. Unlike charged cosmic rays, they are not deflected by magnetic fields, preserving their original direction.

Breaking into the exascale calls for new thinking

High-energy neutrinos therefore offer a unique window into some of the most profound questions in modern physics. Are there new particles beyond the Standard Model at the highest energies? What acceleration mechanisms allow nature to propel them to such extraordinary energies? And is dark matter implicated in these extreme events? With the observation of a 220⁺⁵⁷⁰_–110 PeV neutrino confounding the limits set by prior observatories and opening up the era of ultra-high-energy neutrino astronomy (CERN Courier March/April 2025 p7), the time is ripe for a new generation of neutrino detectors on an even grander scale (see “Thinking big” table).

A cubic-kilometre ice cube

Detecting high-energy neutrinos is a serious challenge. Though the neutrino–nucleon cross section increases a little less than linearly with neutrino energy, the flux of cosmic neutrinos drops as the inverse square or faster, reducing the event rate by nearly an order of magnitude per decade. A cubic-kilometre-scale detector is required to measure cosmic neutrinos beyond 100 TeV, and Earth starts to be opaque as energies rise beyond a PeV or so, when the odds of a neutrino being absorbed as it passes through the planet are roughly even depending on the direction of the event.

The journey of cosmic neutrino detection began off the coast of the Hawaiian Islands in the 1980s, led by John Learned of the University of Hawaii at Mānoa. The DUMAND (Deep Underwater Muon And Neutrino Detector) project sought to use both an array of optical sensors to measure Cherenkov light and acoustic detectors to measure the pressure waves generated by energetic particle cascades in water. It was ultimately cancelled in 1995 due to engineering difficulties related to deep-sea installation, data transmission over long underwater distances and sensor reliability under high pressure.

The next generation of cubic-kilometre-scale neutrino detectors built on DUMAND’s experience. The IceCube Neutrino Observatory has pioneered neutrino astronomy at the South Pole since 2011, probing energies from 10 GeV to 100 PeV, and is now being joined by experiments under construction such as KM3NeT in the Mediterranean Sea, which observed the 220 PeV candidate, and Baikal–GVD in Lake Baikal, the deepest lake on Earth. All three experiments watch for the deep inelastic scattering of high-energy neutrinos, using optical sensors to detect Cherenkov photons emitted by secondary particles.

A decade of data-taking from IceCube has been fruitful. The Milky Way has been observed in neutrinos for the first time. A neutrino candidate event has been observed that is consistent with the Glashow resonance – the resonant production in the ice of a real W boson by a 6.3 PeV electron–antineutrino – confirming a longstanding prediction from 1960. Neutrino emission has been observed from supermassive black holes in NGC 1068 and TXS 0506+056. A diffuse neutrino flux has been discovered beyond 10 TeV. Neutrino mixing parameters have been measured. And flavour ratios have been constrained: due to the averaging of neutrino oscillations over cosmological distances, significant deviations from a 1:1:1 ratio of electron, muon and tau neutrinos could imply new physics such as the violation of Lorentz invariance, non-standard neutrino interactions or neutrino decay.

The sensitivity and global coverage of water-Cherenkov neutrino observatories is set to increase still further. The Pacific Ocean Neutrino Experiment (P-ONE) aims to establish a cubic-kilometre-scale deep-sea neutrino telescope off the coast of Canada; IceCube will expand the volume of its optical array by a factor eight; and the TRIDENT and HUNT experiments, currently being prototyped in the South China Sea, may offer the largest detector volumes of all. These detectors will improve sky coverage, enhance angular resolution, and increase statistical precision in the study of neutrino sources from 1 TeV to 10 PeV and above.

Breaking into the exascale calls for new thinking.

Into the exascale

Optical Cherenkov detectors have been exceptionally successful in establishing neutrino astronomy, however, the attenuation of optical photons in water and ice requires the horizontal spacing of photodetectors to a few hundred metres at most, constraining the scalability of the technology. To achieve sensitivity to ultra-high energies measured in EeV (10¹⁸ eV), an instrumented area of order 100 km² would be required. Constructing an optical-based detector on such a scale is impractical.

One solution is to exchange the tracking volume of IceCube and its siblings with a larger detector that uses the atmosphere as a calorimeter: the deposited energy is sampled on the Earth’s surface.

The Pierre Auger Observatory in Argentina epitomises this approach. If IceCube is presently the world’s largest detector by volume, the Pierre Auger Observatory is the world’s largest detector by area. Over an area of 3000 km², 1660 water Cherenkov detectors and 24 fluorescence telescopes sample the particle showers generated when cosmic rays with energies beyond 10 EeV strike the atmosphere, producing billions of secondary particles. Among the showers it detects are surely events caused by ultra-high-energy neutrinos, but how might they be identified?

Out on a limb

One of the most promising approaches is to filter events based on where the air shower reaches its maximum development in the atmosphere. Cosmic rays tend to interact after traversing much less atmosphere than neutrinos, since the weakly interacting neutrinos have a much smaller cross-section than the hadronically interacting cosmic rays. In some cases, tau neutrinos can even skim the Earth’s atmospheric edge or “limb” as seen from space, interacting to produce a strongly boosted tau lepton that emerges from the rock (unlike an electron) to produce an upward-going air shower when it decays tens of kilometres later – though not so much later (unlike a muon) that it has escaped the atmosphere entirely. This signature is not possible for charged cosmic rays. So far, Auger has detected no neutrino candidate events of either topology, imposing stringent upper limits on the ultra-high-energy neutrino flux that are compatible with limits set by IceCube. The AugerPrime upgrade, soon expected to be fully operational, will equip each surface detector with scintillator panels and improved electronics.

Experiments in space are being developed to detect these rare showers with an even larger instrumentation volume. POEMMA (Probe of Extreme Multi-Messenger Astrophysics) is a proposed satellite mission designed to monitor the Earth’s atmosphere from orbit. Two satellites equipped with fluorescence and Cherenkov detectors will search for ultraviolet photons produced by extensive air showers (see “Exascale from above” figure). EUSO-SPB2 (Extreme Universe Space Observatory on a Super Pressure Balloon 2) will test the same detection methods from the vantage point of high-atmosphere balloons. These instruments can help distinguish cosmic rays from neutrinos by identifying shallow showers and up-going events.

Another way to detect ultra-high-energy neutrinos is by using mountains and valleys as natural neutrino targets. This Earth-skimming technique also primarily relies on tau neutrinos, as the tau leptons produced via deep inelastic scattering in the rock can emerge from Earth’s crust and decay within the atmosphere to generate detectable particle showers in the air.

The Giant Radio Array for Neutrino Detection (GRAND) aims to detect radio signals from these tau-induced air showers using a large array of radio antennas spread over thousands of square kilometres (see “Earth skimming” figure). GRAND is planned to be deployed in multiple remote, mountainous locations, with the first site in western China, followed by others in South America and Africa. The Tau Air-Shower Mountain-Based Observatory (TAMBO) has been proposed to be deployed on the face of the Colca Canyon in the Peruvian Andes, where an array of scintillators will detect the electromagnetic signals from tau-induced air showers.

Another proposed strategy that builds upon the Earth-skimming principle is the Trinity experiment, which employs an array of Cherenkov telescopes to observe nearby mountains. Ground-based air Cherenkov detectors are known for their excellent angular resolution, allowing for precise pointing to trace back to the origin of the high-energy primary particles. Trinity is a proposed system of 18 wide-field Cherenkov telescopes optimised for detecting neutrinos in the 10 PeV–1000 PeV energy range from the direction of nearby mountains – an approach validated by experiments such as Ashra–NTA, deployed on Hawaii’s Big Island utilising the natural topography of the Mauna Loa, Mauna Kea and Hualālai volcanoes.

All these ultra-high-energy experiments detect particle showers as they develop in the atmosphere, whether from above, below or skimming the surface. But “Askaryan” detectors operate deep within the ice of the Earth’s poles, where both the neutrino interaction and detection occur.

In 1962 Soviet physicist Gurgen Askaryan reasoned that electromagnetic showers must develop a net negative charge excess as they develop, due to the Compton scattering of photons off atomic electrons and the ionisation of atoms by charged particles in the shower. As the charged shower propagates faster than the phase velocity of light in the medium, it should emit radiation in a manner analogous to Cherenkov light. However, there are key differences: Cherenkov radiation is typically incoherent and emitted by individual charged particles, while Askaryan radiation is coherent, being produced by a macroscopic buildup of charge, and is significantly stronger at radio frequencies. The Askaryan effect was experimentally confirmed at SLAC in 2001.

Optimised arrays

Because the attenuation length of radio waves is an order of magnitude longer than for optical photons, it becomes feasible to build much sparser arrays of radio antennas to detect the Askaryan signals than the compact optical arrays used in deep ice Cherenkov detectors. Such detectors are optimised to cover thousands of square kilometres, with typical energy thresholds beyond 100 PeV.

The Radio Neutrino Observatory in Greenland (RNO-G) is a next-generation in-ice radio detector currently under construction on the ~3 km-thick ice sheet above central Greenland, operating at frequencies in the 150–700 MHz range. RNO-G will consist of a sparse array of 35 autonomous radio detector stations, each separated by 1.25 km, making it the first large-scale radio neutrino array in the northern hemisphere.

In the southern hemisphere, the proposed IceCube-Gen2 will complement the aforementioned eightfold expanded optical array with a radio component covering a remarkable 500 km². The cold Antarctic ice provides an optimal medium for radio detection, with radio attenuation lengths of roughly 2 km facilitating cost-efficient instrumentation of the large volumes needed to measure the low ultra-high-energy neutrino flux. The radio array will combine in-ice omnidirectional antennas 150 m below the surface with high-gain antennas at a depth of 15 m and upward-facing antennas on the surface to veto the cosmic-ray background.

The IceCube-Gen2 radio array will have the sensitivity to probe features of the spectrum of astrophysical neutrino beyond the PeV scale, addressing the tension between upper limits from Auger and IceCube, and KM3NeT’s 220 ⁺⁵⁷⁰_–110PeV neutrino candidate – the sole ultra-high-energy neutrino yet observed. Extrapolating an isotropic and diffuse flux, IceCube should have detected 75 events in the 72–2600 PeV energy range over its operational period. However, no events have been observed above 70 PeV.

Perhaps the most ambitious way to observe ultra-high-energy neutrinos is to use the Moon as a target

If the detected KM3NeT event has a neutrino energy of around 100 PeV, it could originate from the same astrophysical sources responsible for accelerating ultra-high-energy cosmic rays. In this case, interactions between accelerated protons and ambient photons from starlight or synchrotron radiation would produce pions that decay into ultra-high-energy neutrinos. Alternatively, if its true energy is closer to 1 EeV, it is more likely cosmogenic: arising from the Greisen–Zatsepin–Kuzmin process, in which ultra-high-energy cosmic rays interact with cosmic microwave background photons, producing a Δ-resonance that decays into pions and ultimately neutrinos. IceCube-Gen2 will resolve the spectral shape from PeV to 10 EeV and differentiate between these two possible production mechanisms (see “Diffuse neutrino landscape” figure).

Moonshots

Remarkably, the Radar Echo Telescope (RET) is exploring using radar to actively probe the ice for transient signals. Unlike Askaryan-based detectors, which passively listen for radio pulses generated by charge imbalances in particle cascades, RET’s concept is to beam a radar signal and watch for reflections off the ionisation caused by particle showers. SLAC’s T576 experiment demonstrated the concept in the lab in 2022 by observing a radar echo from a beam of high-energy electrons scattering off a plastic target. RET has now been deployed in Greenland, where it seeks echoes from down-going cosmic rays as a proof of concept.

Perhaps the most ambitious way to observe ultra-high-energy neutrinos foresees using the Moon as a target. When neutrinos with energies above 100 EeV interact near the rim of the Moon, they can induce particle cascades that generate coherent Askaryan radio emission which could be detectable on Earth (see “Moon skimming” figure). Observations could be conducted from Earth-based radio telescopes or from satellites orbiting the Moon to improve detection sensitivity. Lunar Askaryan detectors could potentially be sensitive to neutrinos up to 1 ZeV (10²¹ eV). No confirmed detections have been reported so far.

Neutrino network

Proposed neutrino observatories are distributed across the globe – a necessary requirement for full sky coverage, given the Earth is not transparent to ultra-high-energy neutrinos (see “Full-sky coverage” figure). A network of neutrino telescopes ensures that transient astrophysical events can always be observed as the Earth rotates. This is particularly important for time-domain multi-messenger astronomy, enabling coordinated observations with gravitational wave detectors and electromagnetic counterparts. The ability to track neutrino signals in real time will be key to identifying the most extreme cosmic accelerators and probing fundamental physics at ultra-high energies.

The post Discovering the neutrino sky appeared first on CERN Courier.

Particle physics today benefits from a wealth of high-quality data at the same time as powerful new ideas are boosting the accuracy of theoretical predictions. These are particularly important while the international community discusses future projects, basing projections on current results and technology. The conference heard how theoretical investigations of specific models and “catch all” effective field theories are being sharpened to constrain a broader spectrum of possible extensions of the Standard Model. Theoretical parametric uncertainties are being greatly reduced by collider precision measurements and lattice QCD. Perturbative calculations of short-distance amplitudes are reaching to percent-level precision, while hadronic long-distance effects are being investigated both in B-, D- and K-meson decays, as well as in the modelling of collider events.

Comprehensive searches

Throughout Moriond 2025 we heard how a broad spectrum of experiments at the LHC, B factories, neutrino facilities, and astrophysical and cosmological observatories are planning upgrades to search for new physics at both low- and high-energy scales. Several fields promise qualitative progress in understanding nature in the coming years. Neutrino experiments will measure the neutrino mass hierarchy and CP violation in the neutrino sector. Flavour experiments will exclude or confirm flavour anomalies. Searches for QCD axions and axion-like particles will seek hints to the solution of the strong CP problem and possible dark-matter candidates.

The Standard Model has so far been confirmed to be the theory that describes physics at the electroweak scale (up to a few hundred GeV) to a remarkable level of precision. All the particles predicted by the theory have been discovered, and the consistency of the theory has been proven with high precision, including all calculable quantum effects. No direct evidence of new physics has been found so far. Still, big open questions remain that the Standard Model cannot answer, from understanding the origin of neutrino masses and their hierarchy, to identifying the origin and nature of dark matter and dark energy, and explaining the dynamics behind the baryon asymmetry of the universe.

Several fields promise qualitative progress in understanding nature in the coming years

The discovery of the Higgs boson has been crucial to confirming the Standard Model as the theory of particle physics at the electroweak scale, but it does not explain why the scalar Brout–Englert–Higgs (BEH) potential takes the form of a Mexican hat, why the electroweak scale is set by a Higgs vacuum expectation value of 246 GeV, or what the nature of the Yukawa force is that results in the bizarre hierarchy of masses coupling the BEH field to quarks and leptons. Gravity is also not a component of the Standard Model, and a unified theory escapes us.

At the LHC today, the ATLAS and CMS collaborations are delivering Run 1 and 2 results with beyond-expectation accuracies on Higgs-boson properties and electroweak precision measurements. Projections for the high-luminosity phase of the LHC are being updated and Run 3 analyses are in full swing. The LHCb collaboration presented another milestone in flavour physics for the first time at Moriond 2025: the first observation of CP violation in baryon decays. Its rebuilt Run 3 detector with triggerless readout and full software trigger reported its first results at this conference.

Several talks presented scenarios of new physics that could be revealed in today’s data given theoretical guidance of sufficient accuracy. These included models with light weakly interacting particles, vector-like fermions and additional scalar particles. Other talks discussed how revisiting established quantum properties such as entanglement with fresh eyes could offer unexplored avenues to new theoretical paradigms and overlooked new-physics effects.

The post Planning for precision at Moriond appeared first on CERN Courier.

On 13 February 2023, strings of photodetectors anchored to the seabed off the coast of Sicily detected the most energetic neutrino ever observed, smashing previous records. Embargoed until the publication of a paper in Nature last month, the KM3NeT collaboration believes their observation may have originated in a novel cosmic accelerator, or may even be the first detection of a “cosmogenic” neutrino.

“This event certainly comes as a surprise,” says KM3NeT spokesperson Paul de Jong (Nikhef). “Our measurement converted into a flux exceeds the limits set by IceCube and the Pierre Auger Observatory. If it is a statistical fluctuation, it would correspond to an upward fluctuation at the 2.2σ level. That is unlikely, but not impossible.” With an estimated energy of a remarkable 220 PeV, the neutrino observed by KM3NeT surpasses IceCube’s record by almost a factor of 30.

The existence of ultra-high-energy cosmic neutrinos has been theorised since the 1960s, when astrophysicists began to conceive ways that extreme astrophysical environments could generate particles with very high energies. At about the same time, Arno Penzias and Robert Wilson discovered “cosmic microwave background” (CMB) photons emitted in the era of recombination, when the primordial plasma cooled down and the universe became electrically neutral. Cosmogenic neutrinos were soon hypothesised to result from ultra-high-energy cosmic rays interacting with the CMB. They are expected to have energies above 100 PeV (10¹⁷ eV), however, their abundance is uncertain as it depends on cosmic rays, whose sources are still cloaked in intrigue (CERN Courier July/August 2024 p24).

A window to extreme events

But how might they be detected? In this regard, neutrinos present a dichotomy: though outnumbered in the cosmos only by photons, they are notoriously elusive. However, it is precisely their weakly interacting nature that makes them ideal for investigating the most extreme regions of the universe. Cosmic neutrinos travel vast cosmic distances without being scattered or absorbed, providing a direct window into their origins, and enabling scientists to study phenomena such as black-hole jets and neutron-star mergers. Such extreme astrophysical sources test the limits of the Standard Model at energy scales many times higher than is possible in terrestrial particle accelerators.

Because they are so weakly interacting, studying cosmic neutrinos requires giant detectors. Today, three large-scale neutrino telescopes are in operation: IceCube, in Antarctica; KM3NeT, under construction deep in the Mediterranean Sea; and Baikal–GVD, under construction in Lake Baikal in southern Siberia. So far, IceCube, whose construction was completed over 10 years ago, has enabled significant advancements in cosmic-neutrino physics, including the first observation of the Glashow resonance, wherein a 6 PeV electron antineutrino interacts with an electron in the ice sheet to form an on-shell W boson, and the discovery of neutrinos emitted by “active galaxies” powered by a supermassive black hole accreting matter. The previous record-holder for the highest recorded neutrino energy, IceCube has also searched for cosmogenic neutrinos but has not yet observed neutrino candidates above 10 PeV.

Its new northern-hemisphere colleague, KM3NeT, consists of two subdetectors: ORCA, designed to study neutrino properties, and ARCA, which made this detection, designed to detect high-energy cosmic neutrinos and find their astronomical counterparts. Its deep-sea arrays of optical sensors detect Cherenkov light emitted by charged particles created when a neutrino interacts with a quark or electron in the water. At the time of the 2023 event, ARCA comprised 21 vertical detection units, each around 700 m in length. Its location 3.5 km deep under the sea reduces background noise, and its sparse set up over one cubic kilometre optimises the detector for neutrinos of higher energies.

The event that KM3NeT observed in 2023 is thought to be a single muon created by the charged-current interaction of an ultra-high-energy muon neutrino. The muon then crossed horizontally through the entire ARCA detector, emitting Cherenkov light that was picked up by a third of its active sensors. “If it entered the sea as a muon, it would have travelled some 300 km water-equivalent in water or rock, which is impossible,” explains de Jong. “It is most likely the result of a muon neutrino interacting with sea water some distance from the detector.”

The network will improve the chances of detecting new neutrino sources

The best estimate for the neutrino energy of 220 PeV hides substantial uncertainties, given the unknown interaction point and the need to correct for an undetected hadronic shower. The collaboration expects the true value to lie between 110 and 790 PeV with 68% confidence. “The neutrino energy spectrum is steeply falling, so there is a tug-of-war between two effects,” explains de Jong. “Low-energy neutrinos must give a relatively large fraction of their energy to the muon and interact close to the detector, but they are numerous; high-energy neutrinos can interact further away, and give a smaller fraction of their energy to the muon, but they are rare.”

More data is needed to understand the sources of ultra-high-energy neutrinos such as that observed by KM3NeT, where construction has continued in the two years since this remarkable early detection. So far, 33 of 230 ARCA detection units and 24 of 115 ORCA detection units have been installed. Once construction is complete, likely by the end of the decade, KM3NeT will be similar in size to IceCube.

“Once KM3NeT and Baikal–GVD are fully constructed, we will have three large-scale neutrino telescopes of about the same size in operation around the world,” adds Mauricio Bustamante, theoretical astroparticle physicist at the Niels Bohr Institute of the University of Copenhagen. “This expanded network will monitor the full sky with nearly equal sensitivity in any direction, improving the chances of detecting new neutrino sources, including faint ones in new regions of the sky.”

The post Cosmogenic candidate lights up KM3NeT appeared first on CERN Courier.

“The upgraded detectors are precision detectors for a precision-physics era,” says international co-spokesperson Kendall Mahn (Michigan State). “Our current systematic constraint is at the level of a few percent. To make progress we need to be able to probe regions we’ve not probed before.”

T2K studies the oscillations of 600 MeV neutrinos that have travelled 295 km from the J-PARC accelerator complex in Tokai to Super-Kamiokande – a 50 kton gadolinium-doped water-Cherenkov detector in Kamioka that has also been used to perform seminal measurements of atmospheric neutrino oscillations and constrain proton decay. Since the start of data taking in 2010, the collaboration made the first observation of the appearance of a neutrino flavour due to quantum-mechanical oscillations and the most precise measurement of the θ₂₃ parameter in the neutrino mixing matrix. As well as placing limits on sterile-neutrino oscillation parameters, the collaboration has constrained a wide range of the parameters that describe neutrino interactions with matter. The uncertainties of such measurements typically limit the precision of fits to the fundamental parameters of the three-neutrino paradigm, and constraining neutrino-interaction systematics is the main purpose of near detectors in superbeam experiments such as T2K and NOvA, and the future ones Hyper-Kamiokande and DUNE.

T2K’s near-detector upgrade improves the acceptance and precision of particle reconstruction for neutrino interactions. A new fine-grained “SuperFGD” detector (see pink rectangle, left, on “New and improved” image) serves as the target for neutrino interactions in the new experimental phase. Comprised of two million 1 cm³ cubes of scintillator strung with optical fibres, SuperFGD lowers the detection threshold for protons ejected from nuclei to 300 MeV/c, improving the reconstruction of neutrino energy. Two new time-projection chambers flank it above and below to more closely mimic the isotropic reconstruction of Super-Kamiokande. Finally, six new scintillator planes suppress particle backgrounds from outside the detector by measuring time of flight.

Following construction and testing at CERN’s neutrino platform, the new detectors were successfully integrated in the experiment’s global DAQ and slow-control system. The first neutrino-beam data with the fully upgraded detector was collected in June, with the collaboration also benefitting from an upgraded neutrino beam with 50% greater intensity. Beam intensity is set to increase further in the coming years, in preparation for commissioning the new 260 kton Hyper-Kamiokande water Cherenkov detector. Cavern excavation is underway in Kamioka, with first data-taking planned for 2027.

But much can already be accomplished in the new phase of the T2K experiment, says the team. As well as improving precision on θ₂₃ and another key mixing parameter Δm²₂₃, and refining the theoretical models used in neutrino generators, T2K will improve its fit to δ_CP, the fundamental parameter describing CP violation in the leptonic sector. Measuring its value could shed light on the question of why the  universe is dominated by matter.

“T2K’s current best fit to δ_CP is –1.97,” says Mahn. “We expect to be able to observe leptonic CP violation at 3σ significance if the true value of δ_CP is –π/2.”

The post Near-detector upgrade in place at T2K appeared first on CERN Courier.

At the same time, Steven Weinberg and Abdus Salam were carrying out major construction work on what would become the Standard Model of particle physics, building the Higgs mechanism into Sheldon Glashow’s unification of the electromagnetic and weak interactions. The Standard Model is still bulletproof today, with one proven exception: the nonzero neutrino masses for which Davis’s observations were in hindsight the first experimental evidence.

Today, neutrinos are still one of the most promising windows into physics beyond the Standard Model, with the potential to impact many open questions in fundamental science (CERN Courier May/June 2024 p29). One of the most ambitious experiments to study them is currently taking shape in the same gold mine as Davis’s experiment more than half a century before.

Deep underground

In February this year, the international Deep Underground Neutrino Experiment (DUNE) completed the excavation of three enormous caverns 1.5 kilometres below the surface at the new Sanford Underground Research Facility (SURF) in the Homestake mine. 800,000 tonnes of rock have been excavated over two years to reveal an underground campus the size of eight soccer fields, ready to house four 17,500 tonne liquid–argon time-projection chambers (LArTPCs). As part of a diverse scientific programme, the new experiment will tightly constrain the working model of three massive neutrinos, and possibly even disprove it.

DUNE will measure the disappearance of muon neutrinos and the appearance of electron neutrinos over 1300 km and a broad spectrum of energies. Given the long journey of its accelerator-produced neutrinos from the Long Baseline Neutrino Facility (LBNF) at Fermilab in Illinois to SURF in South Dakota, DUNE will be uniquely sensitive to asymmetries between the appearance of electron neutrinos and antineutrinos. One predicted asymmetry will be caused by the presence of electrons and the absence of positrons in the Earth’s crust. This asymmetry will probe neutrino mass ordering – the still unknown ordering of narrow and broad mass splittings between the three tiny neutrino masses. In its first phase of operation, DUNE will definitively establish the neutrino mass ordering regardless of other parameters.

If CP symmetry is violated, DUNE will then observe a second asymmetry between electron neutrinos and antineutrinos, which by experimental design is not degenerate with the first asymmetry. Potentially the first evidence for CP violation by leptons, this measurement will be an important experimental input to the fundamental question of how a matter–antimatter asymmetry developed in the early universe.

If CP violation is near maximal, DUNE will observe it at 3σ (99.7% confidence) in its first phase. In DUNE and LBNF’s recently reconceptualised second phase, which was strongly endorsed by the US Department of Energy’s Particle Physics Project Prioritization Panel (P5) in December (CERN Courier January/February 2024 p7), 3σ sensitivity to CP violation will be extended to more than 75% of possible values of δ_CP, the complex phase that parameterises this effect in the three-massive-neutrino paradigm.

Combining DUNE’s measurements with those by fellow next-generation experiments JUNO and Hyper-Kamiokande will test the three-flavour paradigm itself. This paradigm rotates three massive neutrinos into the mixtures that interact with charged leptons via the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix, which features three angles in addition to δ_CP.

As well as promising world-leading resolution on the PMNS angle θ₂₃, DUNE’s measurements of θ₁₃ and the Δm²₃₂ mass splitting will be different and complementary to those of JUNO in ways that could be sensitive to new physics. JUNO, which is currently under construction in China, will operate in the vicinity of a flux of lower-energy electron antineutrinos from nuclear reactors. DUNE and Hyper-Kamiokande, which is currently under construction in Japan, will both study accelerator-produced sources of muon neutrinos and antineutrinos, though using radically different baselines, energy spectra and detector designs.

Innovative and impressive

DUNE’s detector technology is innovative and impressive, promising millimetre-scale precision in imaging the interactions of neutrinos from accelerator and astrophysical sources (see “Millimetre precision” image). The argon target provides unique sensitivity to low-energy electron neutrinos from supernova bursts, while the detectors’ imaging capabilities will be pivotal when searching for beyond-the-Standard-Model physics such as dark matter, sterile-neutrino mixing and non-standard neutrino interactions.

First proposed by Nobel laureate Carlo Rubbia in 1977, LArTPC technology demonstrated its effectiveness as a neutrino detector at Gran Sasso’s ICARUS T600 detector more than a decade ago, and also more recently in the MicroBooNE experiment at Fermilab. Fermilab’s short-baseline neutrino programme now includes ICARUS and the new Short Baseline Neutrino Detector, which is due to begin taking neutrino data this year.

The first phase of DUNE will construct one LArTPC in each of the two detector caverns, with the second phase adding an additional detector in each. A central utility cavern between the north and south caverns will house infrastructure to support the operation of the detectors.

Following excavation by Thyssen Mining, final concrete work was completed in all the underground caverns and drifts, and the installation of power, lighting, plumbing, heating, ventilation and air conditioning is underway. 90% of the subcontracts for the installation of the civil infrastructure have already been awarded, with LBNF and DUNE’s economic impact in Illinois and South Dakota estimated to be $4.3 billion through fiscal years 2022 to 2030.

Once the caverns are prepared, two large membrane cryostats will be installed to house the detectors and their liquid argon. Shipment of material for the first of the two cryostats being provided by CERN is underway, with the first of approximately 2000 components having arrived at SURF in January; the remainder of the steel for the first cryostat was due to have been shipped from its port in Spain by the end of May. The manufacture of the second cryostat by Horta Coslada is ongoing (see “Cryostat creation” image).

Procedures for lifting and manipulating the components will be tested in South Dakota in spring 2025, allowing the collaboration to ensure that it can safely and efficiently handle bulky components with challenging weight distributions in an environment where clearances can reach as little as 3 inches on either side. Lowering detector components down the Homestake mine’s Ross shaft will take four months.

Two configurations

The two far-detector modules needed for phase one of the DUNE experiment will use the same LArTPC technology, though with different anode and high-voltage configurations. A “horizontal-drift” far detector will use 150 6 m-by-2.3 m anode plane assemblies (APAs). Each will be wound with 4000 150 μm diameter copper-beryllium wires to collect ionisation signals from neutrino interactions with the argon.

A second “vertical-drift” far detector will instead use charge readout planes (CRPs) – printed circuit boards perforated with an array of holes to capture the ionisation signals. Here, a horizontal cathode plane will divide the detector into two vertically stacked volumes. This design yields a slightly larger instrumented volume, which is highly modular in design, and simpler and more cost-effective to construct and install. A small amount of xenon doping will significantly enhance photo detection, allowing more light to be collected beyond a drift length of 4 m.

The construction of the horizontal-drift APAs is well underway at STFC Daresbury Laboratory in the UK and at the University of Chicago in the US. Each APA takes several weeks to produce, motivating the parallelisation of production across five machines in Daresbury and one in Chicago. Each machine automates the winding of 24 km of wire onto each APA (see “Wind it up” image). Technicians then solder thousands of joints and use a laser system to ensure the wires are all wound to the required tension.

Two large ProtoDUNE detectors at CERN are an essential part of developing and validating DUNE’s detector design. Four APAs are currently installed in a horizontal-drift prototype that will take data this summer as a final validation of the design of the full detector. A vertical-drift prototype (see “Vertical drift” image) will then validate the production of CRP anodes and optimise their electronics. A full-scale test of vertical-drift-detector installation will take place at CERN later this year.

Phase transition

Alongside the deployment of two additional far-detector modules, phase two of the DUNE experiment will include an increase in beam power beyond 2 MW and the deployment of a more capable near detector (MCND) featuring a magnetised high-pressure gaseous-argon TPC. These enhancements pursue increased statistics, lower energy thresholds, better energy resolution and lower intrinsic backgrounds. They are key to DUNE’s measurement of the parameters governing long-baseline neutrino oscillations, and will expand the experiment’s physics scope, including searches for anomalous tau-neutrino appearance, long-lived particles, low-mass dark matter and solar neutrinos.

Phase-one vertical-drift technology is the starting point for phase-two far-detector R&D – a global programme under ECFA in Europe and CPAD in the US that seeks to reduce costs and improve performance. Charge-readout R&D includes improving charge-readout strips, 3D pixel readout and 3D readout using high-performance fast cameras. Light-readout R&D seeks to maximise light coverage by integrating bare silicon photomultipliers and photoconductors into the detector’s field-cage structure.

A water-based liquid scintillator module capable of separately measuring scintillation and Cherenkov light is currently being explored as a possible alternative technology for the fourth “module of opportunity”. This would require modifications to the near detector to include corresponding non-argon targets.

Intense work

At Fermilab, site preparation work is already underway for LBNF, and construction will begin in 2025. The project will produce the world’s most intense beam of neutrinos. Its wide-band beam will cover more than one oscillation period, allowing unique access to the shape of the oscillation pattern in a long-baseline accelerator-neutrino experiment.

LBNF will need modest upgrades to the beamline to handle the 2 MW beam power from the upgrade to the Fermilab accelerator complex, which was recently endorsed by P5. The bigger challenge to the facility will be the proton-target upgrades needed for operation at this beam power. R&D is now taking place at Fermilab and at the Rutherford Appleton Laboratory in the UK, where DUNE’s phase-one 1.2 MW target is being designed and built.

The next generation of big neutrino experiments promises to bring new insights into the nature of our universe

DUNE highlights the international and collaborative nature of modern particle physics, with the collaboration boasting more than 1400 scientists and engineers from 209 institutions in 37 countries. A milestone was achieved late last year when the international community came together to sign the first major multi-institutional memorandum of understanding with the US Department of Energy, affirming commitments to the construction of detector components for DUNE and pushing the project to its next stage. US contributions are expected to cover roughly half of what is needed for the far detectors and the MCND, with the international community contributing the other half, including the cryostat for the third far detector.

DUNE is now accelerating into its construction phase. Data taking is due to start towards the end of this decade, with the goal of having the first far-detector module operational before the end of 2028.

The next generation of big neutrino experiments promises to bring new insights into the nature of our universe – whether it is another step towards understanding the preponderance of matter, the nature of the supernovae explosions that produced the stardust of which we are all made, or even possible signatures of dark matter… or something wholly unexpected!

The post A gold mine for neutrino physics appeared first on CERN Courier.

How big is a neutrino? Though the answer depends on the physical process that created it, knowledge of the size of neutrino wave packets is at present so wildly unconstrained that every measurement counts. New results from the Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment in TRIUMF, Canada, set new lower limits on the size of the neutrino’s wave packet in terrestrial experiments – though theorists are at odds over how to interpret the data.

Neutrinos are created as a mixture of mass eigenstates. Each eigenstate is a wave packet with a unique group velocity. If the wave packets are too narrow, they eventually stop overlapping as the wave evolves, and quantum interference is lost. If the wave packets are too broad, a single mass eigenstate is resolved by Heisenberg’s uncertainty principle, and quantum interference is also lost. No quantum interference means no neutrino oscillations.

“Coherence conditions constrain the lengths of neutrino wave packets both from below and above,” explains theorist Evgeny Akhmedov of MPI-K Heidelberg. “For neutrinos, these constraints are compatible, and the allowed window is very large because neutrinos are very light. This also hints at an answer to the frequently asked question of why charged leptons don’t oscillate.”

The spatial extent of the neutrino wavepacket has so far only been constrained to within 13 orders of magnitude by reactor-neutrino oscillations, say the BeEST team. If wave-packet sizes were at the experimental lower limit set by the world’s oscillation data, it could have impacted future oscillation experiments, such as the Jiangmen Underground Neutrino Observatory (JUNO) that is currently under construction in China.

“This could have destroyed JUNO’s ability to probe the neutrino mass ordering,” says Akhmedov, “however, we expect the actual sizes to be at least six orders of magnitude larger than the lowest limit from the world’s oscillation data. We have no hope of probing them in terrestrial oscillation experiments, in my opinion, though the situation may be different for astrophysical and cosmological neutrinos.”

BeEST uses a novel method to constrain the size of the neutrino wavepacket. The group creates electron neutrinos via electron capture on unstable ⁷Be nuclei produced at the TRIUMF–ISAC facility in Vancouver. In the final state there are only two products: the electron neutrino and a newly transmuted ⁷Li daughter atom that receives a tiny energy “kick” by emitting the neutrino. By embedding the ⁷Be isotopes in superconducting quantum sensors at 0.1 K, the collaboration can measure this low-energy recoil to high precision. Via the uncertainty principle, the team infers a limit on the spatial localisation of the entire final-state system of 6.2 pm – more than 1000 times larger than the nucleus itself.

Consensus has not been reached on how to infer the new lower limit on the size of the neutrino wave packet, with the preprint quoting two lower limits in the vicinity of 10^–11 m and 10^–8 m based on different theoretical assumptions. Although they differ dramatically, even the weaker limit improves upon all previous reactor oscillation data by more than an order of magnitude, and is enough to rule out decoherence effects as an explanation for sterile-neutrino anomalies, says the collaboration.

“I think the more stringent limit is correct,” says Akhmedov, who points out that this is only about 1.5 orders of magnitude lower than some theoretical predictions. “I am not an experimentalist and therefore cannot judge whether an improvement of 1.5 orders of magnitude can be achieved in the foreseeable future, but I very much hope that this is possible.”

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The problem lies with how neutrinos acquire mass. Nonzero neutrino masses are not possible without the existence of new fundamental fields, beyond those that are part of the SM. And we know virtually nothing about the particles associated with them. They could be bosons or fermions, light or heavy, charged or neutral, and experimentally accessible or hopelessly out of reach.

This is the neutrino mass puzzle. At its heart is the particle’s uniquely elusive nature, which is both the source of the problem and the main challenge in resolving it.

Mysterious and elusive

Despite outnumbering other known massive particles in the universe by 10 orders of magnitude, neutrinos are the least understood of the matter particles. Unlike electrons, they do not participate in electromagnetic interactions. Unlike quarks, they do not participate in the strong interactions that bind protons and neutrons together. Neutrinos participate only in aptly named weak interactions. Out of the trillions of neutrinos that the Sun beams through you each second, only a handful will interact with your body during your lifetime.

Neutrino physics has therefore had a rather tortuous and slow history. The existence of neutrinos was postulated in 1930 but only confirmed in the 1950s. The hypothesis that there are different types of neutrinos was first raised in the 1940s but only confirmed in the 1960s. And the third neutrino type, postulated when the tau lepton was discovered in the 1970s, was only directly observed in the year 2000. Nonetheless, over the years neutrino experiments have played a decisive role in the development of the most successful theory in modern physics: the SM. And at the turn of the 21st century, neutrino experiments revealed that there is something missing in its description of particle physics.

Neutrinos are fermions with spin one-half that interact with the charged leptons (the electron, muon and tau lepton) and the particles that mediate the weak interactions (the W and Z bosons). There are three neutrino types, or flavours: electron-type (ν_e), muon-type (ν_μ) and tau-type (ν_τ), and each interacts exclusively with its namesake charged lepton. One of the predictions of the SM is that neutrino masses are exactly zero, but a little over 25 years ago, neutrino experiments revealed that this is not exactly true. Neutrinos have tiny but undeniably nonzero masses.

Mixing it up

The search for neutrino masses is almost as old as Pauli’s 93-year-old postulate that neutrinos exist. They were ultimately discovered around the turn of the millennium through the observation of neutrino flavour oscillations. It turns out that we can produce one of the neutrino flavours (for example ν_μ) and later detect it as a different flavour (for example ν_e) so long as we are willing to wait for the neutrino flavour to change. The probability associated with this phenomenon oscillates in spacetime with a characteristic distance that is inversely proportional to the differences of the squares of the neutrino masses. Given the tininess of neutrino masses and mass splittings, these distances are frequently measured in hundreds of kilometres in particle-physics experiments.

Neutrino oscillations also require the leptons to mix. This means that the neutrino flavour states are not particles with a well defined mass but are quantum superpositions of different neutrino states with well defined masses. The three mass eigenstates are related to the three flavour eigenstates via a three-dimensional mixing matrix, which is usually parameterised in terms of mixing angles and complex phases.

In the last few decades, precision measurements of neutrinos produced in the Sun, in the atmosphere, in nuclear reactors and in particle accelerators in different parts of the world, have measured the mixing parameters at the several percent level. Assuming the mixing matrix is unitary, all but one have been shown to be nonzero. The measurements have revealed that the three neutrino mass eigenvalues are separated by two different mass-squared differences: a small one of order 10^–4 eV² and a large one of order 10^–3 eV². Data therefore reveal that at least two of the neutrino masses are different from zero. At least one of the neutrino masses is above 0.05 eV, and the second lightest is at least 0.008 eV. While neutrino oscillation experiments cannot measure the neutrino masses directly, precise measurements of beta-decay spectra and constraints from the large-scale structure of the universe offer complementary upper limits. The nonzero neutrino masses are constrained to be less than roughly 0.1 eV.

These masses are tiny when compared to the masses of all the other particles (see “Chasm” figure). The mass of the lightest charged fermion, the electron, is of order 10⁶ eV. The mass of the heaviest fermion, the top quark, is of order 10¹¹ eV, as are the masses of the W, Z and Higgs bosons. These particle masses are all at least seven orders of magnitude heavier than those of the neutrinos. No one knows why neutrino masses are dramatically smaller than those of all other massive particles.

The Standard Model and mass

To understand why the SM predicts neutrino masses to be zero, it is necessary to appreciate that particle masses are complicated in this theory. The reason is as follows. The SM is a quantum field theory. Interactions between the fields are strictly governed by their properties: spin, various “local” charges, which are conserved in interactions, and – for fermions like the neutrinos, charged leptons and quarks – another quantum number called chirality.

In quantum field theories, mass is the interaction between a right-chiral and a different left-chiral field. A naive picture is that the mass-interaction constantly converts left-chiral states into right-chiral ones (and vice versa) and the end result is a particle with a nonzero mass. It turns out, however, that for all known fermions, the left-chiral and right-chiral fermions have different charges. The immediate consequence of this is that you can’t turn one into the other without violating the conservation of some charge so none of the fermions are allowed to have mass: the SM naively predicts that all fermion masses are zero!

The Higgs field was invented to fix this shortcoming. It is charged in such a way that some right-chiral and left-chiral fermions are allowed to interact with one another plus the Higgs field which, uniquely among all known fields, is thought to have been turned on everywhere since the phase transition that triggered electroweak symmetry breaking very early in the history of the universe. In other words, so long as the vacuum configuration of the Higgs field is not trivial, fermions acquire a mass thanks to these interactions.

This is not only a great idea; it is also at least mostly correct, as spectacularly confirmed by the discovery of the Higgs boson a little over a decade ago. It has many verifiable consequences. One is that the strength with which the Higgs boson couples to different particles is proportional to the particle ’s mass – the Higgs prefers to interact with the top quark or the Z or W bosons relative to the electron or the light quarks. Another consequence is that all masses are proportional to the value of the Higgs field in the vacuum (10¹¹ eV) and, in the SM, we naively expect all particle masses to be similar.

Neutrino masses are predicted to be zero because, in the SM, there are no right-chiral neutrino fields and hence none for the left-chiral neutrinos – the ones we know about – to “pair up” with. Neutrino masses therefore require the existence of new fields, and hence new particles, beyond those in the SM.

Wanted: new fields

The list of candidate new fields is long and diverse. For example, the new fields that allow for nonzero neutrino masses could be fermions or bosons; they could be neutral or charged under SM interactions, and they could be related to a new mass scale other than the vacuum value of the SM Higgs field (10¹¹ eV), which could be either much smaller or much larger. Finally, while these new fields might be “easy” to discover with the current and near-future generation of experiments, they might equally turn out to be impossible to probe directly in any particle-physics experiment in the foreseeable future.

Though there are too many possibilities to list, they can be classified into three very broad categories: neutrinos acquire mass by interacting with the same Higgs field that gives mass to the charged fermions; by interacting with a similar Higgs field with different properties; or through a different mechanism entirely.

At first glance, the simplest idea is to postulate the existence of right-chiral neutrino fields and further assume they interact with the Higgs field and the left-chiral neutrinos, just like right-chiral and left-chiral charged leptons and quarks. There is, however, something special about right-chiral neutrino fields: they are completely neutral relative to all local SM charges. Returning to the rules of quantum field theory, completely neutral chiral fermions are allowed to interact “amongst themselves” independent of whether there are other right-chiral or left-chiral fields around. This means the right-chiral neutrino fields should come along with a different mass that is independent from the vacuum value of the Higgs field of 10¹¹ eV.

To prevent this from happening, the right-chiral neutrinos must possess some kind of conserved charge that is shared with the left-chiral neutrinos. If this scenario is realised, there is some new, unknown fundamental conserved charge out there. This hypothetical new charge is called lepton number: electrons, muons, tau leptons and neutrinos are assigned charge plus one, while positrons, antimuons, tau antileptons and antineutrinos have charge minus one. A prediction of this scenario is that the neutrino and the antineutrino are different particles since they have different lepton numbers. In more technical terms, the neutrinos are massive Dirac fermions, like the charged leptons and the quarks. In this scenario, there are new particles associated with the right-chiral neutrino field, and a new conservation law in nature.

Accidental conservation

As of today, there is no experimental evidence that lepton number is not conserved, and readers may question if this really is a new conservation law. In the SM, however, the conservation of lepton number is merely “accidental” – once all other symmetries and constraints are taken into account, the theory happens to possess this symmetry. But lepton number conservation is no longer an accidental symmetry when right-chiral neutrinos are added, and these chargeless and apparently undetectable particles should have completely different properties if it is not imposed.

If lepton number conservation is imposed as a new symmetry of nature, making neutrinos pure Dirac fermions, there appears to be no observable consequence other than nonzero neutrino masses. Given the tiny neutrino masses, the strength of the interaction between the Higgs boson and the neutrinos is predicted to be at least seven orders of magnitude smaller than all other Higgs couplings to fermions. Various ideas have been proposed to explain this remarkable chasm between the strength of the neutrino’s interaction with the Higgs field relative to that of all other fermions. They involve a plurality of theoretical concepts including extra-dimensions of space, mirror copies of our universe and dark sectors.

A second possibility is that there are more Higgs fields in nature and that the neutrinos acquire a mass by interacting with a Higgs field that is different from the one that gives a mass to the charged fermions. Since the neutrino mass is proportional to the vacuum value of a different Higgs field, the fact that the neutrino masses are so small is easy to tolerate: they are simply proportional to a different mass scale that could be much smaller than 10¹¹ eV. Here, there are no right-chiral neutrino fields and the neutrino masses are interactions of the left-chiral neutrino fields amongst themselves. This is possible because, while the neutrinos possess weak-force charge they have no electric charge. In the presence of the nontrivial vacuum of the Higgs fields, the weak-force charge is effectively not conserved and these interactions may be allowed. The fact that the Higgs particle discovered at the LHC – associated with the SM Higgs field – does not allow for this possibility is a consequence of its charges. Different Higgs fields can have different weak-force charges and end up doing different things. In this scenario, the neutrino and the antineutrino are, in fact, the same particle. In more technical terms: the neutrinos are massive Majorana fermions.

Neutrino masses require the existence of new fields, and hence new particles, beyond those in the Standard Model

One way to think about this is as follows: the mass interaction transforms left-chiral objects into right-chiral objects. For electrons, for example, the mass converts left-chiral electrons into right-chiral electrons. It turns out that the antiparticle of a left-chiral object is right-chiral and vice versa, and it is tempting to ask whether a mass interaction could convert a left-chiral electron into a right-chiral positron. The answer is no: electrons and positrons are different objects and converting one into the other would violate the conservation of electric charge. But this is no barrier for the neutrino, and we can contemplate the possibility of converting a left-chiral neutrino into its right-chiral antiparticle without violating any known law of physics. If this hypothesis is correct, the hypothetical lepton-number charge, discussed earlier, cannot be conserved. This hypothesis is experimentally neither confirmed nor contradicted but could soon be confirmed with the observation of neutrinoless double-beta decays – nuclear decays which can only occur if lepton-number symmetry is violated. There is an ongoing worldwide campaign to search for the neutrinoless double-beta decay of various nuclei.

A new source of mass

In the third category, there is a source of mass different from the vacuum value of the Higgs field, and the neutrino masses are an amalgam of the vacuum value of the Higgs field and this new source of mass. A very low new mass scale might be discovered in oscillation experiments, while consequences of heavier ones may be detected in other types of particle-physics experiments, including measurements of beta and meson decays, charged-lepton properties, or the hunt for new particles at high-energy colliders. Searches for neutrinoless double-beta decay can reveal different sources for lepton-number violation, while ultraheavy particles can leave indelible footprints in the structure of the universe through cosmic collisions. The new physics responsible for nonzero neutrino masses might also be related to grand-unified theories or the origin of the matter–antimatter asymmetry of the universe, through a process referred to as leptogenesis. The range of possibilities spans 22 orders of magnitude (see “eV to ZeV” figure).

Challenging scenarios

Since the origin of the neutrino masses here is qualitatively different from that of all other particles, the values of the neutrino masses are expected to be qualitatively different. Experimentally, we know that neutrino masses are much smaller than all charged- fermion masses, so many physicists believe that the tiny neutrino masses are strong indirect evidence for a source of mass beyond the vacuum value of the Higgs field. In most of these scenarios, the neutrinos are also massive Majorana fermions. The challenge here is that if a new mass scale exists in fundamental physics, we know close to nothing about it. It could be within direct reach of particle-physics experiments, or it could be astronomically high, perhaps as large as 10¹² times the vacuum value of the SM’s Higgs field.

Searching for neutrinoless double-beta decay is the most promising avenue to reveal whether neutrinos are Majorana or Dirac fermions

How do we hope to learn more? We need more experimental input. There are many outstanding questions that can only be answered with oscillation experiments. These could provide evidence for new neutrino-like particles or new neutrino interactions and properties. Meanwhile, searching for neutrinoless double-beta decay is the most promising avenue to experimentally reveal whether neutrinos are Majorana or Dirac fermions. Other activities include high-energy collider searches for new Higgs bosons that like to talk to neutrinos and new heavy neutrino-like particles that could be related to the mechanism of neutrino mass generation. Charged-lepton probes, including measurements of the anomalous magnetic moment of muons and searches for lepton-flavour violation, may provide invaluable clues, while surveys of the cosmic microwave background and the distribution of galaxies could also reveal footprints of the neutrino masses in the structure of the universe.

We still know very little about the new physics uncovered by neutrino oscillations. Only a diverse experimental programme will reveal the nature of the new physics behind the neutrino mass puzzle.

The post The neutrino mass puzzle appeared first on CERN Courier.

IceCube’s 5160 optical sensors positioned deep within the Antarctic ice detect around 100,000 neutrinos per year, some of which are the most energetic events ever recorded. To make sure that the detector is operational throughout the year, people are required to spend extended periods at the South Pole, where temperatures are on average around –60°C during the winter.

Marc Jacquart was one of two “winterovers” for IceCube during the season November 2022 to November 2023. Having completed his master’s degree, during which he analysed IceCube data, he saw an internal email about the position and applied: “It was a long-time dream-come-true. I had wanted to go to the South Pole since I heard about IceCube six years earlier.” First he had to pass medical tests, a routine requirement for winterovers because it is difficult to evacuate people during the winter. His next stop was the University of Wisconsin–Madison, the lead institution for the IceCube collaboration, where he and his colleague Hrvoje Dujmović received three months’ training on how to operate, troubleshoot, calibrate and repair IceCube’s hardware and software components using a small replica of the data centre. “Our job is to ensure the highest detector uptime, so we need to know how to fix a problem immediately if something breaks.”

The pair made their way to McMurdo Station on the shores of Antarctica closest to New Zealand in early November 2022. From there, a plane took them 1350 km to the Amundsen–Scott station, located 2835 m above sea level and only 150 m from the geographic South Pole. During the summer, up to 150 people stay at the station to make major repairs and upgrades to the research facilities, which also include the South Pole Telescope, BICEP and an atmospheric research observatory. By mid-February, most people leave. “We were only 43 winterovers left, and that’s when you can help each other and busy yourself with all kinds of things,” says Marc.

Part of station life is volunteering for teams, which in Marc’s case included the fire fighters, amongst others. To bide their time during a nearly six-month-long night, the inhabitants can go to the library, music room or grow vegetables in a repurposed biology experiment to freshen up the preserved foods. While winter in the Antarctic Circle is harsh outside, says Marc, it has one major highlight: the southern lights. “I remember one time, they were just dancing, moving and very bright. We stayed outside for a full hour packed in layers and layers of clothes!”

The only real downtime for the detector is when operators perform a full restart every 32 hours

As a winterover, Marc ensured that the IceCube detector worked 24/7 and recorded every incoming neutrino. “Usually, we have 99.9% uptime. If there is something wrong, we have a pager that pings us, even in the middle of the night.” To ensure that the rarest high-energy neutrinos are recorded, the only real downtime for the detector, he says, is when operators perform a full restart every 32 hours. For such events, which could point to high-energy phenomena in the universe, IceCube sends a real-time alert to other experiments. About 200 machines are located in the data centre and collect 1 TB of data per day, only 10% of which are sent north to a data centre in the US due to satellite-bandwidth limitations. The remaining data gets stored on hard drives, which must be swapped manually by the winterovers every two weeks. During the summer, when aircraft can reach the South Pole on a regular basis, boxes stashed with hard drives are taken back for thorough data analysis and archiving.

Since returning home to Switzerland, Marc is considering his next steps. “I have the opportunity to work on a radio observatory in the US next year. After a year operating the IceCube detector, I’m interested to work with hardware more. And I am definitely considering a PhD with IceCube afterwards, as there is a lot coming up.” Currently, the IceCube collaboration is working towards IceCube-Gen2, with the first step being to add seven strings with improved optical modules to the existing underground complex. In a second step, 120 further cables with refined light sensors will optimise the detector, and two radio detectors as well as an extended array will be placed on the surface. The upgrades will enlarge IceCube’s coverage from one to eight cubic kilometres, offering more than enough tasks for future winterovers during the decade . “Maybe in a few years I would be keen to return to the South Pole. It’s a very special place.”

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The aims of the Magnificent CEvNS workshop, named after the Hollywood Western, are to bring together the broad community of researchers working on CEvNS and promote student engagement and connection among experimentalists, theorists and phenomenologists in this new field. The first workshop was held in 2018 in Chicago, and the most recent in Munich from 22 to 24 March with 96 participants.

Examining CEvNS opens a multitude of promising ways to look for BSM interactions. Improved limits on generalised neutrino interactions, new light mediators and sterile neutrinos derived from the complete COHERENT dataset were presented. These data enable the nuclear radius to be probed in a new way. More physics potential was highlighted in talks showing limits on the Weinberg angle and dark matter (axion-like particles). Notable advances by reactor experiments include new limits on CEvNS on germanium by the CONUS and NuGen experiments, which disagree with the previously published Dresden-II results.

The talks underlined the large experimental effort toward a complete mapping of the neutron and energy dependence of the CEvNS cross section. The observation of CEvNS on CsI and Ar by the COHERENT experiment will be complemented with future measurements on targets ranging from light (sodium) to heavy (tungsten) elements in COHERENT and new facilities such as NUCLEUS and Ricochet. Precision will be achieved by increasing statistics in CEvNS events with larger target masses, lower detection thresholds and increased neutrino flux. Reducing systematic effects by characterising backgrounds and detector responses is also critical. The growing precision will trigger studies on BSM physics in the near future, complementing high-energy experimental efforts.

A half-day satellite workshop “Into the Blue Sky” was dedicated to new ideas related to the CEvNS community. These included measurements of neutrino-induced fission, and detector concepts based on latent damage to the crystalline structure of minerals and superconducting crystals. The workshop was followed by a school organised by the Collaborative Research Center “Neutrinos and Dark Matter in Astro- and Particle Physics” at TU Munich from 27 to 29 March. Six lectures covered the fundamentals of low-energy neutrino physics with a focus on CEvNS, backgrounds, neutrino sources and detectors. The 40 participants then applied this knowledge by creating a fictional micro-CEvNS experiment.

Half a century since it was proposed theoretically, the physics accessible with CEvNS is proving to be extensive. The next Magnificent CEvNS workshop will take place next year at a new location and the participants are looking forward to further exploration of the CEvNS frontier.

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To improve the latter further, the T2K collaboration decided in 2016 to upgrade ND280 with a novel scintillator tracker, two TPCs and a time-of-flight system. This upgrade, in combination with an increase in neutrino beam power from the current 500 kW to 1.3 MW, will increase the statistics by a factor of about four and reduce the systematic uncertainties from 6% to 4%. The upgraded ND280 is also expected to serve as a near detector of the next generation long-baseline neutrino oscillation experiment Hyper-Kamiokande. 

Meanwhile, R&D and testing for the prototype detectors for the DUNE experiment at the Long Baseline Neutrino Facility at Fermilab/SURF in the US is entering its final stages. 

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Since their discovery 67 years ago, neutrinos from a range of sources – solar, atmospheric, reactor, geological, accelerator and astrophysical – have provided ever more powerful probes of nature. Although neutrinos are also produced abundantly in colliders, until now no neutrinos produced in such a way had been detected, their presence inferred instead via missing energy and momentum. 

A new LHC experiment called FASER, which entered operations at the start of Run 3 last year, has changed this picture with the first observation of collider neutrinos. Announcing the result on 19 March at the Rencontres de Moriond, and in a paper submitted to Physical Review Letters on 24 March, the FASER collaboration reconstructed 153 candidate muon neutrino and antineutrino interactions in its spectrometer with a significance of 16 standard deviations above the background-only hypothesis. Being consistent with the characteristics expected from neutrino interactions in terms of secondary-particle production and spatial distribution, the results imply the observation of both neutrinos and antineutrinos with an incident neutrino energy significantly above 200 GeV. In addition, an ongoing analysis of data from an emulsion/tungsten subdetector called FASERν revealed a first electron–neutrino interaction candidate (see image). 

“FASER has directly observed the interactions of neutrinos produced at a collider for the first time,” explains co-spokesperson Jamie Boyd of CERN. “This result shows the detector worked perfectly in 2022 and opens the door for many important future studies with high-energy neutrinos at the LHC.” 

The extreme luminosity of proton–proton collisions at the LHC produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a compact apparatus. FASER is one of two new forward experiments situated at either side of LHC Point 1 to detect neutrinos produced in proton–proton collisions in ATLAS. The other, SND@LHC, also reported its first results at Moriond. The team found eight muon–neutrino candidate events against an expected background of 0.2, with an evaluation of systematic uncertainties ongoing. 

Covering energies between a few hundred GeV and several TeV, FASER and SND@LHC narrow the gap between fixed-target and astrophysical neutrinos. One of the unexplored physics topics to which they will contribute is the study of high-energy neutrinos from astrophysical sources. Since the production mechanism and energy of neutrinos at the LHC is similar to that of very-high-energy neutrinos from cosmic-ray collisions with the atmosphere, FASER and SND@LHC can be used to precisely estimate this background. Another application is to measure and compare the production rate of all three types of neutrinos, providing an important test of the Standard Model.

Beyond neutrinos, the two experiments open new searches for feebly interacting particles and other new physics. In a separate analysis, FASER presented first results from a search for dark photons decaying to an electron-positron pair. No events were seen in an almost background-free analysis, yielding new constraints on dark photons with couplings of 10^–5 to 10^–4 and masses of between 10 and 100 MeV, in a region of parameter space motivated by dark matter. 

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Neutrinoless double-beta decay (0νββ) remains as elusive as ever, following publication of the final results from the Majorana Demonstrator experiment at SURF, South Dakota, in February. Based on six years’ monitoring of ultrapure ⁷⁶Ge crystals, corresponding to an exposure of 64.5 kg × yr, the collaboration has confirmed that the half-life of 0νββ in this isotope is greater than 8.3 × 10²⁵ years. This translates to an upper limit of an effective neutrino mass m_ββ of 113–269 meV, and complements a number of other 0νββ experiments that have recently concluded data-taking. 

Whereas double-beta decay is known to occur in several nuclides, its neutrinoless counterpart is forbidden by the Standard Model. That’s because it involves the simultaneous decay of two neutrons into two protons with the emission of two electrons and no neutrinos, which is only possible if neutrinos and antineutrinos are identical “Majorana” particles such that the two neutrinos from the decay cancel each other out. Such a process would violate lepton-number conservation, possibly playing a role in the matter–antimatter asymmetry in the universe, and be a direct sign of new physics. The discovery that neutrinos have mass, which is a necessary condition for them to be Majorana particles, motivated experiments worldwide to search for 0νββ in a variety of candidate nuclei.

Germanium-based detectors have an excellent energy resolution, which is key to be able to resolve the energy of the electrons emitted in potential 0νββ decays. The Majorana Demonstrator is also located 1.5 km underground, with low-noise electronics and ultrapure in-house-grown electroformed copper surrounding the detectors to shield it from background events. Despite a lower exposure, the collaboration was able to achieve similar limits to the GERDA experiment at Gran Sasso National Laboratory, which set a lower limit on the ⁷⁶Ge 0νββ half-life of 1.8 × 10²⁶ yr. Also among the projects of the collaboration is an ongoing search for the influence of dark-matter particles in the decay of metastable ^180mTa – nature’s rarest isotope. Although no hints have been found so far, the search has already improved the sensitivity of dark-matter searches in nuclei significantly. 

The search has already improved the sensitivity of dark-matter searches in nuclei significantly

Other experiments, such as KamLAND- ZEN and EXO-200, use ¹³⁶Xe to search for 0νββ. While the former recently set the most stringent limit of 2.3 × 10²⁶ yr and is ongoing, the latter arrived at a value of 3.5 × 10²⁵ yr with a total ¹³⁶Xe exposure of 234.1 kg × yr based on its full dataset. Searches at Gran Sasso with CUORE using 1t × yr exposure of ¹³⁰Te led to a half-life of 2.2 × 10²⁵ yr and at CUORE’s successor, CUPID-0, which used ⁸²Se with a total exposure of 8.82 kg × yr, of the order 10²³ yr.

Having demonstrated the required sensitivity for 0νββ detection in ⁷⁶Ge, the designs of Majorana Demonstrator and GERDA have been incorporated into the next-generation experiment LEGEND-200, which uses high-purity germanium detectors surrounded by liquid argon. The experiment, based at Gran Sasso, started operations last spring and could have initial results later this year, says co-spokesperson Steven Elliot (LANL): “Once all the detectors are installed, we plan to run for five years, while the next stage, LEGEND-1000, is proceeding through the DOE Critical Decision process. We hope to begin construction in summer 2026, with first data available early next decade.”

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Since its inception in 2013, the CERN Neutrino Platform has evolved into a worldwide hub for both experimental and theoretical neutrino physics. Besides its multifaceted activities in hardware development – including most notably the ProtoDUNE detectors for the international long-baseline neutrino programme in the US – the platform also hosts a vibrant group of theorists.

From 13 to 17 March this group once again hosted the CERN Neutrino Platform Pheno Week, after a COVID-related hiatus of more than three years. With about 100 in-person participants and 200 more on Zoom, the meeting has become one of the largest in the field – a testament to the ever-growing popularity of neutrinos among particle physicists, even though neutrinos are the most elusive among all known elementary particles.

Talks at the March event reflected the full breadth of the subject, with the first days devoted to novel theoretical models explaining the peculiar relations observed among neutrino masses and mixing angles, and to understanding the way in which neutrinos interact with nuclei. The latter topic is particularly complex, given the vast range of energies in which neutrinos are studied – from non-relativistic cosmic background neutrinos with sub-meV energies to PeV-scale neutrinos observed in neutrino telescopes. An especially popular topic has also been the possibility of discovering physics beyond the Standard Model in the neutrino sector. In fact, because of their ability to mix with hypothetical “dark sector” fermions – that is, fermions potentially related to the physics of dark matter, or even dark matter itself – neutrinos offer a unique window to new physics.

The second part of the workshop was devoted to the neutrino’s role in astrophysics and cosmology. “There’s actually a two-way relationship between neutrinos and the cosmos,” explained invited speaker John Beacom (Ohio State University). “On the one hand, astrophysical and cosmological observations can teach us a lot about neutrino properties. On the other, neutrinos are unique cosmic messengers, and from observations at neutrino telescopes we can learn fascinating things about stars, galaxies and the evolution of the universe.” In recent years, for instance, neutrinos have allowed physicists to shed new light on the century-old problem of where ultra-high-energy cosmic rays come from. And the next galactic supernova – an event that happens on average every 30 to 100 years – will be a treasure trove of new information, given that we expect to observe tens of thousands of neutrinos from such an event. At the same time, cosmology sets the strongest upper limits on the absolute scale of neutrino masses, and with the next generation of cosmological surveys we have every expectation to achieve an actual measurement of this quantity. This is interesting because neutrino oscillations, while establishing that neutrinos have non-zero mass, are only sensitive to differences of squared masses, not to the absolute mass scale.

The programme of the Neutrino Platform Pheno Week closed with a tour of the ProtoDUNE experiments, giving the mostly theory-oriented audience an impression of how the magnificent machines testing our theories of the neutrino sector are being developed and assembled.

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By 2006, based on results from scattering and spectroscopic measurements, the Committee on Data for Science and Technology (CODATA) had established the proton charge radius to be 0.8760(78) fm. Then, in 2010, came a surprise: the CREMA collaboration at the Paul Scherrer Institut (PSI) reported a value of 0.8418(7) fm based a novel, high-precision spectroscopic measurement of muonic hydrogen. Disagreeing with previous spectroscopic measurements, and lying more than 5σ below the CODATA world average, the result gave rise to the “proton radius puzzle”. While the most recent electron–proton scattering and hydrogen-spectroscopy measurements are in closer agreement with the latest muonic-hydrogen results, the discrepancies with earlier experiments are not yet fully understood.

Now, the MINERνA collaboration has brought a new tool to gauge the proton’s size: neutrino scattering. Whereas traditional scattering measurements probe the proton’s electric or magnetic charge distributions, which are encoded in vector form factors, scattering by neutrinos allows the analogous axial-vector form factor F_A, which characterises the proton’s weak charge distribution, to be measured. In addition to providing a complementary probe of proton structure, F_A is key to precise measurements of neutrino-oscillation parameters at experiments such as DUNE, Hyper-K, NOvA and T2K.

MINERνA is a segmented scintillator detector with hexagonal planes made from strips of triangular cross-section, which are assembled into planes perpendicular to the incoming beam. By studying how a beam of muon antineutrinos produced by Fermilab’s NuMI neutrino beamline interacts with a polystyrene target, which contains hydrogen closely bonded to carbon, the MINERνA researchers were able to make the first high-statistics measurement of the ν_μ p → μ⁺ n cross-section using the hydrogen atom in polystyrene. Extracting F_A from 5580 ± 180 signal events (observed over an estimated background of 12,500), they measured the nucleon axial charge radius to be 0.73(17) fm, in agreement with the electric charge radius measured with electron scattering.

“If we weren’t optimists, we’d say [this measurement] was impossible,” says lead author Tejin Cai, who proposed the idea of using a polystyrene target to access neutrino-hydrogen scattering while a PhD student at the University of Rochester. “The hydrogen and carbon are chemically bonded, so the detector sees interactions on both at once. But then, I realised that the very nuclear effects that made scattering on carbon complicated also allowed us to select hydrogen and would allow us to subtract off the carbon interactions.”

A new experiment called AMBER, at the M2 beamline of CERN’s Super Proton Synchrotron, is about to open another perspective on the proton charge radius. AMBER is the successor to COMPASS, which played a major role towards resolving the proton “spin crisis” (the finding, by the European Muon Collaboration in 1987, that quarks account for less than a third of the total proton spin) by studying the contribution to the proton spin from gluons. Instead of electrons, AMBER will use muon scattering at unprecedented energies (around 100 GeV) to access the small momentum-transfer needed to measure the proton radius. A future experiment at PSI called MUSE, meanwhile, aims to determine the proton radius through simultaneous measurements of muon– and electron–proton scattering.

AMBER is scheduled to start with a pilot run in September 2023 and to operate for up to three years, with the goal to find a value for the proton radius in the range 0.84–0.88 fm, as expected from previous experiments, and with an uncertainty of about 0.01 fm. “Some colleagues say that there is no proton-radius puzzle, only problematic measurements,” says AMBER spokesperson Jan Friedrich of TU Munich. “The discrepancy between theory and experiments, as well as between individual experiments, will have to shrink and align as much as possible. After all, there is only one true proton radius.” 

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The STEREO experiment, located at the high-flux research reactor at the Institut Laue-Langevin (ILL), Grenoble, is the latest to cast doubt on the existence of an additional, sterile neutrino state. Based on the full dataset collated from October 2017 until the experiment shut down in November 2020, the results support the conclusions of a global analysis of all neutrino data, that a normalisation bias in the beta-decay spectrum of ²³⁵U is the most probable explanation for a deficit of electron neutrinos seen at reactor experiments during the past decade.

The confirmation of neutrino oscillations 25 years ago showed that the lepton content of a given neutrino evolves as it propagates, generating a change of flavour. Numerous experiments based on solar, atmospheric, accelerator, reactor and geological neutrino sources have determined the oscillation parameters in detail, reaffirming the three-neutrino picture obtained by precise measurements of the Z boson’s decay width at LEP. However, several anomalies have also shown up, one of the most prominent being the so-called reactor antineutrino anomaly. Following a re-evaluation of the expected ν_e flux from nuclear reactors by a team at CEA and Subatech in 2011, a deficit in the number of ν_e  detected by reactor neutrino experiments appeared. Combined with a longstanding anomaly reported by short-baseline accelerator-neutrino experiments such as LSND and a deficit in ν_e  seen in calibration data for the solar-neutrino detectors GALLEX and SAGE, excitement grew that an additional neutrino state – a sterile or right-handed neutrino with non-standard interactions that arises in many extensions of the Standard Model – might be at play. 

We anticipate that this result will allow progress towards finer tests of the fundamental properties of neutrinos

Designed specifically to investigate the sterile-neutrino hypothesis, STEREO was positioned about 10 m from the ILL reactor core to measure the evolution of the antineutrino energy spectrum from ²³⁵U fission at short distances with high precision. Comprising six cells filled with gadolinium-doped liquid scintillator positioned at different distances from the reactor core, producing six spectra, the setup allows the hypothesis that ν_e undergo a fast oscillation into a sterile neutrino to be tested independently of the predicted shape of the emitted ν_e spectrum.

The measured antineutrino energy spectrum, based on 107,558 detected antineutrinos, suggests that the previously reported anomalies originate from biases in the nuclear experimental data used for the predictions, while rejecting the hypothesis of a light sterile neutrino with a mass of about 1 eV. “Our result supports the neutrino content of the Standard Model and establishes a new reference for the ²³⁵U antineutrino energy spectrum,” writes the team. “We anticipate that this result will allow progress towards finer tests of the fundamental properties of neutrinos but also to benchmark models and nuclear data of interest for reactor physics and for observations of astrophysical or geoneutrinos.”

Gallium remains

STEREO’s findings fit those reported recently by other neutrino-oscillation experiments. A 2021 analysis by the MicroBooNE collaboration at Fermilab, for example, favoured the Standard Model over an anomalous signal seen by its nearby experiment MiniBooNE, assuming the latter was due to the existence of a non-standard neutrino. Yet the story of the sterile neutrino is not over. In 2022, new results from the Baksan Experiment on Sterile Transitions (BEST) further confirmed the deficit in the ν_e flux emitted from radioactive sources as seen by the SAGE and GALLEX experiments – the so-called gallium anomaly – which, if interpreted in the context of neutrino oscillations, is consistent with ν_e → ν_s oscillations with a relatively large squared mass difference and mixing angle. 

“Under the sterile neutrino hypothesis, a signal in MicroBooNE, MiniBooNE or LSND would require the sterile neutrino to mix with both ν_e and ν_μ, whereas for the gallium anomaly, mixing with ν_e alone is sufficient,” explains theorist Joachim Kopp of CERN. “Even though the reactor anomaly seems to be resolved, we’d still like to understand what’s behind the others.” 

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Neutrinos are the least understood of all elementary particles, and the fact that they have mass is a firm indication of physics beyond the Standard Model. Decades of effort have been devoted to exploring the properties of neutrinos. However, there are still many important questions to address. For example, little is known about the absolute mass scale and neutrino-mass ordering. Also, we have not achieved a decent measurement of the CP phase in the neutrino mixing “PMNS” matrix. Furthermore, the nature of neutrino masses, i.e. whether they are Dirac or Majorana, remains unknown.

From 30 July to 6 August the 23rd NuFACT workshop hosted by the University of Utah reviewed recent developments in neutrino physics, particle physics and astroparticle physics. The workshop brought together experts from all leading neutrino experiments and discussed theoretical aspects, with the aim of facilitating new connections between different disciplines and theorists and experimentalists.

Talking points

NuFACT2022 topics were spread into seven working groups: neutrino oscillations; neutrino scattering physics; accelerator physics; muon physics; neutrinos beyond PMNS; detectors; and inclusion, diversity, equity, education and outreach. The latter was newly established at this year’s workshop to become an integral part of the series.

Three mini-workshops took place. One explored plans for the second phase of the European Spallation Source neutrino Super Beam (ESSνSB) project, for which the European Union has recently decided to continue its support for another four years. This second phase will study new components that open additional physics opportunities including muon studies, precise neutrino cross-section measurements and sterile-neutrino searches.

The two-day mini-workshop “Multi- messenger Tomography of the Earth”, involving 22 talks, saw leading neutrino physicists and geoscientists exchange ideas on how Earth’s interior models may impact high-precision measurements of neutrino oscillation parameters. Participants also addressed the potential of using neutrino absorption at high energies (PeV–TeV) and neutrino oscillation at low energies (~GeV) inside Earth to locate the core–mantle boundary, determine the density of the core and mantle, and measure the chemical composition of the core. A third workshop targeted career development, with the aim of improving communication and negotiation skills among early-career scientists.

Progress in using neutrino-oscillation measurements to search for hints of new physics and symmetries in nature was discussed extensively. Central questions to be addressed include: is the neutrino-mixing angle θ₂₃ exactly 45°, which might hint at a new symmetry in nature? Is the PMNS matrix unitary or could it indicate there are additional neutrinos or something fundamentally wrong with our understanding of the neutrino sector? Are there more than the three active neutrinos? Do we see indications for CP violation in the neutrino sector or is it even maximal? Do neutrino-mass eigenstates follow the same “normal” ordering as observed for quarks, for which there is currently a slight preference in the global fit data ? 

The latest results from leading experiments including IceCube, KM3NeT/ORCA, NOvA,Super-K and T2K were presented. T2K presented a new analysis using the same data runs as last year, but using more data from the near and far detector samples combined with upgraded cross-section and flux models. T2K and NOvA data preferences on δ_CP and sin²θ₂₃ are broadly compatible and joint fit results can be expected for late 2022. For the normal-mass ordering case, the most probable regions are distinct, and the significant contour overlap of 1σ, while no preference on CP violation can be inferred. For the inverted mass ordering case, T2K and NOvA contours overlap and are consistent with maximal CP violation in the neutrino sector.

Particularly competitive results of neutrino oscillation-parameter measurements with neutrino telescopes are available from IceCube–DeepCore and ORCA, and are now approaching the precision of accelerator-based neutrino experiments.

Various theoretical aspects of neutrino physics were covered. The nature of the neutrino mass, either Dirac or Majorana, remains a key focus. Different see-saw mechanism types and their experimental consequences were intensively discussed. In particular, recent progress in Majorana neutrino tests using both neutrinoless double-beta decay experiments as well as LHC measurements by the new FASER experiment were reported. Connecting neutrino and muon experiments, such as charged-lepton-flavour violation and the application of a possible muon collider to neutrino physics, were extensively addressed. The existence of sterile neutrinos and their properties remain of high importance to the field and future experimental results are highly anticipated, such as the short-baseline program at Fermilab and JSNS² at J-PARC. Alternative explanations for various neutrino anomalies were also discussed, including more general dark-sector searches using neutrino experiments. The electron low-energy excess at MicroBooNE in particular draws attention. The focus is on improved event reconstructions, which may unveil the nature of this anomalous excess. Assuming the existence of one species of sterile neutrino, 3+1 oscillation analyses have been carried out to interpret the anomaly and compare with results from other experiments. Although inconclusive, this anomaly triggers many interesting ideas that will motivate follow-up studies.

Taking place shortly after the Snowmass Summer Meeting in Seattle (see Charting the future of US particle physics), NuFACT2022 also offered an opportunity to summarise the scientific vision for the future of neutrino physics in the US. The neutrino frontier in Snowmass has 10 topical groups, with physics beyond the Standard Model and neutrinos as messengers emerging as major focuses. Many possible synergies between neutrino physics and other branches of physics were also highlighted. 

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KM3NeT (the Cubic Kilometre Neutrino Telescope) is the successor to the ANTARES neutrino telescope, which operated continuously from 2008 and has recently been decommissioned (see “The ANTARES legacy” panel). KM3NeT comprises two detectors: ARCA (Astroparticle Research with Cosmics in the Abyss), located at a depth of 3500 m offshore from Sicily, and ORCA (Oscillation Research with Cosmics in the Abyss), located at a depth of 2450 m offshore from southern France. ARCA is a sparse detector of about 1 km³ that is optimised for the detection of TeV–PeV neutrinos, while ORCA is a 7 Mt-dense detector optimised for sub-TeV neutrinos. The KM3NeT collaboration comprises more than 250 scientists from 16 countries.

The key technology is the digital optical module (DOM) – a pressure-resistant glass sphere hosting 31 three-inch photomutiplier tubes, various calibration devices and the readout electronics (see “Modular” image). A total of 18 DOMs are hosted on a single detection line, and the lines are anchored to the seafloor and held taut by a submerged buoy. The ORCA detector will comprise around 100 lines and the ARCA detector will have twice as many. The bases of the lines are connected via cables on the seafloor to junction boxes, from which electro-optical  cables many tens of kilometres long bring the data to shore along optical fibres. Information on every single photon is transmitted to the shore stations, where trigger algorithms are applied to select interesting events for offline analysis.

From the light pattern recorded by the DOMs, the energy and the direction of a neutrino can be estimated. Furthermore, the neutrino flavour can also be distinguished; muon neutrino charged–current (CC) interactions produce an extended track-like signature (see “Subsea shower” image) whereas electron– and tau–neutrino CC interactions, as well as neutral-current interactions, produce more compact shower-like events. By selecting up-going neutrinos, i.e. those that have travelled from the other side of Earth, the large background from down-going atmospheric muons can be rejected and a clean sample of neutrinos obtained. 

The first KM3NeT detection line was connected in 2016 and currently a total of 32 lines are operating at the two sites. The first science results with these partial detectors have already been obtained. 

Fundamental neutrino properties

Sixty-six years after their discovery, neutrinos remain the most mysterious of the fermions. As they whiz through the universe, barely interacting with any other particles, they have the unique ability to oscillate between their three different types or flavours (electron, muon and tau). The observation of neutrino oscillations in the late 1990s implies that neutrinos have a non-zero mass, contrary to the Standard Model expectation. Understanding the origin and order of the neutrino masses could therefore unlock a path to new physics. Numerous neutrino experiments around the world are closing in on the neutrino’s properties, using both artificial (accelerator and reactor) and natural (atmospheric and extraterrestrial) neutrino sources. 

The KM3NeT/ORCA array is optimised for the detection of atmospheric neutrinos, produced when cosmic rays strike atomic nuclei at an altitude of around 15 km. Such interactions produce a cascade of particles on Earth’s surface, mostly pions and kaons, which decay to neutrinos capable of traversing the entire planet. About two thirds of these are muon neutrinos and antineutrinos, and the remainder are electron neutrinos and antineutrinos. 

Measuring the directions and energies of the detected atmospheric neutrinos allows the oscillatory behaviour of neutrinos to be studied, and thus elements of the leptonic “PMNS” mixing matrix to be determined. The measured direction is used as a proxy for the distance the atmospheric neutrino has travelled through Earth between its points of production and detection. First preliminary results with six ORCA lines and one year of data clearly show the expected disappearance of muon neutrinos with increasing baseline/energy. The corresponding constraints on θ₂₃ (the mixing angle between the m₂ and m₃ states) and Δm²₃₂ (the mass difference of the squared masses) already start to be competitive with multi-year results from the current long-baseline accelerator experiments (see “Physics debut” figure). 

The ANTARES legacy

Building a telescope anchored deep at the bottom of the sea requires skill, patience and expertise. KM3NeT would not be on its way without the invaluable experience gained from its older sibling, the ANTARES telescope. ANTARES operated continuously for more than 15 years, and pioneered solutions to construct and operate a neutrino detector in the challenging environment of the deep sea. Despite ANTARES containing only 12 detector lines compared to 86 in IceCube, its superior angular resolution (due to the intrinsic water properties) and its Northern Hemisphere location provided competitive results and valuable insights and constraints in various domains.

Following IceCube’s discovery of a diffuse flux of cosmic neutrinos, the ANTARES all-flavour neutrino data sample revealed a mild (1.8σ) excess of high-energy events consistent with the neutrino signal detected by IceCube. ANTARES also contributed strongly to the multi-messenger endeavour, participating in the search for a neutrino counterpart to major alerts from the LIGO/Virgo gravitational-wave interferometers, IceCube, ground-based imaging air Cherenkov telescopes, as well as X- and gamma-ray satellites. For instance, the TXS0506+056 blazar is the second most significant point source, with a local significance of 2.8σ, strengthening its case as the first high-energy neutrino source. ANTARES also distributed its own neutrino alerts with an unprecedented low latency for a neutrino telescope.

Its energy threshold of a few tens of GeV allowed the study of atmospheric muon neutrino disappearance due to neutrino oscillations and to constrain the “3+1” neutrino model. In this domain, results consistent with world best-fit values were obtained, as well as competitive limits on non-standard interactions. The data were also used to search for dark-matter particles that would have accumulated in astrophysical bodies like the Sun or the galactic centre before annihilating or decaying into neutrinos. Since no excesses were found, competitive limits were set that reduce the parameter space to be explored by direct, indirect (including KM3NeT) and collider dark-matter experiments.

Recently superseded in sensitivity by KM3NeT, ANTARES was finally decommissioned in February 2022.

A longer-term physics goal of KM3NeT is to determine the neutrino mass ordering, i.e. whether the third neutrino mass eigenstate is heavier or lighter than the first two. This is important to help constrain the plethora of theoretical models proposed to explain the neutrino masses. Due to the large distances travelled by atmospheric neutrinos as they pass through Earth’s mantle and core, subtle matter effects come into play and distort the expected oscillation pattern in the zenith angle/energy plane. By comparing the observed distortions to those expected for either “normal” or “inverted” mass ordering, and thanks to the large neutrino sample collected, the neutrino mass ordering can be determined. 

A 115-line configuration of ORCA operating for three years is expected to provide a three-sigma sensitivity for most θ₂₃ values. KM3NeT could therefore be the first detector to unambiguously determine the neutrino mass ordering, on a time scale in advance of the planned long-baseline accelerator experiments. New-physics scenarios (for example, non-standard interactions, neutrino decays and sterile neutrinos) that modify the oscillation patterns recorded in both ORCA and ARCA have already been explored. While no significant deviations from the Standard Model have been observed, the enhanced sensitivity as the detectors grow will push the existing limits and probe uncharted territories.

Neutrino astronomy

At the beginning of the 1960s, it was realised that the neutrino could play a special role in the study of the universe at large. Weakly interacting with matter and electrically neutral, it enables exploration at greater distances and higher energies than is possible with conventional electromagnetic probes. In addition, neutrinos are the unambiguous smoking gun of hadronic acceleration processes occurring at their source. 

Since the observation of a significant flux of cosmic high-energy neutrinos in the TeV–PeV range by the IceCube Neutrino Observatory at the South Pole in 2013, the focus of neutrino astronomers has been to identify the astrophysical origins of these neutrinos. Amongst the diverse possible sources, a multi-messenger approach has identified the first: the flaring blazar TXS0506+056. While other source candidates have appeared, such as tidal disruption events and radio-bright blazars, the currently identified source population(s) cannot fully explain the detected flux. Having a neutrino telescope with a sensitivity similar to that of IceCube and with a complementary field of view allows the full neutrino sky to be continuously monitored. KM3NeT’s location in the Northern Hemisphere provides an optimal view of the galactic plane and makes it the ideal instrument to detect, characterise and resolve sources that may emit galactic neutrinos. 

Soon, KM3NeT will start sending alerts to its multi- messenger partners – including conventional electromagnetic telescopes but also other neutrino telescopes such as IceCube and Baikal/GVD – when a neutrino candidate with a high probability of astrophysical origin is detected. This is right on time for the fourth observing run of the LIGO, Virgo and KAGRA gravitational-wave interferometers. While so far no neutrinos have been observed from binary compact systems detected through gravitational waves, a joint detection would reveal unique information on the high-energy processes in the environment of the mergers. Furthermore, the exceptional pointing resolution of KM3NeT would significantly reduce the region of interest where electromagnetic partners should search for a counterpart. The ARCA detector, for example, will benefit from the low optical scattering of deep seawater to reconstruct the direction of muon-neutrino events to less than 0.1 degrees at 100 TeV and around 1 degree for the electron/tau neutrino flavours. 

Last but not least, KM3NeT is already waiting for the next close-by core-collapse supernova. Such astrophysical events are rare: the first and only one ever detected in neutrinos, SN1987a, occurred 35 years ago. The KM3NeT DOMs are continuously monitoring for a short-duration increase in counting rates on many DOMs simultaneously – the signature of a flash of MeV supernova neutrinos passing through the detectors – and the detector is networked with other neutrino telescopes via the SuperNova Early Warning System (SNEWS). If a galactic supernova would happen today, the number of neutrinos detected by SNEWS would be four orders of magnitude more than for SN1987a! 

Whether the cosmic-neutrino sources are point-like, extended, transient or variable, the KM3NeT collaboration has developed reconstruction techniques, event selections and statistical frameworks to identify them and determine their characteristics. Disentangling the galactic from the extragalactic components, the steady from the transient and the electromagnetically bright from the obscure are on KM3NeT’s to-do list for the coming decade.

Marine science 

KM3NeT is important not only for particle physics, but is also a powerful tool for marine sciences. The acquisition of long-term oceanographic data helps researchers understand and eventually mitigate the harmful effects of global processes, such as climate change and anthropogenic impact, as well as study episodic events such as earthquakes, tsunamis, biodiversity changes and pollution – all of which are difficult to study with short-term conventional marine expeditions. To this end, the seafloor infrastructures of first the ANTARES and now the KM3NeT sites are unique cabled marine observatories. They are open to all scientific communities, and as such are important nodes of the European Multidisciplinary Seafloor and water-column Observatory, EMSO.

Sixty-six years after their discovery, neutrinos remain the most mysterious of the fermions

Furthermore, the KM3NeT optical sensors and the acoustics sensors (used for the positioning of the DOMs) themselves provide unique information on deep-sea bioluminescence and bioacoustics. The ANTARES collaboration has several publications studying deep-sea bioluminescence and acoustic detection of cetaceans, and recently KM3NeT invited citizen scientists to analyse its optical and acoustic data via the Zooniverse platform in the context of the EU project REINFORCE.

The KM3NeT detectors will continue to grow in size and sensitivity as additional new lines are installed over the next five years. With three major neutrino telescope facilities now online – Baikal/GVD, IceCube and KM3NeT – neutrino astronomy is truly entering its golden era. 

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Every Nobel Prize comes with a story, and Leonard A Cole’s Chasing the Ghost offers a new perspective on that of Fred Reines, best known for discovering the electron neutrino with Clyde Cowan in 1956. While Cowan passed away in 1974, Reines went on to win the Nobel Prize in Physics for their discovery in 1995. Cole, Reines’s cousin, describes the life of Fred Reines – focusing on both his scientific career and extracurricular interests – in a personal way, showing obvious admiration for his elder cousin.

After participating in the Manhattan Project and assisting in developing nuclear weapons in the 1940s, Reines pivoted to study neutrinos, the fundamental particles emitted in nearly every nuclear reaction, which he describes as “the tiniest quantity of reality ever imagined by a human being”. While being tiny quantities, neutrinos are abundant, yet mysterious, and Reines’s work opened the door to better understand these particles. His research spanned the next five decades, and positions at universities and laboratories across the US, and the techniques that he developed to study neutrinos are used to this day.

Rainbows and Things

Throughout Chasing the Ghost, Cole splits his time between describing Reines’s career and his extracurricular pursuits. Even among his colleagues, Reines was known to be a prolific singer, performing with groups including the Los Alamos Light Opera Association and the Cleveland Orchestra Chorus. Time spent pondering these activities allowed Reines to connect better with non-science-major students when lecturing at universities. Reines famously taught his course “Rainbows and Things” to much acclaim at the University of California, Irvine, where he encouraged students to think deeply about the connection between classroom physics and the natural world. Cole explains that the course name, and much of its philosophy, stems from the play Finian’s Rainbow, which Reines performed in 1955.

Throughout his later life, it became apparent that Reines thought his accomplishments deserved more praise than they had received. In fact, it was only after he gave up hope of winning the Nobel Prize that he won it in 1995. Reines had been passed up on many occasions, including in 1988 when the team that discovered the second type of neutrinos was awarded the prize before him. Cole shares a humorous anecdote (in hindsight): at a CERN conference with both Reines and 1988 laureate Leon Lederman in attendance, a speaker suggested an experiment to search for the third type of neutrino, the tau neutrino. However, as the speaker lamented, it seemed as if no one would perform this type of experiment, “because evidently they only give a Nobel Prize for the detection of every other neutrino.” While the room may have burst into laughter, Fred Reines didn’t budge.

Regardless, Reines’s dedication to understand neutrinos persisted until the end of his life. Shortly before passing, when he heard of the ground-breaking news from Super-Kamiokande that neutrinos oscillate, he astutely asked “What’s the mass?”, understanding the implications of this result.

The work spearheaded by Reines and his contemporaries has made a lasting impact on the field of particle physics, that continues today. As Cole explains, the subfield of neutrino physics has blossomed to include large, international experimental collaborations, which have found even more unexpected results. Those results have spurred investigators to plan ambitious projects, such as the IceCube experiment in Antarctica, the DUNE experiment in the US, and Hyper-Kamiokande in Japan.

Inspiration

Today’s neutrino detectors are getting bigger and bigger. However, their forerunners can still serve a purpose: inspiration. Several detectors from Reines’s era are now exhibited, such as the Gargamelle detector at CERN. After discovering the electron neutrino, the race was on to build experiments to better understand neutrino properties, and Gargamelle was one such detector. Today, it is on display at the CERN Microcosm, perhaps inspiring a new generation of neutrino physicists.

Overall, Leonard A Cole’s Chasing the Ghost will inspire readers, especially those new to thinking about neutrino physics. Fred Reines’s work, with its focus on a deep understanding of these mysterious, abundant particles, continues to bear fruit to this day. There is no telling what the next neutrino experiments will uncover, but it’s a guarantee that sharp thinkers like Reines will be necessary in this field in the generations to come.

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In the 1960s, the creators of the Standard Model made a smart choice: while all charged fermions came in pairs, with left-handed and right-handed components, neutrinos were only left-handed. This “handicap” of neutrinos allowed physicists to accommodate in the most economical way important features of the experimental data at that time. First, such left-handed-only neutrinos are naturally massless, and second, individual leptonic flavours (electron, muon and tau) are automatically conserved.

It is now well established that neutrinos have masses and that the neutrino flavours mix with each other, in similarity with quarks. If this were known 55 years ago, Weinberg’s seminal 1967 work “A Model of Leptons” would be different: in addition to the left-handed neutrinos, it would very likely also contain their right-handed counterparts. The structure of the Standard Model (SM) dictates that these new states, if they exist, are the only singlets with respect to weak-isospin and hyper-charge gauge symmetry and thus do not participate directly in electroweak interactions (see “On the other hand” figure). This makes right-handed neutrinos (also referred to as sterile neutrinos, singlet fermions or heavy neutral leptons) very special: unlike charged quarks and leptons, which get their masses from the Yukawa interaction with the Brout–Englert–Higgs field, the masses of right-handed neutrinos depend on an additional parameter – the Majorana mass – which is not related to the vacuum expectation value and which results in the violation of lepton-number conservation. As such, right-handed neutrinos are also sometimes referred to as Majorana leptons or Majorana fermions.

Leaving aside the possible signals of eV-scale neutrino states reported in recent years, all established experimental signatures of neutrino oscillations can be explained by the SM with the addition of two heavy-neutral leptons (HNLs). If there were only one HNL, then two out of three SM neutrinos would be massless; with two HNLs, only one of the SM neutrinos is massless – this is not excluded experimentally. Any larger number of HNLs is also possible.

The simplest way to extend the SM in the neutrino sector is to add several HNLs and no other new particles. Already this class of theories is very rich (different numbers of HNLs and different values of their masses and couplings imply very different phenomenology), and contains several different scenarios explaining not only the observed masses and flavour oscillations of the SM neutrinos but also other phenomena that are not accommodated by the SM. The scenario in which the Majorana masses of right-handed neutrinos are much higher than the electroweak scale is known as the “type I see-saw model”, first put forward in the late 1970s. The theory with three right-handed neutrinos (the same as the number of generations in the SM) with their masses below the electroweak scale is called the neutrino minimal standard model (νMSM), and was proposed in the mid-2000s.

Would these new particles be useful for anything else besides neutrino physics? The answer is yes. The first, lightest HNL N₁ may serve as a dark-matter particle, whereas the other two HNLs N_2,3 not only “give” masses to active neutrinos but can also lead to the matter–antimatter asymmetry of the universe. In other words, the SM extended by just three HNLs could solve the key outstanding observational problems of the SM, provided the masses and couplings of the HNLs are chosen in a specific domain. 

The leptonic extension of the SM by right-handed neutrinos is quite similar to the gradual adaptation of electroweak theory to experimental data during the past 50 years. While the bosonic sector of the electroweak model remains intact from 1967, with the discoveries of the W and Z bosons in 1983 and the Higgs boson in 2012, the fermionic sector evolved from one to two to three generations, revealing the remarkable symmetry between quarks and leptons. It took about 20 years to find all the quarks and leptons of the third generation. How much time it will take to discover HNLs, if they indeed exist, depends crucially on their masses.

The value of the Majorana mass, and therefore the physical mass of an HNL, is arbitrary from a theoretical point of view and cannot be found from neutrino-oscillation experiments. The famous see-saw formula that relates the observed masses of the active neutrinos to the Majorana masses of HNLs has a degeneracy: change the Yukawa couplings of HNLs to neutrinos by a factor x and the HNL masses by a factor x², and the active neutrino masses and the physics of their oscillations remain intact. The scale of HNL masses thus can be any number from a fraction of an eV to 10¹⁵ GeV (see “Options abound” figure). Moreover, there could be several HNLs with very different masses. Indeed, even in the SM the masses of charged fermions, though they share a similar origin, differ by almost six orders of magnitude. 

Motivated by the value of the active neutrino masses, the HNL could be light, with masses of the order of 1 eV. Alternatively, similar to the known quarks and charged leptons, they could be somewhere around the GeV or Fermi scale. Or they could be close to the grand unification scale, 10¹⁵ GeV, where the strong and electromagnetic interactions are thought to be unified. These possibilities have different theoretical and experimental consequences. 

The case of the light sterile neutrino

The see-saw formula tells us that if the mass of HNLs is around 1 eV, their Yukawa couplings should be of the order of 10^–12. Such light sterile neutrinos can be potentially observed in neutrino experiments, as they can be involved in the oscillations together with the three active neutrino species. Several experiments – including LSND, GALLEX, SAGE, MiniBooNE and BEST – have reported anomalies in neutrino-oscillation data (the so-called short-baseline, gallium and reactor anomalies) that could be interpreted as a signal for the existence of light sterile neutrinos. However, it looks difficult, if not impossible, to reconcile the existence of these states with recent negative results of other experiments such as MINOS+, MicroBooNE and IceCUBE, accounting for additional constraints coming from β-decay, neutrinoless double-β decay and cosmology.

The parameters of light sterile neutrinos required to explain the experimental anomalies are in strong tension with the cosmological bounds (see “Cosmological bounds” figure). For example, their mixing angle with the ordinary neutrinos should be sufficiently large that these states would have been produced abundantly in the early universe, affecting its expansion rate during Big Bang nucleosynthesis and thus changing the abundances of the light elements. In addition, light sterile neutrinos would affect the formation of structure. Having been created in the hot early universe with relativistic velocities, they would have escaped from forming structures until they cooled down in much later epochs. This so-called “hot dark matter” scenario would mean that the smallest structures, which form first, and the larger ones, which require much more time to develop, would experience different amounts of dark matter. Moreover, the presence of such particles would affect baryon acoustic oscillations and therefore impact the value of the Hubble constant deduced from them.

Besides tensions between the experiments and cosmological bounds, light sterile neutrinos do not provide any solution to the outstanding problems of the SM. They cannot be dark-matter particles because they are too light, nor can they produce the baryon asymmetry of the universe as their Yukawa couplings are too small to give any substantial contribution to lepton-number violation at the temperatures (> 160 GeV) at which the anomalous electroweak processes with baryon non-conservation have a chance to convert a lepton asymmetry into a baryon asymmetry. 

Three Fermi-scale heavy neutral leptons

Another possible scale for HNL masses is around a GeV, plus or minus a few orders of magnitude. Right-handed neutrinos with such masses do not interfere with active-neutrino oscillations because the corresponding length over which these oscillations may occur is far too small. As only two active-neutrino mass differences are fixed by neutrino-oscillation experiments, it is sufficient to have two HNLs N_2,3 with appropriate Yukawa couplings to active neutrinos: to get the correct neutrino masses, they should not be smaller than ~10^–8 (compared to the electron Yukawa coupling of ~10^–6). These two HNLs may produce the baryon asymmetry of the universe, as we explain later, whereas the lightest singlet fermion, N₁, may interact with neutrinos much more weakly and thus can be a dark-matter particle (although unstable, its lifetime can greatly exceed the age of the universe). 

Three main considerations determine the possible range of masses and couplings of the dark-matter sterile neutrino (see “Dark-matter constraints” figure). The first is cosmological production. If N₁ interact too strongly, they would be overproduced in ℓ⁺ℓ^– → N₁ν reactions and make the abundance of dark matter larger than what is inferred by observations, providing an upper limit on their interaction strength. Conversely, the requirement to produce enough dark matter results in a lower bound on the mixing angle that depends on the conditions in the early universe during the epoch of N₁ production. Moreover, the lower bound completely disappears if N₁ can also be produced at very high temperatures by interactions related to gravity or at the end of cosmological inflation. The second consideration is X-ray data. Radiative N₁ → γν decays produce a narrow line that can be detected by X-ray telescopes such as XMM–Newton or Chandra, resulting in an upper limit on the mixing angle between sterile and active neutrinos. While this upper limit depends on the uncertainties in the distribution of dark matter in the Milky Way and other nearby galaxies and clusters, as well as on the modelling of the diffuse X-ray background, it is possible to marginalise these to obtain very robust constraints. 

The third consideration for the sterile neutrino’s properties is structure formation. If N₁ is too light, a very large number-density of such particles is required to make an observed halo of a small galaxy. As HNLs are fermions, however, their number density cannot exceed that of a completely degenerate Fermi gas, placing a very robust lower bound on the N₁ mass. This bound can be further improved by taking into account that light dark-matter particles remain relativistic until late epochs and therefore suppress or erase density perturbations on small scales. As a result, they would affect the inner structure of the halos of the Milky Way and other galaxies, as well as the matter distribution in the intergalactic medium, in ways that can be observed via gravitational-lensed galaxies, gaps in the stellar streams in galaxies and the spectra of distant quasars. 

Neutrino experiments and robust conclusions from observational cosmology call for extensions of the SM

The upper limits on the interaction strength of sterile neutrinos fixes the overall scale of active neutrino masses in the νMSM. The dark-matter sterile neutrino effectively decouples from the see-saw formula, making the mass of one of the active neutrinos much smaller than the observed solar and atmospheric neutrino-mass differences and fixing the masses of the two other active neutrinos to approximately 0.009 eV and 0.05 eV (for the normal ordering) and to the near-degenerate value 0.05 eV for the inverted ordering.

HNLs at the GeV scale and beyond 

Our universe is baryon-asymmetric – it does not contain antimatter in amounts comparable with the matter. Though the SM satisfies all three “Sakharov conditions” necessary for baryon-asymmetry generation (baryon number non-conservation, C and CP-violation, and departure from thermal equilibrium), it cannot explain the observed baryon asymmetry. The Kobayashi–Maskawa CP-violation is too small to produce any substantial effects, and departures from thermal equilibrium are tiny at the temperatures at which the anomalous fermion-number non-conserving processes are active. This is not the case with two GeV-scale HNLs: these particles are not in thermal equilibrium for temperatures above a few tens of GeV, and CP violation in their interactions with leptons can be large. As a result, a lepton asymmetry is produced, which is converted into baryon asymmetry by the baryon-number violating reactions of the SM.

The requirement to get baryon asymmetry in the νMSM puts stringent constraints on the masses and coupling of HNLs (see “Baryon-asymmetry constraints” figure). The mixing angle of these particles cannot be too large, otherwise they equilibrate and erase the baryon asymmetry, and it cannot be below a certain value because it would make the active neutrino masses too small. We know that their mass should be larger than that of the pion, otherwise their decays in the early universe would break the success of Big Bang nucleosynthesis. In addition, the masses of two HNLs should be close to each other so as to enhance CP-violating effects. Interestingly, the HNLs with these properties are within the experimental reach of existing and future accelerators, as we shall see.

The final possible choice of HNL masses is associated with the grand unification scale, ~10¹⁵ GeV. To get the correct neutrino masses, the Yukawa couplings of a pair of these superheavy particles should be of the order of one, in which case the baryon asymmetry of the universe can be produced via thermal leptogenesis and anomalous baryon- and lepton-number non-conservation at high temperatures. The third HNL, if interacting extremely weakly, may play the role of a dark-matter particle, as described previously. Another possibility is that there are three superheavy HNLs and one light one, to play the role of dark matter. This model, as well as that with HNL masses of the order of the electroweak scale, may therefore solve the most pressing problems of the SM. The only trouble is that we will never be able to test it experimentally, since the masses of N_2,3 are beyond the reach of any current or future experiment.

Experimental opportunities

It is very difficult to detect HNLs experimentally. Indeed, if the masses of these particles are within the reach of current and planned accelerators, they must interact orders of magnitude more weakly than the ordinary weak interactions. As for the dark-matter sterile neutrino, the most promising route is indirect detection with X-ray space telescopes. The new X-ray spectrometer XRISM, which is planned to be launched this year, has great potential to unambiguously detect a signal from dark-matter decay. Like many astrophysical observatories, however, it will not be able to determine the particle origin of this signal. Thus, complementary laboratory searches are needed. One experimental proposal that claims a sufficient sensitivity to enter into the cosmologically relevant region is HUNTER, based on radio­active atom trapping and high-resolution decay-product spectrometry. Sterile neutrinos with masses of around a keV can also show up as a kink in the β-decay spectrum of radioactive nuclei, as discussed by the ambitious PTOLEMY proposal. The current generation of experiments that study β-decay spectra – KATRIN and Troitsk nu-mass – also perform searches for keV HNLs, but they are sensitive to significantly larger mixing angles than required for a dark-matter particle. Extending the KATRIN experiment with a multi-pixel silicon drift detector, TRISTAN, will significantly improve the sensitivity here.

The most promising perspectives to find N_2,3 responsible for neutrino masses and baryogenesis are experiments at the intensity frontier. For HNL masses below 5 GeV (the beauty threshold) the best strategy is to direct proton beams at a target to create K, D or B mesons that decay producing HNLs, and then to search for HNL decays through “nothing → leptons and hadrons” processes in a near detector. This strategy was used in the previous PS191 experiment at CERN’s Proton Synchrotron (PS), NOMAD, BEBC and CHARM at the Super Proton Synchrotron (SPS) and NuTeV at Fermilab. There are several proposals for future experiments along these lines. The proposed SHiP experiment at the SPS Beam Dump Facility has the best potential as it can potentially cover almost all parameter space down to the lowest bound on coupling constants coming from neutrino masses. The SHiP collaboration has already performed detailed studies and beam tests, and the experiment is under consideration by the SPS and PS experiments committee. A smaller-scale proposal, SHADOWS, covers part of the interesting parameter space.

The search for HNLs can be carried out at the near detectors of DUNE at Fermilab and T2K/T2HK in Japan, which are due to come online later this decade. The LHC experiments ATLAS, CMS, LHCb, FASER and SND, as well as the proposed CODEX-b facility, can also be used, albeit with fewer chances to enter deeply into the cosmologically interesting part of the HNL parameter space. The decays of HNLs can also be searched for at future huge detectors such as MATHUSLA. And, going to larger HNL masses, breakthroughs can be made at the proposed Future Circular Collider FCC-ee, studying the processes Z → νN with a displaced vertex (DV) corresponding to the subsequent decay of N to available channels (see “Electron coupling” figure).

Conclusions

Neutrino experiments and robust conclusions from observational cosmology call for extensions of the SM. But the situation is very different from that in the period preceding the discovery of the Higgs boson, where the consistency of the SM together with other experimental results allowed us to firmly conclude that either the Higgs boson had to be discovered at the LHC, or new physics beyond the SM must show up. Although we know for sure that the SM is incomplete, we do not have a firm prediction about where to search for new particles nor what their masses, spins, interaction types and strengths are.

Experimental guidance and historical experience suggest that the SM should be extended in the fermion sector, and the completion of the SM with three Majorana fermions solves the main observational problems of the SM at once. If this extension of the SM is correct, the only new particles to be discovered in the future are three Majorana fermions. They have remained undetected so far because of their extremely weak interactions with the rest of the world.

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“MicroBooNE has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques,” says co-spokesperson Bonnie Fleming of Yale. “They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino.” 

The collaboration says that the analyses favour the Standard Model over the anomalous signal seen by sibling-experiment MiniBooNE at more than 99% confidence, should its true origin be electrons from a neutrino oscillation via a hitherto-undetected sterile neutrino. “But that earlier data from MiniBooNE doesn’t lie,” says former co-spokesperson Sam Zeller of Fermilab. “There’s something really interesting happening that we still need to explain.”

There’s something really interesting happening that we still need to explain

Sam Zeller

Neutrinos suffer from an identity crisis regarding their mass. As a result, the three known flavours morph into each other as phase differences develop between three mass eigenstates. However, well before this model solidified around the turn of the millennium, a measurement by the LSND collaboration at Los Alamos in the US suggested the existence of an additional neutrino which had to be “sterile” with respect to the weak, electromagnetic and strong interactions, and much more massive, given how rapidly the oscillation developed. Since this first hint, the tale of the sterile neutrino has taken multiple twists and turns.

Twists and turns

​​In the mid-1990s, LSND reported seeing a 3.8σ excess of electron antineutrinos in a beam of accelerator-generated muon antineutrinos, but the KARMEN experiment at the Rutherford Appleton Laboratory in the UK failed to reproduce the effect. Evidence for an eV-scale sterile neutrino mounted with the observation of a deficit of electron neutrinos from ³⁷Ar and ⁵¹Cr electron-capture decays at Gran Sasso in Italy and at the Baksan Neutrino Observatory in Russia (the gallium anomaly), and a reported deficit of electron antineutrinos from nuclear reactors (the reactor anomaly). Troublingly, however, long-baseline accelerator neutrino experiments such as MINOS+ do not observe the requisite “disappearance” of muon neutrinos required by the principle of unitarity, and the existence of such a sterile neutrino is also starkly incompatible with current models of cosmology. While the gallium anomaly should soon be probed definitively by the BEST experiment at Baksan (Phys. Rev. D 2018 97 073001), recent calculations of reactor fluxes may now be dissolving the reactor anomaly (see, for example, arXiv:2110.06820). But the most compelling single piece of evidence in favour of sterile neutrinos came when the MiniBooNE experiment at Fermilab tried to reproduce the LSND effect. In November 2018, the collaboration reported a 4.5σ excess of electron neutrinos and antineutrinos compared to Standard-Model expectations.

Few neutrino physicists foresaw that MicroBooNE would disfavour both hypotheses

Sibling experiment MicroBooNE has now released its first round of tests of the MiniBooNE anomaly. Equipped with a cutting-edge liquid-argon time-projection chamber, the collaboration observed neutrino interactions at the level of individual particle tracks – a key advantage compared to a Cherenkov detector such as MiniBooNE, which could not distinguish electrons from photons. The collaboration has now used half of its available data to probe which particle is the true origin of the anomaly. Earlier this month, MicroBooNE tested the hypothesis that MiniBooNE’s excess was actually due to an underestimated single-photon background, perhaps caused by a difficult-to-model rare decay of a Δ resonance. Now, MicroBooNE has tested the hypothesis that the MiniBooNE excess was caused by single electrons, most likely the result of neutrino oscillations via an eV-scale sterile neutrino. Few neutrino physicists foresaw that MicroBooNE would disfavour both hypotheses.

“Every time we look at neutrinos, we seem to find something new or unexpected,” says MicroBooNE co-spokesperson Justin Evans of the University of Manchester. “MicroBooNE’s results are taking us in a new direction, and our neutrino programme is going to get to the bottom of some of these mysteries.” The collaboration will now investigate whether more exotic topologies such as electron-positron pairs could be the source of the MiniBooNE anomaly. Such a final state might suggest the existence of heavier sterile neutrinos, say theorists.

“eV-scale sterile neutrinos no longer appear to be experimentally motivated, and never solved any outstanding problems in the Standard Model,” says theorist Mikhail Shaposhnikov of EPFL. “But GeV-to-keV-scale sterile neutrinos – so-called Majorana fermions – are well motivated theoretically and do not contradict any existing experiment. They can explain neutrino masses and oscillations, give a dark-matter candidate, and produce a baryon asymmetry in the universe: all the problems that the Standard Model is incapable of addressing. Experimental efforts at the intensity frontier should now be concentrated in this direction.”

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Excitement is building in the search for sterile neutrinos – long-predicted particles which would constitute physics beyond the Standard Model. Although impervious to the electromagnetic, weak and strong interactions, such a fourth “right-handed” neutrino flavour could reveal itself by altering the rate of standard-neutrino oscillations – tantalising hints of which were reported by Fermilab’s MiniBooNE experiment in 2007. In a preprint published last week, sibling experiment MicroBooNE strongly disfavours a mundane explanation for such hints, with further scrutiny by the collaboration expected to be announced later this month.

“If the MiniBooNE effect is indeed a sterile neutrino, this of course would be a major discovery which would revolutionise particle physics, opening up a whole new sector to explore,” says MicroBooNE co-spokesperson Justin Evans of the University of Manchester.

The story of the sterile neutrino began in the 1990s, when the ​​LSND experiment at Los Alamos reported seeing 88±23 (3.8σ) more electron antineutrinos than expected in a beam of accelerator-generated muon antineutrinos. This apparent short-baseline oscillation from muon to electron antineutrinos was incompatible with the oscillation rates established by Super-Kamiokande in 1998 and SNO in 2002, and would have to occur via an unknown intermediate neutrino flavour with a mass of about an electron-Volt. This hypothesised neutrino was dubbed sterile, as it would have to be insensitive to all interactions but gravity for it to have remained undiscovered this long.

The photon hypothesis

The plot thickened in 2007 when the MiniBooNE experiment at Fermilab tried to reproduce the LSND anomaly. The team also saw an excess of electron-like signals, though not quite at the energy corresponding to the LSND effect. The significance of the MiniBooNE anomaly grew to 4.5σ by the time the experiment finished running in November 2018. But a mundane possible explanation poured cold water on hopes for new physics: as a mineral-oil Cherenkov detector, MiniBooNE could not differentiate electrons from photons, and one particularly tricky-to-model background process might be contributing more photons than expected.

Many of us suspected that there could be something wrong with predictions for this background

Joachim Kopp

“High-energy single photons can be produced when a neutrino scatters on a nucleon via a neutral-current interaction and excites the nucleon to a Δ(1232) resonance,” explains CERN theorist Joachim Kopp. “Most of the time, the resonance decays to a pion and a nucleon, but there is a rare decay mode to a nucleon and a photon. The rate for this mode is very hard to predict, and many of us suspected that there could be something wrong with predictions for this background.”

Enter MicroBooNE, a liquid-argon time-projection-chamber sibling experiment to MiniBooNE which is capable of studying neutrino interactions in photographic detail, and differentiating the two signals. Having detected its first neutrino interactions in 2015, the MicroBooNE team has now set a limit on the neutral-current Δ→Nγ process is more than a factor of 50 better than existing constraints, explains Evans. “With this MicroBooNE result, we reject a Δ→Nγ model of the low-energy excess at 94.8% confidence, a strong indication that we must look elsewhere for the source of the excess.”

The electron hypothesis

Now that MicroBooNE has strongly disfavoured a leading-photon model for the MiniBooNE anomaly, attention shifts to the electron hypothesis – which would hint at the existence of a sterile neutrino, or something more exotic, if proven. And we don’t have long to wait. The MicroBooNE collaboration plans to release its search for an electron-like low-energy excess on 27 October, with results from three independent analyses looking at a range of inclusive and exclusive channels.

Beyond that, there is more to come, says Evans. “Our current round of results use only the first half of the total MicroBooNE data-set, and this is a programme that is only just beginning, with ICARUS and SBND within Fermilab’s short-baseline programme now coming online to turn this into a multi-baseline exploration of the richness of neutrino physics with unparalleled detail.”

The global picture is complex. In 2019, for example, the MINOS+ experiment failed to confirm the MiniBooNE signal (CERN Courier March/April 2019 p7). Were the sterile neutrino to exist, it should also have significant cosmological consequences which remain unobserved. But the anomalies are accumulating, says Kopp.

“LSND and MiniBooNE are quite consistent, and the short-baseline reactor experiments require parameters in the same region of parameter space, though these results are very much in flux and it’s not clear which ones are trustworthy, so it’s hard to make precise statements. The good news is that there’s realistic hope of resolving these puzzles over the next few years. ”

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Having set the scene, director Geneva Guerin, co-founder of Canadian production company Cinécoop, cuts to a wide expanse: a climber scaling a rock face near the French–Swiss border. Francesca Stocker, the star of the film and then a PhD student at the University of Bern, narrates, relating the scientific method to rock climbing. Stocker and her fellow protagonists are engaging, and the film vividly captures the human spirit surrounding the birth of a modern particle-physics detector.

I don’t think it is possible to explain a neutrino for a general audience

Geneva Guerin

But the viewer is not allowed to settle for long in any one location. After zipping to CERN, and a tour through its corridors accompanied by eerie cello music, we meet Stocker in her home kitchen, explaining how she got interested in science as a child. Next, we hop to Federico Sánchez, spokesperson of the T2K experiment in Japan, explaining the basics of the Standard Model. 

T2K, and its successor Hyper-Kamiokande, DUNE’s equal in ambition and scope, both feature in the one-hour-long film. But the focus is on the development of the prototype DUNE detector modules that have been designed, built and tested at the CERN Neutrino Platform – and here the film is at its best. Guerin had full access to protoDUNE activities, allowing her to immerse the viewer with the peculiar but oddly fitting accompaniment of a solo didgeridoo inside the protoDUNE cryostat. We gatecrash celebrations when the vessel was filled with liquid argon and the first test-beam tracks were recorded. The film focuses on detailed descriptions of the workings of TPCs and other parts of the apparatus rather than accessible explanations of the neutrino’s fascinating and mysterious nature. Unformatted plots and graphics are pulled from various sources. While authentic, this gives the film an unpolished, home-made feel.

Given the density of the exposition in some parts, beyond the most enthusiastic popular-science fans, Ghost Particle seems best tailored for physics students encountering experimental neutrino physics for the first time – a point that Guerin herself made during a live Q&A following the CineGlobe screening: “I was aiming at people like me – those who love science documentaries,” she told the capacity crowd. “Originally I envisaged a three-part series over a decade or more, but I realised that I don’t think it is possible to explain a neutrino for a general audience, so maybe it’s something for educational purposes, to help future generations get introduced to this exciting programme.”

The film ends as it began, with the rickety elevator continuing its 12-minute descent into the bowels of the Earth.

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At the time of the first NuFact in 1999, it wasn’t at all clear that accelerator experiments could address leptonic CP violation in neutrinos. Fits still ignored θ₁₃, which expresses the relatively small coupling between the third neutrino mass eigenstate and the electron, and the size of the solar-oscillation mass splitting, which drives the CP-violating amplitude. Today, leading experiments testify to a precision era of neutrino physics where every parameter in the neutrino mixing matrix must be fitted. TK2, NOvA and MINERvA all reported new analyses and speakers from Fermilab updated the conference on the commissioning of the laboratory’s short-baseline experiments ICARUS, MicroBooNE and SBND, which seek to clarify experimental hints of additional “sterile” neutrinos. After a long journey from CERN to Fermilab, the ICARUS detector, the largest and most downstream of the three liquid-argon detectors in the programme, has been filled with liquid argon, and data taking is now in full swing.

g-2 anomaly

As we strive to pin down the values of the neutrino mixing matrix with a precision approaching that of the CKM matrix, NuFact serves as a key forum for collaborations between theorists and experimentalists. Simon Corrodi (Argonne) showed how the latest results from Fermilab on the g-2 anomaly may suggest new physics in lepton couplings, with potential implications for neutrino couplings and neutrino propagation. Collaboration with accelerator physicists is also important. After the discovery in 2012 that θ₁₃ is nonzero, the focus of experiments with artificial sources of neutrinos turned to the development of multi-MW beams and the need for new facilities. Keith Gollwitzer (Fermilab) kicked off the discussion by summarising Fermilab’s outstanding programme at the intensity frontier, paving the way for DUNE, and Megan Friend (KEK) presented impressive progress in Japan last year. The J-PARC accelerator complex is being upgraded to serve the new T2K near detector, for which the final TPC anode and cathode are now being tested at CERN. The J-PARC luminosity upgrade will also serve the Hyper-Kamiokande experiment, which is due to come online on approximately the same timeline as DUNE. Though the J-PARC neutrino beam will be less intense and by design more monochromatic than that from Fermilab to DUNE, the Hyper-Kamiokande detector will be both closer and larger, promising comparable statistics to DUNE, and addressing the same physics questions at a lower energy.

ENUBET and nuSTORM could operate in parallel with DUNE and Hyper-Kamiokande

A lively round-table discussion featured a dialogue between two of the experiments’ co-spokespersons, Stefan Söldner-Rembold (Manchester) and Francesca Di Lodovico (King’s College London). Both emphasised the complementarity of DUNE and Hyper-Kamiokande, and the need to reduce systematic uncertainties with ad-hoc experiments. J-PARC director Takahashi Kobayashi explored this point in the context of data-driven models and precision experiments such as ENUBET and nuSTORM. Both experiments are in the design phase, and could operate in parallel with DUNE and Hyper-Kamiokande in the latter half of this decade, said Sara Bolognesi (Saclay) and Kenneth Long (Imperial). A satellite workshop focused on potential synergies between these two CERN-based projects and a muon-collider demonstrator, while another workshop explored physics goals and technical challenges for “ESSnuSB” – a proposed neutrino beam at the European Spallation Source in Lund, Sweden. In a plenary talk, Nobel laureate and former CERN Director-General Carlo Rubbia went further still, exploring the possibility of a muon collider at the same facility.

The next NuFact will take place in August 2022 in Salt Lake City, Utah.

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The Proton Improvement Plan II (PIP-II) is an essential upgrade – and ambitious reimagining – of the Fermilab accelerator complex. An all-new, leading-edge superconducting linear accelerator, combined with a comprehensive overhaul of the laboratory’s existing circular accelerators, will deliver multimegawatt proton beam power and, in turn, enable the world’s most intense beam of neutrinos for the international Long Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE). While positioning Fermilab at the forefront of accelerator-based neutrino research, PIP-II will also provide the “engine room” for a diverse – and scalable – experimental programme in US particle physics for decades to come. Put simply, PIP-II will be the highest-energy and highest-power continuous-wave (CW) proton linac ever built, capable of delivering both pulsed and continuous particle beams.

Another unique aspect of PIP-II is that it is the first US Department of Energy (DOE)-funded particle accelerator that will be built with significant international participation. With major “in-kind” contributions from institutions in India, Italy, the UK, France and Poland, the project’s international partners bring wide-ranging expertise and know-how in core accelerator technologies along with an established track-record in big-physics initiatives. What’s more, PIP-II is not going to be the last DOE project to benefit from international collaboration – there will be more to come – so a near-term priority is to provide a successful template that others can follow. 

Deconstructing neutrino physics

Operationally, LBNF/DUNE is a global research endeavour comprising three main parts: the experiment itself (DUNE); the facility that produces the neutrino beam plus associated infrastructure to support the experiment (LBNF); and the PIP-II upgrade to the Fermilab accelerator complex, which will power the neutrino beam. 

At Fermilab, PIP-II will accelerate protons and smash them into an ultrapure graphite target. The resulting beam of neutrinos will travel through the DUNE near detector on the Fermilab site, then through 1300 km of earth (no tunnel required), and finally through the DUNE far detector at Sanford Lab in South Dakota (see figure). Data from neutrino interactions collected by the experiment’s detectors will be analysed by a network of more than 1000 DUNE collaborators around the world.

In this way, DUNE will enable a comprehensive programme of precision neutrino-oscillation measurements using ν_μ and ν_μ beams from Fermilab. Key areas of activity will include tests of leptonic charge-parity conservation; determining the neutrino mass ordering; measuring the angle θ₂₃ in the Pontecorvo–Maki–Nakagawa–Sakata mixing matrix; and probing the three-neutrino paradigm. Furthermore, DUNE will search for proton decay in several decay modes and potentially detect and measure the ν_e flux from any supernovae that take place in our galaxy. 

To provide unprecedented detail in the reconstruction of neutrino events, the DUNE experiment will exploit liquid-argon time-projection-chamber (LArTPC) detectors on a massive scale (technology itself that was first deployed at scale in 2010 for the ICARUS detector as part of the CERN Neutrinos to Gran Sasso facility). The LArTPC implementation for DUNE is currently being developed in two prototype detectors at CERN via the CERN Neutrino Platform, an initiative inaugurated in 2014 following the recommendations of the 2013 European Strategy for Particle Physics to provide a focal point for Europe’s contributions to global neutrino research. 

In addition to the prototype DUNE detectors, the CERN Neutrino Platform is contributing to the long-baseline Tokai-to-Kamioka (T2K) and future Hyper-Kamiokande experiments in Japan. Construction of the underground caverns for DUNE and Hyper-Kamiokande is under way, with both experiments chasing similar physics goals and offering valuable scientific complementarity when they come online towards the end of the decade. 

A key driver of change was the recommendation of the 2014 US Particle Physics Project Prioritization Panel (P5) that the US host a world-leading international programme in neutrino physics. “Its centrepiece,” the P5 report asserts, “would be a next-generation long-baseline neutrino facility (LBNF). LBNF would combine a high-intensity neutrino beam and a large-volume precision detector [DUNE] sited underground a long distance away to make accurate measurements of the oscillated neutrino properties… A powerful, wideband neutrino beam would be realised with Fermilab’s PIP-II upgrade project, which provides very high intensities in the Fermilab accelerator complex.”

Fast forward to December 2020 and full DOE approval of the PIP-II baseline plan, at a total project cost of $978m and with completion scheduled for 2028. Initial site preparation actually started in March 2019, while construction of the cryoplant building got under way in July 2020. Commissioning of PIP-II is planned for the second half of this decade, with the first delivery of neutrino beam to LBNF/DUNE in the late 2020s (see “Deconstructing neutrino physics” panel). With the help of Fermilab’s network of international partners, a highly capable, state-of-the-art accelerator will soon be probing new frontiers in neutrino physics and, more broadly, redefining the roadmap for US high-energy physics.  

Then, now, next

If that’s the future, what of the back-story? Fermilab’s particle-accelerator complex originally powered the Tevatron, the first machine to break the TeV energy barrier and the world’s most powerful accelerator before CERN’s Large Hadron Collider (LHC) came online a decade ago. The Tevatron was shut down in 2011 after three illustrious decades at the forefront of particle physics, with notable high-points including discovery of the top quark in 1995 and direct discovery of the tau neutrino in 2000. 

A powerful, wideband neutrino beam would be realised with Fermilab’s PIP-II upgrade project

Today, about 4000 scientists from more than 50 countries rely on Fermilab’s accelerators, detectors and computing facilities to support their cutting-edge research. The laboratory comprises four interlinking accelerators and storage rings: a 400 MeV room-temperature linac; an 8 GeV Booster synchrotron; an 8 GeV fixed-energy storage ring called the Recycler; and a 60–120 GeV Main Injector synchrotron housed in the same tunnel with the Recycler. The Main Injector generates more than 800 kW of proton beam power, in turn yielding the world’s most intense beams of neutrinos for Fermilab’s flagship NOvA experiment (with the far detector located in Ash River, Minnesota), while supporting a multitude of other research programmes exploring fundamental particles and forces down to the smallest scales.

A leading-edge SRF proton linac

The roll-out of PIP-II will make the Fermilab complex more powerful again. Replacing the 50-year-old linear accelerator with a high-intensity, superconducting radio­frequency (SRF) linac will enable Fermilab to deliver 1.2 MW of proton beam power to the LBNF target, providing a platform for scale-up to multimegawatt levels and the capability for high-power operation across multiple particle-physics experiments simultaneously. 

Deconstructed, the PIP-II linac is an 800 MeV, 2 mA H^– machine consisting of a room-temperature front-end (up to 2.1 MeV) followed by an SRF section designed to operate in CW mode. The CW operation, and the requirements it places on the SRF systems, present some unprecedented challenges in terms of machine design. 

The H^– source (capable of 15 mA beam current) is followed by a low-energy beam transport (LEBT) section and a radio­frequency quadrupole (RFQ) that operates at a frequency of 162.5 MHz and is capable of 10 mA CW operation. The RFQ bunches, focuses and accelerates the beam from 30 keV to 2.1 MeV. Subsequently, the PIP-II MEBT includes a bunch-by-bunch chopping system that removes undesired bunches of arbitrary patterns from the CW beam exiting the RFQ. This is one of several innovative features of the PIP-II linac design that enables not only direct injection into the Booster RF bucket – thereby mitigating beam losses at injection – but also delivery of tailored bunch patterns for other experiments. The chopper system itself comprises a pair of wideband kickers and a 20 kW beam absorber.

In terms of the beam physics, the H^– ions are non-relativistic at 2.1 MeV and their velocity changes rapidly with acceleration along the linac. To achieve efficient acceleration to 800 MeV, the PIP-II linac employs several families of accelerating cavities optimised for specific velocity regimes – i.e. five different types of SRF cavities at three RF frequencies. Although this arrangement ensures efficient acceleration, it also increases the technical complexity of the project owing to the unique challenges associated with the design, fabrication and commissioning of a portfolio of accelerating systems.

Mapped versus increasing energy, the PIP-II linac consists of a half-wave resonator (HWR) operating at 162.5 MHz at optimal beta-value of 0.112; two types of single-spoke resonators (SSR1, SSR2) at 325 MHz and optimal betas equal to 0.222 and 0.472, respectively; and two types of elliptical cavities with low and high beta at 650 MHz (LB650, HB650) and optimal betas equal to 0.65 and 0.971. The HWR cryomodule has been built by the DOE’s Argonne National Laboratory (Lemont, Illinois), while an SSR1 prototype cryomodule was constructed by Fermilab, with a cavity provided by India’s Department of Atomic Energy. Both cryomodules have now been tested successfully with beam by the PIP-II accelerator physics team. 

Innovation yields acceleration

Each of the five accelerating systems comes with unique technical challenges and requires dedicated development to validate performance requirements. In particular, the CW RF mode of operation necessitates SRF cavities with high-quality factors at high gradient, thereby minimising the cryogenic load. For the SSR2, LB650 and HB650 cavities, the Q_o and accelerating gradient specifications are: 0.82 × 10¹⁰ and 11.4 MV/m; 2.4 × 10¹⁰ and 16.8 MV/m; 3.3 × 10¹⁰ and 18.7 MV/m, respectively – figures of merit that are all beyond the current state-of-the-art. Nitrogen doping will enable the elliptical cavities to reach this level of performance, while the SSR2 cavities will undergo a rotational-buffered chemical polishing treatment. 

PIP-II prioritises international partnerships

PIP-II is the first DOE-funded particle accelerator to be built with significant international participation, leveraging in-kind contributions of equipment, personnel and expertise from a network of partners across six countries. It’s a similar working model to that favoured by European laboratories like CERN, the European X-ray Free Electron Laser (XFEL) and the European Spallation Source (ESS) – all of which have shared their experiences with Fermilab to inform the PIP-II partnership programme. 

US

Partners: Argonne National Laboratory; Fermilab (lead partner); Lawrence Berkeley National Laboratory; Thomas Jefferson National Accelerator Facility

Key inputs: HWR, RFQ and resonance control systems

INDIA

Partners: Bhabha Atomic Research Centre (BARC); Inter-University Accelerator Centre (IUAC); Raja Ramanna Centre for Advanced Technology (RRCAT); Variable Energy Cyclotron Centre (VECC)

Key inputs: room-temperature and superconducting magnets, SRF cavities, cryomodules, RF amplifiers

ITALY

Partner: Italian Institute for Nuclear Physics (INFN)

Key inputs: SRF cavities (LB650) 

UK

Partner: Science and Technology Facilities Council as part of UK Research and Innovation (STFC UKRI)

Key inputs: SRF cryomodules (HB650)

FRANCE

Partners: French Alternative Energies and Atomic Energy Commission (CEA); French National Centre for Scientific Research/National Institute of Nuclear and Particle Physics (CNRS/IN2P3) 

Key inputs: cryomodules (LB650) and SRF cavity testing (SSR2)

POLAND

Partners: Wrocław University of Science and Technology; Warsaw University of Technology; Lodz University of Technology

Key inputs: cryogenic distribution systems and high-performance electronics (e.g. low-level RF and RF protection instrumentation).

A further design challenge is to ensure that the cavity resonance is as narrow as possible – something that is necessary to minimise RF power requirements when operating in CW mode. However, a narrow-bandwidth cavity is prone to detuning owing to small acoustic disturbances (so-called microphonic noise), with adverse effects on the required phase, amplitude stability and ultimately RF power consumption. The maximum detuning requirement for PIP-II is 20 Hz – achieved via a mix of passive approaches (e.g. cryomodule design, decoupling cavities from sources of vibration and more rigid cavity design) and active intervention (e.g. adaptive detuning control algorithms). 

Another issue in the pulsed RF regime is Lorentz force cavity detuning, in which the thin walls of the SRF cavities are deformed by forces from electromagnetic fields inside the cavity. This phenomenon can be especially severe in the SSR2 and LB650 cavities – where detuning may be approximately 10 times larger than the cavity bandwidth – though initial operation of PIP-II in CW RF and pulsed beam mode will help to mitigate any detuning effects.

The management of risk 

Given the scale and complexity of the linac development programme, the Fermilab project team has constructed the PIP-II Injector Test facility (also known as PIP2IT) as a systems engineering testbed for PIP-II’s advanced technologies. Completed last year, PIP2IT is a near-full-scale prototype of the linac’s room-temperature front-end, which accelerates protons up to 2.1 MeV, and the first two PIP-II cryomodules (HWR and SSR1) that then take the beam up to about 20 MeV. 

The testbed is all about risk management: on the one hand, validating design choices and demonstrating that core enabling technologies will meet PIP-II performance goals in an operational setting; on the other, ensuring seamless integration of the in-kind contributions (including SRF cavities, magnets and RF amplifiers) from PIP-II’s network of international partners (see “PIP-II prioritises international partnerships”). Beam commissioning in PIP2IT was completed earlier this year, with notable successes versus a number of essential beam manipulations and technology validations including: the PIP-II design beam parameters; the bunch-by-bunch chopping pattern required for injection into the Booster; and acceleration of beam to 17.2 MeV in the first two PIP-II cryomodules. Significant progress was also registered with successful testing of the SRF/cryomodule technologies, first operation of the laser-wire profile monitor, and the application of machine-learning algorithms to align the orbit through the cryomodules. 

There’s no duplication of effort here either. Post-commissioning, after completion of full system and design validation, the PIP2IT accelerator will be disassembled, moved and reinstalled in the PIP-II facility as the SRF linac’s upstream front-end. The testbed location, meanwhile, is being transformed into the PIP-II Cryomodule Test Facility, where most of the cryomodules will be tested with full RF power before being installed in the tunnel. 

Notwithstanding construction of the new SRF linac, PIP-II also involves fundamental upgrades to Fermilab’s existing circular accelerators – the Booster, Recycler Ring and Main Injector – to enable the complex to achieve at least 1.2 MW of proton beam power while providing a scalable platform towards multi-MW capability. More specifically, the path to 1.2 MW from Fermilab’s Main Injector, over the energy range 60 to 120 GeV, requires a number of deliverables to come together: increase of the Fermilab Booster beam intensity by roughly 50% compared to current operation (i.e. an increase in the number of protons extracted per Booster cycle from 4.3 × 10¹² to 6.3 × 10¹²); reduction of the Main Injector cycle from 1.33 to 1.2 s; and an increase of the Booster repetition rate from 15 to 20 Hz. 

Right now, beam losses in the Booster – which occur during injection, transition and extraction – prevent the intensity increase and limit the performance of the accelerator complex to roughly 900 kW. The PIP-II SRF linac injection into the Booster mitigates high-intensity effects and reduces losses on two fronts: first, the higher injection energy (800 MeV vs 400 MeV) will mitigate space-charge forces at higher beam intensities; second, the high-quality, lower-emittance beam will allow “beam painting” at injection in all three degrees of freedom, further reducing space-charge forces and beam losses at high intensity. Other upgrades are also in the works to further reduce and control losses, with some of them to be made available early, several years before PIP-II commissioning, to benefit the NOvA experiment. 

In PIP-II, the 8 GeV Booster beam will be injected into the Fermilab Recycler ring – equipped with new 53 MHz RF cavities capable of larger beam current – where 12 Booster transfer batches are accumulated and slip-stacked. Next, the Recycler beam will enter Fermilab’s Main Injector – equipped with double the number of power amplifiers and vacuum tubes – which accelerates this intense beam anywhere from 60 to 120 GeV, delivering at least 1.2 MW of beam power at 120 GeV. Further, the Booster upgrade to 20 Hz will support an 8 GeV science programme, including Fermilab’s muon-to-electron conversion experiment (Mu2e) and studies of short-baseline neutrinos (see “PIP-II: a flexible, versatile design”). 

International collaboration  

Over the next decade, the PIP-II roadmap is clear. Phase one of the project will see the front-end of the Fermilab accelerator complex replaced with an 800 MeV SRF linac while performing necessary upgrades to the existing rings. Completion will see PIP-II deliver an initial beam power of 1.2 MW on the LBNF target, though the longer-term objective is to upgrade to 2.4 MW through replacement of the Booster synchrotron.

Operationally, it’s worth reiterating that PIP-II is very much a collective endeavour

Operationally, it’s worth reiterating that PIP-II is very much a collective endeavour – in fact, the first US accelerator to be built with the help of a network of international partnerships. In this way, PIP-II is very much a trail-blazer, with the excellence and sustained commitment of the project’s international partners essential for the construction – and ultimately the successful delivery – of this next-generation accelerator complex by the end of the decade. 

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The Deep Underground Neutrino Experiment (DUNE) in the US is set to replicate that marvel of model-making, the ship-in-a-bottle, on an impressive scale. More than 3000 tonnes of steel and other components for DUNE’s four giant detector modules, or cryostats, must be lowered 1.5 km through narrow shafts beneath the Sanford Lab in South Dakota, before being assembled into four 66 × 19 × 18 m³ containers. And the maritime theme is more than a metaphor: to realise DUNE’s massive cryostats, each of which will keep 17.5 kt of liquid argon (LAr) at a temperature of –200°, CERN is working closely with the liquefied natural gas (LNG) shipping industry.

Since it was established in 2013, CERN’s Neutrino Platform has enabled significant European participation in long-baseline neutrino experiments in the US and Japan. For DUNE, which will beam neutrinos 1300 km through the Earth’s crust from Fermilab to Sanford, CERN has built and operated two large-scale prototypes for DUNE’s LAr time-projection chambers (TPCs). All aspects of the detectors have been validated. The “ProtoDUNE” detectors’ cryostats will now pave the way for the Neutrino Platform team to design and engineer cryostats that are 20 times bigger. CERN had already committed to build the first of these giant modules. In June, following approval from the CERN Council, the organisation also agreed to provide a second.

Scaling up

Weighing more than 70,000 tonnes, DUNE will be the largest ever deployment of LAr technology, which serves as both target and tracker for neutrino interactions, and was proposed by Carlo Rubbia in 1977. The first large-scale LAr TPC – ICARUS, which was refurbished at CERN and shipped to Fermilab’s short-baseline neutrino facility in 2017 – is a mere twentieth of the size of a single DUNE module.

Scaling LAr technology to industrial levels presents several challenges, explains Marzio Nessi, who leads CERN’s Neutrino Platform. Typical cryostats are carved from big chunks of welded steel, which does not lend itself to a modular design. Insulation is another challenge. In smaller setups, a vacuum installation comprising two stiff walls would be used. But at the scale of DUNE, the cryostats will deform by tens of cm when cooled from room temperature, potentially imperilling the integrity of instrumentation, and leading CERN to use an active foam with an ingenious membrane design.

The nice idea from the liquefied-natural-gas industry is to have an internal membrane which can deform like a spring

Marzio Nessi

“The nice idea from the LNG industry is that they have found a way to have an internal membrane, which can deform like a spring, as a function of the thermal conditions. It’s a really beautiful thing,” says Nessi. “We are collaborating with French LNG firm GTT because there is a reciprocal interest for them to optimise the process. They never went to LAr temperatures like these, so we are both learning from each other and have built a fruitful ongoing collaboration.”

Having passed all internal reviews at CERN and in the US, the first cryostat is now ready for procurement. Several different industries across CERN’s member states and beyond are involved, with delivery and installation at Sanford Lab expected to start in 2024. The cryostat is only one aspect of the ProtoDUNE project: instrumentation, readout, high-voltage supply and many other aspects of detector design have been optimised through more than five years of R&D. Two technologies were trialled at the Neutrino Platform: single- and dual-phase LAr TPCs. The single-phase design has been selected as the design for the first full-size DUNE module. The Neutrino Platform team is now qualifying a hybrid single/dual-phase version based on a vertical drift, which may prove to be simpler, more cost effective and easier-to-install.

Step change

In parallel with efforts towards the US neutrino programme, CERN has developed the BabyMIND magnetic spectrometer, which sandwiches magnetised iron and scintillator to detect relatively low-energy muon neutrinos, and participates in the T2K experiment, which sends neutrinos 295 km from Japan’s J-PARC accelerator facility to the Super-Kamiokande detector. CERN will contribute to the upgrade of T2K’s near detector, and a proposal has been made for a new water Cherenkov test-beam experiment at CERN, to later be placed about 1 km from the neutrino beam source of the Hyper Kamiokande experiment . Excavation of underground caverns for Hyper Kamiokande and DUNE has already begun.

DUNE and Hyper-Kamiokande, along with short-baseline experiments and major non-accelerator detectors such as JUNO in China, will enable high-precision neutrino-oscillation measurements to tackle questions such as leptonic CP violation, the neutrino mass hierarchy, and hints of additional “sterile” neutrinos, as well as a slew of questions in multi-messenger astronomy. Entering operation towards the end of the decade, Hyper-Kamiokande and DUNE will mark a step-change in the scale of neutrino experiments, demanding a global approach.

“The Neutrino Platform has become one of the key projects at CERN after the LHC,” says Nessi. “The whole thing is a wonderful example – even a prototype – for the global participation and international collaboration that will be essential as the field strives to build ever more ambitious projects like a future collider.”

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Think “neutrino detector” and images of giant installations come to mind, necessary to compensate for the vanishingly small interaction probability of neutrinos with matter. The extreme luminosity of proton-proton collisions at the LHC, however, produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a much more compact apparatus.

In March, the CERN research board approved the Scattering and Neutrino Detector (SND@LHC) for installation in an unused tunnel that links the LHC to the SPS, 480 m downstream from the ATLAS experiment. Designed to detect neutrinos produced in a hitherto unexplored pseudo-rapidity range (7.2 < ? < 8.6), the experiment will complement and extend the physics reach of the other LHC experiments — in particular FASERν, which was approved last year. Construction of FASERν, which is located in an unused service tunnel on the opposite side of ATLAS along the LHC beamline (covering |?|>9.1), was completed in March, while installation of SND@LHC is about to begin.

Both experiments will be able to detect neutrinos of all types, with SND@LHC positioned off the beamline to detect neutrinos produced at slightly larger angles. Expected to commence data-taking during LHC Run 3 in spring 2022, these latest additions to the LHC-experiment family are poised to make the first observations of collider neutrinos while opening new searches for feebly interacting particles and other new physics.

Neutrinos galore
SND@LHC will comprise 800 kg of tungsten plates interleaved with emulsion films and electronic tracker planes based on scintillating fibres. The emulsion acts as vertex detector with micron resolution while the tracker provides a time stamp, the two subdetectors acting as a sampling electromagnetic calorimeter. The target volume will be immediately followed by planes of scintillating bars interleaved with iron blocks serving as a hadron calorimeter, followed downstream by a muon-identification system.

During its first phase of operation, SND@LHC is expected to collect an integrated luminosity of 150 fb^-1, corresponding to more than 1000 high-energy neutrino interactions. Since electron neutrinos and antineutrinos are predominantly produced by charmed-hadron decays in the pseudorapidity range explored, the experiment will enable the gluon parton-density function to be constrained in an unexplored region of very small x. With projected statistical and systematic uncertainties of 30% and 22% in the ratio between ?_e and ?_?, and about 10% for both uncertainties in the ratio between ?_e and ?_? at high energies, the Run-3 data will also provide unique tests of lepton flavour universality with neutrinos, and have sensitivity in the search for feebly interacting particles via scattering signatures in the detector target.

“The angular range that SND@LHC will cover is currently unexplored,” says SND@LHC spokesperson Giovanni De Lellis. “And because a large fraction of the neutrinos produced in this range come from the decays of particles made of heavy quarks, these neutrinos can be used to study heavy-quark particle production in an angular range that the other LHC experiments can’t access. These measurements are also relevant for the prediction of very high-energy neutrinos produced in cosmic-ray interactions, so the experiment is also acting as a bridge between accelerator and astroparticle physics.”

A FASER first
FASERν is an addition to the Forward Search Experiment (FASER), which was approved in March 2019 to search for light and weakly interacting long-lived particles at solid angles beyond the reach of conventional collider detectors. Comprising a small and inexpensive stack of emulsion films and tungsten plates measuring 0.25 x 0.25 x 1.35 m and weighing 1.2 tonnes, FASERν is already undergoing tests. Smaller than SND, the detector is positioned on the beam-collision axis to maximise the neutrino flux, and should detect a total of around 20,000 muon neutrinos, 1300 electron neutrinos and 20 tau neutrinos in an unexplored energy regime at the TeV scale. This will allow measurements of the interaction cross-sections of all neutrino flavours, provide constraints on non-standard neutrino interactions, and improve measurements of proton parton-density functions in certain phase-space regions.

The final detector should do much better — it will be a hundred times bigger

Jamie Boyd

In May, based on an analysis of pilot emulsion data taken in 2018 using a target mass of just 10 kg, the FASERν team reported the detection of the first neutrino-interaction candidates, based on a measured 2.7σ excess of a neutrino-like signal above muon-induced backgrounds. The result paves the way for high-energy neutrino measurements at the LHC and future colliders, explains FASER co-spokesperson Jamie Boyd: “The final detector should do much better — it will be a hundred times bigger, be exposed to much more luminosity, have muon identification capability, and be able to link observed neutrino interactions in the emulsion to the FASER spectrometer. It is quite impressive that such a small and simple detector can detect neutrinos given that usual neutrino detectors have masses measured in kilotons.”

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The XIX International Workshop on Neutrino Telescopes (NeuTel) attracted 1000 physicists online from 18 to 26 February, under the organisation of INFN Sezione di Padova and the Department of Physics and Astronomy of the University of Padova.

The opening session featured presentations by Sheldon Lee Glashow, on the past and future of neutrino science, Carlo Rubbia, on searches for neutrino anomalies, and Barry Barish, on the present and future of gravitational-wave detection. This session was a propitious moment for IceCube principal investigator Francis Halzen to give a “heads-up” on the first observation, in the South-Pole detector, of a so-called Glashow resonance – the interaction of an electron antineutrino with an atomic electron to produce a real W boson, as the eponymous theorist predicted back in 1960. According to Glashow’s calculations, the energy at which the resonance shall happen depends on the mass of the W boson, which was discovered in 1983 by Rubbia and his team. 

The first edition of NeuTel saw the birth of the idea of instrumenting a large volume of Antarctic ice

The first edition of NeuTel saw the birth of the idea of instrumenting a large volume of Antarctic ice to capture high-energy neutrinos – a “Deo volente” (God willing) detector, as Halzen and collaborators then dubbed it. Thirty-three years later, as the detection of a Glashow resonance demonstrates, it is possible to precisely calibrate the absolute energy scale of these gigantic instruments for cosmic particles, and we have achieved several independent proofs of the existence of high-energy cosmic neutrinos, including first confirmations by ANTARES and Baikal-GVD.

Astrophysical models describing the connections between cosmic neutrinos, photons and cosmic rays were discussed in depth, with special emphasis on blazars, starburst galaxies and tidal-distribution events. Perspectives for future global multi-messenger observations and campaigns, including gravitational waves and networks of neutrino instruments over a broad range of energies, were illustrated, anticipating core-collapse supernovae as the most promising sources. The future of astroparticle physics relies upon very large infrastructures and collaborative efforts on a planetary scale. Next-generation neutrino telescopes might follow different strategic developments. Extremely large volumes, equipped with cosmic-ray-background veto techniques and complementary radio-sensitive installations might be the key to achieving high statistics and high-precision measurements over a large energy range, given limited sky coverage. Alternatively, a network of intermediate-scale installations, like KM3NeT, distributed over the planet and based on existing or future infrastructures, might be better suited for population studies of transient phenomena. Efforts are currently being undertaken along both paths, with a newborn project, P-ONE, exploiting existing deep-underwater Canadian infrastructures for science to operate strings of photomultipliers.

T2K and NOvA did not update last summer’s leptonic–CP–violation results. The tension of their measurements creates counter-intuitive fit values when a combination is tried, as discussed by Antonio Marrone of the University of Bari. The most striking example is the neutrino mass hierarchy: both experiments in their own fits favour a normal hierarchy, but their combination, with a tension in the value of the CP phase, favours an inverted hierarchy.

The founder of the Borexino experiment, Gianpaolo Bellini, discussed the results of the experiment together with the latest exciting measurements of the CNO cycle in the Sun. DUNE, Hyper-K, and JUNO presented progress towards the realisation of these leading projects, and speakers discussed their potential in many aspects of new-physics searches, astrophysics investigations and neutrino–oscillation sensitivities. The latest results of the reactor–neutrino experiment Neutrino-4, which about one year ago claimed 3.2σ evidence for an oscillation anomaly that could be induced by sterile neutrinos, were discussed in a dedicated session. Both ICARUS and KATRIN presented their sensitivities to this signal in two completely different setups.

Marc Kamionkowski (John Hopkins University) and Silvia Galli (Institut d’Astrophysique de Paris) both provided an update on the “Hubble tension”: an approximately 4σ difference in the Hubble constant when determined from angular temperature fluctuations in the cosmic microwave background (probing the expansion rate when the universe was approximately 380,000 years old) and observing the recession velocity of supernovae (which provides its current value). This Hubble tension could hint at new physics modifying the thermal history of our universe, such as massive neutrinos that influence the early-time measurement of the Hubble parameter.

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In October 2007, neutrino physicists broke ground 55 km north-east of Hong Kong to build the Daya Bay Reactor Neutrino Experiment. Comprising eight 20-tonne liquid-scintillator detectors sited within 2 km of the Daya Bay nuclear plant, its aim was to look for the disappearance of electron antineutrinos as a function of distance to the reactor. This would constitute evidence for mixing between the electron and the third neutrino mass eigenstate, as described by the parameter θ₁₃. Back then, θ₁₃ was the least well known angle in the Pontecorvo–Maki–Nakagawa–Sakata matrix, which quantifies lepton mixing, with only an upper limit available. Today, it is the best known angle by some margin, and the knowledge that it is nonzero has opened the door to measuring leptonic CP violation at long-baseline accelerator-neutrino experiments.

Daya Bay was one of a trio of experiments located in close proximity to nuclear reactors, along with RENO in South Korea and Double Chooz in France, which were responsible for this seminal measurement. Double Chooz published the first hint that θ₁₃ was nonzero in 2011, before Daya Bay and RENO established this conclusively the following spring. The experiments also failed to dispel the reactor–antineutrino anomaly, whereby observed neutrino fluxes are a few percent lower than calculations predict. This has triggered a slew of new experiments located mere metres from nuclear-reactor cores, in search of evidence for oscillations involving additional, sterile light neutrinos. As the Daya Bay experiment’s detectors are dismantled, after almost a decade of data taking, the three collaborations can reflect on the rare privilege of having pencilled the value of a previously unknown parameter into the Standard- Model Lagrangian.

Particle physics is fundamental and influential, and deserves to be supported

Yi-Fang Wang

Founding Daya Bay co-spokesperson Yi-Fang Wang says the experiment has had a transformative effect on Chinese particle physics, emboldening the country to explore major projects such as a circular electron–positron collider. “One important lesson we learnt from Daya Bay is that we should just go ahead and do it if it is a good project, rather than waiting until everything is ready. We convinced our government that we could do a great job, that world-class jobs need to be international, and that particle physics is fundamental and influential, and deserves to be supported.”

JUNO

The experiment has also paved the way for China to build a successor, the Jiangmen Underground Neutrino Observatory (JUNO), for which Wang is now spokesperson. JUNO will tackle the neutrino mass hierarchy – the question of whether the third neutrino mass eigenstate is the most or least massive of the three. An evolution of Daya Bay, the new experiment will also measure a deficit of electron antineutrinos, but at a distance of 53 km, seeking to resolve fast and shallow oscillations that are expected to differ depending on the neutrino mass hierarchy. Excavation of a cavern for the 20 kilotonne liquid-scintillator detector 700 m beneath the Dashi hill in Guangdong was completed at the end of 2020. The construction of a concrete water pool is the next step.

The next steps in reactor-neutrino physics will involve an extraordinary miniaturisation

Thierry Lasserre

The detector concept that the three experiments used to uncover θ₁₃ was designed by the Double Chooz collaboration. Thierry Lasserre, one of the experiment’s two founders, recalls that it was difficult, 20 years ago, to convince the community that the measurement was possible at reactors. “It should not be forgotten that significant experimental efforts were also undertaken in Angra dos Reis, Braidwood, Diablo Canyon, Krasnoyarsk and Kashiwazaki,” he says. “Reactor neutrino detectors can now be used safely, routinely and remotely, and some of them can even be deployed on the surface, which will be a great advantage for non-proliferation applications.” The next steps in reactor-neutrino physics, he explains, will now involve an extraordinary miniaturisation to cryogenic detectors as small as 10 grams, which take advantage of the much larger cross section of coherent neutrino scattering.

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Founded on 24 December 1970, the Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS) is a large centre for particle physics in Moscow with wide participation in international projects. The INR RAS conducts work on cosmology, neutrino physics, astrophysics, high-energy physics, accelerator physics and technology, neutron research and nuclear medicine. It is most well-known for its unique research facilities that are spread all across Russia, and its large-scale collaborations in neutrino and high-energy physics. This includes experiments such as the Baksan Neutrino Observatory, and collaborations with a number of CERN experiments including CMS, ALICE, LHCb, NA61 and NA64.

The Institute was founded by the Decree of the Presidium of the USSR Academy of Sciences in accordance with the decision of the government. Theoretical physicist Moisey Markov had a crucial role in establishing the Institute and influenced the research that would later be undertaken. His ambition is seen in the decision to base INR RAS on three separate nuclear laboratories of the P.N. Lebedev Institute of Physics of the Academy of Sciences of the USSR. Each laboratory had a leading physicist in charge: the Atomic Nucleus Laboratory headed by Nobel laureate Ilya Frank; the Photonuclear Reactions Laboratory under the direction of Lyubov Lazareva; and a neutrino laboratory headed by Georgy Zatsepin and Alexander Chudakov. The man overseeing it all was the first director of INR RAS, Albert Tavkhelidze, a former researcher at the Joint Institute for Nuclear Research (JINR, Dubna). In 1987 he was replaced as director by Victor Matveev, then in 2014 by Leonid Kravchuk. Since 2020 the director of INR RAS is Maxim Libanov.

It (Troitsk) has the most powerful linear proton accelerator in the Euro-Asian region

From the very beginning, major efforts were focused on the construction and operation of large-scale research facilities. The hub of INR RAS was built 20 km outside of Moscow, in a town called Troitsk. In 1973 an accelerator division was created, with a long-term goal of creating a meson facility that would house a 600 MeV linear accelerator for protons and H- ions. The first beam was eventually accelerated to 20 MeV in 1988 and the facility was fully operational by 1993. Now known as the Moscow Meson Facility, it has the most powerful linear proton accelerator in the Euro-Asian region, providing fundamental and applied research in nuclear and neutron physics, condensed matter, development of technologies for the production of a wide range of radioisotopes, operation of a radiation therapy complex and many other applications.

A town called Neutrino
Over 1000 miles south from the Troitsk laboratory, an underground tunnel in the Caucasus mountains is the base of another INR RAS facility, the Baksan Neutrino Observatory (BNO). The facility was established in 1967 and the Baksan Underground Scintillation Telescope (BUST) started taking data in 1978. A town sensibly called “Neytrino” (Russian for neutrino) was constructed in parallel with the facility, and was where scientists and their families could live 1700 m above sea level next to the observatory. In 1987 BUST was one of the four neutrino detectors to first directly observe neutrinos from supernova SN1987A.

The observatory did not finish there, and the next step was the gallium-germanium neutrino telescope (GGNT), which was home to the Soviet–American Gallium Experiment (SAGE). The experiment contributed heavily towards solving the solar neutrino problem and simultaneously gave rise to a new problem known as the gallium anomaly, which is yet to be explained. SAGE is still well and truly alive, and with a recent upgrade of the GGNT completed in 2019, the team will now hunt for sterile neutrinos.

By 1990 another neutrino detector was under construction, following the original proposal of Markov and Chudakov. In collaboration with JINR, plans for an underwater neutrino telescope located at the world’s largest freshwater lake, Lake Baikal, took shape. Underwater telescopes use glass spheres that house photomultiplier tubes to detect Cherenkov light from the charged particles emerging from neutrino interactions in the lake water. The first detector developed for Lake Baikal was the NT200, which was constructed over five years from 1993–1998 and detected cosmic neutrinos for more than a decade. It has now been replaced with the Gigaton Volume Detector (Baikal-GVD), and plans were concluded in 2015 for the first phase of the telescope to be completed by 2021. Baikal-GVD has an effective volume of 1 km³ and is designed to register and study ultrahigh-energy neutrino fluxes from astrophysical sources.

Left a mark
There is no doubt that INR RAS has left its mark on high-energy physics. While the Institute’s most recognised work will be in neutrino physics, the Moscow Meson Facility has also contributed largely to other areas of the field. An experiment was created for direct measurement of the mass of the electron antineutrino via the beta decay of tritium. The “Troitsk nu-mass” experiment started in 1985 and its limit on the electron antineutrino mass was the world’s best for years. The improvement of this result became possible only in 2019 with the large-scale KATRIN experiment in Germany that was created in participation with INR RAS. In fact, the Troitsk nu-mass experiment was considered as a prototype for KATRIN.

Experimental data have been obtained on nuclear reactions with the participation of protons and neutrons of medium energies along with data on photonuclear reactions, including the study of the spin structure of a proton using an active polarised target. New effects in collisions of relativistic nuclei have been observed and a new scientific direction has been started, “nuclear photonics”. Two effects in astroparticle physics have been named after scientists from INR RAS: the “GZK cut-off”, which is high-energy cut-off in the spectrum of the ultrahigh-energy cosmic rays named after Kenneth Greisen (US), Georgy Zatsepin and Vadim Kuzmin (INR RAS); and the “Mikheyev–Smirnov–Wolfenstein effect” concerning neutrino oscillations in matter, named after Stanislav Mikheyev, Alexei Smirnov (INR RAS) and Lincoln Wolfenstein (US).

Theoretical studies at INR RAS are also widely known, including the development of perturbation theory methods, study of the ground state (vacuum) in gauge theories, methods for studying the dynamics of strong interactions of hadrons outside the framework of perturbation theory, the first ever brane-world models and the development of principles and the search for mechanisms for the formation of the baryon asymmetry of the universe.

There are plans to construct a large centre for nuclear medicine at the base of the linear accelerator centre

Scientists from INR RAS take an active part in the work of a number of large international experiments at CERN, JINR, Germany, Japan, Italy, USA, China, France, Spain and other countries. The Institute also conducts educational activities, having its own graduate school and teaching departments in nearby institutes such as the Moscow Institute for Physics and Technology.

The future of INR RAS is deeply rooted in its new large-scale infrastructures. Baikal-GVD will, along with the IceCube experiment at the South Pole, be able to register neutrinos of astrophysical origin in the hope of establishing their nature. A project has been prepared to modernise the linear proton accelerator in Troitsk using superconducting radio-frequency cavities, while there are also plans to construct a large centre for nuclear medicine at the base of the linear accelerator centre. There is a proposal to build the Baksan Large-Volume Scintillator Detector at BNO containing 10 ktons of ultra-pure liquid scintillator, which would be able to register neutrinos from the carbon–nitrogen–oxygen (CNO) fusion cycle in the Sun with a precision sufficient to discriminate between various solar models.

The past 50 years have seen consistent growth at INR RAS, and with world-leading future projects on the horizon, the Institute has no signs of slowing down.

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Detecting LLPs at the LHC experiments requires a paradigm shift with respect to the usual data-analysis and trigger strategies. To that end, more than 200 experimentalists and theorists met online from 16 to 19 November for the eighth workshop of the LHC LLP community.

Dark quarks would undergo fragmentation and hadronisation, resulting in “dark showers”

Strong theoretical motivations underpin searches for LLPs. For example, dark matter could be part of a larger dark sector, parallel to the Standard Model (SM), with new particles and interactions. If dark quarks could be produced at the LHC, they would undergo fragmentation and hadronisation in the dark sector resulting in characteristic “dark showers” — one of the focuses of the workshop. Collider signatures for dark showers depend on the fraction of unstable particles they contain and their lifetime, with a range of categories presenting their own analysis challenges: QCD-like jets, semi-visible jets, emerging jets, and displaced vertices with missing transverse energy. Delegates agreed on the importance of connecting collider-level searches for dark showers with astrophysical and cosmological scales. In a similar spirit of collaboration across communities, a joint session with the HEP Software Foundation focused on triggering and reconstruction software for dedicated LLP detectors.

Heavy neutral leptons

The discovery of heavy neutral leptons (HNLs) could address different open questions of the SM. For example, neutrinos are expected to be left-handed and massless in the SM, but oscillate between flavours as their wavefunction evolves, providing evidence for as-yet immeasurably small masses. One way to fix this problem is to complete the field pattern of the SM with right-handed HNLs. The number and other characteristics of HNLs depend on the model considered, but in many cases HNLs are long-lived and connect to other important questions of the SM, such as dark matter and the baryon asymmetry of the universe. There are many ongoing searches for HNLs at the LHC and many more proposed elsewhere. During the November workshop the discussion touched on different models and simulations, reviewing what is available and what is needed for the different signal benchmarks.

Another focus was the reinterpretation of previous LLP searches. Recasting public results is common practice at the LHC and a good way to increase physics impact, but reinterpreting LLP searches is more difficult than prompt searches due to the use of non-standard selections and analysis-specific objects.

The latest results from CERN experiments were presented. ATLAS reported the first LHC search for sleptons using displaced-lepton final states, greatly improving sensitivity compared to LEP. CMS presented a search for strongly interacting massive particles with trackless jets, and a search for long-lived particles decaying to jets with displaced vertices. LHCb reported searches for low -mass di-muon resonances and a search for heavy neutrinos in the decay of a W boson into two muons and a jet, and the NA62 experiment at CERN’s SPS presented a search for π⁰ decays to invisible particles. These results bring important new constraints on the properties and parameters of LLP models.

Dedicated detectors

A series of dedicated LLP detectors at CERN — including the Forward Physics Facility for the HL-LHC, the CMS forward detector, FASER, Codex-b and Codex-ß, MilliQan, MoEDAL-MAPP, MATHUSLA, ANUBIS, SND@LHC, and FORMOSA – are in different stages between proposal and operation. These additional detectors, located at various distances from the LHC experiments, have diverse strengths: some, like MilliQan, look for specific particles (milli-charged particles, in that case), whereas others, like Mathusla, offer a very low background environment in which to search for neutral LLPs. These complementary efforts will, in the near future, provide all the different pieces needed to build the most complete picture possible of a variety of LLP searches, from axion-like particles to exotic Higgs decays, potentially opening the door to a dark sector.

ATLAS reported the first LHC search for sleptons using displaced-lepton final states

The workshop featured a dedicated session on future colliders for the first time. Designing these experiments with LLPs in mind would radically boost discovery chances. Key considerations will be tracking and the tracking volume, timing information, trigger and DAQ, as well as potential additional instrumentation in tunnels or using the experimental caverns.

Together with the range of new results presented and many more in the pipeline, the 2020 LLP workshop was representative of a vibrant research community, constantly pushing the “lifetime frontier”.

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Since the discovery of the Higgs boson in 2012, great progress has been made in our understanding of the Standard Model (SM) and the prospects for the discovery of new physics beyond it. Despite excellent advances in Higgs-sector measurements, searches for WIMP dark matter and exploration of very rare processes in the flavour realm, however, no unambiguous signals of new fundamental physics have been seen. This is the reason behind the explosion of interest in feebly interacting particles (FIPs) over the past decade or so.

The inaugural FIPs 2020 workshop, hosted online by CERN from 31 August to 4 September, convened almost 200 physicists from around the world. Structured around the four “portals” that may link SM particles and fields to a rich dark sector – axions, dark photons, dark scalars and heavy neutral leptons – the workshop highlighted the synergies and complementarities among a great variety of experimental facilities, and called for close collaboration across different physics communities.

Today, conventional experimental efforts are driven by arguments based on the naturalness of the electroweak scale. They result in searches for new particles with sizeable couplings to the SM, and masses near the electroweak scale. FIPs represent an alternative paradigm to the traditional beyond-the-SM physics explored at the LHC. With masses below the electroweak scale, FIPs could belong to a rich dark sector and answer many open questions in particle physics (see “Four portals” figure). Diverse searches using proton beams (CERN and Fermilab), kaon beams (CERN and JPARC), neutrino beams (JPARC and Fermilab) and muon beams (PSI) today join more idiosyncratic experiments across the globe in a worldwide search for FIPs.

FIPs can arise from the presence of feeble couplings in the interactions of new physics with SM particles and fields. These may be due to a dimensionless coupling constant or to a “dimensionful” scale, larger than that of the process being studied, which is defined by a higher dimension operator that mediates the interaction. The smallness of these couplings can be due to the presence of an approximate symmetry that is only slightly broken, or to the presence of a large mass hierarchy between particles, as the absence of new-physics signals from direct and indirect searches seems to suggest.

Take the axion, for example. This is the particle that may be responsible for the conservation of charge–parity symmetry in strong interactions. It may also constitute a fraction or the entirety of dark matter, or explain the hierarchical masses and mixings of the SM fermions – the flavour puzzle.

Or take dark photons or dark Z′ bosons, both examples of new vector gauge bosons. Such particles are associated with extensions of the SM gauge group, and, in addition to indicating new forces beyond the four we know, could lead to evidence of dark-matter candidates with thermal origins and masses in the MeV to GeV range.

Exotic Higgs bosons could also have been responsible for cosmological inflation

Then there are exotic Higgs bosons. Light dark scalar or pseudoscalar particles related to the SM Higgs may provide novel ways of addressing the hierarchy problem, in which the Higgs mass can be stabilised dynamically via the time evolution of a so-called “relaxion” field. They could also have been responsible for cosmological inflation.

Finally, consider right-handed neutrinos, often referred to as sterile neutrinos or heavy neutral leptons, which could account for the origin of the tiny, nearly-degenerate masses of the neutrinos of the SM and their oscillations, as well as providing a mechanism for our universe’s matter–antimatter asymmetry.

Scientific diversity

No single experimental approach can cover the large parameter space of masses and couplings that FIPs models allow. The interconnections between open questions require that we construct a diverse research programme incorporating accelerator physics, dark-matter direct detection, cosmology, astrophysics, and precision atomic experiments, with a strong theoretical involvement. The breadth of searches for axions or axion-like particles (ALPs) is a good indication of the growing interest in FIPs (see “Scaling the ALPs” figure). Experimental efforts here span particle and astroparticle physics. In the coming years, helioscopes, which aim to detect solar axions by their conversion into photons (X-rays) in a strong magnetic field, will improve the sensitivity by more than 10 orders of magnitude in mass in the sub-eV range. Haloscopes, which work by converting axions into visible photons inside a resonant microwave cavity placed inside a strong magnetic field, will complement this quest by increasing the sensitivity for small couplings by six orders of magnitude (down to the theoretically motivated gold band in a mass region where the axions can be a dark-matter candidate). Accelerator-based experiments, meanwhile, can probe the strongly motivated QCD scale (MeV–GeV) and beyond for larger couplings. All these results
will be complemented by a lively theo­retical activity aimed at interpreting astrophysical signals within axion and ALP models.

FIPs 2020 triggered lively discussions that will continue in the coming months via topical meetings on different subjects. Topics that motivated particular interest between communities included possible ways of comparing results from direct-detection dark-matter experiments in the MeV–GeV range against those obtained at extracted beam line and collider experiments; the connection between right-handed neutrino properties and active neutrino parameters; and the interpretation of astrophysical and cosmological bounds, which often overwhelm the interpretation of each of the four portals.

The next FIPs workshop will take place at CERN next year.

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This was the first ICHEP meeting since the publication of the update of the European strategy for particle physics, which presented an ambitious vision for the future of CERN. Though VIP-guest Peter Gabriel – rock star and human rights advocate – may not have been aware of this when delivering his opening remarks, his urging that delegates speak up for science and engage with politicians resonated with the physicists virtually present.

Many scientific highlights were covered at ICHEP and it is only possible to scratch the surface here. The results from all four major LHC experiments were particularly impressive and the collective progress in understanding the properties of neutrinos shows no sign of slowing down.

Higgs physics
ATLAS and CMS presented the first evidence for the decay of the Higgs boson into a pair of muons. Combined, the results provide strong evidence for the coupling of the Higgs boson to the muon, with the strength of the coupling consistent with that predicted in the Standard Model. Prior to these new results, the Higgs had only been observed to couple to the much heavier third-generation fermions and the W and Z gauge bosons. The measurements also provide further evidence for the linearity of the Higgs coupling, now over four orders of magnitude (from the muon to top quark), indicating the universality of the Standard-Model Higgs boson as the mechanism through which all Standard Model particles acquire mass. These are highly non-trivial statements.

ATLAS also presented a combined measurement of the Higgs signal strength, which describes a common scaling of the expected Higgs-boson yields in all processes, of 1.06 ± 0.07. In this measurement, the experimental and theoretical uncertainties are now roughly equal, emphasising the ever-increasing importance of theoretical developments in keeping up with the experimental progress; a feature that will ultimately determine the precision that will be reached by the LHC and high-luminosity LHC (HL-LHC) Higgs physics programmes.

The range of Standard Model measurements presented at ICHEP 2020 by ATLAS and CMS was truly impressive

More generally, the precision we are seeing from the ATLAS and CMS Run 2 proton–proton data is truly impressive, and an exciting indication of what is to come as the integrated luminosity accumulated by the experiments ramps up, and then ramps up again in the HL-LHC era. One interesting new example was the first observation of WW production from photon–photon collisions, where the photons are radiated from the incoming proton beams. This is a neat measurement that demonstrates the breadth of physics accessible at the LHC.

Overall, the range of Standard Model measurements presented at ICHEP 2020 by ATLAS and CMS was truly impressive and we should not forget that it is still relatively early in the LHC programme. In parallel, direct searches for new phenomena, such as supersymmetry and the “unexpected”, continues apace. Results from direct searches at the energy frontier were covered in numerous parallel session presentations. The current status was summarised succinctly by Paris Sphicas (Athens) in his conference summary talk: “Looked for a lot of possible new things. Nothing has turned up yet. Still looking intensively.”

Flavour physics
Over the last few years, a number of deviations from theoretical predictions have been observed in B-meson decays to final states with leptons. Discrepancies have been observed in ratios of decays to different lepton species, and in the angular distribution of decay products. Taken alone, each of these discrepancies are not particularly significant, but collectively they may be telling us something new about nature. At ICHEP 2020, the LHCb experiment presented their recently published results on the angular analysis in B⁰ → K^*0 μ⁺μ^–. The overall picture remains unchanged. The full analysis of the LHCb Run-2 data set, including updated measurements of the relative rates of the muon and electron decay modes (R_K and R_K*), is eagerly awaited.

The search for rare kaon decays continues to attract interest

The search for rare kaon decays continues to attract interest. One of the most impressive results presented at ICHEP was the recent observation by NA62 of the extremely rare kaon decay, K⁺ → π⁺νν̄. Occurring only once in every 10 billion decays, this is an incredibly challenging measurement and the new NA62 result is the first statistically significant observation of this decay, based on just 17 events. Whilst the observed rate is consistent with the Standard Model expectation, its observation opens up a new future avenue for exploring the possible effects of new physics.

Neutrino physics
Neutrino physics continues to be one of the most active areas of research in particle physics, so it was not surprising that the neutrino parallel sessions were the best attended of the conference. This is a particularly interesting time, with long-baseline neutrino oscillation experiments becoming sensitive to the neutrino mass ordering, and beginning to provide constraints on CP violation. Updates were presented by the NOvA experiment in the USA and the T2K experiment in Japan. Both experiments favour the normal-ordering hypothesis, although not definitively, and there is currently a slight tension between the CP violation results from the two experiments. It is worth noting that the combined interpretation of the two experiments is quite complex. The NOvA and T2K collaborations are working on a combined analysis to clarify the situation.

There were also a number of presentations on the next generation of long-baseline neutrino oscillation experiments, DUNE in the US and Hyper-Kamiokande in Japan, which aim to make the definitive discovery of CP violation in the neutrino sector. In the context of DUNE, the progress with liquid-argon time-projection- chamber (LArTPC) detector technology is impressive. It was particularly pleasing to see a number of physics results from MicroBooNE at Fermilab, and the single-phase DUNE detector prototype at CERN (ProtoDUNE-SP), that are based on the automatic reconstruction of LArTPC images – a longstanding challenge.

Virtual success
A vast range of high-qualify scientific research was covered in the 800 parallel session presentations and summarised in the 44 plenary talks at ICHEP 2020. The quality of the presentations was high, and speakers coped well with the challenge of pre-recording talks. The “replay” sessions worked extremely well too – an innovation that is likely to persist in the post-COVID world. About 3000 people registered for the meeting, which is more than double the previous two events. It was particularly pleasing to learn that almost 2500 connected to the parallel sessions.

Despite the many successes, we all missed the opportunity to meet colleagues in person; it is often the informal discussions over coffee or in restaurants and bars that generate new ideas and, importantly, lead to new collaborations. Whilst virtual conferences are likely to remain a feature in the post- COVID world, they will not replace in-person events.

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A highlight of the conference was the first observation of solar CNO neutrinos

A highlight of the conference was the first observation of solar CNO neutrinos by the Borexino collaboration, which operates a 280-tonne liquid-scintillator detector in Italy’s Gran Sasso Laboratory. Dominant in stars more than 1.3 times the mass of the sun, the CNO cycle accounts for about 1% of the sun’s energy and generates a difficult-to-detect neutrino flux similar to backgrounds due to decays in the detector of ²¹⁰Bi and its daughter nucleus ²¹⁰Po. Gioacchino Ranucci (INFN, Milano) explained that the spectral fit to the observed data returns only the sum of CNO and ²¹⁰Bi neutrinos. “The quest for CNO is turned into the quest for ²¹⁰Bi through ²¹⁰Po,” he emphasised. “With this outcome, Borexino has completely unravelled the two processes powering the Sun—the pp chain and the CNO cycle.” The final data analysis yielded a 5.1σ statistic against a hypothesis of no CNO neutrinos, and a CNO flux at the Earth of 7.0_-1.9^+2.9 × 10⁸ cm^-2 s^-1.

Another highlight from Gran Sasso was the report from the Gerda collaboration on the search for neutrino-less double beta decay. If observed, this process would confirm the long-suspected Majorana rather than Dirac-fermion nature of neutrinos – a beyond the Standard Model feature with intriguing implications for why the neutrino mass is so small. Since Neutrino 2018, Gerda has nearly doubled its Phase 2 exposure and added a liquid-argon veto and a new detector string. The now complete Phase 2 result is a 90% confidence level half-life of >1.8 x 10²⁶ years according to a frequentist analysis, or >1.4 x 10²⁶ years, according to a Bayesian analysis with additional prior assumptions. Talks describing a half-dozen other double-beta-decay experiments displayed the high level of interest in this field.

Sterile neutrinos

Searches for additional “sterile” neutrinos with no Standard-Model gauge interactions were also featured. Takasumi Maruyama (KEK) described the liquid-scintillator JSNS² experiment as a direct test of the controversial LSND Experiment result, first reported about 25 years ago. JSNS² collected its first data during the three weeks before Neutrino 2020. Adrien Hourlier (MIT) reported on the now complete analysis of data from MiniBooNE that was collected during the past 17 years. Combining neutrino and anti-neutrino modes, MiniBooNE reports a 4.8σ excess. Hourlier presented soon-to-be published detailed distributions which the collaboration hopes “will guide theorists to explain our data”. Minerba Betancourt (Fermilab) then described the Fermilab Short-Baseline Neutrino (SBN) programme, which will use three detectors to obtain a definitive result on neutrino oscillations for an LSND and MiniBooNE-like ratio of oscillation distance to energy of ~1 m/MeV. The beam neutrino energy peaks at 700 MeV. A new liquid-argon near detector (SBND) will be placed 110 m from the target. The existing MicroBooNE is located at 470 m and the ICARUS Detector, moved from Gran Sasso, is installed at 600 m. Thomas Carroll (Wisconsin) reported on sterile-neutrino limits by muon disappearance determined by the now completed long-baseline MINOS/MINOS+ collaboration. These limits are in tension with the appearance data from both LSND and MiniBooNE when analysed as evidence for sterile neutrinos.

Two talks described the world’s two hundred-kilometre-scale neutrino-oscillation experiments, NOvA and T2K. The degeneracy of mass difference, mixing angle, hierarchy and possible CP violation make interpretation of these experiments’ results quite complex. Interestingly, there is mild tension, albeit only at the 1σ level, between the NOvA and T2K results regarding leptonic CP conservation and the neutrino mass hierarchy. The two collaborations are now working together on a combined analysis. Several talks discussed future initiatives. Lia Merminga (Fermilab) reported on LBNF and PIP-II, which will result in a new neutrino beam from Fermilab to the Sanford Laboratory in South Dakota for the DUNE experiment. Combined, these two projects will result in beam power of 2.4 MW, more than three times the intensity of the current NuMI beam. Michael Mooney (Colorado State) reported on the enormous progress of the DUNE project with two successful prototype detectors operating at CERN and pre-excavation work progressing at Sanford Laboratory. Complementary to the liquid-argon technology of DUNE is the recently approved Hyper-Kamiokande water-Cherenkov detector, which was described by Masaki Ishitsuka (Tokyo University of Science). Hyper-K will have a total mass of 260 kilo-tonne and 8.4 times the fiducial volume of the current Super-Kamiokande detector.

The VR feature attracted 3,409 conference participants

While much of Neutrino 2020 was modelled after the usual features of an in-person conference, the Virtual Reality (VR) poster presentation was novel and unique. Marco Del Tutto (Fermilab) created multiple virtual “rooms” for five posters each, along with additional rooms for topical discussions, sightseeing in Chicago and visiting Fermilab. The most enabling feature of the VR was that the software facilitated dialogue between participants whose avatars could move around the space and speak with one another. For example, if a group of avatars clustered around a poster, the participants could discuss the poster as a group. The VR feature attracted 3,409 participants. The VR was also supplemented by two-minute videos from presenters which enabled 5,800 YouTube views and 60,600 web displays.

In closing remarks, the organisers acknowledged the challenges of an online conference, but also emphasised the strengths of this novel approach. The exciting physics of Neutrino 2020 was made available to an extensive and diverse audience, including many scientists who would not have been able to attend an in-person conference because of funding, visas, family concerns or other issues. About 60% of participants were students or post-docs and the conference reached participants from 67 countries. The Slack discussions and posts on social media indicated wide-spread praise that the online format worked as well as it did. Some aspects of Neutrino 2020 may well affect the planning and organisation of future in-person and online conferences.

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In traditional Balinese music, instruments are made in pairs, with one tuned slightly higher in frequency than its twin. The notes are indistinguishable to the human ear when played together, but the sound recedes and swells a couple of times each second, encouraging meditation. This is a beating effect: fast oscillations at the mean frequency inside a slowly oscillating envelope. Similar physics is at play in neutrino oscillations. Rather than sound intensity, it’s the probability to observe a neutrino with its initial flavour that oscillates. The difference is how long it takes for the interference to make itself felt. When Balinese musicians strike a pair of metallophones, the notes take just a handful of periods to drift out of phase. By contrast, it takes more than 10²⁰ de Broglie wavelengths and hundreds of kilometres for neutrinos to oscillate in experiments like the planned mega-projects Hyper-Kamiokande and DUNE.

The zeitgeist began to shift to artificially produced neutrinos

Neutrino oscillations revealed a rare chink in the armour of the Standard Model: neutrinos are not massless, but are evolving superpositions of at least three mass eigenstates with distinct energies. A neutrino is therefore like three notes played together: frequencies so close, given the as-yet immeasurably small masses involved, that they are not just indistinguishable to the ear, but inseparable according to the uncertainty principle. As neutrinos are always ultra-relativistic, the energies of the mass eigenstates differ only due to tiny mass contributions of m²/2E. As the mass eigenstates propagate, phase differences develop between them proportional to squared-mass splittings Δm². The sought-after oscillations range from a few metres to the diameter of Earth.

Orthogonal mixtures

The neutrino physics of the latter third of the 20th century was bookended by two anomalies that uncloaked these effects. In 1968 Ray Davis’s observation of a deficit of solar neutrinos prompted Bruno Pontecorvo to make public his conjecture that neutrinos might oscillate. Thirty years later, the Super-Kamiokande collaboration’s analysis of a deficit of atmospheric muon neutrinos from the other side of the planet posthumously vindicated the visionary Italian, and later Soviet, theorist’s speculation. Subsequent observations have revealed that electron, muon and tau neutrinos are orthogonal mixtures of mass eigenstates ν₁ and ν₂, separated by a small so-called solar splitting Δm²₂₁, and ν₃, which is separated from that pair by a larger “atmospheric” splitting usually quantified by Δm²₃₂ (see “Little and large” figure). It is not yet known if ν₃ is the lightest or the heaviest of the trio. This is called the mass-hierarchy problem.

“In the first two decades of the 21st century we have achieved a rather accurate picture of neutrino masses and mixings,” says theorist Pilar Hernández of the University of Valencia, “but the ordering of the neutrino states is unknown, the mass of the lightest state is unknown and we still do not know if the neutrino mixing matrix has imaginary entries, which could signal the breaking of CP symmetry,” she explains. “The very different mixing patterns in quarks and leptons could hint at a symmetry relating families, and a more accurate exploration of the lepton-mixing pattern and the neutrino ordering in future experiments will be essential to reveal any such symmetry pattern.”

Today, experiments designed to constrain neutrino mixing tend to dispense with astrophysical neutrinos in favour of more controllable accelerator and reactor sources. The experiments span more than four orders of magnitude in size and energy and fall into three groups (see “Not natural” figure). Much of the limelight is taken by experiments that are sensitive to the large mass splitting Δm²₃₂, which include both a cluster of current (such as T2K) and future (such as DUNE) accelerator-neutrino experiments with long baselines and high energies, and a high-performing trio of reactor-neutrino experiments (Daya Bay, RENO and Double Chooz) with a baseline of about a kilometre, operating just above the threshold for inverse beta decay. The second group is a beautiful pair of long-baseline reactor-neutrino experiments (KamLAND and the soon-to-be-commissioned JUNO), which join experiments with solar neutrinos in having sensitivity to the smaller squared-mass splitting Δm²₂₁. Finally, the third group is a host of short-baseline accelerator-neutrino experiments and very-short-baseline reactor neutrino experiments that are chasing tantalising hints of a fourth “sterile” neutrino (with no Standard-Model gauge interactions), which is split from the others by a squared-mass splitting of the order of 1 eV².

Artificial sources

Experiments with artificial sources of neutrinos have a storied history, dating from the 1950s, when physicists toyed with the idea of detecting neutrinos created in the explosion of a nuclear bomb, and eventually observed them streaming from nuclear reactors. The 1960s saw the invention of the accelerator neutrino. Here, proton beams smashed into fixed targets to create a decaying debris of charged pions and their concomitant muon neutrinos. The 1970s transformed these neutrinos into beams by focusing the charged pions with magnetic horns, leading to the discovery of weak neutral currents and insights into the structure of nucleons. It was not until the turn of the century, however, that the zeitgeist of neutrino-oscillation studies began to shift from naturally to artificially produced neutrinos. Just a year after the publication of the Super-Kamiokande collaboration’s seminal 1998 paper on atmospheric–neutrino oscillations, Japanese experimenters trained a new accelerator-neutrino beam on the detector.

Operating from 1999 to 2006, the KEK-to-Kamioka (K2K) experiment sent a beam of muon neutrinos from the KEK laboratory in Tsukuba to the Super-Kamiokande detector, 250 km away under Mount Ikeno on the other side of Honshu. K2K confirmed that muon neutrinos “disappear” as a function of propagation distance over energy. The experiments together supported the hypothesis of an oscillation to tau neutrinos, which could not be directly detected at that energy. By increasing the beam energy well above the tau-lepton mass, the CERN Neutrinos to Gran Sasso (CNGS) project, which ran from 2006 to 2012, confirmed the oscillation to tau neutrinos by directly observing tau leptons in the OPERA detector. Meanwhile, the Main Injector Neutrino Oscillation Search (MINOS), which sent muon neutrinos from Fermilab to northern Minnesota from 2005 to 2012, made world-leading measurements of the parameters describing the oscillation.

With ν_μ → ν_τ oscillations established, the next generation of experiments innovated in search of a subtler effect. T2K (K2K’s successor, with the beam now originating at J-PARC in Tokai) and NOvA (which analyses oscillations over the longer baseline of 810 km between Fermilab and Ash River, Minnesota) both have far detectors offset by a few degrees from the direction of the peak flux of the beams. This squeezes the phase space for the pion decays, resulting in an almost mono-energetic flux of neutrinos. Here, a quirk of the mixing conspires to make the musical analogy of a pair of metallophones particularly strong: to a good approximation, the muon neutrinos ring out with two frequencies of roughly equal amplitude, to yield an almost perfect disappearance of muon neutrinos – and maximum sensitivity to the appearance of electron neutrinos.

Testing CP symmetry

The three neutrino mass eigenstates mix to make electron, muon and tau neutrinos according to the Pontecorvo– Maki–Nakagawa–Sakata (PMNS) matrix, which describes three rotations and a complex phase δ_CP that can cause charge–parity (CP) violation – a question of paramount importance in the field due to its relevance to the unknown origin of the matter–antimatter asymmetry in the universe. Whatever the value of the complex phase, leptonic CP violation can only be observed if all three of the angles in the PMNS matrix are non-zero. Experiments with atmospheric and solar neutrinos demonstrated this for two of the angles. At the beginning of the last decade, short-baseline reactor-neutrino experiments in China (Daya Bay), Korea (RENO) and France (Double Chooz) were in a race with T2K to establish if the third angle, which leads to a coupling between ν₃ and electrons, was also non-zero. In the reactor experiments this would be seen as a small deficit of electron antineutrinos a kilometre or so from the reactors; in T2K the smoking gun would be the appearance of a small number of electron neutrinos not present in the initial muon-neutrino-dominated beam.

After data taking was cut short by the great Sendai earthquake and tsunami of March 2011, T2K published evidence for the appearance of six electron-neutrino events, over the expected background of 1.5 ± 0.3 in the case of no coupling. Alongside a single tau-neutrino candidate in OPERA, these were the first neutrinos seen to appear in a detector with a new flavour, as previous signals had always registered a deficit of an expected flavour. In the closing days of the year, Double Chooz published evidence for 4121 electron–antineutrino events, under the expected tally for no coupling of 4344 ± 165, reinforcing T2K’s 2.5σ indication. Daya Bay and RENO put the matter to bed the following spring, with 5σ evidence apiece that the ν₃-electron coupling was indeed non-zero. The key innovation for the reactor experiments was to minimise troublesome flux and interaction systematics by also placing detectors close to the reactors.

Since then, T2K and NOvA, which began taking data in 2014, have been chasing leptonic CP violation – an analysis that is out of the reach of reactor experiments, as δ_CP does not affect disappearance probabilities. By switching the polarity of the magnetic horn, the experiments can compare the probabilities for the CP-mirror oscillations ν_μ → ν_e and ν_μ → ν_e directly. NOvA data are inconclusive at present. T2K data currently err towards near maximal CP violation in the vicinity of δ_CP= –π/2. The latest analysis, published in April, disfavours leptonic CP conservation (δ_CP = 0, ±π) at 2σ significance for all possible mixing parameter values. Statistical uncertainty is the biggest limiting factor.

Major upgrades planned for T2K next year target statistical, interaction-model and detector uncertainties. A substantial increase in beam intensity will be accompanied by a new fine-grained scintillating target for the ND280 near-detector complex, which will lower the energy threshold to reconstruct tracks. New transverse TPCs will improve ND280’s acceptance at high angles, yielding a better cancellation of systematic errors with the far detector, Super-Kamiokande, which is being upgraded by loading 0.01% gadolinium salts into the otherwise ultrapure water. As in reactor-neutrino detectors, this will provide a tag for antineutrino events, to improve sample purities in the search for leptonic CP violation.

T2K and NOvA both plan to roughly double their current data sets, and are working together on a joint fit, in a bid to better understand correlations between systematic uncertainties, and break degeneracies between measurements of CP violation and the mass hierarchy. If the CP-violating phase is indeed maximal, as suggested by the recent T2K result, the experiments may be able to exclude CP conservation with more than 99% confidence. “At this point we will be in a transition from a statistics-dominated to a systematics-dominated result,” says T2K spokesperson Atsuko Ichikawa of the University of Kyoto. “It is difficult to say, but our sensitivity will likely be limited at this stage by a convolution of neutrino-interaction and flux systematics.”

The next generation

Two long-baseline accelerator-neutrino experiments roughly an order of magnitude larger in cost and detector mass than T2K and NOvA have received green lights from the Japanese and US governments: Hyper-Kamiokande and DUNE. One of their primary missions is to resolve the question of leptonic CP violation.

Hyper-Kamiokande will adopt the same approach as T2K, but will benefit from major upgrades to the beam and the near and far detectors in addition to those currently underway in the present T2K upgrade. To improve the treatment of systematic errors, the suite of near detectors will be complemented by an ingenious new gadolinated water-Cherenkov detector at an intermediate baseline: by spanning a range of off-axis angles, it will drive down interaction-model systematics by exploiting previously neglected information on the how the flux varies as a function of the angle relative to the centre of the beam. Hyper-Kamiokande’s increased statistical reach will also be impressive. The power of the Japan Proton Accelerator Research Complex (J-PARC) beam will be increased from its current value of 0.5 MW up to 1.3 MW, and the new far detector will be filled with 260,000 tonnes of ultrapure water, yielding a fiducial volume 8.4 times larger than that of Super-Kamiokande. Procurement of the photo-multiplier tubes will begin this year, and the five-year-long excavation of the cavern has already begun. Data taking is scheduled to commence in 2027. “The expected precision on δ_CP is 10–20 degrees, depending on its true value,” says Hyper-Kamiokande international co-spokesperson Francesca di Lodovico of King’s College, London.

In the US, the Deep Underground Neutrino Experiment (DUNE) will exploit the liquid-argon–TPC technology first deployed on a large scale by ICARUS – OPERA’s sister detector in the CNGS project. The idea for the technology dates back to 1977, when Carlo Rubbia proposed using liquid rather than gaseous argon as a drift medium for ionisation electrons. Given liquid-argon’s higher density, such detectors can serve as both target and tracker, providing high-resolution 3D images of the interactions – an invaluable tool for reducing systematics related to the murky world of neutrino–nucleus interactions.

Spectacular performance

The technology is currently being developed in two prototype detectors at CERN. The first hones ICARUS’s single-phase approach. “The performance of the prototype has been absolutely spectacular, exceeding everyone’s expectations,” says DUNE co-spokesperson Ed Blucher of the University of Chicago. “After almost two years of operation, we are confident that the liquid–argon technology is ready to be deployed at the huge scale of the DUNE detectors.” In parallel, the second prototype is testing a newer dual-phase concept. In this design, ionisation charges drift through an additional layer of gaseous argon before reaching the readout plane. The signal can be amplified here, potentially easing noise requirements for the readout electronics, and increasing the maximum size of the detector. The dual-phase prototype was filled with argon in summer 2019 and is now recording tracks.

The final detectors will have about twice the height and 10 to 20 times the footprint. Following the construction of an initial single-phase unit, the DUNE collaboration will likely pick a mix of liquid-argon technologies to complete their roster of four 10 kton far-detector modules, set to be installed a kilometre underground at the Sanford Underground Research Laboratory in Lead, South Dakota. Site preparation and pre-excavation activities began in 2017, and full excavation work is expected to begin soon, with the goal that data-taking begin during the second half of this decade. Work on the near-detector site and the “PIP-II” upgrade to Fermilab’s accelerator complex began last year.

Though similar to Hyper-Kamiokande at first glance, DUNE’s approach is distinct and complementary. With beam energy and baseline both four times greater, DUNE will have greater sensitivity to flavour-dependent coherent-forward-scattering with electrons in Earth’s crust – an effect that modifies oscillation probabilities differently depending on the mass hierarchy. With the Fermilab beam directed straight at the detector rather than off-axis, a broader range of neutrino energies will allow DUNE to observe the oscillation pattern from the first to the second oscillation maximum, and simultaneously fit all but the solar mixing parameters. And with detector, flux and interaction uncertainties all distinct, a joint analysis of both experiments’ data could break degeneracies and drive down systematics.

“If CP violation is maximal and the experiments collect data as anticipated, DUNE and Hyper-Kamiokande should both approach 5σ significance for the exclusion of leptonic CP conservation in about five years,” estimates DUNE co-spokesperson Stefan Söldner-Rembold of the University of Manchester, noting that the experiments will also be highly complementary for non-accelerator topics. The most striking example is supernova-burst neutrinos, he says, referring to a genre of neutrinos only observed once so far, during 15 seconds in 1987, when neutrinos from a supernova in the Large Magellanic Cloud passed through the Earth. “While DUNE is primarily sensitive to electron neutrinos, Hyper-Kamiokande will be sensitive to electron antineutrinos. The difference between the timing distributions of these samples encodes key information about the dynamics of the supernova explosion.” Hyper-Kamiokande spokesperson Masato Shiozawa of ICRR Tokyo also emphasises the broad scope of the physics programmes. “Our studies will also encompass proton decay, high-precision measurements of solar neutrinos, supernova-relic neutrinos, dark-matter searches, the possible detection of solar-flare neutrinos and neutrino geophysics.”

Half a century since Ray Davis and two co-authors published evidence for a 60% deficit in the flux of solar neutrinos compared to John Bahcall’s prediction, DUNE already boasts more than a thousand collaborators, and Hyper-Kamiokande’s detector mass is set to be 500 times greater than Davis’s tank of liquid tetrachloroethylene. If Ray Davis was the conductor who set the orchestra in motion, then these large experiments fill out the massed ranks of the violin section, poised to deliver what may well be the most stirring passage of the neutrino-oscillation symphony. But other sections of the orchestra also have important parts to play.

Mass hierarchy

The question of the neutrino mass hierarchy will soon be addressed by the Jiangmen Underground Neutrino Observatory (JUNO) experiment, which is currently under construction in China. The project is an evolution of the Daya Bay experiment, and will seek to measure a deficit of electron antineutrinos 53 km from the Yangjiang and Taishan nuclear-power plants. As the reactor neutrinos travel, the small kilometre-scale oscillation observed by Daya Bay will continue to undulate with the same wavelength, revealed in JUNO as “fast” oscillations on a slower and deeper first oscillation maximum due to the smaller solar mass splitting Δm²₂₁ (see “An oscillation within an oscillation” figure).

“JUNO can determine the neutrino mass hierarchy in an unambiguous and definite way, independent from the CP phase and matter effects, unlike other experiments using accelerator or atmospheric neutrinos,” says spokesperson Yifang Wang of the Chinese Academy of Sciences in Beijing. “In six years of data taking, the statistical significance will be higher than 3σ.”

JUNO has completed most of the digging of the underground laboratory, and equipment for the production and purification of liquid scintillator is being fabricated. A total of 18,000 20-inch photomultiplier tubes and 26,000 3-inch photomultiplier tubes have been delivered, and most of them have been tested and accepted, explains Wang. The installation of the detector is scheduled to begin next year. JUNO will arguably be at the vanguard of a precision era for the physics of neutrino oscillations, equipped to measure the mass splittings and the solar mixing parameters to better than 1% precision – an improvement of about one order of magnitude over previous results, and even better than the quark sector, claims Wang, somewhat provocatively. “JUNO’s capabilities for supernova-burst neutrinos, diffused supernova neutrinos and geoneutrinos are unprecedented, and it can be upgraded to be a world-best double-beta-decay detector once the mass hierarchy is measured.”

With JUNO, Hyper-Kamiokande and DUNE now joining a growing ensemble of experiments, the unresolved leitmotifs of the three-neutrino paradigm may find resolution this decade, or soon after. But theory and experiment both hint, quite independently, that nature may have a scherzo twist in store before the grand finale.

A rich programme of short-baseline experiments promises to bolster or exclude experimental hints of a fourth sterile neutrino with a relatively large mixing with the electron neutrino that have dogged the field since the late 1990s. Four anomalies stack up as more or less consistent among themselves. The first, which emerged in the mid-1990s at Los Alamos’s Liquid Scintillator Neutrino Detector (LSND), is an excess of electron antineutrinos that is potentially consistent with oscillations involving a sterile neutrino at a mass splitting Δm² ∼ 1 eV². Two other quite disparate anomalies since then – a few-percent deficit in the expected flux from nuclear reactors, and a deficit in the number of electron neutrinos from radioactive decays in liquid-gallium solar-neutrino detectors – could be explained in the same way. The fourth anomaly, from Fermilab’s MiniBooNE experiment, which sought to replicate the LSND effect at a longer baseline and a higher energy, is the most recent: a sizeable excess of both electron neutrinos and antineutrinos, though at a lower energy than expected. It’s important to note, however, that experiments including KARMEN, MINOS+ and IceCube have reported null searches for sterile neutrinos that fit the required description. Such a particle would also stand in tension with cosmology, notes phenomenologist Silvia Pascoli of Durham University, as models predict it would make too large a contribution to hot dark matter in the universe today, unless non-standard scenarios are invoked.

Three different types of experiment covering three orders of magnitude in baseline are now seeking to settle the sterile-neutrino question in the next decade. A smattering of reactor-neutrino experiments a mere 10 metres or so from the source will directly probe the reactor anomaly at Δm² ∼ 1 eV². The data reported so far are intriguing. Korea’s NEOS experiment and Russia’s DANSS experiment report siren signals between 1 and 2 eV², and NEUTRINO-4, also based in Russia, reports a seemingly outlandish signal, indicative of very large mixing, at 7 eV². In parallel, J-PARC’s JSNS² experiment is gearing up to try to reproduce the LSND effect using accelerator neutrinos at the same energy and baseline. Finally, Fermilab’s short-baseline programme will thoroughly address a notable weakness of both LSND and MiniBooNE: the lack of a near detector.

The Fermilab programme will combine three liquid-argon TPCs – a bespoke new short-baseline detector (SBND), the existing MicroBooNE detector, and the refurbished ICARUS detector – to resolve the LSND anomaly once and for all. SBND is currently under construction, MicroBooNE is operational, and ICARUS, removed from its berth at Gran Sasso and shipped to the US in 2017, has been installed at Fermilab, following work on the detector at CERN. “The short-baseline neutrino programme at Fermilab has made tremendous technical progress in the past year,” says ICARUS spokesperson and Nobel laureate Carlo Rubbia, noting that the detector will be commissioned as soon as circumstances allow, given the coronavirus pandemic. “Once both ICARUS and SBND are in operation, it will take less than three years with the nominal beam intensity to settle the question of whether neutrinos have an even more mysterious character than we thought.”

Muon neutrinos ring out with two frequencies of roughly equal amplitude, to yield almost perfect disappearance

Outside of the purview of oscillation experiments with artificially produced neutrinos, astrophysical observatories will scale a staggering energy range, from the PeV-scale neutrinos reported by IceCube at the South Pole, down, perhaps, to the few-hundred-μeV cosmic neutrino background sought by experiments such as PTOLEMY in the US. Meanwhile, the KATRIN experiment in Germany is zeroing in on the edges of beta-decay distributions to set an absolute scale for the mass of the peculiar mixture of mass eigenstates that make up an electron antineutrino (CERN Courier January/February 2020 p28). At the same time, a host of experiments are searching for neutrinoless double-beta decay – a process that can only occur if the neutrino is its own antiparticle. Discovering such a Majorana nature for the neutrino would turn the Standard Model on its head, and offer grist for the mill of theorists seeking to explain the tininess of neutrino masses, by balancing them against still-to-be-discovered heavy neutral leptons.

Indispensable input

According to Mikhail Shaposhnikov of the Swiss Federal Institute of Technology in Lausanne, current and future reactor- and accelerator-neutrino experiments will provide an indispensable input for understanding neutrino physics. And not in isolation. “To reach a complete picture, we also need to know the mechanism for neutrino-mass generation and its energy scale, and the most important question here is the scale of masses of new neutrino states: if lighter than a few GeV, these particles can be searched for at new experiments at the intensity frontier, such as SHiP, and at precision experiments looking for rare decays of mesons, such as Belle II, LHCb and NA62, while the heavier states may be accessible at ATLAS and CMS, and at future circular colliders,” explains Shaposhnikov. “These new particles can be the key in solving all the observational problems of the Standard Model, and require a consolidated effort of neutrino experiments, accelerator-based experiments and cosmological observations. Of course, it remains to be seen if this dream scenario can indeed be realised in the coming 20 years.”

• This article was updated on 6 July, to reflect results presented at Neutrino 2020

The post Tuning in to neutrinos appeared first on CERN Courier.

Luckily for us, there is presently almost no antimatter in the universe. This makes it possible for us – made of matter – to live without being annihilated in matter–antimatter encounters. However, cosmology tells us that just after the cosmic Big Bang, the universe contained equal amounts of matter and antimatter. Obviously, for the universe to have evolved from that early state to the present one, which contains quite unequal amounts of matter and antimatter, the two must behave differently. This implies that the symmetry CP (charge conjugation × parity) must be violated. That is, there must be physical systems whose behaviour changes if we replace every particle by its antiparticle, and interchange left and right.

In 1964, Cronin, Fitch and colleagues discovered that CP is indeed violated, in the decays of neutral kaons to pions – a phenomenon that later became understood in terms of the behaviour of quarks. By now, we have observed quark CP violation in the strange sector, the beauty sector and most recently in the charm sector (CERN Courier May/June 2019 p7). The observations of CP violation in B (beauty) meson decays have been particularly illuminating. Everything we know about quark CP violation is consistent with the hypothesis that this violation arises from a single complex phase in the quark mixing matrix. This matrix gives the amplitude for any particular negatively-charged quark, whether down, strange or bottom, to convert via a weak interaction into any particular positively-charged quark, be it up, charm or top. Just two parameters in the quark mixing matrix, ρ and η, whose relative size determines the complex phase, account very successfully for numerous quark phenomena, including both CP-violating ones and others. This is impressively demonstrated by a plot of all the experimental constraints on these two parameters (figure 1). All the constraints intersect at a common point.

Of course, precisely which (ρ, η) point is consistent with all the data is not important. Lincoln Wolfenstein, who created the quark-mixing-matrix parametrisation that includes ρ and η, was known to say: “Look, I invented ρ and η, and I don’t care what their values are, so why should you?”

Having observed CP violation among quarks in numerous laboratory experiments of today, we might be tempted to think that we understand how CP violation in the early universe could have changed the world from one with equal quantities of matter and antimatter to one in which matter dominates very heavily over antimatter. However, scenarios that tie early-universe CP violation to that seen among the quarks today, and do not add new physics to the Standard Model of the elementary particles, yield too small a present-day matter–antimatter asymmetry. This leads one to wonder whether early-universe CP violation involving leptons, rather than quarks, might have led to the present dominance of matter over antimatter. This possibility is envisaged by leptogenesis, a scenario in which heavy neutral leptons that were their own antiparticles lived briefly in the early universe, but then underwent CP-asymmetric decays, creating a world with unequal numbers of particles and antiparticles. Such heavy neutral leptons are predicted by “see-saw” models, which explain the extreme lightness of the known neutrinos in terms of the extreme heaviness of the postulated heavy neutral leptons. Leptogenesis can successfully account for the observed size of the present matter–antimatter asymmetry.

Deniable plausibility

In the straightforward version of this picture, the heavy neutral leptons are too massive to be observable at the LHC or any foreseen collider. However, since leptogenesis requires leptonic CP violation, observing this violation in the behaviour of the currently observed leptons would make it more plausible that leptogenesis was indeed the mechanism through which the present matter–antimatter asymmetry of the universe arose. Needless to say, observing leptonic CP violation would also reveal that the breaking of CP symmetry, which before 1964 one might have imagined to be an unbroken, fundamental symmetry of nature, is not something special to the quarks, but is participated in by all the constituents of matter.

To find out if leptons violate CP, we are searching for what is traditionally described as a difference between the behaviour of neutrinos and that of antineutrinos. This description is fine if neutrinos are Dirac particles – that is, particles that are distinct from their antiparticles. However, many theorists strongly suspect that neutrinos are actually Majorana particles – that is, particles that are identical to their antiparticles. In that case, the traditional description of the search for leptonic CP violation is clearly inapplicable, since then the neutrinos and the antineutrinos are the same objects. However, the actual experimental approach that is being pursued is a perfectly valid probe of leptonic CP violation regardless of whether neutrinos are of Dirac or of Majorana character. In fact, this approach is completely insensitive to which of these two possibilities nature has chosen.

Through a glass darkly

The pursuit of leptonic CP violation is based on comparing the rates for two CP mirror-image processes (figure 2). In process A, the initial state is a π⁺ and an undisturbed detector. The final state consists of a μ⁺, an e^–, and a nucleus in the detector that has been struck by an intermediate-state neutrino beam particle that travelled a long distance from its source to the detector. Since the neutrino was born together with a muon, but produced an electron in the detector, and the probability for this to have happened oscillates as a function of the distance the neutrino travels divided by its energy, the process is commonly referred to as muon–neutrino to electron–neutrino oscillation.

Leptogenesis can account for the matter–antimatter asymmetry

In process B, the initial and final states are the same as in process A, but with every particle replaced by its antiparticle. In addition, owing to the character of the weak interactions, the helicity (the projection of the spin along the momentum) of every fermion is reversed, so that left and right are interchanged. Thus, regardless of whether neutrinos are identical to their antiparticles, processes A and B are CP mirror images, so if their rates are unequal, CP invariance is violated. Moreover, since the probability of a neutrino oscillation involves the weak interactions of leptons, but not those of quarks, this violation of CP invariance must come from the weak interactions of leptons.

Of course, we cannot employ an anti-detector in process B in practice. However, the experiment can legitimately use the same detector in both processes. To do that, it must take into account the difference between the cross sections for the beam particles in processes A and B to interact in this detector. Once that is done, the comparison of the rates for processes A and B remains a valid probe of CP non-invariance.

The matrix reloaded

Just as quark CP violation arises from a complex phase in the quark mixing matrix, so leptonic CP violation in neutrino oscillation can arise from a complex phase, δ_CP, in the leptonic mixing matrix, which is the leptonic analogue of the quark mixing matrix. However, if, as suggested by several short-baseline oscillation experiments, there exist not only the three well-established neutrinos, but also additional so-called “sterile” neutrinos that do not participate in Standard Model weak interactions, then the leptonic mixing matrix is larger than the quark one. As a result, while the quark mixing matrix is permitted to contain just one complex phase, its leptonic analogue may contain multiple complex phases that can contribute to CP violation in neutrino oscillations.

Leptonic CP violation is being sought by two current neutrino-oscillation experiments. The NOvA experiment in the US has reported results that are consistent with either the presence or absence of CP violation. The T2K experiment in Japan reports that the complete absence of CP violation is excluded at 95% confidence. Assuming that the leptonic mixing matrix is the same size as the quark one, so that it may contain only one complex phase relevant to neutrino oscillations, the T2K data show a preference for values of that phase, δ_CP, that correspond to near maximal CP violation. Of course, as Lincoln Wolfenstein would doubtless point out, the precise value of δ_CP is not important. What counts is the extremely interesting experimental finding that the behaviour of leptons may very well violate CP. In the future, the oscillation experiments Hyper-Kamiokande in Japan and DUNE in the US will probe leptonic CP violation with greater sensitivity, and should be capable of observing it even if it should prove to be fairly small (see Tuning in to neutrinos).

By searching for leptonic CP violation, we hope to find out whether the breaking of CP symmetry occurs among all the constituents of matter, including both the leptons and the quarks, or whether it is a feature that is special to the quarks. If leptonic CP violation should be definitively shown to exist, this violation might be related to the reason that the universe contains matter, but almost no antimatter, so that life is possible.

The post The search for leptonic CP violation appeared first on CERN Courier.

Who needs the WIMP if we can have the axion?

Elena Aprile

“Thanks to our unprecedented low event rate in electronic recoils background, and thanks to our large exposure, both in detector mass and time, we could afford to look for signatures of rare and new phenomena expected at the lowest energies where one usually finds lots of background,” says XENON spokesperson Elena Aprile, of Columbia University in New York. “I am especially intrigued by the possibility to detect axions produced in the Sun,” she says. “Who needs the WIMP if we can have the axion?”

The XENON collaboration has been in pursuit of WIMPs, a leading bosonic cold-dark-matter candidate, since 2005 with a programme of 10 kg, 100 kg and now 1 tonne liquid-xenon TPCs. Particles scattering in the liquid xenon create both scintillation light and ionisation electrons; the latter drift upwards in an electric field towards a gaseous phase where electroluminescence amplifies the charge signal into a light signal. Photomultiplier tubes record both the initial scintillation light and the later electroluminescence, to reveal 3D particle tracks, and the relative magnitudes of the two signals allows nuclear and electronic recoils to be differentiated. XENON1T derives its world-leading limit on WIMPs – the strictest 90% confidence limit being a cross-section of 4.1×10⁻⁴⁷ cm² for WIMPs with a mass of 30 GeV – from the very low rate of nuclear recoils observed by XENON1T from February 2017 to February 2018.

A surprise was in store, however, in the same data set, which also revealed 285 electronic recoils at the lower end of XENON1T’s energy acceptance, from 1 to 7 keV, over the expected background of 232±15. The sole background-modelling explanation for the excess that the collaboration has not been able to rule out is a minute concentration of tritium in the liquid xenon. With a half-life of 12.3 years and a relatively low amount of energy liberated in the decay of 18.6 keV, an unexpected contribution of tritium decays is favoured over XENON1T’s baseline background model at approximately 3σ. “We can measure extremely tiny amounts of various potential background sources, but unfortunately, we are not sensitive to a handful of tritium atoms per kilogram,” explains deputy XENON1T spokesperson Manfred Lindner, of the Max Planck Institute for Nuclear Physics in Heidelberg. Cryogenic distillation plus running the liquid xenon through a getter is expected to remove any tritium below the level that would be relevant, he says, but this needs to be cross-checked. The question is whether a minute amount of tritium could somehow remain in liquid xenon or if some makes it from the detector materials into the liquified xenon in the detector. “I personally think that the observed excess could equally well be a new background or new physics. About 3σ implies of course a certain statistical chance for a fluctuation, but I find it intriguing to have this excess not at some random place, but towards the lower end of the spectrum. This is interesting since many new-physics scenarios generically lead to a 1/E or 1/E² enhancement which would be cut off by our detection threshold.”

Solar axions

One solution proposed by the collaboration is solar axions. Axions are a consequence of a new U(1) symmetry proposed in 1977 to explain the immeasurably small degree of CP violation in quantum chromodynamics – the so-called strong CP problem — and are also a dark-matter candidate. Though XENON1T is not expected to be sensitive to dark-matter axions, should they exist they would be produced by the sun at energies consistent with the XENON1T excess. According to this hypothesis, the axions would be detected via the “axioelectric” effect, an axion analogue of the photoelectric effect. Though a good fit phenomenologically, and like tritium favoured over the background-only hypothesis at approximately 3σ, the solar-axion explanation is disfavoured by astrophysical constraints. For example, it would lead to a significant extra energy loss in stars.

Axion helioscopes such as the CERN Axion Solar Telescope (CAST) experiment, which directs a prototype LHC dipole magnet at the Sun and could convert solar axions into X-ray photons, will help in testing the hypothesis. “It is not impossible to have an axion model that shows up in XENON but not in CAST,” says deputy spokesperson Igor Garcia Irastorza of University of Zaragoza, “but CAST already constraints part of the axion interpretation of the XENON signal.” Its successor, the International Axion Observatory (IAXO), which is set to begin data taking in 2024, will have improved sensitivity. “If the XENON1T signal is indeed an axion, IAXO will find it within the first hours of running,” says Garcia Irastorza.

A second new-physics explanation cited for XENON1T’s low-energy excess is an enhanced rate of solar neutrinos interacting in the detector. In the Standard Model, neutrinos have a negligibly small magnetic moment, however, should they be Majorana rather than Dirac fermions, and identical to their antiparticles, their magnetic moment should be larger, and proportional to their mass, though still not detectable. New physics beyond the Standard Model could, however, enhance the magnetic moment further. This leads to a larger interaction cross section at low energies and an excess of low-energy electron recoils. XENON1T fits indicate that solar Majorana neutrinos with an enhanced magnetic moment are also favoured over the background-only hypothesis at the level of 3σ.

The absorption of dark photons could explain the observed excess.

Joachim Kopp

The community has quickly chimed in with additional ideas, with around 40 papers appearing on the arXiv preprint server since the result was released. One possibility is a heavy dark-matter particle that annihilates or decays to a second, much lighter, “boosted dark-matter” particle which could scatter on electrons via some new interaction, notes CERN theorist Joachim Kopp. Another class of dark-matter model that has been proposed, he says, is “inelastic dark matter”, where dark-matter particles down-scatter in the detector into another dark-matter state just a few keV below the original one, with the liberated energy then seen in the detector. “An explanation I like a lot is in terms of dark photons,” he says. “The Standard Model would be augmented by a new U(1) gauge symmetry whose corresponding gauge boson, the dark photon, would mix with the Standard-Model photon. Dark photons could be abundant in the Universe, possibly even making up all the dark matter. Their absorption in the XENON1T detector could explain the observed excess.”

“The strongest asset we have is our new detector, XENONnT,” says Aprile. Despite COVID-19, the collaboration is on track to take first data before the end of 2020, she says. XENONnT will boast three times the fiducial volume of XENON1T and a factor six reduction in backgrounds, and should be able to verify or refute the signal within a few months of data taking. “An important question is if the signal has an annual modulation of about 7% correlated to the distance of the sun,” notes Lindner. “This would be a strong hint that it could be connected to new physics with solar neutrinos or solar axions.”

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Almost 90 years since Pauli postulated its existence, much remains to be learnt about the neutrino. The observation in 1998 of neutrino oscillations revealed that the particle’s flavour and mass eigenstates mix and oscillate. At least two must be massive, like the other known fermions, though with far smaller masses. The need for a mechanism to generate such small masses strongly hints at the existence of new physics beyond the Standard Model. Faced with such compelling questions, neutrino experiments are springing up at an unprecedented rate, from a plethora of searches for neutrinoless double-beta decay to gigantic astrophysical–neutrino detectors at the South Pole (IceCube) and soon in the Mediterranean Sea (KM3NeT), and two projects of enormous scope on the horizon in DUNE and Hyper-Kamiokande. Now, then, is a timely moment for the publication of a tutorial for graduate students and young researchers who are entering this fast-moving field.

Access all areas

Edited by former spokesperson of the OPERA experiment Antonio Ereditato, The State of the Art of Neutrino Physics provides an historical account and introduction to basic concepts, reviews of the various subfields where neutrinos play a significant role, and gives a detailed account of the data produced by present experiments in operation. An extremely valuable compilation of topical articles, the book covers essentially all areas of research in experimental neutrino physics, from astrophysical, solar and atmospheric neutrinos to accelerator and reactor neutrinos. The large majority of the articles are written in a didactic style by leading experts in the field, allowing young researchers to acquaint themselves with the diverse research in the field. In particular the chapter describing the formalism of neutrino oscillations should be required reading for all aspiring neutrino physicists. In all cases special attention is given to experimental challenges.

From the theory side, chapters cover measurements at neutrino experiments of the low-energy interactions of neutrinos with nuclei (a key way to reduce systematic uncertainties), the phenomenology and consequences of the yet-to-be-determined neutrino-mass hierarchy, and the possibility of CP violation in the lepton sector. A very detailed account of solar neutrinos and matter effects in the Sun is written by Alexei Smirnov, one of the inventors of the celebrated Mikheyev–Smirnov–Wolfenstein effect, which describes how weak interactions with electrons modify oscillation probabilities for the various neutrino flavours. More speculative scenarios, for example on the possibility of the existence of sterile neutrinos, are discussed as well.

For a book like this, which has the ambition to address a broad palette of neutrino questions, it is always difficult to be totally complete, but it comes close. Some topics have evolved in the details since 2016, when the material upon which the book is based was written, but that doesn’t take away from the book’s value as a tutorial. I recommend it very highly to young and not-so-young aspiring
neutrino aficionados alike.

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These exciting results are thanks to the hard work of hundreds of T2K collaborators

Federico Sanchez

“These exciting results are thanks to the hard work of hundreds of T2K collaborators involved in the construction, data collection and data analysis for T2K over the past two decades,” says T2K international co-spokesperson Federico Sanchez of the University of Geneva.

Discovered in 1964, CP violation has so far only been observed in the weak interactions of quarks, mostly recently in the charm system by the LHCb collaboration. Since the size of the effect in quarks is too small to explain the observed matter-antimatter disparity in the universe, finding additional sources of CP violation is one of the outstanding mysteries in particle physics. The quantum mixing of neutrino flavours as neutrinos travel over large distances, the discovery of which was marked by the 2015 Nobel Prize in Physics, provides a way to probe another potential source of CP violation: a complex phase, δ_CP, in the neutrino mixing matrix. Though models indicate that no value of δ_CP could explain the cosmological matter-antimatter asymmetry without new physics, the observation of leptonic CP violation would make models such as leptogenesis, which feature heavy Majorana partners for the Standard Model neutrinos, more plausible.

Long baseline

The T2K (Tokai-to-Kamioka) experiment uses the Super Kamiokande detector to observe neutrinos and antineutrinos generated by a proton beam at the J-PARC accelerator facility 295 km away. As the beams travel through Earth, a fraction of muon neutrinos and antineutrinos in the beam oscillate into electron neutrinos that are recorded via nuclear-recoil interactions in Super Kamiokande’s 50,000-tonne tank of ultrapure water, where the charged lepton generated by the weak interaction creates a Cherenkov ring which can be distinguished as being created by an electron or muon (see image above). Since the beam-line and detector components are made out of matter and not antimatter, the observation of neutrinos is already enhanced. The T2K analysis therefore includes corrections based on data from magnetised near-detectors (ND280, which uses the magnet originally built for the UA1 detector at CERN’s Spp̄S collider) placed 280m from the target.

The δ_CP parameter is a cyclic phase: if δ_CP=0, neutrinos and antineutrinos will change from muon- to electron-types in the same way during oscillation; any other value would enhance the oscillations of either neutrinos or antineutrinos, violating CP symmetry. Analysing data with 1.49×10²¹ and 1.64×10²¹ protons produced in neutrino- and antineutrino-beam mode respectively, T2K observed 90 electron-neutrino candidates and 15 electron-antineutrino candidates. This may be compared with the 56 and 22 events expected for maximal antineutrino enhancement (δ_CP=+π/2), and the 82 and 17 events expected for maximal neutrino enhancement (δ_CP=−π/2). Being most compatible with the latter scenario, the T2K data disfavour almost half of the possible values of δ_CP at 3σ confidence. For the “normal” neutrino-mass ordering favoured by T2K and other experiments, and averaged over all other oscillation parameters, the measured 3σ confidence-level interval for δ_CP is [−3.41, −0.03], while for the “inverted” mass ordering (in which the first mass splitting is greater than the second) it is [−2.54, −0.32]. Averaged over all oscillation parameters, δ_CP=0 is now disfavoured at 3σ confidence, though it is still within the 3σ bound for some allowed values of the mixing angle θ₂₃ (see figure, above).

“Our results show the strongest constraint yet on the parameter governing CP violation in neutrino oscillations, one of the few parameters governing fundamental particle interactions that has not yet been precisely measured,” continues Sanchez. “These results indicate that CP violation in neutrino mixing may be large, and T2K looks forward to continued operation with the prospect of establishing evidence for CP violation in neutrino oscillations.”

Next steps

To further improve the experimental sensitivity to a potential CP-violating effect, the collaboration plans to upgrade the near detector to reduce systematic uncertainties and to accumulate more data, while J-PARC will increase the beam intensity by upgrading its accelerator and beam line.

“This is the first time ever CP-violation is glimpsed in the lepton sector and it has the potential of being a very large effect,” says Albert De Roeck, group leader of the CERN neutrino group, which has participated in the T2K experiment since last year. “Future neutrino CP violation measurements will be further performed by currently running neutrino experiments, and then the torch will be passed to the planned high precision neutrino experiments DUNE and Hyper-Kamiokande that will provide measurements of the exact degree of CP violation in the neutrino system.”

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The neutrino community’s current challenge is to understand the origin of neutrino masses and lepton mixing. This means establishing whether neutrinos are Dirac or Majorana fermions, their absolute mass scale, the order of the measured mass splittings (the neutrino mass ordering), whether there is leptonic CP violation, the precise value of other parameters in the neutrino mixing matrix, and, finally, whether there is an indication of physics beyond the standard three-neutrino paradigm, for example through the detection of sterile neutrinos.

Construction of the Hyper-Kamiokande experiment will begin in 2020

2015 Nobel laureate Takaaki Kajita (University of Tokyo) opened the conference by confirming that construction of the Hyper-Kamiokande experiment will begin in 2020, following the allocation by the Japanese government of a supplementary budget on 13 December. Hyper-Kamiokande will be a water-Cherenkov detector with a total mass of 260 kton — almost an order of magnitude larger than its famous predecessor Super-Kamiokande, where atmospheric neutrino oscillations were discovered, and far larger than KamiokaNDE, which observed solar neutrinos and supernova SN1987A. Hyper-Kamiokande will eventually replace Super-Kamiokande as the far detector for the upgraded J-PARC neutrino beam, which is situated on the far side of Japan (essentially a comprehensive upgrade of the T2K experiment), with the aim of measuring CP violation in the leptonic sector. It will also provide high statistics for proton-decay searches, supernova neutrino bursts, atmospheric and solar neutrinos, and indirect searches for dark matter. Hyper-Kamiokande will therefore soon join DUNE in the US as a next-generation long-baseline neutrino-oscillation experiment under construction. Together the detectors will provide a far wider coverage of physics signals than either could manage alone.

Critical mass

News of KATRIN’s record-breaking new upper limit on the electron-antineutrino mass was complemented by a report by Joseph Formaggio (MIT) on the successful “Project 8” demonstration in the US of a new approach to directly measuring neutrino masses wherein the energies of beta-decay electrons are determined from the frequency of cyclotron radiation as the electrons spiral in a magnetic field. This work will be complemented by the JUNO experiment in China which will in 2021 begin to constrain the ordering of the neutrino-mass eigenvalues.

The search for neutrinoless double-beta decay also has the potential to provide information on neutrino masses. A potentially unambiguous indication of lepton-number violation and the postulated Majorana nature of neutrinos, it is being pursued aggressively as experiments compete to reduce backgrounds and increase detector masses to the ton-scale. Several talks emphasised the complementary progress by the theory community to better estimate nuclear effects, and reduce the errors arising from the differences between different nuclear models and different isotopes. These calculations are equally important for NOvA and T2K, which is now beginning to probe leptonic CP conservation at the 3? level.

The cosmological upper limit on the sum of neutrino masses could be relaxed upwards

Current and future cosmological constraints of neutrino properties were reviewed by Eleonora Di Valentino (Manchester), whose recent work with Alessandro Melchiorri and Joe Silk reinterprets Planck-satellite data to favour a closed universe at more than 99% significance – an inference which could lead to the current cosmological upper limit on the sum of neutrino masses being relaxed upwards if it is accepted by the community. Conversely, astrophysical neutrinos are also powerful tools for studying astrophysical objects. One key development in this field is the doping of Super-Kamiokande with gadolinium, currently underway in Japan. This will soon give the detector sensitivity to the diffuse supernova-neutrino background.

The next edition of NuPhys will take place in London from 16 to 18 December 2020.

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In the long history of measuring the tritium beta-decay spectrum to determine the neutrino mass, the ensuing weeks of KATRIN’s first data-taking opened a new chapter. Everything worked as expected, and KATRIN’s initial measurements have already propelled it into the top ranks of neutrino experiments. The aim of this ultra-high-precision beta-decay spectroscope, more than 15 years in the making, is to determine, by the mid-2020s, the absolute mass of the neutrino.

Massive discovery

Since the discovery of the oscillation of atmospheric neutrinos by the Super-Kamiokande experiment in 1998, and of the flavour transitions of solar neutrinos by the SNO experiment shortly afterwards, it was strongly implied that neutrino masses are not zero, but big enough to cause interference between distinct mass eigenstates as a neutrino wavepacket evolves in time. We know now that the three neutrino flavour states we observe in experiments – ν_e, ν_μ and ν_τ – are mixtures of three neutrino mass states.

Though not massless, neutrinos are exceedingly light. Previous experiments designed to directly measure the scale of neutrino masses in Mainz and Troitsk produced an upper limit of 2 eV for the neutrino mass – a factor 250,000 times smaller than the mass of the otherwise lightest massive elementary particle, the electron. Nevertheless, neutrino masses are extremely important for cosmology as well as for particle physics. They have a number density of around 336 cm^–3, making them the most abundant particles in the universe besides photons, and therefore play a distinct role in the formation of cosmic structure. Comparing data from the Planck satellite together with data from galaxy surveys (baryonic acoustic oscillations) with simulations of the evolution of structure yields an upper limit on the sum of all three neutrino masses of 0.12 eV at 95% confidence within the framework of the standard Lambda cold-dark matter (ΛCDM) cosmological model.

Considerations of “naturalness” lead most theorists to speculate that the exceedingly tiny neutrino masses do not arise from standard Yukawa couplings to the Higgs boson, as per the other fermions, but are generated by a different mass mechanism. Since neutrinos are electrically neutral, they could be identical to their antiparticles, making them Majorana particles. Via the so-called seesaw mechanism, this interesting scenario would require a new and very high particle mass scale to balance the smallness of the neutrino masses, which would be unreachable with present accelerators.

As neutrino oscillations arise due to interference between mass eigenstates, neutrino-oscillation experiments are only able to determine splittings between the squares of the neutrino mass eigenstates. Three experimental avenues are currently being pursued to determine the neutrino mass. The most stringent upper limit is currently the model-dependent bound set by cosmological data, as already mentioned, which is valid within the ΛCDM model. A second approach is to search for neutrinoless double-beta decay, which allows a statement to be made about the size of the neutrino masses but presupposes the Majorana nature of neutrinos. The third approach – the one adopted by KATRIN – is the direct determination of the neutrino mass from the kinematics of a weak process such as beta decay, which is completely model-independent and depends only on the principle of energy and momentum conservation.

The direct determination of the neutrino mass relies on the precise measurement of the shape of the beta electron spectrum near the endpoint, which is governed by the available phase space (figure 1). This spectral shape is altered by the neutrino mass value: the smaller the mass, the smaller the spectral modification. One would expect to see three modifications, one for each neutrino mass eigenstate. However, due to the tiny neutrino mass differences, a weighted sum is observed. This “average electron neutrino mass” is formed by the incoherent sum of the squares of the three neutrino mass eigenstates, which contribute to the electron neutrino according to the PMNS neutrino-mixing matrix. The super-heavy hydrogen isotope tritium is ideal for this purpose because it combines a very low endpoint energy, E_o, of 18.6 keV and a short half-life of 12.3 years with a simple nuclear and atomic structure.

KATRIN is born

Around the turn of the millennium, motivated by the neutrino oscillation results, Ernst Otten of the University of Mainz and Vladimir Lobashev of INR Troitsk proposed a new, much more sensitive experiment to measure the neutrino mass from tritium beta decay. To this end, the best methods from the previous experiments in Mainz, Troitsk and Los Alamos were to be combined and upscaled by up to two orders of magnitude in size and precision. Together with new technologies and ideas, such as laser Raman spectroscopy or active background reduction methods, the apparatus would increase the sensitivity to the observable in beta decay (the square of the electron antineutrino mass) by a factor of 100, resulting in a neutrino-mass sensitivity of 0.2 eV. Accordingly, the entire experiment was designed to the limits of what was feasible and even beyond (see “Technology transfer delivers ultimate precision” box).

Technology transfer delivers ultimate precision

Many technologies had to be pushed to the limits of what was feasible or even beyond. KATRIN became a CERN-recognised experiment (RE14) in 2007 and the collaboration worked with CERN experts in many areas to achieve this. The KATRIN main spectrometer is the largest ultra-high vacuum vessel in the world, with a residual gas pressure in the range of 10^–11 mbar – a pressure that is otherwise only found in large volumes inside the LHC ring – equivalent to the pressure recorded at the lunar surface.

Even though the inner surface was instrumented with a complex dual-layer wire electrode system for background suppression and electric-field shaping, this extreme vacuum was made possible by rigorous material selection and treatment in addition to non-evaporable getter technology developed at CERN. KATRIN’s almost 40 m-long chain of superconducting magnets with two large chicanes was put into operation with the help of former CERN experts, and a ²²³Ra source was produced at ISOLDE for background studies at KATRIN. A series of ^83mKr conversion electron sources based on implanted ⁸³Rb for calibration purposes was initially produced at ISOLDE. At present these are produced by KATRIN collaborators and further developed with regard to line stability.

Conversely, the KATRIN collaboration has returned its knowledge and methods to the community. For example, the ISOLDE high-voltage system was calibrated twice with the ppm-accuracy KATRIN voltage dividers, and the magnetic and electrical field calculation and tracking programme KASSIOPEIA developed by KATRIN was published as open source and has become the standard for low-energy precision experiments. The fast and precise laser Raman spectroscopy developed for KATRIN is also being applied to fusion technology.

KIT was soon identified as the best place for such an experiment, as it had the necessary experience and infrastructure with the Tritium Laboratory Karlsruhe. The KIT board of directors quickly took up this proposal and a small international working group started to develop the project. At a workshop at Bad Liebenzell in the Black Forest in January 2001, the project received so much international support that KIT, together with nearly all the groups from the previous neutrino-mass experiments, founded the KATRIN collaboration. Currently, the 150-strong KATRIN collaboration comprises 20 institutes from six countries.

It took almost 16 years from the first design to complete KATRIN, largely because many new technologies had to be developed, such as a novel concept to limit the temperature fluctuations of the huge tritium source to the mK scale at 30 K or the high-voltage stabilisation and calibration to the 10 mV scale at 18.6 kV. The experiment’s two most important and also most complex components are the gaseous, windowless molecular tritium source (WGTS) and the very large spectrometer. In the WGTS, tritium gas is introduced in the midpoint of the 10 m-long beam tube, where it flows out to both sides to be pumped out again by turbomolecular pumps. After being partially cleaned it is re-injected, yielding a closed tritium cycle. This results in an almost opaque column density with a total decay rate of 10¹¹ per second. The beta electrons are guided adiabatically to a tandem of a pre- and a main spectrometer by superconducting magnets of up to 6 T. Along the way, differential and cryogenic pumping sections including geometric chicanes reduce the tritium flow by more than 14 orders of magnitude to keep the spectrometers free of tritium (figure 2).

Filtration

The KATRIN spectrometers operate as so-called MAC-E filters, whereby electrons are guided by two superconducting solenoids at either end and their momenta are collimated by the magnetic field gradient. This “magnetic bottle” effect transforms almost all kinetic energy into longitudinal energy, which is filtered by an electrostatic retardation potential so that only electrons with enough energy to overcome the barrier are able to pass through. The smaller pre-spectrometer blocks the low-energy part of the beta spectrum (which carries no information on the neutrino mass), while the 10 m-diameter main spectrometer provides a much sharper filter width due to its huge size.

The transmitted electrons are detected by a high-resolution segmented silicon detector. By varying the retarding potential of the main spectrometer, a narrow region of the beta spectrum of several tens of eV below the endpoint is scanned, where the imprint of a non-zero neutrino mass is maximal. Since the relative fraction of the tritium beta spectrum in the last 1 eV below the endpoints amounts to just 2 × 10^–13, KATRIN demands a tritium source of the highest intensity. Of equal importance is the high precision needed to understand the measured beta spectrum. Therefore, KATRIN possesses a complex calibration and monitoring system to determine all systematics with the highest precision in situ, e.g. the source strength, the inelastic scattering of beta electrons in the tritium source, the retardation voltage and the work functions of the tritium source and the main spectrometer.

Start-up and beyond

After intense periods of commissioning during 2018, the tritium source activity was increased from its initial value of 0.5 GBq (which was used for the inauguration measurements) to 25 GBq (approximately 22% of nominal activity) in spring 2019. By April, the first KATRIN science run had begun and everything went like clockwork. The decisive source parameters – temperature, inlet pressure and tritium content – allowed excellent data to be taken, and the collaboration worked in several independent teams to analyse these data. The critical systematic uncertainties were determined both by Monte Carlo propagation and with the covariance-matrix method, and the analyses were also blinded so as not to generate bias. The excitement during the un-blinding process was huge within the KATRIN collaboration, which gathered for this special event, and relief spread when the result became known. The neutrino-mass square turned out to be compatible with zero within its uncertainty budget. The model fits the data very well (figure 3) and the fitted endpoint turned out to be compatible with the mass difference between ³He and tritium measured in Penning traps. The new results were presented at the international TAUP 2019 conference in Toyama, Japan, and have recently been published.

This first result shows that all aspects of the KATRIN experiment, from hardware to data-acquisition to analysis, works as expected. The statistical uncertainty of the first KATRIN result is already smaller by a factor of two compared to previous experiments and systematic uncertainties have gone down by a factor of six. A neutrino mass was not yet extracted with these first four weeks of data, but an upper limit for the neutrino mass of 1.1 eV (90% confidence) can be drawn, catapulting KATRIN directly to the top of the world of direct neutrino-mass experiments. In the mass region around 1 eV, the limit corresponds to the quasi-degenerated neutrino-mass range where the mass splittings implied by neutrino-oscillation experiments are negligible compared to the absolute masses.

The neutrino-mass result from KATRIN is complementary to results obtained from searches for neutrinoless double beta decay, which are sensitive to the “coherent sum” m_ββ of all neutrino mass eigenstates contributing to the electron neutrino. Apart from additional phases that can lead to possible cancellations in this sum, the values of the nuclear matrix elements that need to be calculated to connect the neutrino mass m_ββ with the observable (the half-life) still possess uncertainties of a factor two. Therefore, the result from a direct neutrino-mass determination is more closely connected to results from cosmological data, which give (model-dependent) access to the neutrino-mass sum.

A sizeable influence

Currently, KATRIN is taking more data and has already increased the source activity by a factor of four to close to its design value. The background rate is still a challenge. Various measures, such as out-baking and using liquid-nitrogen cooled baffles in front of the getter pumps, have already yielded a background reduction by a factor 10, and more will be implemented in the next few years. For the final KATRIN sensitivity of 0.2 eV (90% confidence) on the absolute neutrino-mass scale, a total of 1000 days of data are required. With this sensitivity KATRIN will either find the neutrino mass or will set a stringent upper limit. The former would confront standard cosmology, while the latter would exclude quasi-degenerate neutrino masses and a sizeable influence of neutrinos on the formation of structure in the universe. This will be augmented by searches for physics beyond the Standard Model, such as for sterile neutrino admixtures with masses from the eV to the keV scale.

Neutrino-oscillation results yield a lower limit for the effective electron-neutrino mass to manifest in direct neutrino-mass experiments of about 10 meV (50 meV) for normal (inverse) mass ordering. Therefore, many plans exist to cover this region in the future. At KATRIN, there is a strong R&D programme to upgrade the MAC-E filter principle from the current integral to a differential read-out, which will allow a factor-of-two improvement in sensitivity on the neutrino mass. New approaches to determine the absolute neutrino-mass scale are also being developed: Project 8, a radio-spectroscopy method to eventually be applied to an atomic tritium source; and the electron-capture experiments ECHo and HOLMES, which intend to deploy large arrays of cryogenic bolometers with the implanted isotope ¹⁶³Ho. In parallel, the next generation of neutrinoless double beta decay experiments like LEGEND, CUPID or nEXO (as well as future xenon-based dark-matter experiments) aim to cover the full range of inverted neutrino-mass ordering. Finally, refined cosmological data should allow us to probe the same mass region (and beyond) within the next decades, while long-baseline neutrino-oscillation experiments, such as JUNO, DUNE and Hyper-Kamiokande, will probe the neutrino-mass ordering implemented in nature. As a result of this broad programme for the 2020s, the elusive neutrino should finally yield some of its secrets and inner properties beyond mixing.

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Since 1993 the Rencontres du Vietnam have fostered exchanges between scientists in the Asia-Pacific region and colleagues from other parts of the world. The 15th edition, which brought together more than 50 physicists in Quy Nhon, Vietnam from 4–10 August, celebrated the 30th anniversary of the start of the Large Electron Positron collider (LEP) in 1989, which within a mere three weeks of running had established that the number of species of light active neutrinos is three (CERN Courier September/October 2019 p32). This was a great opportunity to emphasise the important role that colliders have played and will continue to play in neutrino physics. Before the three-neutrino measurement of LEP, the tau neutrino had been established in the years 1975–1986 by a combination of e⁺e^–-collider observations of tau decays, pp collisions (the W → τν_τ decay was observed at CERN in 1985) and neutrino-beam experiments, where it was observed that taus are never produced by electron– or muon–neutrino beams (for example Fermilab’s E531 experiment in 1986).

A neutrino-oscillation industry then sprang into being, following the discovery that neutrinos have mass. An abundance of recent results on oscillation parameters were presented from accelerator-neutrino beams, nuclear reactors, and atmospheric and astrophysical neutrinos. Interestingly, the data now seem to indicate at > 3σ that neutrinos follow the natural (rather than inverted) mass ordering, in which the most electron-like neutrino has a mass smaller than that of the muon and tau neutrinos. The next 10 years should see this question resolved, as well as a determination of the CP-violating phase of the neutrino-mixing matrix, with a precision of 5–10 degrees.

The fact that neutrinos have mass requires an addition to the Standard Model (SM), wherein neutrinos are massless by definition. There are several solutions, of which a minimal modification is to introduce right-handed neutrinos in addition to the normal ones, which have left-handed chirality. The properties of these heavy neutral leptons would be very well predicted were it not that their mass can lie anywhere from less than an eV to 10¹⁰ GeV or more. Being sterile they only couple to SM particles via mixing with normal neutrinos. Consequently, they should be very rare and have long lifetimes, perhaps allowing a spectacular observation in either fixed-target or high-luminosity colliders at the electroweak scale. One possible low-energy indication could be the existence of neutrinoless double-beta decay. Such decays, currently being searched for directly in dedicated experiments worldwide, violate lepton–number conservation in the case where neutrinos possess a Majorana mass term that transforms neutrinos into antineutrinos.

For a fortunate combination of parameters, this could lead to a spectacular signature

The meeting reviewed the status of all aspects of massive neutrinos, from direct mass measurements of the sort successfully executed shortly after the conference by the KATRIN experiment (see KATRIN sets first limit on neutrino mass) to the search for heavy sterile neutrinos in ATLAS and CMS. A new feature of the field is the abundance of experimental projects searching for very weakly, or, to use the newly coined parlance, “feebly”, interacting particles. These range from CERN’s SHiP experiment to future LHC projects such as FASER and Mathusla (for masses from the pion to the B meson); proposed high-luminosity and high-energy colliders such as the Future Circular Collider would extend the search up to the Z mass for mixings between the heavy and light neutrinos down to 10^–11. Until recently classified as exotic, these experiments could yield the long-sought-after explanation for the matter–antimatter asymmetry of the universe by combining CP violation with an interaction that transforms particles into antiparticles. For a fortunate combination of parameters, this could lead to a spectacular signature: the production of a heavy neutrino in a W decay, tagged by an associated charged lepton, and followed by its transformation into its antineutrino, which could then be identified by its decay into a lepton of the same sign as that initially tagged (and possibly of a different flavour).

The meeting was thus concluded in continuity with its initial commemoration: could the physics of neutrinos be one of the highlights of future high-energy colliders?

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The 16th International Conference on Topics in Astroparticle and Underground Physics (TAUP 2019) was held in Japan from 9–13 September, attracting a record 540 physicists from around 30 countries. The 2019 edition of the series, which covered recent experimental and theoretical developments in astroparticle physics, was hosted by the Institute for Cosmic Ray Research of the University of Tokyo, and held in Toyama – the gateway city to the Kamioka experimental site.

Discussions first focused on gravitational-wave observations. During their first two observing runs, reported Patricia Schmidt from Radboud University, LIGO and Virgo confidently detected gravitational waves from 10 binary black-hole coalescenses and one binary neutron star inspiral, seeing one gravitational-wave event every 15 days of observation. It was also reported that, during the ongoing third observing run, LIGO and Virgo have already observed 26 candidate events. Among them is the first signal from a black hole–neutron star merger.

Guido Drexlin revealed the first measurement results on the upper limit of the neutrino mass

The programme continued with presentations from various research fields, a highlight being a report on the first result of the KATRIN experiment (KATRIN sets first limit on neutrino mass). Co-spokesperson Guido Drexlin revealed the first measurement results on the upper limit of the neutrino mass: < 1.1 eV at 90% confidence. This world-leading direct limit – which measures the neutrino mass by precisely measuring the kinematics of the electrons emitted from tritium beta decays – was obtained based on only four weeks of data. With the continuation of the experiment, it is expected that the limit will be reduced further, or even – if the neutrino mass is sufficiently large – the actual mass will be determined. Due to their oscillatory nature, it has been known since 1998 that neutrinos have tiny, but non-zero, masses. However, their absolute values have not yet been measured.

Diversity is a key feature of the TAUP conference. Topics discussed included cosmology, dark matter, neutrinos, underground laboratories, new technologies, gravitational waves, high-energy astrophysics and cosmic rays. Multi-messenger astronomy – which combines information from gravitational-wave observation, optical astronomy, neutrino detection and other electromagnetic signals – is quickly becoming established and is expected to play an even more important role in the future in gaining a deeper understanding of the universe.

The next TAUP conference will be held in Valencia, Spain, from 30 August to 3 September 2021.

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Neutrinos are among the least well understood particles in the Standard Model. Their three known mass eigenstates do not match up with the better-known flavour eigenstates, but mix according to the PMNS matrix, resulting in the flavour transmutations seen by neutrino-oscillation experiments. Despite their success in constraining neutrino mixing, such experiments are sensitive only to squared mass differences between the eigenstates, and not to the neutrino masses themselves.

Physicists have pursued direct mass measurements since Reines and Cowan observed electron antineutrinos in inverse beta decays in 1956. The direct mass measurement method hinges on precisely measuring the energy spectrum of beta-decay electrons, and is considered model independent as the extracted neutrino mass depends only on the kinematics of the decay. KATRIN is now the most precise experiment of this kind. It builds on the invention of gaseous molecular tritium sources and spectrometers based on the principle of magnetic adiabatic collimation with electrostatic filtering. The combination of these methods culminated in the previous best limits of 2.3 eV at 95% confidence in 2005, and 2.05 eV at 95% confidence in 2011, by physicists working in Mainz, Germany and Troitsk, Russia, respectively. The KATRIN analysis improves on these experimental results, with systematic uncertainties reduced by a factor of six and statistical uncertainties reduced by a factor of two.

These are exciting times for the collaboration

Guido Drexlin

“These are exciting times for the collaboration,” said KATRIN co-spokesperson Guido Drexlin. “The first KATRIN result is based on a measurement campaign of only four weeks at reduced source activity, equivalent to five days at nominal activity.” To reach its final sensitivity, KATRIN will collect data for 1000 days, and systematic errors will be reduced. “This will allow us to probe neutrino masses down to 0.2 eV,” continued Drexlin, “as well as many other interesting searches for beyond-the-Standard-Model physics, such as for admixtures of sterile neutrinos from the eV up to the keV scale.”

Conceived almost two decades ago, KATRIN operates using a high-resolution, large-acceptance and low-background measurement of the decay spectrum of tritium ³H → ³He e^– ν̄_e. Electrons are transported to the spectrometer via a beamline that was completed in autumn 2016, allowing experimenters to search for distortions in the tail of the electron energy distribution that depend on the absolute mass of the neutrino. KATRIN collaborators are now looking forward to a two-month measurement campaign, which will start in a few days. It will feature a signal-to-background ratio that is expected to be about one order of magnitude better than the initial measurements, due to an increase in source activity, and a decrease in background due to hardware upgrades. The goal is to achieve an activity of 10¹¹ beta-decay electrons per second, while reducing the current background level by about a factor of two.

Direct measurements are not the only handle on neutrino masses available to physicists, though they are certainly the most model independent. Experiments searching for neutrinoless double beta-decay offer a complementary limit, but must assume that the neutrino is a Majorana fermion.

The tightest limit on neutrino masses comes from cosmology. Comparing data from the Planck satellite with simulations of the development of structure in the early universe yields an upper limit on the sum of all three neutrino masses of 0.17 eV at 95% confidence.

The Planck limit is fairly robust, and one would have to go to great lengths to avoid it

Joachim Kopp

“The Planck limit is fairly robust, and one would have to go to great lengths to avoid it – but it’s not impossible to do so,” says CERN theorist Joachim Kopp. For example, it would be invalidated by a scenario where as-yet-undiscovered right-handed neutrinos couple to a new scalar field with a vacuum expectation value that evolves over cosmological timescales. “Planck data tell us what neutrinos were like in the early universe,” says Kopp. “The value of KATRIN lies in testing neutrinos now.”

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PHYSTAT-nu 2019 was held at CERN from 22 to 25 January. Counted among the 130 participants were LHC physicists and professional statisticians as well as neutrino physicists from across the globe. The inaugural meeting took place at CERN in 2000 and PHYSTAT has gone from strength to strength since, with meetings devoted to specific topics in data analysis in particle physics. The latest PHYSTAT-nu event is the third of the series to focus on statistical issues in neutrino experiments. The workshop focused on the statistical tools used in data analyses, rather than experimental details and results.

Modern neutrino physics is geared towards understanding the nature and mixing of the three neutrinos’ mass and flavour eigenstates. This mixing can be inferred by observing “oscillations” between flavours as neutrinos travel through space. Neutrino experiments come in many different types and scales, but they tend to have one calculation in common: whether the neutrinos are created in an accelerator, a nuclear reactor, or by any number of astrophysical sources, the number of events expected in the detector is the product of the neutrino flux and the interaction cross section. Given the ghostly nature of the neutrino, this calculation presents subtle statistical challenges. To cancel common systematics, many facilities have two or more detectors at different distances from the neutrino source. However, as was shown for the NOVA and T2K experiments, competitors to observe CP violation using an accelerator-neutrino beam, it is difficult to correlate the neutrino yields in the near and far detectors. A full cancellation of the systematic uncertainties is complicated by the different detector acceptances, possible variations in the detector technologies, and the compositions of different neutrino interaction modes. In the coming years these two experiments plan to combine their data in a global analysis to increase their discovery power – lessons can be learnt from the LHC experience.

The problem of modelling the interactions of neutrinos with nuclei – essentially the problem of calculating the cross section in the detector – forces researchers to face the thorny statistical challenge of producing distributions that are unadulterated by detector effects. Such “unfolding” corrects kinematic observables for the effects of detector acceptance and smearing, but correcting for these effects can cause huge uncertainties. To counter this, strong “regularisation” is often applied, biasing the results towards the smooth spectra of Monte Carlo simulations. The desirability of publishing unregularised results as well as unfolded measurements was agreed by PHYSTAT-nu attendees. “Response matrices” may also be released, allowing physicists outside of an experimental collaboration to smear their own models, and compare them to detector-level data. Another major issue in modeling neutrino–nuclear interactions is the “unknown unknowns”. As Kevin McFarland of the University of Rochester reflected in his summary talk, it is important not to estimate your uncertainty by a survey of theory models. “It’s like trying to measure the width of a valley from the variance of the position of sheep grazing on it. That has an obvious failure mode: sheep read each other’s papers.”

An important step for current and future neutrino experiments could be to set up a statistics committee, as at the Tevatron, and, more recently, the LHC experiments. This PHYSTAT-nu workshop could be the first real step towards this exciting scenario.

The next PHYSTAT workshop will be held at Stockholm University from 31 July to 2 August on the subject of statistical issues in direct-detection dark-matter experiments.

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Neutrinos, discovered in 1956, play an exceptional role in particle and nuclear physics, as well as astrophysics, and their study has led to the award of several Nobel prizes. In recognition of their importance, the first International Conference on the History of the Neutrino took place at the Université Paris Diderot in Paris on 5–7 September 2018.

The purpose of the conference, which drew 120 participants, was to cover the main steps in the history of the neutrino since 1930, when Wolfgang Pauli postulated its existence to explain the continuous energy spectrum of the electrons emitted in beta decay. Specifically, for each topic in neutrino physics, the aim was to pursue an historical approach and follow as closely as possible the discovery or pioneering papers. Speakers were chosen as much as possible for their roles as authors or direct witnesses, or as players in the main events.

The first session, “Invention of a new particle”, started with the prehistory of the neutrino – that is, the establishment of the continuous energy spectrum in beta decay – before moving into the discoveries of the three flavour neutrinos. The second session, “Neutrinos in nature”, was devoted to solar and atmospheric neutrinos, as well as neutrinos from supernovae and Earth. The third session covered neutrinos from reactors and beams including the discovery of neutral-current neutrino interactions, in which the neutrino is not transformed into another particle like a muon or an electron. This discovery was made in 1973 by the Gargamelle bubble chamber team at CERN after a race with the HPWF experiment team at Fermilab.

The major theme of neutrino oscillations from the first theoretical ideas of Bruno Pontecorvo (1957) to the Mikheyev–Smirnov–Wolfenstein effect (1985), which can modify the oscillations when neutrinos travel through matter, was complemented by talks on the discovery of neutrino oscillations by Nobel laureates Takaaki Kajita and Art McDonald. In 1998, the Super-Kamiokande experiment, led by Kajita, observed the oscillation of atmospheric neutrinos, and in 2001 the Sudbury Neutrino Observatory experiment, led by McDonald, observed the oscillation of solar neutrinos.

The role of the neutrino in the Standard Model was discussed, as was its intrinsic nature. Although physicists have observed the rare process of double beta decay with neutrinos in the final state, neutrinoless double beta decay with no neutrinos produced has been searched  for for more than 30 years because its observation would prove that the neutrino is Majorana-type (its own antiparticle) and not Dirac-type.

To complete the panorama, the conference discussed neutrinos as messengers from the wider universe, from the Big Bang to violent phenomena such as gamma-ray bursts or active galactic nuclei. Delegates also discussed wrong hints and tracks, which play a positive role in the development of science, and the peculiar sociological aspects that are common to particle physics and astrophysics.

Following the conference, a website dedicated to the history of this fascinating particle was created: https://neutrino-history.in2p3.fr.

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Newly published results from the MINOS+ experiment at Fermilab in the US cast fresh doubts on the existence of the sterile neutrino – a hypothetical fourth neutrino flavour that would constitute physics beyond the Standard Model. MINOS+ studies how muon neutrinos oscillate into other neutrino flavours as a function of distance travelled, using magnetised-iron detectors located 1 and 735 km downstream from a neutrino beam produced at Fermilab.

Neutrino oscillations, predicted more than 60 years ago, and finally confirmed in 1998, explain the observed transmutation of neutrinos from one flavour to another as they travel. Tantalising hints of new-physics effects in short-baseline accelerator-neutrino experiments have persisted since 1995, when the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory reported an 88±23 excess in the number of electron antineutrinos emerging from a muon–antineutrino beam. This suggested that muon antineutrinos were oscillating into electron antineutrinos along the way, but not in the way expected if there are only three neutrino flavours.

The plot thickened in 2007 when another Fermilab experiment, MiniBooNE, an 818 tonne mineral-oil Cherenkov detector located 541 m downstream from Fermilab’s Booster neutrino beamline, began to see a similar effect. The excess grew, and last November the MiniBooNE collaboration reported a 4.5σ deviation from the predicted event rate for the appearance of electron neutrinos in a muon neutrino beam. In the meantime, theoretical revisions in 2011 meant that measurements of neutrinos from nuclear reactors also show deviations suggestive of sterile-neutrino interference: the so-called “reactor anomaly”.

Tensions have been running high. The latest results from MINOS+, first reported in 2017 and recently accepted for publication in Physical Review Letters, fail to confirm the MiniBooNE signal. The MINOS+ results are also consistent with those from a comparable analysis of atmospheric neutrinos in 2016 by the IceCube detector at the South Pole. “LSND, MiniBooNE and the reactor data are fairly compatible when interpreted in terms of sterile neutrinos, but they are in stark conflict with the null results from MINOS+ and IceCube,” says theorist Joachim Kopp of CERN. “It might be possible to come up with a model that allows compatibility, but the simplest sterile neutrino models do not allow this.” In late February, the long-baseline T2K experiment in Japan joined the chorus of negative searches for the sterile neutrino, although excluding a different region of parameter space.

Whereas MiniBooNE and LSND sought to observe a second-order flavour transition (in which a muon neutrino morphs into a sterile and then electron neutrino), MINOS+ and IceCube are sensitive to a first-order muon-to-sterile transition that would reduce the expected flux of muon neutrinos. Such “disappearance” experiments are potentially more sensitive to sterile neutrinos, provided systematic errors are carefully modelled.

“The MiniBooNE observations interpreted as a pure sterile neutrino oscillation signal are incompatible with the muon-neutrino disappearance data,” says MINOS+ spokesperson Jenny Thomas of University College London. “In the event that the most likely MiniBooNE signal were due to a sterile neutrino, the signal would be unmissable in the MINOS/MINOS+ neutral-current and charged-current data sets.” Taking into account simple unitarity arguments, adds Thomas, the latest MINOS+ analysis is incompatible with the MiniBooNE result at the 2σ level and at 3σ sigma below a “mass-splitting” of 1 eV² (see figure 1).

The sterile-neutrino hypothesis is also in tension with cosmological data, says theorist Silvia Pascoli of Durham University. “Sterile neutrinos with these masses and mixing angles would be copiously produced in the early universe and would make up a significant fraction of hot dark matter. This is somewhat at odds with cosmological observations.”

One possibility for the surplus electron–neutrino-like events in MiniBooNE is insufficient accuracy in the way neutrino–nucleus interactions in the detector are modelled – a challenge for neutrino-oscillation experiments generally. According to MiniBooNE collaborator Teppei Katori, one effect proposed to account for the MiniBooNE anomaly is neutral-current single-gamma production. “This rare process has many theoretical interests, both within and beyond the Standard Model, but the calculations are not yet tractable at low energies (around 1 GeV) as they are in the non-perturbative QCD region,” he says.

MINOS+ is now analysing its final dataset and working on a direct comparison with MiniBooNE to look for electron-neutrino appearance as well as the present study on muon-neutrino disappearance. Clarification could also come from other short-baseline experiments at Fermilab, in particular MicroBooNE, which has been operating since 2015, and two liquid-argon detectors ICARUS and SBND (CERN Courier June 2017 p25). The most exciting possibility is that new physics is at play. “One viable explanation requires a new neutral-current interaction mediated by a new GeV-scale vector boson and sterile neutrinos with masses in the hundreds of MeV,” explains Pascoli. “So far this has not been excluded. And it is theoretically consistent. We have to wait and see.”

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Deep in a mine in Greater Sudbury, Ontario, Canada, you will find the deepest flush toilets in the world. Four of them, actually, ensuring the comfort of the staff and users of SNOLAB, an underground clean lab with very low levels of background radiation that specialises in neutrino and dark-matter physics.

Toilets might not be the first thing that comes to mind when discussing a particle-physics laboratory, but they are one of numerous logistical considerations when hosting 60 people per day at a depth of 2 km for 10 hours at a time. SNOLAB is the world’s deepest cleanroom facility, a class-2000 cleanroom (see panel below) the size of a shopping mall situated in the operational Vale Creighton nickel mine. It is an expansion of the facility that hosted the Sudbury Neutrino Observatory (SNO), a large, heavy-water detector designed to detect neutrinos from the Sun. In 2001, SNO contributed to the discovery of neutrino oscillations, leading to the joint award of the 2015 Nobel Prize in Physics to SNO spokesperson Arthur B McDonald and Super-Kamiokande spokesperson Takaaki Kajita.

Initially, there were no plans to maintain the infrastructure beyond the timeline of SNO, which was just one experiment and not a designated research facility. However, following the success of the SNO experiment, there was increased interest in low-background detectors for neutrino and dark-matter studies.

Building on SNO’s success

The SNO collaboration was first formed in 1984, with the goal of solving the solar neutrino problem. This problem surfaced during the 1960s, when the Homestake experiment in the Homestake Mine at Lead, South Dakota, began looking for neutrinos created in the early stages of solar fusion. This experiment and its successors, using different target materials and technologies, consistently observed only 30–50% of the neutrinos predicted by the standard solar model. A seemingly small nuisance posed a large problem, which required a large-scale solution.

SNO used a 12 m-diameter spherical vessel containing 1000 tonnes of heavy water to count solar neutrino interactions. Canada had vast reserves of heavy water for use in its nuclear reactors, making it an ideal location for such a detector. The experiment also required an extreme level of cleanliness, so that the signals physicists were searching for would not be confused with background events coming from dust, for instance. The SNO collaboration also had to develop new techniques to measure the inherent radioactivity of their detector materials and the heavy water itself.

Using heavy water gave SNO the ability to observe three different neutrino reactions: one reaction could only happen with electron neutrinos; one was sensitive to all neutrino flavours (electron, muon and tau); and the third provided the directionality pointing back to the Sun. These three complementary interactions let the team test the hypothesis that solar neutrinos were changing flavour as they travelled to Earth. In contrast to previous experiments, this approach allowed SNO to make a measurement of the parameters describing neutrino oscillations that didn’t depend on solar models. SNO’s data confirmed what previous experiments had seen and also verified the predictions of theories, implying that neutrinos do indeed oscillate during their Sun–Earth journey. The experiment ran for seven years and produced 178 papers accumulating more than 275 authors.

In 2002, the Canadian community secured funding to create an extended underground laboratory with SNO as the starting point. Construction of SNOLAB’s underground facility was completed in 2009 and two years later the last experimental hall entered “cleanroom” operation. Some 30 letters of interest were received from different collaborations proposing potential experiments, helping to define the requirements of the new lab.

SNOLAB’s construction was made possible by capital funds totalling CAD$73 million, with more than half coming from the Canada Foundation for Innovation through the International Joint Venture programme. Instead of a single giant cavern, local company Redpath Mining excavated several small and two large halls to hold experiments. The smaller halls helped the engineers manage the enormous stress placed on the rock in larger underground cavities. Bolts 10 m long stabilise the rock in the ceilings of the remaining large caverns, and throughout the lab the rock is covered with a 10 cm-thick layer of spray-on concrete for further stability, with an additional hand-troweled layer to help keep the walls dust-free. This latter task was carried out by Béton Projeté MAH, the same company that finished the bobsled track in the 2010 Vancouver Winter Olympics.

In addition to the experimental halls, SNOLAB is equipped with a chemistry laboratory, a machine shop, storage areas, and a lunchroom. Since the SNO experiment was still running when new tunnels and caverns were excavated, the connection between the new space and the original clean lab area was completed late in the project. The dark-matter experiments DEAP-1 and PICASSO were also already running in the SNO areas before construction of SNOLAB was completed.

Dark matter, neutrinos, and more

Today, SNOLAB employs a staff of over 100 people, working on engineering design, construction, installation, technical support and operations. In addition to providing expert and local support to the experiments, SNOLAB research scientists undertake research in their own right as members of the collaborations.

With so much additional space, SNOLAB’s physics programme has expanded greatly during the past seven years. SNO has evolved into SNO+, in which a liquid scintillator replaces the heavy water to increase the detector’s sensitivity. The scintillator will be doped with tellurium, making SNO+ sensitive to the hypothetical process of neutrinoless double-beta decay. Two of tellurium’s natural isotopes (¹²⁸Te and ¹³⁰Te) are known to undergo conventional double-beta decay, making them good candidates to search for the long-sought neutrinoless version. Detecting this decay would violate lepton-number conservation, proving that the neutrino is its own antiparticle (a Majorana particle). SNO+ is one of several experiments currently hunting this process down.

Another active SNOLAB experiment is the Helium and Lead Observatory (HALO), which uses 76 tons of lead blocks instrumented with 128 helium-3 neutron detectors to capture the intense neutrino flux generated when the core of a star collapses at the early stages of a supernova. Together with similar detectors around the world, HALO is part of a supernova early-warning system, which allows astronomers to orient their instruments to observe the phenomenon before it is visible in the sky.

With no fewer than six active projects, dark-matter searches comprise a large fraction of SNOLAB’s physics programme. Many different technologies are employed to search for the dark-matter candidate of choice: the weakly interacting massive particle (WIMP). The PICASSO and COUPP collaborations were both using bubble chambers to search for WIMPS, and merged into the very successful PICO project. Through successive improvements, PICO has endeavoured to enhance the sensitivity to WIMP spin-dependent interactions by an order of magnitude every couple of years. Its sensitivity is best for WIMP masses around 20 GeV/c². Currently the PICO collaboration is developing a much larger version with up to 500 litres of active-mass material.

DEAP-3600, successor to DEAP-1, is one of the biggest dark-matter detectors ever built, and it has been taking data for almost two years now. It seeks to detect spin-independent interactions between WIMPs and 3300 kg of liquid argon contained in a 1.7 m-diameter acrylic vessel. The best sensitivity will be achieved for a WIMP mass of 100 GeV/c². Using a different technology, the DAMIC (Dark Matter In CCDs) experiment employs CCD sensors, which have low intrinsic noise levels, and is sensitive to WIMP masses as low as 1 GeV/c².

Although the science at SNOLAB primarily focuses on neutrinos and dark matter, the low-background underground environment is also useful for biology experiments. REPAIR explores how low radiation levels affect cell development and repair from DNA damage. One hypothesis is that removing background radiation may be detrimental to living systems. REPAIR can help determine whether this hypothesis is correct and characterise any negative impacts. Another experiment, FLAME, studies the effect of prolonged time spent underground on living organisms using fruit flies as a model. The findings from this research could be used by mining companies to support
a healthier workforce.

Future research

There are many exciting new experiments under construction at SNOLAB, including several dark-matter experiments. While the PICO experiment is increasing its detector mass, other experiments are using several different technologies to cover a wide range of possible WIMP masses. The SuperCDMS experiment and CUTE test facility use solid-state silicon and germanium detectors kept at temperatures near absolute zero to search for dark matter, while the NEWS-G experiment will use gasses such as hydrogen, helium and neon in a 1.4 m-diameter copper sphere.

SNOLAB still has space available for additional experiments requiring a deep underground cleanroom environment. The Cryopit, the largest remaining cavern, will be used for a next-generation double-beta-decay experiment. Additional spaces outside the large experimental halls can host several small-scale experiments. While the results of today’s experiments will influence future detectors and detector technologies, the astroparticle physics community will continue to demand clean underground facilities to host the world’s most sensitive detectors. From an underground cavern carved out to host a novel neutrino detector to the deepest cleanroom facility in the world, SNOLAB will continue to seek out and host world-class physics experiments to unravel some of the universe’s deepest mysteries.

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The world’s largest liquid-argon neutrino detector has recorded its first particle tracks in tests at CERN, marking an important step towards the international Deep Underground Neutrino Experiment (DUNE) under preparation in the US. The enormous ProtoDUNE detector, designed and built at CERN’s neutrino platform, is the first of two prototypes for what will be a much larger DUNE detector. Situated deep beneath the Sanford Underground Research Facility in South Dakota, four final DUNE detector modules (each 20 times larger than the current prototypes and containing a total of 70,000 tonnes of liquid argon) will record neutrinos sent from Fermilab’s Long Baseline Neutrino Facility some 1300 km away.

DUNE’s scientific targets include CP violation in the neutrino sector, studies of astrophysical neutrino sources, and searches for proton decay. When neutrinos enter the detector and strike argon nuclei they produce charged particles, which leave ionisation traces in the liquid from which a 3D event can be reconstructed. The first ProtoDUNE detector took two years to build and eight weeks to fill with 800 tonnes of liquid argon, which needs to be cooled to a temperature below –184 degrees. It adopts a single-phase architecture, which is an evolution from the 170 tonne MicroBooNE detector at Fermilab’s short-baseline neutrino facility. The second ProtoDUNE module adopts a different, dual-phase, scheme with a second detection chamber.

The construction and operation of ProtoDUNE will allow researchers to validate the membrane cryostat technology and associated cryogenics for the final detector, in addition to the networking and computing infrastructure. Now that the first tracks have been seen, from beam tests involving cosmic rays and charged-particle beams from CERN’s SPS, ProtoDUNE’s operation will be studied in greater depth. The charged-particle beam test enables critical calibration measurements necessary for precise calorimetry, and will also produce valuable data for optimising event-reconstruction algorithms. These and other measurements will help quantify and reduce systematic uncertainties for the DUNE far detector and significantly improve the physics reach of the experiment. “Seeing the first particle tracks is a major success for the entire DUNE collaboration,” said DUNE co-spokesperson Stefan Soldner-Rembold of the University of Manchester, UK.

More than 1000 scientists and engineers from 32 countries in five continents are working on the development, design and construction of the DUNE detectors. For CERN, it is the first time the European lab has invested in infrastructure and detector development for a particle-physics project in the US. “Only two years ago we completed the new building at CERN to house two large-scale prototype detectors that form the building blocks for DUNE,” said Marzio Nessi, head of the neutrino platform at CERN. “Now we have the first detector taking beautiful data, and the second detector, which uses a different approach to liquid-argon technology, will be online in a few months.”

In July, the US Department of Energy also formally approved PIP-II, an accelerator upgrade project at Fermilab required to deliver the high-power neutrino beam required for DUNE. First data at DUNE is expected in 2026. Meanwhile, in Japan, an experiment with similar scientific goals and also with scientific links to the CERN neutrino platform – Hyper-Kamiokande – has recently been granted seed funding for construction to begin in 2020 (CERN Courier October 2018 p11). Together with several other experiments such as KATRIN in Germany, physicists are closing in on the neutrino’s mysteries two decades after the discovery of neutrino oscillations (CERN Courier July/August 2018 p5).

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On 12 September, the Japanese government granted seed funding towards the construction of the Hyper-Kamiokande experiment, a next-generation detector for the study of neutrinos. Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) allocated $700,000 within its budget request for the 2019 fiscal year, which will enable progress in preparatory work for construction and efforts to secure international collaboration.

Coinciding with the MEXT announcement, the University of Tokyo pledged to ensure that construction of the Hyper-Kamiokande detector commences in April 2020. According to a statement from university president Makoto Gonokami: “The University of Tokyo has made this decision in recognition of both the project’s importance and value both nationally and internationally. … Seed fundings in the past projects usually lead to full funding in the following year, as was the case for the Super-Kamiokande project.”

Hyper-Kamiokande (Hyper-K) is a water Cherenkov detector centered on a huge underground tank containing 300,000 tonnes of water, with a sensitive volume about a factor of 10 larger than its predecessor Super-Kamiokande (Super-K). Like Super-K, Hyper-K will be located in Kamioka on the west coast of Japan directly in the path of a neutrino beam generated 295 km away at the J-PARC facility in Tokai, allowing it to make high-statistics measurements of neutrino oscillations. Together with a near-detector located close to J-PARC, Super-K formed the “T2K” long-baseline neutrino programme. An order of magnitude bigger than Super-K, Hyper-K will serve as the next far-detector at T2K, with a rich physics portfolio. This ranges from the study of the CP violation in the leptonic sector and measurements of neutrino-mixing parameters, to studies of proton decay, atmospheric neutrinos and neutrinos from astrophysical sources.

It was at Super-K in 1998 that researchers discovered neutrino oscillations, proving that neutrinos are massive and leading to the award of the 2015 Nobel Prize in Physics to Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen’s University in Canada. The Japanese neutrino programme has progressed steadily since the 1998 discovery (CERN Courier July/August 2016 p29). Hyper-K was discussed as long ago as 2002 and a letter of intent was published in 2011, following the first measurement of the neutrino mixing angle θ₁₃ at T2K, which boosted the expectation of a discovery of leptonic CP violation by Hyper-K. The experiment was placed in Japan’s list of priority projects in 2014 but was not short-listed. The project was proposed again in 2017, this time making the short-list of seven projects to be funded by MEXT. The Hyper-K conceptual design report was published earlier this year (see further reading).

“Hyper-Kamiokande now moves from planning to construction,” said Hyper-K project co-leader Francesca Di Lodovico of Queen Mary University of London, in a statement released by the Kavli Institute for the Physics and Mathematics of the Universe in Japan on behalf of the Hyper-K collaboration. “The collaboration will now work on finalising designs, and is very open to more international partners joining this exciting, far-reaching new experiment.” The Hyper-K proto-collaboration was formed in 2015 and currently comprises around 300 members from 73 institutes in 15 countries. Many European institutes are involved, including the CERN neutrino group, which is already participating in the upgrade of the T2K near detector to serve Hyper-K. To this end, in the summer of
last year a detector called Baby MIND that was designed and built at CERN was shipped to J-PARC (CERN Courier July/August 2017 p12).

“Hyper-K is the next step in the Japanese neutrino adventure,” says Baby MIND spokesperson and Hyper-K collaborator Alain Blondel of the University of Geneva. “This success comes from wise choices and intelligent planning. The increase in the far-detector mass is exciting: demonstration of an asymmetry between neutrinos and antineutrinos was identified as the ‘great discovery’ goal as soon as neutrino oscillations were discovered, although it presents a challenge regarding systematics. And if a proton decay is detected or a supernova strikes, it will be fireworks!”

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Since the discovery of high-energy extragalactic neutrinos by the IceCube collaboration in 2013, the hunt for the sources of such extreme cosmic events has been a major focus of neutrino astronomy. Now, in a multi-messenger measurement campaign involving more than 1000 scientists, IceCube and 18 independent partner observatories have identified such a cosmic particle accelerator – providing a first answer to the 100-year-old question concerning the origin of cosmic rays.

On 22 September 2017, IceCube – a cubic-kilometre neutrino detector installed in the 2.8 km-thick ice at the South Pole – registered a neutrino of likely astrophysical origin with a reconstructed energy of about 300 TeV. Within less than a minute from detection, IceCube’s automatic alert system sent a notice to the astronomical community, triggering worldwide follow-up observations. The notice was the 10th alert of this type sent by IceCube to the international astronomy community so far.

The neutrino event pointed to a 0.15 square-degree area in the sky, consistent with the position of a blazar called TXS 0506+056, an active galaxy whose jet points precisely towards Earth. The Fermi gamma-ray satellite found the blazar to be in a flaring state with a rare seven-fold increase in activity around the time of the neutrino event, making it one of the brightest objects in the gamma-ray sky at that moment. The MAGIC gamma-ray telescope in La Palma, Spain, then also recorded gamma rays with energies exceeding hundreds of GeV from the same region.

The convergence of observations convincingly implicates the blazar as the most likely source. A worldwide team from the various observatories involved conducted a statistical analysis to determine whether the correlation between the neutrino and the gamma-ray observations was perhaps just a coincidence, and found the chance for this to be around one in 1000.

Following the 22 September detection, the IceCube team searched the detector’s archival data and discovered a flare of more than a dozen lower-energy neutrinos detected in late 2014 and early 2015, which were also coincident with the blazar position. This independent observation greatly strengthened the initial detection of a single high-energy neutrino and was the start of a growing body of evidence for TXS 0506+056 being the first identified source of high-energy cosmic neutrinos. Furthermore, the distance to the blazar was determined to be about 4 billion light years (redshift z = 0.34) in the course of the follow-up observations, allowing the first luminosity determination for both gamma rays and neutrinos.

“It came as no surprise that on 12 July, the date of the release of the new observations, and on the following days, several studies were published devoted to modelling the source,” says IceCube member Marek Kowalski from DESY. “It will be exciting to watch the high-energy astronomy community develop a coherent picture of the source over the next few months, as well as new strategies to identify similar events more frequently in the future.”

The concerted observational campaign uses instruments located all over the globe and in space, spanning an energy range from radio waves, through visible light to X-rays and gamma rays, as well as neutrinos. It is thus a significant achievement for the nascent field of multi-messenger astronomy. Since neutrinos are produced through the collisions of charged cosmic rays, the new observation implies that active galaxies are also accelerators of charged cosmic-ray particles. “More than a century after the discovery of cosmic rays by Victor Hess in 1912, the IceCube findings have therefore for the first time pinpointed a compelling candidate for an extragalactic source of these high-energy particles,” says IceCube principal investigator Francis Halzen.

The post IceCube neutrino points to origin of cosmic rays appeared first on CERN Courier.

On 28 June, the US Department of Energy and the Italian Embassy, on behalf of the Italian Ministry of Education, Universities and Research, signed a collaboration agreement concerning the international Short Baseline Neutrino (SBN) programme hosted at Fermilab. The SBN programme, started in 2015, comprises the development, installation and operation of three neutrino detectors on the Fermilab site: the Short Baseline Near Detector, located 110 m from the neutrino beam source; MicroBooNE, located 470 m from the source; and ICARUS, located 600 m from the source. ICARUS was refurbished at CERN last year after a long and productive scientific life at Gran Sasso National Laboratory.

The SBN programme aims to search for exotic and highly non-reactive sterile neutrinos and resolve anomalies observed in previous experiments (CERN Courier June 2017 p25). Due to their different distances from the source, but employing the same liquid-argon technology, the three SBN detectors will be able to distinguish whether their measurements are due to transformations between neutrino types involving a sterile neutrino or are due to other previously-unknown neutrino interactions.

The signing of the SBN programme agreement is an addendum to a broader collaboration agreement on neutrino research that the US and Italy signed in 2015.

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The 28th International Conference on Neutrino Physics and Astrophysics took place in Heidelberg, Germany, on 4–9 June. It was organised by the Max Planck Institute for Nuclear Physics and the Karlsruhe Institute of Technology. With 814 registrations, 400 posters and the presence of Nobel laureates, Art McDonald and Takaaki Kajita, it was the most attended of the series to date – showcasing many new results.

Several experiments presented their results for the first time at Neutrino 2018. T2K in Japan and NOvA in the US updated their results, strengthening their indication of leptonic CP violation and normal-neutrino mass ordering, and improving their precision in measuring the atmospheric oscillation parameters. Taken together with the Super-Kamiokande results of atmospheric neutrino oscillations, these experiments provide a 2σ indication of leptonic CP violation and a 3σ indication of normal mass ordering. In particular, NOvA presented the first 4σ evidence of ν̅_μ– ν̅_e transitions compatible with three-neutrino oscillations.

The next-generation long-baseline experiments DUNE and Hyper-Kamiokande in the US and Japan, respectively, were discussed in depth. These experiments have the capability to measure CP violation and mass ordering in the neutrino sector with a sensitivity of more than 5σ, with great potential in other searches like proton decay, supernovae, solar and atmospheric neutrinos, and indirect dark-matter searches.

All the reactor experiments – Daya Bay, Double Chooz and Reno – have improved their results, providing precision measurements of the oscillation parameter θ₁₃ and of the reactor antineutrino spectrum. The Daya Bay experiment, integrating 1958 days of data taking, with more than four million antineutrino events on tape, is capable of measuring the reactor mixing angle and the effective mass splitting with a precision of 3.4% and 2.8%, respectively. The next-generation reactor experiment JUNO, aiming at taking data in 2021, was also presented.

The third day of the conference focused on neutrinoless double-beta decay (NDBD) experiments and neutrino telescopes. EXO, KamLAND-Zen, GERDA, Majorana Demonstrator, CUORE and SNO+ presented their latest NDBD search results, which probe whether neutrinos are Majorana particles, and their plans for the short-term future. The new GERDA results pushed their NDBD lifetime limit based on germanium detectors to 0.9 × 10²⁶ years (90% CL), which represents the best real measurement towards a zero-background next-generation NDBD experiment.  CUORE also updated its results based on tellurium to 0.15 × 10²⁶ years.

Neutrino telescopes are of great interest for multi-messenger studies of astrophysical objects at high energies. Both IceCube in Antarctica and ANTARES in the Mediterranean were discussed, together with their follow-up IceCube Gen2 and KM3NeT facilities. IceCube has already collected 7.5 years of data, selecting 103 events (60 of which have an energy of more than 60 TeV) and a best-fit power law of E^–2.87. IceCube does not provide any evidence for neutrino point sources and the measured ν_e:ν_μ:ν_τ neutrino-flavour composition is 0.35:0.45:0.2. A recent development in neutrino physics has been the first observation of coherent elastic neutrino–nucleus scattering as discussed by the COHERENT experiment (CERN Courier October 2017 p8), which opens the possibility of searches for new physics.

A very welcome development at Neutrino 2018 was the presentation of preliminary results from the KATRIN collaboration about the tritium beta-decay end-point spectrum measurement, which allows a direct measurement of neutrino masses. The experiment has just been inaugurated at KIT in Germany and aims to start data taking in early 2019 with a sensitivity of about 0.24 eV after five years. The strategic importance of a laboratory measurement of neutrino masses cannot be overestimated.

A particularly lively session at this year’s event was the one devoted to sterile-neutrino searches. Five short-baseline nuclear reactor experiments (DANSS, NEOS, STEREO, PROSPECT and SoLid) presented their latest results and plans regarding the so-called reactor antineutrino anomaly. These are experiments aimed at detecting the oscillation effects of sterile neutrinos at reactors free from any assumption about antineutrino fluxes. There was no reported evidence for sterile oscillations, with the exception of the DANSS experiment reporting a 2.8σ effect, which is not in good agreement with previous measurements of this anomaly. These experiments are only at the beginning of data taking and more refined results are expected in the near future, even though it is unlikely that any of them will be able to provide a final sterile-neutrino measurement with a sensitivity much greater than 3σ.

Further discussion was raised by the results reported by MiniBooNE at Fermilab, which reports a 4.8σ excess of electron-like events by combining their neutrino and antineutrino runs. The result is compatible with the 3.8σ excess reported by the LSND experiment about 20 years ago in an experiment taking data in a neutrino beam created by pion decays at rest at Los Alamos. Concerns are raised by the fact that even sterile-neutrino oscillations do not fit the data very well, while backgrounds potentially do (and the MicroBooNE experiment is taking data at Fermilab with the specific purpose of precisely measuring the MiniBooNE backgrounds). Furthermore, as discussed by Michele Maltoni in his talk about the global picture of sterile neutrinos, no sterile neutrino model can, at the same time, accommodate the presumed evidence of ν_μ–ν_e oscillations by MiniBooNE and the null results reported by several different experiments (among which is MiniBooNE itself) regarding ν_μ disappearance at the same Δm².

The lively sessions at Neutrino 2018, summarised in the final two beautiful talks by Francesco Vissani (theory) and Takaaki Kajita (experiment), reinforce the vitality of this field at this time (see A golden age for neutrinos).

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On 3 July 1998, researchers working on the Super-Kamiokande experiment in Japan announced the first evidence for atmospheric-neutrino flavour oscillations. Since neutrinos can only oscillate among different flavours if at least some of them have a non-zero mass, the result proved that neutrinos are massive, albeit with very small mass values. This is not expected in the Standard Model.

Neutrino physics was already an active field, but the 1998 observation sent it into overdrive. The rich scientific programme and record attendance of the Neutrino 2018 conference in Heidelberg last month (see Neutrino physics shines bright in Heidelberg) is testament to our continued fascination with neutrinos. Many open questions remain: what generates the tiny masses of the known neutrinos, and what is their mass ordering? Are there more than the three known neutrino flavours, such as additional sterile or right-handed versions? Is there CP violation in the neutrino sector and, if so, how large is it? In addition, there are solar neutrinos, atmospheric neutrinos, cosmic/supernova neutrinos, relic neutrinos, geo-neutrinos, reactor neutrinos and accelerator-produced neutrinos – allowing for a plethora of experimental and theoretical activity.

Many of these questions are expected to be answered in the next decade thanks to vigorous experimental efforts. Concerning neutrino-flavour oscillations, new results are anticipated in the short term from the accelerator-based T2K and NOvA experiments in Japan and the US, respectively. These experiments probe the CP-violating phase in the neutrino-flavour mixing matrix and the ordering of the neutrino mass states; evidence for large CP violation could be established, in particular thanks to the planned ND280 near-detector upgrade of T2K.

The next generation of accelerator-based experiments is already under way. The Deep Underground Neutrino Experiment (DUNE) in South Dakota, US, which will use a neutrino beam sent from Fermilab, is taking shape and two large prototypes of the DUNE far detector are soon to be tested at CERN. In Japan, plans are shaping up for Hyper-Kamiokande, a large detector with a fiducial volume around 10 times larger than that of Super-Kamiokande, and this effort is complemented with other sensitivity improvements and a possible second detector in Korea for analysing a neutrino beam sent from J-PARC in Japan. These experiments, which are planned to come online in 2026, will allow precision neutrino-oscillation measurements and provide decisive statements on the neutrino mass hierarchy and CP-violating phase.

Important insights are also expected from reactor sources. In China, the JUNO experiment should start in 2021 and could settle the mass-hierarchy question and determine complementary oscillation parameters. Meanwhile, very-short-baseline reactor experiments – such as PROSPECT, STEREO, SoLid, NEOS and DANSS – are soon to join the hunt for sterile neutrinos. Together with detectors at the short-baseline neutrino beam at Fermilab (SBND, MicroBooNE and ICARUS), the next few years should see conclusive results on the existence of sterile neutrinos. In particular, the recently reported update on the intriguing excess seen by the MiniBooNE experiment will be scrutinised.

Together with the ever-increasing sensitivities achieved by double-beta-decay experiments, which test whether neutrinos have a Majorana mass term, the SHiP experiment is proposed to search for right-handed neutrinos, while KATRIN in Germany has just started its campaign to measure the mass of the electron antineutrino with sub-eV precision. The interplay with astronomy and cosmology, using detectors such as IceCUBE and KM3NeT, which survey atmospheric neutrinos, further underlines the vibrancy and breadth of modern neutrino physics. Also, the European Spallation Source, under construction in Sweden, is investigating the possibility of a precise neutrino-measurement programme.

Neutrino experiments are spread around the globe, but Europe is a strong player. A discussion forum on neutrino physics for the update of the European strategy for particle physics will be hosted by CERN on 22–24 October. Clearly, neutrino science promises many exciting results in the near future.

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The OPERA experiment, located at the Gran Sasso Laboratory of the Italian National Institute for Nuclear Physics (INFN), was designed to conclusively prove that muon-neutrinos can oscillate into tau-neutrinos by studying beams of muons sent from CERN 730 km away.

In a paper published on 22 May, describing the very final results of the experiment on neutrino oscillations, the OPERA collaboration has reported the observation of a total of 10 candidate events for a muon- to tau-neutrino conversion. This result demonstrates unambiguously that muons morph into tau neutrinos on their way from CERN to Gran Sasso.

The OPERA collaboration observed the first tau-neutrino event (evidence of muon-neutrino oscillation) in 2010, followed by four additional events reported between 2012 and 2015. A new analysis strategy applied to the full data sample collected between 2008 and 2012 led to the new total of 10 candidate events, with an extremely high level of significance. “We also report the first direct observation of the tau-neutrino lepton number, the parameter that discriminates neutrinos from antineutrinos,” says Giovanni de Lellis, OPERA spokesperson. “It is extremely gratifying to see today that our legacy results largely exceed the level of confidence we had envisaged in the experiment proposal.”

Beyond its contribution to neutrino physics, OPERA pioneered the use of large-scale emulsion films with fully automated and high-speed readout technologies with submicrometre accuracy. These technologies are now used in a wide range of other scientific areas, from dark-matter searches to investigations of volcanoes, and from the optimisation of hadron therapy for cancer treatment to the exploration of secret chambers in the Great Pyramid. The OPERA collaboration has also made its data public through the CERN open data portal, allowing researchers outside the collaboration to conduct novel research and offering tools such as a visualiser to help adapt the datasets for educational use.

The post OPERA concludes on tau appearance appeared first on CERN Courier.

On 16 April, US energy secretary Rick Perry and Indian Atomic Energy Secretary Sekhar Basu signed an agreement in New Delhi to expand the two countries’ collaboration in neutrino science. It opens the way for jointly advancing the Long-Baseline Neutrino Facility (LBNF) and the international Deep Underground Neutrino Experiment (DUNE) in the US and the India-based Neutrino Observatory (INO).

More than 1000 scientists from over 170 institutions in 31 countries work on LBNF/DUNE, construction for which got under way in July 2017. The project will direct the world’s most intense beams of neutrinos from Fermilab accelerators (driven by the new PIP-II machine) to detectors 1300 km away. INO scientists, meanwhile, will observe neutrinos that are produced in Earth’s atmosphere. Scientists from more than 20 institutions are working on INO, which is currently going through approval procedures.

The India–US agreement builds on one signed in 2013 authorising the joint development and construction of particle-accelerator components. Scientists from four institutions in India – BARC in Mumbai, IUAC in New Delhi, RRCAT in Indore and VECC in Kolkata – are contributing to the design and construction of magnets and superconducting particle-accelerator components for PIP-II at Fermilab and the next generation of particle accelerators in India.

Under the new agreement, US and Indian institutions will expand this to include neutrino research projects. DUNE, located about 1.5 km underground, will use almost 70,000 tonnes of liquid argon to detect neutrinos; and an additional detector will measure the neutrino beam at Fermilab as it leaves the accelerator complex. Prototype neutrino detectors are already under construction at CERN, which is also a partner in LBNF/DUNE. INO will use a different technology: an iron calorimeter. Its detector will feature what could be the world’s biggest magnet, allowing INO to be the first experiment able to distinguish signals produced by atmospheric neutrinos and antineutrinos produced when cosmic rays strike the atmosphere.

More than a dozen Indian institutions are involved in the collaboration on neutrino research. According to former INO spokesperson Naba Monda of the Saha Institute of Nuclear Physics, “this agreement is a positive step towards making INO a global centre for fundamental research. Students working at INO will get opportunities to interact with international experts.”

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Researchers at TRIUMF in Canada have reported the first production of ultracold neutrons (UCN), marking an important step towards a future neutron electric dipole moment (nEDM) experiment at the Vancouver laboratory. Precision measurements of the nEDM are a sensitive probe of physics beyond the Standard Model: if a nonzero value were to be measured, it would suggest a new source of CP violation, possibly related to the baryon asymmetry of the universe.

The TUCAN collaboration (TRIUMF UltraCold Advanced Neutron source) aims to measure nEDM a factor 30 better than the present best measurement, which has a precision of 3 × 10^–26 e cm and is consistent with zero. For this to be possible, physicists need to provide the world’s highest density of ultracold neutrons. In 2010 a collaboration between Canada and Japan was established to realise such a facility and a prototype UCN source was shipped to Canada and installed at TRIUMF in early 2017.

The setup uses a unique combination of proton-induced spallation and a superfluid helium UCN source that was pioneered in Japan. A tungsten block stops a beam of protons, producing a stream of fast neutrons that are then slowed in moderators and converted to ultracold speeds (less than around 7 ms^–1) by phonon scattering in superfluid helium. The source is based on a non-thermal down-scattering process in superfluid helium below 1 K, which gives the neutrons an effective temperature of a few mK. The ultracold temperature is below the neutron optical potential for many materials, which means the neutrons are totally reflected for all angles of incidence and can be stored in bottles for periods of up to hundreds of seconds.

Tests late last year demonstrated the highest current operation of this particular source, resulting in the most UCNs it has ever produced (> 300,000) in a single 60-second-long irradiation at a 10 µA proton beam current. This is a record for TRIUMF, but the UCN source intensity is still two orders of magnitude below what is needed for the nEDM experiment.

Funding of C$15.7 million to upgrade the UCN facility, a large proportion of which was granted by the Canada Foundation for Innovation in October 2017, will enable the TUCAN team to increase the production of neutrons at higher beam current to levels competitive with other planned nEDM experiments worldwide. These include proposals at the Paul Scherrer Institute in Switzerland, Los Alamos National Laboratory in the US, the Institut Laue–Langevin in France and others in Germany and Russia. The neutron EDM is experiencing intense competition, with most projects differing principally in the way they propose to produce the ultracold neutrons (CERN Courier September 2016 p27).

The nEDM experimental campaign at TRIUMF is scheduled to start in 2021. “The TRIUMF UCN source is the only one combining a spallation source of neutrons with a superfluid helium production volume, providing the project its uniqueness and competitive edge,” says team member Beatrice Franke.

The post Neutrons cooled for interrogation appeared first on CERN Courier.

Neutrinos are popularly thought to penetrate everything owing to their extremely weak interactions with matter. A recent analysis by the IceCube neutrino observatory at the South Pole proves this is not the case, confirming predictions that the neutrino–nucleon interaction cross section rises with energy to the point where even an object as tiny as the Earth can stop high-energy neutrinos in their tracks.

By studying a sample of 10,784 neutrino events, the IceCube team found that neutrinos with energies between 6.3 and 980 TeV were absorbed in the Earth. From this, they concluded that the neutrino–nucleon cross-section was 1.30^+0.21_–0.19 (stat) ^+0.39_–0.43 (syst) times the Standard Model (SM) cross-section in that energy range. IceCube did not observe a large increase in the cross-section as is predicted in some models of physics beyond the SM, including those with leptoquarks or extra dimensions.

The analysis used the 1km ³ volume of IceCube to collect a sample of upward-going muons produced by neutrino interactions in the rock and ice below and around the detector, selecting 10,784 muons with an energy above 1 TeV. Since the zenith angles of these neutrinos are known to about one degree, the absorber thickness can be precisely determined. The data were compared to a simulation containing atmospheric and astrophysical neutrinos, including simulated neutrino interactions in the Earth such as neutral-current interactions. Consequently, IceCube extended previous accelerator measurements upward in energy by several orders of magnitude, with the result in good agreement with the SM prediction (see figure, above).

Neutrinos are key to probing the deep structure of matter and the high-energy universe, yet until recently their interactions had only been measured at laboratory energies up to about 350 GeV. The high-energy neutrinos detected by IceCube, partially of astrophysical origin, provide an opportunity to measure their interactions at higher energies.

In an additional analysis of six years of IceCube data, Amy Connolly and Mauricio Bustamante of Ohio State University employ an alternative approach which uses 58 IceCube-contained events (in which the neutrino interaction took place within the detector) to measure the neutrino cross-section. Although these events mostly have well-measured energies, their neutrino zenith angles are less well known and they are also much less numerous, limiting the statistical precision.

Nevertheless, the team was able to measure the neutrino cross-section in four energy bins from 18 TeV to 2 PeV with factor-of-ten uncertainties, showing for the first time that the energy dependence of the cross section above 18 TeV agrees with the predicted softer-than-linear dependence and reaffirming the absence of new physics at TeV energy scales.

Future analyses from the IceCube Collaboration will use more data to measure the cross-sections in narrower bins of neutrino energy and to reach higher energies, making the measurements considerably more sensitive to beyond-SM physics. Planned larger detectors such as IceCube-Gen2 and the full KM3NeT can push these measurements further upwards in energy, while even larger detectors would be able to search for the coherent radio Cherenkov pulses produced when neutrinos with energies above 10¹⁷ eV interact in ice.

Proposals for future experiments such as ARA and ARIANNA envision the use of relatively-inexpensive detector arrays to instrument volumes above 100 km³, enough to measure “GZK” neutrinos produced when cosmic-rays interact with the cosmic-microwave background radiation. At these energies, the Earth is almost opaque and detectors should be able to extend cross-section measurements above 10¹⁹ eV, thereby probing beyond LHC energies.

These analyses join previous results on neutrino oscillations and exotic particle searches in showing that IceCube can also contribute to nuclear and particle physics, going beyond its original mission of studying astrophysical neutrinos.

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Researchers at the Cryogenic Underground Observatory for Rare Events (CUORE), located at Gran Sasso National Laboratories (LNGS) in Italy, have reported the latest results in their search for neutrinoless double beta-decay based on CUORE’s first full data set. This exceedingly rare process, which is predicted to occur less than once about every 10²⁶ years in a given nucleus, if it occurs at all, involves two neutrons in an atomic nucleus simultaneously decaying into two protons with the emission of two electrons and no neutrinos. This is only possible if neutrinos and antineutrinos are identical or “Majorana” particles, as posited by Ettore Majorana 80 years ago, such that the two neutrinos from the decay cancel each other out.

The discovery of neutrinoless double beta-decay (NDBD) would demonstrate that lepton number is not a symmetry of nature, perhaps playing a role in the observed matter–antimatter asymmetry in the universe, and constitute firm evidence for physics beyond the Standard Model. Following the discovery two decades ago that neutrinos have mass (a necessary condition for them to be Majorana particles), several experiments worldwide are competing to spot this exotic decay using a variety of techniques and different NDBD candidate nuclei.

CUORE is a tonne-scale cryogenic bolometer comprising 19 copper-framed towers that each house a matrix of 52 cube-shaped crystals of highly purified natural tellurium (containing more than 34% tellurium-130). The detector array, which has been cooled below a temperature of 10 mK and is shielded from cosmic rays by 1.4 km of rock and thick lead sheets, was designed and assembled over a 10 year period. Following initial results in 2015 from a CUORE prototype containing just one tower, the full detector with 19 towers was cooled down in the CUORE cryostat one year ago and the collaboration has now released its first publication, submitted to Physical Review Letters, with much higher statistics. The large volume of detector crystals greatly increases the likelihood of recording a NDBD event during the lifetime of the experiment.

Based on around seven weeks of data-taking, alternated with an intense programme of commissioning of the detector from May to September 2017 and corresponding to a total tellurium exposure of 86.3 kg per year, CUORE finds no sign of NDBD, placing a lower limit of the decay half-life of NDBD in tellurium-130 of 1.5 × 10²⁵ years (90% C.L.). This is the most stringent limit to date on this decay, says the team, and suggests that the effective Majorana neutrino mass is less than 140−400 meV, where the large range results from the nuclear matrix-element estimates employed. “This is the first preview of what an instrument this size is able to do,” says CUORE spokesperson Oliviero Cremonesi of INFN. “Already, the full detector array’s sensitivity has exceeded the precision of the measurements reported in April 2015 after a successful two-year test run that enlisted one detector tower.”

Over the next five years CUORE will collect around 100 times more data. Combined with search results in other isotopes, the possible hiding places of Majorana neutrinos will shrink much further.

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The UK is to invest £65 million (€74 million) in the Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) in the US, announced UK science minister Jo Johnson on 20 September. Currently under construction at Fermilab and Sanford Underground Research Laboratory, LBNF/DUNE will investigate crucial questions about neutrinos such as their mass ordering and CP-violating properties.

The latest investment makes the UK, already a major scientific contributor with 14 universities and two Science and Technology Facilities Council laboratories providing expertise and components, the largest country in the LBNF/DUNE project outside of the US.

“We have been working towards this for a long time and it is important both for the UK and for DUNE overall,” says co-spokesperson of the DUNE collaboration Mark Thomson of the University of Cambridge. “Specifically, the investment will allow the UK to play a major role in the construction of the DUNE far detector (read-out TPC wire planes and DAQ system) and in the neutrino beamline (super-conducting RF for the PIP-II LINAC and the LBNF neutrino target).”

LBNF/DUNE is the first major project to be addressed by a broader UK–US science and technology agreement, the first between the two countries, signed in January to strengthen UK–US co-operation.

CERN is an important partner in LBNF/DUNE and is developing prototype liquid-argon detectors for the project as part of its dedicated neutrino platform.

The post UK neutrino investment steps up appeared first on CERN Courier.

This review volume is motivated by the 2014 observation of a high-energy neutrino flux of extraterrestrial origin by the IceCube experiment at the South Pole. The energy of the events recorded ranges from 30 to 2000 TeV, with the latter marking the highest-energy neutrino interaction ever observed. The study of neutrinos originating from violent astrophysical sources enhances our knowledge not only of cosmological phenomena but also of neutrinos themselves.

This book gives an overview of the current status of research in the field and of existing and future neutrino observatories. The first group of chapters present the physics of potential sources of high-energy neutrinos, including gamma-ray bursts, active galactic nuclei, star-forming galaxies and sources in the Milky Way. A chapter is then dedicated to the measurements performed by IceCube, the results of which are discussed in terms of energy spectrum, flavour-ratio and arrival-direction isotropy. Following this, the results of two deep-sea neutrino experiments, ANTARES and Baikal, are presented.

After a brief discussion of other research topics in which the study of high-energy astrophysical neutrinos can play an important role, such as the quest for dark matter, the book examines the next generation of cosmic neutrino detectors. In particular, the future KM3NeT experiment, which will consist of a network of underwater telescopes located in the Mediterranean Sea, and IceCube-Gen2, characterised by unprecedented sensitivity and higher angular resolution compared to IceCube, are described.

Finally, a review of present and in-planning experiments aiming at detecting radio emissions from high-energy neutrino interactions concludes the volume.

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The Deep Underground Neutrino Experiment (DUNE) in the US, the cavern for which entered construction this summer, will make precision studies of neutrinos produced 1300 km away at Fermilab as part of the international Long-Baseline Neutrino Facility. The DUNE far detector will be the largest liquid-argon (LAr) neutrino detector ever built, comprising four cryostats holding 68,000 tonnes of liquid, and prototype detectors called protoDUNE are being built at CERN.

Each protoDUNE detector comprises a 10 × 10 × 10 m LAr time projection chamber with a single-phase (SP) or dual-phase (DP) configuration, containing about 800 tonnes of LAr. While the two big cryostats housing the detectors are about to be completed, the construction of the protoDUNE detectors themselves has just started. The first of six anode-plane-assembly modules for the protoDUNE-SP detector, which will detect electrons produced by ionising particles passing through the detector (pictured) recently arrived at CERN. The module will be tested, together with its electronics, and then installed in its final position inside the cryostat.

In parallel with the anode-plane-assembly, other parts of the protoDUNE-SP detector are being assembled at CERN, including the field cage, which keeps the electric field uniform inside the volume of the detector. Around a quarter of the 28 field-cage modules have already been assembled and are stored in CERN’s EHN1 hall, ready to be installed. The assembly and installation of the detector parts is expected to be completed by spring next year, in order for protoDUNE-SP to take data in autumn 2018.

The protoDUNE detectors are among several major activities taking place at the CERN neutrino platform, which was initiated in 2013 to develop detector technology for neutrino experiments in the US and Japan.

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The COHERENT collaboration at Oak Ridge National Laboratory (ORNL) in the US has detected coherent elastic scattering of neutrinos off nuclei for the first time. The ability to harness this process, predicted 43 years ago, offers new ways to study neutrino properties and could drastically reduce the scale of neutrino detectors.

Neutrinos famously interact very weakly, requiring very large volumes of active material to detect their presence. Typically, neutrinos interact with individual protons or neutrons inside a nucleus, but coherent elastic neutrino-nucleus scattering (CEνNS) occurs when a neutrino interacts with an entire nucleus. For this to occur, the momentum exchanged must remain significantly small compared to the nuclear size. This restricts the process to neutrino energies below a few tens of MeV, in contrast to the charged-current interactions by which neutrinos are usually detected. The signature of CEνNS is a low-energy nuclear recoil with all nucleon wavefunctions remaining in phase, but until now the difficulty in detecting these low-energy nuclear recoils has prevented observations of CEνNS – despite the predicted cross-section for this process being the largest of all low-energy neutrino couplings.

The COHERENT team, comprising 80 researchers from 19 institutions, used ORNL’s Spallation Neutron Source (SNS), which generates the most intense pulsed neutron beams in the world while simultaneously creating a significant yield of low-energy neutrinos. Approximately 5 × 10²⁰ protons are delivered per day, each returning roughly 0.08 isotropically emitted neutrinos per flavour. The researchers placed a detector, a caesium-iodide scintillator crystal doped with sodium, 20 m from the neutrino source with shielding to reduce background events associated with the neutron-induced nuclear recoils produced from the SNS. The results favour the presence of CEνNS over its absence at the 6.7σ level, with 134±22 events observed versus 173±48 predicted.

Crucially, the result was achieved using the world’s smallest neutrino detector, with a mass of 14.5 kg. This is a consequence of the large nuclear mass of caesium and iodine, which results in a large CEνNS cross-section.

The intense scintillation of this material for low-energy nuclear recoils, combined with the large neutrino flux of the SNS, also contributed to the success of the measurement. In effect, CEνNS allows the same detection rates as conventional neutrino detectors that are 100 times more massive.

“It is a nearly ideal detector choice for coherent neutrino scattering,” says lead designer Juan Collar of the University of Chicago. “However, other new coherent neutrino-detector designs are appearing over the horizon that look extraordinarily promising in order to further reduce detector mass, truly realising technological applications such as reactor monitoring.”

Yoshi Uchida of Imperial College London, who was not involved in the study, says that detecting neutrinos via the neutral-current process as opposed to the usual charged-current process is a great advantage because it is “blind” to the type of neutrino being produced and is sensitive at low energies. “So in combination with other types of detection, it could tell us a lot about a particular neutrino source of interest.” However, he adds that the SNS set-up is very specific and that, outside such ideal conditions, it might be difficult to scale a similar detector in a way that would be of practical use. “The fact that the COHERENT collaboration already has several other target nuclei (and detection methods) being used in their set-up means there will be more to come on this subject in the near future.”

The post Miniature detector first to spot coherent neutrino-nucleus scattering appeared first on CERN Courier.

A major focus of experiments at the Large Hadron Collider (LHC) is to search for new phenomena that cannot be explained by the Standard Model of particle physics. In addition to sophisticated analysis routines, this requires detailed measurements of particle tracks and energy deposits produced in large detectors by the LHC’s proton–proton collisions and, in particular, precise knowledge of the collision energy. The LHC’s counter-rotating proton beams each carry an energy of 6.5 TeV and this quantity is known to a precision of about 0.1 per cent – a feat that requires enormous technical expertise and equipment.

So far, no clear signs of physics beyond the Standard Model (BSM) have been detected at the LHC or at other colliders where a precise knowledge of the beam energy is needed. Indeed, the only evidence for BSM physics has come from experiments in which the beam energy is known very poorly. In 1998, in work that would lead to the 2015 Nobel Prize in Physics, researchers discovered that neutrinos have mass and that, therefore, these elementary particles cannot be purely left-handed, as had been assumed by the Standard Model. The discovery came from the observation of oscillations of atmospheric and solar neutrinos. The energies of the latter are determined by various elementary processes in the Sun and cover a wide range from a few keV up to about 20 MeV.

Since then, dedicated “long-baseline” experiments have started to explore neutrino oscillations under controlled conditions by sending neutrino beams produced in accelerator labs to detectors located hundreds of kilometres away. The T2K experiment in Japan shoots a beam from JPARC into the Super-Kamiokande underground detector about 300 km away, while the NOvA experiment in the US aims a beam produced at Fermilab to an above-ground detector near the Canadian border about 800 km away. Finally, the international Deep Underground Neutrino Experiment (DUNE), for which prototype detectors are being assembled at CERN (see “Viewpoint: CERN’s recipe for knowledge transfer”), will send a neutrino beam from Fermilab over a distance of 1300 km to a detector in the old Homestake gold mine in South Dakota. The targets in these experiments are all nuclei, rather than single protons, and the neutrino-beam energies range from a few 100 MeV to about 30 GeV.

Such poor knowledge of neutrino-beam energies is no longer acceptable for the science that awaits us. All long-baseline experiments aim to determine crucial neutrino properties, namely: the neutrino mixing angles; the value of a CP-violating phase in the neutrino sector; and the so far unknown mass ordering of the three neutrino flavours. Extracting these parameters is only possible by knowing the incoming neutrino energies, and these have to be reconstructed from observations of the final state of a neutrino–nucleus reaction. This calls for Monte Carlo generators that, unlike those used in high-energy physics, not only describe elementary particle reactions and their decays but also reactions with the nuclear environment.

Beam-energy problem

Neutrino beams have been produced for more than 50 years, famously allowing the discovery of the muon neutrino at Brookhaven National Laboratory in 1962. The difficulty in knowing the energy of a neutrino beam, as opposed to the situation at colliders such as the LHC, stems from the way the beams are produced. First, a high-current proton beam is fired into a thick target to produce many secondary particles such as charged pions and kaons, which are emitted in the forward direction. A device called a magnetic horn, invented by Simon van der Meer at CERN in 1961, then bundles the charged particles into a given direction as they decay into neutrinos and their corresponding charged leptons. Once the leptons have been removed by appropriate absorber materials, a neutrino beam emerges.

Whereas a particle beam in a high-energy accelerator has a diameter of about 10 μm, the width of the neutrino beam at its origin is determined by the dimensions of the horn, which is typically of the order of 1 m. Since the pions and kaons are produced with their own energy spectra, which have been measured by experiments such as HARP and NA61/SHINE at CERN, their two-body decays into a charged lepton and a neutrino lead to a broad neutrino-energy distribution. By the time a neutrino beam reaches a long-baseline detector, it may be as wide as a few kilometres and its energy is known only in broad ranges. For example, the beam envisaged for DUNE will have a distribution of energies with a maximum at about 2.5 GeV, with tails all the way down to 0 GeV on one side and 30 GeV on the other. While the high-energy tail may be small, the neutrino–nucleon cross-section in this region scales roughly linearly with the neutrino energy, so that even small tails contribute to interactions in the detector (figure 1).

The neutrino energy is a key parameter in the formula governing neutrino-oscillation probabilities and must be reconstructed on an event-by-event basis. In a “clean” two-body reaction such as ν_μ+ n → μ + p, where a neutrino undergoes quasi-elastic (QE) scattering off a neutron at rest, the neutrino energy can be determined from the kinematics (energy and angle) of the outgoing muon alone. This kinematical or QE-based approach requires a sufficiently good detector to make sure that no inelastic excitations of the nucleon have taken place. Alternatively, the so-called calorimetric method measures the energies of all the outgoing particles to yield the incoming neutrino energy. Since both methods suffer from less-than-perfect detectors with limitations in acceptance and efficiency, the reconstructed energy may not be equal to the true energy and detector simulations are therefore essential.

A major additional complication comes about because all modern neutrino experiments use nuclear targets, such as water in T2K and argon in DUNE. Even assuming that the neutrino–nucleus interaction can be described as a superposition of quasi-free interactions of the neutrino with individual nucleons, the latter are bound and move with their Fermi motion. As a result, the kinematical method suffers because the initial-state neutron is no longer at rest but moves with a momentum of up to about 225 MeV, smearing the reconstructed neutrino energy around its true value by a few tens of MeV. Furthermore, final-state interactions concerning the hadrons produced – both between themselves and with the nuclear environment of the detector – significantly complicate the energy reconstruction procedure. Even true initial QE events cannot be distinguished from events in which first a pion is produced and then is absorbed in the nuclear medium (figure 2), and the kinematical approach to energy reconstruction necessarily leads to a wrong neutrino energy.

The calorimetric method, on the other hand, suffers because detectors often do not see all particles at all energies. Here, the challenge to determine the neutrino energy is to “calculate backwards” from the final state, which is only partly known due to detector imperfections, to the incoming state of the reaction. One can gain an impression of how good this backwards calculation has to be by considering figure 3, which shows the sensitivity of the oscillation signal to changes in the CP-violating phase angle: for DUNE and T2K an accuracy of about 100 MeV and 50 MeV is required, respectively, to distinguish between the various curves showing the expected oscillation signal for different assumptions about the phase and the neutrino mass-ordering. At the same time, one sees that the oscillation maxima have to be determined within about 20% to be able to measure the phase and mass-ordering.

Detectors near and far

Neutrino physicists working on long-baseline experiments have long been aware of the problems in extracting the oscillation signal. The standard remedy is to build a detector not only at the oscillation distance (called the far detector, FD) but also one close to the neutrino production target (the near detector, ND). By dividing the event rates seen in the FD by those in the ND, the oscillation probability follows directly. The division also leads one to hope that uncertainties in our knowledge of cross-sections and reaction mechanisms cancel out, making the resulting probability less sensitive to uncertainties in the energy reconstruction. In practice, however, there are obstacles to such an approach. For instance, often the ND contains a different active material and has a different geometry to the FD, the latter simply because of the significant broadening of the neutrino beam with distance between the ND and the FD. Furthermore, due to the oscillation the energy spectrum of neutrinos is different in the ND than it is in the FD. It is therefore vital that we have a good understanding of the interactions in different target nuclei and energy regimes because the energy reconstruction has to be done separately both at the ND and the FD.

To place neutrino–nucleus reactions on a more solid empirical footing, neutrino researchers have started to measure the relevant cross-sections to a much higher accuracy than was possible at previous experiments such as CERN’s NOMAD. MiniBooNE at Fermilab has provided the world’s largest sample of charged-current events (QE-like reactions and pion production) on a target consisting of mineral oil, for example, and the experiment is now being followed by the nearby MicroBooNE, which uses an argon target. At higher energies, the MINERvA experiment (also at Fermilab) is dedicated to determining neutrino–nucleus cross-sections in an energy distribution that peaks at about 3.5 GeV and is thus close to that expected for DUNE. Cross-section measurements are also taking place in the NDs of T2K and NOvA, which are crucial to our understanding of neutrino–nucleus interactions and for benchmarking new neutrino generators.

Neutrino generators provide simulations of the entire neutrino–nucleus interaction, from the very first initial neutrino–nucleon interaction to the final state of many outgoing and interacting hadrons, and are needed to perform the backwards computation from the final state to the initial state. Such generators are also needed to estimate effects of detector acceptance and efficiency, similar to the role of GEANT in other nuclear and high-energy experiments. These generators should be able to describe all of the interactions over the full energy range of interest in a given experiment, and should also be able to describe neutrino–nucleus and hadron–nucleus interactions involving different nuclei. Obviously, the generators should therefore be based on state-of-the-art nuclear physics, both for the nuclear structure and for the actual reaction process.

Presently used neutrino generators, such as NEUT or GENIE, deal with the final-state interactions by employing Monte Carlo cascade codes in which the nucleons move freely between collisions and nuclear-structure information is kept at a minimum. The challenge here is to deal correctly with questions such as relativity in many-body systems, nuclear potentials and possible in-medium changes of the hadronic interactions. Significant progress has been made recently in describing the structure of target nuclei and their excitations in so-called Green’s function Monte Carlo theory. A similarly sophisticated approach to the hadronic final-state interactions is provided by the non-equilibrium Green’s function method. This method, the foundations of which were written down half a century ago, is the only known way to describe high-multiplicity events while taking care of possible in-medium effects on the interaction rates and the off-shell transport between collisions. Only during the last two decades have numerical implementations of this quantum-kinetic transport theory become possible. A neutrino generator built on this method (GiBUU) has recently been used to explore the uncertainties that are inherent in the kinematical-energy reconstruction method for the very same process shown in figure 3, and the result of that study (figure 4) gives an idea of the errors inherent in such an energy reconstruction.

Generators that contain all the physics of neutrino–nucleus interactions are absolutely essential to get at the neutrino’s intriguing properties in long-baseline experiments. The situation is comparable to that in experiments at the LHC and at the Relativistic Heavy Ion Collider at Brookhaven that study the quark–gluon plasma (QGP). Here, the existence and properties of the QGP can be inferred only by calculating backwards from the final-state observations with “normal” hadrons to the hot and dense collision zone with gluons and quarks. Without a full knowledge of the neutrino–nucleus interactions the neutrino energy in current and future long-baseline neutrino experiments cannot be reliably reconstructed. Thus, generators for neutrino experiments should clearly be of the same quality as the corresponding experimental apparatus. This is where the expertise and methods of nuclear physicists are needed in experiments with neutrino beams.

A natural test bed for these generators is provided by data from electron–nucleus reactions. These test the vector part of the neutrino–nucleus interaction and thus constitute a mandatory test for any generator. Experiments with electrons at JLAB are presently in a planning stage. Since the energy reconstruction has to start from the final state of the reaction, the four-vectors of all final-state particles are needed for the backwards calculation to the initial state. Inclusive lepton–nucleus cross-sections, with no information on the final state, are therefore not sufficient.

Call to action

All of this has been realised only recently and there is now a growing community that tries to bring together experimental high-energy physicists that work on long-baseline experiments with nuclear theorists. There is a dedicated conference series called NUINT, in addition to meetings such as WIN or NUFACT, which now all have sessions on neutrino–nucleus interactions.

We face a challenge that is completely new to high-energy physics experiments: the reconstruction of the incoming energy from the final state requires a good description of the nuclear ground state, control of neutrino–nucleus interactions and, on top of all this, control of the final-state interactions of the hadrons when they cascade through the nucleus after the primary neutrino–nucleon interaction. Neutrino generators that fulfil all of these requirements can minimise the uncertainties in the energy reconstruction. They should therefore attract the same attention and support as the development of new equipment for long-baseline neutrino experiments, since their quality ultimately determines the precision of the extracted neutrino properties.

The post Neutrinos on nuclei appeared first on CERN Courier.

On 12 June, two large detector modules for the ICARUS experiment were loaded onto trucks at CERN to begin a six-week journey to Fermilab in the US. ICARUS will form part of Fermilab’s short-baseline neutrino programme, which aims to make detailed measurements of neutrino interactions and search for eV-scale sterile neutrinos (CERN Courier June 2017 p25).

Based on advanced liquid-argon time projection technology, ICARUS began its life under a mountain at the Gran Sasso National Laboratory in Italy in 2010, recording data from neutrino beams sent from CERN. Since 2014, it has been at CERN undergoing an upgrade and refurbishment at the CERN Neutrino Platform (CERN Courier July/August 2016 p21). It left CERN in two parts by road and boarded a boat on the Rhine to a port in Antwerp, Belgium, where it was loaded onto a ship. As the Courier went to press, ICARUS was already heading across the Atlantic to Fermilab via the Great Lakes, equipped with a GPS unit that allows its progress to be tracked in real time (icarustrip.fnal.gov).

Just two days after ICARUS left CERN, another key component of the CERN Neutrino Platform was on the move, albeit on a smaller lorry. Baby MIND, a 75 tonne prototype for a magnetised iron neutrino detector that will precisely identify and track muons, was moved from its construction site in building 180 to the East Hall of the Proton Synchrotron. Following commissioning and full characterisation in the T9 test beam, at the end of July Baby MIND will be transported to Japan to be part of the WAGASCI experiment at JPARC, where it will contribute to a better understanding of neutrino interactions for the T2K experiment.

The post Neutrino detectors on the move appeared first on CERN Courier.

This summer, two 270 m³ steel containment vessels are making their way by land, sea and river from CERN in Europe to Fermilab in the US, a journey that will take five weeks. Each vessel houses one of the 27,000-channel precision wire chambers of the ICARUS detector, which uses advanced liquid-argon technology to detect neutrinos. Having already operated successfully in the CERN to Gran Sasso neutrino beam from 2010 to 2012, and spent the past two years being refurbished at CERN, ICARUS will team up with two similar detectors at Fermilab to deliver a new physics opportunity: the ability to resolve some intriguing experimental anomalies in neutrino physics and perform the most sensitive search to date for eV-scale sterile neutrinos. This new endeavour, comprised of three large liquid-argon detectors (SBND, MicroBooNE and ICARUS) sitting in a single intense neutrino beam at Fermilab, is known as the Short-Baseline Neutrino (SBN) programme.

The sterile neutrino is a hypothetical particle, originally introduced by Bruno Pontecorvo in 1967, which doesn’t experience any of the known forces of the Standard Model. Sterile-neutrino states, if they exist, are not directly observable since they don’t interact with ordinary matter, but the phenomenon of neutrino oscillations provides us with a powerful probe of physics beyond the Standard Model. Active–sterile mixing, just like standard three-neutrino mixing, could generate additional oscillations among the standard neutrino flavours but at wavelengths that are distinct from the now well-measured “solar” and “atmospheric” oscillation effects. Anomalies exist in the data of past neutrino experiments that present intriguing hints of possible new physics. We now require precise follow-up experiments to either confirm or rule out the existence of additional, sterile-neutrino states.    

On the scent of sterile states

The discovery nearly two decades ago of neutrino-flavour oscillations led to the realisation that each of the familiar flavours (ν_e, ν_μ, ν_τ ) is actually a linear superposition of states of distinct masses (ν₁, ν₂, ν₃ ). The wavelength of an oscillation is determined by the difference in the squared masses of the participating mass states, m²_i – m²_j. The discoveries that were awarded the 2015 Nobel Prize in Physics correspond to the atmospheric mass-splitting Δm²_ATM = |m²₃ – m²₂| = 2.5 × 10^–3 eV² and the solar mass-splitting Δm²_SOLAR = m²₂ – m²₁ = 7.5 × 10^–5 eV², so-named because of how they were first observed. Any additional and mostly sterile mass states, therefore, could generate a unique oscillation driven by a new mass scale in the neutrino sector: m²_{mostly sterile} – m²_{mostly active}.

The most significant experimental hint of new physics comes from the LSND experiment performed at the Los Alamos National Laboratory in the 1990s, which observed a 3.8σ excess of electron antineutrinos appearing in a mostly muon antineutrino beam in a region where standard mixing would predict no significant effect. Later, in the 2000s, the MiniBooNE experiment at Fermilab found excesses of both electron neutrinos and electron antineutrinos, although there is some tension with the original LSND observation. Other hints come from the apparent anomalous disappearance of electron antineutrinos over baselines less than a few hundred metres at nuclear-power reactors (the “reactor anomaly”), and the lower than expected rate in radioactive-source calibration data from the gallium-based solar-neutrino experiments GALLEX and SAGE (the “gallium anomaly”). Numerous other searches in appearance and disappearance channels have been conducted at various neutrino experiments with null results (including ICARUS when it operated in the CERN to Gran Sasso beam), and these have thus constrained the parameter space where light sterile neutrinos could still be hiding. A global analysis of the available data now limits the possible sterile–active mass-splitting, m²_{mostly sterile} – m²_{mostly active}, to a small region around 1–2 eV². 

Long-baseline accelerator-based neutrino experiments such as NOvA at Fermilab, T2K in Japan, and the future Deep Underground Neutrino Experiment (DUNE) in the US, which will involve detectors located 1300 km from the source, are tuned to observe oscillations related to the atmospheric mass-splitting, Δm²_ATM ~ 10^–3 eV². Since the mass-squared difference between the participating states and the length scale of the oscillation they generate are inversely proportional to one another, a short-baseline accelerator experiment such as SBN, with detector distances of the order 1 km, is most sensitive to an oscillation generated by a mass-squared difference of order 1 eV² – exactly the region we want to search.

Three detectors, one beam

The SBN programme has been designed to definitively address this question of short-baseline neutrino oscillations and test the existence of light sterile neutrinos with unprecedented sensitivity. The key to SBN’s reach is the deployment of multiple high-precision neutrino detectors, all of the same technology, at different distances along a single high-intensity neutrino beam. Use of an accelerator-based neutrino source has the bonus that both electron-neutrino appearance and muon-neutrino disappearance oscillation channels can be investigated simultaneously.

The neutrino source is Fermilab’s Booster Neutrino Beam (BNB), which has been operating at high rates since 2002 and providing beam to multiple experiments. The BNB is generated by impinging 8 GeV protons from the Booster onto a beryllium target and magnetically focusing the resulting hadrons, which decay to produce a broad-energy neutrino beam peaked around 700 MeV that is made up of roughly 99.5% muon neutrinos and 0.5% electron neutrinos.

The three SBN detectors are each liquid-argon time projection chambers (LArTPCs) located along the BNB neutrino path (see images above). MicroBooNE, an 87 tonne active-mass LArTPC, is located 470 m from the neutrino production target and has been collecting data since October 2015. The Short-Baseline Near Detector (SBND), a 112 tonne active-mass LArTPC to be sited 110 m from the target, is currently under construction and will provide the high-statistics characterisation of the un-oscillated BNB neutrino fluxes that is needed to control systematic uncertainties in searches for oscillations at the downstream locations. Finally, ICARUS, with 476 tonnes of active mass and located 600 m from the BNB target, will achieve a sufficient event rate at the downstream location where a potential oscillation signal may be present. Many of the upgrades to ICARUS implemented during its time at CERN over the past few years are in response to unique challenges presented by operating a LArTPC detector near the surface, as opposed to the underground Gran Sasso laboratory where it operated previously. The SBN programme is being realised by a large international collaboration of researchers with major detector contributions from CERN, the Italian INFN, Swiss NSF, UK STFC, and US DOE and NSF. At Fermilab, new experimental halls to house the ICARUS and SBND detectors were constructed in 2016 and are now awaiting the LArTPCs. ICARUS and SBND are expected to begin operation in 2018 and 2019, respectively, with approximately three years of ICARUS data needed to reach the programme’s design sensitivity.

A rich physics programme

In a combined analysis, the three SNB detectors allow for the cancellation of common systematics and can therefore test the ν_μ → ν_e oscillation hypothesis at a level of 5σ or better over the full range of parameter space originally allowed at 99% C.L. by the LSND data. Recent measurements, especially from the NEOS, IceCube and MINOS experiments, have constrained the possible sterile-neutrino parameters significantly and the sensitivity of the SBN programme is highest near the most favoured values of Δm². In addition to ν_e appearance, SBN also has the sensitivity to ν_μ disappearance needed to confirm an oscillation interpretation of any observed appearance signal, thus providing a more robust result on sterile-neutrino-induced oscillations (figure 1).

SBN was conceived to unravel the physics of light sterile neutrinos, but the scientific reach of the programme is broader than just the searches for short-baseline neutrino oscillations. The SBN detectors will record millions of neutrino interactions that can be used to make precise measurements of neutrino–argon interaction cross-sections and perform detailed studies of the rather complicated physics involved when neutrinos scatter off a large nucleus such as argon. The SBND detector, for example, will see of the order 100,000 muon-neutrino interactions and 1000 electron-neutrino interactions per month. For comparison, existing muon-neutrino measurements of these interactions are based on only a few thousand total events and there are no measurements at all with electron neutrinos. The position of the ICARUS detector also allows it to see interactions from two neutrino beams running concurrently at Fermilab (the Booster and Main Injector neutrino beams), allowing for a large-statistics measurement of muon and electron neutrinos in a higher-energy regime that is important for future experiments.

In fact, the science programme of SBN has several important connections to the future long-baseline neutrino experiment at Fermilab, DUNE. DUNE will deploy multiple 10 kt LArTPCs 1.5 km underground in South Dakota, 1300 km from Fermilab. The three detectors of SBN present an R&D platform for advancing this exciting technology and are providing direct experimental activity for the global DUNE community. In addition, the challenging multi-detector oscillation analyses at SBN will be an excellent proving ground for sophisticated event reconstruction and data-analysis techniques designed to maximally exploit the excellent tracking and calorimetric capabilities of the LArTPC. From the physics point of view, discovering or excluding sterile neutrinos plays an important role in the ability of DUNE to untangle the effects of charge-parity violation in neutrino oscillations, a primary physics goal of the experiment. Also, precise studies of neutrino–argon cross-sections at SBN will help control one of the largest sources of systematic uncertainties facing long-baseline oscillation measurements.    

Closing in on a resolution

The hunt for light sterile neutrinos has continued for several decades now, and global analyses are regularly updated with new results. The original LSND data still contain the most significant signal, but the resolution on Δm² was poor and so the range of values allowed at 99% C.L. spans more than three orders of magnitude. Today, only a small region of mass-squared values remain compatible with all of the available data, and a new generation of improved experiments, including the SBN programme, are under way or have been proposed that can rule on sterile-neutrino oscillations in exactly this region.

There is currently a lot of activity in the sterile-neutrino area. The nuPRISM and JSNS² proposals in Japan could also test for ν_μ → ν_e appearance, while new proposals like the KPipe experiment, also in Japan, can contribute to the search for ν_μ disappearance. The MINOS+ and IceCube detectors, both of which have already set strong limits on ν_μ disappearance, still have additional data to analyse. A suite of experiments is already currently under way (NEOS, DANSS, Neutrino-4) or in the planning stages (PROSPECT, SoLid, STEREO) to test for electron-antineutrino disappearance over short baselines at reactors, and others are being planned that will use powerful radioactive sources (CeSOX, BEST). These electron-neutrino and -antineutrino disappearance searches are highly complementary to the search modes being explored at SBN. 

The Fermilab SBN programme offers world-leading sensitivity to oscillations in two different search modes at the most relevant mass-splitting scale as indicated by previous data. We will soon have critical new information regarding the possible existence of eV-scale sterile neutrinos, resulting in either one of the most exciting discoveries across particle physics in recent years or the welcome resolution of a long-standing unresolved puzzle in neutrino physics.

LArTPCs rule the neutrino-oscillation waves

A schematic diagram of the ICARUS liquid-argon time projection chamber (LArTPC) detector, where electrons create signals on three rotated wire planes. The concept of the LArTPC for neutrino detection was first conceived by Carlo Rubbia in 1977, followed by many years of pioneering R&D activity and the successful operation of the ICARUS detector in the CNGS beam from 2010 to 2012, which demonstrated the effectiveness of single-phase LArTPC technology for neutrino physics. A LArTPC provides both precise calorimetric sampling and 3D tracking similar to the extraordinary imaging features of a bubble chamber, and is also fully electronic and therefore potentially scalable to large, several-kilotonne masses. Charged particles propagating in the liquid argon ionise argon atoms and free electrons drift under the influence of a strong, uniform electric field applied across the detector volume. The drifted ionisation electrons induce signals or are collected on planes of closely spaced sense wires located on one side of the detector boundary, with the wire signals proportional to the amount of energy deposited in a small cell. The very low electron drift speeds, in the range of 1.6 mm/μs, require a continuous read-out time of 1–2 milliseconds for a detector a few metres across. This creates a challenge when operating these detectors at the surface, as the SBN detectors will be at Fermilab, so photon-detection systems will be used to collect fast scintillation light and time each event.

The post Search for sterile neutrinos triples up appeared first on CERN Courier.

On 29 June 1967, the Soviet government issued a document that gave the go-ahead to build a brand new underground facility for neutrino physics in the Baksan valley in the mountainous region of the Northern Caucasus. Construction work began straight away on the tunnels under the 4000 m-high peak of Mount Andyrchi that would contain the experimental halls, and 10 years later, the laboratory’s first neutrino telescope started operation. Today, a varied experimental programme continues at the Baksan Neutrino Observatory, which is operated by the Institute for Nuclear Research (INR) of the Russian Academy of Sciences (RAS). And there is the promise of more to come.

The detailed proposal for the Baksan Neutrino Observatory was put together by “the father of neutrino astronomy”, Moisei Markov, and his younger colleagues, Alexander Chudakov, George Zatsepin and Alexander Pomansky, together with many others. The decision to construct a dedicated underground facility rather than use an existing mine – something that had never been done before – gave the scientists the freedom to choose the location and the structure of their future laboratory to maximise its scientific output. Their proposal to house it in an almost horizontal tunnel under a steep mountain decreased the construction costs by a factor of six with respect to a mine, while maintaining higher safety standards. They selected Andyrchi – one of a series of peaks dominated by Europe’s highest mountain, Mount Elbrus (5642 m) – from many potential sites. The entrance to the laboratory tunnel is located in the valley below the peaks, which is well known to mountaineers, hikers and skiers, at an altitude of 1700 m. A small village called Neutrino was built to accommodate scientists and engineers working for the observatory, with office and laboratory buildings, some surface installations, living quarters and related infrastructure.

The basic idea of underground neutrino detection is to use soil and rock to shield the installations from muons produced in cosmic-ray interactions with the atmosphere – the main background for neutrino detection. The underground complex at Baksan contains two interconnected tunnels (having two is a safety requirement) with laboratory halls situated at various distances along the tunnels, corresponding to different shielding conditions below the mountain. At the end of the 4 km-long tunnels, the flux of the muons is suppressed by almost 10 million times with respect to the surface.

Experiments past

The first experiment to start at Baksan, back in 1973, was not however underground. The Carpet air-shower experiment completely covered an area of around 200 m² with 400 liquid-scintillator detectors, identical to those of the first neutrino telescope “BUST” (see below). Its key task was a detailed study of the central part of air showers produced by cosmic particles in the atmosphere. One of its first results, based on the interpretation of shower sub-cores as imprints of jets with high transverse-momenta – born in the primary interactions of the cosmic rays – was on the production cross-section of these jets for leading-particle energies up to 500 GeV. This result was published earlier than the corresponding measurement at CERN’s Super Proton Synchrotron and confirmed predictions of quantum chromodynamics. Carpet’s discoveries of astrophysical importance included a puzzling giant flare in the Crab Nebula in 1989.

The Baksan Underground Scintillator Telescope (BUST) started operation in 1977. A multipurpose detector, it is located in an artificial cavern with a volume of 12,000 m³ located 550 m from the tunnel entrance. The telescope is a four-level underground building 11.1 m high with a base area of 280 m². The building, made of low-radioactivity concrete, houses 3180 detectors containing 330 tonnes of liquid scintillator. Sensitive to cosmic neutrinos with energies of dozens of MeV, the detector is well suited to the search for supernova neutrinos, and on 23 February 1987 it was one of four detectors in the world that registered the renowned neutrino signal from the supernova 1987A in the Large Magellanic Cloud. The results obtained with the telescope have been used for cosmic-ray studies, searches for exotic particles (notably, magnetic monopoles) and neutrino bursts.

Neutrinos with lower energies were the target of the Gallium–Germanium Neutrino Telescope, a pioneering device to search for solar neutrinos in the SAGE (Soviet–American Gallium Experiment) project. The first experiments to detect neutrinos from the Sun – Homestake in the US and Kamiokande II in Japan – registered neutrinos with energies of a few MeV, which are mainly produced in the decay of boron-8 and constitute less than 1% of the total solar-neutrino flux. These Nobel-prize-winning experiments revealed the solar-neutrino deficit, subsequently interpreted in terms of neutrino oscillations, the only firm laboratory indication so far for the incompleteness of the Standard Model of particle physics. However, to assess the problem fully, it was necessary to find out what happens with the bulk (86% of the total flux) of the solar neutrinos, which come from proton–proton (pp) fusion reactions and have energies below about 0.4 MeV.

In 1965, Vadim Kuzmin proposed using the reaction ⁷¹Ga + ν_e → ⁷¹Ge + e⁻ to detect the low-energy solar neutrinos. This idea was implemented in two experiments: GALLEX in the Gran Sasso National Laboratory and SAGE at Baksan. SAGE, which has been in operation since 1986 and is led by Vladimir Gavrin, is located 3.5 km from the tunnel entrance, where the cosmic-ray muon flux is suppressed by a factor of several million. About 50 tonnes of liquid gallium are used as a target; amazingly, a special factory was built to produce this amount of gallium, which exceeded the total consumed by the Soviet Union at the time. A unique chemical technology was developed to allow about 15 germanium atoms to be extracted from the 50 tonnes of gallium every month.

SAGE and GALLEX were the first experiments to detect solar pp neutrinos and to confirm the solar-neutrino deficit for the bulk of the flux. Combined with results from other experiments to subtract sub-leading contributions from other channels, SAGE found the solar pp neutrino flux to be 6.0±0.8 × 10¹⁰ cm^–2 s^–1. This agrees nicely with the solar-model prediction (taking into account neutrino oscillations) of 5.98±0.04 × 10¹⁰ cm^–2 s^–1 and the result has been confirmed by the 2014 measurement by Borexino, using a different method, which gives 6.6±0.7 × 10¹⁰ cm^–2 s^–1.

The unique underground conditions at Baksan also allowed the creation of several ultra-low-background laboratories where, in addition to the natural shielding, materials with extremely low radioactivity were used in construction. There are three shielded chambers at different depths where rare nuclear processes have been searched for and a number of low-background experiments performed, including a precise measurement of the isotopic composition of the lunar soil delivered by the Luna-16, Luna-20 and Luna-24 spacecraft.

Current experiments

Now 50, the Baksan Neutrino Observatory continues to probe the neutrino frontier. The scintillator telescope is still monitoring the universe for neutrino bursts, its almost 40 year exposure time setting stringent constraints on the rate of core-collapse supernova in the Milky Way. The non-observation of neutrinos associated with the gravitational-wave event of 15 September 2015, detected by the LIGO Observatory, puts a unique constraint on the associated flux of neutrinos with energies of 1–100 GeV, complementary to constraints from larger experiments at different energies.

Calibration of the gallium solar-neutrino experiments, SAGE and GALLEX, with artificial neutrino sources has revealed the so-called gallium anomaly, which can be understood in terms of a new, sterile-neutrino state. A new experiment called the Baksan Experiment on Sterile Transitions (BEST), has been instigated to check the anomaly and thus test the sterile-neutrino hypothesis. This will be based on a ⁵¹Cr artificial neutrino source with an intensity of around 100 PBq, placed in the centre of a spherical gallium target of two concentric zones with equal neutrino mean-free-paths; any significant difference in the rate of neutrino capture in the inner and outer zones would indicate the existence of a sterile neutrino. CrSOX, a similar experiment with the Borexino detector at Gran Sasso, might become competitive with BEST but only in its full-scale configuration with the 400 PBq neutrino source. Reactor experiments would provide complementary information about a sterile antineutrino.

BEST is now fully constructed and is awaiting the artificial neutrino source. Meanwhile, ultra-pure gallium is still used in the SAGE experiment, confirming the stability of the solar-neutrino flux over decades: fortunately, the Sun is not about to change its power output.

Numerous experiments are being carried out in the low-background laboratory, thanks to a new experimental hall – Low Background Lab 3 on the figure above – located 3.67 km from the tunnel entrance (providing shielding equivalent of 4900 m of water). One of them searches for solar axions via their resonant reconversion on ⁸³Kr, and this experiment has already resulted in the world’s best constraint on certain couplings of the hadronic axion.

Among the surface-based experiments, the Carpet air-shower array is undergoing the most intense development. Equipped with a brand new muon detector with an area of 410 m², this old cosmic-ray installation is starting a new life as a sophisticated sub-PeV gamma-ray telescope. A world-best sensitivity to the diffuse gamma-ray flux above 100 TeV, which could be achieved by the end of 2017, would be sufficient to decide between the galactic and extragalactic origin of the high-energy astrophysical neutrinos detected by the IceCube neutrino observatory at the South Pole.

Other experiments are also ready to produce interesting results. The Andyrchi air-shower array located on the slope of the mountain above BUST works in coincidence with the telescope, which serves as a muon detector with a 120 GeV threshold. A small gravitational-wave detector, OGRAN, capable of registering a galactic supernova, makes Baksan a true multi-messenger observatory. In addition, important interdisciplinary studies are taking place at the border with geophysics. They include not only deep-underground precise monitoring of seismic and magnetic parameters close to the sleeping volcano Elbrus, but also, for example, studies of atmospheric electricity and its relation to the cosmic-ray muon flux.

Future prospects

Looking ahead, the Baksan Neutrino Observatory could host new breakthrough experiments. The many planned projects include a further upgrade of the Carpet array with the increase in both the surface-array and muon-detector areas for the purposes of sub-PeV gamma-ray astronomy; a new resonant-reconversion solar axion experiment with a sensitivity an order of magnitude better than the present one; and a circular laser interferometer – or Sagnac gyroscope – for geophysics and fundamental-physics measurements.

However, the main project for the observatory is the Baksan Large-Volume Scintillator Detector (BLVSD, although the name of the experiment is yet to be fixed). This detector, currently at the R&D stage, should contain 10–20 kilotonnes of ultra-pure liquid scintillator and could be located at the end of the observatory tunnel. There, unused artificial caverns exist in which a Cl–Ar solar-neutrino experiment was originally planned, but was replaced by the SAGE Ga–Ge detector in a different cave. This large-volume detector should be able to detect not only neutrinos from a galactic core-collapse supernova, but also the composite neutrino background of numerous distant explosions, thus making it possible to study supernova neutrinos in the unlucky, but probable, case that no galactic explosion happens in the coming decades. In the opposite case, the large neutrino statistics from a nearby explosion would open up possibilities for a detailed study.

For solar neutrinos, BLVSD would be capable of measuring the neutrino flux from the carbon–nitrogen–oxygen (CNO) fusion cycle in the Sun with a precision sufficient to discriminate between various solar models and therefore solve experimentally the present-day contradiction between results from helioseismology and those from chemical-composition studies of the solar surface. A primary target for BLVSD would be the study of geoneutrinos, which are produced in nuclear decays in the Earth’s interior. Clearly, the detector could also be used for a precise study of neutrino oscillations, in particular with a dedicated long-distance accelerator beam.

BLVSD would join a global network of large-scale neutrino detectors, if created. Such joint operation would open possibilities to solve many interesting problems. It would allow, for instance, the inclusion of effects of the inhomogeneous structure of the Earth’s crust in geoneutrino studies, or the determination of the direction of a supernova that is obscured and visible only in neutrinos. The unique conditions at the Baksan observatory would also make the solo operation of BLVSD efficient. Not only do the existing infrastructure and experience allow for ultra-low-background experiments, but the geographical position in the Northern Caucasus guarantees a large distance from nuclear reactors. For geoneutrinos, estimates of the ratio of the signal counting-rate to the background from artificial reactors give a value around 5 for Baksan, compared to around 1.1 for Borexino and around 0.15 for KamLand. It is the low background that would allow a precise measurement of the solar CNO flux, which is barely possible in any of the currently operating experiments.

The large-scale BLVSD project is still in its infancy, and numerous efforts in R&D, fundraising and construction are still to be made. The Baksan Neutrino Observatory is fully open for worldwide collaboration and co-operation, both in this and in other scientific projects. Happy birthday, Baksan.

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An experiment in Korea designed to search for light sterile neutrinos has published its first results, further constraining the possible properties of such a particle. Even though the number of light neutrinos cannot exceed three, it is still possible to have additional neutrinos if they are “sterile”. Such particles, which are right-handed singlets under the electromagnetic, strong and weak interactions, are predicted by extensions of the Standard Model and would reveal themselves by altering the rate of oscillation between the three standard neutrino flavours. An early hint for such a state came from observations of the mixing between electron and muon neutrinos by the LSND experiment, although more recent results from other experiments are so far inconclusive.

The NEOS detector is a Gd-loaded liquid scintillator located just 24 m from the core of the 2.8 GW Hanbit nuclear power plant in South Korea, which generates a high flux of antineutrinos. Based on precise measurements of an antineutrino energy spectrum over an eight-month period, the NEOS team found no evidence for oscillations involving sterile neutrinos. On the other hand, the team recorded a small excess of antineutrinos above an energy of around 5 MeV that is consistent with anomalies seen at longer-baseline neutrino experiments.

With no strong evidence for “3 + 1” neutrino oscillations, the new results set stringent upper limits on the θ₁₄ mixing angle (see figure) of sin²2θ₁₄ less than 0.1 for Δm²₄₁ ranging from 0.2–2.3 eV² at 90% confidence level. The results further improve the constraints to the LSND anomaly parameter space, say the team. With the NEOS experiment now completed, the team is discussing a further reactor neutrino programme using commercial reactors to be built in the near future in Korea.

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The SNOLAB laboratory in Ontario, Canada, has received a grant of $28.6m to help secure its next three years of operations. The facility is one of 17 research facilities to receive support through Canada’s Major Science Initiative (MSI) fund, which exists to secure state-of-the-art national research facilities.

SNOLAB, which is located in a mine 2 km beneath the surface, specialises in neutrino and dark-matter physics and claims to be the deepest cleanroom facility in the world. Current experiments located there include: PICO and DEAP-3600, which search for dark matter using bubble-chamber and liquid-argon technology, respectively; EXO, which aims to measure the mass and nature of the neutrino; HALO, designed to detect supernovae; and a new neutrino experiment SNO+ based on the existing SNO detector.

The new funds will be used to employ the 96-strong SNOLAB staff and support the operations and maintenance of the lab’s facilities.

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This 11 m-high structure with thick steel walls will soon contain a prototype detector for the Deep Underground Neutrino Experiment (DUNE), a major international project based in the US for studying neutrinos and proton decay. It is being assembled in conjunction with CERN’s Neutrino Platform, which was established in 2014 to support neutrino experiments hosted in Japan and the US (CERN Courier July/August 2016 p21), and is pictured here in December as the roof of the structure was lowered into place. Another almost identical structure is under construction nearby and will house a second prototype detector for DUNE. Both are being built at CERN’s new “EHN1” test facility, which was completed last year at the north area of the laboratory’s Prévessin site.

DUNE, which is due to start operations in the next decade, will address key outstanding questions about neutrinos. In addition to determining the ordering of the neutrino masses, it will search for leptonic CP violation by precisely measuring differences between the oscillations of muon-type neutrinos and antineutrinos into electron-type neutrinos and antineutrinos, respectively (CERN Courier December 2015 p19). To do so, DUNE will consist of two advanced detectors placed in an intense neutrino beam produced at Fermilab’s Long-Baseline Neutrino Facility (LBNF). One will record particle interactions near the source of the beam before the neutrinos have had time to oscillate, while a second, much larger detector will be installed deep underground at the Sanford Underground Research Laboratory in Lead, South Dakota, 1300 km away.

The outer structure of the cryostat  (red, pictured at top) for the single-phase protoDUNE module is now complete, and an equivalent structure for the double-phase module is taking shape just a few metres away and is expected to be complete by March. In addition, a smaller technology demonstrator for the double-phase protoDUNE detector is complete and is currently being cooled down at a separate facility on the CERN site (image above). The 3 × 1 × 1 m³ module will allow the CERN and DUNE teams to perfect the double-phase concept, in which a region of gaseous argon situated above the usual liquid phase provides additional signal amplification.

The large protoDUNE modules are planned to be ready for test beam by autumn 2018 at the EHN1 facility using dedicated beams from the Super Proton Synchrotron. Given the intensity of the future LBNF beam, for which Fermilab’s Main Injector recently passed an important milestone by generating a 700 kW, 120 GeV proton beam for a period of more than one hour, the rate and volume of data produced by the DUNE detectors will be substantial. Meanwhile, the DUNE collaboration continues to attract new members and discussions are now under way to share responsibilities for the numerous components of the project’s vast far detectors (see “DUNE collaboration meeting comes to CERN” in this month’s Faces & Places).

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On 14 October, the KArlsruhe TRItium Neutrino (KATRIN) experiment, which is presently being assembled at Tritium Laboratory Karlsruhe on the KIT Campus North site, Germany, celebrated “first light”. For the first time, electrons were guided through the 70 m-long beamline towards a giant spectrometer, which allows the kinetic energy of the beta electron from tritium beta decays to be determined very precisely. Although actual measurements will not get under way until next year, it marks the beginning of KATRIN operation.

The goal of the technologically challenging KATRIN experiment, which has been a CERN-recognised experiment since 2007, is to determine the absolute mass scale of neutrinos in a model-independent way. Previous experiments using the same technique set an upper limit to the electron antineutrino mass of 2.3 eV/c², but KATRIN will either improve on this by one order of magnitude or, if neutrinos weigh more than 0.35 eV/c², discover the actual mass.

KATRIN involves more than 150 scientists, engineers and technicians from 12 institutions in Germany, the UK, Russia, the Czech Republic and the US.

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In July 1956, in a brief paper published in Science, a small team based at the Los Alamos National Laboratory in the US presented results from an experiment at a new, powerful fission reactor at the Savannah River Plant, in South Carolina. The work, they wrote, “verifies the neutrino hypothesis suggested by Pauli”. Clyde Cowan, Fred Reines, Kiko Harrison, Herald Kruse and Austin McGuire had demonstrated for the first time that it was possible to detect neutrinos, setting in motion the new field of neutrino physics. The key ingredients were an intense source and a big detector, with more than a touch of ingenuity and patience.

More than two decades previously, in 1930, Wolfgang Pauli had proposed that the “energy crisis” in nuclear beta decay – presented by the continuous energy spectrum of the emitted electron – would be solved if the decaying nucleus also emitted a second, undetected particle. This would allow the energy released to be shared between three objects, including the recoiling nucleus, and so yield electrons with a range of energies, just as observed. The new particle had to be neutral and have a relatively small mass. Pauli called his proposal “a desperate remedy”, in part because he thought that if such a particle did indeed exist, then it “would probably have long ago been seen”.

Nevertheless, Enrico Fermi took the possibility seriously and based his seminal work on beta decay, published in 1934, on a point-contact interaction in which a neutron decays to a proton, electron and (anti)neutrino: n → p e^– ν. Soon afterwards, Hans Bethe and Rudolf Peierls calculated the cross-section for the inverse reaction in which a neutrino is absorbed, but when they found a value of about 10^–44 cm², the pair concluded that no one would be able to detect neutrinos (Bethe and Peierls 1934). What they did not count on was the discovery of nuclear fission – which on a macroscopic scale produces copious numbers of neutrinos – or the ingenuity of experimentalists and, later, accelerator physicists.

Notoriously, nuclear fission was first applied in the atomic bombs used towards the end of the Second World War. A few years later, in 1951, Fred Reines, a physicist who had worked on the Manhattan Project at Los Alamos, began to think about how to harness the neutrinos produced during tests of atomic bombs to make a direct detection of the elusive particle. He was soon joined in this strange pursuit by Clyde Cowan, a fellow researcher at Los Alamos, after they were stranded together at Kansas Airport, where the conversation turned to the “supreme challenge” of detecting neutrinos.

Reines had an idea to place a detector close to a bomb-test tower and use the timing of the detonation as a “gate” to minimise background. But what kind of detector? He and Cowan decided on the recently developed medium of liquid scintillator, which could both act as a target for the inverse beta-decay reaction ν p → e⁺ n, and detect the emitted positrons via their annihilation to gamma rays. It was an audacious plan, not only in taking advantage of a bomb test but also in scaling up the use of liquid scintillator, which until then had been used only in quantities of about a litre. Reines and Cowan named it “Project Poltergeist”, to reflect the neutrino’s ghostly nature.

Remarkably, the Los Alamos director gave approval for the experiment. However, in late 1952, Cowan and Reines were urged to reconsider the more practical idea of using antineutrinos from a nuclear reactor. The challenge was to work out how to reduce the backgrounds, because the antineutrino flux from a reactor would be thousands of times smaller than that from a nuclear explosion. Reines and Cowan realised that in addition to looking for positron annihilation, they could also detect the neutrons through neutron capture – a process that is delayed for several microseconds, thanks to the neutron’s random walk through a medium prior to interacting with a nucleus. In particular, the addition of cadmium to the detector would increase the likelihood of capture and lead to the emission of gamma rays. The signature for inverse beta decay would then be a delayed coincidence between two sets of gamma rays: one from the positron’s annihilation and the other from the neutron’s capture.

The detector for Project Poltergeist contained 300 litres of liquid scintillator with added cadmium chloride, viewed by 90 photomultiplier tubes, and was set up in 1953 at a new reactor at the Hanford Engineering Works in Washington State. This initial experiment showed a small increase in delayed coincidences when the reactor was operating compared with the situation when it was turned off, but it was set against a cosmic-ray background that was more than 10 times higher than the expected signal rate (Reines and Cowan 1953).

This tantalising result encouraged a still more determined effort, with a new detector design that was basically a sandwich with three layers of liquid scintillator and two layers of water with added cadmium chloride to act as the target (figure 1). Positrons produced in a neutrino interaction would be detected almost immediately via two back-to-back gamma rays in the adjacent scintillator tanks, which would be followed a few microseconds later by another burst of gamma rays in the same two scintillator tanks, this time from neutron capture.

The second experiment ran at the newly completed Savannah River Plant for a total of 1371 hours in 1956 and, when the reactor was on, it recorded nearly three delayed coincidences per hour (Cowan et al. 1956). After completing many checks, on 14 June 1956 Reines and Cowan sent a jubilant telegram to Pauli in Zurich, informing him that they had “definitely detected neutrinos from fission fragments by observing inverse beta decay of protons”. At the time, Pauli was in fact at a meeting at CERN, to where the telegram was forwarded, and he reportedly interrupted the meeting to read out the good news, later celebrating with a case of champagne (Reines 1979).

The move to accelerators

At the time of the neutrino’s discovery, laboratories such as CERN and Brookhaven were on their way to building proton synchrotrons that would have sufficient energy and intensity to form beams of neutrinos via decays of pions and kaons produced when protons strike a suitable target. The muons produced in the decays could be stopped by large amounts of shielding, allowing only neutrinos to penetrate to experiments beyond. At Brookhaven, this led to the discovery at the Alternating Gradient Synchrotron (AGS) in 1962 that the neutrinos produced in association with electrons (as in beta decay) are different from those produced in association with muons (as in pion decay): a second type of neutrino, the muon neutrino, had been discovered.

In 1963, an ingenious way to produce neutrino beams of greater intensity first came into use at the Proton Synchrotron (PS) at CERN, where Simon van der Meer had described his concept of the neutrino horn a couple of years earlier (van der Meer 1961). Because neutrinos are electrically neutral, they cannot be focused into a beam using magnets, so he devised instead a way to focus the parent pions and kaons using magnetic fields set up by currents circulating in a metallic cone-shaped “horn” (CERN Courier June 2011 p24). The device concentrated neutrinos produced as the charged particles decayed in flight into a beam, and because it could focus either positive or negative particles, it produced an almost pure beam of neutrinos (from positive parents) or antineutrinos (negative parents). A second technical innovation at CERN enabled the horn to become a formidable device: the technique of “fast ejection”, devised by Berend Kuiper and Günther Plass, could direct all of the protons from one cycle of the PS onto the target at the mouth of the horn (Kuiper and Plass 1959). By mid-1963, thanks to these innovations, CERN had what was at the time the world’s most intense neutrino beam.

In the 1970s, the combination of the neutrino beam from the PS and Gargamelle – the large bubble chamber built at the Saclay Laboratory by a team led by André Lagarrigue – led to the discovery of weak neutral currents (CERN Courier September 2009 p25), thereby providing crucial experimental support for the unification of the weak and electromagnetic forces. The neutrino experiments with Gargamelle also produced key evidence about the existence of quarks and, in particular, their fractional charges (CERN Courier April 2014 p24). Then, in 1977, the Super Proton Synchrotron (SPS) became the source of neutrino beams at higher energies, and for the next 21 years a series of experiments in CERN’s West Area used neutrinos in experiments covering a broad range of physics, from neutral currents and the quark structure of matter through quantum chromodynamics to neutrino oscillations (CERN Courier December 1998 p28).

Around that time, physicists at Fermilab were closing in on a third neutrino type. The DONUT experiment (Direct Observation of the NU Tau) detected neutrinos produced at the Tevatron, and in 2000, the collaboration announced the discovery of the tau neutrino. Although experiments at CERN’s Large Electron–Positron collider had already established from precise measurements of the Z boson that there are three light neutrino types, the observation of the tau neutrino completed the leptonic sector of the Standard Model.

Ten years later, CERN was again setting records for neutrino beams, with the CERN Neutrinos to Gran Sasso (CNGS) project, which directed an intense beam of muon-neutrinos (ν_μ) to two experiments, ICARUS and OPERA, in the Gran Sasso National Laboratory in Italy about 730 km away. CNGS followed the same principle as CERN’s early record-breaking beam, this time with protons from the SPS. Following first commissioning in 2006 (CERN Courier November 2006 p20), the facility ran for physics from 2008 to the end of 2012, and achieved a maximum beam power of 480 kW – the most powerful at the time. A total of 18.24 × 10¹⁹ protons were delivered on target, and the OPERA experiment detected 19,500 neutrino events – with five among them identified as a tau neutrino (ν_τ), thereby firmly establishing the direct observation of ν_μ→ ν_τ oscillations (CERN Courier July/August 2015 p6).

A bountiful legacy

Since the first glimpses of antineutrino interactions 60 years ago in reactor experiments, experiments have gone on to detect neutrinos and antineutrinos produced in a variety of ways – both in beams created at particle accelerators and also naturally by reactions in the Sun, interactions of cosmic rays in the Earth’s atmosphere and, most recently, astrophysical processes. We now know that neutrinos exist not only in three flavour eigenstates – electron (ν_e), muon (ν_μ) and tau (ν_τ) – but also in different mass eigenstates (ν₁, ν₂ and ν₃) with very small masses, and that they can oscillate from one flavour to another through quantum-mechanical mixing (see “Japan eyes up its future”).

Reactor experiments – in particular Double Chooz in France, the Daya Bay Reactor Neutrino Experiment in China (figure 2) and the Reactor Experiment for Neutrino Oscillation (RENO) in South Korea – are still as relevant now as they were in Cowan and Reines’ day. Modern nuclear power plants produce about 10²⁰ electron antineutrinos (ν_e) per second and experiments based on the same liquid-scintillator concept continue to provide essential contributions to neutrino physics by looking for the “disappearance” of the ν_e.

Sixty years after the first detection of the neutrino, and more than 80 years after the particle was tentatively predicted, experiments with neutrinos continue to have a leading role in particle physics. Today, experimentalists around the world are vying to determine precisely the mixing parameters of the neutrino, including the masses. The measurements may prove to hold the answers to some key questions in the field – ensuring that the “supreme challenge” of creating and detecting neutrinos will remain a worthwhile and exciting pursuit for the foreseeable future.

The post Ghosts in the machine appeared first on CERN Courier.

When CERN was founded in 1954, the neutrino was technically still a figment of theorists’ imaginations. Six decades later, neutrinos have become the most studied of all elementary particles. Several new and upgraded neutrino-beam experiments planned in Japan and the US, in addition to the reactor-based JUNO experiment in China, aim to measure vital parameters such as the ordering of the neutrino masses and potential CP-violating effects in the neutrino sector. In support of this effort, CERN is mounting a significant R&D programme called the CERN Neutrino Platform to strengthen European participation in neutrino physics.

CERN has a long tradition in neutrino physics. It was the study of neutrino beams with the Gargamelle detector at CERN in 1973 that provided the first evidence for the weak neutral current, and in the late 1970s, three experiments – BEBC, CDHS and CHARM – used a beam from the SPS to further unveil the neutrino’s identity. A milestone came in 1989, when precise measurements at the Large Electron–Positron Collider showed that there are three, and only three, types of light neutrinos that couple to the Z boson. This was followed by searches for neutrino oscillations at NOMAD (also known as WA96) and CHORUS (WA95) during the 1990s, which were eventually established by the Super-Kamiokande collaboration in Japan and the Sudbury Neutrino Observatory in Canada. More recently, from 2006 to 2012, CERN sent a muon-neutrino beam to the ICARUS and OPERA detectors at the Gran Sasso National Laboratory, 732 km away in Italy. The main goal was to observe the transformation of muon neutrinos into tau neutrinos, which was confirmed by the OPERA collaboration in 2015.

Following the recommendations of the European Strategy for Particle Physics in 2013, CERN inaugurated the neutrino platform at the end of 2014. Its aim is to provide a focal point for Europe’s contributions to global neutrino research by developing and prototyping the next generation of neutrino detectors. So far, around 50 European institutes have signed up as members of the neutrino platform, which sees CERN shift from its traditional role of providing neutrino beams to one where it shares its expertise in detectors, infrastructure and international collaboration.

“The neutrino platform pulls together a community that is scattered across the world and CERN has committed significant resources to support R&D in all aspects of neutrino research,” says project leader Marzio Nessi. Specifically, he explains, CERN is using the organisational model of the LHC to help in

developing an international project on US soil and to contribute to neutrino programmes in Japan and elsewhere. “This is precisely what CERN is about,” says Nessi. “The platform provides a structure at CERN to foster active involvement of Europe and CERN in the US and Japanese facilities.”

In December 2014, CERN and the Italian National Institute for Nuclear Physics (INFN) took delivery of the 760 tonne ICARUS detector, which formerly was located at Gran Sasso. The detector is currently being refurbished by the neutrino platform’s WA104 team and in 2017 it will be shipped to Fermilab in the US to become part of a dedicated short-baseline neutrino (SBN) programme there. This programme was approved following unexpected results from the LSND experiment at Los Alamos National Laboratory in the 1990s, which hinted at the existence of a fourth – possibly “sterile” – type of neutrino. The result was followed up by the MiniBooNE experiment at Fermilab, which also saw deviations – albeit different again – from the expected signal.

ICARUS will be installed just behind the previous MiniBooNE site, some 600 m downstream from the source of the beam at Fermilab’s booster ring. It will be the farthest of three detectors in the line of the beam after the Short Baseline Neutrino Detector (SBND, which is currently under design) and MiniBooNE’s successor MicroBooNE (which is already operational). All three detectors employ liquid-argon time projection chambers (LAr-TPCs) to study neutrino oscillations in detail. ICARUS comprises two 270 m³  modules filled with liquid argon: when an energetic charged particle passes through its volume it ionises the liquid and a uniform electric field causes electrons to drift towards the end plates, where three layers of parallel wire planes oriented at different angles (together with the drift time) allow researchers to reconstruct a 3D image of the event.

The refurbishing campaign at CERN concerns many parts of the ICARUS experiment: the photomultipliers, the read-out electronics, the cathode plane and the argon recirculating system. Moreover, it will benefit from European expertise in automatic event reconstruction and the handling of large data sets. Finally, the unique cryostat in which ICARUS will be placed is also being assembled at CERN. “Improving the performances of a detector already successfully operating in the Gran Sasso underground laboratory is extremely challenging in many respects,” says ICARUS technical co-ordinator Claudio Montanari. “Indeed, in order to make it fully functional to operate on surface, many different aspects including data acquisition, background rejection, timing and event reconstruction needed to be rethought.”

Going deeper

Rapid progress made in understanding neutrino oscillations during the past 15 years has also provided a strong case for long-baseline neutrino programmes. A major new international project called DUNE (Deep Underground Neutrino Experiment), which is estimated to begin operations by approximately 2026 as part of Fermilab’s Long Baseline Neutrino Facility (LBNF), will take the form of a near and a far multikiloton detector. The far detector will consist of four 10 kt active LAr-TPC modules sited in a 1.5 km-deep cavern at the Sanford lab in South Dakota, 1300 km away, at which neutrino beams with unprecedented intensities will be fired through the Earth from Fermilab. While the three experiments in the SBN programme will look for the disappearance of electron and muon neutrinos to search for sterile neutrinos, they will also serve as a stepping stone to the large LAr modules required by LBNF. The LBNF/DUNE experiment will allow not just the neutrino-mass hierarchy to be determined but also CP violation to be looked for in the leptonic sector, which could help to explain the missing baryonic matter in the universe.

The CERN Neutrino Platform is building two large-scale prototypes – single-phase and double-phase ProtoDUNE modules – to enable LAr detectors to be scaled up to the multikiloton level. The cryostat for such giant detectors is a particular challenge, and led physicists to explore a novel technological solution inspired by the liquified-natural-gas (LNG) shipping industry. CERN is currently collaborating with French firm Gaztransport & Technigaz, which owns the patent for a membrane-type containment system with two cryogenic liners that support and insulate the liquid cargo. Although this containment system has the advantage of being modular, the challenge in a particle-physics setting is that the cryostats not only have to contain the liquid argon but also all of the detectors and read-out electronics.

Global connection

While the single-phase ProtoDUNE detector uses technology that is very similar to that in ICARUS, a second neutrino-platform project called WA105 aims to prototype the new concept of a “dual-phase” LAr time projection chamber (DLAr-TPC), which is being considered for one or more of the DUNE far-detector 10 kt modules. In a DLAr chamber, a region of gaseous argon resides above the usual liquid phase. Ionisation electrons drift up through the detector volume and are accelerated into the gaseous region near the top of the cryostat by a strong electric field. Here, large electron multipliers amplify the signals, while the anode collects the charged particles and provides the spatial read-out. “The ProtoDUNE tests foreseen at the CERN Neutrino Platform represent the culmination of more than a decade of R&D towards the feasibility of very large liquid-argon time projection chambers for next-generation long-baseline experiments,” says André Rubbia, co-spokesperson of the DUNE collaboration.

ProtoDUNE and WA105 are planned to be ready for test beam by 2018 at a new EHN1 test facility currently under construction in the north area of CERN’s Prévessin site. Most of the civil-engineering work to extend the EHN1 building is complete and all components are under procurement or installation, with staff expected to move in towards the end of the year. The test facility was financed by CERN, with two beamlines due to be commissioned in late 2017.

As ICARUS prepares for its voyage across the Atlantic, and the detectors for the next-generation of US neutrino experiments takes shape, the CERN Neutrino Platform is also working on components for Japan’s neutrino programme (see “NOvA releases new bounds on neutrino mixing parameters”). The Baby-MIND collaboration aims to construct a muon spectrometer – a state-of-the-art prototype for a would-be Magnetized Iron Neutrino Detector (MIND) – and characterise it in a charged-particle beam at CERN. The system will be assembled at CERN during the winter and tested in May next year, before being shipped to Japan in the summer of 2017. Once there, it will become part of the WAGASCI experiment, where it will contribute to a better understanding of the systematics for the T2K neutrino and antineutrino oscillation analysis. Baby-MIND was approved by the CERN research board in December last year as a Neutrino Platform experiment. “Other projects for the Japanese neutrino programme are also under discussion,” says Baby-MIND spokesperson Alain Blondel of the University of Geneva.

Finally, in June it was decided that the CERN Neutrino Platform will also involve a neutrino-theory working group to strengthen the connections between CERN and the worldwide community and help to promote research in theoretical neutrino physics at CERN. “Fundamental questions in neutrino physics, such as the existence of leptonic CP violation, the Majorana nature of neutrinos and the origin of neutrino masses and mixings, will be at the centre of research activities,” explains group-convener Pilar Hernández. “The answers to these questions could have essential implications in other areas of high-energy physics, from collider physics to indirect searches, as well as in our understanding of the universe.”

• CERN Neutrino Platform: cenf.web.cern.ch.

• Theory working group: th-dep.web.cern.ch/cern-neutrino-platform-theory-working-group-cenf-th.

The post Neutrinos take centre stage appeared first on CERN Courier.

In 1976, neutrinos did not yet have the prominent role in particle physics that they play today. Postulated by Pauli in 1930, they had been said to be undetectable due to their tiny interaction probability, and were only first observed in the mid-1950s by Fred Reines and Clyde Cowan using a detector located close to a military nuclear reactor. In 1962, researchers at Brookhaven National Laboratory discovered a second type of neutrino, the muon neutrino, but the third (tau) neutrino would not be seen directly for a further 38 years. On the other hand, the theory of electroweak interactions mediated by the W and Z bosons was firming up, and measurements of neutrinos played a significant role in this context.

The first naturally generated neutrinos, originating from cosmic-ray collisions in the Earth’s atmosphere, were observed in 1965 in deep gold mines located in South Africa and India. Also in the late 1960s, Ray Davis was beginning his famous solar-neutrino observations. The time was right to start thinking seriously about neutrino astronomy.

Enter DUMAND

Plans that ultimately would shape the present-day neutrino industry blossomed in September 1976 at a meeting in Waikiki, Hawaii. It was here that some of the first ideas for large detectors such as the gigaton DUMAND (Deep Underwater Muon and Neutrino Detector) array, which eventually morphed into the present-day IceCube experiment at the South Pole, were envisioned. The technology for smaller water-filled detectors such as IMB (Irvine–Michigan–Brookhaven) and, later, Kamiokande in Japan, were also laid out in detail for the first time. Moreover, totally new concepts such as particle detection via sound waves or radio waves were explored.

The first organised stirrings of what was to become DUMAND took place at a cosmic-ray conference in Denver, Colorado, in 1973, which led to a preliminary workshop at Western Washington University in 1975. It was at this meeting that the detection of Cherenkov radiation in water was chosen as the most viable method to “see” neutrino interactions in a transparent medium, with some deep lakes and the ocean near Hawaii considered to be good locations. This detection principle goes back to Russian physicists Moisey Markov and Igor Zheleznykh in 1960: water provides the target for neutrinos, which create charged particles that generate a flash of light in approximate proportion to the neutrino energy. The water also shields the many downward-moving muons from cosmic-ray interactions in the atmosphere. With light detectors, such as a basketball-sized photomultiplier tube, one could register the light flash over large distances.

The vision of building devices larger than any yet dreamed about, and placing them in the deep ocean to study the cosmos above, attracted many adventurous souls from around the US and elsewhere, a number of whom dedicated years to realising this dream. One of us (JL) joined Reines, along with Howard Blood of the US navy and other ocean-engineering aficionados – in particular, George Wilkins, who was responsible for the first undersea fibre-optic cables. Inventor of the wetsuit and former bubble-chamber developer Hugh Bradner became another driving force behind the project. Theorists, cosmic-ray experts, particle physicists, astronomers and astrophysicists all played important roles (see photograph). The event captured a spirit of adventure and worldwide co-operation, particularly concerning interactions between US and Soviet colleagues, and the special nature of our unusual international physics collaboration brought a certain level of spice to relations.

Searching the sky

A significant problem of the era was to know what sources of neutrinos the detectors should be looking for. Astrophysicists and astronomers seldom thought about neutrinos at the time, and neutrinos were neglected in calculations of radiation and power in the universe. They were, however, included in studies of solar burning and supernovae, and it was from these efforts that neutrinos began to appear in the astronomer’s lexicon. For the first time, a survey of possible astrophysical sources of lower-energy neutrinos (typically 1–100 MeV) was produced at the 1976 meeting, which were organised by Craig Wheeler into seven possible steady sources and eight potential burst sources.

For the steady sources, only solar neutrinos and terrestrial radioactivity appeared practical for detecting neutrinos. For the bursting neutrino sources, galactic gravitational collapses clearly yielded the most available total power for neutrinos. The others – namely type I supernovae, solar flares, gamma- and X-ray bursts and mini-black-hole evaporation – did not seem to have enough power and proximity to be competitive. Even today, these sources have not been observed in terms of neutrinos.

There was also great interest in targeting high-energy (TeV and above) neutrinos, but predictions of the strengths of the potential sources were plagued by huge uncertainties. It was known that the number of cosmic rays impinging upon the Earth decreases as a function of energy up to values of around 10²⁰ eV, whereupon the spectrum was predicted to show a cut-off known as the Greisen–Zatsepin–Kuzmin (GZK) limit caused by high-energy protons being degraded by resonant collisions with the cosmic microwave background. Veniamin Berezinsky of the Lebedev Institute in Moscow put forward some prescient models of ultra-high-energy neutrino generation, in particular, those generated in GZK processes, and also gave first estimates of upper bounds on neutrinos from star formation in the early universe. Credible sources of neutrinos with energies far beyond the TeV scale were probably too far ahead of people’s visions at the time, while neutrino cross-sections and even the production dynamics were not well defined at the highest energies.

By the end of the Hawaii workshop, which lasted for two weeks, everyone considered low-energy neutrino detection to be the most worthwhile cosmic-neutrino-detection goal. Aside from solar neutrinos, it was also agreed that neutrinos from supernova collapses were the most likely to be seen (as they later were in 1987, when a burst of neutrinos was observed by the IMB, Kamiokande and Baksan detectors). But it was also realised that this effort requires kiloton detectors with threshold sensitivities of about 10 MeV to guarantee a few supernovae per century. Regarding the detection of high-energy neutrinos in the TeV range, as expected from acceleration processes in galactic and extragalactic objects, everyone understood the necessity of detectors in the megaton to gigaton class. This was so far beyond the reach of technology at the time, however, that people realised they had to start small and work upwards in target mass.

Interestingly, neutrinos generated in the atmosphere with energies in the GeV range were regarded as the least interesting target. Nobody at that time would have expected that precisely these neutrinos, together with solar neutrinos, would demonstrate for neutrino oscillations and lead to the award of the 2015 Nobel Prize in Physics.

Detector evolution

Considering the vast target volumes required for high-energy neutrino astronomy, it became clear that the most promising – and most affordable – detection method was to register the Cherenkov light in natural water. With the intensity of Cherenkov light being around 30 times weaker than that from a scintillator, however, the design of the optical detectors became paramount. One group of attendees from the 1976 workshop aimed for the use of wavelength shifters that absorb blue light and re-emit green light, allowing the use of modest photodetectors, while another pushed for the development of photomultipliers larger than the 25 cm-diameter versions available at the time – as did in fact transpire. The one serious alternative to optical Cherenkov light detection, building on a concept developed by Gurgen Askaryan in 1957, was to utilise the pulse of sound made by neutrino progeny after neutrino collision with a nucleus of water or another medium such as ice.

Following the 1976 event, annual neutrino workshops were held for about a decade, eventually blending with DUMAND collaboration meetings. The 1978 workshop, held in La Jolla, California, took place in three sessions over a six-week period, and attracted more neutrino converts from physics, astrophysics and ocean engineering. The following year’s event, which was held at Khabarovsk and Lake Baikal in Siberia, offered some physicists their first chance to interact with Soviet physicists who had not been able to travel. Indeed, by the end of 1979, international politics and in particular the Soviet–Afghan war had forced the separation of the Russian and US DUMAND efforts. Russian DUMANDers decided to push ahead with a detector array deployed from ice in the world’s deepest freshwater lake, Lake Baikal, and one of us (CS) joined the Baikal collaboration in 1988. A few years later, the first underwater neutrino events were identified in Lake Baikal, and the principle of this detection technique was finally proven (followed by more statistics from AMANDA at the South Pole and ANTARES in the Mediterranean Sea). The heroic efforts of Russian physicists through difficult times have continued for 35 years, and Baikal researchers have just deployed the first subunit of a cubic-kilometre array similar to IceCube.

The formal outcome of the 1976 workshop was a joint resolution and plans for ocean studies and further workshops. The major vision for the high-energy DUMAND detector itself (see diagram) was an array of bottom-moored strings carrying some 22,000 optical detectors distributed in a volume slightly larger than one cubic kilometre – quite similar to the eventual IceCube array. The DUMAND project carried out many ocean studies and also accomplished some physics offshore in Hawaii, measuring muons and the lack of large bursts.

Alas, DUMAND was cancelled by the US Department of Energy in 1995, prior to starting full deployment. One can debate the causes – the failure of the first deployed string was certainly one aspect. It may be noted, however, that the Superconducting Supercollider had just been cancelled and that the main funding agencies were simply not supportive of non-accelerator research until after SuperKamiokande’s discovery of neutrino oscillations in 1998. The DUMAND project was also ahead of its time in terms of its detection scheme, the use of undersea fibre optics and robotic module deployment.

The DUMAND legacy

Attendees of DUMAND’76 realised the importance of the venture, but probably came away with varying levels of belief in its practicality. Perhaps the greatest legacy of the 1976 workshop was bringing natural neutrino studies to the attention of a wide research community, and astrophysicists in particular. The breadth of ideas that this allowed, and the spirit of interdisciplinarity and international co-operation, has continued in the neutrino community. Moreover, initially still-born alternatives for neutrino detection have been revived in the last two decades.

The neutrino-detection method using acoustic signals was taken up in the late 1990s and early 2000s using military hydrophone arrays close to the Bahamas and to Kamchatka, as well as dedicated test set-ups in Lake Baikal, the Mediterranean Sea and in Antarctic ice – namely, the South Pole Acoustic Test Setup (SPATS). More profitably, the radio technique of detecting high-energy neutrino interactions via the negative charge excess in a very-high-energy particle shower induced by a neutrino interaction has allowed stringent upper limits to be placed on neutrino fluxes at the highest energies. The Askaryan Radio Array (ARA), the balloon-borne ANITA project and ARIANNA all focus on radio detection in Antarctic ice. Indeed, ANITA has just reported the first candidate for a super-high-energy neutrino event emerging from the Earth at a large angle.

Probably the most important lineage descending from the 1976 workshop is in the exploitation of the large water Cherenkov detectors located underground: IMB (1983), Kamiokande (1985) and, most notably, SuperKamiokande (1996). Some other experiments can be claimed as at least partial progeny of the early DUMAND enterprises, such as NESTOR in the Mediterranean, the Sudbury Neutrino Observatory (SNO) in Canada and KamLAND in Japan. The links become more tenuous, but the people engaged all owe much stimulation and experience to the explorations of these problems and detector solutions 40 years ago.

As for DUMAND itself, its spirit lives on in present-day neutrino observatories. IceCube at the South Pole is certainly the most extreme realisation of the DUMAND concept, and DUMAND spherical phototube modules are still the archetypal unit for this and many other detectors, such as ANTARES and GVD in Lake Baikal. A further array currently being installed deep in the Mediterranean, KM3NeT, has varied the principle by arranging many small phototubes inside of the glass sphere instead of a single large tube, while IceCube is exploring multiphototube modules as well as wavelength-shifter solutions for its next-generation incarnation.

Following IceCube’s discovery of the first extraterrestrial high-energy neutrinos in 2013, we are finally realising the decades-old dream of seeing the universe in neutrinos. Today, with thousands of researchers undertaking neutrino studies, the revelations for particle physics from neutrinos seem to be unending. The experimental road has been full of surprises, and neutrino physics and astronomy remain some of the most exciting games in town.

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Japan has been a leader in the global neutrino community since the 1980s, breaking ground (both literally and figuratively) with multiple generations of massive underground experiments. These experiments, which although sited in Japan are built and operated by international collaborations, went on to make some of the most surprising discoveries in the history of particle physics. In doing so, they pointed the way to new experiments, and garnered the most prestigious accolades for Japanese physicists and their international partners.

Today, Japan is undertaking two major projects – T2K and Hyper-Kamiokande – to delve deeper into the neutrino’s properties. These and other global neutrino projects were the subject of discussions at the Third International Meeting for Large Neutrino Infrastructures, which took place at the KEK laboratory on 30–31 May (see panel below).

From Kamiokande to Super-K

Japan’s neutrino odyssey began with the Kamioka Nucleon Decay Experiment (Kamiokande), a 3000 tonne water Cherenkov experiment in the Kamioka mine in Japan’s Gifu prefecture, which started collecting data in search of proton decay in 1983. Although the experiment did not observe proton decay, it did make history with novel observations of solar neutrinos and, unexpectedly, 11 neutrino interactions from a supernova (SN1987a). These observations led to the 2002 Nobel Prize in Physics for Masatoshi Koshiba of the University of Tokyo, shared with the late Ray Davis Jr, and paved the way to a second-generation experiment.

Following Kamiokande’s success, in the 1990s, the late Yoji Totsuka led the construction of a 50,000 tonne water Cherenkov detector called Super-Kamiokande (Super-K). Like its predecessor, Super-K is also a proton-decay experiment that became famous for its measurements of neutrinos – both solar and atmospheric. Atmospheric neutrinos come from high-energy cosmic-ray interactions in the Earth’s atmosphere, predominately from charged-pion decays that result in a two-to-one mix of muon and electron neutrinos that can pass straight through the Earth before interacting in the Super-K detector.

Although the cosmic-ray flux impinging on the Earth is isotropic, Super-K data indicated that the flux of atmospheric neutrinos is not. In 1998, the Super-K collaboration showed that muon neutrinos coming from above the detector outnumbered those coming from below. The muon neutrinos that travel from the other side of the Earth transform into tau neutrinos and effectively disappear, because they are not energetic enough to interact and produce charged tau particles. This process, called neutrino oscillation, can only happen if neutrinos have mass – in contradiction to the Standard Model of particles physics. The discovery of atmospheric-neutrino oscillations led to the 2015 Nobel Prize in Physics for Super-K leader Takaaki Kajita and also Arthur McDonald of the Sudbury Neutrino Observatory (SNO) in Canada, for the concurrent observations of solar-neutrino oscillations.

From KEK to Kamioka to T2K

Two new experiments, K2K and KamLAND, were built in Japan to follow up the discovery of neutrino oscillations. The KEK-to-Kamioka (K2K) collaboration built at the KEK laboratory an accelerator-based neutrino beam aimed at Super-K, 250 km away, and also a suite of near-detectors. The collaboration solved several technical difficulties to confirm that nature’s most elusive particle was being created in their accelerator beam and was definitely interacting in Super-K, with careful comparisons from the near detectors at KEK showing that some muon neutrinos were indeed disappearing as they travelled, just as the Super-K collaboration predicted.

The Kamioka Liquid Scintillator AntiNeutrino Detector (KamLAND) experiment was built in the cavern that originally held Kamiokande, and offered sensitivity to electron antineutrinos from Japan’s nuclear reactors. KamLAND found that the neutrinos were indeed oscillating in a manner exactly consistent with the solar-neutrino oscillation observed by SNO and Super-K. These two international experiments in Japan, K2K and KamLAND, confirmed that neutrino oscillations were the explanation for the surprising observations of Super-K and SNO. But all of these experiments had seen only the disappearance of neutrinos, and it was therefore time for an experiment to observe the appearance of neutrinos.

In 2009, the Tokai-to-Kamioka (T2K) collaboration, comprising 500 scientists from 11 nations, built an experiment to observe the appearance of electron neutrinos in a muon-neutrino beam. The concept is similar to that of K2K, but with higher beam power and higher precision in the near detectors. To achieve higher beam power, T2K uses the new accelerator complex at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village on the east coast of Japan, about 70 km from KEK. The neutrino beam is directed in an off-axis configuration, which allows precise control of the neutrino energy spectrum, towards the Super-K detector 295 km away. In 2011, T2K reported the appearance of electron neutrinos from a muon-neutrino beam, which was the first significant neutrino-flavour appearance signal. For this result, which completed the picture of neutrino oscillations with the three known Standard Model neutrino flavours, T2K and K2K founding spokesperson Koichiro Nishikawa was awarded a share of the 2016 Breakthrough Prize in Fundamental Physics, along with all T2K and K2K collaboration members. The Super-K, KamLAND, SNO and Daya Bay collaborations also shared in the award.

As often happens in particle physics, the previous generation’s discovery becomes the current generation’s tool to search for new discoveries. In 2014, T2K began to operate with a muon antineutrino beam. By comparing oscillations of antineutrinos with oscillations of neutrinos, it might be possible to find a clue to one of the most profound mysteries in science: why does the universe appear to be composed entirely of matter, when it is believed that equal quantities of matter and antimatter were created in the Big Bang? Differences between the oscillations of neutrinos and antineutrinos could provide the answer to this, and, if they exist, these differences would be an example of CP violation (CPV).

In late June 2016, the T2K collaboration submitted a proposal to J-PARC requesting an extension of its neutrino (and antineutrino) data run that would give the collaboration significant (potentially 3σ) sensitivity to CPV by 2025. The physics reach of this extended run has been boosted by news that MEXT, the Japanese science funding agency, has approved the first step of an upgrade of the main-ring accelerator at J-PARC. This facility will house a new power supply system with which the repetition rate of the main ring will be doubled, resulting in 750 kW beam power for T2K with the potential to exceed 1 MW.

To Hyper-K and beyond

For discovery-level sensitivity to CPV (5σ or above), the next-generation water Cherenkov detector, Hyper-Kamiokande, is currently being designed. To reduce costs while maximising physics potential, the Hyper-K detector design has changed from the original concept of horizontal cylinders to vertical cylinders similar to Super-K, taking advantage of newly developed high-efficiency photo sensors. The international Hyper-K collaboration was formed in 2015, with agreements between the University of Tokyo’s Institute for Cosmic Ray Research (ICRR), which runs the Kamioka Observatory, and KEK.

Hyper-K, when exposed to the 1 MW J-PARC neutrino beam, will make precise measurements of neutrino and antineutrino oscillations as it searches for CPV with high significance, as well as perform the most sensitive searches for proton decay yet. Hyper-K construction could start as early as 2018, with physics data-taking in 2026. Combined with long- and short-baseline programmes in the US, Japan’s next generation of neutrino observatories should help physicists to answer most of the remaining questions about neutrino oscillations.

Planning ahead: Third International Meeting for Large Neutrino Infrastructures

A major upgrade to the T2K experiment and the ongoing design of the Hyper-K detector were among many large neutrino projects discussed at the Third International Meeting for Large Neutrino Infrastructures, which took place at the KEK laboratory on 30–31 May. The event followed the previous instance of the meeting at Fermilab in April 2015 and aims to strengthen global co-ordination on large neutrino infrastructures, not just in Japan but the world over. The third meeting, which was organised by KEK, ICRR, Fermilab, the Astroparticle Physics European Consortium (APPEC), the ICFA Neutrino Panel and the IUPAP Astroparticle Physics International Committee (APPIC), evaluated progress made since last year, discussed strategy toward the realisation of next-generation large neutrino infrastructures, and reviewed the programme of supporting measurements, prototyping and R&D. The first day of the event addressed major accelerator-based programmes worldwide, focusing on Hyper-Kamiokande and the DUNE experiment planned in the US. The second day was devoted to examining the non-accelerator physics potential of the various large neutrino infrastructures, for which it was agreed that closer co-ordination is needed. Presenting progress towards its road-map document, the ICFA Neutrino Panel discussed long-term opportunities such as the Neutrino Factory and ESSnuSB. It also identified 2020 as the approximate date when the future of sterile-neutrino searches and cross-section measurement programmes should be defined, and recommended that experiments such as nuSTORM and IsoDAR be evaluated by then. It was proposed that the next “Infra” meeting be held in Europe in 2017.

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The breakthrough results from the Super-Kamiokande and SNO experiments, which showed that neutrinos oscillate between their three flavours, marked the start of nearly two decades of tremendous progress in neutrino physics. The basic features of the three-flavour neutrino oscillation framework have been fleshed out, and the NOvA experiment in the US – which presented its latest results at the Neutrino 2016 conference in London earlier this month – is poised to address many of the remaining unknowns related to neutrino masses and their mixing.

Among these is whether neutrinos obey a “normal” or “inverted” mass hierarchy: that is, whether the mass eigenstate with the least ν_e content (called ν₃) is the heaviest or lightest of the three (see “A portal to new physics”). A second set of questions relate to the flavour admixture of the ν₃ state. Past experimental data are consistent with ν₃ being equal parts ν_μ and ν_τ, in addition to a small amount of ν_e. But is there a new symmetry that underlies this apparent ν_μ/ν_τ equality? And if the equality breaks down as measurements improve, which flavour will dominate? A third major unknown is whether neutrinos violate CP symmetry, as allowed by the complex phase δ of the leptonic mixing matrix.

Addressing the unknown

NOvA was conceived to address these unknowns using two detectors together with the intense beam of muon neutrinos provided by Fermilab’s NuMI neutrino source. NOvA’s 300 tonne near detector, which is located 1 km downstream of the neutrino source, measures the rate, energy spectrum and flavour composition of the neutrino beam prior to significant flavour oscillations, while the 14,000 tonne far detector is located 810 km downstream in northern Minnesota. The detectors are identical in their structure, consisting of 4 × 6 cm liquid scintillator-filled PVC cells in alternating planes in order to provide two orthogonal 2D views of particle trajectories.

NOvA has been collecting data with the NuMI beam since February 2014, and full operations began the following October upon completion of the far detector (CERN Courier July/August 2014 p30). As of May 2016, the experiment has accumulated 16% of its planned total. The results released at Neutrino 2016 are based on this data set, and highlighted here are the measurements of ν_μ → ν_μ (corresponding to muon-neutrino survival) and ν_μ → ν_e (electron-neutrino appearance).

To identify the flavour of an interacting neutrino, researchers look for tell-tale signs of a muon or an electron in the recorded event. Muons produced in charged-current ν_μ interactions in NOvA leave long straight tracks of detector activity that can span hundreds of cells (see the image of a muon-neutrino interaction in the NOvA detector). Electrons, in contrast, create more compact electromagnetic showers with well-characterised longitudinal and transverse profiles. An important background to both the ν_μ and ν_e charged-current channels comes from neutral-current interactions, whereby the neutrino exits the detector and leaves behind only a hadronic recoil system. Neutral pions in these recoil systems can mimic electrons, while charged pions can mimic muons.

To keep this background at bay in the ν_μ→ ν_μ measurement, each recorded track is assigned a muon likelihood based on key track features such as overall length and the rate of energy deposition. Additionally, each event must be far enough from the detector edges to ensure that the entire final state has been recorded and that the event is not due to an incoming cosmic ray. For the latest data, the NOvA team predicts 470 selected ν_μ charged current interactions in the far detector if neutrino oscillations do not occur. Only 78 such interactions were observed, in line with the well-established result originally observed by Super-Kamiokande that muon neutrinos are indeed oscillating into other flavours as they travel.

Non-maximal mixing

The real value of these data, though, comes from examining the precise energy dependence and amplitude of the ν_μ disappearance signal, which depends most strongly on the mass splitting |Δm²₃₂| and the mixing angle θ₂₃. The ranges of these parameters allowed by the latest NOvA data are shown above. NOvA’s results are consistent with prior measurements but show an intriguing preference for non-maximal mixing – that is, a preference for sin²θ₂₃ ≠ 0.5 and thus a break in the ν₃ state’s apparent flavour symmetry. Whether this preference becomes conclusive or fades away will be addressed as NOvA continues to accumulate data.

On the ν_e appearance side, distinguishing signal and background is a trickier affair due to their stronger mutual similarities and to the low probability for ν_μ→ ν_e oscillations. For its latest 2016 ν_e analysis, the NOvA team has developed an event-classification algorithm based on techniques from the image analysis community, notably convolutional neural networks and deep learning. A total of 33 candidate ν_e events were isolated in the latest data, which is far above the expected background of eight, and therefore represents a clear observation of ν_e appearance – in line with data from T2K.

The ν_μ→ ν_e oscillation probability, and therefore this measurement, is a function of several key unknowns that NOvA is pursuing. The probability will be 40 –70% higher for a normal mass hierarchy than for an inverted one, with the opposite correspondence holding for antineutrinos. This dependence stems from the so-called matter effect arising from neutrinos scattering off electrons in the Earth, and the effect is intentionally made large in NOvA by choosing the longest-distance baseline possible. Next, the phase δ of the leptonic mixing matrix can either increase or decrease the ν_μ→ ν_e probability by a similar amount, and this effect is also opposite for neutrinos and antineutrinos. Finally, the probability increases or decreases for neutrinos and antineutrinos alike, in step with sin²θ₂₃. This last dependence is complementary to the behaviour of the ν_μ→ ν_μ channel, which is better at detecting non-maximal mixing but cannot on its own distinguish which way the ν₃ flavour mixing breaks.

The present electron-neutrino appearance results, which point to a probability on the higher end of the range, have already started carving up parameter space. But NOvA must collect both neutrino and antineutrino data to disentangle all the above effects, particularly in light of possible non-maximal mixing. The first large antineutrino run for the experiment is slated to begin next spring.

In addition to accruing neutrino and antineutrino exposure for the flagship oscillation measurements, this summer’s results also included a first look at the total neutral current rate in the far detector, for which a deficit could suggest mixing with light sterile neutrinos. No deviation is seen thus far, but with both detectors operating smoothly and the NuMI source running at high power, NOvA is set to play a central role in illuminating the neutrino sector in the coming years.

• CERN Courier went to press just as Neutrino 2016 got under way. Other expected highlights of the conference include new neutrino oscillation measurements from the T2K experiment in Japan.

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The 1998 discovery that neutrinos can oscillate between different flavours, by the Super-Kamiokande experiment in Japan and subsequently by the SNO experiment in Canada, marked a turning point in our understanding of elementary particles. For many theorists, it represents the first hard particle-physics evidence for the existence of new degrees of freedom (d.o.f.) beyond the known fundamental particles and, most probably, of new physics beyond the Standard Model (SM). The hunt to uncover the new theory is the main focus of the neutrino theoretical community.

First postulated by Pauli in 1930, neutrinos have always played the role of the elusive particle. Their interactions were soon understood thanks to Fermi’s beta-decay theory, but searching for them seemed more like science fiction, at the time. In 1946, however, Pontecorvo suggested that nuclear reactors and the Sun are copious sources of neutrinos, and proposed a radiochemical method for the detection of neutrinos.

A decade later, Reines and Cowan established the neutrino’s existence by performing a reactor neutrino experiment, and the search for astrophysical neutrinos began soon afterwards. Ray Davis Jr and collaborators later detected solar neutrinos, reporting in 1972 a flux that was significantly smaller than predicted by the most sophisticated solar models developed by John Bahcall. This “solar-neutrino puzzle” was eventually explained in terms of the proposal by Bruno Pontecorvo in 1957 and 1958, applied by him to solar neutrinos in 1967, that neutrinos may oscillate between their different types. Combined with the 1962 proposal by Maki, Nakagawa and Sakata – inspired by the discovery of the muon neutrino in the well-known Brookhaven experiment – that there exists mixing between flavour and massive neutrino states, the stage was set to catch and “see” an oscillating neutrino.

Proving this elegant theoretical picture took a further 36 years of experimental innovation. In the end, it relied on the observation of atmospheric neutrinos produced by collisions of cosmic rays in the upper atmosphere, which produce showers of pions and kaons whose subsequent decays give muon and electron neutrinos. Atmospheric neutrinos were first detected in 1965 by the Kolar Gold Fields experiment in India and another experiment at the East Rand gold mine in South Africa. Then, in 1998, in a momentous discovery, Super-Kamiokande showed that muon neutrinos disappear as a function of distance travelled (see figure 1). Just a few years later, thanks to measurements of the total solar-neutrino flux, the SNO experiment confirmed that solar neutrinos can transform from electron neutrinos into muon and tau neutrinos. These two milestones and subsequent results, which have been recognised by a number of prestigious awards, ushered in a mesmerising period of new results that continues to this day.

Neutrino oscillations are the transformation of a neutrino from one flavour into another as it propagates. It is a fundamentally quantum-mechanical process arising from a misalignment of flavour states, ν_e, ν_μ and ν_τ, which describe neutrinos in production and detection, compared with the mass eigenstates, ν₁, ν₂ and ν₃. At the source, a flavour neutrino is the coherent superposition of mass states, which propagate with different phases due to their different masses. As neutrinos travel, the shift in phase results in a different combination of flavour neutrinos. The existence of neutrino oscillations necessarily requires neutrinos to have masses and to mix, which is different from the prediction of the SM (at least in its minimal form). This is why it is widely believed that neutrino oscillations are the first and so far only concrete evidence for physics beyond the SM provided by particle-physics experiments.

The decade following the discovery of neutrino oscillations did not disappoint. In 2002, the KamLAND experiment in Japan reported the first oscillations of man-made neutrinos, produced by nuclear reactors, while the K2K and MINOS experiments detected neutrinos from accelerator-produced beams. Together with the ongoing T2K and NOvA experiments, as well as atmospheric-neutrino observations at Super-Kamiokande, these experiments have established that there are two mass-squared differences that drive different neutrino oscillations and imply the existence of at least three neutrino mass eigenstates. The data also show that neutrino mixing is described by a 3 × 3 unitary matrix parameterised by three well-measured angles: θ₁₂, θ₂₃ and θ₁₃.

We have now an incredibly rich picture of neutrino properties that was unthinkable at the time of the Super-Kamiokande results and that is very different from that of the quarks. But despite these immense achievements, we are still in need of crucial pieces of information to reach a complete understanding of neutrino properties.

The most important question about neutrinos concerns the type of masses they have. So far, all the known fermions are of the Dirac type: their particles and antiparticles have opposite charges and they possess a Dirac mass that arises from the coupling to the Higgs field. Neutrinos could behave in the same way, but because they are electrically neutral it is possible that neutrinos acquire mass via a different mechanism. Indeed, neutrinos and antineutrinos might be indistinguishable, constituting what is called a Majorana particle after Ettore Majorana who proposed the concept in 1937. Unlike Dirac fields, which have four components, Majorana fields have only two d.o.f. Such a particle cannot possess any charge, not even a lepton number.

A matter of conservation

The question of the nature of neutrinos is therefore intrinsically related to the conservation of the lepton number. In the SM, the lepton number is a global accidental symmetry that happens to be preserved thanks to the gauge symmetries and particle content, but it does not have a dynamic role because there are no associated gauge bosons. The question arises whether the ultimate theory of particles and their interactions is lepton-number violating or not. The most promising way to answer this question is to search for neutrinoless double-beta decay, whereby certain nuclei spontaneously undergo two beta decays at once, without producing any neutrinos. This process directly violates lepton-number conservation and would imply that neutrinos are Majorana particles, motivating a broad international experimental programme (see panel below).

A second major question is whether the CP symmetry is violated in the lepton sector, as it is in the quark one. CP violation is one of the three key ingredients in baryogenesis and leptogenesis, which are needed to dynamically explain the observed matter–antimatter asymmetry of the universe (see panel below). There are three possible sources of CP violation in the lepton sector: the Dirac phase, which is the analogue of the one in the quark sector, and two Majorana phases that appear only if neutrinos are Majorana particles. If neutrinos are Dirac particles, the latter can be rotated away as is done in the quark sector.

The first hints of leptonic CP violation came recently from combining data from China’s Daya Bay experiment with measurements at long-baseline accelerator facilities, in particular T2K and NOvA. These seem to indicate a preference for a nonzero value of the CP-violating Dirac phase (see figure 2). It is too early to tell, but very ambitious plans – including the proposed Deep Underground Neutrino Experiment (DUNE) in the US and T2HK in Japan – aim to settle the issue by allowing both neutrino and antineutrino oscillations to be studied. The latter behave differently if Dirac CP violation is present, with oscillations that are being enhanced or suppressed, depending on the values of the Dirac phase.

The other mixing parameters, namely the three mixing angles, are already quite well-determined. Angle θ₁₃ went from being unknown just over four years ago to being the best-measured, thanks to results from the Daya Bay as well as RENO and Double Chooz experiments, while the JUNO experiment in China plans to reach a sub-per-cent accuracy for the θ₁₂ angle after a few years of operation. θ₂₃ is particularly interesting because it could be exactly maximal, therefore pointing towards a symmetry in the lepton-flavour sector, or could deviate from this by several degrees. Current and future long-baseline oscillation experiments will have the best chance of determining θ₂₃, which will be critical for disentangling the different models proposed to explain the observed mixing pattern.

Massive considerations

As for the values of the neutrino masses themselves, we already have a very precise measurement of the absolute values of the two mass-squared differences – which differ by a factor of about 30 (figure 2). But we still lack key pieces of information, namely which neutrino is the lightest, defining the neutrino mass ordering, and what its mass scale is. The sign of the solar mass-squared difference is determined by solar-neutrino oscillations, but that of the atmospheric one is unknown. If it turns out to be positive, corresponding to m₃ > m₁, neutrino masses exhibit the so-called “normal” ordering. The alternative scenario, m₃ < m₁, implies an “inverted” ordering (figure 3).

Knowing the mass ordering and scale is important for theorists because different theoretical models predict different patterns, and also for experimentalists searching for specific signatures. It strongly affects the rate of neutrinoless double-beta decay, substantially impacting on the prospects of discovering the Majorana nature of neutrinos, while in the early universe heavier neutrinos suppress the growth of large-scale structures at small scales. The ordering of the masses also changes the way in which neutrinos propagate over long distances in media such as the Earth, due to weak interactions with the background of electrons, protons and neutrons. This gives neutrinos an effective mass that modifies their energies and the mixing: neutrino oscillations are enhanced for normal mass ordering and suppressed for inverted ordering, with the opposite happening in the case of antineutrinos.

Experiments such as the long-baseline experiment NOvA, which measures a neutrino beam produced 810 km away at Fermilab, exploit these effects to hunt for the neutrino mass ordering (see “NOvA releases new bounds on neutrino mixing parameters”). With DUNE, which will operate at a distance of 1300 km, and new atmospheric-neutrino observatories such as PINGU, ORCA and INO, as well as JUNO, we expect to resolve this issue in the next 5–10 years.

However, even knowing the neutrino mass ordering still leaves open the question of the overall neutrino mass scale. So far, we know that neutrino masses cannot be too large. They are restricted to be smaller than 2.2 eV by the Troitsk and Mainz experiments, and well below this limit if one considers cosmological observations, which suggest a conservative bound on the sum of the masses of around 0.7 eV in the standard cosmological model. The KArlsruhe TRItium Neutrino (KATRIN) experiment currently being commissioned in Germany aims to determine the absolute mass scale by searching for a small deformation of the electron energy spectrum in beta decays and will be sensitive to neutrino masses as small as 0.2 eV. It is expected to take data very soon.

The standard neutrino picture comprises three neutrino flavour states and correspondingly three light-mass eigenstates. In many extensions of the SM this is not the case because new degrees of freedom and/or new interactions can be added (see panel below). The simplest extension is that of sterile-neutrinos, which do not experience SM interactions. The corresponding nearly sterile neutrino mass could take any value, from the very small to the GUT scale, but in many phenomenological studies is around the eV scale and therefore could induce short-baseline oscillations.

Results from the LSND and MiniBooNE experiments in the US, as well as some reactor neutrino ones, have hinted at precisely such a signal. But the results are still controversial, and there is tension with other searches of sterile-neutrinos from short-baseline muon neutrino experiments. New short-baseline reactor and radioactive-source neutrino experiments, in addition to the dedicated short-baseline accelerator programme at Fermilab involving the MicroBooNE, ICARUS and SBND detectors, will shed light on these results and possibly hunt for nearly sterile-neutrinos with even smaller mixing angles. A positive signal would be groundbreaking, forcing us to rethink the theoretical framework for light neutrinos and posing new questions about the nature, masses, mixing and CP-violating properties of the new states.

Indeed, neutrinos remain the most intriguing and elusive of all known fermions and are an ideal portal to explore new physics beyond the Standard Model. Despite impressive progress in the past 20 years, going from not knowing if neutrino oscillations took place to having measured most of the oscillation parameters with great precision, many key phenomenological and theoretical questions remain open and urgently require answers. Fortunately, a broad and exciting experimental programme is under way and, as is often the case in research, the focus of our theoretical work could change in an instant. With the LHC now into its high-energy run, for example, it is possible that we will discover entirely new particles and phenomena beyond the SM. We would then need to establish what connection – if any – exists between this and the already new physics of neutrino masses. Or perhaps neutrino masses come from a secluded sector, possibly at energy scales so high that we cannot test it directly. These and many other questions, informed by the current wealth of new and upcoming experiments, promise to keep neutrino theorists occupied for the foreseeable future.

Searching for the neutrino’s fundamental nature

Two-neutrino double beta decay (DBD) is a very rare Standard Model process that causes two neutrons simultaneously to decay into two protons and two electrons, with the emission of two electron antineutrinos. If neutrinos are Majorana particles, however, instead of being emitted, the Majorana neutrinos can mediate a new process called neutrinoless double-beta decay (NDBD), which is not allowed in the Standard Model. Observing this process would be groundbreaking because it would imply that the lepton number is violated and provide crucial information about neutrino masses. More than a dozen experiments worldwide are searching for NDBD which, like DBD, can be observed in nuclei in which ordinary beta decay is kinematically forbidden. Because NDBD produces no neutrinos to carry off energy, all events will be concentrated at the end point of the two-electron energy spectrum – unlike the case for DBD, in which the spectrum is a continuum. Being an extremely rare process, NDBD searches require sufficiently large detector volumes, very good energy resolution, a location deep underground and extremely low backgrounds. A number of different experimental techniques are being employed. Liquid-scintillator detectors such as KamLAND-Zen in Japan and SNO+ in Canada offer large target masses, and currently KamLAND-Zen provides the strongest bound on NDBD with a half-life greater than 1.1 × 1026 years. Germanium detectors such as GERDA and MAJORANA are more compact and ensure very good energy resolution, while planned experiments such as SuperNEMO and DCBA can track both electrons and could reconstruct their angular distribution. Time projection chambers, such as nEXO in the US and NEXT in Spain, can simultaneously track the electrons and allow large target volumes, while bolometers such as CUORE and AMoRE benefit from very high energy resolution. Despite this impressive armoury, NDBD hunters are at the mercy of the neutrino masses and mixing parameters (see main text). The NDBD rate depends crucially on the combination of masses and mixing parameters, the so-called effective Majorana mass parameter. If neutrinos exhibit an inverted mass ordering, the predicted lower bound on the decay rate will be just within reach of the next-to-next generation of experiments. If they adopt the normal mass ordering, the decay rate could be anywhere between the current bounds and zero, if a specific cancellation between the three massive neutrinos is at work (see figure).

Messengers from beyond the Standard Model

The origin of neutrino masses and mixing is still unresolved, and necessarily requires new degrees of freedom and new interactions. The simplest extension of the Standard Model assumes the existence of right-handed (RH) neutrinos, which behave as singlets with respect to the Standard Model gauge group. Unless specific symmetries are imposed, Yukawa couplings with the lepton doublet and the Higgs will be allowed and the lepton number will be preserved. Dirac masses therefore arise for neutrinos as they do for all the other known fermions, but this mechanism provides no insight as to why neutrino masses are so small (the Yukawa coupling needed would be 12 orders of magnitude smaller than that of the top quark). One could simply accept such extreme fine-tuning as a fact of nature, but this would naively lead one to expect the same mixing in the lepton sector as in the quark one and a similar mass ordering, neither of which is observed. The alternative option is that neutrinos are Majorana particles. Majorana neutrinos will have a mass term in the Lagrangian that breaks lepton-number conservation. Although this mass term is forbidden by the gauge group of the SM, it could arise as the low-energy realisation of a higher-energy theory. This can explain both the existence of neutrino masses and their smallness, because a strong suppression is induced by the new heavy scale. Theorists are working hard to understand what the new theory at high energy might be. The ultimate theory behind neutrino masses must also explain the observed mixing structure, the presence of CP violation (if observed), and why the lepton sector contains large angles that are different to the quark sector. Many approaches have been proposed, for instance the use of continuous or discrete flavour symmetries, but no unique underlying principle has yet been identified. The simplest and most studied extension beyond the SM for neutrino masses is the “see-saw type I” mechanism. Because RH neutrinos are completely neutral with respect to the SM gauge symmetries, they could be much heavier than the other known fermions. The Lagrangian would then contain both a Yukawa coupling with the Higgs, as for the quarks, and a Majorana mass term, M, for the RH neutrinos. Once the neutral Higgs boson gets a vacuum expectation value, light masses for the neutrinos arise that are proportional to the square of the Yukawa couplings and suppressed by M. Taking an order-one Yukawa coupling and M of around 1014 GeV, we obtain a sub-eV neutrino mass scale as required by the data and, because the lepton number is violated by M, the light neutrinos will be Majorana particles. This is by no means the only way to give origin to neutrino masses. First of all, since the RH neutrino masses can take any value, the scale of the see-saw mechanism could be lowered even below the electroweak scale, allowing some models to be tested at the LHC. Typical signatures are same-sign dileptons with no missing energy, indicating lepton-number violation, and flavour-violating multi-lepton events. Several searches have been conducted by the LHC’s ATLAS and CMS collaborations, but so far no positive hint has been found. The heavy particles responsible for the see-saw mechanism could also be different: a fermion triplet in see-saw type III and a scalar triplet in see-saw type II models. Some models, such as radiative and R-parity-violating supersymmetric models, do not invoke the see-saw mechanism at all. With so many possibilities, clearly one needs to hunt for other beyond-SM signatures to try to identify the origin of neutrino masses. Leptogenesis is a key one. To generate dynamically a baryon asymmetry in the early universe, the three Sakharov conditions need to be satisfied: lepton or baryon number violation, C and CP violation, and an out-of-equilibrium state (satisfied by the expansion of the universe). The see-saw mechanism can satisfy all of these conditions. In the early universe, RH neutrinos got out of equilibrium once the temperature dropped below their mass. Thanks to their decays into leptons and Higgs bosons, a net lepton asymmetry could arise if the rate in one channel and the conjugated one are different due to CP violation. This asymmetry would then be converted into a baryon asymmetry by non-perturbative SM effects. Observing CP violation in future neutrino-oscillation experiments and lepton-number violation in neutrinoless double-beta-decay searches would therefore provide strong hints that leptogenesis is at the origin of the baryon asymmetry of the universe.

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The IceCube experiment at the South Pole has been one of the pioneers of the field of neutrino astronomy. During a seven-year-long construction campaign that ended in 2010, the 325 strong IceCube collaboration transformed a cubic kilometre of ultra-transparent Antarctic ice into a giant Cherenkov detector. Today, 5160 optical sensors are suspended beneath the ice to detect Cherenkov light from charged particles produced when high-energy neutrinos from the cosmos interact with nuclei in the detector. So far, IceCube has detected neutrinos with energies in the range 10¹¹–10¹⁶ eV, which include the most energetic neutrinos ever recorded (see image of the proposed Gen2 array). However, we do not yet know where these neutrinos come from. For this reason, the IceCube collaboration is developing designs for an expanded “Gen2” detector.

IceCube observes astrophysical neutrinos in two ways. The first approach selects upgoing events by using the Earth to filter out the large flux of cosmic-ray muons. At low energies (below 100 TeV), the measured flux of muon neutrinos is consistent with an atmospheric origin, whereas at higher energies, a clear excess of events with a significance of 5.6σ is observed. The second approach selects neutrinos that interact inside the detector. A total of 54 cosmic-neutrino events with energies ranging from 30–2000 TeV were detected during four years of operation, excluding a purely atmospheric explanation at the level of 6.5σ. Although there is some tension between the results from the two approaches, a combined analysis finds that the data are consistent with an at-Earth flux equally shared between three neutrino flavours, as is expected for neutrinos originating in cosmic sources.

Towards a new detector

Despite multiple searches for the locations of these sources, however, the IceCube team has yet to find any statistically significant associations. Searches for neutrinos from gamma-ray bursts and some classes of galaxies have also come up empty. Although these observations have disfavoured many promising models of the origin of cosmic rays, the ultimate goal of neutrino astronomy is to detect multiple neutrinos from a single source. This requires many hundreds of events, which would take an array of the scale of IceCube at least 20 years to detect.

To speed up data collection, an expanded IceCube collaboration is planning a greatly enhanced instrument (see image of the proposed Gen2 array) with multiple elements: an enlarged array to search for high-energy astrophysical neutrinos; a dense infill array to determine the neutrino properties (PINGU); a larger surface air-shower array to veto downgoing atmospheric neutrinos; and possibly an array of radio detectors targeting neutrinos with energies above 10¹⁷ eV. Most importantly, thanks to the clarity of the Antarctic ice, we would be able to increase the instrumented volume of this next-generation array by a factor of 10 without a corresponding increase in the number of deployed sensors – or in the cost. The Gen2 proposal would therefore see an instrumented volume of approximately 10 km³ comprising strings of optical modules, but with improved hardware and deployment methods compared with IceCube.

For the in-ice component PINGU (Precision IceCube Next Generation Upgrade), the Gen2 collaboration is exploring a number of optimised designs for the optical modules, as well as longer strings deployed with improved drilling methods. Photomultipliers (PMTs) with higher quantum efficiency will be used, as is already the case for DeepCore in IceCube, and pressure spheres with improved glass and optical gel will improve sensitivity by transmitting more ultraviolet Cherenkov light. Some designs include more than one phototube per optical module (see image), while more radical concepts envision the addition of long cylindrical wavelength shifters to improve information about the photon arrival direction. Many-PMT designs were pioneered by the KM3NeT collaboration, which is proposing to build a cubic-kilometre-sized European neutrino Cherenkov telescope in the Mediterranean Sea, but are also attractive to IceCube.

The increased complexity of these approaches would be offset by new electronics, and increased computing power will allow the use of more sophisticated software algorithms that better account for the positional dependence of the optical properties of the ice and the stochastic nature of muon energy loss. This will result in improved pointing and energy resolution of both tracks and showers and better identification of tau neutrinos. IceCube has produced a white paper for the Gen2 proposal (arXiv:1412.5106) that fits well with the US National Science Foundation’s recent identification of multi-wavelength astronomy as one of six future priorities, and a formal proposal will be completed in the next few years.

Physics in order

PINGU will build on the success of DeepCore in measuring atmospheric neutrino-oscillation parameters. It consists of a dense infill array in the centre of DeepCore with a threshold of a few GeV, allowing the ordering of the neutrino masses to be determined by matter-induced oscillations of the atmospheric neutrino flux. By precisely measuring the oscillation probability as a function of neutrino energy and zenith angle, PINGU will be able to determine which neutrino is lightest.

Like the present IceTop (a surface air-shower array that covers IceCube’s surface), an expanded surface array will tag and veto downgoing atmospheric neutrinos that are accompanied by cosmic-ray air showers. Current Gen2 designs envision a 75 km² surface array that would allow IceCube to collect a clean sample of astrophysical neutrinos over a much larger solid angle, including the galactic centre. It will also result in much improved cosmic-ray studies and more sensitive searches for PeV photons from galactic sources. To study the highest-energy (above typically 10¹⁷ eV) neutrinos, Gen2 may also include an array of radio detectors to observe the coherent radio Cherenkov emission from neutrino-induced showers. Radio detection is now pursued by the ARA (the Askaryan Radio Array at the South Pole) and ARIANNA (located on Antarctica’s Ross Ice Shelf) experiments, but coincident observations with IceCube Gen2 would be preferable.

Of course, IceCube is not the only neutrino telescope in town. ANTARES has been taking data in the Mediterranean Sea since 2008 and will be followed by KM3NeT (CERN Courier March 2016 p12), while the Gigaton Volume Detector (Baikal-GVD) is currently being built in Lake Baikal, Russia (CERN Courier July/August 2015 p23). Seawater, lake water and Antarctic ice present different challenges and advantages to cosmic-neutrino observatories, and sites in the Northern Hemisphere benefit because the galactic centre is below the horizon. While we all benefit from friendly competition and from sharing R&D resources, size has undeniable advantages. IceCube-Gen2, should the project go ahead, will be larger than any of the proposed alternatives, and is therefore well placed to write the next chapter in neutrino astronomy.

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When I meet Jack in his office in Building 2, he has just returned from a “splendid” birthday celebration – a classical-music concert “with a lady conductor”, he is quick to add. It had been organised by members of his town of birth, Bad Kissingen in South Germany, and was held at the local gymnasium that bears his name. Steinberger’s memories of the town are those of a 13 year-old child in pre-war Germany during the Nazi election propaganda. “Hitler was psychopathic when it came to Jews,” he says. “In making me leave, however, he did me a great favour because I had a wonderful education in America.”

Talking to this extraordinary man and physicist – who is too modest to dwell on the 1962 discovery of the muon neutrino that won him, Leon Lederman and Melvin Schwartz the 1988 Nobel Prize in Physics – is like taking a trip back in the history of particle physics. With the help of a scholarship from the University of Chicago, Steinberger completed a first degree in chemistry in 1942. He owes his first contact with physics to Ed Purcell and Julian Schwinger, with whom he worked at the MIT radiation laboratory where he had been assigned a military role in 1941 – the year that Japan attacked the US at Pearl Harbour.

“We were making bombsights for bombers, something that could be mounted on airplanes and could see the ground with radar and so you could find military targets,” he explains. “The bombsight we succeeded in developing had a very limited accuracy and you couldn’t see a military target, but you could see cities.” With a heavy heart, Steinberger adds that the radar system was used in the infamous Dresden bombing. “That was my contribution during the war,” he states flatly.

The Fermi years

When the war ended, Steinberger went back to Chicago with the intention of completing a thesis in theoretical physics. Then he met Enrico Fermi. “Fermi was the biggest luck I had in my life!” he exclaims, with a spark in his striking blue eyes. “He asked me to look into a problem raised by an experiment by Rossi and Sands on stopping cosmic-ray muons, and suggested that I do an experiment instead of waiting for a theoretical topic to surface,” recalls Steinberger. At the time, most experiments required just a handful of Geiger counters and a detector measuring about 20 cm long, he says. “The experiment I wanted to do required 80 of those and was 50 cm long, so it was not trivial to build it.”

It was the time before computers, when vacuum tubes were the height of technology, and Fermi had identified the resources required in the physics department of the University of Chicago. Once the experiment was up and running, however, Fermi suggested it would produce results more quickly if it were located on top of a mountain, where there would be more mesons from cosmic rays. “He found a young driver – I didn’t know how to drive, it was the beginning of cars – who took me to the only mountain in the US with a road to the top,” says Steinberger. “It was almost as high as Mt Blanc, and I could do the experiment faster by being on top of that thing.”

The experiment showed that the energy spectrum of the electron in certain meson decays is continuous. It suggested that the muon undergoes a three-body decay, probably into an electron and two neutrinos, and helped to lay the experimental foundation for the concept of a universal weak interaction. What followed is history, leading to the discovery of the muon neutrino (see “DUMAND and the origins of large neutrino detectors”). “It is likely that we had no prejudice on the question of whether the neutrino in muon decay is the same as the one in beta decay.”

Apart from the discovery of the muon neutrino, Steinberger’s pioneering work in physics overlaps 40 years of history of electroweak theory and experiment. At each turn of a decade, Steinberger was the first user of the latest device available for experimentalists, starting with McMillan’s electron synchrotron when it had just been completed in 1949, or Columbia’s 380 MeV cyclotron in 1950. In 1954, he published the first bubble-chamber paper with Leitner, Samios and Schwartz, making a substantial contribution to the technique itself and achieving important results on the properties of the new unstable (strange) particles.

Lasting legacy

What brought Steinberger to CERN in 1968 was the availability of Charpak’s wire chamber, which he realised was a much more powerful way to study K⁰ decays – to which he says he had “become addicted”. Then he conceived and led the ALPEH experiment at the Large Electron–Positron (LEP) collider. The results of this and the other LEP experiments, he says, “dominated CERN physics, perhaps the world’s, for a dozen or more years, with crucial precise measurements that confirmed the Standard Model of the unified electroweak and strong interactions”.

These days, Jack still comes to CERN with the same curiosity for the field that he always had. He says he is “trying to learn astrophysics, in spite of my mental deficiencies”, and thinks that the most interesting question today is dark matter. “You have a Standard Model which does not predict everything and it does not predict dark matter, but you can conceive of mechanisms for making dark matter in the Standard Model,” he says. “You don’t know if you really understand it, but you can imagine it. And I am not the only one who doesn’t know.”

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CERN and Fermilab have a rich history of scientific accomplishment. Fermilab, which is currently the only US laboratory fully devoted to particle physics, tends to favour fermions: the top and bottom quarks were discovered here, as was the tau neutrino. CERN seems to prefer bosons: the W, Z and Higgs bosons were all discovered at the European lab. Both labs also have ambitious plans for the future that build on a history of close collaboration. A recent example is the successful test of a novel high-field quadrupole superconducting magnet made from Nb₃Sn as part of the R&D programme for the High-Luminosity Large Hadron Collider (HL-LHC). The highly successful team behind this technology (the Fermilab-led LHC Accelerator Research Programme, which includes Berkeley and Brookhaven national labs) is also committed to developing 16 T magnets for a high-energy LHC and a possible larger circular collider.

Our laboratories and their global communities are now moving even closer together. At a ceremony held at the White House in Washington, DC in May 2015, representatives from the US Department of Energy (DOE), the US National Science Foundation and CERN signed a co-operation agreement for continued joint research in particle physics and computing, both at CERN and in the US. This was followed by a ceremony at CERN in December, at which the US ambassador to the United Nations and the former CERN Director-General signed five formal agreements that will serve as the framework for future US–CERN collaboration. The new agreements enable US scientists to continue their vital contribution to the LHC and its upgrade programme, while for the first time enabling CERN participation in experiments hosted in the US.

The US physics community and DOE are committed to the success of CERN. Physicists migrated from the US to CERN en masse following the 1993 cancellation of the Superconducting Super Collider. In 2008, lack of clarity about the future of US particle physics contributed to budget cuts, which together brought us to a low point for our field. These painful periods taught us that a unified scientific community and strong partnerships are vital to success.

Fortunately, the tides have now turned, in particular thanks to two important planning reports. The first was the 2013 European Strategy Report, which for the first time recommended that CERN supports physics programmes, particularly regarding neutrinos, outside of its laboratory. The following year, this bold proposal led the US Particle Physics Project Prioritisation panel to strongly recommended a continued partnership with CERN on the LHC and to pursue an ambitious long-baseline neutrino programme hosted by Fermilab, for which international participation and contributions are vital.

CERN’s support and European leadership are critical to the success of the ambitious Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) being hosted by Fermilab. In partnership with the Italian Institute for Nuclear Physics, CERN is also upgrading the ICARUS detector for our short-baseline neutrino programme. Thanks largely to this partnership with CERN, the US particle-physics community is now enjoying a sense of optimism and increasing budgets.

Fermilab and CERN have always worked together at some level, but the high-level agreements between CERN and the DOE will reach decades into the future. CERN recognises the extensive technical capability of Fermilab and the US community, which are currently working to help upgrade CMS and ATLAS as well as accelerator magnets for the HL-LHC, while the US recognises CERN’s leadership in high-energy collider physics, and more than 1000 US physicists call CERN their scientific home.

Yet, not everyone agrees that our laboratories should be intertwined. Some in the US think too much money is sent abroad and believe that the funds could be used for particle physics at “home”, or for other uses entirely. On the other side of the Atlantic, some might wonder why they should work outside of CERN or, worse, outside of Europe. These views are short-sighted. The best science is best achieved through collaborative global partnerships. For this reason, CERN and Fermilab will be intertwined for a long time to come.

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The planned Deep Underground Neutrino Experiment (DUNE) (CERN Courier December 2015 p19) will require 70,000 tonnes of liquid argon, making it the largest experiment of its kind – 100 times larger than the liquid-argon particle detectors that came before. Scientists recently began taking data using a 35 tonne test version of their detector – a significant step towards building the four massive detectors at the Sanford Underground Research Facility (SURF), which will hold the 70,000 tonnes of liquid argon.

Built at the Department of Energy’s Fermi National Accelerator Laboratory, the 35 tonne prototype allows researchers to check that the various detector elements are working properly and to start formal studies. Scientists also use the prototype to assess detector components that have not been tried before. The new parts include redesigned photodetectors – long rectangular prisms with a special coating that changes invisible light to a visible wavelength and bounces the collected light to the detector’s electronic components.

DUNE scientists are also paying special attention to the prototype’s wire planes – pieces that hold the thin wires strung across the detector to pick up electrons. To ensure the frames will fit down the narrow mineshaft at SURF and avoid having to stretch the wires across the long DUNE detectors, risking sagging, scientists plan to use a series of independent 6 m-long and 2.3 m-wide frames. These wire planes should measure tracks in the liquid argon, both in front of and behind them, unlike other detectors.

Engineers have also moved some of the detector’s electronic parts inside the cryostat, which holds liquid argon at –184 °C.

Much like the full detectors, development of the components of the 35 tonne prototype depends on teamwork. For the prototype, Brookhaven and SLAC national laboratories in the US provided much of the electronic equipment; Indiana University, Colorado State University, Louisiana State University and Massachusetts Institute of Technology worked on the light detectors; and the universities of Oxford, Sussex and Sheffield helped to make special digital cameras that can survive in liquid argon, and wrote the software to make sense of the data. Fermilab was responsible for the cryostat and cryogenic support systems.

Scientists will use what they learn from this small prototype version to build one of the full-scale modules for a larger, 400 tonne prototype currently under construction at the CERN Neutrino Platform. A second 400 tonne module using dual-phase technology will also be built at CERN. These will be the final tests before installation of the four huge detectors at SURF for the actual experiment, which is scheduled to start in 2021/2022.

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In the early morning of 3 December, scientists and engineers started the installation of KM3NeT (CERN Courier July/August 2012 p31). Once completed, it will be the largest detector of neutrinos in the Northern Hemisphere. Located in the depths of the Mediterranean Sea, the infrastructure will be used to study the fundamental properties of neutrinos and to map the high-energy cosmic neutrinos emanating from extreme cataclysmic events in space.

Neutrinos are the most elusive of elementary particles and their detection requires the instrumentation of enormous volumes: the KM3NeT neutrino telescope will occupy more than a cubic kilometre of seawater. It comprises a network of several hundred vertical detection strings, anchored to the seabed and kept taut by a submerged buoy. Each string hosts 18 light-sensor modules, equally spaced along its length. In the darkness of the abyss, the sensor modules register the faint flashes of Cherenkov light that signal the interaction of neutrinos with the seawater surrounding the telescope.

On board the Ambrosius Tide deployment boat, the first string – wound, like a ball of wool, around a spherical frame – arrived at the location of the KM3NeT-Italy site, south of Sicily. It was anchored to the seabed at a depth of 3500 m and connected to a junction box, already present on the sea floor, using a remotely operated submersible. The junction box is connected by a 100 km cable to the shore station located in Portopalo di Capo Passero in the south of Sicily.

After verification of the quality of the power and fibre-optic connections to the shore station, the go-ahead was given to trigger the unfurling of the string to its full 700 m height. During this process, the deployment frame is released from its anchor and floats towards the surface while slowly rotating. In doing so, the string unwinds from the spherical frame, eventually leaving behind a vertical string. The string was then powered on from the shore station, and the first data from the sensor modules started streaming to shore.

The successful acquisition of data from the abyss with the novel technology developed by the KM3NeT collaboration is a major milestone for the project. It represents the culmination of more than 10 years of research and development by the many research institutes that make up the international collaboration.

The post Construction of KM3NeT, a next-generation neutrino telescope, has begun appeared first on CERN Courier.

SHiP is an experiment aimed at exploring the domain of very weakly interacting particles and studying the properties of tau neutrinos. It is designed to be installed downstream of a new beam-dump facility at the Super Proton Synchrotron (SPS). The CERN SPS and PS experiments Committee (SPSC) has recently completed a review of the SHiP Technical and Physics Proposal, and it recommended that the SHiP collaboration proceed towards preparing a Comprehensive Design Report, which will provide input into the next update of the European Strategy for Particle Physics, in 2018/2019.

Why is the SHiP physics programme so timely and attractive? We have now observed all the particles of the Standard Model, however it is clear that it is not the ultimate theory. Some yet unknown particles or interactions are required to explain a number of observed phenomena in particle physics, astrophysics and cosmology, the so-called beyond-the-Standard Model (BSM) problems, such as dark matter, neutrino masses and oscillations, baryon asymmetry, and the expansion of the universe.

While these phenomena are well-established observationally, they give no indication about the energy scale of the new physics. The analysis of new LHC data collected at √ = 13 TeV will soon have directly probed the TeV scale for new particles with couplings at O(%) level. The experimental effort in flavour physics, and searches for charged lepton flavour violation and electric dipole moments, will continue the quest for specific flavour symmetries to complement direct exploration of the TeV scale.

However, it is possible that we have not observed some of the particles responsible for the BSM problems due to their extremely feeble interactions, rather than due to their heavy masses. Even in the scenarios in which BSM physics is related to high-mass scales, many models contain degrees of freedom with suppressed couplings that stay relevant at much lower energies.

Given the small couplings and mixings, and hence typically long lifetimes, these hidden particles have not been significantly constrained by previous experiments, and the reach of current experiments is limited by both luminosity and acceptance. Hence the search for low-mass BSM physics should also be pursued at the intensity frontier, along with expanding the energy frontier.

SHiP is designed to give access to a large class of interesting models. It has discovery potential for the major observational puzzles of modern particle physics and cosmology, and can explore some of the models down to their natural “bottom line”. SHiP also has the unique potential to test lepton flavour universality by comparing interactions of muon and tau neutrinos.

SPS: the ideal machine

SHiP is a new type of intensity-frontier experiment motivated by the possibility to search for any type of neutral hidden particle with mass from sub-GeV up to O(10) GeV with super-weak couplings down to 10^–10. The proposal locates the SHiP experiment on a new beam extraction line that branches off from the CERN SPS transfer line to the North Area. The high intensity of the 400 GeV beam and the unique operational mode of the SPS provide ideal conditions. The current design of the experimental facility and estimates of the physics sensitivities assume the SPS accelerator in its present state. Sharing the SPS beam time with other SPS fixed-target experiments and the LHC should allow 2 × 10²⁰ protons on target to be produced in five years of nominal operation.

The key experimental parameters in the phenomenology of the various hidden-sector models are relatively similar. This allows common optimisation of the design of the experimental facility and of the SHiP detector. Because the hidden particles are expected to be predominantly accessible through the decays of heavy hadrons and in photon interactions, the facility is designed to maximise their production and detector acceptance, while providing the cleanest possible environment. As a result, with 2 × 10²⁰ protons on target, the expected yields of different hidden particles greatly exceed those of any other existing and planned facility in decays of both charm and beauty hadrons.

As shown in the figure (left), the next critical component of SHiP after the target is the muon shield, which deflects the high flux of muon background away from the detector. The detector for the hidden particles is designed to fully reconstruct the exclusive decays of hidden particles and to reject the background down to below 0.1 events in the sample of 2 × 10²⁰ protons on target. The detector consists of a large magnetic spectrometer located downstream of a 50 m-long and 5 × 10 m-wide decay volume. To suppress the background from neutrinos interacting in the fiducial volume, the decay volume is maintained under a vacuum. The spectrometer is designed to accurately reconstruct the decay vertex, mass and impact parameter of the decaying particle at the target. A set of calorimeters followed by muon chambers provide identification of electrons, photons, muons and charged hadrons. A dedicated high-resolution timing detector measures the coincidence of the decay products, which allows the rejection of combinatorial backgrounds. The decay volume is surrounded by background taggers to detect neutrino and muon inelastic scattering in the surrounding structures, which may produce long-lived SM V⁰ particles, such as K_L, etc. The experimental facility is also ideally suited for studying interactions of tau neutrinos. The facility will therefore host a tau-neutrino detector largely based on the Opera concept, upstream of the hidden-particle decay volume (CERN Courier November 2015 p24).

Global milestones and next steps

The SHiP experiment aims to start data-taking in 2026, as soon as the SPS resumes operation after Long Shutdown 3 (LS3). The 10 years consist, globally, of three years for the comprehensive design phase and then, following approval, a bit less than five years of civil engineering, starting in 2021, in parallel with four years for detector production and staged installation of the experimental facility, and two years to finish the detector installation and commissioning.

The key milestones during the upcoming comprehensive design phase are aimed at further optimising the layout of the experimental facility and the geometry of the detectors. This involves a detailed study of the muon-shield magnets and the geometry of the decay volume. It also comprises revisiting the neutrino background in the fiducial volume, together with the background detectors, to decide on the required type of technology for evacuating the decay volume. Many of the milestones related to the experimental facility are of general interest beyond SHiP, such as possible improvements to the SPS extraction, and the design of the target and the target complex. SHiP has already benefitted from seven weeks of beam time in test beams at the PS and SPS in 2015, for studies related to the Technical Proposal (TP). A similar amount of beam time has been requested for 2016, to complement the comprehensive design studies.

The SHiP collaboration currently consists of almost 250 members from 47 institutes in 15 countries. In only two years, the collaboration has formed and taken the experiment from a rough idea in the Expression of Interest to an already mature design in the TP. The CERN task force, consisting of key experts from CERN’s different departments, which was launched by the CERN management in 2014 to investigate the implementation of the experimental facility, brought a fundamental contribution to the TP. The SHiP physics case was demonstrated to be very strong by a collaboration of more than 80 theorists in the SHiP Physics Proposal.

The intensity frontier greatly complements the search for new physics at the LHC. In accordance with the recommendations of the last update of the European Strategy for Particle Physics, a multi-range experimental programme is being actively developed all over the world. Major improvements and new results are expected during the next decade in neutrino and flavour physics, proton-decay experiments and measurements of the electric dipole moments. CERN will be well-positioned to make a unique contribution to exploration of the hidden-particle sector with the SHiP experiment at the SPS.

• For further reading, see cds.cern.ch/record/2007512.

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MicroBooNE, an experiment designed to measure neutrinos and antineutrinos generated by Fermilab’s Booster accelerator (CERN Courier September 2014 p8), has recorded its first neutrino events. MicroBooNE is the first of three neutrino detectors of the lab’s new short-baseline neutrino (SBN) programme, recommended by the 2014 report of the US Particle Physics Project Prioritization Panel (P5). The ICARUS detector (being refurbished at CERN) as far detector, MicroBooNE as intermediate detector and SBND as near detector will compose the SBN project.

Designed to search for sterile neutrinos and other new physics phenomena in low-energy neutrino oscillations, the SBN programme aims to confirm or refute the hints of a fourth type of neutrino first reported by the LSND collaboration at Los Alamos National Laboratory, and resolve the origin of a mysterious low-energy excess of particle events seen by the MiniBooNE experiment, which used the same short-baseline neutrino beam line at Fermilab.

MicroBooNE uses a 10.4 m-long liquid-argon time-projection chamber (TPC) filled with 170 tonnes of liquid argon. The TPC probes neutrino oscillations by reconstructing particle tracks as finely detailed 3D images. When a neutrino hits the nucleus of an argon atom, its collision creates a spray of subatomic particles. Tracking and identifying those particles allows scientists to reveal the type and properties of the neutrino that produced them.

The MicroBooNE time-projection chamber is the largest ever built in the US and is equipped with 8256 delicate gold-plated wires. The three layers of wires capture pictures of particle interactions at different points in space and time. The superb resolution of the time-projection chamber will allow scientists to check whether the excess of MiniBooNE events – recorded with a Cherenkov detector filled with mineral oil – is due to photons or electrons.

MicroBooNE will collect data for several years, and computers will sift through thousands of neutrino interactions recorded every day. It will be the first liquid-argon detector to measure neutrino interactions from a neutrino beam with particle energies of less than 800 MeV.

Construction is under way for the two buildings that will house the other detectors of the SBN programme: the new 260 tonne Short-Baseline Near Detector (110 m from the neutrino production target) and the 760 tonne ICARUS detector (600 m) that took data at the Gran Sasso National Laboratory in Italy from 2009 to 2012. Like MicroBooNE (470 m from the target), they are both liquid-argon TPCs.

The MicroBooNE collaboration comprises 138 scientists from 28 institutions, while more than 200 scientists from 45 institutions are collaborating on the SBN programme. The experience and knowledge they will gain is relevant for the forthcoming Deep Underground Neutrino Experiment (DUNE), which will use four 10,000 tonne liquid-argon TPCs to examine neutrino oscillations over a much longer distance (1300 km) and a much higher and broader energy range (0.5–10 GeV).

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The DUNE collaboration came together in response to the US P5 report on the “Strategic Plan for US Particle Physics in the Global Context”, published in 2014, and the recommendations of the European Strategy for Particle Physics to freeze the development of neutrino beams at CERN. The P5 report called for the previously US-dominated LBNE experiment to be reformulated as a truly international scientific endeavour, incorporating the scientific goals and expertise of the worldwide neutrino-physics community, in particular those developed by LBNO in Europe. As a result, the international DUNE collaboration was formed and structured following a model that was successfully adopted by the LHC experiments.

The DUNE collaboration currently consists of almost 800 scientists and engineers from 145 institutes in 26 nations. The rapid development of this large collaboration is indicative of the global interest in neutrino physics and the innovative science made possible with the DUNE near and far detectors and the proposed Long-Baseline Neutrino Facility (LBNF) at Fermilab. The strong partnership between the US DOE and CERN already established in the LHC programme is also one of the essential components for the success of DUNE/LBNF. Construction of the CERN facility that will host two large-scale DUNE prototype detectors and a test beam has already begun.

So what is DUNE/LBNF? LBNF is a new 60–120 GeV beamline at Fermilab that can produce either an intense beam of muon neutrinos or antineutrinos. The initial beam power will be 1.2 MW (compared with the maximum planned for Fermilab’s existing NuMI beam of 700 kW for the NOvA experiment). This is just the first step for LBNF and the beam is being designed to be upgradable to at least 2.4 MW.

The neutrino beam will be directed towards a near and a far detector. DUNE’s far detector will be located 1.5 km underground at the Sanford Underground Research Facility (SURF) in South Dakota. Neutrinos will travel a distance of 1300 km through the Earth’s crust, therefore allowing the neutrino flavours to oscillate. The DUNE far detector consists of four 10 kton (fiducial) liquid-argon time projection chambers (LAr-TPCs). These detectors are very large – each will be approximately 62 × 15 × 14 m. The advantage of the LAr-TPC technology is that it allows 3D bubble-chamber-like imaging of neutrino interactions (or proton decay) in the vast detector volume. The DUNE near detector on the Fermilab site will observe the unoscillated neutrino beam, providing constraints on experimental uncertainties. By the standards of neutrino physics, the near-detector event rates are incredible – it will detect hundreds of millions of neutrino interactions. This will enable a diverse and world-leading neutrino-physics programme.

DUNE is not only about neutrinos. The large far detector with bubble-chamber-like imaging capability, located deep underground, provides an opportunity to search for proton decay. In particular, DUNE is able to search for proton-decay modes with kaons (such as the p → K⁺ antineutrino), which are favoured in many SUSY scenarios. The clear topological and ionisation (dE/dx) signature of these decay modes allows for a near-background-free search – a significant advantage in capability over large water Cherenkov detectors. Furthermore, DUNE will provide unique capabilities for the observation of neutrinos from core-collapse supernova bursts (SNBs). While water Cherenkov detectors are primarily sensitive to electron antineutrinos from SNBs, DUNE is mostly sensitive to the electron neutrinos. This would enable DUNE to directly observe the neutron-star-formation stage (p + e^– → n + ν_e) in “real time”, albeit delayed by the time that it takes for neutrinos to reach the Earth – this would be a truly remarkable observation. There is even the possibility to observe the formation of a black hole as a sharp cut-off in the time spectrum of the SNB neutrinos, if the black hole were to form a few seconds after the stellar-core collapse.

CERN’s role

CERN is playing a crucial role in prototyping the DUNE far detector and in the detailed understanding of its performance. Following the recommendations of the European Strategy document, CERN has set up a programme to fulfil the needs of large-scale neutrino-detector prototyping. In the framework of this programme, a new neutrino “platform” is being brought to light in the North Area. The new CERN facility will be available for experiments in the autumn of 2016 and will include a 70 m extension of the EHN1 experimental hall, which will host the large experimental apparatus and expose them to charged-particle test beams. The plan is to operate the first charged-particle beams in 2017 after the civil engineering and infrastructure work needed to upgrade the experimental hall has been completed.

ProtoDUNE is the engineering prototype for the single-phase far-detector design currently planned for the first 10 kton far-detector module. ProtoDUNE is based on the pioneering work carried out for the ICARUS detector operated at the Gran Sasso underground laboratory. The ICARUS detector, with its 600 tonnes of liquid argon, took data from 2010 to 2012. It demonstrated that a liquid-argon TPC detector can provide detailed images of charged particles and electromagnetic showers, with excellent spatial and calorimetric resolution. ICARUS also demonstrated the long-term stability of the LAr-TPC concept.

The WA105 demonstrator will be based on the novel dual-phase liquid-argon time projection chamber that was developed by the European LAGUNA-LBNO consortium, with R&D efforts located at CERN for more than a decade. The dual-phase approach, which offers potential advantages over the single-phase read-out, is being considered by DUNE for one or more of the DUNE far-detector 10 kton modules. The WA105 collaboration is currently building a smaller-scale 25 tonne prototype at CERN, to be operated in 2016. The larger 300 tonne WA105 demonstrator should be ready for test beam by 2018 in the EHN1 extension of the North Area at CERN.

The goal of these prototypes is to validate the construction techniques that will be adopted for the deep-underground installation at SURF, and to measure the performance of full-scale modules. In addition, the EHN1 test beams will provide the unique capability to collect and analyse charged-particle data necessary to understand the response of these detectors, with the high precision required for the DUNE science programme. The CERN neutrino platform will also serve additional R&D efforts, in particular for the DUNE near detector, where the current design utilises a straw-tube tracking chamber (inspired by the earlier NOMAD experiment at CERN), but other options, such as a high-pressure gaseous-argon TPC, are being studied.

The DUNE/LBNF scientific programme has broad support from partners in the Americas, Asia and Europe, and the collaboration is expected to grow. Progress in the last year has been rapid; DUNE/LBNF produced a four-volume Conceptual Design Report (CDR) in July 2015, detailing the design of the DUNE near and far detectors and the design of LBNF, which encompasses both the new neutrino beamline at Fermilab and civil facilities for the DUNE detectors. The CDR was a crucial element of the DOE CD-1 review of the cost range for the project. DUNE/LBNF is currently seeking DOE CD-3a approval for the underground excavation of the far-site facility that would host the four far-detector modules. The timescales are relatively short, with the start of the excavation project planned for 2017 and installation of the first far-detector module planned to start in 2021, with first commissioning for physics starting soon after. The strong role of CERN in this programme is crucial to its success.

• For further details, see lbnf.fnal.gov and www.dunescience.org.

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In July, the Borexino collaboration reported a geoneutrino signal from the Earth’s mantle with 98% C.L. Geoneutrinos are electron antineutrinos produced by β decays of ²³⁸U and ²³²Th chains, and ⁴⁰K. These isotopes are naturally present in the interior of the Earth and have lifetimes compatible with the age of the planet. Their radioactive decays contribute significantly to the heat released by the planet. Therefore, the detection of antineutrinos can give geophysicists key information about the relative distribution of the various components in specific layers of the Earth’s interior (crust and mantle).

In Borexino, geoneutrinos are detected in the 278 tonnes of ultra-pure organic liquid scintillator via the inverse β-decay process, ν_e + p → e⁺ + n, with a threshold in the neutrino energy of 1.806 MeV. Data reported in the recent publication were collected between 15 December 2007 and 8 March 2015 for a total of 2055.9 days before any selection cut. In this data set, the total geoneutrino signal (from the crust and mantle) has been measured for the first time at more than 5σ.

The signal disentanglement from background is obtained by applying selection cuts based on the properties of the interaction process. The combined efficiency of the cuts, determined by Monte Carlo techniques, is estimated to be (84.2±1.5)%. A total of 77 antineutrino candidates survived the cuts. They include signals from the Earth and background events. The latter are mainly composed of antineutrinos coming from the nuclear reactors. Their signal, corresponding to some 53 events, has been calculated and based on the data from the International Atomic Energy Agency. From previous studies, the contribution from the crust is estimated to be (23.4±2.8) terrestrial neutrino units (TNU), corresponding to 13 events. To estimate the significance of a positive signal from the mantle, the collaboration has determined the likelihood of S_geo(mantle) = S_geo – S_geo(crust) using the experimental likelihood profile of S_geo and a Gaussian approximation for the crust contribution. This approach gives a signal from the mantle equal to S_geo(mantle) = 20.9^+15.1_–10.3 TNU (corresponding to 11 events), with the null hypothesis rejected at 98% C.L.

Although limited by the detection volume and the exposure time, the Borexino researchers could also perform spectroscopy studies (figure 1) that show how their detection technique allows separation of the contributions from uranium (the dark-blue area) and thorium (the light-blue area).

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The Daya Bay Reactor Neutrino Experiment has recently published a new measurement of the disappearance of electron antineutrinos emitted by nuclear reactors. The observation improves the precision of the mixing angle θ₁₃ and the associated mass-squared difference |Δm²_ee| by almost a factor of two.

This is the first measurement obtained with the completed Daya Bay detector configuration consisting of eight modular antineutrino detectors, providing a total target mass of 160 tonnes. The gadolinium-doped organic liquid scintillator detects electron antineutrinos via inverse beta decay (ν_e + p → e⁺ + n). Oscillation converts some of the ν_e to ν_μ and ν_τ, reducing the ν_e flux. Six commercial pressurised-water nuclear reactors (17.4 GW of thermal power in total) of the Daya Bay Nuclear Power Complex are an intense source, producing about 10²¹ electron antineutrinos per second. Four detectors located around 300 to 500 m from the reactors measure the initial ν_e rate from the reactors, while four detectors at around 1.6 km from the reactors observe the subsequent disappearance.

This result builds on previous measurements by the Daya Bay and RENO experiments, which provided the first proof that θ₁₃ is nonzero. The improved statistical precision came from a 3.6 times increase in exposure, generating a data sample of 1.2 million ν_e interactions. The systematic uncertainties were also reduced through improved characterisation of the detectors and reduction of background.

The analysis found sin²(2θ₁₃) = 0.084±0.005 from the amplitude of anti-ν_e disappearance, while the energy dependence of this disappearance provided a measurement of oscillation frequency expressed in terms of the effective mass-squared difference |Δm²_ee| = (2.42±0.11) × 10^–3 eV² (see figure 1). This is actually related to the two almost-equal neutrino mass-squared differences |Δm²₃₁| and |Δm²₃₁| = |Δm²₃₂ + Δm²₂₁|. One measure of how far neutrino physics has progressed is that the interpretation of this mixing parameter is now a step closer to being sensitive to the neutrino mass hierarchy. If the mass hierarchy is normal, then |Δm²₃₂| = (2.37±0.11) × 10^–3 eV², while if it is inverted, |Δm²₃₂| = (2.47±0.11) × 10^–3 eV².

The Daya Bay Reactor Neutrino Experiment continues to collect data, and aims at achiving a further factor of two improvement in precision by 2017.

Weblink

• dx.doi.org/10.1103/PhysRevLett.115.111802

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Developed in the late 1990s, the OPERA detector design was based on a hybrid technology, using both real-time detectors and nuclear emulsions. The construction of the detector at the Gran Sasso underground laboratory in Italy started in 2003 and was completed in 2007 – a giant detector of around 4000 tonnes, with 2000 m³ volume and nine million photographic films, arranged in around 150,000 target units, the so-called bricks. The emulsion films in the bricks act as tracking devices with micrometric accuracy, and are interleaved with lead plates acting as neutrino targets. The longitudinal size of a brick is around 10 radiation lengths, allowing for the detection of electron showers and the momentum measurement through the detection of multiple Coulomb scattering. The experiment took data for five years, from June 2008 until December 2012, integrating 1.8 × 10²⁰ protons on target.

The aim of the experiment was to perform the direct observation of the transition from muon to tau neutrinos in the neutrino beam from CERN. The distance from CERN to Gran Sasso and the SPS beam energy were just appropriate for tau-neutrino detection. In 1999, intense discussions took place between CERN management and Council delegations about the opportunity of building the CERN Neutrino to Gran Sasso (CNGS) beam facility and the way to fund it. The Italian National Institute for Nuclear Physics (INFN) was far-sighted in offering a sizable contribution. Many delegations supported the idea, and the CNGS beam was approved in December 1999. Commissioning was performed in 2006, when OPERA (at that time not fully equipped yet) detected the first muon-neutrino interactions.

With the CNGS programme, CERN was joining the global experimental effort to observe and study neutrino oscillations. The first experimental hints of neutrino oscillations were gathered from solar neutrinos in the 1970s. According to theory, neutrino oscillations originate from the fact that mass and weak-interaction eigenstates do not coincide and that neutrino masses are non-degenerate. Neutrino mixing and oscillations were introduced by Pontecorvo and by the Sakata group, assuming the existence of two sorts (flavours) of neutrinos. Neutrino oscillations with three flavours including CP and CPT violation were discussed by Cabibbo and by Bilenky and Pontecorvo, after the discovery of the tau lepton in 1975. The mixing of the three flavours of neutrinos can be described by the 3 × 3 Pontecorvo–Maki–Nakagawa–Sakata matrix with three angles – that have since been measured – and a CP-violating phase, which remains unknown at present. Two additional parameters (mass-squared differences) are needed to describe the oscillation probabilities.

Several experiments on solar, atmospheric, reactor and accelerator neutrinos have contributed to the understanding of neutrino oscillations. In the atmospheric sector, the strong deficit of muon neutrinos reported by the Super-Kamiokande experiment in 1998 was the first compelling observation of neutrino oscillations. Given that the deficit of muon neutrinos was not accompanied by an increase of electron neutrinos, the result was interpreted in terms of ν_μ → ν_τ oscillations, although in 1998 the tau neutrino had not yet been observed. The first direct evidence for tau neutrinos was announced by Fermilab’s DONuT experiment in 2000, with four reported events. In 2008, the DONuT collaboration presented its final results, reporting nine observed events and an expected background of 1.5. The Super-Kamiokande result was later confirmed by the K2K and MINOS experiments with terrestrial beams. However, for an unambiguous confirmation of three-flavour neutrino oscillations, the appearance of tau neutrinos in ν_μ → ν_τ oscillations was required.

OPERA comes into play

OPERA reported the observation of the first tau-neutrino candidate in 2010. The tau neutrino was detected by the production and decay of a τ^– in one of the lead targets, where τ^– → ρ^–ν_τ. A second candidate, in the τ^– → π^–π⁺π^–ν_τ channel, was found in 2012, followed in 2013 by a candidate in the fully leptonic τ^– → μ⁻ν_μν_τ decay. A fourth event was found in 2014 in the τ^– → h^–ν_τ channel (where h^– is a pion or a kaon), and a fifth one was reported a few months ago in the same channel. Given the extremely low expected background of 0.25±0.05 events, the direct transition from muon to tau neutrinos has now been measured with the 5σ statistical precision conventionally required to firmly establish its observation, confirming the oscillation mechanism.

The extremely accurate detection technique provided by OPERA relies on the micrometric resolution of its nuclear emulsions, which are capable of resolving the neutrino-interaction point and the vertex-decay location of the tau lepton, a few hundred micrometres away. The tau-neutrino identification is first topological, then kinematical cuts are applied to suppress the residual background, thus giving a signal-to-noise ratio larger than 10. In general, the detection of tau neutrinos is extremely difficult, due to two conflicting requirements: a huge, massive detector and the micrometric accuracy. The concept of the OPERA detector was developed in the late 1990s with relevant contributions from Nagoya – the emulsion group led by Kimio Niwa – and from Naples, under the leadership of Paolo Strolin, who led the initial phase of the project.

The future of nuclear emulsions

Three years after the end of the CNGS programme, the OPERA collaboration – about 150 physicists from 26 research institutions in 11 countries – is finalising the analysis of the collected data. After the discovery of the appearance of tau neutrinos from the oscillation of muon neutrinos, the collaboration now plans to further exploit the capability of the emulsion detector to observe all of the three neutrino flavours at once. This unique feature will allow OPERA to constrain the oscillation matrix by measuring tau and electron appearance together with muon-neutrino disappearance.

An extensive development of fully automated optical microscopes for the scanning of nuclear-emulsion films was carried out along with the preparation and running of the OPERA experiment. These achievements pave the way for using the emulsion technologies in forthcoming experiments, including SHiP (Search for Hidden Particles), a new facility that was recently proposed to CERN. If approved, SHiP will not only search for hidden particles in the GeV mass range, but also study tau-neutrino physics and perform the first direct observation of tau antineutrinos. The tau-neutrino detector of the SHiP apparatus is designed to use nuclear emulsions similar to those used by OPERA. The detector will be able to identify all three neutrino flavours, while the study of muon-neutrino scattering with large statistics is expected to provide additional insights into the strange-quark content of the proton, through the measurement of neutrino-induced charmed hadron production.

Currently, the R&D work on emulsions continues mainly in Italy and Japan. Teams at Nagoya University have successfully produced emulsions with AgBr crystals of about 40 nm diameter – one order of magnitude smaller than those used in OPERA. In parallel, significant developments of fully automated optical-scanning systems, carried out in Italy and Japan with innovative analysis technologies, have overcome the intrinsic optical limit and achieved the unprecedented position resolution of 10 nm. Both achievements make it possible to use emulsions for the detection of sub-micrometric tracks, such as those left by nuclear recoils induced by dark-matter particles (Weakly Interacting Massive Particles, WIMPs). This paves the way for the first large-scale dark-matter experiment with directional information. The NEWS experiment (Nuclear Emulsions for WIMP Search) plans to carry out this search at the Gran Sasso underground laboratory.

Thanks to their extreme accuracy and capability of identifying particles, nuclear emulsions are also successfully employed in fields beyond particle physics. Exploiting the cosmic-ray muon radiography technique, sandwiches of OPERA-like emulsion films and passive materials were used to image the shallow-density structure beneath the Asama Volcano in Japan and, more recently, to image the crater structure of the Stromboli volcano in Italy. Detectors based on nuclear emulsions are also used in hadron therapy to characterize the carbon-ion beams and their secondary interactions in human tissues. The high detection accuracy provided by emulsions allows experts to better understand the secondary effects of radiation, and to monitor the released dose with the aim of optimizing the planning of medical treatments.

• For more information, visit http://operaweb.lngs.infn.it/.

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This is an event view of the highest energy neutrino detected so far by the IceCube experiment based at the South Pole (CERN Courier December 2014 p30). Each sphere is one optical sensor; the coloured spheres show those that observed light from this event. The sizes show how many photons each module observed, while the colour gives some idea of the arrival time of the first photon, from red (earliest) to blue (latest). It is easy to see that the neutrino is going slightly upward (by about 11.5°), so the muon cannot be from a cosmic-ray air shower; it must be from a neutrino. The event, detected on 11 June 2014, was in the form of a through-going muon, which means that the track originated and ended outside of the detector’s volume. So, IceCube cannot measure the total energy of the neutrino, but rather its specific energy loss (dE/dx). While the team is still working on estimating the neutrino energy, the total energy loss visible in the detector was 2.6±0.3 PeV.

• From “Neutrino Hunting in Antarctica” by Spencer Klein, see antarcticaneutrinos.blogspot.ch/.

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In June, Fermilab’s Main Injector accelerator sustained a 521 kW proton beam, and set a world record for the production of high-energy neutrinos with a proton accelerator. The 120 GeV proton beam is used to provide high-energy neutrinos or antineutrinos to three experiments at the laboratory: the long-baseline experiments MINOS+ and NOvA (CERN Courier June 2015 p17) and the neutrino-interaction experiment MINERvA (CERN Courier April 2014 p26).

The record beam surpasses that of the proton beam of more than 400 kW achieved at CERN for the CERN Neutrinos to Gran Sasso (CNGS) beamline, which provided neutrinos for the ICARUS and OPERA long-baseline experiments. The highest beam powers for fixed-target proton beams are achieved with protons in the giga-electron-volt range. Both the Spallation Neutron Source at Oak Ridge National Laboratory and the cyclotron facility at the Paul Scherrer Institute in Switzerland create proton beams with powers in excess of 1 MW. In the 1990s, Los Alamos National Laboratory operated a 0.8 GeV proton beam at about 800 kW for its low-energy neutrino experiment, LSND.

The power of the proton beam is a key element in producing neutrinos at accelerators: the more protons packed in the beam, the higher the number of neutrinos and antineutrinos produced and the better the chance to record neutrino interactions. The protons strike a target to create pions and other short-lived particles; the higher the proton energy, the larger the number of pions produced. Magnetic-focusing horns direct the charged pions into a vacuum pipe that is centred along the desired neutrino-beam direction. As the pions decay, they produce neutrinos and antineutrinos that are boosted in the direction of the original pions.

Since 2011, Fermilab has made significant upgrades to its accelerators and reconfigured the complex to provide the best possible particle beams for neutrino and muon experiments. The next goal for the 3.3 km circumference Main Injector accelerator is to deliver 700 kW in 2016 – double the beam power produced in the Tevatron era.

Fermilab plans to make additional upgrades to its accelerator complex over the next decade. The Proton Improvement Project-II includes the construction of a 800 MeV superconducting linac. Its beam would enable the Main Injector to provide more than 1.2 MW of proton beam power for the international Deep Underground Neutrino Experiment (CERN Courier April 2015 p20).

Fermilab also operates a second neutrino beamline, powered by its 8 GeV booster accelerator. This provides neutrinos for the Short Baseline Neutrino programme, which comprises three neutrino detectors: MicroBooNE (construction complete), ICARUS (upgrades underway at CERN) and the Short Baseline Neutrino Detector (construction to start in 2016).

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The international OPERA experiment, which involves about 140 physicists from 26 research institutes in 11 countries, was designed to observe this exceptionally rare phenomenon, gathering data in the neutrino beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project (CERN Courier November 2006 p24). A small fraction of the incoming neutrinos interacted with the giant detector, consisting of more than 4000 tonnes of material, with a volume of some 2000 m³ and some nine million photographic plates, to produce the particles observed. After detecting the first few ν_μ produced at CERN in 2006, the experiment has collected data for five years, from 2008 to the end of 2012. The first ν_τ was observed in 2010. The second and third events were reported in 2012 and 2013, respectively, while the fourth one was announced in 2014 (CERN Courier May 2014 p9).

The OPERA collaboration will continue to analyse the data collected, searching for other ν_μ to ν_τ transitions, and possibly also measure the oscillation parameters, for the first time using oscillated ν_τ.

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In early April, members of the Baikal collaboration deployed and started operation of the first cluster of the Gigaton Volume Detector (Baikal-GVD). Named “Dubna”, the cluster comprises 192 optical modules arranged at depths down to 1300 m. The modules are glass spheres that house photomultiplier tubes to detect Cherenkov light from the charged particles emerging from neutrino interactions in the water of the lake. By 2020, GVD is set to consist of 10–12 clusters covering a total volume of about 0.4 km³ (GVD phase-1). This is about half the size of the present world leader – the IceCube Neutrino Observatory at the South Pole (CERN Courier December 2014 p30). A planned further extension should then lead towards a second stage containing 27 clusters in a telescope with a total volume of about 1.5 km³.

Neutrino detection in Lake Baikal will be an important part of the effort to understand better the high-energy processes that occur in far-distant astrophysical sources, to determine the origin of cosmic particles of the highest energies ever registered, to search for dark matter, to study properties of elementary particles, and to learn a great deal of new information about the structure and evolution of the universe as a whole. Together with KM3NeT in the Mediterranean Sea, the other future Northern-hemisphere neutrino telescope (CERN Courier July/August 2012 p31), GVD will allow an optimal view to the central parts of the Galaxy.

The start of the Baikal neutrino experiment dates back to 1 October 1980, when a laboratory of high-energy neutrino astrophysics was established at the Institute for Nuclear Research of the former Academy of Sciences of the USSR in Moscow – now the Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS). This laboratory later became the core of the Baikal collaboration, including at various times the Joint Institute for Nuclear Research (JINR) in Dubna, Irkutsk State University, Moscow State University, DESY-Zeuthen, the Nizhni Novgorod State Technical University, the Saint Petersburg State Marine Technical University, and other scientific research organizations in Russia, Hungary and Germany. At present, the participation of institutes from the Czech Republic, Slovakia and Poland is under discussion.

The idea to register neutrinos in large-scale Cherenkov detectors in natural water was expressed for the first time by Moisey Markov, then at Dubna, at the 10th International Conference on High-Energy Physics, in 1960. Two decades later, Alexander Chudakov, of INR, proposed using Lake Baikal as a site both for tests and for future large-scale neutrino telescopes. The choice of this lake – the largest and deepest freshwater reservoir in the world – was determined by the high transparency of its water, its depth, and the ice cover that allows the installation of deep-water equipment during two months in winter.

The predecessor of GVD was constructed during 1993–1998. Named NT200, it comprised 192 photodetectors placed on eight vertical strings at a depth of 1100–1200 m. NT200 covered some 100,000 m³ of fresh water (an order of magnitude less than the present Dubna cluster). Already in 1994, data taken with only 36 of the final 192 photodetectors showed two neutrino events. These two events were the first of several-hundred-thousand atmospheric neutrinos since recorded by deep-underwater and under-ice experiments. Scientific research with NT200 covered a wide programme, most notably the search for a cosmic diffuse neutrino flux leading to tight limits on that flux (CERN Courier July/August 2005 p24). Moreover, limits were derived on the flux of magnetic monopoles and on muons from dark-matter annihilation in the centre of the Earth and the Sun. Last but not least, the NT200 infrastructure was used for innovative environmental studies.

A notable breakthrough in the field came in 2012, when IceCube detected the first high-energy “astrophysical” neutrinos, i.e. high-energy neutrinos generated beyond the solar system (CERN Courier July/August 2013 p35). That marked the birth of high-energy neutrino astronomy, and underlined the need to develop neutrino telescopes of similar capacity in the Northern hemisphere, to be able to study high-energy neutrino sources across the whole celestial sphere. JINR, with many years of experience as a participant in the Baikal neutrino project, recognized this opportunity and decided to treat activities related to Baikal-GVD as a scientific priority.

Baikal-GVD will have a modular structure formed from functionally independent clusters of vertical strings of optical modules. This modular structure will allow data acquisition at early stages in the construction of the facility. The choice of the telescope structure will also allow adjustment of its configuration in response to changes in scientific priorities at different times.

Prototypes of all of the basic elements of the GVD telescope system were designed, manufactured and tested during 2006–2010. The final stage of complex in-situ testing started in 2011 and finished in 2015 with the development of the Dubna cluster. Its 192 optical modules are arranged down to depths of 1300 m on eight vertical strings, each 345-m long. Different from NT200, the optical modules are not grouped in pairs, resulting in 192 space points per cluster (instead of only 96 for NT200). Moreover, the former custom-made, hybrid QUASAR phototube has been replaced by a conventional 10-inch photomultiplier with a high-sensitivity photocathode. The mechanical structure has been simplified compared with NT200, and a totally new system for front-end and trigger electronics and for data acquisition has been designed and implemented.

Deployment of the Dubna cluster is an exciting step towards a next-generation neutrino telescope in Lake Baikal. Such a telescope will be a cornerstone of a future worldwide neutrino observatory, with detectors at the South Pole, in the Mediterranean Sea and in Lake Baikal. The Baikal collaboration pioneered this technology in the 1980s and 1990s, and measured neutrinos generated in the Earth’s atmosphere. Two decades later, the long-awaited discovery by IceCube of the first high-energy neutrinos from far beyond the Earth and the solar system has given increased motivation to projects for similar large detectors in the Northern hemisphere. IceCube has lifted the curtain that hides the high-energy neutrino universe, but just by a little. In the future, Baikal-GVD will help to chart this new cosmic territory fully.

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Over the past decade and more, cosmology on one side and particle physics on the other have approached what looks like a critical turning point. The theoretical models that for many years have been the backbone of research carried out in both fields – the Standard Model for particle physics and the Lambda cold dark matter (ΛCDM) model for cosmology – are proving insufficient to describe more recent observations, including those of dark matter and dark energy. Moreover, the most important “experiment” that ever happened, the Big Bang, remains unexplained. Physicists working at both extremes of the scale – the infinitesimally small and the infinitely large – face the same problem: they know that there is much to search for, but their arms seem too short to reach still further distances. So, while researchers in the two fields maintain their specific interests and continue to build on their respective areas of expertise, they are also looking increasingly at each other’s findings to reconstitute the common mosaic.

Studies on the nature of dark matter are the most natural common ground between cosmology and particle physics. Run 2 of the LHC, which has just begun, is expected to shed some light on this area. Indeed, while the main outcome of Run 1 was undoubtedly the widely anticipated discovery of a Higgs boson, Run 2 is opening the door to uncharted territory. In practical and experimental terms, exploring the properties and the behaviour of nature at high energy consists in understanding possible signals that include “missing energy”. In the Standard Model, this energy discrepancy is associated with neutrinos, but in physics beyond the Standard Model, the missing energy could also be the signature of many undiscovered particles, including the weakly interacting massive particles (WIMPs) that are among the leading candidates for dark matter. If WIMPs exist, the LHC’s collisions at 13 TeV may reveal them, and this will be another huge breakthrough. Because supersymmetry has not yet been ruled out, the high-energy collisions might also eventually unveil the supersymmetric partners of the known particles, at least the lighter ones. Missing energy could also account for the escape of a graviton into extra dimensions, or a variety of other possibilities. Thanks to the LHC’s Run 1 and other recent studies, the Standard Model is so well known that future observation of an unknown source of missing energy could be confidently linked to new physics.

Besides the search for dark matter, another area where cosmology and particle physics meet is in neutrino physics. The most recent result that collider experiments have published for the number of standard (light) neutrino types is N_ν = 2.984±0.008 (ALEPH et al. 2006). While the search for a fourth right-handed neutrino is continuing with ground-based experiments, satellite experiments have shown that they can also have their say. Indeed, recent results from ESA’s Planck mission yield N_eff = 3.04±0.18 for the effective number of relativistic degrees of freedom, and the sum of neutrino masses is constrained to Σm_ν < 0.17 eV. These values, derived from Planck’s data of temperature and polarization CMB anisotropies in combination with data from baryonic acoustic oscillation experiments, are consistent with standard cosmological and particle-physics predictions in the neutrino sector (Planck Collaboration 2015a). Although these values do not completely rule out a sterile neutrino, especially if thermalized at a different background temperature, its existence is disfavoured by the Planck data (figure 1).

Ground-based experiments have observed the direct oscillation of neutrinos, which proves that these elusive particles have a nonzero mass.

Working out absolute neutrino masses is no easy task. Ground-based experiments have observed the direct oscillation of neutrinos, which proves that these elusive particles have a nonzero mass. However, no measurement of absolute masses has been performed yet, and the strongest upper limit (about one order of magnitude more accurate than direct-detection measurements) on their sum comes from cosmology. Because neutrinos are the most abundant particles with mass in the universe, the influence of their absolute mass on the formation of structure is as big as their role in many physics processes observed at small scales. The picture in the present Standard Model might suggest (perhaps naively) that the mass distribution among the neutrinos could be similar to the mass distribution among the other particles and their families, but only experiments such as KATRIN – the Karslruhe Tritium Neutrino experiment – are expected to shed some light on this topic.

In recent years, cosmologists and particle physicists have shown a common interest in testing Lorentz and CPT invariances. The topic seems to be particularly relevant for theorists working on string theories, which sometimes involve mechanisms that lead to a spontaneous breaking of these symmetries. To find possible clues, satellite experiments are probing the cosmic microwave background (CMB) to investigate the universe’s birefringence, which would be a clear signature of Lorentz invariance and, therefore, CPT violation. So far, the CMB experiments WMAP, QUAD and BICEP1 have found a value of α – the rotation angle of the photon-polarization plane – consistent with zero. Results from Planck on the full set of observations are expected later this year.

Since its discovery in 2012, the Higgs boson found at the LHC has been in the spotlight for physicists studying both extremes of the scale. Indeed, in addition to its confirmed role in the mass mechanism, recent papers have discussed its possible role in the inflation of the universe. Could a single particle be the Holy Grail for cosmologists and particle physicists alike? It is a fascinating question, and many studies have been published about the particle’s possible role in shaping the early history of the universe, but the theoretical situation is far from clear. On one side, the Higgs boson and the inflaton share some basic features, but on the other side, the Standard Model interactions do not seem sufficient to generate inflation unless there is an anomalously strong coupling between the Higgs boson and gravity. Such strong coupling is a highly debated point among theoreticians. Also in this case, the CMB data could help to rule out or disentangle models. Recent full mission data from Planck clearly disfavour natural inflation compared with models that predict a smaller tensor-to-scalar ratio, such as the Higgs inflationary model (Planck Collaboration 2015b). However, the question remains open, and subject to new information coming from the LHC’s future runs and from new cosmological missions.

AMS now has results based on more than 6 × 10¹⁰ cosmic-ray events.

In the meantime, astroparticle physics is positioning itself as the area where both cosmology and particle physics could find answers to the open questions. An event at CERN in April provided a showcase for experiments on cosmic rays and dark matter, in particular the latest results from the Alpha Magnetic Spectrometer (AMS) collaboration on the antiproton-to-proton ratio in cosmic rays and on the proton and helium fluxes. Following earlier measurements by PAMELA – the Payload for Antimatter Matter Exploration and Light nuclei Astrophysics – which took data in 2006–2011, AMS now has results based on more than 6 × 10¹⁰ cosmic-ray events (electrons, positrons, protons and antiprotons, as well as nuclei of helium, lithium, boron, carbon, oxygen…) collected during the first four years of AMS-02 on board the International Space Station. With events at energies up to many tera-electron-volts, and with unprecedented accuracy, the AMS data provide systematic information on the deepest nature of cosmic rays. The antiproton-to-proton ratio measured by AMS in the energy range 0–500 GeV shows a clear discrepancy with existing models (figure 2). Anomalies are also visible in the behaviour of the fluxes of electrons, positrons, protons, helium and other nuclei. However, although a large part of the scientific community tends to interpret these observations as a new signature of dark matter, the origin of such unexpected behaviour cannot be easily identified, and discussions are still ongoing within the community.

It may seem that the universe is playing hide-and-seek with cosmologists and particle physicists alike as they probe both ends of the distance scale. However, the two research communities have a new smart move up their sleeves to unveil its secrets – collaboration. Bringing together the two ends of the scales probed by the LHC and by Planck will soon bear its fruits. Watch this space!

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IceCube records more than 100,000 atmospheric neutrinos a year, most of them muon neutrinos, and its sub-detector DeepCore allows the detection of neutrinos with energies from 100 GeV down to 10 GeV. These lower-energy neutrinos are key to IceCube’s oscillation studies. Based on current best-fit oscillation parameters, IceCube should see fewer muon neutrinos at energies around 25 GeV reaching the detector after passing through the Earth. Using data taken between May 2011 and April 2014, the analysis selected muon-neutrino candidates in DeepCore with energies in the region of 6–56 GeV. The detector surrounding DeepCore was used as a veto to suppress the atmospheric muon background. Nearly 5200 neutrino candidates were found, compared with the 6800 or so expected in the non-oscillation scenario. The reconstructed energy and arrival time for these events were used to obtain values for the neutrino-oscillation parameters, Δm₃₂² = 2.72^+0.19_–0.20 × 10^–3 ev² and sin² θ₂₃ = 0.53^+0.09_–0.12. These results are compatible and comparable in precision to those of dedicated oscillation experiments.

The collaboration is currently planning the Precision IceCube Next Generation Upgrade (PINGU), in which a much higher density of optical modules in the whole central region will reduce the energy threshold to a few giga-electron-volts. By carefully measuring coherent neutrino interactions with electrons in the Earth (the Mikheyev–Smirnov–Wolfenstein effect), this should allow determination of the neutrino-mass hierarchy, and which neutrino flavour is heaviest.

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Neutrinos have kept particle physicists excited for at least the past 20 years. After they were finally proved to be massive, two mass-squared differences and all three mixing angles have now been determined, the final remaining angle, θ₁₃, in 2012 by the three reactor experiments: Daya Bay, RENO and Double Chooz. As neutrino masses are expected to be linked intimately to physics beyond the Standard Model that can be probed at the LHC, and as neutrinos are about to start a “second career” as astrophysical probes, it seems a perfect time to publish a new textbook on the elusive particle. The authors Jose Vallé and Jorge Romão are leading protagonists in the field who have devoted most of their careers to the puzzling neutrino. In this new book they share their experience of many years at the forefront of research.

They begin with a brief historical introduction, before reviewing the Standard Model and its problems and discussing the quantization of massive neutral leptons. The next three chapters deal with neutrino oscillations and absolute neutrino masses – the mass being one of the fundamental properties of neutrinos that is still unknown. Here the authors give a detailed discussion of the lepton-mixing matrix – the basic tool to describe oscillations – and seesaw models of various types. An interesting aspect is the thorough discussion of what could be called “Majorananess” and its relation to neutrino masses, lepton-number violation and neutrinoless double beta decay – for example, in the paragraphs dealing with the Majorana–Dirac confusion and black-box theorems, a point that is rarely covered in text books and often results in confusion.

Next, the book discusses how neutrino masses are implemented in the Standard Model’s SU(2) × U(1) gauge theory and the relationship to Higgs physics. This is followed by a detailed treatment of neutrinos and physics beyond the Standard Model (supersymmetry, unification and the flavour problem), which constitutes almost half of the entire book. Here the text exhibits its particular strength – also in comparison to the competing books by Carlo Giunti and Chung Kim, and by Vernon Barger, Danny Marfatia and Kerry Whisnant, both of which concentrate more on neutrino oscillation phenomenology – by discussing exhaustively how neutrino physics is linked to physics beyond Standard Model phenomenology, such as lepton-flavour violation or collider processes. The inclusion of a detailed discussion of these topics is a good choice and it makes the book valuable as a textbook, although it does make this part rather long and encyclopedic. Another strong point is the focus on model building. For example, the book discusses in detail the challenges in flavour-symmetry model building to accommodate a non-zero θ₁₃, and the deviation of the lepton-mixing matrix from the simple tri-bi-maximal form.

The authors end with a brief chapter on cosmology, concentrating mainly on dark matter and its connection to neutrinos. While this chapter obviously cannot replace a dedicated introduction to cosmology, a few more details such as an introduction of the Friedmann equation could have been helpful here. In general, the treatment of astroparticle physics is shorter than expected from the title of the book. For example, the detection of extragalactic neutrinos at IceCube is not covered – indeed, IceCube is only mentioned in passing as an experiment that is sensitive to the indirect detection of dark matter. Also leptogenesis and supernova neutrinos are mentioned only briefly.

The book mainly serves as a detailed and concise, thorough and pedagogical introduction to the relationship of neutrinos to physics beyond the Standard Model, and in particular the related particle-physics phenomenology. This subject is highly topical and will be more so in the years to come. As such, Neutrinos in High Energy and Astroparticle Physics does an excellent job and belongs on the bookshelf of every graduate student and researcher who is seriously interested in this interdisciplinary and increasingly important topic.

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The neutrino is the most abundant matter-particle in the universe and the lightest, most weakly interacting of the fundamental fermions. The way in which a neutrino’s flavour changes (oscillates) as it propagates through space implies that there are at least three different neutrino masses, and that mixing of the different mass states produces the three known neutrino flavours. The consequences of these observations are far reaching because they imply that the Standard Model is incomplete; that neutrinos may make a substantial contribution to the dark matter known to exist in the universe; and that the neutrino may be responsible for the matter-dominated flat universe in which we live.

This wealth of scientific impact justifies an energetic programme to measure the properties of the neutrino, interpret these properties theoretically, and understand their impact on particle physics, astrophysics and cosmology. The scale of investment required to implement such a programme requires a coherent, international approach. In July 2013, the International Committee for Future Accelerators (ICFA) established its Neutrino Panel for the purpose of promoting both international co-operation in the development of the accelerator-based neutrino-oscillation programme and international collaboration in the development of a neutrino factory as a future intense source of neutrinos for particle-physics experiments. The Neutrino Panel’s initial report, presented in May 2014, provides a blueprint for the international approach (Cao et al. 2014).

The accelerator-based contributions to this programme must be capable both of determining the neutrino mass-hierarchy and of seeking for the violation of the CP symmetry in neutrino oscillations. The complexity of the oscillation patterns is sufficient to justify two complementary approaches that differ in the nature of the neutrino beam and the neutrino-detection technique (Cao et al. 2015). In one approach, which is adopted by the Hyper-K collaboration, a neutrino beam of comparatively low energy and narrow energy spread (a narrow-band beam) is used to illuminate a “far” detector at a distance of approximately 300 km from the source (see “Proto-collaboration formed to promote Hyper-Kamiokande”). In a second approach, a neutrino beam with a higher energy and a broad spectrum (a wide-band beam) travels more than 1000 km through the Earth before being detected.

Since summer 2014, a global neutrino collaboration has come together to pursue the second approach, using Fermilab as the source of a wide-band neutrino beam directed at a far detector located deep underground in South Dakota. In addition to measuring the neutrinos in the beam, the experiment will study neutrino astrophysics and nucleon decay. This experiment will be of an unprecedented scale and dramatically improve the understanding of neutrinos and the nature of the universe.

This new collaboration – currently dubbed ELBNF for Experiment at the Long-Baseline Neutrino Facility – has an ambitious plan to build a modular liquid-argon time-projection chamber (LAr-TPC) with a fiducial mass of approximately 40 kt as the far detector and a high-resolution “near” detector. The collaboration is leveraging the work of several independent efforts from around the world that have been developed through many years of detailed studies. These groups have now converged around the opportunity provided by the megawatt neutrino-beam facility that is planned at Fermilab and by the newly planned expansion with improved access of the Sanford Underground Research Facility in South Dakota, 1300 km from Fermilab. To give a sense of scale, to house this detector some 1.5 km underground requires a hole that is approximately 120,000 m³ in size – nearly equivalent to the volume of Wimbledon’s centre-court stadium.

The principal goals of ELBNF are to carry out a comprehensive investigation of neutrino oscillations, to search for CP-invariance violation in the lepton sector, to determine the ordering of the neutrino masses and to test the three-neutrino paradigm. In addition, with a near detector on the Fermilab site, the ELBNF collaboration will perform a broad set of neutrino-scattering measurements. The large volume and exquisite resolution of the LAr-TPC in its deep underground location will be exploited for non-accelerator physics topics, including atmospheric-neutrino measurements, searches for nucleon decay, and measurement of astrophysical neutrinos (especially those from a core-collapse supernova).

The new international team has the necessary expertise, technical knowledge and critical mass to design and implement this exciting “discovery experiment” in a relatively short time frame. The goal is the deployment of a first detector with 10 kt fiducial mass by 2021, followed by implementation of the full detector mass as soon as possible. The accelerator upgrade of the Proton Improvement Plan-II at Fermilab will provide 1.2 MW of power by 2024 to drive a new neutrino beam line at the laboratory. There is also a plan that could further upgrade the Fermilab accelerator complex to enable it to provide up to 2.4 MW of beam power by 2030 – an increase of nearly a factor of seven on what is available today. With the possibility of space for expansion at the Sanford Underground Research Facility, the new international collaboration will develop the necessary framework to design, build and operate a world-class deep-underground neutrino and nucleon-decay observatory. This plan is aligned with both the updated European Strategy for Particle Physics and the report of the Particle Physics Project Prioritization Panel (P5) written for the High Energy Physics Advisory Panel in the US.

A letter of intent (LoI) was developed during autumn 2014, and the first collaboration meeting was held in mid January at Fermilab. Sergio Bertolucci, CERN’s director for research and computing, and interim chair of the institutional board, ran the meeting. More than 200 participants from around the world attended, and close to 600 physicists from 140 institutions and 20 countries have signed the LoI, to date. The collaboration has chosen its new spokespersons – Mark Thomson of Cambridge University and André Rubbia of ETH Zurich – and will begin the process of securing the early funding needed to excavate the cavern in a timely fashion so that detector installation can begin in the early 2020s.

Mounting such a significant experiment on such a compressed time frame will require all three world regions – Asia, the Americas and Europe – to work in concert. The pioneering work on the liquid-argon technique was carried out at CERN and implemented in the ICARUS detector, which ran for many years at Gran Sasso. To deliver the ELBNF far detector requires that the LAr-TPC technology be scaled up to an industrial scale. To deliver the programme required to produce the large LAr-TPC, neutrino platforms are being constructed at Fermilab and at CERN. A team led by Marzio Nessi is working hard to make this resource available at CERN and to complete in the next few years the R&D needed for both the single- and dual-phase liquid-argon technologies that are being proposed on a large scale (see box).

The steps taken by the neutrino community during the nine months or so since summer 2014 have put the particle-physics community on the road towards an exciting and vibrant programme that will culminate in exquisitely precise measurements of neutrino oscillation. It will also establish in the US one of the flagships of the international accelerator-based neutrino programme called for by the ICFA Neutrino Panel. In addition, ELBNF will be a world-leading facility for the study of neutrino astrophysics and cosmology.

With such a broad and exciting programme, ELBNF will be at the forefront of the field for several decades. The remarkable success of the LHC programme has demonstrated that a facility of this scale can deliver exceptional science. The aim is that ELBNF will provide a second example of how the world’s high-energy-physics community can come together to deliver an amazing scientific programme. New collaborators are still welcome to join in the pursuit.

• In March the new collaboration chose the name DUNE for Deep Underground Neutrino Experiment.

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The discovery of high-energy astrophysical neutrinos initially announced by IceCube in 2013 provided an added boost to the planning for new, larger facilities that could study the signal in detail and identify its origins. Three large projects – KM3NeT in the Mediterranean Sea, IceCube-Gen2 at the South Pole and the Gigaton Volume Detector (GVD) in Lake Baikal – are already working together in the framework of the Global Neutrino Network (CERN Courier December 2014 p11).

In December, the RWTH Aachen University hosted a workshop on these projects and their low-energy sub-detectors, ORCA and PINGU, which aim at determination of the neutrino-mass hierarchy through precision measurements of atmospheric-neutrino oscillations. Some 80 participants from 11 different countries came to discuss visionary strategies for detector optimization and technological aspects common to the high-energy neutrino telescopes.

Photodetection techniques, as well as trigger and readout strategies, formed one particular focus. All of the detectors are based on optical modules consisting of photomultiplier tubes (PMTs) housed in a pressure-resistant glass vessel together with their digitization and read-out electronics. Representatives of the experiments shared their experiences on the development, in situ performance and mass-production of the different designs. While the baseline design for IceCube-Gen2 follows the proven IceCube modules closely, KM3NeT has successfully deployed and operated prototypes of a new design consisting of 31 3″ PMTs housed in a single glass sphere, which offer superior timing and intrinsic directional information. Adaption of this technology for IceCube is under investigation.

New and innovative designs for optical modules were also reviewed, for example a large-area sensor employing wavelength-shifting and light-guiding techniques to collect photons in the blue and UV range and guide them to a small-diameter low-noise PMT. Presentations from Hamamatsu Photonics and Nautilus Marine Service on the latest developments in photosensors and glass housings, respectively, complemented the other talks nicely.

In addition, discussions centred on auxiliary science projects that can be carried out at the planned infrastructures. These can serve as a test bed for completely new detection technologies, such as acoustic neutrino detection, which is possible in water and ice, or radio neutrino detection, which is limited to ice as the target medium. Furthermore, IceCube-Gen2 at the South Pole offers the unique possibility to install detectors on the surface above the telescope deep in the ice, the latter acting as a detector for high-energy muons from cosmic-ray-induced extensive air showers. Indeed, the interest in cosmic-ray detectors on top of an extended IceCube telescope reaches beyond the communities of the three big projects.

The second focus of the workshop addressed the physics potential of cosmic-ray detection on the multi-kilometre scale, and especially the use of a surface array as an air-shower veto for the detection of astrophysical neutrinos from the southern sky at the South Pole. The rationale for surface veto techniques is the fact that the main background to extraterrestrial neutrinos from the upper hemisphere consists of muons and neutrinos produced in the Earth’s atmosphere. These particles are correlated to extended air showers, which can be tagged by a surface array. While upward-moving neutrinos have to traverse the entire Earth and are absorbed above some 100 TeV energy, downward-moving neutrinos do not suffer from absorption. Therefore a surface veto is especially powerful for catching larger numbers of cosmic neutrinos at the very highest energies.

The capabilities of these surface extensions together with deep-ice components will be evaluated in the near future. Presentations at the workshop on various detection techniques – such as charged-particle detectors, imaging air-Cherenkov telescopes and Cherenkov timing arrays – allowed detailed comparisons of their capabilities. Parameters of interest are duty cycle, energy threshold and the cost for construction and installation. The development of different detectors for applications in harsh environments is already on its way and the first prototypes are scheduled to be tested in 2015.

• The Detector Design and Technology for Next Generation Neutrino Observatories workshop was supported by the Helmholtz Alliance for Astroparticle Physics (HAP), RWTH Aachen University, and Hamamatsu Photonics. For more information, visit hap2014.physik.rwth-aachen.de.

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On 10 January, the ground-breaking ceremony for the Jiangmen Underground Neutrino Observatory (JUNO) took place in Jiangmen City, Guangdong Province, China. More than 300 scientists and officials from China and other countries attended and witnessed this historical moment.

JUNO is the second China-based neutrino project, following the Daya Bay Reactor experiment, and is designed to determine the neutrino mass-hierarchy via precision measurements of the reactor-neutrino energy spectrum. The experiment is scheduled to start data-taking in 2020 and is expected to operate for at least 20 years. The neutrino detector, which is the experiment’s core component, will be the world’s largest and highest-precision liquid scintillator detector.

After the determination of the θ₁₃ mixing angle by Daya Bay and other experiments, the next challenge to the international neutrino community is to determine the neutrino-mass hierarchy. Sensitivity analysis shows that the preferred range for the experiment stations is 50–55 km from a nuclear reactor. Jinji Town, the detector site chosen for the JUNO experiment, is 53 km from both Yangjiang and Taishan Nuclear Power Plants, which provide a total thermal power of 35.8 GW. By 2020, the effective power will be the highest in the world.

The JUNO international collaboration, established on 28 July 2014, already consists of more than 300 members from 45 institutions in nine countries and regions, and more than 10 institutions from five countries are planning to join.

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On 20–12 September, CERN hosted the fifth annual Mediterranean-Antarctic Neutrino Telescope Symposium (MANTS). For the first time, the meeting was organized under the GNN umbrella.

The idea to link more closely the various neutrino telescope projects under both water and ice has been a topic for discussion in the international community of high-energy neutrino astrophysicists for several years. On 15 October 2013, representatives of the ANTARES, BAIKAL, IceCube and KM3NeT collaborations signed a memorandum of understanding for co-operation within a Global Neutrino Network (GNN). GNN aims for extended inter-collaboration exchanges, more coherent strategy planning and exploitation of the resulting synergistic effects.

No doubt, the evidence for extraterrestrial neutrinos recently reported by IceCube at the South Pole (“Cosmic neutrinos and more: IceCube’s first three years”) has given wings to GNN, and is encouraging the KM3NeT (in the Mediterranean Sea) and GVD (Lake Baikal) collaborations in their efforts to achieve appropriate funding to build northern-hemisphere cubic-kilometre detectors. IceCube is also working towards an extension of its present configuration.

One focus of the MANTS meeting was, naturally, on the most recent results from IceCube and ANTARES, and their relevance for future projects. The initial configurations of KM3NeT (with three to four times the sensitivity of ANTARES) and GVD (with sensitivity similar to ANTARES) could provide additional information on the characteristics of the IceCube signals, first because they look at a complementary part of the sky, and second because water has optical properties that are different from ice. Cross-checks with different systematics are of the highest importance for these detectors in natural media. As an example, KM3NeT will measure down-going muons from cosmic-ray interactions in the atmosphere with superb precision. This could help in determining more precisely the flux of atmospheric neutrinos co-generated with those muons, in particular those from the decay of charmed mesons, which are expected to have particularly high energies and therefore could mimic an extraterrestrial signal.

A large part of the meeting was devoted to finding the best “figures of merit” characterizing the physics capabilities of the detectors. These not only allow comparison of the different projects, but also provide an important tool to optimize future detector configurations. The latter also concerns the two sub-projects that aim to determine the neutrino mass hierarchy using atmospheric neutrinos. These are both small, high-density versions of the huge kilometre-scale arrays: PINGU at the South Pole and ORCA in the Mediterranean Sea. In this effort a particularly close co-operation has emerged during the past year, down to technical details.

Combining data from different detectors is another aspect of GNN. A recent common analysis of IceCube and ANTARES sky maps has provided the best sensitivity ever for point sources in certain regions of the sky, and will be published soon. Further goals of GNN include the co-ordination of alert and multimessenger policies, exchange and mutual checks of software, creation of a common software pool, development of standards for data representation, cross-checks of results with different systematics, and the organization of schools and other forums for exchanging expertise and experts. Mutual representation in the experiments’ science advisory committees is another way to promote close contact and mutual understanding.

Contingent upon availability of funding, the mid 2020s could see one Global Neutrino Observatory, with instrumented volumes of 5–8 km³ in each hemisphere. This would, finally, fully raise the curtain just lifted by IceCube, and provide a rich view on the high-energy neutrino sky.

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For the past four years, the IceCube Neutrino Observatory, located at the South Pole, has been collecting data on some of the most violent collisions in the universe. Fulfilling its pre-construction aspirations, the detector has observed astrophysical neutrinos with energies above 60 TeV, at the “magic” 5σ significance. The most energetic neutrino observed had an energy of about 2 PeV (2 × 10¹⁵ eV) – 250 times higher than the beam energy of the LHC.

These neutrinos are just one highlight of IceCube’s broad physics programme, which encompasses searches for astrophysical neutrinos, searches for neutrinos from dark matter, studies of neutrino oscillations, cosmic-ray physics, and searches for supernovae and a variety of exotica. All of these studies take advantage of a unique detector at a unique location: the South Pole.

IceCube observes the Cherenkov light emitted by charged particles produced in neutrino interactions in 1 km³ of transparent Antarctic ice. The detector is the ice itself, and is read out by 5160 optical sensors. Figure 1 shows how the optical sensors are distributed throughout the 1 km³ of ice, 1.5 km beneath the geographic South Pole. They are deployed 17 m apart, on 86 vertical cables or “strings”. Seventy-eight of the strings are spaced horizontally, 125 m apart in a grid of equilateral triangles forming a hexagonal array across an area of a square kilometre. The remaining eight strings form a more densely instrumented sub-array called DeepCore. In DeepCore, most of the sensors are concentrated in the lower 350 m of the detector.

Each sensor, or digital optical module (DOM), is like a miniature satellite made up of a 10 inch (25 cm) photomultiplier tube together with data-acquisition and control electronics. These include a custom 300 megasample/s waveform digitizer with 14 bits of dynamic range, plus light sources for calibrations, all consuming a power of less than 5 W. The hardware is protected by a centimetre-thick pressure vessel.

The ice in IceCube formed from compacted snow that fell on Antarctica 100,000 years ago.

The ice in IceCube formed from compacted snow that fell on Antarctica 100,000 years ago. Its properties vary with depth, with layers reflecting the atmospheric conditions when the snow first fell. Measuring the optical properties of this ice has been one of the major challenges of IceCube, involving custom “dust loggers”, studies with LED “flashers” and cosmic-ray muons. During the past decade, the collaboration has found that the ice is layered, that the layers are not perfectly flat and, most recently, that the light scattering is somewhat anisotropic. Each insight has led to a better understanding of the detector and to smaller systematic uncertainties. Fortunately, advances in computing technology have allowed IceCube’s simulations to keep up, more or less, with the increasingly complex models of light propagation in the ice.

The distributed sensors give IceCube strong pattern-recognition capabilities. The three neutrino flavours – ν_e, ν_μ and ν_τ – each leave different signatures in the detector. Charged-current ν_μ produce high-energy muons, which leave long tracks. All ν_e interactions, and all neutral-current interactions, produce hadronic or electromagnetic showers. High-energy ν_τ produce a characteristic “double-bang” signature – one shower when the ν_τ interacts and a second when the τ decays. More complex topologies have also been studied, including tracks that start in the detector as well as pairs of parallel tracks.

Despite past doubts, IceCube works and works well. More than 98% of the sensors are fully operational, and another 1% are usable – most of the failures occurred during deployment. The post-deployment attrition rate is a few DOMs per year, so IceCube will be able to operate for as long as required. The “live” times are also impressive – in the range of 99%.

IceCube has excellent reconstruction capabilities. For kilometre-long muon tracks, the angular resolution is better than 0.4°, verified by studying the shadow of the Moon cast by cosmic rays. For high-energy contained events, the angular resolution can reach 15°, and at high energies the visible energy can be determined to better than 15%.

Cosmic neutrinos

The detector’s dynamic range covers from 10 GeV to infinity. The higher energy the neutrino, the easier it is to detect. Every six minutes, IceCube records an atmospheric neutrino, from the decay of pions, kaons and heavier particles produced in cosmic-ray air showers. These 100,000 neutrinos collected every year are interesting in their own right, but they are also the background to any search for cosmic neutrinos. On top of this, the detector records about 3000 atmospheric muons every second. This is a painful background for neutrino searches, but a gold mine for cosmic-ray physics.

Although IceCube has an extremely rich physics programme, the centrepiece is clearly the search for cosmic neutrinos. Many signatures have been proposed for these neutrinos: point source searches, a high-energy diffuse flux, identified ν_τ, and others. IceCube has looked for all of these.

Point-source searches are the simplest strategy conceptually – just create a sky map showing the arrival directions of all of the detected neutrinos. Figure 2 shows the IceCube sky map containing 400,000 events gathered across four years (Aartsen et al. 2014c). In the southern hemisphere, the large background of downgoing muons is only partially counteracted by selecting high-energy muons, which are less likely to be of atmospheric origin. The 177,544 events in the northern-hemisphere sample are mostly from ν_μ. So far, there is no statistically significant evidence for any hot spots, even in searches for spatially extended sources. IceCube has also looked for variable sources, whether episodic or periodic, with similar results. These limits constrain theoretical models, especially those involving gamma-ray bursts.

If there are enough weak sources in the cosmos, they should be visible as an aggregate, diffuse flux. This diffuse flux is expected to have a harder energy spectrum than do atmospheric neutrinos. Calculations have indicated that IceCube would be more sensitive to this diffuse flux than to point sources, which is indeed the case. Several early searches, using the partially completed detector, turned up intriguing hints of an excess over the expected atmospheric neutrino flux. Then the search diverged from the anticipated script.

One of the first searches for diffuse neutrinos with the complete detector looked for ultra-high-energy cosmogenic neutrinos – neutrinos produced when ultra-high-energy cosmic-ray protons (E > 4 × 10¹⁹ eV) interact with photons of around 10^–4 eV in the cosmic-microwave background, exciting them to a Δ⁺ resonance. The decay products of the pion produced in the Δ’s decay include a neutrino with a typical energy of 10¹⁸ eV (1 EeV). The search found two spectacular events, one of which is shown in figure 3. Both events were well contained within the detector – clearly neutrinos. Both had energies around 1 PeV – spectacular, but too low to be produced by cosmic rays interacting with CMB photons. Such events were completely unexpected.

Inspired by these events, the IceCube collaboration instigated a follow-up search that used two powerful techniques (Aartsen et al. 2013). The first was a filter to identify neutrino interactions that originate inside the detector, as distinct from events originating outside it. The filter divides the instrumented volume into an outer-veto shield and a 420 megatonne inner active volume. Figure 4 shows how this veto works: by rejecting events with significant in-time energy deposition in the veto region, neutrino interactions within the detector’s fiducial volume can be separated from backgrounds. For neutrinos that are contained within the instrumented volume of ice, the detector functions as a total absorption calorimeter, measuring energy with 15% resolution. It is flavour-blind, equally sensitive to hadronic or electromagnetic showers and to muon tracks. This veto analysis also used a “tagging” approach to estimate the atmospheric-muon background using the data, rather than relying on simulations. Because of the veto, the analysis could observe neutrinos from all directions in the sky.

The second innovation was to take advantage of the fact that downgoing atmospheric neutrinos should be accompanied by a cosmic-ray air shower depositing one or more muons inside IceCube. In contrast, cosmic neutrinos should be unaccompanied. A very high-energy, isolated downgoing neutrino is highly likely to be cosmic.

The follow-up search found 26 additional events. Although no new events had an energy near 1 PeV, the analysis produced evidence for cosmic neutrinos at the 4σ level. To clinch the case, the collaboration added a third year of data, pushing the significance above the “magic” 5σ level (Aartsen et al. 2014a). One of the new events had an energy above 2 PeV, making it the most energetic neutrino ever seen.

The observation of a flux of cosmic neutrinos was soon confirmed by the independent and more traditional analysis recording the diffuse flux of muon neutrinos penetrating the Earth. Both observations are consistent with a diffuse flux composed equally of the three neutrino flavours. No statistically significant hot spots were seen. The observed flux is consistent with that expected from cosmic accelerators producing equal energies in gamma rays, neutrinos and, possibly, cosmic rays.

Newer studies are shedding more light on these events, extending contained-event studies down to lower energies and adding flavour identification. At energies above 10 TeV, the astrophysical neutrino flux can be fit by a single power-law spectrum that is significantly harder than the background cosmic-ray muon spectrum:
φ_ν = 2.06^+0.4_–0.3 × 10^–18 (E_v/100TeV)^{–2.46±0.12} GeV^–1 cm^–2 sr^–1 s (Aartsen et al. 2014d).

Within the limited statistics, the flux appears isotropic and consistent with the ν_e:ν_μ:ν_τ ratio of 1:1:1 that is expected for cosmic neutrinos. The majority of the events appear to be extragalactic. Some might originate in the Galaxy, but there is no compelling statistical evidence for that at this point.

Many explanations have been proposed for the IceCube observations, ranging from the relativistic particle jets emitted by active galactic nuclei to gamma-ray bursts, to starburst galaxies to magnetars. IceCube’s dedicated searches do, however, disfavour gamma-ray bursts as the source. A spectral index of –2 (dN_ν/dE ~ E^–2), predicted by Fermi shock-acceleration models, is also disfavoured, but many other scenarios are possible. Of course, the answer is clear: more data are needed.

Other physics

The 100,000 neutrinos and 85 × 10⁹ cosmic-ray events recorded each year provide ample opportunities to search for dark matter and to study cosmic rays as well as neutrinos themselves. IceCube has measured the cosmic-ray spectrum and composition and observed anisotropies in the spectrum at the 10^–4 level that have thus far defied explanation. It has also studied atypical events, such as muon-free showers expected from photons with peta-electron-volt energies, produced in the Galaxy, and investigated isolated muons produced in air showers. The latter have separations that shift from an exponential decrease to a power-law separation spectrum, as predicted by perturbative QCD.

IceCube observes atmospheric neutrinos across an energy range from 10 GeV to 100 TeV – at higher energies, the atmospheric flux is swamped by the flux of cosmic neutrinos. As figure 5 shows, the flux is consistent with expectations across a large energy range. Lower-energy neutrinos are of particular interest because they are sensitive to neutrino oscillations. For neutrinos passing vertically through the Earth, the ν_μ flux develops a first minimum at 28 GeV.

Figure 6 shows the observed ν_μ flux, seen in one year of data, using well-reconstructed events contained within DeepCore. The change in flux with distance travelled/energy (L/E) is consistent with neutrino oscillations and inconsistent with a no-oscillation scenario. IceCube constraints on the mixing angle θ₂₃ and |Δm²₃₂| are comparable to constraints from other experiments.

IceCube also searched for neutrinos from dark-matter annihilation. Dark matter can be gravitationally captured by the Earth, the Sun, or in the centre or halo of the Galaxy. It then accumulates and the dark-matter particles annihilate, producing neutrinos. IceCube has searched for signatures of this annihilation, and has set limits. The Sun is a particularly interesting option, producing a characteristic dark-matter signature that cannot be explained by any astrophysical scenario. It is also mostly protons, allowing IceCube to set the world’s best limits on the spin-dependent cross-section for the interaction of dark-matter particles with ordinary matter.

The collaboration has also looked for even more exotic signatures, such as magnetic monopoles and pairs of upgoing particles. One particularly spectacular and interesting signature could come from the next supernova in the Galaxy. These explosions produce a blast of neutrinos with 10–50 MeV energy. This energy level is far too low to trigger IceCube directly, but the neutrinos would be visible as a collective increase in the singles rate in the buried IceCube photomultipliers. Moreover, IceCube has a huge effective area, which will allow measurements of the time structure of the supernova-neutrino pulse with millisecond precision.

IceCube is still a novel instrument unlikely to have exhausted its discovery potential. However, at high energies, it might not be big enough. Doing neutrino astronomy could require samples of 1000 or more, high-energy neutrino events. In addition, some key physics questions require a detector with a lower energy threshold. These two considerations are driving two different upgrade projects.

The IceCube high-energy extension (IceCube-gen2) aims for a detector with a 10-times-larger instrumented volume.

DeepCore has demonstrated that IceCube is capable of making precise measurements of neutrino-oscillation parameters. If precision studies can be extended to neutrino energies below 10 GeV, it will be possible to determine the neutrino-mass hierarchy. Neutrinos passing through the Earth interact coherently with matter electrons, modifying the oscillation pattern in a way that differs for normal and inverted hierarchies. In addition to a threshold of a few giga-electron-volts, this measurement requires improved control of systematic uncertainties. An expanded collaboration has come together to pursue the construction of a high-density infill array called Precision In Ice Next-Generation Upgrade, or PINGU (Aartsen et al. 2014b). The present design consists of 40 additional high-sensitivity strings equipped with improved calibration devices. PINGU should be able to determine the mass hierarchy with 3σ significance within about three years, independent of the value of the CP-violation phase.

The IceCube high-energy extension (IceCube-gen2) aims for a detector with a 10-times-larger instrumented volume, albeit with a higher energy threshold. It will explore the observed cosmic neutrino flux and pin down its origin. With a sample of more than 100 cosmic neutrinos per year, it will be possible to observe multiple neutrinos from the same sources, and so do astronomy. The instrument will also have an improved sensitivity to study the ultra-high-energy neutrinos produced in the interactions of cosmic rays with microwave photons.

Of course, IceCube is not the only collaboration studying high-energy neutrinos. Projects on the cubic-kilometre scale are also being prepared in the Mediterranean Sea (KM3NeT) and in Lake Baikal (GVD), with a field of view complementary to that of IceCube. Within KM3NeT, ORCA, a proposed low-threshold detector, would pursue the same physics as PINGU. And the radio-detection experiments ANITA, ARA, GNO and ARIANNA are beginning to explore the neutrino sky at energies above 10¹⁷ eV.

After a decade of construction, the completed IceCube detector came on line in December 2010. It has achieved the outstanding goal of observing cosmic neutrinos and has produced important results in diverse areas: cosmic-ray physics, dark-matter searches and neutrino oscillations, not to mention its contributions to glaciology and solar physics. The observation of cosmic neutrinos at the peta-electron-volt energy scale has attracted enormous attention, with many suggestions about the location of the requisite cosmic accelerators.

Looking ahead, IceCube anticipates two important extensions: PINGU, which will determine the neutrino-mass hierarchy, and IceCube-gen2, which will expand a discovery instrument into an astronomical telescope.

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When neutrinos scatter coherently off an entire nucleus, the exchange of a Z⁰ or W^± boson can lead to the production of a pion with the same charge. The first observations of such interactions came in the early 1980s from the Aachen–Padova experiment at CERN’s Proton Synchrotron, followed by an analysis of earlier data from Gargamelle. A handful of other experiments at CERN, Fermilab and Serpukhov provided additional measurements before the end of the 1990s. These experiments determined interaction cross-sections for high-energy neutrinos (5–100 GeV), which were in good agreement with the model of Deiter Rein and Lalit Sehgal of Aachen. Published shortly after the first measurements were made, their model is still used in some Monte Carlo simulations.

More recently, the SciBooNE and K2K collaborations attempted to measure the coherent production of charged pions at lower neutrino energies (less than 2 GeV). However, they found no evidence of the interaction, and published upper limits below Rein and Sehgal’s original estimation. These results, together with recent observations of coherent production of neutral pions by the MiniBooNE and NOMAD collaborations, have now motivated renewed interest and new models of coherent pion production.

In the NuMI beamline at Fermilab – which has a peak energy of 3.5 GeV and energies beyond 20 GeV – coherent charged-current pion production accounts for only 1% of all of the ways that a neutrino can interact. Nevertheless, both the ArgoNeuT and MINERvA collaborations have now successfully measured the cross-sections for charged-current pion production by recording the interactions of neutrinos and antineutrinos.

ArgoNeuT uses a liquid-argon time-projection chamber (TPC), and has results for coherent interactions of antineutrinos and neutrinos at mean energies of 3.6 GeV and 9.6 GeV, respectively (Acciarri et al. 2014). A very limited exposure produced only 30 candidates for coherent interactions of antineutrinos and 24 for neutrinos (figure 1), but a measurement was possible thanks to the high resolution and precise calorimetry achieved by the TPC. It is the first time that this interaction has been measured in a liquid-argon detector. ArgoNeuT’s results agree with the state-of-the-art theoretical predictions (figure 2), but its small detector size (<0.5 tonnes) limits the precision of the measurements.

MINERvA uses a fine-grained scintillator tracker to fully reconstruct and select the coherent interactions in a model-independent analysis. With 770 antineutrino and 1628 neutrino candidates, this experiment measured the cross-section as a function of incident antineutrino and neutrino energy (figure 2). The measured spectrum and angle of the coherently produced pions are not consistent with models used by oscillation experiments (Higuera et al. 2014), and they will be used to correct those models.

The techniques developed during both the ArgoNeuT and MINERvA analyses will be used by larger liquid-argon experiments, such as MicroBooNE, that are part of the new short-baseline neutrino programme at Fermilab. While these experiments will focus on neutrino oscillations and the search for new physics, they will also provide more insight into coherent pion production.

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The Borexino experiment at the INFN Gran Sasso National Laboratories has measured the energy of the Sun in real time, showing for the first time that the energy released today at its centre is exactly the same as that produced 100,000 years ago. This has been possible through the experiment’s direct detection of the low-energy neutrinos produced in the initial nuclear reactions occurring in the solar core.

Previous measurements of solar energy have always been made on the radiation (photons) that currently illuminate and heat the Earth. The energy of this radiation originates in the Sun’s nuclear reactions, but, on average, has taken 100,000 years to travel through the dense solar matter and reach the surface. Neutrinos produced by the same nuclear reactions, on the other hand, take only a few seconds to escape from the Sun before making the eight-minute journey to Earth. The comparison between the neutrino measurement now published by the Borexino collaboration and the previous measurements on the emission of radiant energy from the Sun shows that solar activity has not changed during the past 100,000 years.

Borexino is an ultra-sensitive liquid-scintillator detector designed to detect low-energy neutrino events in real time at a high rate, in contrast to earlier radioachemical experiments such as Homestake, GALLEX and SAGE. The experiment previously has focussed on measurements of neutrinos from ⁷Be and ⁸B – nuclei formed in certain branches of the principal chain of reactions that converts hydrogen to helium at the heart of the Sun. The ⁷Be neutrinos constitute only 7% of the neutrino flux from the Sun and the ⁸B neutrinos even less, but they have been key to the discovery and study of the phenomenon of neutrino oscillations, most recently by Borexino. In contrast in this latest work, Borexino has focused on the neutrinos from the fusion of two hydrogen nuclei (protons) to form deuterium – the seed reaction of the nuclear-fusion cycle that produces about 99% of the solar power, some 3.84 × 10³³ ergs/s.

The difficulty of the new measurement lies in the extremely low energy of these so-called pp neutrinos, which is smaller than that of the others emitted by the Sun. The capability to do this successfully makes the Borexino detector unique, and has also allowed the study of neutrinos produced by the Earth.

The Borexino experiment is the result of a collaboration between European countries (Italy, Germany, France, Poland), the US and Russia, and it will take data for at least another four years, improving the accuracy of measurements already made and addressing others of great importance, for both particle physics as well as astrophysics.

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The particle detector for MicroBooNE, a new short-baseline neutrino experiment at Fermi National Accelerator Laboratory, was gently lowered into place on 23 June. It is expected to detect its first neutrinos this winter.

The detector – a time-projection chamber surrounded by a 12-m-long cylindrical vessel – was carefully transported by truck across the Fermilab site, from the assembly building where the detector was constructed to the experimental hall nearly 5 km away. The 30-tonne object was then hoisted up by a crane, lowered through the open roof of a new building and placed into its permanent home, directly in the path of Fermilab’s Booster neutrino beamline.

When filled with 170 tonnes of liquid argon, MicroBooNE will look for low-energy neutrino oscillations to help to resolve the origin of a mysterious low-energy excess of particle events seen by the MiniBooNE experiment, which used the same beam line and relied on a Cherenkov detector filled with mineral oil.

The MicroBooNE time-projection chamber is the largest ever built in the US and is equipped with 8256 delicate gold-plated wires. The three layers of wires will capture pictures of particle interactions at different points in space and time. The superb resolution of the time-projection chamber will allow scientists to check whether the excess of MiniBooNE events is due to photons or electrons.

Using one of the most sophisticated processing programs ever designed for a neutrino experiment, computers will sift through the thousands of neutrino interactions recorded every day and create 3D images of the most interesting ones. The MicroBooNE team will use that data to learn more about neutrino oscillations and to narrow the search for a hypothesized fourth type of neutrino.

MicroBooNE is a cornerstone of Fermilab’s short-baseline neutrino programme, which could also see the addition of two more neutrino detectors along the Booster neutrino beamline, to refute or confirm hints of a fourth type of neutrino first reported by the LSND collaboration at Los Alamos National Laboratory. In its recent report, the Particle Physics Project Prioritization Panel (P5) expressed strong support for a short-baseline neutrino programme at Fermilab. The report was commissioned by the High Energy Physics Advisory Panel, which advises both the US Department of Energy and the National Science Foundation on funding priorities.

The detector technology used in MicroBooNE will serve as a prototype for a much larger liquid-argon detector that has been proposed as part of a long-baseline neutrino facility to be hosted at Fermilab. The P5 report strongly supports this larger experiment, which will be designed and funded through a global collaboration.

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The P5 report culminates a process open to all members of the US particle-physics community that lasted more than a year. It was presented to the High Energy Physics Advisory Panel (HEPAP), a body that formally advises the US Department of Energy Office of Science and the National Science Foundation.

The plan recommends a US particle-physics programme that will pursue research related to the Higgs boson, neutrinos, dark matter, dark energy and inflation, and as-yet-undiscovered particles, interactions and physical principles. It advises increasing investment in the construction of new experimental facilities.

The P5 panel envisions the US as the host of an international programme of neutrino research that will operate the world’s most powerful neutrino beam and, with international partners, build a major long-baseline neutrino facility complemented by multiple small, short-baseline neutrino experiments. Launching this programme will involve a change in direction, because the panel recommends reformulating the currently planned Long-Baseline Neutrino Experiment as an internationally designed, co-ordinated and funded programme called the Long-Baseline Neutrino Facility, or LBNF. The facility would use a neutrino beam at Fermilab, upgraded through the proposed project called the Proton Improvement Plan II, together with a massive liquid-argon neutrino detector placed underground, probably at the Sanford Underground Research Facility in South Dakota, and a smaller detector placed nearer to the source of the beam.

The plan emphasizes the need for the US to begin several planned second-generation dark-matter experiments immediately, with a vision to build at least one large, third-generation experiment in the US near the beginning of the next decade. It also recommends increasing funding for the particle-physics components of cosmic surveys.

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NOvA, Fermilab’s new flagship neutrino-oscillation experiment, has recorded its first neutrinos and is now poised to make precision measurements of electron-neutrino (ν_e) appearance and muon-neutrino (ν_μ) disappearance. These data will help to unravel remaining unknowns in understanding neutrino masses and mixing. In the now standard picture of neutrinos, the three electroweak flavour states (ν_e, ν_μ and ν_τ) are mixtures of the mass eigenstates (ν₁, ν₂ and ν₃) related by a unitary matrix that is parameterized by three mixing angles and a charge-parity (CP) violating phase. Neutrinos are produced and detected in flavour eigenstates but propagate in mass eigenstates. Interference among the mass states means that a neutrino created in a definite flavour state can later be detected in a different flavour state. This oscillation probability is determined by the sizes of the mixing angles, the splittings in the neutrino masses, the energy of the neutrino and the distance it has travelled. Measurements of the oscillation probabilities of neutrinos of known energy that travel a known distance reveal the underlying mass-splittings and mixings.

Thanks to experiments using neutrinos produced in the Sun, in the atmosphere, at particle accelerators and in nuclear reactors, researchers have found out a great deal about neutrino masses and mixing. We know that two neutrinos are relatively close in mass and that the third is relatively far away in mass. We know that the mixing angles are all relatively large, in contrast to mixing angles in the quark sector, which are small. We also know that the two neutrinos that are relatively close in mass contain most of the electron-neutrino flavour, and that the third is a nearly equal combination of muon and tau flavour. However, we do not know if the third mass eigenstate is composed of more ν_μ or ν_τ, or if a new symmetry keeps these two contributions equal. We do not know if neutrinos violate CP symmetry, and we do not know the ordering of the neutrino masses.

Neutrinos could follow a normal hierarchy, with most of the ν_e content contained in the lightest two states, or they could follow an inverted hierarchy with the ν_e content predominantly in the heaviest two states. The neutrino-mass hierarchy is one specific prediction of different grand-unification theories, with implications for cosmological measurements of the absolute scale of neutrino mass. The hierarchy, in combination with results from neutrinoless double-beta decay experiments, plays an important role in determining the Dirac or Majorana nature of the neutrino.

NOvA will use two detectors to measure oscillation probabilities in Fermilab’s NuMI (Neutrinos at the Main Injector) muon-neutrino beam. When neutrinos travel the 810 km between Fermilab and Ash River, Minnesota, through the crust of the Earth, scattering of ν_e on atomic electrons can either enhance or suppress the oscillation probability, depending on the mass hierarchy. The effect is opposite in neutrinos compared with antineutrinos, so by comparing the oscillation probability measured in neutrinos with that measured with antineutrinos, NOvA can determine the mass hierarchy, resolve the nature of ν₃, and begin the study of CP violation in neutrinos.

To achieve these goals, NOvA requires an intense neutrino and antineutrino source. NuMI had routinely delivered 320 kW of beam power to the MINOS and MINERvA experiments during operation of the Tevatron. However, with Tevatron operations now ended, the accelerator complex has been reconfigured to provide twice the beam power to the NuMI beamline. During a shutdown of a year and a half starting in the spring of 2012, a major RF upgrade in the Main Injector was accomplished, reducing its cycle time from 2.2 s to 1.67 s. Additionally, the Recycler ring, which was key to antiproton generation for the Tevatron, was converted to a proton accumulator so that protons can be integrated and stored during the Main Injector ramp from 8 GeV at injection to 120 GeV.

At the same time, the NuMI beamline underwent a transformation to accommodate the higher proton intensities required for NOvA. The neutrino target and focusing horns were replaced and repositioned. The new beam provides higher-energy neutrinos on-axis, but at 14 mrad off the beam axis – where the NOvA detectors are located – the neutrino energy spectrum is peaked narrowly at 2 GeV, the perfect energy for the long-baseline oscillations that NOvA will study.

Beam began circulating again in the Main Injector in September 2013 and work started on commissioning the new accelerator in the Recycler ring. The Recycler is now normally included in operations, and work is underway to “slip stack” routinely in this new machine – a delicate manoeuvre where one bunch is injected then shifted to a different orbit to make room for a second bunch in the same RF bucket. Once the two bunches are merged, they are accelerated together. This work is expected to bring the NuMI intensity to 450 kW by the end of the year, and ongoing upgrades to the Booster ring that feeds this complex are expected to bring the intensity to 700 kW within another year. Since coming back up from the shutdown, the complex has achieved a peak beam power of more than 300 kW and delivered almost 2.5 × 10²⁰ protons to NOvA and the other two neutrino experiments sharing the beam, MINOS+ and MINERvA.

The NOvA detector must be big to overcome the small size of the neutrino-interaction cross-section and the 810 km distance from the neutrino source

In addition to an intense beam, NOvA also requires a massive far detector and a functionally identical near detector. Like all neutrino detectors, the NOvA detector must be big to overcome the small size of the neutrino-interaction cross-section and the 810 km distance from the neutrino source. Being big, however, is not enough. The detector must also be highly segmented to prevent the numerous cosmic rays that cross the detector from interfering with neutrino events from Fermilab. Furthermore, to separate electromagnetic showers from electron-neutrino events from similar showers from other sources, especially the decays of π⁰ mesons, heavy materials of high atomic number (Z) such as steel – which are normally used to build large structures – have not been employed.

The NOvA detectors (figures 1 and 2) are a unique solution to the particular challenges of observing ν_e appearance using the NuMI neutrino beamline. The NOvA far detector is a 14,000 tonne detector, using 9000 tonnes of liquid scintillator – the largest quantity of liquid scintillator ever produced for a physics experiment – to record the tracks of charged particles. The scintillator is contained in a 15.6 × 15.6 × 60 m³, 5000-tonne PVC structure constructed from modules assembled at a factory operated by collaborators at the University of Minnesota. A crew of more than 700 undergraduate students directed by 10 full-time staff members ran the factory. These pieces were shipped to the Ash River Laboratory in Northern Minnesota, where another 45 full-time staff members built the 28 free-standing blocks that make up the detector. The 190-tonne blocks were constructed horizontally on an enormous table, which later pivoted them into a vertical position and placed them in the experimental hall.

In addition to containing the scintillator, the PVC structure segments the detector into 4 cm × 6 cm × 15.6 m channels. Light produced in these channels by the charged particles that traverse them bounces 10 times, on average, before it is captured in a wavelength-shifting fibre. To ensure that enough light is captured in the fibre, a special PVC formulation with enhanced reflectivity had to be developed. The large size of the detector and the large number of channels required more than 10,000 km of wavelength-shifting fibre – enough to stretch from the supplier in Japan to the Ash River Laboratory in a single unbroken thread.

This large-scale assembly project is now finished. The last detector block was put in place in February of this year and the last of the 11 million litres of scintillator made for the experiment was delivered in April. While the task of outfitting the detector with electronics is continuing through the summer, the experiment recorded its first neutrino event in November last year, and has analysed millions of cosmic-ray tracks. This analysis has verified that the scintillator, PVC, fibre and electronics work together as designed to move the scintillation light from the far end of the detection channels to where it can be recorded. As figure 3 shows, the efficiency for detecting a minimum-ionizing particle crossing a cell at the furthest end from the read-out is above 90%, which is key to the tracking and particle-identification performance of the detector.

First events

Cosmic-ray interactions are an excellent source for detector calibration, but they are also a potential background to the neutrino selection. While the NuMI beam is delivered in regular bursts, 10 μs in duration, the high cosmic rate on the surface means that about 1.5 cosmic interactions are expected in the detector during the spill. On the other hand, after oscillations, a NuMI neutrino interacts in the far detector once every 12,000 spills, or only about once every four hours. Containment and directional cuts suppress the cosmic rate by about a factor of 10⁵, with only minimal loss of neutrino events. Figure 4 shows a charged-current ν_μ interaction identified in the NOvA far detector, along with two cosmic-ray muons zipping through during the beam spill. Figure 5 shows the same event, reconstructed, as well as a timing distribution of other neutrino candidates found in the far detector. The neutrino candidates pile up at the arrival time measured in the NOvA prototype detector delayed by the neutrino flight time between the two sites, confirming that NOvA can identify neutrinos among the cosmic-ray backgrounds.

In May, one sixth of the full near detector was turned on for the first time, and neutrinos were seen in the first spills

While relatively simple cuts can be used to separate beam neutrino events from cosmogenic events, further suppression of cosmic rays is required to achieve the oscillation physics goals. Multivariate event-selection algorithms tuned to recognize the differing topologies of ν_μ and ν_e charged-current and and neutral-current interactions suppress the cosmic-ray background rate by a further two orders of magnitude. Data collected when the beam is known to be off confirm that the necessary level of rejection can be achieved: the cosmic-ray background in a one-year exposure is predicted to be one event in the ν_μ sample and 0.5 events in the ν_e sample, well below the expected signal rates of 75 and 15 neutrinos in these samples.

The NOvA collaboration is now eagerly awaiting data from the near detector, which are needed to measure the beam composition and energy spectrum before oscillations have developed. The near-detector data will set the background expectation in the far detector for the ν_e appearance channel, and determine the unoscillated event rate as a function of energy for the ν_μ disappearance channel. In May, one sixth of the full near detector was turned on for the first time, and neutrinos were seen in the first spills. NOvA researchers are looking forward to an exciting summer.

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The OPERA experiment at the INFN Gran Sasso Laboratory has detected a fourth example of neutrino oscillation, with a muon neutrino (ν_μ) produced at CERN detected as a τ neutrino (ν_τ) after travelling a distance of 730 km.

The international OPERA experiment, which involves 140 physicists from 28 research institutes in 11 countries, was designed to observe this exceptionally rare phenomenon, gathering data in the neutrino beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project. Generated by decays of pions and kaons made in the interactions of a proton beam from the Super Proton Synchrotron with a graphite target, the beam consisted mainly of ν_μ that would pass unhindered through the Earth’s crust towards Gran Sasso. The appearance and subsequent decay of a τ lepton in the OPERA experiment provides the telltale sign of ν_μ to ν_τ oscillation through a charged-current interaction.

After the first neutrinos arrived at the Gran Sasso Laboratory in 2006, the experiment gathered data for five consecutive years, from 2008 to 2012, during which the CNGS beam delivered a total of 17.97 × 10¹⁹ protons on target, yielding 19,500 neutrino events in the detector. The first ν_τ was observed in 2010, the second and third ones in 2012 and 2013, respectively.

The detection of the fourth ν_τ is important confirmation of the events seen previously. It means that the ν_μ to ν_τ transition has been seen for the first time with a statistical significance exceeding the 4σ level, so that OPERA can now claim the observation of this extremely rare phenomenon. The collaboration will continue to search for ν_τ in the data that remain to be analysed.

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On 11 February, the NOvA collaboration announced the detection of the first neutrinos in the long-baseline experiment’s far detector in northern Minnesota. The neutrino beam is generated at Fermilab and sent 800 km through the Earth’s surface to the far detector. Once completed, the near and far detectors will weigh 300 and 14,000 tonnes, respectively. Installation of the last module of the far detector is scheduled for early this spring and outfitting of both detectors with electronics should be completed in summer.

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On 7 April 1934, the journal Nature published a paper – The “Neutrino” – in which Hans Bethe and Rudolf Peierls considered some of the consequences of Wolfgang Pauli’s proposal that a lightweight neutral, spin 1/2 particle is emitted in beta decay together with an electron (Bethe and Peierls 1934). Enrico Fermi had only recently put forward his theory of beta decay, in which he considered both the electron and the neutral particle – the neutrino – not as pre-existing in the nucleus, but as created at the time of the decay. As Bethe and pointed out, such a creation process implies annihilation processes, in particular one in which a neutrino interacts with a nucleus and disappears, giving rise to an electron (or positron) and a different nucleus with a charge changed by one unit. They went on to estimate the cross-section for such a reaction and argued that for a neutrino energy of 2.3 MeV it would be less than 10^–44 cm² – “corresponding to a penetrating power of 10¹⁴ km in solid matter”. This led them to conclude that even with a cross-section rising with energy as expected in Fermi’s theory, “it seems highly improbable that, even for cosmic ray energies, the cross-section becomes large enough to allow the process to be observed”.

However, as Peierls commented 50 years later, they had not allowed for “the existence of nuclear reactors producing neutrinos in vast quantities” or for the “ingenuity of experimentalists” (Peierls 1983). These two factors combined to underpin the first observation of neutrinos by Clyde Cowan and Fred Reines at the Savannah River nuclear reactor in 1956, and during the following years the continuing ingenuity of the particle-physics community led to the production of neutrinos with much higher energies at particle accelerators. With the reasonably large numbers of neutrinos that could be produced at accelerators, and cross-sections increasing with energy, their measurement became a respectable line of research, and ingenious experimentalists began to turn neutrinos into a tool to investigate different aspects of particle physics. Following the idea of Mel Schwarz, studies with neutrino beams began at the Alternating Gradient Synchrotron at Brookhaven and at the Proton Synchrotron (PS) at CERN in the early 1960s, and were taken to higher energies at Fermilab and at CERN’s Super Proton Synchrotron in the 1970s. They continue today, using high-intensity beams produced at Fermilab and the Japan Proton Accelerator Research Complex.

At CERN, the story began in earnest in 1963 with an intense neutrino beam provided courtesy of Simon van der Meer’s invention of the neutrino horn – a magnetic device that focuses the charged particles (pions and kaons) whose decays give rise to the neutrinos – coupled with a scheme for fast ejection of the proton beam from the PS devised by Berend Kuiper and Günther Plass in 1959. First in line to receive the neutrinos was the 500-litre Heavy-Liquid Bubble Chamber (HLBC) built by a team led by Colin Rammm

The combination worked well, allowing the measurement of neutrino cross-sections for various kinds of interactions. Studies of quasi-electric scattering, such as ν + n → μ + p, mirrored for the weak interaction – the only way that neutrinos can interact – measurements that had been made for several years in elastic electron-nucleon scattering at Stanford. The cross-sections measured in electron scattering were used to derive electromagnetic “form factors” – an expression of how much the scattering is “smeared out” by an extended object, in comparison with the expectation from point-like scattering. The early results from the HLBC showed the weak form factors to be similar to those measured in electron scattering. Electrons and neutrinos were apparently “seeing” the same thing in (quasi-)elastic scattering.

Less easy to understand at the time were the “deep” inelastic events where the nucleus was more severely disrupted and several pions produced, as in ν + N → μ + N + nπ. The measurements of such events revealed a cross-section that increased with neutrino energy, rising to more than 10 times the quasi-elastic cross-section. Don Perkins of Oxford University reported on these results at a conference in Siena in 1963. “They were clearly trying to tell us a very simple thing,” he recalled nearly 40 years later, “but unfortunately, we were just not listening!” (Perkins 2001)

Indeed, most physicists thought that this sub-structure (quarks) was more of a mathematical convenience

The following year, Murray Gell-Mann and George Zweig put forward their ideas about a new substructure to matter – the “quarks” or “aces” that made up the hadrons, including the protons and neutrons of the nucleus. Today, this sub-structure is a fundamental part of the Standard Model of particle physics, and many young people learn about quarks as basic building blocks of matter while still at school. At the time, however, it was a different story because there was no evidence for real particles with charges of 1/3 and 2/3 that the proposals required. Indeed, most physicists thought that this sub-structure was more of a mathematical convenience.

The picture began to change at a conference in Vienna in 1968, when deep-inelastic electron-scattering measurements at SLAC’s 3 km linear accelerator by the SLAC-MIT experiment – the direct descendent of the earlier experiments in Stanford – made people sit up and listen. The deep-inelastic cross-section divided by the cross-section expected from a point charge (Mott scattering) showed a surprisingly flat dependence on the square of the momentum transfer (q²). This was consistent with scattering from points within the nucleons rather than the smeared-out structure seen in elastic scattering, which gives a cross-section that falls away rapidly with q². Moreover, the measurements yielded a structure function – akin to the form factor of elastic scattering – that depended very little on q² at large values of energy transfer, ν. Indeed, the data appeared consistent with a proposal by James Bjorken that in the limit of high q², the deep-inelastic structure functions would depend only on a dimensionless variable, x = q²/2Mν, for a target nucleon mass M. This behaviour, called “scaling”, implied point-like scattering.

What did this imply for neutrinos? If they really were seeing the same structure as electrons – if the deep-inelastic structure function depended only on the dimensionless variable x – then the total cross-section should simply rise linearly with neutrino energy. As soon as Perkins saw the first results from SLAC in 1968, he quickly revisited the data from the Heavy-Liquid Bubble Chamber and found that this was indeed the case (Perkins 2001).

The “points” in the nucleons became known as partons – a name coined by Richard Feynman, who had been trying to understand high-energy proton–proton collisions in terms of point-like constituents. A key question to be resolved was whether the partons had the attributes of quarks, such as spin 1/2 and the predicted fractional charges. The SLAC-MIT group went on to make an outstanding series of systematic measurements over the next couple of years, which provided undisputable evidence for the point-like structure within the nucleon – and led in 1990 to the award of the Nobel Prize in Physics to Jerome Friedman, Henry Kendall and Richard Taylor. This wealth of data included results that clearly indicated that the partons must have spin 1/2.

In the meantime, a new heavy-liquid bubble chamber had been installed at the PS at CERN. Gargamelle was 4.8 m long and contained 18 tonnes of Freon, and had been designed and built at Orsay under the inspired leadership of André Lagarrigue, of the Ecole Polytechnique. It was to become famous for the first observation of weak neutral currents in 1973 (CERN Courier September 2009 p25). The same year saw the first publication of total cross-sections measured in Gargamelle, based on a few thousand events, not only with neutrinos but also antineutrinos. The results had in fact been aired first the previous year at Fermilab, at the 16th International Conference on High-Energy Physics (ICHEP). They showed clearly the linear rise with energy consistent with point-like scattering. Moreover, the neutrino cross-section was around three times larger than that for antineutrinos, which confirmed that neutrinos and antineutrinos were also seeing structure with spin 1/2.

However, there was still more. With data from both neutrinos and antineutrinos, the team could derive one of the structure functions that was also measured in deep-inelastic electron-scattering. Electrons scatter electromagnetically in proportion to the square of the charge of whatever is doing the scattering. Neutrinos, by contrast, are blind to charge and scatter only weakly. A comparison of the two structure functions should depend only on the mean charge squared seen by the electrons, which for quarks of charges 2/3 and –1/3 in equal numbers in the deuterium target used in the experiment at SLAC would be 5/18. So, the structure function from neutrino scattering, with no charge dependence, should be 18/5 of that for electron-scattering. As Feynman himself said: ” If you never did believe that ‘nonsense’ that quarks have non-integral charges, we have a chance now, in comparing neutrino to electron scattering, to finally discover for the first time whether the idea…is physically sensible, physically sound; that’s exciting.” (Feynman 1974)

At the 17th ICHEP held in London in 1974, particle physicists from around the world were able to see the results from Gargamelle for themselves – the neutrino structure function, when multiplied by 18/5, did indeed fit closely with the data from the SLAC-MIT experiment (see figure). Forty years on from the paper by Bethe and Peierls, neutrino cross-sections were not only being measured, they were revealing a more fundamental layer to nature – the quarks.

These early experiments were just the beginning of what became a prodigious effort, mainly at CERN and Fermilab, using neutrinos to probe the structure of the nucleon within the context of quantum chromodynamics, the theory of quarks and the gluons that bind them together. And the effort is not finished, because neutrinos are still being used to understand puzzles that remain in the structure of the nucleus. But that is another story.

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Neutrino physicists enjoy a challenge, and the members of the MINERvA (Main INjector ExpeRiment for v-A) collaboration at Fermilab are no exception. MINERvA seeks to make precise measurements of neutrino reactions using the Neutrinos at the Main Injector (NuMI) beam on both light and heavy nuclei. Does this goal reflect the wisdom of the collaboration’s namesake? Current and future accelerator-based neutrino-oscillation experiments must precisely predict neutrino reactions on the nuclei if they are to search successfully for CP violation in oscillations. Understanding matter–antimatter asymmetries might in turn lead to a microphysical mechanism to answer the most existential of questions: why are we here? Although MINERvA might provide vital assistance in meeting this worthy goal, neutrinos never yield answers easily. Moreover, using neutrinos to probe the dynamics of reactions on complicated nuclei convolutes two challenges.

The history of neutrinos is wrought with theorists underestimating the persistence of experimentalists (Close 2010). Wolfgang Pauli’s quip about the prediction of the neutrino, “I have done a terrible thing. I have postulated a particle that cannot be detected,” is a famous example. Nature rejected Enrico Fermi’s 1933 paper explaining β decay, saying it “contained speculations too remote from reality to be of interest to readers”. Eighty years ago, when Hans Bethe and Rudolf Peierls calculated the first prediction for the neutrino cross-section, they said, “there is no practical way of detecting a neutrino” (p23). But when does practicality ever stop physicists? The theoretical framework developed during the following two decades predicted numerous measurements of great interest using neutrinos, but the technology of the time was not sufficient to enable those measurements. The story of neutrinos across the ensuing decades is that of many dedicated experimentalists overcoming these barriers. Today, the MINERvA experiment continues Fermilab’s rich history of difficult neutrino measurements.

Neutrinos at Fermilab

Fermilab’s research on neutrinos is as old as the lab itself. While it was still being built, the first director, Robert Wilson, said in 1971 that the initial aim of experiments on the accelerator system was to detect a neutrino. “I feel that we then will be in business to do experiments on our accelerator…[Experiment E1A collaborators’] enthusiasm and improvisation gives us a real incentive to provide them with the neutrinos they are waiting for.” The first experiment, E1A, was designed to study the weak interaction using neutrinos, and was one of the first experiments to see evidence of the weak neutral current. In the early years, neutrino detectors at Fermilab were both the “15 foot” (4.6 m) bubble chamber filled with neon or hydrogen, and coarse-grained calorimeters. As the lab grew, the detector technologies expanded to include emulsion, oil-based Cherenkov detectors, totally active scintillator detectors, and liquid-argon time-projection chambers. The physics programme expanded as well, to include 42 neutrino experiments either completed (37), running (3) or being commissioned (2). The NuTeV experiment collected an unprecedented million high-energy neutrino and antineutrino interactions, of both charged and neutral currents. It provided precise measurements of structure functions and a measurement of the weak mixing angle in an off-shell process with comparable precision to contemporary W-mass measurements (Formaggio and Zeller 2013). Then in 2001, the DONuT experiment observed the τ neutrino – the last of the fundamental fermions to be detected.

While much of the progress of particle physics has come by making proton beams of higher and higher energies, the most recent progress at Fermilab has come from making neutrino beams of lower energies but higher intensities. This shift reflects the new focus on neutrino oscillations, where the small neutrino mass demands low-energy beams sent over long distances. While NuTeV and DONuT used beams of 100 GeV neutrinos in the 1990s, the MiniBooNE experiment, started in 2001, used a 1 GeV neutrino beam to search for oscillations over a short distance. The MINOS experiment, which started in 2005, used 3 GeV neutrinos and measured them both at Fermilab and in a detector 735 km away, to study oscillations that were seen in atmospheric neutrinos. MicroBooNE and NOvA – two experiments completing construction at the time of this article – will place yet more sensitive detectors in these neutrino beamlines. Fermilab is also planning the Long-Baseline Neutrino Experiment to be broadly sensitive to resolve CP violation in neutrinos.

A spectrum of interactions

Depending on the energy of the neutrino, different types of interactions will take place (Formaggio and Zeller 2013, Kopeliovich et al. 2012). In low-energy interactions, the neutrino will scatter from the entire nucleus, perhaps ejecting one or more of the constituent nucleons in a process referred to as quasi-elastic scattering. At slightly higher energies, the neutrinos interact with nucleons and can excite a nucleon into a baryon resonance that typically decays to create new final-state hadrons. In the high-energy limit, much of the scattering can be described as neutrinos scattering from individual quarks in the familiar deep-inelastic scattering framework. MINERvA seeks to study this entire spectrum of interactions.

To measure CP violation in neutrino-oscillation experiments, quasi-elastic scattering is an important channel. In a simple model where the nucleons of the nucleus live in a nuclear binding potential, the reaction rate can be predicted. In addition, an accurate estimate of the energy of the incoming neutrino can be made using only the final-state charged lepton’s energy and angle, which are easy to measure even in a massive neutrino-oscillation experiment. However, the MiniBooNE experiment at Fermilab and the NOMAD experiment at CERN both measured the quasi-elastic cross-section and found contradictory results in the framework of this simple model (Formaggio and Zeller 2013, Kopeliovich et al. 2012).

One possible explanation of this discrepancy can be found in more sophisticated treatments of the environment in which the interaction occurs (Formaggio and Zeller 2013, Kopeliovich et al. 2012). The simple relativistic Fermi-gas model treats the nucleus as quasi-free independent nucleons with Fermi motion in a uniform binding potential. The spectral-function model includes more correlation among the nucleons in the nucleus. However, more complete models that include the interactions among the many nucleons in the nucleus modify the quasi-elastic reaction significantly. In addition to modelling the nuclear environment on the initial reaction, final-state interactions of produced hadrons inside the nucleus must also be modelled. For example, if a pion is created inside the nucleus, it might be absorbed on interacting with other nucleons before leaving the nucleus. Experimentalists must provide sufficient data to distinguish between the models.

The ever-elusive neutrino has forced experimentalists to develop clever ways to measure neutrino cross-sections, and this is exactly what MINERvA is designed to do with precision. The experiment uses the NuMI beam – a highly intense neutrino beam. The MINERvA detector is made of finely segmented scintillators, allowing the measurement of the angles and energies of the particles within. Figures 1 and 2 show the detector and a typical event in the nuclear targets. The MINOS near-detector, located just behind MINERvA, is used to measure the momentum and charge of the muons. With this information, MINERvA can measure precise cross-sections of different types of neutrino interactions: quasi-elastic, resonance production, and deep-inelastic scatters, among others.

The MINERvA collaboration began by studying the quasi-elastic muon neutrino scattering for both neutrinos (MINERvA 2013b) and antineutrinos (MINERvA 2013a). By measuring the muon kinematics to estimate the neutrino energies, they were able to measure the neutrino and antineutrino cross-sections. The data, shown in figure 3, suggest that the nucleons do spend some time in the nucleus joined together in pairs. When the neutrino interacts with the pair, the pair is kicked out of the nucleus. Using the visible energy around the nucleus allowed a search for evidence of the pair of nucleons. Experience from electron quasi-elastic scattering leads to an expectation of final-state proton–proton pairs for neutrino quasi-elastic scattering and neutron–neutron pairs for antineutrino scattering. MINERvA’s measurements of the energy around the vertex in both neutrino and antineutrino quasi-elastic scattering support this expectation (figure 3, right).

A 30-year-old puzzle

Another surprise beyond the standard picture in lepton–nucleus scattering emerged 30 years ago in deep-inelastic muon scattering. The European Muon Collaboration (EMC) observed a modification of the structure functions in heavy nuclei that is still theoretically unresolved, in part because there is no other reaction in which an analogous effect is observe. Neutrino and antineutrino deep-inelastic scattering might see related effects with different leptonic currents, and therefore different couplings to the constituents of the nucleus (Gallagher et al. 2010, Kopeliovich et al. 2012). MINERvA has begun this study using large targets of active scintillator and passive graphite, iron and lead (MINERvA 2014). Figure 4 shows the ratio of lead to scintillator and illustrates behaviour that is not in agreement with a model based on charged-lepton scattering modifications of deep-inelastic scattering and the elastic physics described above. Similar behaviour, but with smaller deviations from the model, is observed in the ratio of iron to scintillator. MINERvA’s investigation of this effect will benefit greatly from its current operation in the upgraded NuMI beam for the NOvA experiment, which is more intense and higher in (the beamline’s on-axis) energy. Both features will allow more access to the kinematic regions where deep-inelastic scattering dominates. By including a long period of antineutrino operation needed for NOvA’s oscillation studies, an even more complete survey of the nucleons can be done. The end result of these investigations will be a data set that can offer a new window on the process behind the EMC effect.

Initially in the history of the neutrino, theory led experiment by several decades

Initially in the history of the neutrino, theory led experiment by several decades. Now, experiment leads theory. Neutrino physics has repeatedly identified interesting and unexpected physics. Currently, physics is trying to understand how the most abundant particle in the universe interacts in the simplest of situations. MINERvA is just getting started on answering these types of questions and there are many more interactions to study. The collaboration is also looking at what happens when neutrinos make pions or kaons when they hit a nucleus, and how well they can measure the number of times a neutrino scatters off an electron – the only “standard candle” in this business.

Time after time, models fail to predict what is seen in neutrino physics. The MINERvA experiment, among others, has shown that quasi-elastic scattering is a wonderful tool to study the nuclear environment. Maybe the use of neutrinos, once thought to be impossible to detect, as a probe to study inside the nucleus, would make Pauli, Fermi, Bethe, Peierls and the rest chuckle.

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The IceCube collaboration has reported evidence, at the 4σ level, for a diffuse (i.e. isotropic) flux of high-energy extra-terrestrial neutrinos, mostly above 60 TeV (Aartsen et al. 2013). Using two years of data, the analysis selected 28 events – including the two events previously reported with energies above 1 PeV. This is substantially above the background estimate of 12.1 events.

In the energy range 60 TeV to 2 PeV, the data are well described by a neutrino energy spectrum that varies as E^–2, with a flux E_ν²φ <1.2±0.4 × 10^–8 GeV cm^–2 s^–1 sr^–1. This is near the Waxman–Bahcall bound – the flux expected if cosmic-ray nuclei undergoing acceleration interact strongly in their sources and transfer most of their energy to secondary particles (mainly π^± and K^±) whose decays produce neutrinos. For an E^–2 spectrum, the data indicate that there must be a cut-off at a few peta-electron-volts, otherwise more energetic events would have been seen. Alternatively, the energy spectrum might be somewhat softer: an E^–2.2 spectrum fits the data well.

The analysis combined multiple techniques to isolate the 28 events from a much larger background of downward-going cosmic-ray muons and atmospheric neutrinos. The event selection was simple. It involved choosing events that originated within the detector and produced more than 6000 observed photoelectrons. The origination criteria used the outer portion of the detector as a veto, therefore removing events with early light, which could be from entering tracks. The analysis estimated the muon backgrounds using two independent, nested veto regions around a smaller fiducial volume. Events tagged in the outer veto that missed the inner veto were used to determine the veto-miss fraction. The veto also eliminated energetic, downward-going atmospheric neutrinos, which should be accompanied by a cosmic-ray air shower with energetic muons that should trigger the veto.

The selection criteria were largely insensitive to the event topology, so the analysis selected ν_e, ν_μ and ν_τ interactions, providing they occurred inside the detector. The events fall into two classes: long tracks (muons) from ν_μ charged-current interactions, plus cascades, electromagnetic or hadronic showers from ν_e and most ν_τ charged-current interactions, and neutral-current interactions of any flavour. Most of the events that IceCube sees are atmospheric ν_μ charged-current interactions, but the requirement that the events originate within the detector, depositing 6000 photoelectrons, changes the fraction. Of the 28 events found, only seven are classed as track-like. While this is consistent with the 1:1:1 ratio of ν_e:ν_μ:ν_τ, it is a lower fraction of tracks than expected for atmospheric neutrinos, which are mostly ν_μ.

Figure 1(a) shows the deposited energy for the 28 events, together with the expected backgrounds for muons, conventional atmospheric neutrinos and prompt atmospheric neutrinos from the decay of charmed particles. The atmospheric neutrino fluxes include the effect of the downward-going veto. There is a substantial uncertainty for the prompt flux, which has not yet been observed – the range is based on theoretical estimates, with upper limits from previous IceCube studies. Although the two 1 PeV neutrinos are prominent, the signal rises above the background at energies above 60 TeV. The black line shows the best fit to an E^–2 astrophysical signal.

Figure 1(b) compares the zenith angle distribution of the data with the same background estimates. The muon background is entirely downward-going, while the atmospheric neutrino background is largely upward-going, owing to a combination of the downward-going veto and the absorption of high-energy neutrinos in the Earth. An isotropic extra-terrestrial signal would also be mostly downward-going because of this absorption. Of the 28 selected events, 24 are downward-going, which is more than expected from the background plus the astrophysical component from the fit. The excess is about 1.5σ. The angular agreement for a purely atmospheric neutrino flux is even worse.

This analysis shows that cosmic accelerators emit a significant fraction of their energies as neutrinos. The collaboration has also studied the arrival directions of the events, but observes no significant clusters. However, follow-up studies should further characterize the radiation and pin down its source. Already, some tantalizing hints have been presented at the 2013 International Cosmic Ray Conference.

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Paris, dans les années 1950. Michel a 11 ans et voit des bulles, ce dont il est très fier. Terme résolument non scientifique, le mot ” bulle ” désigne pour le narrateur – Michel – ” une myriade de points lumineux dansant dans tous les sens “, points lumineux qui se révèleront être, au fil des pages, des ” neutrinos “. Nous y voilà.

Vous l’aurez compris, bien qu’écrit par un physicien des particules spécialisé en physique des neutrinos, ce livre est un roman. L’objectif n’étant pas de vous en apprendre des kilomètres sur ces fameux neutrinos, mais de vous embarquer dans une histoire dont ils sont les protagonistes. Et si l’histoire est contée par un jeune narrateur passionné de physique, il n’en reste pas moins qu’il s’agit d’un enfant, et non pas (encore) d’un physicien des particules.

L’intrigue, si je puis donner à l’histoire cette connotation très romanesque, est somme toute assez simple. Michel, écolier plutôt mauvais en maths mais bon en imagination, vit dans un minuscule appartement parisien avec ses parents. Il va à l’école à pied, troue ses chaussettes, accompagne sa mère au marché le jeudi et à la messe le dimanche, passe ses vacances d’été à la campagne, collectionne les timbres, adore les truffes au chocolat, et se délecte des histoires de science de son oncle Albert, fonctionnaire tire-au-flanc et lecteur assidu de magazines de vulgarisation scientifique. Mais ce qui anime surtout Michel, moins son histoire, c’est cette étrange capacité à voir des neutrinos.

Mais ne vous méprenez pas, les neutrinos de Michel sont loin de coller à l’idée que l’on s’en fait au CERN. Pour Michel, ce ne sont en effet ni plus ni moins que les constituants de l’âme des êtres vivants, ou, comme les décrit encore le narrateur, ” notre carburant spirituel “. Ce qui explique d’ailleurs que les jeunes en émettent plus que les vieux, et que ceux qui n’en émettent plus sont morts. CQFD.

Au final, ce livre est un long voyage dans la tête d’un gamin de 11 ans, à la rencontre de ses idées farfelues, de ses expérimentations et déductions scientifiques, de ses découvertes triomphantes et de ses confrontations au monde des adultes. Certains passages sont franchement réjouissants, et l’on finit par se prendre d’affection pour le jeune Michel, qui garde précieusement au fond de sa poche, un marron, une bille et une boîte pleine de neutrinos.

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In August, after a 16-month shutdown, Fermilab resumed operation of its Neutrinos at the Main Injector (NuMI) beamline and sent the first muon neutrinos to three neutrino experiments: MINERvA, MINOS+ and the new NOvA experiment. Numerous upgrades to the Fermilab accelerator complex have laid the groundwork for increasing the beam power of the NuMI beamline from about 350 kW to 700 kW. In addition, Fermilab has changed the NuMI horn and target configurations to deliver a higher-energy neutrino beam compared with pre-shutdown operation.

The NOvA experiment – still under construction – will study the properties of neutrinos, especially the elusive transition of muon neutrinos into electron neutrinos. The results will help to answer questions about the neutrino-mass hierarchy, neutrino oscillations and the role that neutrinos might have played in the evolution of the universe. The construction of the NOvA near and far detectors, both located 14 milliradians off the NuMI beam axis, is advancing quickly.

The near detector – located 100 m underground in a new cavern that has been excavated at Fermilab – has more than a quarter of its structure in place. Meanwhile, 810 km away in northern Minnesota, technicians have installed more than three quarters of the plastic structure that is the skeleton of the huge, 14,000 tonne far detector. More than 70% of the far detector’s plastic modules have been filled with 5.7 million litres of liquid scintillator and the first modules are recording data. The first part of the near detector will turn on before the end of the year.

The MINOS+ experiment uses the existing MINOS near and far detectors and takes advantage of the fact that the post-shutdown NuMI neutrino beam differs from earlier operation. The new beam, which is optimized for the NOvA experiment, yields higher-energy neutrinos at the location of the MINOS detector and should not show measurable oscillations. This means that MINOS+ can look for surprises. New types of neutrino interactions could deform the spectrum at the far detector’s distance of 735 km and the observation of additional neutrinos would indicate new physics. The experiment can even search for extra dimensions.

MINERvA – located in front of the MINOS near detector – is a dedicated neutrino-interaction experiment designed to study a range of nuclei. These measurements will not only improve understanding of the nucleus but will also be important inputs to neutrino-oscillation experiments. The MINERvA detector has several targets including helium, carbon, scintillator, water, steel and lead, followed by precise tracking and calorimetry. Previously, MINERvA took data in a beam around 3 GeV, where quasi-elastic, resonance and deep-inelastic scattering processes contribute roughly equally to the event rates. With the new, higher-energy neutrino beam, the event rate is much higher and the events are dominated by deep-inelastic scattering. While MINERvA will study all processes at higher energy, the huge increase in deep-inelastic scattering events in particular will allow precise measurements of the nuclear structure-functions.

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The international Daya Bay collaboration has announced new results, including their first data on how neutrino oscillations vary with neutrino energy, which allows them to measure mass splitting between different neutrino types. Mass splitting represents the frequency of neutrino oscillation while mixing angles represent the amplitude and both are crucial for understanding the nature of neutrinos.

The Daya Bay experiment, which is run by a collaboration of more than 200 scientists from six regions and countries, is located close to the Daya Bay and Ling Ao nuclear power plants, 55 km north-east of Hong Kong. It measures neutrino oscillation using electron antineutrinos created by six powerful nuclear reactors. Because the antineutrinos travel up to 2 km to underground detectors, some transform to another type and therefore apparently disappear. The rate at which they transform is the basis for measuring the mixing angle, while the mass splitting is determined by studying how the rate of transformation depends on the antineutrino energy.

Daya Bay’s first results were announced in March 2012 and established an unexpectedly large value for the mixing angle θ₁₃ – the last of three long-sought neutrino mixing angles. The new results, which were announced at the XVth International Workshop on Neutrino Factories, Super Beams and Beta Beams (NuFact2013) in Beijing, give a more precise value – sin² 2θ₁₃ = 0.090±0.009. The improvement in precision is a result both of having more data to analyse and of having the additional measurements on how the oscillation process varies with neutrino energy.

The KamLAND experiment in Japan and other solar neutrino experiments have previously measured the mass splitting Δm²₂₁ by observing the disappearance of electron antineutrinos from reactors some 160 km from the detector and the disappearance of electron neutrinos from the Sun. The long-baseline experiments MINOS in the US and Super-Kamiokande and T2K in Japan have determined the effective mass splitting |Δm²_μμ| using muon neutrinos. The Daya Bay collaboration has now measured the magnitude of the mass splitting |Δm²_ee| to be (2.54±0.20) × 10^–3 eV².

The result establishes that the electron neutrino has all three mass states and is consistent with that from muon neutrinos measured by MINOS. Precision measurements of the energy dependence should further the goal of establishing a hierarchy of the three mass states for each neutrino flavour.

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When the Swedish warship Vasa capsized in Stockholm harbour on her maiden voyage in 1628, many hearts must have also sunk metaphorically, as they did at CERN in September 2008 when the LHC’s start-up came to an abrupt end. Now, the raised and preserved Vasa is the pride of Stockholm and the LHC – following a successful restart in 2009 – is leading research in particle physics at the high-energy frontier. This year the two icons crossed paths when the International Europhysics Conference on High-Energy Physics, EPS-HEP 2013, took place in Stockholm on 18–24 July, hosted by the KTH (Royal Institute of Technology) and Stockholm University. Latest results from the LHC experiments featured in many of the parallel, plenary and poster sessions – and the 750 or so participants had the opportunity to see the Vasa for themselves at the conference dinner. There was, of course, much more and this report can only touch on some of the highlights.

Coming a year after the first announcement of the discovery of a “Higgs-like” boson on 4 July 2012, the conference was the perfect occasion for a birthday celebration for the new particle. Not only has its identity been more firmly established in the intervening time – it almost certainly is a Higgs boson – but many of its attributes have been measured by the ATLAS and CMS experiments at the LHC, as well as by the CDF and DØ collaborations using data collected at Fermilab’s Tevatron. At 125.5 GeV/c², its mass is known to within 0.5% precision – better than for any quark – and several tests by ATLAS and CMS show that its spin-parity, J^P, is compatible with the 0⁺ expected for a Standard Model Higgs boson. These results exclude other models to greater than 95% confidence level (CL), while a new result from DØ rejects a graviton-like 2⁺ at >99.2% CL.

The mass of the top quark is in fact so large – 173 GeV/c² – that it decays before forming particles

The new boson’s couplings provide a crucial test of whether it is the particle responsible for electroweak-symmetry breaking in the Standard Model. A useful parameterization for this test is the ratio of the observed signal strength to the Standard Model prediction, μ = (σ × BR)/(σ × BR)_SM, where σ is the cross-section and BR the branching fraction. The results for the five major decay channels measured so far (γγ, WW*, ZZ*, bb and ττ) are consistent with the expectations for a Standard Model Higgs boson, i.e. μ = 1, to 15% accuracy. Although it is too light to decay to the heaviest quark – top, t – and its antiquark, the new boson can in principle be produced together with a tt pair, so yielding a sixth coupling. While this is a challenging channel, new results from CMS and ATLAS are starting to approach the level of sensitivity for the Standard Model Higgs boson, which bodes well for its future use.

The mass of the top quark is in fact so large – 173 GeV/c² – that it decays before forming particles, making it possible to study the “bare” quark. At the conference, the CMS collaboration announced the first observation, at 6.0σ, of the associated production of a top quark and a W boson, in line with the Standard Model’s prediction. Both ATLAS and CMS had previously found evidence for this process but not to this significance. The DØ collaboration presented latest results on the lepton-based forward–backward lepton asymmetry in tt- production, which had previously indicated some deviation from theory. The new measurement, based on the full data set of 9.7 fb^–1 of proton–antiproton collisions at the Tevatron, gives an asymmetry of (4.7±2.3 stat.+1.1–1.4 syst.)%, which is consistent with predictions from the Standard Model to next-to-leading order.

The study of B hadrons, which contain the next heaviest quark, b, is one of the aspects of flavour physics that could yield hints of new physics. One of the highlights of the conference was the announcement of the observation of the rare decay mode B⁰_s → μμ by both the LHCb and CMS collaborations, at 4.0 and 4.3σ, respectively. While there had been hopes that this decay channel might open a window on new physics, the long-awaited results align with the predictions of the Standard Model. The BaBar and Belle collaborations also reported on their precise measurements of the decay B → D^(*)τν_τ at SLAC and KEK, respectively, which together disagree with the Standard Model at the 4.3σ level. The results rule out one model that adds a second Higgs doublet to the Standard Model (2HDM type II) but are consistent with a different variant, 2HDM type III – a reminder that the highest energies are not the only place where new physics could emerge.

Precision, precision

Precise measurements require precise predictions for comparison and here theoretical physics has seen a revolution in calculating next-to-leading order (NLO) effects, involving a single loop in the related Feynman diagrams. Rapid progress during the past few years has meant that the experimentalists’ wish-list for QCD calculations at NLO relevant to the LHC is now fulfilled, including such high-multiplicity final states as W + 4 jets and even W + 5 jets. Techniques for calculating loops automatically should in future provide a “do-it-yourself” approach for experimentalists. The new frontier for the theorists, meanwhile, is at next-to-NLO (NNLO), where some measurements – such as pp → tt – are already at an accuracy of a few per cent and some processes – such as pp → γγ – could have large corrections, up to 40–50%. So a new wish-list is forming, which will keep theorists busy while the automatic code takes over at NLO.

With a measurement of the mass for the Higgs boson, small corrections to the theoretical predictions for many measurable quantities – such as the ratio between the masses of the W and the top quark – can now be calculated more precisely. The goal is to see if the Standard Model gives a consistent and coherent picture when everything is put together. The GFitter collaboration of theorists and experimentalists presented its latest global Standard Model fit to electroweak measurements, which includes the legacy both from the experiments at CERN’s Large Electron–Positron Collider and from the SLAC Large Detector, together with the most recent theoretical calculations. The results for 21 parameters show little tension between experiment and the Standard Model, with no discrepancy exceeding 2.5σ, the largest being in the forward–backward asymmetry for bottom quarks.

There is more to research at the LHC than the deep and persistent probing of the Standard Model. The ALICE, LHCb, CMS and ATLAS collaborations presented new results from high-energy lead–lead and proton–lead collisions at the LHC. The most intriguing results come from the analysis of proton–lead collisions and reveal features that previously were seen only in lead–lead collisions, where the hot dense matter that was created appears to behave like a perfect liquid. The new results could indicate that similar effects occur in proton–lead collisions, even though far fewer protons and neutrons are involved. Other results from ALICE included the observation of higher yields of J/ψ particles in heavy-ion collisions at the LHC than at Brookhaven’s Relativistic Heavy-Ion Collider, although the densities are much higher at the LHC. The measurements in proton–lead collisions should cast light on this finding by allowing initial-state effects to be disentangled from those for cold nuclear matter.

Supersymmetry and dark matter

The energy frontier of the LHC has long promised the prospect of physics beyond the Standard Model, in particular through evidence for a new symmetry – supersymmetry. The ATLAS and CMS collaborations presented their extensive searches for supersymmetric particles in which they have explored a vast range of masses and other parameters but found nothing. However, assumptions involved in the work so far mean that there are regions of parameter space that remain unexplored. So while supersymmetry may be “under siege”, its survival is certainly still possible. At the same time, creative searches for evidence of extra dimensions and many kinds of “exotics” – such as excited quarks and leptons – have likewise produced no signs of anything new.

However, evidence that there must almost certainly be some kind of new particle comes from the existence of dark, non-hadronic matter in the universe. Latest results from the Planck mission show that this should make up some 26.8% of the universe – about 4% more than previously thought. This drives the search for weakly interacting particles (WIMPs) that could constitute dark matter, which is becoming a worldwide effort. Indeed, although the Higgs boson may have been top of the bill for hadron-collider physics, more generally, the number of papers with dark matter in the title is growing faster than those on the Higgs boson.

While experiments at the LHC look for the production of new kinds of particles with the correct properties to make dark matter, “direct” searches seek evidence of interactions of dark-matter particles in the local galaxy as they pass through highly sensitive detectors on Earth. Such experiments are showing an impressive evolution with time, increasing in sensitivity by about a factor of 10 every two years and now reaching cross-sections down to 10^–8 pb. Among the many results presented, an analysis of 140.2-kg days of data in the silicon detectors of the CDMS II experiment revealed three WIMP-candidate events with an expected background of 0.7. A likelihood analysis gives a 0.19% probability for the known background-only hypothesis.

Neutrinos are the one type of known particle that provide a view outside the Standard Mode

“Indirect” searches, by contrast, involve in particular the search from signals from dark-matter annihilation in the cosmos. In 2012, an analysis of publically available data from 43 months of the Fermi Large Area Telescope (LAT) indicated a puzzling signal at 130 GeV, with the interesting possibility that these γ rays could originate from the annihilation of dark-matter particles. A new analysis by the Fermi LAT team of four years’ worth of data gives preliminary indications of an effect with a local significance of 3.35σ but the global significance is less than 2σ. The HESS II experiment is currently accumulating data and might soon be able to cross-check these results.

With their small but nonzero mass and consequent oscillations from one flavour to another, neutrinos are the one type of known particle that provide a view outside the Standard Model. At the conference, the T2K collaboration announced the first definitive observation at 7.5σ of the transition ν_μ → ν_e in the high-energy ν_μ beam that travels 295 km from the Japan Proton Accelerator Complex to the Super-Kamiokande detector. Meanwhile, the Double CHOOZ experiment, which studies ν_e produced in a nuclear reactor, has refined its measurement of θ₁₃, one of the parameters characterizing neutrino oscillations, by using two independent methods that allow much better control of the backgrounds. The GERDA collaboration uses yet another means to investigate if neutrinos are their own antiparticles, by searching for the neutrinoless double-beta decay of the isotope ⁷⁶Ge in a detector in the INFN Gran Sasso National Laboratory. The experiment has completed its first phase and finds no sign of this process but now provides the world’s best lower limit for the half-life at 2.1 × 10²³ years.

On the other side of the world, deep in the ice beneath the South Pole, the IceCube collaboration has recently observed oscillations of neutrinos produced in the atmosphere. More exciting, arguably, is the detection of 28 extremely energetic neutrinos – including two with energies above 1 PeV – but the evidence is not yet sufficient to claim observation of neutrinos of extraterrestrial origin.

Towards the future

In addition to the sessions on the latest results, others looked to the continuing health of the field with presentations of studies on novel ideas for future particle accelerators and detection techniques. These topics also featured in the special session for the European Committee for Future Accelerators, which looked at future developments in the context of the update of the European Strategy for Particle Physics. A range of experiments at particle accelerators currently takes place on two frontiers – high energy and high intensity. Progress in probing physics that lies at the limit of these experiments will come both from upgrades of existing machines and at future facilities. These will rely on new ideas being investigated in current accelerator R&D and will also require novel particle detectors that can exploit the higher energies and intensities.

For example, two proposals for new neutrino facilities would allow deeper studies of neutrinos – including the possibility of CP violation, which could cast light on the dominance of matter over antimatter in the universe. The Long-Baseline Neutrino Experiment (LBNE) would create a beam of high-energy ν_μ at Fermilab and detect the appearance of ν_e with a massive detector that is located 1300 km away at the Sanford Underground Research Facility. A test set-up, LBNE10, has received funding approval. A complementary approach providing low-energy neutrinos is proposed for the European Spallation Source, which is currently under construction in Lund. This will be a powerful source of neutrons that could also be used to generate the world’s most intense neutrino beam.

The LHC was first discussed in the 1980s, more than 25 years before the machine produced its first collisions. Looking to the long-term future, other accelerators are now on the drawing board. One possible option is the International Linear Collider, currently being evaluated for construction in Japan. Another option is to create a large circular electron–positron collider, 80–100 km in circumference, to produce Higgs bosons for precision studies.

The main physics highlights of the conference were reflected in the 2013 EPS-HEP prizes, awarded in the traditional manner at the start of the plenary sessions. The EPS-HEP prize honoured both ATLAS and CMS – for the discovery of the new boson – and three of their pioneering leaders (Michel Della Negra, Peter Jenni and Tejinder Virdee). François Englert and Peter Higgs were there to present this major prize and took part later in a press conference together with the prize winners. Following the ceremony, Higgs gave a talk, “Ancestry of a New Boson”, in which he recounted what led to his paper of 1963 and also cast light on why his name became attached to the now-famous particle. Other prizes acknowledged the measurement of the all-flavour neutrino flux from the Sun, as well as the observation of the rare decay B⁰_s → μμ, work in 4D field theories and outstanding contributions to outreach. In a later session, a prize sponsored by Elsevier was awarded for the best four posters out of the 130 that were presented by young researchers in the dedicated poster sessions.

To close the conference, Nobel Laureate Gerard ‘t Hooft presented his outlook for the field. This followed the conference summary by Sergio Bertolucci, CERN’s director for research and computing, in which he also thanked the organizers for the “beautiful venue, the fantastic weather and the perfect organization” and acknowledged the excellent presentations from the younger members of the community. The baton now passes to the organizing committees of the next EPS-HEP conference, which will take place in Vienna on 22–29 July 2015.

• This article has touched on only some of the physics highlights of the conference. For all of the talks, see http://eps-hep2013.eu/.

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The GERDA (GERmanium Detector Array) experiment, which is operated at the underground INFN Laboratori Nazionali del Gran Sasso, is looking for double beta decay processes in the germanium isotope ⁷⁶Ge, both with and without the emission of neutrinos. For ⁷⁶Ge, normal beta decay is energetically forbidden, but the simultaneous conversion of two neutrons with the emission of two neutrinos is possible. This has been measured by GERDA with unprecedented precision with a half-life of about 2 × 10²¹ years, making it one of the rarest decays ever observed. However, if neutrinos are Majorana particles, neutrinoless double beta decay should also occur, at an even lower rate. In this case, the antineutrino from one beta decay is absorbed as a neutrino by the second beta-decaying neutron, which is possible if the neutrino is its own antiparticle.

In GERDA germanium crystals are both source and detector. ⁷⁶Ge has an abundance of about 8% in natural germanium and its fraction was therefore enriched more than 10-fold before the special detector crystals were grown. To help to minimize the backgrounds from environmental radioactivity, the GERDA detector crystals and the surrounding detector parts have been carefully selected and processed. In addition, the detectors are located in the centre of a huge vessel filled with extremely clean liquid argon, lined by ultrapure copper, which in turn is surrounded by a 10-m diameter tank filled with high purity water. Last, but not least, it is all located underground below 1400 m of rock. The combination of all of these techniques has made it possible to reduce the background to unprecedented levels.

Data taking started in autumn 2011 using eight detectors if 2 kg each. Subsequently, five additional detectors were commissioned. Until recently, the signal region was blinded and the researchers focused on the optimization of the data analysis procedures. The experiment has now completed its first phase, with 21 kg years of accumulated data. The analysis, in which all calibrations and cuts had been defined before the data in the signal region were processed, revealed no signal of neutrinoless double beta decay in ⁷⁶Ge, which leads to the world’s best lower limit for the half-life of 2.1 × 10²⁵ years. Combined with information from other experiments, this result rules out an earlier claim for a signal by others.

The next steps for GERDA will be to add new detectors, effectively doubling the amount of ⁷⁶ Ge. Data taking will then continue in a second phase after some further improvements are implemented to achieve even stronger background suppression.

• GERDA is a European collaboration with scientists from 19 research institutes or universities in Germany, Italy, Russia, Switzerland, Poland and Belgium.

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The international T2K collaboration chose the EPSHEP2013 meeting in Stockholm as the forum to announce its definitive observation of the transformation of muon-neutrinos to electron-neutrinos, ν_μ→ν_e.

In 2011, the collaboration announced the first signs of this process – at the time a new type of neutrino oscillation. Now with 3.5 times more data, T2K has firmly established the transformation at a 7.5σ significance level.

In the T2K experiment, a ν_μ beam is produced in the Japan Proton Accelerator Research Complex (J-PARC) in Tokai on the east coast of Japan. The beam – monitored by a near detector in Tokai – is aimed at the Super-Kamiokande detector, which lies underground in Kamioka near the west coast, 295 km away. Analysis of the data from Super-Kamiokande reveals that there are more ν_e (a total of 28 events) than would be expected (4.6 events) without the transformation process.

Observation of this type of neutrino oscillation opens the way to new studies of charge-parity (CP) violation in neutrinos, which may be linked to the domination of matter over antimatter in the present-day universe. The T2K collaboration expects to collect 10 times more data in the near future, including data with an antineutrino beam for studies of CP violation.

In announcing the discovery, the collaboration paid tribute to the unyielding and tireless effort by the J-PARC staff and management to deliver high-quality beam to T2K after the devastating earthquake in eastern Japan in March 2011. The earthquake caused severe damage to the accelerator complex and abruptly halted the data-taking run of the T2K experiment.

• The T2K experiment was constructed and is operated by an international collaboration, which currently consists of more than 400 physicists from 59 institutions in 11 countries: Canada, France, Germany, Italy, Japan, Poland, Russia, Switzerland, Spain, UK and the US.

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Neutrino experiments – thanks to the nature of the particles themselves – are notoriously difficult and experiments that make use of the natural source of particles within the cosmic radiation face problems of their own. In detecting cosmic neutrinos, the IceCube Neutrino Observatory at the South Pole successfully contends with both of these challenges, as two papers to appear in Physical Review Letters reveal. They illustrate the observatory’s capabilities in particle physics and in astroparticle physics.

The potential for IceCube to meet its aim of detecting neutrinos from astrophysical sources has been boosted by the observation of two neutrino events with the highest energies ever seen. The events have estimated energies of 1.04±0.16 and 1.14±0.17 PeV – hundreds of times greater than the energy of protons at the LHC. The expected number of atmospheric background events at these energies is 0.082±0.004 (stat.)^+0.04_–0.057 (syst.) and the probability that the two observed events are background is 2.9 × 10^–3, giving the signal a significance of 2.8σ (Aartsen et al. 2013a). While this is not sufficient to indicate a first observation of astrophysical neutrinos, the closeness in energy of the two events is intriguing and is already attracting the attention of theorists.

The analysis revealed the disappearance of low-energy, upwards-moving muon neutrinos and rejected the non-oscillation hypothesis with a significance of more than 5σ

Meanwhile, measurements of lower-energy neutrinos produced in the atmosphere have enabled the IceCube collaboration to make the first statistically significant detection of neutrino oscillations in the high-energy region (20–100 GeV). The data used for this analysis were collected between May 2010 and May 2011 by the IceCube and DeepCore detectors, which together make up the IceCube Neutrino Observatory. The IceCube detector consists of an array with 86 strings of digital sensors deployed in Antarctica’s ice sheet at depths in the range 1450–24507 m. This main array defines the high-energy detector, designed to detect neutrinos with energies from hundreds to millions of giga-electron-volts – that is, up to the peta-electron-volts and more of the observed high-energy events. The DeepCore subdetector adds eight additional strings near the centre of this array, six of which were deployed during the period covered by this analysis. The denser core allows lowering the energy threshold to about 20 GeV.

The analysis revealed the disappearance of low-energy, upwards-moving muon neutrinos and rejected the non-oscillation hypothesis with a significance of more than 5σ. This result verifies the first, lower-significance indication reported by the ANTARES collaboration. Using a two-neutrino flavour formalism, the IceCube collaboration derived a new estimation of the oscillation parameters, |Δm²₂₃| = 2.3^+0.6_–0.5 × 10^–3 eV² and sin²2θ₂₃ > 0.93, with maximum mixing favoured. These values are in good agreement with previous measurements by the MINOS and Super-Kamiokande experiments.

More efficient event-reconstruction methods are being tested, which together with new data sets will increase the sensitivity of the IceCube and DeepCore detectors to atmospheric neutrino oscillations. As a result of these improvements, the IceCube collaboration is expecting to set further constraints on the oscillation parameters in the coming months.

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It is more than six years since Uppsala University was host to the first Workshop on Exotic Physics with Neutrino Telescopes. Since then, the large neutrino telescopes IceCube and ANTARES have been completed and indirect searches for dark matter, monopoles and other aspects of physics beyond the Standard Model are proceeding at full strength. Indeed, some theoretical models have already been called into question by recent results from these detectors. Meanwhile, searches for dark-matter candidates and indications of physics beyond the Standard Model in experiments at the LHC have set stringent constraints on many models, complementing those derived from the neutrino telescopes. The time was therefore ripe for a second workshop, with the Centre for Particle Physics of Marseilles (CPPM) as host, bringing together 47 experts on 3–6 April.

Dark matter

Review talks on supersymmetric dark-matter candidates and the status of experimental searches opened the first day’s sessions. Supersymmetry – still a well motivated candidate for physics beyond the Standard Model – has been put to the first serious tests at the LHC. The discovery there of a Higgs boson at a mass of 126 GeV can be seen either as just another confirmation of the Standard Model or as a first glimpse of physics beyond it. The lack of evidence so far for supersymmetry from direct searches at the LHC raises the limits of supersymmetric particle masses to the scale of tera-electron-volts and has implications for the dark-matter candidates arising in supersymmetric models. The current preferred mass-range for the lightest, stable neutralino is in the region of hundreds of giga-electron-volts. This is good news for neutrino telescopes, which – by design – are sensitive to high-energy particles. The downside is that the predicted rates from annihilation of neutralinos accumulated in heavy celestial objects are low if the constraints from the Wilkinson Microwave Anisotropy Probe and the LHC are taken into account. Only a handful of minimal supersymmetric Standard Model variants predict rates in cubic-kilometre neutrino telescopes that are of the order of 100 events per year or higher.

However, the neutralino in minimal supersymmetry is not the only viable candidate for dark matter. In models with R-parity violation, a long-lived but unstable gravitino with a mass between a few and a few hundred giga-electron-volts could be a component of the dark matter in the halo of galaxies. Neutrinos of any flavour can be produced in gravitino decay and can be detected by neutrino telescopes. A feature of gravitino dark matter is that it would leave no signal in direct-detection experiments because the cross-section for the interaction between a gravitino and normal matter is suppressed by the Planck mass to the fourth power.

Models with extra dimensions of sizes in the range 10^–3–10^–15 m can also provide dark-matter candidates. Extra dimensions can be accommodated (or even required) in supersymmetry, string-theory or M-theory, where they give rise to branons – weakly interacting and massive fluctuations of the field that represents the 3D brane on which the standard world lives. As stable and weakly interacting objects, branons make a good candidate for dark matter, following the usual scenario: relic branons left over after a freeze-out period during the evolution of the universe accumulate gravitationally in the halos of galaxies, where they annihilate into Standard Model particles that can be detected by gamma-ray telescopes, surface arrays or neutrino telescopes.

From the experimental side, the IceCube, ANTARES, Baikal, Baksan and Super-Kamiokande collaborations presented their latest results on the search for dark matter from different potential sources – the Sun, the Galaxy or nearby galaxies. Their search techniques are similar and based on looking for an excess of neutrinos over the known atmospheric-neutrino background from the direction of the sources. DeepCore, the denser array in the centre of IceCube, which was not part of the original design, has proved extremely useful in lowering the energy reach of the detector. It has opened up the possibility of pursuing physics topics that would otherwise be impossible with a detector designed for tera-electron-volt neutrino astrophysics. Using the surrounding strings of IceCube as a veto, starting and contained tracks can be defined, therefore turning IceCube into a 4π detector with an energy threshold of around 10 GeV, with access to the Galactic centre and the whole Southern Sky.

However, none of the experiments report any excess, and upper limits on the neutrino flux and on the cross-section for interactions between weakly interacting massive particles (WIMPs) and nucleons have been calculated over an ample range of WIMP masses, from about 1 GeV (Super-Kamiokande) to 10 TeV (IceCube). An example of the long-term search capability, as well as consistency in data analysis, was presented for the Baksan experiment. Although it has the smallest of the detectors mentioned above, it has gathered data over 24 years, from 1978 to 2009.

Monopoles, nuclearites and more

Monopoles and heavy, highly ionizing particles leave a unique signal in a neutrino telescope: a strong light-yield along the path of the particle, which is much more intense than the usual track-pattern of a minimum-ionizing muon. If the particle is nonrelativistic, then the separation of such a signal from relativistic muons traversing the detector is even easier. However, dedicated online or offline triggers are needed because for a nonrelativistic particle, light is deposited in the detector over a time of up to tens of milliseconds, instead of a few microseconds for a relativistic muon.

The best limit for fast (β > 0.75) monopoles, at a level of about 3 × 10^–18 cm^–2 s^–1, was presented by the IceCube collaboration using data from its 40-string configuration, although the ANTARES limit – at a level of around 7 × 10^–17 cm^–2 s^–1 – remains the best so far, at between 0.65 < β < 0.75. However, the sensitivity of the full IceCube detector could extend to β = 0.60 and reach a level of between 2 × 10^–18 cm^–2 s^–1and 10^–17 cm^–2 s^–1 in a one-year exposure, depending on the assumptions on the monopole spin. Results are expected soon, when the ongoing data analysis is finalized.

The Super-Kamiokande collaboration presented a novel way to search for monopoles using the Sun as the target. The idea is that super-heavy monopoles that have been gravitationally trapped in the Sun will induce proton decay along their orbits. Neutrinos with an energy of tens of mega-electron-volts will then be emitted by the decays of the muons and pions produced as the protons decay. This is a low-energy signal that is well below the threshold of large-scale neutrino telescopes but for which Super-Kamiokande has sensitivity. Indeed, this experiment provides the best limit so far on the flux of super-heavy monopoles in the range 10^–5 < β < 10^–2. At the other end of the kinematic spectrum, radio-Cherenkov detectors such as RICE and ANITA provide the best limits for ultrarelativistic monopoles of intermediate mass, at the level of 10^–19 cm^–2 s^–1.

Another bright signature, although from a different process, is produced by slowly moving heavy nuclearites. These massive stable lumps of up, down and strange quarks could be detected in neutrino telescopes through the blackbody radiation emitted by the overheated matter along their path. From the analysis of 310 days of live time in the years 2007–2008, the ANTARES collaboration reported a flux limit at the level of 10^–17 cm^–2 s^–1 sr^–1 for nuclearite masses larger than 10¹⁴ GeV and β around 10^–3. Indeed, the limit improves previous results from the MACRO experiment by a factor of between three and an order of magnitude, depending on the nuclearite mass.

The atmosphere, acting as a target for ultra-high-energy cosmic rays, can be a useful source for searches of physics beyond the Standard Model

The atmosphere, acting as a target for ultra-high-energy cosmic rays, can be a useful source for searches of physics beyond the Standard Model. The interaction of a cosmic ray of energy around 10¹¹ GeV with a nucleon in the atmosphere takes place at a much higher centre-of-mass energy than is achievable in accelerator laboratories and a wealth of physics can be extracted from such collisions. Supersymmetric particles can be produced in pairs and, except for the lightest, they can be charged. Even if unstable, they can, because of the boost in the interaction, reach the depths of a detector and emit Cherenkov light as they traverse an array. The signature is two minimum-ionizing, parallel, coincident tracks separated by more than 100 m. These types of searches are being carried out by the two large neutrino-telescope collaborations, IceCube and ANTARES.

The same interactions of cosmic rays with the atmosphere can also be used to probe non-standard neutrino interactions arising from the effects of tera-electron-volt gravity and/or extra dimensions. At high energies, neutrino interactions with matter may become stronger and the atmosphere can become opaque to neutrinos with energies of peta-electron-volts. A signature in a neutrino telescope would be an absence of regular neutrinos with ultra-high energies accompanied by an excess of muon bundles at horizontal zenith angles. The same effect would take place with a cosmogenic neutrino flux – that is, the flux of neutrinos produced by the interactions of ultra-high-energy cosmic rays with the cosmic microwave background radiation. In the absence of a discovery so far, this flux can be assumed to be at a level compatible with gamma-ray constraints from the Fermi Gamma-ray Space Telescope. The neutrino-nucleon cross-section will depend on the number of extra dimensions, N_D, and a lack of events over the expected flux can be transformed into a limit on N_D. However, the effect in neutrino telescopes with volumes of a cubic kilometre or so is not big. For values of N_D not excluded by the LHC, fewer than one event a year is estimated for IceCube. Only with the larger radio arrays is the expected number of events of the order of 10 per year.

The recent two peta-electron-volt events announced by the IceCube collaboration have already been used to set stringent limits on the violation of Lorentz invariance. If strict Lorentz invariance does not hold, then neutrino bremsstrahlung of electron–positron pairs (ν → νe⁺e^–) is possible, so extragalactic neutrinos would rapidly lose energy via such a process. This would lead to a depletion of the ultra-high-energy neutrino flux at the Earth. Assuming that the IceCube events are, indeed, extragalactic (that is, they have travelled of the order of megaparsecs from the sources to the Earth) and that the extragalactic high-energy neutrino flux is at most at the level of the current IceCube limit of 2 × 10^–8 cm^–2 s^–1 sr^–1, a limit can be set on Lorentz invariance violation, parameterized by the factor δ, defined as (dE/dp)²-1. Under these assumptions, the bound obtained from the two IceCube events is δ <10^–18, which is orders of magnitude smaller than the current best limit of 10^–13.

Even if conventionally produced, the absolute normalization of the atmospheric lepton spectrum is not well understood

High-energy atmospheric muons and neutrinos present a background to many of the topics discussed in the workshop. Even if conventionally produced, the absolute normalization of the atmospheric lepton spectrum is not well understood – in particular the contribution from prompt charm decays. Calculations of an atmospheric lepton component, which is rarely considered, from the decays of unflavoured mesons (η, η’, ρ, ω, φ), were presented at the workshop. These mesons decay rapidly to μ⁺μ^– pairs and in very-high-energy cosmic-ray interactions the products of the decays can be the dominant muon flux at energies above 10⁶ GeV, forming a background that must be taken into account in exotic searches.

One of the unexpected developments in the field since the first ideas of building neutrino telescopes has been their use in neutrino-oscillation physics. On one hand, the detectors can probe oscillation physics at energies not reachable by the smaller detectors. On the other, an aggressive plan to lower the energy threshold of IceCube and the proposed KM3NeT array to the few-giga-electron-volt region is underway, and IceCube has already produced physics results with its low-energy subarray, DeepCore. Plans to build megatonne water-Cherenkov detectors with a giga-electron-volt energy threshold – PINGU at the South Pole and ORCA in the Mediterranean – were also discussed in the workshop. These detectors consist of about 20–50 strings of optical modules with an inter-string separation of 20 m, to be compared, for example, with the 125 m inter-string separation of IceCube or the 70 m inter-string separation of DeepCore. Such detectors may address the issue of the neutrino mass hierarchy at a relatively low cost and on a short timescale because the technology exists already and the deployment techniques are the same as in IceCube and ANTARES.

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The international Borexino collaboration has released results from a new measurement of geoneutrinos corresponding to 1352.60 live days and about 187 tonnes of liquid scintillator after all selection criteria have been applied (3.7 × 10³¹ proton × year). This corresponds to a 2.4 times higher exposure with respect to the measurement made in 2010.

Borexino is a liquid-scintillator detector built principally underground at INFN Gran Sasso National Laboratory in central Italy to detect solar neutrinos. However, because of its high level of radiopurity – unmatched elsewhere in the world – it can also detect rare events such as the interactions of geoneutrinos. These are electron-antineutrinos that are produced in the decays of long lived radioactive elements (⁴⁰K, ²³⁸U and ²³²Th) in the Earth’s interior.

From the data collected, 46 electron-antineutrino candidates have been found, about 30% of them geoneutrinos. Borexino has also detected electron-antineutrinos from nuclear power plants around the world. These latter antineutrinos give a signal of about 31 events, which is in good agreement with the number expected from the 446 nuclear cores operating during the period of interest (December 2007 to August 2012) and from current knowledge of the parameters of neutrino oscillations. The total expected background for electron-antineutrinos in Borexino is determined to be about 0.7 events. The small background is a result of the high level of radiopurity of the liquid scintillator. For the current measurement, the null geoneutrino hypothesis has a probability of 6 × 10^–6.

The detection of geoneutrinos offers a unique tool to probe uranium and thorium abundances within the mantle. By considering the contribution from the local crust (around the Gran Sasso region) and the rest of the crust to the geoneutrino signal, the signal from the radioactivity of uranium and thorium in the mantle can be extracted. The latest results from Borexino, together with the measurement by the KamLAND experiment in Japan, indicate a signal from the mantle of 14.1±8.1 TNU (1 TNU = 1 event/year/10³² protons).

These new results mark a breakthrough in the comprehension of the origin and thermal evolution of the Earth. The good agreement between the ratios of thorium to uranium determined from geoneutrino signals and the value obtained from chondritic meteorites has fundamental implications for cosmochemical models and the processes of planetary formation in the early Solar System.

By measuring the geoneutrino flux at the surface, the contribution of radioactive elements to the Earth’s heat budget can be explored. The radiogenic heat is of great interest for understanding a number of geophysical processes, such as mantle convection and plate tectonics. For the first time two independent geoneutrino detectors – Borexino and KamLAND, which are placed in different sites around the planet – are providing the same constraints on the radiogenic heat power of the Earth set by the decays of uranium and thorium. With these latest results, the Borexino collaboration finds that the data fit to a possible georeactor with an upper limit on the output power of 4.5 TW at 95% confidence level.

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OPERA, which is run by an international experiment involving 140 physicists from 28 research institutes in 11 countries, was set up for the specific purpose of discovering neutrino oscillations of this kind. A beam of neutrinos produced at CERN travels towards the INFN Gran Sasso National Laboratory some 730 km away. Thanks to their weak interactions, the neutrinos arrive almost unperturbed at the giant OPERA detector, which consists of more than 4000 tonnes of material, has a volume of some 2000 m³ and contains nine million photographic plates. After the first neutrinos arrived at Gran Sasso in 2006, the experiment gathered data for five consecutive years, from 2008 to 2012. The first τ neutrino was observed in 2010, the second in 2012.

The arrival of the τ neutrino is an important confirmation of the two previous observations. Statistically, the observation of three τ neutrinos enables the collaboration to claim confidently that muon neutrinos oscillate to τ neutrinos. Data analysis is set to continue for another two years.

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When Spanish particle-physicist Juan José Gómez Cadenas arrived at CERN as a summer student, the passion that he already had for physics turned into an infatuation. Thirty years later and back in his home country, Gómez Cadenas is pursuing one of nature’s most elusive particles, the neutrino, by looking where it is expected not to appear at all – in neutrinoless double-beta decay. Moreover, fiction has become entwined with fact, as he was recently invited to write a novel set at CERN. The result, Materia Extraña (Strange matter), is a scientific thriller that has already been translated into Italian.

Critical point

“Particle physicists were a rare commodity in Spain when the country first joined CERN in 1961,” Cecilia Jarlskog noted 10 years ago after a visit to “a young and rapidly expanding community” of Spanish particle physicists. Indeed, the country left CERN in 1969, when Juan was only nine years old and Spain was still under the Franco regime. Young Juan – or “JJ” as he later became known – initially wanted to become a naval officer, like his father, but in 1975 he was introduced to the wonders of physics by his cousin; Bernardo Llanas had just completed his studies with the Junta de Energía Nuclear (the forerunner of CIEMAT, the Spanish research centre for energy, the environment and technology) at the same time as Juan Antonio Rubio, who was to do so much to re-establish particle physics in Spain. The young JJ set his sights on the subject – “Suddenly the world became magic,” he recalls, “I was lost to physics” – and so began the love affair that was to take him to CERN and, in a strange twist, to write his first novel.

The critical point came in 1983. JJ was one of the first Spanish students to gain a place in CERN’s summer student programme when his country rejoined the organization. It was an amazing time to be at the laboratory: the W and Z bosons had just been discovered and the place was buzzing. “I couldn’t believe this place, it was the beginning of an absolute infatuation,” he says. That summer he met two people who were to influence his career: “My supervisor, Peter Sonderegger, with whom I learnt the ropes as an experimental physicist, and Luis Álvarez-Gaume, a rising star who took pity on the poor, hungry fellow-Spaniard hanging around at night in the CERN canteen.” After graduating from Valencia University, JJ’s PhD studies took him to the DELPHI experiment at CERN’s Large Electron–Positron collider. With the aid of a Fulbright scholarship, he then set off for America to work on the Mark II experiment at SLAC. From there it was back to CERN and DELPHI again, but in 1994 he left once more for the US, this time following his wife, Pilar Hernandez, to Harvard. An accomplished particle-physics theorist, she converted her husband to her speciality, neutrino physics, thus setting him on the trail that would lead him through the NOMAD, HARP and K2K experiments to the challenge of neutrinoless double-beta decay.

The neutrinoless challenge

Established for 15 years as professor of physics at the Institute of Nuclear and Particle Physics (IFIC), a joint venture between the University of Valencia and the Spanish research council (CSIC), he is currently leading NEXT – the Neutrino Experiment with a Xenon TPC. The aim is to search for neutrinoless double-beta decay using a high-pressure xenon time-projection chamber (TPC) in the Canfranc Underground Laboratory in the Spanish Pyrenees. JJ believes that the experiment has several advantages in the hunt for this decay mode, which would demonstrate that the neutrino must be its own antiparticle, as first proposed by Ettore Majorana (whose own life ended shrouded in mystery). The experiment uses xenon, which is relatively cheap and also cheap to enrich because it is a nobel gas. Moreover, NEXT uses gaseous xenon, which gives 10 times better energy resolution for the decay electrons than the liquid form. By using a TPC, it also provides a topological signature for the double-beta decay.

The big challenge was to find a way to amplify the charge in the xenon gas without inducing sparks. The solution came when JJ talked to David Nygren, inventor of the TPC at Berkeley. “It was one of those eureka moments,” he recalls. “Nygren proposed using electroluminescence, where you detect light emitted by ionization in a strong field near the anode. You can get 1000 UV photons for each electron. He immediately realized that we could get the resolution that way.” JJ then came up with an innovative scheme to detect those electrons in the tracking plane using light-detecting pixels (the silicon photomultiplier) – and the idea for NEXT was born. “It is hard for me not to believe in the goddess of physics,” says JJ. “Every time that I need help, she sends me an angel. It was Abe Seiden in California, Gary Feldman in Boston, Luigi di Lella and Ormundur Runolfson at CERN, Juan Antonio Rubio in Spain … and then Dave. Without him, I doubt NEXT would have ever materialized.” The collaboration now involves not only Spain and the US but also Colombia, Portugal and Russia. The generous help of a special Spanish funding programme, called CONSOLIDER-INGENIO, provided the necessary funds to get it going. “More angels came to help here,” he explains, “all of them theorists: José Manuel Labastida, at the time at the ministry of science, Álvaro de Rújula, my close friend Concepción González-García … really, the goddess gave us a good hand there.”

Despite the financial problems in Spain, JJ says that “there is a lot of good will” in MINECO, the Ministry of Economy, which currently handles science in Spain. He points out that there has already been a big investment in the experiment and that there is full support from the Canfranc Laboratory. He is particularly grateful for the “huge support and experience” of Alessandro Bettini, the former director of the Gran Sasso National Laboratory in Italy, who is now in charge at Canfranc. JJ finds Bettini and Nygren – both in their mid-seventies – inspirational characters, calling them the “Bob Dylans” of particle physics. Indeed, he set up an interview with both of them for the online cultural magazine, Jotdown – where he regularly contributes with a blog called “Faster than light”.

In many ways, JJ’s trajectory through particle physics is similar to that of any talented, energetic particle physicist pursuing his passion. So what about the novel? When did an interest in writing begin? JJ says that it goes back to when his family eventually settled in the town of Sagunto, near Valencia, when he was 15. An ancient city where modern steel-making stands alongside Roman ruins, he found it “a crucible of ideas”, where writers and artists mingled with the steel-workers, who wanted a more intellectual lifestyle for their children – especially after the return of democracy with the new constitution in 1978, following Franco’s death. JJ started writing poetry while studying physics in Sagunto, and when physics took him to SLAC in 1986, as a member of Stanford University, he was allowed to sit in on the creative-writing workshop. “I was not only the only non-native American but also the only physicist,” he recalls. “I’m not sure that they knew what to make of me.” Years later, he continued his formal education as a writer at the prestigious Escuela de Letras in Madrid.

A novel look at CERN

Around 2003, CERN was starting to become bigger news, with the construction of the LHC, experiments on antimatter and an appearance in Dan Brown’s mystery-thriller Angels & Demons. Having already written a book of short stories, La agonía de las libélulas (Agony of the dragonflies), published in 2000, JJ was approached by the Spanish publisher Espasa to write a novel that would involve CERN. Of course, the story would require action but it would also be a personal story, imbued with JJ’s love for the place. Materia Extraña, published in 2008, “deals with how someone from outside tries to come to grips with CERN,” he explains, “and also with the way that you do science.” It gives little away to say that at one and the same time it is CERN – but not CERN. For example, the director-general is a woman, with an amalgam of the characteristics that he observes to be necessary for women to succeed in physics. “The novel was presented in Madrid by Rubio,” says JJ. “At the time, we couldn’t guess he had not much time left.” (Rubio was to pass away in 2010.)

When asked by Espasa to write another book, JJ turned from fiction to fact and the issue of energy. Here he encountered “a kind of Taliban of environmentalism” and became determined to argue a more rational case. The result was El ecologista nuclear (The Nuclear Environmentalist, now published in English) in which he sets down the issues surrounding the various sources of energy. Comparing renewables, fossil fuels and nuclear power, he puts forward the case for an approach based on diversity and a mixture of sources. “The book created a lot of interest in intellectual circles in Spain,” he says. “For example, Carlos Martínez, who was president of CSIC and then secretary of state (second to the minister) liked it quite a bit. Cayetano López, now director of CIEMAT, and an authority in the field, was kind enough to present it in Madrid. It has made some impact in trying to put nuclear energy into perspective.”

So how does JJ manage to do all of this while also developing and promoting the NEXT experiment? “The trick is to find time,” he reveals. ‘We have no TV and I take no lunch, although I go for a swim.” He is also one of those lucky people who can manage with little sleep. “I write generally between 11 p.m. and 2 a.m.,” he explains, “but it is not like a mill. I’m very explosive and sometimes I go at it for 12 hours, non-stop.”

He is now considering writing about nuclear energy, along the lines of the widely acclaimed Sustainable Energy – without the hot air by Cambridge University physicist David MacKay, who is currently the chief scientific adviser at the UK’s Department of Energy and Climate Change. “The idea would be to give the facts without the polemic,” says JJ, “to really step back.” He has also been asked to write another novel, this time aimed at young adults, a group where publisher Espasa is finding new readers. While his son is only eight years old, his daughter is 12 and approaching this age group. This means that he is in touch with young-adult literature, although he finds that at present “there are too many vampires” and admits that he will be “trying to do better”. That he is a great admirer of the writing of Philip Pullman, the author of the bestselling trilogy for young people, His Dark Materials, can only bode well.

• For more about the NEXT experiment see the recent CERN Colloquium by JJ Gómez Cadenas at http://indico.cern.ch/conferenceDisplay.py?confId=225995. For a review of El ecologista nuclear see the Bookshelf section of this issue.

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The first results from the Enriched Xenon Observatory 200 (EXO-200) on the search for neutrinoless double beta decay show no evidence for this hypothesised process, which would shed new light on the nature of the neutrino. Located in the US Department of Energy’s Waste Isolation Pilot Plant in New Mexico, EXO-200 is a large beta-decay detector. In 2011 it was the first to measure two-neutrino double beta decay in ¹³⁶Xe; now it has set a lower limit for neutrinoless double beta decay for the same isotope.

Double beta decay, first observed in 1986, occurs when a nucleus is energetically unable to decay via single beta decay, but can instead lose energy through the conversion of two neutrons to protons, with the emission of two electrons and two antineutrinos. The related process without the emission of antineutrinos is theoretically possible but only if the neutrino is a “Majorana” particle, i.e. it is its own antiparticle.

EXO-200 uses 200 kg of ¹³⁶Xe to search for double beta decay. Xenon can be easily purified and reused, and it can be enriched in the ¹³⁶Xe isotope using Russian centrifuges, which makes processing large quantities feasible. It also has a decay energy – Q-value – of 2.48 MeV, high enough to be above many of the uranium emission lines. Using ¹³⁶Xe as a scintillator gives excellent energy resolution through the collection both of ionization electrons and of scintillation light. Finally, using xenon allows for complete background elimination through tagging of the daughter barium ion. This tagging, combined with the detector’s location more than 650 m underground and the use of materials selected and screened for radiopurity, ensures that other traces of radioactivity and cosmic radiation are eliminated or kept to a minimum. The latest results reflect this low background activity and high sensitivity – as only one event was recorded in the region where neutrinoless double beta decay was expected.

In the latest result, no signal for neutrinoless double beta decay was observed for an exposure of 32.5 kg/y, with a background of about 1.5 × 10^–3 kg^–1y^–1keV^–1. This sets a lower limit on the half-life of neutrinoless double beta decay in ¹³⁶Xe to greater than 1.6 × 10²⁵ y, corresponding to effective Majorana masses of less than 140–380 meV, depending on details of the calculation (Auger et al. 2012).

The EXO collaboration announced the results at Neutrino 2012, the 25th International Conference on Neutrino Physics and Astrophysics, held in Kyoto, on 3–9 June. This dedicated conference for the neutrino community provided the occasion for many neutrino experiments to publicize their latest results. In the case of the MINOS collaboration, these included the final results from the first phase of the experiment, which studies oscillations between neutrino types.

In 2010 the MINOS collaboration caused a stir when it announced the observation of a surprising difference between neutrinos and antineutrinos. Measurements of a key parameter used in the study of oscillations – Δm², the difference in the squares of the masses of two oscillating types – gave different values for neutrinos and antineutrinos. In 2011, additional statistics brought the values closer together and, with twice as much antineutrino data collected since then, the gap has now closed. From a total exposure of 2.95 × 10²⁰ protons on target, a value was found for muon antineutrinos of Δm² = 2.62+0.31–0.28(stat.)±0.09(syst.) and the antineutrino “atmospheric” mixing angle was constrained with sin²2θ greater than 0.75 at 90% confidence level (Adamson et al. 2012). These values are in agreement with those measured for muon neutrinos.

Since its debut in 2006, the OPERA experiment in the Gran Sasso National Laboratory has been searching for neutrino oscillations in which muon-neutrinos transform into τ-neutrinos as they travel the 730 km of rock between CERN, where they originate, and the laboratory in Italy. At the conference, the OPERA collaboration announced the observation of their second τ-neutrino, after the first observation two years ago. This new event is an important step towards the accomplishment of the final goal of the experiment.

Results on the time of flight of neutrinos from CERN to the Gran Sasso were also presented by CERN’s director for research and scientific computing, Sergio Bertolucci, on behalf of four experiments. All four – Borexino, ICARUS, LVD and OPERA – measure a neutrino time of flight that is consistent with the speed of light. The indications are that a measurement by OPERA announced last September can be attributed to a faulty element of the experiment’s fibre-optic timing system.

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Particle physicists – like many other scientists – are used to working under well controlled laboratory conditions, with constant temperature, controlled humidity and perhaps even a clean-room environment. They would consider crazy anyone who tried to install an experiment in the field outside the lab environment, without shelter against wind and weather. So what must they think of a group of physicists and engineers planning to install a huge, highly complex detector on the bottom of the open sea?

This is exactly what the KM3NeT project is about: a neutrino telescope that will consist of an array of photo-sensors instrumenting several cubic kilometres of water deep in the Mediterranean Sea (figure 1). The aim is to detect the faint Cherenkov light produced as charged particles emerge from the reactions of high-energy neutrinos in the instrumented volume of ocean or the rock beneath it. Most of the neutrinos that are detected will be “atmospheric neutrinos”, originating from the interactions of charged cosmic rays in the Earth’s atmosphere. Hiding among these events will be a few that have been induced by neutrinos of cosmic origin, and these are the prime objects that the experimenters desire.

Ideal messengers

Why are a few cosmic neutrinos worth the huge effort to construct and operate such an instrument? A century after the discovery of cosmic rays, the start of construction of the KM3NeT neutrino telescope marks a big step forwards in understanding their origin and solving the mystery of the astrophysical processes in which they acquire energies that are many orders of magnitude beyond the reach of terrestrial particle accelerators. This is because neutrinos are ideal messengers from the universe: they are neither absorbed nor deflected, i.e. they can escape from dense environments that would absorb all other particles; they point back to their origin; and they are produced inevitably if protons or heavier nuclei with the energies typical of cosmic rays – up to eight orders of magnitude above the LHC beam energy – scatter on other nuclei or on photons and thereby signal astrophysical acceleration of nuclei.

Only a handful of neutrinos assigned to an astrophysical source would convey the unambiguous message that this source accelerates nuclei – a finding that can not be achieved any other way. Of course, much more can be studied with neutrino telescopes. Cosmic neutrinos might signal annihilations of dark-matter particles, and their isotropic flux provides information about sources that cannot be resolved individually. Moreover, atmospheric neutrinos could be used to make measurements of unique importance for particle physics, such as the determination of the neutrino-mass hierarchy.

Driven by the fundamental significance of neutrino astronomy, a first generation of neutrino telescopes with instrumented volumes up to about a per cent of a cubic kilometre was constructed over the past two decades: Baikal, in the homonymous lake in Siberia; AMANDA, in the deep ice at the South Pole; and ANTARES, off the French Mediterranean coast. These detectors have proved the feasibility of neutrino detection in the respective media and provided a wealth of experience on which to build. However, they have not – yet – identified any neutrinos of cosmic origin.

These results and the evolution of astrophysical models of potential classes of neutrino sources over the past few years indicate that, in fact, much larger target volumes are necessary for neutrino astronomy. The first neutrino telescope of cubic-kilometre size, the IceCube observatory at the South Pole, was completed in December 2010. Its integrated exposure is growing rapidly and the discovery of a first source may be just round the corner.

Why then start constructing another large neutrino telescope? Would it not be better to wait and see what IceCube finds? To answer this question it is important to understand in somewhat more detail the way in which neutrinos are actually measured.

The key reaction is the charged-current (mostly deep-inelastic) scattering of a muon-neutrino or muon-antineutrino on a target nucleus. In such a reaction, an outgoing muon is produced that, on average, carries a large fraction of the neutrino energy and is emitted with only a small angular deflection from the neutrino direction. The muon trajectory – and thus the neutrino direction – is reconstructed from the arrival times of the Cherenkov light in the photo-sensors and the positions of the sensors. This method is suitable for the identification of neutrinos if they come from the opposite hemisphere, i.e. through the Earth. If they come from above, then the resulting muons are barely distinguishable from “atmospheric” muons that penetrate to the detector and are much more numerous. Neutrino telescopes therefore look predominantly “downwards” and do not cover the full sky. IceCube, being at the South Pole, can thus observe the Northern sky but not the Galactic centre and the largest part of the Galactic plane (figure 2).

The KM3NeT telescope will have the Galactic centre and central plane of the Galaxy in its field of view and will be optimized to discover and investigate the neutrino flux from Galactic sources. Shell-type supernova remnants are a particularly interesting kind of candidate source. In these objects the supernova ejecta hit interstellar material, such as molecular clouds, and form shock fronts. Gamma-ray observations show that these are places where particles are accelerated to very high energies – but there is an intense debate as to whether these gamma rays stem from accelerated electrons and positrons or hadrons. The only way to give a conclusive answer is through observing neutrinos. Figure 3 shows the sensitivity of KM3NeT and other different experiments to neutrino point sources. According to simulations based on model calculations using gamma-ray measurements by the High Energy Stereoscopic System (HESS) – an air Cherenkov telescope – KM3NeT could make an observation of the supernova remnant RX J1713.7-3946 (figure 4) with a significance of 5σ within 5 years, if the emission process is purely hadronic.

The construction of a neutrino telescope of this sensitivity within a realistic budget faces a number of challenges. The components have to withstand the hostile environment with several hundred bar of static pressure and extremely aggressive salt water. That limits the choice of materials, in particular as maintenance is difficult or even impossible. In addition, background light from the radioactive decay of potassium-40 and bioluminescence causes high rates of photomultiplier hits, while the deployment of the detector requires tricky sea operations and the use of unmanned submersibles to make cable connections.

When the KM3NeT design effort started out with an EU-funded Design Study (2006–2009), a target cost of €200 million for a cubic-kilometre detector was defined. At the time, this was considered utterly optimistic in view of the investment cost for ANTARES of about €20 million. Now, in 2012, the collaboration is confident that it can construct a detector of 5–6 km³ for €220–250 million. This enormous development is partly a result of optimizing the neutrino telescope for slightly higher energies, which implies larger horizontal and vertical distances between the photo-sensors. The main progress, however, has been in the technical design. Almost all of the components have been newly designed, in many cases pursuing completely new approaches.

The design of the optical module is a prime example. Instead of a large, hemispherical photomultiplier (8- or 10-inch diameter) in a glass sphere (17-inch diameter), the design now uses as many as 31 photomultipliers of 3-inch diameter per sphere (figure 5). This triples the photocathode area for each optical module, allows for a clean separation of hits with one or two photo-electrons and adds some directional sensitivity.

All data, i.e. all photomultiplier hits, will be digitized in the optical modules and sent to shore via optical fibres. At the shore station, a data filter will run on a computer cluster and select the hit combinations in which the hit pattern and timing are compatible with particle-induced events.

Three countries (France, Italy and the Netherlands) have committed major contributions to an overall funding of €40 million for a first construction phase; others (Germany, Greece, Romania and Spain) are contributing at a smaller level or have not yet made final decisions. It is expected that final prototyping and validation activities will be concluded by 2013 and that construction will begin in 2013–2014. The installation will soon substantially exceed any existing northern-hemisphere instruments in sensitivity, thus providing discovery potential from an early stage.

Last, astroparticle physicists are not alone in looking forward to KM3NeT. For scientists from various areas of underwater research, the detectors will provide access to long-term, continuous measurements in the deep sea. It will provide nodes in a global network of deep-ocean observatories and thus be a truly multidisciplinary research infrastructure.

• For more information, see the KM3NeT Technical Design Report at www.km3net.org.

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The IceCube collaboration, with a detector that looks at a cubic kilometre of ice at the South Pole, has searched for evidence of neutrinos associated with gamma-ray bursts (GRBs). They find none at a level 3.7 times lower than models predict, indicating that cosmic rays with energies above 10⁸ TeV originate from some other source.

Where nature accelerates particles to 10⁸ TeV has been one of the long-standing questions of extreme astrophysics. Although the flux of the highest-energy cosmic rays arriving at Earth is small, it pervades the universe and corresponds to a large amount of energy. Equally mysterious in origin, gamma-ray bursts (GRBs), some associated with the collapse of massive stars to black holes, have released a small fraction of a solar mass of radiation more than once a day since the Big Bang. The assumption is that they invest a similar amount of energy in the acceleration of protons, which explains the observed cosmic-ray flux. This leads to the 15-year-old prediction that when protons and gamma rays co-exist in the GRB fireball they photoproduce pions that decay into neutrinos. The prediction is quantitative (albeit with astrophysical ambiguities) because astronomers can calculate the number of photons in the fireball, and the observed cosmic-ray flux dictates the number of protons. Textbook particle physics then predicts the number of neutrinos.

With 5160 photomultiplier tubes, the IceCube experiment has transformed a cubic kilometre of Antarctic ice into a Cherenkov detector. Even while still incomplete, the instrument reached the sensitivity to observe GRBs, taking data with 40 and 59 of the final number of 86 photomultiplier strings. The measurement is relatively easy because it exploits alerts from the NASA’s Swift satellite and Fermi Gamma-Ray Space Telescope to look for neutrinos arriving from the right direction at the right time. The window is small enough to do a background-free measurement because accidental coincidence with a high-energy atmospheric neutrino is negligible.

During the periods of data-taking, some 307 GRBs had the potential to result in neutrinos that IceCube could detect. However, the experiment found no evidence for any neutrinos that could be associated with the GRBs. This implies either that GRBs are not the only sources of cosmic rays with energies exceeding 10⁸ TeV or that the efficiency of neutrino production is much lower than has been predicted.

With GRBs on probation, the stock rises for the alternative speculation that associates supermassive black holes at the centres of galaxies with the enigmatic cosmic accelerators.

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RENO detects antineutrinos from six reactors, each with a thermal power output of 2.8 GW_th, at Yonggwang Nuclear Power Plant in Korea. The reactors are almost equally spaced in a line about 1.3 km long and the experiment uses two identical detectors located at 294 m and 1383 m on either side of the centre of this line, beneath hills that provide, respectively, 120 and 450 m of water-equivalent of rock overburden to reduce the cosmic backgrounds. This symmetric arrangement of reactors and detectors is useful for minimizing the complexity of the measurement. RENO is the first experiment to measure θ₁₃, the smallest neutrino-mixing angle and the last to be known, with two identical detectors.

In the 229-day data-taking period from 11 August 2011 to 26 March 2012, the far (near) detector observed 17,102 (154,088) electron-antineutrino candidate events with a background fraction of 5.5% (2.7%). During this period, all six reactors were operating mainly at full power, with two reactors being off for a month each for fuel replacement.

The two identical antineutrino detectors allow a relative measurement through a comparison of the observed neutrino rates. Measuring the far-to-near ratio of the reactor neutrinos in this way can considerably reduce several systematic errors. The relative measurement is independent of correlated uncertainties and helps in minimizing uncorrelated reactor uncertainties.

Each detector comprises four layers. At the core lies the target volume of 16.5 tonnes of liquid scintillator that is doped with gadolinium. An electron-antineutrino can interact with a free proton in the scintillator, ν + p → e⁺ + n. The positron from this inverse β-decay annihilates immediately giving a prompt signal. The neutron wanders into the target volume, eventually being captured by the gadolinium – giving a delayed signal. The delayed coincidence between the positron and neutron signals provides the distinctive signature of inverse β-decay.

The central target volume is surrounded by a 60 cm layer of liquid scintillator without gadolinium, which serves to catch γ-rays escaping from the target volume, thus increasing the detection efficiency. Outside this γ-catcher, a 70 cm buffer-layer of mineral oil shields the inner detectors from radioactivity in the surrounding rocks and in the 354 photomultiplier tubes (10-inch) that are installed on the inner wall of the buffer container. The outermost veto layer consists of 1.5 m of pure water, which serves to identify events coming from the outside through their Cherenkov radiation and to shield against ambient γ-rays and neutrons from the surrounding rocks. Both detectors are calibrated using radioactive sources and cosmic-ray induced background samples.

Based on the number of events at the near detector and assuming no oscillation, RENO finds a clear deficit, with a far-to-near ratio R = 0.920 ± 0.009 (stat.) ± 0.014 (syst.). The value of sin²2θ₁₃ is determined from a χ² fit with pull terms on the uncorrelated systematic uncertainties. The number of events in each detector after the background subtraction has been compared with the expected number of events, based on the neutrino flux, detection efficiency, neutrino oscillations and contribution from the reactors to each detector determined by the baselines and reactor fluxes. The best-fit value obtained is sin²2θ₁₃ = 0.113 ± 0.013 (stat.) ± 0.019 (syst.), which excludes the no-oscillation hypothesis at 4.9 σ.

The RENO collaboration consists of about 35 researchers from Seoul National University, Chonbuk National University, Chonnam National University, Chung Ang University, Dongshin University, Gyeongsang National University, Kyungpook National University, Pusan National University, Sejong University, Seokyeong University, Seoyeong University and Sungkyunkwan University.

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The Daya Bay reactor antineutrino experiment has observed the disappearance of electron-antineutrinos at a distance of about 2 km from the reactors. As briefly reported earlier, this provides strong evidence for a new kind of neutrino oscillation through a nonzero neutrino-mixing angle, θ₁₃.

There has been good evidence for more than a decade that the electron-neutrino, muon-neutrino and tau-neutrino can morph into one another. This phenomenon of neutrino oscillation is a consequence of mixing between the three flavours of neutrinos, and oscillations between three neutrinos are described with three mixing angles, two mass-squared differences and one CP-violating phase. Two of the mixing angles, θ₁₂ and θ₂₃, have been measured to good precision but the third mixing angle, θ₁₃, was poorly known.

A decade ago, the CHOOZ experiment set a limit of sin²2θ₁₃ < 0.17. However, newer analyses of the measurements with solar neutrinos and by the KamLAND experiment – as well as data from the T2K, MINOS and Double Chooz experiments – hinted that θ₁₃ could be larger than zero. On 8 March, based on an exposure of 43,000 tonne-GW_th-days, the Daya Bay collaboration reported the result of their measurement, sin²2θ₁₃ = 0.092 ± 0.016 (stat.) ± 0.005 (syst.), concluding that θ₁₃ is significantly different from zero.

The Daya Bay experiment is located at the Daya Bay Nuclear Power Complex in China, 55 km northeast of Hong Kong. About 3.6 × 10²¹ low-energy electron-antineutrinos per second are produced by three pairs of nuclear reactors with a combined maximum thermal-power of 17.4 GW_th. Three underground experimental halls connected by horizontal tunnels will eventually house eight antineutrino detectors (two in each near hall and four in the far site).

In each hall, the antineutrino detectors are submerged in a water pool that is partitioned optically into two zones. These two water-Cherenkov detectors tag cosmic-ray muons, which can generate background that mimics antineutrino interactions. The water also shields the detectors from ambient radiation that can generate background. The experiment identified electron-antineutrinos via the inverse beta-decay reaction ν_e + p → e⁺ + n, with 20 tonnes of 0.1% gadolinium-doped liquid scintillator in each antineutrino detector.

The data used for these first results were obtained with six antineutrino detectors – three deployed in the far hall, two in one of the near halls and one in the other near hall. When the number of detected electron-antineutrino events at the far site was compared with the expected number derived from the measurements in the near sites, a ratio of 0.940 ± 0.011 (stat.) ±0.004 (syst.) was found, indicating neutrino oscillation through θ₁₃. Using the total number of detected events yielded a value of sin²2θ₁₃ that was 5.2 σ from zero.

Figure 1 shows the disappearance of reactor electron-antineutrinos as a function of flux-weighted distance. Further evidence for this new kind of neutrino oscillation comes from the comparison of the observed and predicted energy spectra of the electron-antineutrinos at the far site. As figure 2 shows, the spectral distortion as a function of the prompt (positron) energy is also consistent with oscillation corresponding to sin²2θ₁₃ = 0.092.

Since the announcement of Daya Bay’s measurement of a nonzero value for θ₁₃, the RENO collaboration has reported the observation of the disappearance of electron-antineutrinos by their experiment based in Korea. The value that they find for sin²2θ₁₃ is consistent with the results from Daya Bay.

A nonzero θ₁₃ is crucial for designing experiments to search for CP-violation in the neutrino sector. These next-generation experiments will explore whether neutrinos oscillate differently from antineutrinos and answer the question of whether neutrinos can explain why matter is predominant in the universe. Furthermore, knowing the value of θ₁₃ helps to complete the determination of the neutrino-mixing matrix and constrain models beyond the current Standard Model.

• The Daya Bay collaboration consists of 230 collaborators from 38 institutions worldwide. The experiment is supported by the funding agencies of China, the Czech Republic, Hong Kong, Russia, Taiwan and the US. Daya Bay is currently one of the largest collaborative scientific projects between China and the US.

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The measurements of the electron- and muon-neutrino fluxes published by the Super-Kamiokande collaboration in 1998 marked a turning point in the history of particle physics. This team showed that fewer muon-neutrinos arrive at the surface of the Earth than are produced by cosmic-ray interactions in the upper atmosphere (atmospheric neutrinos). This in turn indicated evidence for neutrino oscillations, the phenomenon in which the flavour of the neutrino changes (oscillates) as the neutrino propagates through space and time. Since the publication of Super-Kamiokande’s seminal paper, the phenomenon of neutrino oscillations has been established through further measurements of atmospheric neutrinos, as well as of neutrinos and antineutrinos produced in the Sun, by nuclear reactors and by high-energy particle accelerators. It is arguably the most significant advance in particle physics of the past decade.

Extending the Standard Model

Neutrino oscillations imply that the Standard Model is incomplete and must be extended to include neutrino mass as well as mixing among the three neutrino flavours. The mechanism by which neutrino mass is generated is not known. An intriguing possibility is that the tiny neutrino mass is the result of physics at extremely high energy scales. Such a “see-saw” mechanism might also help to explain why neutrino mixing is so much stronger than the mixing among quarks. Mixing among three massive neutrinos admits the possibility that symmetry between matter and antimatter (CP-symmetry) is violated via the neutrino mixing matrix. Nonzero neutrino mass implies that lepton number must be used to distinguish a neutrino from an antineutrino. If lepton number is not conserved then a neutrino is indistinguishable from an antineutrino, i.e. the neutrino is a Majorana particle – a completely new state of matter. The determination of the properties of the neutrino, therefore, is fundamental to the development of particle physics.

These exciting new measurements imply that it may be possible to observe CP-violation in neutrino oscillations

Neutrino oscillations are readily described by extending the Standard Model to include three neutrino-mass eigenstates, ν₁, ν₂ and ν₃, such that the neutrino-flavour eigenstates, ν_e, ν_μ and ν_τ, are quantum-mechanical mixtures of the mass eigenstates (figure 1). Neutrino oscillations arise from the “beating” of the phase of the neutrino-mass eigenstates as a neutrino produced as an eigenstate of flavour propagates through space and time. The matrix by which the mass-basis is rotated into the flavour-basis is parameterized in terms of three mixing angles (θ₁₂, θ₂₃ and θ₁₃) and one phase parameter (δ). If δ is nonzero (and not equal to π), then CP-violation in the neutrino sector will occur so long as θ₁₃ > 0. Measurements of neutrino oscillations in vacuum can be used to determine the moduli of the mass-squared differences Δm²₃₁ = m²₃ – m²₁ and Δm²₂₁ = m²₂ – m²₁ and, with the aid of interactions with matter, also the sign.

The bulk of the measurements of neutrino oscillations to date have been collected using the dominant “disappearance” channels ν_e → ν_e and ν_μ → ν_μ. These data have yielded values for the three mixing angles, as well as for the magnitude of the mass-squared differences Δm²₃₁ and Δm²₂₁, and have shown that m₂ > m₁ (i.e. that Δm²₂₁ > 0). Last year, the T2K, MINOS and Double Chooz experiments presented evidence that θ₁₃ may be greater than zero. Then, in March this year, the Daya Bay collaboration reported that sin²2θ₁₃ = 0.092 ± 0.016 (stat.) ± 0.005 (syst.), i.e. that sin²2θ₁₃ = 0 is excluded at 5.2 σ. The announcement was soon followed by the report of a similar result from the RENO experiment. These exciting new measurements imply that it may be possible to observe CP-violation in neutrino oscillations. The challenge for the neutrino community, therefore, is to refine the measurement of θ₁₃ to determine the sign of Δm²₃₁ (the “mass hierarchy”), to discover CP-violation (if, indeed, it does occur) by measuring δ and to improve the accuracy with which θ₂₃ is known.

Over the next few years, several experiments – MINOS, T2K, NOνA, Double Chooz, Daya Bay and RENO – will exploit the ν_μ→ ν_e and ν_e → ν_x channels to improve significantly the precision with which θ₁₃ is known. The NOνA long-baseline experiment might also be able to determine the mass hierarchy. However, it is unlikely that either T2K or NOνA will be able to discover CP-violation, i.e. that δ ≠ 0 or π.

The Neutrino Factory

Neutrino oscillations also have implications well beyond the confines of particle physics. The possibility of CP-violation through the neutrino mixing matrix, combined with the possibility that the neutrino is a Majorana particle, makes it conceivable that the interactions of the neutrino led to the observed domination of matter over antimatter in the universe. The abundance of neutrinos in the universe is second only to that of photons. Even with a tiny mass, the neutrino may make a significant contribution to dark matter and thereby play an important role in determining the structure of the universe.

Such a breadth of impact justifies an ambitious, far-reaching experimental programme. Determining the nature of the neutrino – whether Majorana or Dirac – through the search for neutrinoless double-beta decay (2β0ν) is an important part of this programme. The absolute neutrino mass must also be determined either through observations of 2β0ν decay or from the measurement of the end-point of the electron spectrum in beta decay. Equally important is the accurate determination of the parameters that determine the properties of the neutrino. This requires intense, high-energy neutrino and antineutrino beams – precisely what the Neutrino Factory is designed to produce.

In the Neutrino Factory, beams of ν_e and ν_μ (ν_e,ν_μ) are produced from the decays of μ⁺ (μ^–) circulating in a storage ring. High neutrino-energies can readily be achieved because the neutrinos carry away a substantial fraction of the energy of the muon. Time-dilation is beneficial, allowing sufficient time to produce a pure, collimated beam. The table above lists the oscillation channels that are available at the Neutrino Factory. Charged-current interactions induced by ν_e → ν_μ oscillations – the “gold channel” – produce muons that are opposite in charge to those produced by the ν_μ in the beam, so a magnetized detector is required. The additional capability to investigate the “silver” (ν_e → ν_τ) and “platinum” (ν_μ → ν_e) channels also makes the Neutrino Factory an excellent place to look for oscillation phenomena that are outside the standard three-neutrino mixing paradigm. It would be the ideal facility to serve the precision-era of neutrino-oscillation measurements.

In 2011, the International Design Study for the Neutrino Factory (the IDS-NF) collaboration presented two options for the facility in its Interim Design Report (IDR) (Choubey et al. 2011). The first, optimized for discovery reach at small θ₁₃ (sin²2θ₁₃ < 10^–2), calls for two distant detectors, with baselines of 2500–5000 km and 7000–8000 km, and a stored-muon energy of 25 GeV. The second option, optimized for sensitivity at large θ₁₃, requires a single detector at a distance of around 2000 km and a stored-muon beam with an energy of only 10 GeV. Figure 2 shows the discovery reach of the facility presented in terms of the fraction of all possible values of δ (the “CP fraction”) and plotted as a function of sin²2θ₁₃.

In the past few weeks, the Daya Bay and RENO collaborations have announced the first measurements of sin²2θ₁₃ with a value around 0.1. Figure 2 shows that at such a large value of θ₁₃, excellent performance can be achieved using the “low-energy” option. At such a large value of θ₁₃, the precision and discovery reach of a “low energy” Neutrino Factory is significantly better than the realistic alternatives (IDS-NF 2011).

Novel techniques

The IDS-NF baseline accelerator facility sketched in figure 3 provides a total of 10²¹ muon decays per year, split between the two distant neutrino detectors. The process of creating the muon beam begins with the bombardment of a pion-production target with a pulsed proton beam. The pions are captured in a solenoidal channel in which they decay to produce the muon beam. A sequence of accelerators is then used to manipulate and reduce (cool) the muon-beam phase space and to accelerate the muons to their final energy.

The muon’s short lifetime has required novel techniques to be developed to carry out these steps. Ionization cooling, the technique by which it is proposed to cool the muons, involves passing the beam through a material in which it loses energy through ionization and then re-accelerating it in the longitudinal direction to replace the lost energy. Muon acceleration will be carried out in a series of superconducting linear and recirculating linear accelerators. The final stage of acceleration, from 12.6 GeV to the stored-muon energy of 25 GeV, is provided by a fixed-field alternating-gradient (FFAG) accelerator. The baseline neutrino detector is a MINOS-like iron-scintillator sandwich calorimeter with a sampling fraction optimized for the Neutrino Factory beam. The baseline calls for a fiducial mass of 100 kilotonnes to be placed at the intermediate baseline and a detector of 50 kilotonnes at the magic baseline.

Much of the Neutrino Factory facility, the accelerator complex and the neutrino detectors exploit state-of-the-art technologies. To achieve the ultimate performance (10²¹ muon decays per year) the IDS-NF baseline calls for: a proton-beam power of 4 MW, delivered at a repetition rate of 50 Hz in short (around 2 ns) bunches; a pion-production target capable of accepting the high proton-beam power; an ionization-cooling channel that increases the useful muon flux by a factor of around 2; and an FFAG to boost the beam energy rapidly to 25 GeV. R&D programmes that address each of these issues are underway. CERN, along with other proton-accelerator laboratories, is actively developing the technologies necessary to deliver multimega-watt, pulsed proton beams. The principle of a mercury-jet pion-production target was demonstrated by the MERIT experiment in 2008 that ran in the beamline of n_TOF, the neutron time-of-flight facility at CERN. The nonscaling FFAG accelerator EMMA (the Electron Model of Muon Acceleration, also known as the Electron Model of Many Applications) has been commissioned at the Daresbury Laboratory in the UK and used to demonstrate the “serpentine acceleration” characteristic of the nonscaling FFAG. The international Muon Ionization Cooling Experiment (MICE) at the Rutherford Appleton Laboratory will provide the engineering demonstration of the ionization-cooling technique (see box, previous page).

The Neutrino Factory is the facility of choice for the study of neutrino oscillations. It has excellent discovery reach and offers the best precision on the mixing parameters. The ability to vary the stored-muon energy and, perhaps the detector technology, gives the necessary flexibility to respond to developments in understanding neutrino physics and in the discovery of new phenomena. The R&D programme required to make the Neutrino Factory a reality will directly benefit the development of a muon collider and experiments that seek to discover charged lepton-flavour violation. The case for the Neutrino Factory as part of a comprehensive muon-physics programme is compelling indeed.

I gratefully acknowledge the help, advice, and support of my many colleagues within the IDS-NF, EUROnu and MICE collaborations and the Neutrino Factory community who have freely discussed their results with me and from whose work and results I have drawn freely.

BOX INSET

Cooling at MICE

MICE is a single-particle experiment in which the position and momentum of each muon is measured before it enters the MICE cooling channel and is measured again after it has left (Gregoire et al. 2003 and 2005). Muons with momenta between 140 MeV/c and 240 MeV/c, with normalized emittance between 2 πmm and 10 πmm, will be provided by a purpose-built beamline at the 800 MeV proton synchrotron, ISIS, at the Rutherford Appleton Laboratory.

The MICE cooling channel, a single lattice cell, comprises three 20-l volumes of liquid hydrogen and two short linac modules each consisting of four 201 MHz cavities. Beam transport is achieved by a series of superconducting solenoids: the “focus coils” focus the beam into the liquid-hydrogen absorbers, while a “coupling coil” surrounds each of the linac modules. A particle-identification system, with scintillator time-of-flight (TOF) hodoscopes and threshold Cherenkov counters, upstream of the cooling channel allows a pure muon beam to be selected. Downstream of the cooling channel, a final hodoscope and a calorimeter system allow muon decays to be identified. The calorimeter is composed of a lead-scintillator section, of a similar design to that of the KLOE detector at DAΦNE but with thinner lead foils, followed by a fully active scintillator detector (the electron-muon ranger) in which the muons are brought to rest.

Charged-particle tracking in MICE is provided by two solenoidal spectrometers that together determine the relative change in transverse emittance of the beam, which is expected to be approximately 10%, with a precision of ±1% (i.e. a 0.1% measurement of the change in absolute emittance). The trackers themselves are required to have high track-finding efficiency in the presence of background that is induced by X-rays produced in the RF cavities.

In the first “step” of the experiment, the muon beam for MICE has been characterized using the beamline instrumentation and the TOF, Cherenkov and lead-scintillator systems (figure 5). The results, which are being prepared for publication, show that the muon beam can provide the range of momentum and emittance required by MICE. The trackers and a prototype of the electron-muon ranger have been tested and shown to perform to specification. The cavities that make up the two short linac sections have been manufactured by Lawrence Berkeley National Laboratory (LBNL). The superconducting magnets required for the cooling channel are all under construction. By the end of 2012, the collaboration will commission the two spectrometer modules and the first liquid-hydrogen absorber and focus-coil module. This will allow preliminary studies of the ionization-cooling effect to be performed. The full MICE cooling cell will be constructed once the initial cooling studies are complete.

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The Daya Bay Reactor Neutrino Experiment, a multinational collaboration operating in the south of China, has reported its first results. The team has analysed tens of thousands of interactions of electron-antineutrinos caught by six massive detectors buried in the mountains adjacent to the powerful nuclear reactors of the China Guangdong Nuclear Power Group.

The copious data revealed for the first time a strong signal of the mixing angle θ₁₃, related to the type of neutrino oscillation in which electron-neutrinos morph into the other two flavours. This is the last mixing angle to be measured precisely and could reveal clues leading to an understanding of why matter predominates over antimatter in the universe. Once thought to be near zero, the first results indicate that sin²2θ₁₃ is equal to 0.092 ± 0.017.

The Daya Bay experiment counts the number of electron-antineutrinos detected in the halls nearest the Daya Bay and Ling Ao reactors and calculates how many would reach the detectors in the Far Hall if there were no oscillation. The number that apparently vanish on the way (by oscillating into other flavours) gives the value of θ₁₃. Because of the near-hall/far-hall arrangement, it is unnecessary to have a precise estimate of the antineutrino flux from the reactors.

The initial results will in the coming months and years be honed by collecting more data and reducing statistical and systematic errors. Refined results will open the door to further investigations and influence the design of future neutrino experiments, including how to determine which neutrino flavours are the most massive and whether there is a difference between neutrino and antineutrino oscillations.

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The search for the neutrinoless double-beta decay (ββ0ν decay) aims to solve a long-standing question concerning the nature of neutrinos. The decay, in which a nucleus decays by emitting two electrons but no neutrinos, can occur only if the neutrino is its own antiparticle, i.e. a Majorana particle. If it occurs, it must be extremely rare, with a half-life greater than 10²⁴ years. This poses an enormous experimental challenge regarding its unambiguous detection, with just a few nuclear isotopes offering a useful hunting ground. Now an experiment at the ISOLDE facility at CERN has identified a new potential candidate, the palladium isotope ¹¹⁰Pd.

The signature for ββ0ν decay appears in the sum of the energies of the two emitted electrons, which should have a single peak at the Q value for the decay, i.e. at the energy corresponding to the mass difference between the initial and final nuclide. (In double-beta decay with neutrinos (ββ2ν), the emitted electrons have a broad energy spectrum.) Calorimetric experiments searching for ββ0ν require detectors fabricated from sufficient quantities of the transmuting material to allow the detection of a decay within a reasonable amount of time. In addition, the energy of the decay peak must be known precisely if the detector is to have a high resolution at the correct energy.

With its high natural abundance, ¹¹⁰Pd offers a promising alternative for double-beta decay searches, now that its Q value has been measured directly with unprecedented accuracy. An experiment using the Penning-trap mass spectrometer ISOLTRAP at ISOLDE has determined the Q value from the cyclotron frequency ratio of ¹¹⁰Pd and its decay-product ¹¹⁰Cd by manipulating a few, singly charged ions in an isolated environment (Fink et al. 2012).

In a Penning trap, a charged particle is bound radially on the cyclotron orbit by a homogeneous magnetic field, while an electrostatic potential between the hyperbola-shaped electrodes provides axial confinement (see figure). Since the ions are trapped in three dimensions, they exhibit three eigenmotions (only one of which is shown in the figure for simplicity). An applied radio-frequency field can modify the energy stored in the eigenmotions, resonantly enhancing the energy transfer when it reaches the exact eigenfrequency. This can be measured using a technique known as time-of-flight ion-cyclotron-resonance. Usually, fewer than 10 ions of one species are excited in the trap and the cyclotron frequency is determined. The other species is then loaded into the trap and excited. This measurement cycle is repeated many times in order to collect statistics and minimize systematic effects.

In this experiment, the Q value was determined after roughly two days of measurement to be Q = 2017.85(64) keV. This value is shifted by 14 keV compared with previous results and is 17 times more precise. While the shift leads to a new value for the ¹¹⁰Pd half life, the lower uncertainty should enable future experiments on ββ0ν decay to have higher resolution.

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When Kai Zuber’s text on neutrinos was published in 2003, the author correctly predicted that the field would see tremendous growth in the immediate future. Now, revised and expanded to include the latest research, conclusions and implications, Neutrino Physics, Second Edition delves into neutrino cross-sections, mass measurements, double-beta decay, solar neutrinos, neutrinos from supernovae and high-energy neutrinos, as well as new experimental results in the context of theoretical models. It also provides an entirely new discussion on the resolution of the solar-neutrino problem, the first real-time measurement of solar neutrinos below 1 MeV, geoneutrinos and long-baseline accelerator experiments.

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The Double Chooz experiment, which detects antineutrinos produced in the nearby nuclear reactor at Chooz in the French Ardennes, started data-taking in April 2011. The collaboration announced its first results seven months later at the 2011 LowNu conference held in Seoul, reporting new data consistent with short-range oscillations.

The result, on the disappearance of antineutrinos compared with the expected flux from the reactor, helps to determine the so-far unknown third neutrino-mixing angle, θ₁₃. The observed deficit indicates oscillation with the following value: sin²2θ₁₃ = 0.086 ± 0.041 (stat.) ± 0.030 (syst.) or, at 90% CL, 0.015 < sin²2θ₁₃ < 0.16.

The measurement of this last mixing angle is important for future experiments aimed at measuring leptonic CP violation and relates indirectly to the matter–antimatter asymmetry in the universe.

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Located under 1700 m of rock in the Modane Underground Laboratory (LSM) at the middle of the Fréjus Rail Tunnel, the NEMO 3 experiment was designed to search for neutrinoless double beta decay, with the aim of discovering the nature of the neutrino – whether it is a Majorana or Dirac particle – and measuring its mass. The experiment ran for seven years before it finally stopped taking data in January 2010. While the sought-after decay mode remained elusive, NEMO 3 nevertheless made impressive headway in the study of double beta decay, providing new limits on a number of processes beyond the Standard Model.

Standard double beta decay (ββ2ν) involves the simultaneous disintegration of two neutrons in a nucleus into two protons with the emission of two electrons accompanied by two antineutrinos, (A,Z) → (A,Z+2) + 2e^– +2ν. It is a second-order Standard Model process and for it to occur the transition to the intermediate nucleus accessible by normal beta decay, (A,Z) → (A,Z+1) + e^– + ν, must be forbidden by conservation of either energy or angular momentum. In nature, there are 70 isotopes that can decay by ββ2ν and experiments have observed this process in 10 of these, with half-lives ranging from 10¹⁸ to 10²¹ years. However, ββ2ν decay is not sensitive to the nature or mass of the neutrino, unlike double beta decay with no emitted neutrinos (ββ0ν). This process, (A,Z) → (A,Z+2) + 2e^–, is forbidden by the Standard Model electroweak interaction because it violates the conservation of lepton number (ΔL = 2). Such a decay can occur only if the neutrino is a Majorana particle (a fermion that is its own antiparticle). Non-Standard Model processes that can lead to ββ0ν decay include the exchange of a light neutrino, in which case the inverse of the ββ0ν half-life depends on the square of the effective neutrino mass. Other possible processes involve a right-handed neutrino current, a Majoron coupling or supersymmetric particle exchange.

The experimental signature for double beta-decay processes appears in the sum of the energy of the two electrons. For ββ0ν decay, this would have a peak at the Q_ββ transition energy (typically 2–4 MeV), while for ββ2ν decay it takes the form of a continuous spectrum from zero to Q_ββ. There are also two other observables: the angular distribution between the two electrons and the individual energy of the electrons. These two variables can distinguish which process is responsible for ββ0ν decay, if it is observed.

The NEMO collaboration – where NEMO stands for the Neutrino Ettore Majorana Observatory – has been working on ββ0ν decay since 1989. The design of the NEMO 3 detector, which evolved from two prototypes, NEMO 1 and NEMO 2, began in 1994 and construction started three years later. The method uses a number of thin source foils of enriched double beta-decay emitters surrounded by two tracking volumes and a calorimeter.

The challenge for any search for ββ0ν decay is the control of the backgrounds from cosmic rays, natural radioactivity, neutrons and radon. The background comes from any particle interactions or radioactive decays that can produce two electrons in the source foils. Because the signal level is so low, even third- and fourth-order processes can be a problem. Cosmic rays are suppressed by installing the experiment in a deep underground laboratory, as at the LSM. Natural radioactivity is reduced by material selection and purification of the source isotopes: the source foils in NEMO 3 had a radioactivity level a million times less than the natural level of radioactivity (around 100 Bq/kg). Neutrons and high-energy γ-rays are suppressed by specially designed and adapted shielding.

The NEMO 3 detector

The principle of NEMO 3 was to detect the two emitted electrons and to measure their energy as well as their angular distribution and their individual energies. The identification of the electrons reduces drastically the background compared with the calorimetric techniques of other experiments. The price of this advantage is a rather modest energy resolution, partly as a result of the electron’s energy loss in the source foils. However, the experimental sensitivity for ββ0ν depends on the product of the energy resolution and the number of background events. The source foils in NEMO 3 had a thickness of around 100 μm, which corresponded to a compromise between the amount of radioactive isotope and the electrons’ energy losses.

Another advantage of this experimental technique is the possibility of using different isotopes. The double beta-decay source inside NEMO 3 had a total mass of 10 kg, which was shared as follows: 6.914 kg of ¹⁰⁰Mo, 0.932 kg of ⁸²Se, 0.405 kg of ¹¹⁶Cd, 0.454 kg of ¹³⁰Te, 37.0 g of ¹⁵⁰Nd, 9.4 g of ⁹⁶Zr and 7.0 g of ⁴⁸Ca. These isotopes were enriched in Russia. In addition, two ultrapure sources of copper (0.621 kg) and natural tellurium (0.491 kg) were used to measure the external background. It is the first time that a detector has measured seven different double beta-decay emitters at the same time.

The NEMO 3 detector was made of 20 identical sectors. The tracking volume consisted of 8000 drift chambers working in Geiger mode. The volume was filled with a mixture of helium, 4% alcohol, 1% argon and a few parts per million of water to ensure the stable behaviour of the chamber. Electrons could be tracked with energy down to 100 keV with an efficiency of greater than 99%.The calorimeter was made of 2000 plastic scintillators coupled to low-radioactivity Hamamatsu phototubes. The choice of plastic scintillator was driven by the low Z to reduce back scattering, the low radioactivity and the cost. The calorimeter allowed measurements of both the energy (σ=3.6% at 3 MeV) and the time of flight (σ= 300 ps at 1 MeV).

A coil created a magnetic field of 0.003 T to enable the identification of the sign of the electrons. The shielding was made of 20 cm of iron to reduce γ-ray background and 30 cm of water to reduce the neutron background. A tent flushed with air containing just 15 mBq/m³ of radon surrounded the whole detector.

The unique feature of the NEMO 3 experiment was its ability to identify electrons, positrons, γ-rays and delayed α-particles. Figure 2 shows a typical double beta-decay event in NEMO 3 with two electrons emitted from a source foil, with the track curvature in the magnetic field identifying the charge and the struck scintillator blocks measuring the energy and the time of flight. The timing is important to distinguish a background electron crossing the detector (Δt=4 ns) from two electrons coming from a source foil (Δt=0 ns).

The experiment has measured the background through various analysis channels: single e^–, e^–+γ, e+α, e^–+α+γ, e^–+γ+γ, e^–+e⁺ and so on. This allows measurements to be made of the actual backgrounds from residual contamination of the source foils as well as from the surrounding materials. Figure 3 demonstrates the ability of the experiment to identify the many sources of external background in the e^–γ channel (as an example) for the ¹⁰⁰Mo source foil.

NEMO 3 has produced an impressive list of results. The main result is, of course, related to the search for ββ0ν decay. Figure 4 shows the sum of the electron energy for 7 kg of ¹⁰⁰Mo after 4.5 years of data-taking, zoomed into the region where the signal for ββ0ν decay is expected. The measurement of all of the kinematic parameters and the identification of all of the sources of background allows a 3D likelihood analysis to be performed. The result is a limit on the half-life of T_1/2 > 1×10²⁴ years, corresponding to a neutrino mass limit <m_ν> < 0.3–0.9 eV. The range corresponds to the spread associated with the different nuclear matrix-element calculations that must be used to extract the effective neutrino mass. This limit obtained with 7 kg of ¹⁰⁰Mo is one of the best limits, together with the result of <m_ν> <0.3 – 0.7 eV from the Cuoricino experiment (12 kg of ¹³⁰Te) and of <m_ν> < 0.3–1.0 eV from the Heidelberg-Moscow experiment (11 kg of ⁷⁶Ge).

One possible scenario for ββ0ν involves the emission of the Majoron, the hypothetical massless boson associated with the spontaneous breaking of baryon-number minus lepton-number (B-L) symmetry. NEMO 3 has obtained the best limit so far for the Majoron-neutrino coupling, with g_M < (0.4–1.8) × 10^–4. The experiment has also set a limit on the λ parameter in models where a right-handed current exists for neutrinos, with λ < 1.4 × 10^–6. These limits were obtained by analysing the angular distributions of the decay electrons and they are therefore unique to NEMO 3.

In addition, NEMO 3 has measured the half-lives for seven ββ2ν decays, providing a high-precision test of the Standard Model and nuclear data that can be used in theoretical calculations. In seven years, more than 700,000 events were recorded for ββ2ν emission from ¹⁰⁰Mo. Figure 5 shows the energy spectrum, angular distribution and single energies measured for ¹⁰⁰Mo. The first direct detection of ββ2ν decay to the 0⁺ excited state has also been measured for this nucleus and the first limit on the bosonic component of the neutrino has been obtained.

The NEMO 3 detector has demonstrated a powerful method for searching for neutrinoless double beta decay, with the unique capability of measuring all kinematic parameters of the decay. The next step for the NEMO collaboration is to build the SuperNEMO detector, which will accommodate 100 kg of source foil (⁸²Se, ¹⁵⁰Nd or ⁴⁸Ca) to reach a sensitivity of 50 meV on the effective mass of the neutrino. A demonstrator module is under construction in several laboratories around the world and will start operation in 2013 in the LSM, with 7 kg of ⁸²Se. The main improvement in this larger detector over NEMO 3 will be the energy resolution (σ=1.7% at 3 MeV) and the reduction of the background by a factor of 10. This demonstrator will improve the current limit on the effective neutrino mass and is expected to reach the goal of a zero-background experiment for 7 kg of source and two years of data-taking, which has never been done before. With this demonstration, the collaboration will be ready to build more Super NEMO modules up to the maximum source mass possible.

• The NEMO and SuperNEMO collaboration is formed by laboratories from France, the UK, Russia, the US, Japan, the Czech Republic, Slovakia, Ukraine, Chile and Korea. The LSM is operated by the CNRS and the CEA.

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Accelerator neutrino beams are currently the object of intense discussion and development. They provide a necessary tool for the detailed study of neutrino oscillations and in particular the observation of potential CP-violating effects that are born from the interference of transitions among the three known species of neutrino. Neutrino interaction cross-sections are tiny, so the challenge in studying their properties has been to produce ever increasing beam intensities. The next challenge in neutrino physics will be to establish precisely the parameters of the oscillations and then compare the oscillations of neutrinos with anti-neutrinos (or the oscillation probability as a function of neutrino energy) to search for CP-violation. This will require precise measurements of the transitions of neutrinos into each other, which will in turn demand a much better knowledge of the neutrino beams.

At present – and probably for the next decade – neutrino beams are generated by the conventional technique: a beam of multi-giga-electron-volt protons, as powerful as possible (up to around 500 kW beam power), is directed at a target to produce a large number (10¹² or more) of hadrons, mainly pions with a small admixture (5–10%) of kaons. These are then focused in the direction desired for the neutrino beam and they decay – producing neutrinos – in a decay tunnel.

In the absence of a good theory of hadronic interactions, a precise prediction of the properties of such neutrino beams requires measurements of particle production at an unprecedented level of precision. The role of the NA61/SHINE experiment at CERN’s Super Proton Synchrotron (SPS) is to perform these hadron production measurements. More specifically, it has taken data for the T2K experiment in Japan, both with a thin carbon target and a full replica of the target used in T2K. These data have already proved important for the extraction of the first results on electron-neutrino appearance and muon-neutrino disappearance in T2K. As statistics increase in T2K, they will become more and more essential.

The collaboration behind the SPS Heavy Ion and Neutrino Experiment (SHINE, approved at CERN as NA61) is an unlikely marriage between aficionados of the heaviest and lightest beams on offer. Ions as heavy as lead nuclei have been accelerated in the SPS, while neutrinos have the lightest mass (now famously non-zero) of all particles apart from photons. So what is the unifying concept between these communities that are a priori so different?

The NA49 detector in CERN’s North Area offers excellent tracking with its immense set of time-projection chambers (TPCs), time-of-flight (TOF) detectors and flexible beamline. To perform systematic measurements at energies at the onset of quark–gluon plasma creation, the heavy-ion physicists were interested in upgrading the detector to allow higher event statistics and lower systematic uncertainties. At the same time, neutrino physicists, attracted by the extensive coverage of the detector, were interested in running it in a simple configuration, but also with high statistics, so as to have the first data ready in time for the start of T2K.

The main upgrades relevant for all of the NA61/SHINE physics programmes concerned the TPC read-out, an extension of the TOF detectors and an upgrade of the trigger and data-acquisition system. Figure 1 shows the upgraded detector. Its acceptance fully covers the kinematic region of interest for T2K.

The NA61/SHINE experiment was approved in April 2007 and took data in a pilot run the following September, with 600,000 triggers on the thin carbon target and 200,000 triggers on the replica (long) T2K target. More extensive data-taking for the T2K physics programme took place in 2009 and 2010, both with thin (6 million triggers in 2009) and long targets (10 million triggers in 2010). In parallel, data were recorded for the NA61/SHINE heavy-ion and cosmic-ray programmes.

As a first priority, the cross-sections for producing charged pions from 30 GeV protons on carbon were measured with the thin-target data taken in 2007 (Abgrall et al. 2011). The systematic errors are typically in the range of 5–10% and smaller than the statistical errors. These data have already been used for an improved prediction of the neutrino flux in T2K (Abe et al. 2011). Furthermore, they also provide important input to improve the hadron-production models needed for the interpretation of air showers initiated by ultra-high-energy cosmic rays.

However, these first NA61/SHINE measurements provide only a part of what is needed to predict the neutrino flux in T2K. A substantial fraction of the high-energy flux, and in particular the electron-neutrino contamination, originates from the decay of kaons. Charged kaons are readily identified in NA61/SHINE from the suite of particle-identification techniques – dE/dx in the TPC and the TOF in the upgraded detector (see figure 2) – and a first set of cross-sections has been produced already. Neutral kaons can be reconstructed using the V⁰-like topology of K⁰_S→ π⁺π^– decays.

A large fraction (up to 40%) of the neutrinos originates from particles produced by re-interactions of secondary particles in the target, which for T2K is 90 cm long. This is difficult to calculate precisely and it motivates a careful analysis of the data taken with the long target. Long-target data are notoriously more difficult to reconstruct and analyse but they provide much more directly the information needed for extracting the neutrino flux. The NA61/SHINE collaboration presented a pilot analysis at the NUFACT meeting at CERN in early August (Abgrall 2011). The ultimate precision will come from the full analysis of the long-target data taken in 2010. The collaboration is working hard to complete these analyses in time for the high-statistics measurements that will become possible in T2K when the experiment resumes data-taking after recovering from damage in the massive earthquake in north-eastern Japan that occurred in March this year.

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The result is based on the observation of more than 15,000 neutrino events measured by the experiment, which observes the beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project. Using high-statistics data taken in 2009, 2010 and 2011, the collaboration has measured the velocity of the muon-neutrinos reaching the detector with much higher accuracy than previous studies conducted using accelerator neutrinos. Upgrades to the CNGS timing system and to the OPERA detector, as well as the use of high-precision geodesy to measure the neutrino baseline, allowed the collaboration to achieve comparable systematic and statistical accuracies.

To perform the study, the OPERA collaboration teamed up with experts in metrology from CERN and other institutions to make a series of high-precision measurements of the distance between the source and the detector, and of the neutrinos’ time of flight. The distance between the origin of the neutrino beam and OPERA was measured with an uncertainty of 20 cm over the 730 km travel path. The neutrinos’ flight time was determined with an accuracy of less than 10 ns by using sophisticated instruments, including advanced GPS systems and atomic clocks. The time responses of all of the elements of the CNGS beamline and of the OPERA detector have also been measured with great precision.

The results indicate that neutrinos from CERN arrive early at Gran Sasso by 60.7 ± 6.9 (stat.) ± 7.4 (sys.) ns compared with the time that would be taken assuming the speed of light in vacuum. This anomaly corresponds to a relative difference of the muon-neutrino velocity, v, with respect to the speed of light, c, (v-c)/c = (2.48 ± 0.28 (stat.) ± 0.30 (sys.) × 10^–5.

Given the potential far-reaching consequences of such a result, independent measurements are certainly needed before the effect can either be refuted or firmly established. While OPERA continues to gather more data, the MINOS collaboration in the US is planning to improve its measurement of the neutrino time of flight with the beam from Fermilab to the Soudan Underground Laboratory, about 730 km away.

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The Daya Bay Reactor Neutrino Experiment has begun its quest to answer some of the puzzling questions that still remain about neutrinos. The experiment’s first completed set of twin detectors is now recording interactions of antineutrinos as they travel away from the powerful reactors of the China Guangdong Nuclear Power Group, in southern China.

The start-up of the Daya Bay experiment marks the first step in the international effort of the Daya Bay collaboration to measure a crucial quantity related to the third type of oscillation, in which the electron-neutrinos morph into the other two flavours of neutrino. This transformation occurs through the least known neutrino-mixing angle, θ₁₃, and could reveal clues leading to an understanding of why matter predominates over antimatter in the universe.

The experiment is well positioned for a precise measurement of the poorly known value of θ₁₃ because it is close to some of the world’s most powerful nuclear reactors – the Daya Bay and Ling Ao nuclear power reactors, located 55 km from Hong Kong – and it will take data from a total of eight large, virtually identical detectors in three experimental halls deep under the adjacent mountains. Experimental Hall 1, a third of a kilometre from the twin Daya Bay reactors, is the first to start operating. Hall 2, about a half kilometre from the Ling Ao reactors, will come online in the autumn. Hall 3, the furthest hall, about 2 km from the reactors, will be ready to take data in the summer of 2012.

The Daya Bay experiment is a “disappearance” experiment. The detectors in the two closest halls will measure the flux of electron-antineutrinos from the reactors; the detectors at the far hall will look for a depletion in the expected antineutrino flux. The cylindrical antineutrino detectors are filled with liquid scintillator, while sensitive photomultiplier tubes line the detector walls, ready to amplify and record the telltale flashes of light produced by the rare antineutrino interactions. As a result of the large flux of antineutrinos from the reactors, the twin detectors in each hall will capture more than 1000 interactions a day, while at their greater distance the four detectors in the far hall will measure only a few hundred interactions a day. To measure θ₁₃, the experiment records the precise difference in flux and energy distribution between the near and far detectors.

The experimental halls are deep under the mountain to shield the detectors from cosmic rays and the detectors themselves are submerged in pools of water to shield them from radioactive decays in the surrounding rock. Energetic cosmic rays that make it through the shielding are tracked by photomultiplier tubes in the walls of the water pool and muon trackers in the roof over the pool so that events of this kind can be rejected.

After two to three years of collecting data with all eight detectors, the Daya Bay Reactor Neutrino Experiment should be well positioned to meet its goal of measuring the electron-neutrino oscillation amplitude – and hence sin² 2θ₁₃ – with a sensitivity of 1%.

The start up of the experiment begins after eight years of effort – four years of planning and four years of construction – by hundreds of physicists and engineers from around the globe. China and the US lead the Daya Bay collaboration, which also includes participants from Russia, the Czech Republic, Hong Kong and Taiwan. The Chinese effort is led by project manager Yifang Wang of the Institute of High Energy Physics (IHEP), Beijing, and the US effort is led by project manager Bill Edwards of Lawrence Berkeley National Laboratory and chief scientist Steve Kettell of Brookhaven National Laboratory.

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Since the result announced in June 2010, the experiment has nearly doubled its data set, from 100 antineutrino events to 197 events. While the new results are only about 1 σ away from the previous results, the combination rules out concerns that the previous result could have arisen from detector or calculation errors. Instead, the combined results point to a statistical fluctuation that has lessened as more data have been collected.

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Astrophysical neutrinos are produced in the interactions of cosmic rays with an ambient medium of gas (protons) and photons of different energies. Once produced, these cosmic neutrinos can propagate cosmological distances and reach the Earth practically without interactions. They therefore carry unique information about the sources of cosmic rays, their acceleration and the composition of the most energetic phenomena in the universe.

The neutrino sky “seen” by experiments originates in the atmosphere, which shines day and night in neutrinos. One experiment alone, IceCube at the South Pole, has already detected more than 10⁵ atmospheric neutrino events. However, the hope is to see “stars in broad daylight” through this atmospheric flux – that is, to observe neutrinos of cosmic origins. These include neutrinos from various point-like sources and some extended objects, as well as diffuse neutrino fluxes. The selection of the energy band – cosmic neutrinos should dominate at high energies – together with directional and time features, as well as correlations with known objects, emitting for instance in γ rays, are the main tools for distinguishing atmospheric and cosmic neutrinos.

The NUSKY 2011 international workshop on cosmic rays and cosmic neutrinos took place at the Abdus Salam International Centre for Theoretical Physics, Trieste, on 20–24 June. It attracted around 90 participants and featured some 40 talks by the main players in the field, covering all of the important aspects of the production, propagation and detection of high-energy cosmic neutrinos. Numerous discussions ensued, focusing on the implications of the latest experimental results, as well as on the status and perspectives of the field.

The results from IceCube played a prominent role in the discussions

The workshop took place during a critical period for a field in which the working experiments have reached the sensitivity necessary to probe realistic theoretical predictions. The results from IceCube – the first cubic-kilometre-scale detector ever built – thus played a prominent role in the discussions. Their preliminary results correspond to 40 and 59 detector strings (IC40 and IC59); data from IC79 are being analysed and the complete detector, IC86, is now running. So far, the various searches have found no cosmic-neutrino events.

Diffuse neutrino fluxes include the cosmogenic neutrinos generated in cosmic-ray interactions with the photons of the cosmic microwave background, as well as the integrated fluxes from remote, faint and unresolved objects. The IceCube collaboration finds no deviation of the reconstructed neutrino-energy spectrum from that for atmospheric neutrinos. This gives an upper bound on the neutrino flux in the 0.1–10 PeV energy range that is already below the Waxman-Bahcall limit, derived from the known cosmic-ray flux above 10¹⁹ eV.

As far as individual sources are concerned, the main suspects are objects that are relatively close, where the acceleration of cosmic rays probably occurs. These include supernova remnants (SNRs) in the Galaxy, as well as active galactic nuclei and gamma-ray bursters (GRBs). The IceCube all-sky maps show no statistically significant signal for steady or transient galactic or extragalactic sources. Nor has any neutrino been detected by IceCube (IC40 + IC59) in the so called stacking analysis of the GRBs (more than 100). The limit on the neutrino flux that emerges from this analysis is a factor of 5 below predictions, thus disfavouring the fireball model of GRBs.

The Pierre Auger Observatory in Argentina and ANITA, the balloon-bourne radio-interferometer that flew over Antarctica, are sensitive to the upper end of the cosmic-neutrino spectrum, the most relevant range for cosmogenic neutrinos (i.e. 10¹⁸ eV or 1 EeV). No neutrino-candidate events have been found in Auger data for periods equivalent to two years of the full array. ANITA-II has one candidate event, with one background event expected; cosmogenic models predict from 0.3 to 25 events.

The predictions for atmospheric neutrino fluxes depend on the properties of cosmic rays and on the physical conditions of the sources. In this connection, there are some new and interesting results. IceCube has found cosmic-ray anisotropies in the 20–400 TeV energy range, with a significant angular structure in the southern hemisphere. Anisotropy at higher energies, above 100 TeV, could reveal some connection to nearby SNRs. In addition, the KASCADE-Grande extensive air-shower array has observed structures in the “knee” region of the all-particle cosmic-ray spectrum.

Cosmic-ray origins

Turning to the question of the composition of ultra-high-energy cosmic rays (UHECRs), there had been somewhat contradictory results from the HiRes experiment and the Pierre Auger Observatory. In this connection, the possibilities for UHECR production by sources in the Galaxy (such as past GRBs ), as well as a dominant contribution from Centaurus A, were discussed at the workshop. The basic principles of cosmic-ray acceleration in SNRs are well understood on the basis of the non-linear theory of diffusive acceleration at collisionless Newtonian shocks.

The neutrino−γ-ray connection was at the centre of many discussions as a result of the wealth of new information from γ-ray astronomy. The production of neutrinos should be accompanied by production of γ-rays from π⁰-decay (the hadronic mechanism). However, ultrahigh-energy γs from extragalactic sources and γs of cosmogenic origin can interact with the medium (photons, electrons), giving rise to electromagnetic cascades. Hence, the whole γ spectrum shifts to lower energies in the giga- to tera-electron-volt range, where the Large Area Telescope (LAT) on the Fermi Gamma Ray Telescope gives important bounds. The Fermi-LAT results on the extragalactic γ flux can be translated into bounds on cosmic rays and cosmogenic neutrinos – the so-called “cascade” bound, based on the approximate equality of the energy released in neutrino production and in the electromagnetic cascade process. These data challenge the GRB origin of cosmic rays: if GRBs are the source of cosmic rays, then 10 events are predicted, while nothing appears in the diffuse bound.

One open question concerns the mechanism for the production of photons at the source. Tera-electron-volt γ-rays from transparent galactic sources can provide a direct indication of cosmic-ray acceleration sites. However, γs can be produced by accelerated electrons via the inverse Compton effect and by synchrotron radiation (both leptonic mechanisms). Fermi-LAT has measured γ spectra from a large number of SNRs and it turns out that both leptonic and hadronic γ-ray models work for SNRs on a source-by-source basis. In the case of GRBs, only bright GRBs are favoured by the Fermi-LAT data as the detectable sources. Nevertheless, bright nearby GRBs seem to be rare.

Features of neutrino propagation are a key element when the flavour of neutrino is taken into account in the detection process. The flavour composition and its dependence on neutrino energy are determined by conditions at the neutrino sources, in particular by the strength of magnetic field, the density distribution, etc. Flavour is also affected by neutrino oscillations and therefore depends on neutrino parameters. The expected composition ratio has the form a : 1 : 1 with a around 1, its precise value depending on 1–3 mixing, the deviation of 2–3 mixing from maximal, the neutrino-mass hierarchy and CP-violation. Various effects typical of physics beyond the Standard Model, such as neutrino decay, non-standard neutrino interactions or the presence of new neutrino species, can also modify the ratio. Finally, the ratio is extremely sensitive to possible violations of fundamental symmetries, such as Lorentz symmetry or the equivalence principle, which lead to modifications in the dispersion relations.

Neutrino astronomy enters a new cubic-kilometre era

Another highlight of the workshop was the report on the first-year of data-taking by DeepCore, the inner detector of IceCube, which has a low energy threshold of 10 GeV. The rate of events, which include cascades induced by electron-neutrinos as well as neutral current muon-neutrinos, was shown. DeepCore will detect around 800 neutrino-induced cascades per year. The physics motivations for the Phased IceCube Next Generation Upgrade (PINGU-I and PINGU-II) were also presented.

Neutrino observatories have now reached sufficient sensitivity to constrain multimessenger signals, γ-rays and UHECRs with minimal assumptions. That there is no evidence as yet for astrophysical neutrinos poses a problem for future projects because it means that IceCube will only scratch the surface of neutrino astronomy. The prime targets now are the transient sources.

There are several projects already under consideration or in progress. KM3NeT, a detector for neutrino astronomy under the Mediterranean Sea, which will have an instrumented volume of more than 5 km³, is in its preparatory phase. It will search for neutrino point sources in the energy range 100 GeV – 1 PeV. The Cherenkov Telescope Array is a new instrument for very high-energy (10–10⁵) GeV γ astronomy. JEM-EUSO will detect Cherenkov light coming from the atmosphere using a telescope on the International Space Station that will have an instantaneous aperture of up to 10⁶ km². ANITA-III, approved to fly in 2013–2014, will search for ultra-high-energy neutrinos with 3–5 times higher sensitivity than ANITA-II. The Askaryan Radio Array is a ground-based antenna array at the South Pole covering an area of 100 km². The expected yield is 3–5 neutrinos per year above 10¹⁷ eV, below the bulk of the cosmogenic neutrino predictions.

The NUSKY 2011 workshop was held just as high-energy neutrino astronomy enters a new cubic-kilometre era. Current bounds already have important implications and any further improvement of data will have an impact on the picture of the neutrino sky, with important consequences. The hope is that, with progressively more data from IceCube, a discovery is on the horizon. As Francis Halzen, of the University of Wisconsin-Madison and IceCube, concluded, “Hess 1912… and still no conclusion [on the origins of cosmic rays]; now the instrumentation is in place… SNRs and GRBs are in close range!”

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The T2K and MINOS experiments, which are both designed to study neutrino oscillations over long baselines, have reported results from their searches for the appearance of electron-neutrinos in beams of muon-neutrinos produced at distant locations. On 15 June the T2K collaboration announced that it had observed an indication that muon-neutrinos are able to transform into electron-neutrinos over the 295 km baseline of their experiment in Japan. Ten days later, the MINOS collaboration announced its latest results on the same effect. Both experiments find a non-zero value for the neutrino mixing angle θ₁₃. This would be zero if electron- and muon-neutrinos could not transform into each other.

Oscillations between the three known flavours of neutrino – electron, muon and tau – are described by a mixing matrix, which can be parameterized in terms of three angles, θ₁₂, θ₂₃, θ₁₃, and a CP-violating phase. Observations of oscillations in solar neutrinos and atmospheric neutrinos have determined θ₁₂ and θ₂₃, respectively, leaving θ₁₃ still unknown. The new results provide the first indications that this angle is not zero, via values of sin²2θ₁₃.

The collaboration found 88 neutrino events registered in the Super-Kamiokande detector

T2K (Tokai to Kamioka) uses the Super-Kamiokande detector in Kamioka to detect neutrinos produced at the Japan Proton Accelerator Research Complex (J-PARC) situated 295 km away. The new results are from an analysis based on all of the data collected between January 2010 – when the experiment began full operation – and 11 March 2011, when it was interrupted by the enormous earthquake in East Japan. This corresponds to a total of 1.43 × 10²⁰ protons on the neutrino-production target. The collaboration found 88 neutrino events registered in the Super-Kamiokande detector, six of which are clearly identifiable as candidate electron-neutrino events. The expectation would be for 1.5 such events in this data sample if neutrino oscillations do not take place. The observation implies the appearance of electron-neutrinos in the experiment, with a probability of 99.3%. At 90% confidence level (CL), the data are consistent with 0.03 < sin²2θ₁₃ < 0.28.

The MINOS (Main Injector Neutrino Oscillation Search) in the US sends a muon-neutrino beam 735 km through the Earth from the Main Injector accelerator at Fermilab to a 5000-tonne detector in the Soudan Underground Laboratory in northern Minnesota. In the recently announced analysis, based on 8 × 10²⁰ protons on target, the collaboration found a total of 62 electron neutrino-like events. Only 48 events would be expected if muon-neutrinos do not transform into electron neutrinos.

Compared with T2K, MINOS uses a different method and a different analysis technique to search for electron-neutrino appearance. The MINOS collaboration extracts 2sin²θ₂₃sin²2θ₁₃, and finds that it is less than 0.12 at 90% CL, with a best fit of 2sin²θ₂₃sin²2θ₁₃ = 0.04. This improves on results that the collaboration obtained with smaller data sets in 2009 and 2010. The latest results disfavour θ₁₃ = 0 at 89% CL, with a range that is consistent with that measured by T2K.