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.

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Cold non-baryonic dark matter appears to make up 85% of the matter and 25% of the energy in our universe. However, we don’t yet know what it is. As the opening of many research proposals state, “The nature of dark matter is one of the major open questions in physics.”

The evidence for dark matter comes from astronomical and cosmological observations. Theoretical particle physics provides us with various well motivated candidates, such as weakly interacting massive particles (WIMPs), axions and primordial black holes. Each has different experimental and observational signatures and a wide range of searches are taking place. Dark-matter research spans a very broad range of topics and methods. This makes it a challenging research field to enter and master. Dark Matter: Evidence, Theory and Constraints by David Marsh, David Ellis and Viraf Mehta, the latest addition to the Princeton Series in Astrophysics, clearly presents the relevant essentials of all of these areas.

The book starts with a brief history of dark matter and some warm-up calculations involving units. Part one outlines the evidence for dark matter, on scales ranging from individual galaxies to the entire universe. It compactly summarises the essential background material, including cosmological perturbation theory.

Part two focuses on theories of dark matter. After an overview of the Standard Model of particle physics, it covers three candidates with very different motivations, properties and phenomenology: WIMPs, axions and primordial black holes. Part three then covers both direct and indirect searches for these candidates. I particularly like the schematic illustrations of experiments; they should be helpful for theorists who want to (and should!) understand the essentials of experimental searches.

The main content finishes with a brief overview of other dark-matter candidates. Some of these arguably merit more extensive coverage, in particular sterile neutrinos. The book ends with extensive recommendations for further reading, including textbooks, review papers and key research papers.

Dark-matter research spans a broad range of topics and methods, making it a challenging field to master

The one thing I would argue with is the claim in the introduction that dark matter has already been discovered. I agree with the authors that the evidence for dark matter is strong and currently cannot all be explained by modified gravity theories. However, given that all of the evidence for dark matter comes from its gravitational effects, I’m open to the possibility that our understanding of gravity is incorrect or incomplete. The authors are also more positive than I am about the prospects for dark-matter detection in the near future, claiming that we will soon know which dark-matter candidates exist “in the real pantheon of nature”. Optimism is a good thing, but this is a promise that dark-matter researchers (myself included…) have now been making for several decades.

The conversational writing style is engaging and easy to read. The annotation of equations with explanatory text is novel and helpful, and  the inclusion of numerous diagrams – simple and illustrative where possible and complex when called for – aids understanding. The attention to detail is impressive. I reviewed a draft copy for the publishers, and all of my comments and suggestions have been addressed in detail.

This book will be extremely useful to newcomers to the field, and I recommend it strongly to PhD students and undergraduate research students. It is particularly well suited as a companion to a lecture course, with numerous quizzes, problems and online materials, including numerical calculations and plots using Jupyter notebooks. It will also be useful to those who wish to broaden or extend their research interests, for instance to a different dark-matter candidate.

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With the long shutdown on the horizon, the third run of the LHC is progressing in earnest. Its high-availability operation and mastery of operational risks were highly praised. Run 3 data is of immense importance as it will be the dataset that experiments will work with for the next decade. With the newly collected data at 13.6 TeV, the LHC experiments showed new measurements of Higgs and di-electroweak-boson production, though of course most of the LHC results were based on the Run 2 (2014 to 2018) dataset, which is by now impeccably well calibrated and understood. This also allowed ATLAS and CMS to bring in-depth improvements to reconstruction algorithms.

AI algorithms

A highlight of the conference was the improvements brought by state-of-the-art artificial-intelligence algorithms such as graph neural networks, both at the trigger and reconstruction level. A striking example of this is the ATLAS and CMS flavour-tagging algorithms, which have improved their rejection of light jets by a factor of up to four. This has important consequences. Two outstanding examples are: di-Higgs-boson production, which is fundamental for the measurement of the Higgs boson self-coupling (CERN Courier July/August 2024 p7); and the Higgs boson’s Yukawa coupling to charm quarks. Di-Higgs-boson production should be independently observable by both general-purpose experiments at the HL-LHC, and an observation of the Higgs boson’s coupling to charm quarks is getting closer to being within reach.

The LHC experiments continue to push the limits of precision at hadron colliders. CMS and LHCb presented new measurements of the weak mixing angle. The per-mille precision reached is close to that of LEP and SLD measurements (CERN Courier September/October 2024 p29). ATLAS presented the most precise measurement to date (0.8%) of the strong coupling constant extracted from the measurement of the transverse momentum differential cross section of Drell–Yan Z-boson production. LHCb provided a comprehensive analysis of the B⁰ → K^0* μ⁺μ^– angular distributions, which had previously presented discrepancies at the level of 3σ. Taking into account long-distance contributions significantly weakens the tension down to 2.1σ.

Pioneering the highest luminosities ever reached at colliders (setting a record at 4.7 × 10³⁴ cm^–2 s^–1), SuperKEKB has been facing challenging conditions with repeated sudden beam losses. This is currently an obstacle to further progress to higher luminosities. Possible causes have been identified and are currently under investigation. Meanwhile, with the already substantial data set collected so far, the Belle II experiment has produced a host of new results. In addition to improved CKM angle measurements (alongside LHCb), in particular of the γ angle, Belle II (alongside BaBar) presented interesting new insights in the long standing |V_cb| and |V_ub| inclusive versus exclusive measurements puzzle (CERN Courier July/August 2024 p30), with new |V_cb| exclusive measurements that significantly reduce the previous 3σ tension.

ATLAS and CMS furthered their systematic journey in the search for new phenomena to leave no stone unturned at the energy frontier, with 20 new results presented at the conference. This landmark outcome of the LHC puts further pressure on the naturalness paradigm.

A highlight of the conference was the overall progress in neutrino physics. Accelerator-based experiments NOvA and T2K presented a first combined measurement of the mass difference, neutrino mixing and CP parameters. Neutrino telescopes IceCube with DeepCore and KM3NeT with ORCA (Oscillation Research with Cosmics in the Abyss) also presented results with impressive precision. Neutrino physics is now at the dawn of a bright new era of precision with the next-generation accelerator-based long baseline experiments DUNE and Hyper Kamiokande, the upgrade of DeepCore, the completion of ORCA and the medium baseline JUNO experiment. These experiments will bring definitive conclusions on the measurement of the CP phase in the neutrino sector and the neutrino mass hierarchy – two of the outstanding goals in the field.

The KATRIN experiment presented a new upper limit on the effective electron–anti-neutrino mass of 0.45 eV, well en route towards their ultimate sensitivity of 0.2 eV. Neutrinoless double-beta-decay search experiments KamLAND-Zen and LEGEND-200 presented limits on the effective neutrino mass of approximately 100 meV; the sensitivity of the next-generation experiments LEGEND-1T, KamLAND-Zen-1T and nEXO should reach 20 meV and either fully exclude the inverted ordering hypothesis or discover this long-sought process. Progress on the reactor neutrino anomaly was reported, with recent fission data suggesting that the fluxes are overestimated, thus weakening the significance of the anti-neutrino deficits.

Neutrinos were also a highlight for direct-dark-matter experiments as Xenon announced the observation of nuclear recoil events from⁸B solar neutrino coherent elastic scattering on nuclei, thus signalling that experiments are now reaching the neutrino fog. The conference also highlighted the considerable progress across the board on the roadmap laid out by Kathryn Zurek at the conference to search for dark matter in an extraordinarily large range of possibilities, spanning 89 orders of magnitude in mass from 10^–23 eV to 10⁵⁷ GeV. The roadmap includes cosmological and astrophysical observations, broad searches at the energy and intensity frontier, direct searches at low masses to cover relic abundance motivated scenarios, building a suite of axion searches, and pursuing indirect-detection experiments.

Neutrinos also made the headlines in multi-messenger astrophysics experiments with the announcement by the KM3Net ARCA (Astroparticle Research with Cosmics in the Abyss) collaboration of a muon-neutrino event that could be the most energetic ever found. The energy of the muon from the interaction of the neutrino is compatible with having an energy of approximately 100 PeV, thus opening a fascinating window on astrophysical processes at energies well beyond the reach of colliders. The conference showed that we are now well within the era of multi-messenger astrophysics, via beautiful neutrinos, gamma rays and gravitational-wave results.

The conference saw new bridges across fields being built. The birth of collider-neutrino physics with the beautiful results from FASERν and SND fill the missing gap in neutrino–nucleon cross sections between accelerator neutrinos and neutrino astronomy. ALICE and LHCb presented new results on He³ production that complement the AMS results. Astrophysical He³ could signal the annihilation of dark matter. ALICE also presented a broad, comprehensive review of the progress in understanding strongly interacting matter at extreme energy densities.

The highlight in the field of observational cosmology was the recent data from DESI, the Dark Energy Spectroscopic Instrument in operation since 2021, which bring splendid new data on baryon acoustic oscillation measurements. These precious new data agree with previous indirect measurements of the Hubble constant, keeping the tension with direct measurements in excess of 2.5σ. In combination with CMB measurements, the DESI measurements also set an upper limit on the sum of neutrino masses at 0.072 eV, in tension with the inverted ordering of neutrino masses hypothesis. This limit is dependent on the cosmological model.

In everyone’s mind at the conference, and indeed across the domain of high-energy physics, it is clear that the field is at a defining moment in its history: we will soon have to decide what new flagship project to build. To this end, the conference organised a thrilling panel discussion featuring the directors of all the major laboratories in the world. “We need to continue to be bold and ambitious and dream big,” said Fermilab’s Lia Merminga, summarising the spirit of the discussion.

“As we have seen at this conference, the field is extremely vibrant and exciting,” said CERN’s Fabiola Gianotti at the conclusion of the panel. In these defining times for the future of our field, ICHEP 2024 was an important success. The progress in all areas is remarkable and manifest through the outstanding number of beautiful new results shown at the conference.

The post A rich harvest of results in Prague appeared first on CERN Courier.

In a series of daring balloon flights in 1912, Victor Hess discovered radiation that intensified with altitude, implying extra-terrestrial origins. A century later, experiments with cosmic rays have reached low-Earth orbit, but physicists are still puzzled. Cosmic-ray spectra are difficult to explain using conventional models of galactic acceleration and propagation. Hypotheses for their sources range from supernova remnants, active galactic nuclei and pulsars to physics beyond the Standard Model. The study of cosmic rays in the 1940s and 1950s gave rise to particle physics as we know it. Could these cosmic messengers be about to unlock new secrets, potentially clarifying the nature of dark matter?

The cosmic-ray spectrum extends well into the EeV regime, far beyond what can be reached by particle colliders. For many decades, the spectrum was assumed to be broken into intervals, each following a power law, as Enrico Fermi had historically predicted. The junctures between intervals include: a steepening decline at about 3 × 10⁶ GeV known as the knee; a flattening at about 4 × 10⁹GeV known as the ankle; and a further steepening at the supposed end of the spectrum somewhere above 10¹⁰ GeV (10 EeV).

While the cosmic-ray population at EeV energies may include contributions from extra-galactic cosmic rays, and the end of the spectrum may be determined by collisions with relic cosmic-microwave-background photons – the Greisen–Zatsepin–Kuzmin cutoff – the knee is still controversial as the relative abundance of protons and other nuclei is largely unknown. What’s more, recent direct measurements by space-borne instruments have discovered “spectral curvatures” below the knee. These significant deviations from a pure power law range from a few hundred GeV to a few tens of TeV. Intriguing anomalies in the spectra of cosmic-ray electrons and positrons have also been observed below the knee.

Electron origins

The Calorimetric Electron Telescope (CALET; see “Calorimetric telescope” figure) on board the International Space Station (ISS) provides the highest-energy direct measurements of the spectrum of cosmic-ray electrons and positrons. Its goal is to observe discrete sources of high-energy particle acceleration in the local region of our galaxy. Led by the Japan Aerospace Exploration Agency, with the participation of the Italian Space Agency and NASA, CALET was launched from the Tanegashima Space Center in August 2015, becoming the second high-energy experiment operating on the ISS following the deployment of AMS-02 in 2011. During 2017 a third experiment, ISS-CREAM, joined AMS-02 and CALET, but its observation time ended prematurely.

As a result of radiative losses in space, high-energy cosmic-ray electrons are expected to originate just a few thousand light-years away, relatively close to Earth. CALET’s homogeneous calorimeter (fully active, with no absorbers) is optimised to reconstruct such particles (see “Energetic electron” figure). With the exception of the highest energies, anisotropies in their arrival direction are typically small due to deflections by turbulent interstellar magnetic fields.

Energy spectra also contain crucial information as to where and how cosmic-ray electrons are accelerated. And they could provide possible signatures of dark matter. For example, the presence of a peak in the spectrum could be a sign of dark-matter decay, or dark-matter annihilation into an electron–positron pair, with a detected electron or positron in the final state.

Direct measurements of the energy spectra of charged cosmic rays have recently achieved unprecedented precision thanks to long-term observations of electrons and positrons of cosmic origin, as well as of individual elements from hydrogen to nickel, and even beyond. Space-borne instruments such as CALET directly identify cosmic nuclei by measuring their electric charge. Ground-based experiments must do so indirectly by observing the showers they generate in the atmosphere, incurring large systematic uncertainties. Either way, hadronic cosmic rays can be assumed to be fully stripped of atomic electrons in their high-temperature regions of origin.

A rich phenomenology

The past decade has seen the discovery of unexpected features in the differential energy spectra of both leptonic and hadronic cosmic rays. The observation by PAMELA and AMS of an excess of positrons above 10 GeV has generated widespread interest and still calls for an unambiguous explanation (CERN Courier December 2016 p26). Possibilities include pair production in pulsars, in addition to the well known interactions with the interstellar gas, and the annihilation of dark matter into electron–positron pairs.

Regarding cosmic-ray nuclei, significant deviations of the fluxes from pure power-law spectra have been observed by several instruments in flight, including by CREAM on balloon launches from Antarctica, by PAMELA and DAMPE aboard satellites in low-Earth orbit, and by AMS-02 and CALET on the ISS. Direct measurements have also shown that the energy spectra of “primary” cosmic rays is different from those of “secondary” cosmic rays created by collisions of primaries with the interstellar medium. This rich phenomenology, which encodes information on cosmic-ray acceleration processes and the history of their propagation in the galaxy, is the subject of multiple theoretical models.

An unexpected discovery by PAMELA, which had been anticipated by CREAM and was later measured with greater precision by AMS-02, DAMPE and CALET, was the observation of a flattening of the differential energy spectra of protons and helium. Starting from energies of a few hundred GeV, the proton flux shows a smooth and progressive hardening (increase in gradient) of the spectrum that continues up to around 10 TeV, above which a completely different regime is established. A turning point was the subsequent discovery by CALET and DAMPE of an unexpected softening of proton and helium fluxes above about 10 TeV/Z, where the atomic number Z is one for protons and two for helium. The presence of a second break challenges the conventional “standard model” of cosmic-ray spectra and calls for a further extension of the observed energy range, currently limited to a few hundred TeV.

At present, only two experiments in low-Earth orbit have an energy reach beyond 100 TeV: CALET and DAMPE. They rely on a purely calorimetric measurement of the energy, while space-borne magnetic spectrometers are limited to a maximum magnetic “rigidity” – a particle’s momentum divided by its charge – of a few teravolts. Since the end of PAMELA’s operations in 2016, AMS-02 is now the only instrument in orbit with the ability to discriminate the sign of the charge. This allows separate measurements of the high-energy spectra of positrons and antiprotons – an important input to the observation of final states containing antiparticles for dark-matter searches. AMS-02 is also now preparing for an upgrade: an additional silicon tracker layer will be deployed at the top of the instrument to enable a significant increase in its acceptance and energy reach (CERN Courier March/April 2024 p7).

Pioneering observations

CALET was designed to extend the energy reach beyond the rigidity limit of present space-borne spectrometers, enabling measurements of electrons up to 20 TeV and measurements of hadrons up to 1 PeV. As an all-calorimetric instrument with no magnetic field, its main science goal is to perform precision measurements of the detailed shape of the inclusive spectra of electrons and positrons.

Thanks to its advanced imaging calorimeter, CALET can measure the kinetic energy of incident particles well into TeV energies, maintaining excellent proton–electron discrimination throughout. CALET’s homogeneous calorimeter has a total thickness of 30 radiation lengths, allowing for a full containment of electron showers. It is preceded by a high-granularity pre-shower detector with imaging capabilities that provide a redundant measurement of charge via multiple energy-loss measurements. The calibration of the two instruments is the key to controlling the energy scale, motivating beam tests at CERN before launch.

A first important deviation from a scale-invariant power-law spectrum was found for electrons near 1 TeV. Here, CALET and DAMPE observed a significant flux reduction, as expected from the large radiative losses of electrons during their travel in space. CALET has now published a high-statistics update up to 7.5 TeV, reporting the presence of candidate electrons above the 1 TeV spectral break (see “Electron break” figure).

This unexplored region may hold some surprises. For example, the detection of even higher energy electrons, such as the 12 TeV candidate recently found by CALET, may indicate the contribution of young and nearby sources such as the Vela supernova remnant, which is known to host a pulsar (see “Pulsar home” image).

CALET was designed to extend the energy reach beyond the rigidity limit of present space-borne spectrometers

A second unexpected finding is the observation of a significant reduction in the proton flux around 10 TeV. This bump and dip were also observed by DAMPE and anticipated by CREAM, albeit with low statistics (see “Proton bump” figure). A precise measurement of the flux has allowed CALET to fit the spectrum with a double-broken power law: after a spectral hardening starting at a few hundred GeV, which is also observed by AMS-02 and PAMELA, and which progressively increases above 500 GeV, a steep softening takes place above 10 TeV.

A similar bump and dip have been observed in the helium flux. These spectral features may result from a single physical process that generates a bump in the cosmic-ray spectrum. Theoretical models include an anomalous diffusive regime near the acceleration sources, the dominance of one or more nearby supernova remnants, the gradual release of cosmic rays from the source, and the presence of additional sources.

CALET is also a powerful hunter of heavier cosmic rays. Measurements of the spectra of boron, carbon and oxygen ions have been extended in energy reach and precision, providing evidence of a progressive spectral hardening for most of the primary elements above a few hundred GeV per nucleon. The boron-to-carbon flux ratio is an important input for understanding cosmic-ray propagation. This is because diffusion through the interstellar medium causes an additional softening of the flux of secondary cosmic rays such as boron with respect to primary cosmic rays such as carbon (see “Break in B/C?” figure). The collaboration also recently published the first high-resolution flux measurement of nickel (Z = 28), revealing the element to have a very similar spectrum to iron, suggesting similar acceleration and propagation behaviour.

CALET is also studying the spectra of sub-iron elements, which are poorly known above 10 GeV per nucleon, and ultra-heavy galactic cosmic rays such as zinc (Z = 30), which are quite rare. CALET studies abundances up to Z = 40 using a special trigger with a large acceptance, so far revealing an excellent match with previous measurements from ACE-CRIS (a satellite-based detector), SuperTIGER (a balloon-borne detector) and HEAO-3 (a satellite-based detector decommissioned in the 1980s). Ultra-heavy galactic cosmic rays provide insights into cosmic-ray production and acceleration in some of the most energetic processes in our galaxy, such as supernovae and binary-neutron-star mergers.

Gravitational-wave counterparts

In addition to charged particles, CALET can detect gamma rays with energies between 1 GeV and 10 TeV, and study the diffuse photon background as well as individual sources. To study electromagnetic transients related to complex phenomena such as gamma-ray bursts and neutron-star mergers, CALET is equipped with a dedicated monitor that to date has detected more than 300 gamma-ray bursts, 10% of which are short bursts in the energy range 7 keV to 20 MeV. The search for electromagnetic counterparts to gravitational waves proceeds around the clock by following alerts from LIGO, VIRGO and KAGRA. No X-ray or gamma-ray counterparts to gravitational waves have been detected so far.

On the low-energy side of cosmic-ray spectra, CALET has contributed a thorough study of the effect of solar activity on galactic cosmic rays, revealing charge dependence on the polarity of the Sun’s magnetic field due to the different paths taken by electrons and protons in the heliosphere. The instrument’s large-area charge detector has also proven to be ideal for space-weather studies of relativistic electron precipitation from the Van Allen belts in Earth’s magnetosphere.

The spectacular recent experimental advances in cosmic-ray research, and the powerful theoretical efforts that they are driving, are moving us closer to a solution to the century-old puzzle of cosmic rays. With more than four billion cosmic rays observed so far, and a planned extension of the mission to the nominal end of ISS operativity in 2030, CALET is expected to continue its campaign of direct measurements in space, contributing sharper and perhaps unexpected pictures of their complex phenomenology.

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The first part of the workshop was devoted to dark-matter searches and dielectric laser acceleration (DLA). For dark-matter searches, multiple experiments are proposed across different classes (muons vs electrons and positrons, appearance vs disappearance experiments, etc), and an adequate background rejection is important. Promising advanced accelerator technologies are DLA for single electrons – perhaps also muons – and plasma-wakefield accelerators for muons and pions.

Some dark matter-experiments look for an appearance that requires a high flux of incoming particles. For electrons, the standard is set by BDX at JLab, for protons by the proposed SHiP experiment at CERN, and for photons by the proposed Gamma Factory at CERN. In addition, appearances could be seen at existing collider experiments such as the LHC. Other dark-matter experiments search for disappearance. They rely on DC-like electron beams, with prominent examples being LDMX at SLAC and the newly proposed DLA–DMX at PSI. A DC-like muon beam could be explored by the M3 experiment at Fermilab.

Paolo Crivelli (ETH Zürich) described the NA64 experiment as one of the most prominent examples of ongoing accelerator-based dark-matter searches, and presented the first results using a high-energy muon beam. The proposed LDMX experiment at SLAC, presented by Silke Möbius (University of Bern), may set a new standard for indirect dark-matter searches, while advanced concepts employing dielectric laser acceleration, in particular when integrating the accelerating structure with laser oscillator, could achieve many orders of magnitude higher rates of single high-energy electrons entering into an LDMX-type detector.

Uwe Niedermayer (TU Darmstadt), Stefanie Kraus (University Erlangen-Nürnberg) and Raziyeh Dadashi (PSI/EPFL) reviewed the state of the art in DLA plus future plans. Yves Bellouard (EPFL) discussed advances in high-repetition-rate lasers and micro/nano-structures, which suggests that the proposed combined laser-accelerator structures are within reach. Of course, the detector time resolution would also need to be improved tremendously to keep pace with the higher rate of the accelerator.

Acceleration and decay

The second part of the workshop was devoted to the plasma acceleration of non-ultra relativistic and rapidly decaying particles, such as muons and pions. Vladimir Shiltsev (Fermilab) and Daniel Schulte (CERN) presented tentative parameters and ongoing R&D efforts towards a muon collider. Shiltsev also discussed the intriguing possibility of low-emittance muon sources based on plasma-wakefield accelerators, while Alexander Pukhov (Heinrich Heine University Düsseldorf) and Chiara Badiali (IST Lisbon) discussed how plasma acceleration could bring slow particles, such as muons, to relativistic velocities.

The workshop fostered numerous heated discussions and uncovered unresolved issues, which included the “Bern controversy” regarding the ultimate limits of luminosity for PeV energies. Muons are considered particles of choice for future accelerators at the energy frontier. Both low- and high-energy muons have useful applications. Is there an Angstrom limit to the beam diameter? Are tiny beta functions possible? Can plasmas help to overcome such limitations? Understanding and modelling non-point-like particle luminosity is another important topic, also relevant for the Gamma Factory.

The workshop showed how advanced accelerator concepts can jump-start dark-sector searches

The final part of the workshop assembled a roadmap and perspective. DLA studies are to be maintained and, if possible, accelerated. A reasonable target is achieving a gradient of 500 MeV/m and an energy gain of 0.05 GeV in five years on a single wafer, while an integrated DLA laser oscillator could be foreseen five to seven years from now. Plasma-wakefield acceleration of muons could conceivably be tested either at CERN–AWAKE or PSI. It was proposed, as a first step, to put a solid target or tape into the AWAKE set up.

The gamma factory, presented by Witek Krasny (LPNHE), was recognised as an intense source of polarised muons and positrons. For muon-acceleration studies, the dephasing issue, linked to the muons’ non-ultrarelativistic energy, seems to be resolved. A demonstrator experiment for muon plasma acceleration is called for. Open questions include when and where?

Overall, the Bern workshop showed how advanced accelerator concepts can jump start dark-sector searches and muon/pion acceleration. High-repetition-rate acceleration of single electrons for dark-matter searches, using dielectric laser accelerators, and applying high-gradient plasma acceleration to muon and/or pion beams, are intriguing and far-forward looking topics.

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The base and frame of the ELT’s dome structure on Cerro Armazones in the Chilean Atacama Desert have now been set. Meanwhile at European sites, the five-system mirrors for the ELT are being manufactured. More than 70% of the supports and blanks for the main mirror – which at 39 m across will be the biggest primary mirror ever built – are complete, and mirrors two and three are cast and now in the process of being polished.

Along with six laser guiding sources that will act as reference stars, mirrors four and five form part of a sophisticated adaptive-optics system to correct for atmospheric disturbances. The ELT will observe the universe in the near-infrared and visible regions to track down Earth-like exoplanets, investigate faint objects in the solar system and study the first stars and galaxies. It will also explore black holes, the dark universe and test fundamental constants (CERN Courier November/December 2019 p25).

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The worldwide interest in axions and other weakly interacting slim particles (WISPs) as constituents of a dark sector of nature has strongly increased over the last years. A vibrant community is developing, constructing and operating corresponding experiments, so that most promising parameter regions will be probed within the next 15 years.

Many of these approaches rely on WISPs converting to photons. At DESY in Hamburg, larger-scale projects are pursued: the “light-shining-through-a-wall” experiment, ALPS II in the HERA tunnel, will start data taking soon. The solar helioscope BabyIAXO is nearly ready to start construction, while the dark matter haloscope MADMAX is in the prototyping phase.

This webinar will introduce the physics cases and focus on the axion search activities ongoing at DESY.

Axel Lindner was working in accelerator-based particle physics, astroparticle physics and management before he engaged in WISP searches in 2007 as the spokesperson of the ALPS I experiment. Since 2018 he has been leading a new experimental group at DESY in Hamburg in charge of realizing non-accelerator-based particle physics experiments on-site. Axel has been a member of the MADMAX and IAXO collaborations and spokesperson of ALPS II since 2012.

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The international conference series on the identification of dark matter (IDM) was brought to life in 1996 with the motto that “it is of critical importance now not just to pursue further evidence for its existence but rather to identify what the dark matter is.” Despite earnest attempts to identify what dark matter comprises, the answer to this question remains elusive. Today, the evidence for dark matter is overwhelming; its amount is known to be around 27% of the universe’s energy-density budget. IDM2022 illuminated the dark-matter mystery from all angles, ranging from cosmological evidence via astrophysics to possible dark-matter particle candidates and their detection via indirect searches, direct searches and colliders.

The 14th edition of IDM took place in Vienna, Austria, from 18 to 22 July, attracting about 250 physicists and more than 200 contributions. The conference was initially scheduled for 2020 but changed to an online format due to the pandemic, while the in-person IDM was delayed until 2022. Many young scientists were able to meet the dark-matter community for the first time “in real life”. The Strings 2022 conference took place in Vienna simultaneously, with complementary presentations.

One focus of IDM2022 was the direct detection of dark matter. Tremendous progress in the sensitivity of direct detection experiments has been achieved in the past few decades over a wide dark-matter particle mass range. All major experiments presented their latest results. While in the past, direct searches focused on the classical WIMP region in a mass between a few GeV and several TeV, the search region is now enlarged towards even lighter dark-matter particles down to the keV region. Different mass regions require different technologies and new ideas were presented to increase the sensitivities towards these unexplored mass regions. For GeV WIMP dark-matter searches, the XENON collaboration displayed the first results from their latest setup, XENONnT, which has a significantly lower background level and recently eliminated a previously seen excess in XENON1T. The XENON, Darwin and LZ collaborations recently formed the XLZD collaboration with the aim of building a next-generation liquid-xenon experiment.

While the XENON1T excess is gone, direct-detection experiments exploring the sub-GeV mass regime still face unknown background contributions, especially in solid-state detectors. This is currently one of the biggest obstacles to increasing the sensitivity to even smaller cross-sections. No complete understanding has been achieved so far, but combining the results, knowledge and expertise of the experiments points to stress relaxations in crystals as one primary underlying source. To tackle this tricky problem, a subset of the IDM2022 participants held a dedicated satellite meeting. This EXCESS workshop was the third event of its kind, and the first to take place in person.  

The direct detection experiment DAMA has observed a statistically significant signal of an annual modulated event rate for several years. This observation is consistent with Earth moving through the dark-matter halo, but has not been confirmed by any other experiment. DAMA recently reduced the energy threshold to 0.5 keV electron equivalent by upgrading their readout electronics to further increase sensitivity. Several new dark-matter experiments based on the same target material – NaI – are running or being commissioned to provide more information on the long-standing DAMA observation: ANAIS, COSINE, COSINUS and SABRE. Even lighter forms of dark matter, such as axions and axion-like particles, were discussed, as well as the possibility that dark matter comprises bound states.

Primordial black holes are also attractive potential dark-matter candidates. Astronomical data from, for example,  microlensing, structure formation and gravitational waves hint at their existence. However, current data gives no handle on whether primordial black holes could be responsible for all the universe’s dark-matter content, or only correspond to part of the overall dark-matter density. Besides black-hole mergers, gravitational-wave signals can provide additional information to understand the origin of dark matter. In particular, processes in the early universe detected via gravitational waves could provide new insights into the particle nature of dark matter. With the increased sensitivity of operating and future gravitational-wave detectors, new players will provide additional data to unravel the dark-matter problem.

With a plethora of new ideas and experiments presented at this year’s IDM, the path is prepared for the next edition in L’Aquila, Italy, in 2024.

The post Identifying dark matter appeared first on CERN Courier.

The CMB comprises the photons created in the Big Bang. These photons have therefore experienced the entire history of the universe. Everything that has happened has left an imprint on them in the form of anisotropies in their temperature and polarisation with characteristic amplitudes and angular scales. The early universe was hot enough to be completely ionised, which meant that the CMB photons constantly scattered off free electrons. During this period the primary CMB anisotropies were imprinted, tracing the overall geometry of the universe, the fraction of the energy density in baryons, the number of light-relic particles and the nature of inflation. After about 375,000 years of expansion the universe cooled enough for neutral hydrogen atoms to be stable. With the free electrons rapidly swept up by protons, the CMB photons simply free-streamed in whatever direction they were last moving in. When we observe the CMB today we therefore see a snapshot of this so-called last-scattering surface.

The continued evolution of the universe had two main effects on the CMB photons. First, its ongoing expansion stretched their wavelengths to peak at microwave frequencies today. Second, the growth of structure eventually formed galaxy clusters that changed the direction, energy and polarisation of the CMB photons that pass through them, both from gravitational lensing by their mass and from inverse Compton scattering by the hot gas that makes up the inter-cluster medium. These secondary anisotropies therefore constrain all of the parameters that this history depends on, from the moment the first stars formed to the number of light-relic particles and the masses of neutrinos.  

As noted by the Astro2020 report, the history of CMB research is that of continuously improving ground and balloon experiments, punctuated by comprehensive measurements from the major satellite missions COBE, WMAP and Planck. The increasing temperature and polarisation sensitivity and angular resolution of these satellites is evidenced in the depth and resolution of the maps they produced (see “Relic radiation” image”). However, such maps are just our view of the CMB – one particular realisation of a random process. To derive the underlying cosmology that gave rise to them, we need to measure the amplitude of the anisotropies on various angular scales (see “Power spectra” figure). Following the serendipitous discovery of the CMB in 1965, the first measurements of the temperature anisotropy were made by COBE in 1992. The first peak in the temperature power spectrum was measured by the BOOMERanG and MAXIMA balloons in 2000, followed by the E-mode polarisation of the CMB by the DASI experiment in 2002, and the B-mode polarisation by the South Pole Telescope and POLARBEAR experiments in 2015.

CMB-S4, a joint effort supported by the US Department of Energy (DOE) and the National Science Foundation (NSF), will help write the next chapter in this fascinating adventure. Planned to comprise 21 telescopes at the South Pole and in the Chilean Atacama Desert instrumented with more than 500,000 cryogenically-cooled superconducting detectors, it will exceed the capabilities of earlier generations of experiments by more than an order of magnitude and deliver transformative discoveries in fundamental physics, cosmology, astrophysics and astronomy.

The CMB-S4 challenge 

Three major challenges must be addressed to study the CMB at such levels of precision. Firstly, the signals are extraordinarily faint, requiring massive datasets to reduce the statistical uncertainties. Secondly, we have to contend with systematic effects both from imperfect instruments and from the environment, which must be controlled to exquisite precision if they are not to swamp the signals. Finally, the signals are obscured by other sources of microwave emission, especially galactic synchrotron and dust emission. Unlike the CMB, these sources do not have a black-body spectrum, so it is possible to distinguish between CMB and non-CMB sources if observations are made at enough microwave frequencies to break the degeneracy.

This third challenge actually proves to be an astrophysical blessing as well as a cosmological curse: CMB observations are also excellent legacy surveys of the millimetre-wave sky, which can be used for a host of other science goals. These range from cataloguing galaxy clusters, to studying the Milky Way, to detecting spatial and temporal transients such as gamma-ray bursts via their afterglows.

Coming together

In 2013 the US CMB community came together in the Snowmass planning process, which informs the deliberations of the decadal Particle Physics Project Prioritization Panel (P5). We realised that achieving the sensitivity needed to make the next leap in CMB science would require an experiment of such magnitude (and therefore cost) that it could only be accomplished as a community-wide endeavour, and that we would therefore need to transition from multiple competing experiments to a single collaborative one. By analogy with the US dark-energy programme, this was designated a “Stage 4” experiment, and hence became known as CMB-S4. 

In 2014 a P5 report made the critical recommendation that the DOE should support CMB science as a core piece of its programme. The following year a National Academies report identified CMB science as one of three strategic priorities for the NSF Office of Polar Programs. In 2017 the DOE, NSF and NASA established a task force to develop a conceptual design for CMB-S4, and in 2019 the DOE took “Critical Decision 0”, identifying the mission need and initiating the CMB-S4 construction project. In 2020 Berkeley Lab was appointed the lead laboratory for the project, with Argonne, Fermilab and SLAC all playing key roles. Finally, late last year, the long-awaited Astro2020 report unconditionally recommended CMB-S4 as a joint NSF and DOE project with an estimated cost of $650 million. With these recommendations in place, the CMB-S4 construction project could begin.

From the outset, CMB-S4 was intended to be the first sub-orbital CMB experiment designed to reach specific critical scientific thresholds, rather than simply to maximise the science return under a particular cost cap. Furthermore, as a community-wide collaboration, CMB-S4 will be able to adopt and adapt the best of all previous experiments’ technologies and methodologies – including operating at the site best suited to each science goal. One third of the major questions and discovery areas identified across the six Astro2020 science panels depend on CMB observations.

The critical degrees of freedom in the design of any observation are the sky area, frequency coverage, frequency-dependent depth and angular resolution, and observing cadence. Having reviewed the requirements across the gamut of CMB science, four driving science goals have been identified for CMB-S4. 

For the first time, the entire community is coming together to build an experiment defined by achieving critical science thresholds

The first is to test models of inflation via the primordial gravitational waves they naturally generate. Such gravitational waves are the only known source of a primordial B-mode polarisation signal. The size of these primordial B-modes is quantified by the ratio of their power to that of the temperature power spectrum – the scalar-to-tensor ratio, designated r. For the largest and most popular classes of inflationary models, CMB-S4 will make a 5σ detection of r, while failure to make such a measurement will put an upper limit of r ≤ 0.001 at 95% confidence, setting a rigorous constraint on alternative models (see “Constraining inflation” figure). The large-scale B-mode polarisation signal encoding r is the faintest of all the CMB signals, requiring both the deepest measurement and the widest low-resolution frequency coverage of any CMB-S4 science case.

The second goal concerns the dark universe. Dark matter and dark energy make up 95% of the universe’s mass-energy content, and their particular form and composition impact the growth of structure and thus the small-scale CMB anisotropies. The collective influence of the three known light-relic particles (the Standard Model neutrinos) has already been observed in CMB data, but many new light species, such as axion-like particles and sterile neutrinos, are predicted by extensions of the Standard Model. CMB-S4’s goal, and the most challenging measurement in this arena, is to detect any additional light-relic species with freeze-out temperatures up to the QCD phase-transition scale. This corresponds to constraining the uncertainty on the number of light-relic species N_eff to ≤ 0.06 at 95% confidence (see “Light relics” figure). Precise measurements of the small-scale temperature and E-mode polarisation signals that encode this signal require the largest sky area of any CMB-S4 science case. In addition, since the sum of the masses of the neutrinos impacts the degree of lensing of the E-mode polarisation into small-scale B-modes, CMB-S4 will be able to constrain this sum around a fiducial value of 58 meV with a 1σ uncertainty ≤ 24 meV (in conjunction with baryon acoustic oscillation measurements) and ≤ 14 meV with better measurements of the optical depth to reionisation. 

The third science goal is to understand the formation and evolution of galaxy clusters, and in particular to probe the early period of galaxy formation at redshifts z > 2. This is enabled by the Sunyaev–Zel’dovitch (SZ) effect, whereby CMB photons are up-scattered by the hot, moving gas in the intra-cluster medium. This shifts the CMB photons’ frequency spectrum, resulting in a decrement at frequencies below 217 GHz and an increment at frequencies above, therefore allowing clusters to be identified by matching up the corresponding cold and hot spots. A key feature of the SZ effect is its red-shift independence, allowing us to generate complete, flux-limited catalogues of clusters to the survey sensitivity. The small-scale temperature signals needed for such a catalogue require the highest angular resolution and the widest high-resolution frequency coverage of all the CMB-S4 science cases.

Finally, CMB-S4 aims to explore the mm-wave transient sky, in particular the rate of gamma-ray bursts to help constrain their mechanisms (a few hours to days after the initial event, gamma-ray bursts are observable at longer wavelengths). CMB-S4 will be so sensitive that even its daily maps will be deep enough to detect mm-wave transient phenomena – either spatial from nearby objects moving across our field, or temporal from distant objects exploding in our field. This is the only science goal that places constraints on the survey cadence, specifically on the lag between repeated observations of the same point on the sky. Given its large field of view, CMB-S4 will be an excellent tool for serendipitous discovery of transients but less useful for follow-up observations. The plan is therefore to issue daily alerts for other teams to follow up with targeted observations.

Survey design

While it would be possible to meet all of the CMB-S4 science goals with a single survey, the result – requiring the sensitivity of the inflation survey across the area of the light-relic survey – would be prohibitively expensive. Instead, the requirements have been decoupled into an ultra-deep, small-area survey to meet the inflation goal and a deep, wide-area survey to meet the light-relic goal, the union of these providing a two-tier “wedding cake” survey for the cluster and gamma-ray-burst goals.

Having set the survey requirements, the task was to identify sites at which these observations can most efficiently be made, taking into account the associated cost, schedule and risk. Water vapour is a significant source of noise at microwave frequencies, so the first requirement on any site is that it be high and dry. A handful of locations meet this requirement, and two of them – the South Pole and the high Chilean Atacama Desert – have both exceptional atmospheric conditions and long-standing US CMB programmes. Their positions on Earth also make them ideally suited to CMB-S4’s two-survey strategy: the polar location enables us to observe a small patch of sky continuously, minimising the time needed to reach the required observation depth, and the more equatorial Chilean location enables observations over a large sky area.

Finally, we know that instrumental systematics will be the limiting factor in resolving the extraordinarily faint large-scale B-mode signal. To date, the experiments that have shown the best control of such systematics have used relatively small-aperture (~0.5 m) telescopes. However, the secondary lensing of the much brighter E-mode signal to B-modes, while enabling us to measure the neutrino-mass sum, also obscures the primordial B-mode signal coming from inflation. We therefore need a detailed measurement of this medium- to small-scale lensing signal in order to be able to remove it at the necessary precision. This requires larger, higher-resolution telescopes. The ultra-deep field is therefore itself composed of coincident low- and high-resolution surveys.

A key feature of CMB-S4 is that all of the technologies are already well-proven by the ongoing Stage 3 experiments. These include CMB-S4’s “founding four” experiments, the Atacama Cosmology Telescope (ACT) and POLARBEAR/Simons Array (PB/SA) in Chile, and BICEP/Keck (BK) and the South Pole Telescope (SPT) at the South Pole, which have pairwise-merged into the Simons and South Pole Observatories (SO and SPO). The ACT, PB/SA, BK and SPT are all single-aperture, single-site experiments, while SO and SPO are dual-aperture, single sites. CMB-S4 is therefore the first experiment able to take advantage of both apertures and both sites. 

The key difference with CMB-S4 is that it will deploy these technologies on an unprecedented scale. As a result, the primary challenges for CMB-S4 are engineering ones, both in fabricating detector and readout modules in huge numbers and in deploying them in cryostats on telescopes with unprecedented systematics control. The observatory will comprise: 18 small-aperture refractors collectively fielding about 150,000 detectors across eight frequencies for measuring large angular scales; one large-aperture reflector with about 130,000 detectors across seven frequencies for measuring medium-to-small angular scales in the ultra-deep survey from the South Pole; and two large-aperture reflectors collectively fielding about 275,000 detectors across six frequencies for measuring medium-to-small angular scales in the wide-deep survey from Chile (see “Looking up” image). The final configuration maximises the use of available atmospheric windows to control for microwave foregrounds (particularly synchrotron and dust emission at low and high frequencies, respectively), and to meet the frequency-dependent depth and angular-resolution requirements of the surveys. 

CMB-S4 will be able to adopt and adapt the best of all previous experiments’ technologies and methodologies

Covering the frequency range 20–280 GHz, the detectors employ dichroic pixels at all but one frequency (to maximise the use of the available focal plane) using superconducting transition-edge sensors, which have become the standard in the field. A major effort is already underway to scale up the production and reduce the fabrication variance of the detectors, taking advantage of the DOE national laboratories and industrial partners. Reading out such large numbers of detectors with limited power is a significant challenge, leading CMB-S4 to adopt the conservative but well-proven time-domain multiplexing approach. The detector and readout systems will be assembled into modules that will be cryogenically cooled to 100 mK to reduce instrument noise. Each large-aperture telescope will carry an 85-tube cryostat with a single wafer per optics tube; and each small-aperture telescope will carry a single optics tube with 12 wafers per tube, with three telescopes sharing a common mount. 

Prototyping of detector and readout fabrication lines, and building up module assembly and testing capabilities, is expected to begin in earnest this year. At the same time, the telescope designs will be refined and the data acquisition and management subsystems developed. The current schedule sees a staggered commissioning of the telescopes in 2028–2030, and operations running for seven years thereafter.

Shifting paradigms

CMB-S4 represents a paradigm shift for sub-orbital CMB experiments. For the first time, the entire community is coming together to build an experiment defined by achieving critical science thresholds in fundamental physics, cosmology, astrophysics and astronomy, rather than by its cost cap. CMB-S4 will span the entire range of CMB science in a single experiment, take advantage of the best of all worlds in the design of its observation and instrumentation, and make the results available to the entire CMB community. As an extremely sensitive, two-tiered, multi-wavelength, mm-wave survey, it will also play a key role in multi-messenger astrophysics and transient science. Taken together, these measurements will constitute a giant leap in our study of the history of the universe.

The post Exploring the CMB like never before appeared first on CERN Courier.

After 25 years of development, the James Webb Space Telescope (JWST) successfully launched from Europe’s spaceport in French Guiana on the morning of 25 December. Nerves were on edge as the Ariane 5 rocket blasted its $10 billion cargo through the atmosphere, aided by a velocity kick from its equatorial launch site. An equally nail-biting moment came 27 minutes later, when the telescope separated from the launch vehicle and deployed its solar array. In scenes reminiscent of those at CERN on 10 September 2008 when the first protons made their way around the LHC, the JWST command centre erupted in applause. “Go Webb, go!” cheered the ground team as the craft drifted into the darkness.

The result of an international partnership between NASA, ESA and the Canadian Space Agency, Webb took a similar time to design and build as the LHC and cost almost twice as much. Its science goals are also complementary to particle physics. The 6.2 tonne probe’s primary mirror – the largest ever flown in space, with a diameter of 6.5 m compared to 2.4 m for its predecessor, Hubble – will detect light, stretched to the infrared by the expansion of the universe, from the very first galaxies. In addition to shedding new light on the formation of galaxies and planets, Webb will deepen our understanding of dark matter and dark energy. “The promise of Webb is not what we know we will discover,” said NASA administrator Bill Nelson after the launch. “It’s what we don’t yet understand or can’t yet fathom about our universe. I can’t wait to see what it uncovers!”

The promise of Webb is not what we know we will discover. It’s what we don’t yet understand or can’t yet fathom about our universe

Bill Nelson

Five days after launch, Webb successfully unfurled and tensioned its 300 m² sunshield. Although the craft’s final position at Earth–Sun Lagrange point 2 (L2) ensures that it is sheltered by Earth’s shadow, further protection from sunlight is necessary to keep its four science instruments operating at 34 K. The delicate deployment procedure involved 139 release mechanisms, 70 hinge assemblies, some 400 pulleys and 90 individual cables – each of which was a potential single-point failure. Just over one week later, on 7 and 8 January, the two wings of the primary mirror, which had to be folded in for launch, were opened, involving the final four of a total of 178 release mechanisms. The ground team then began the long procedure of aligning the telescope optics via 126 actuators on the backside of the primary mirror’s 18 hexagonal segments. On 24 January, having completed a 1.51 million-km journey, the observatory successfully inserted itself into its orbit at L2, marking the end of the complex deployment process and the beginning of commissioning activities. The process will take months, with Webb scheduled to return its first science images in the summer.

The 1998 discovery of the accelerating expansion of the universe, which implies that around 70% of the universe is made up of an unknown dark energy, stemmed from observations of distant type-Ia supernovae that appeared fainter than expected. While the primary evidence came from ground-based observations, Hubble helped confirm the existence of dark energy via optical and near-infrared observations of supernovae at earlier times. Uniquely, Webb will allow cosmologists to see even farther, from as early as 200 million years after the Big Bang, while also extending the observation and cross-calibration of other standard candles, such as Cepheid variables and red giants, beyond what is currently possible with Hubble. Operating in the infrared rather than optical regime also means less scattering of light from interstellar gas.

With these capabilities, the JWST should enable the local rate of expansion to be determined to a precision of 1%. This will bring important information to the current tension between the measured expansion rate at early and late times, as quantified by the Hubble constant, and possibly shed light on the nature of dark energy.

Launching Webb is a huge celebration of the international collaboration that made this mission possible

Josef Aschbacher

By measuring the motion and gravitational lensing of early objects, Webb will also survey the distribution of dark matter, and might even hint at what it’s made of. “In order to make progress in the identification of dark matter, we need observations that clearly discriminate among the tens of possible explanations that theorists have put forward in the past four decades,” explains Gianfranco Bertone, director of the European Consortium for Astroparticle Theory. “If dark matter is ‘warm’ for example – meaning that it is composed of particles moving at mildly relativistic speeds when first structures are assembled – we should be able to detect its imprint on the number density of small dark-matter halos probed by the JWST. Or, if dark matter is made of primordial black holes, as suggested in the early 1970s by Stephen Hawking, the JWST could detect the faint emission produced by the accretion of gas onto these objects in early epochs.”

On 11 February, Webb returned images of its first star in the form of 18 blurry white dots, the product of the unaligned primary-mirror segments all reflecting light from the same star back at the secondary mirror and into its near-infrared camera. Though underwhelming at first sight, this and similar images are crucial to allow operators to gradually align and focus the hexagonal mirror segments until 18 images become one. After that, Webb will start downlinking science data at a rate of about 60 GB per day.

“Launching Webb is a huge celebration of the international collaboration that made this next-generation mission possible,” said ESA director-general Josef Aschbacher. “We are close to receiving Webb’s new view of the universe and the exciting scientific discoveries that it will make.”

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LLP9 played host to six new results, three each from ATLAS and CMS. These included a remarkable new ATLAS paper searching for stopped particles – beyond-the-Standard Model (BSM) LLPs that can be produced in a proton–proton collision and then get stuck in the detector before decaying minutes, days or weeks later. Good hypothetical examples are the so-called gluino R-hadrons that occur in supersymmetric models. Also featured was a new CMS search for displaced di-muon resonances using “data scouting” – a unique method of increasing the number of potential signal events kept at the trigger level by reducing the event information that is retained. Both experiments presented new results searching for the Higgs boson decaying to LLPs (see “LLP candidate” figure).

Long-lived particles can also be produced in a collision inside ATLAS, CMS or LHCb and live long enough to drift entirely outside of the detector volume. To ensure that this discovery avenue is also covered for the future of the LHC’s operation, there is a rich set of dedicated LLP detectors either approved or proposed, and LLP9 featured updates from MoEDAL, FASER, MATHUSLA, CODEX-b, MilliQan, FACET and SND@LHC, as well as a presentation about the proposed forward physics facility for the High-Luminosity LHC (HL-LHC).

Reinterpreting machine learning

The liveliest parts of any LLP community workshop are the brainstorming and hands-on working-group sessions. LLP9 included multiple vibrant discussions and working sessions, including on heavy neutral leptons and the ability of physicists who are not members of experimental collaborations to be able to re-interpret LLP searches – a key issue for the LLP community. At LLP9, participants examined the challenges inherent in re-interpreting LLP results that use machine learning techniques, by now a common feature of particle-physics analyses. For example, boosted decision trees (BDTs) and neural networks (NNs) can be quite powerful for either object identification or event-level discrimination in LLP searches, but it’s not entirely clear how best to give theorists access to the full original BDT or NN used internally by the experiments.

LLP searches at the LHC often must also grapple with background sources that are negligible for the majority of searches for prompt objects. These backgrounds – such as cosmic muons, beam-induced backgrounds, beam-halo effects and cavern backgrounds – are reasonably well-understood for Run 2 and Run 3, but little study has been performed for the upcoming HL-LHC, and LLP9 featured a brainstorming session about what such non-standard backgrounds might look like in the future.

Also looking to the future, two very forward-thinking working-group sessions were held on LLPs at a potential future muon collider and at the proposed Future Circular Collider (FCC). Hadron collisions at ~100 TeV in FCC-hh would open up completely unprecedented discovery potential, including for LLPs, but it’s unclear how to optimise detector designs for both LLPs and the full slate of prompt searches.

Simulating dark showers is a longstanding challenge

Finally, LLP9 hosted an in-depth working-group session dedicated to the simulation of “dark showers”, in collaboration with the organisers of the dark-showers study group connected to the Snowmass process, which is currently shaping the future of US particle physics. Dark showers are a generic and poorly understood feature of a potential BSM dark sector with similarities to QCD, which could have its own “dark hadronisation” rules. Simulating dark showers is a longstanding challenge. More than 50 participants joined for a hands-on demonstration of simulation tools and a discussion of the dark-showers Pythia module, highlighting the growing interest in this subject in the LLP community.

LLP9 was raucous and stimulating, and identified multiple new avenues of research. LLPX, the tenth workshop in the series, will be held in November this year.

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Despite the strong indirect evidence for the existence of dark matter, a plethora of direct searches have not resulted in a positive detection. The exception to this are the famous results from the DAMA/NaI experiment at Gran Sasso underground laboratory in Italy, first reported in the late 1990s, which show a modulating signal compatible with Earth moving through a region containing Weakly Interacting Massive Particles (WIMPs). These results were backed-up more recently with measurements from the follow-up DAMA/LIBRA detector. Combining the data in 2018, the evidence reported for a dark-matter signal is as high as 13 sigma.Now, the Annual modulation with NaI Scintillators (ANAIS) collaboration, which aims to directly reproduce the DAMA results using the same detector concept, has published the results from their first three years of operations. The results, which were presented today at Rencontres de Moriond, show a clear contradiction with DAMA, indicating that we are still no closer to finding dark matter.

The DAMA results are based on searches for an annual modulation in the interaction rate of WIMPs in a detector comprising NaI crystals. First theoretically proposed in 1986 by Andrzej Drukier, Katherine Freese and David Spergel, this modulation is a result of the difference in velocity of Earth with respect to the dark-matter halo of the galaxy. On 2 June, the velocities of Earth and the Sun are aligned with respect to the galaxy, whereas half a year later they are oppositely aligned, resulting in a lower cross section for WIMPs with a detector placed on Earth. Although this method has advantages compared to more direct detection methods, it requires that other potential sources of such a seasonal modulation be ruled out. Despite the significant modulation with the correct phase observed by DAMA, its results were not immediately accepted as a clear signal of dark matter due to the remaining possibility of instrumental effects, seasonal background modulation or artifacts from the analysis.

Over the years the significance of the DAMA results has continued to increase while other dark-matter searches, in particular with the XENON1T and LUX experiments, found no evidence of WIMPs capable of explaining the DAMA results. The fact that only the final analysis products from DAMA have been made public has also hampered attempts to prove or disprove alternative origins of the modulation. To overcome this, the ANAIS collaboration set out to reproduce the data with an independent detector intentionally similar to DAMA, consisting of NaI(Tl) scintillators readout by photomultipliers placed in the Canfranc Underground Laboratory deep beneath the Pyrenees in northern Spain. Using this method ANAIS can rule out any instrument-induced effects while producing data in a controlled way and studying it in detail with different analysis procedures.

The ANAIS results agree with the first results published by the COSINE-100 collaboration

The first three years of ANAIS data have now been unblinded, and the results were posted on arXiv on 1 March. None of the analysis methods used show any signs of a modulation, with a statistical analysis ruling out the DAMA results at 99% confidence. The results therefore narrow down the possible causes of the modulation observed by DAMA to either differences in the detector compared to ANAIS, or in the analysis method. One specific issue raised by the ANAIS collaboration regards the background-subtraction method. In the DAMA results the mean background rate for each year is subtracted from the raw data for that full year. In case the background during that year is not constant, however, this will produce an artificial saw-tooth shape which, with the limited statistics, can be fitted with a sinusoidal. This effect was already pointed out in a publication by a group from INFN in 2020, which showed how a slowly increasing background is capable of producing the exact modulation observed by DAMA. The ANAIS collaboration describes their background in detail, shows that it is indeed not constant, and provides suggestions for a more robust handling of the background.

The ANAIS results also agree with the first results published by the COSINE-100 collaboration in 2019 which, again using a NaI-based detector, found no evidence of a yearly modulation. Thanks to the continued experimental efforts of these two groups, and with the ANAIS collaboration planning to make their data public to allow independent analyses, the more than 20 year-old DAMA anomaly looks likely to be settled in the next few years.

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The past century has seen ever stronger links forged between the physics of elementary particles and the universe at large. But the picture is mostly incomplete. For example, numerous observations indicate that 87% of the matter of the universe is dark, suggesting the existence of a new matter constituent. Given a plethora of dark-matter candidates, numerical tools are essential to advance our understanding. Fostering cooperation in the development of such software, the TOOLS 2020 conference attracted around 200 phenomenologists and experimental physicists for a week-long online workshop in November.

The viable mass range for dark matter spans 90 orders of magnitude, while the uncertainty about its interaction cross section with ordinary matter is even larger (see “Theoretical landscape” figure). Dark matter may be new particles belonging to theories beyond-the-Standard Model (BSM), an aggregate of new or SM particles, or very heavy objects such as primordial black holes (PBHs). On the latter subject, Jérémy Auffinger (IP2I Lyon) updated TOOLS 2020 delegates on codes for very light PBHs, noting that “BlackHawk” is the first open-source code for Hawking-radiation calculations.

Flourishing models

Weakly interacting massive particles (WIMPs) have enduring popularity as dark-matter candidates, and are amenable to search strategies ranging from colliders to astrophysical observations. In the absence of any clear detection of WIMPs at the electroweak scale, the number of models has flourished. Above the TeV scale, these include general hidden-sector models, FIMPs (feebly interacting massive particles), SIMPs (strongly interacting massive particles), super-heavy and/or composite candidates and PBHs. Below the GeV scale, besides FIMPs, candidates include the QCD axion, more generic ALPs (axion-like particles) and ultra-light bosonic candidates. ALPs are a class of models that received particular attention at TOOLS 2020, and is now being sought in fixed-target experiments across the globe.

For each dark-matter model, astro­particle physicists must compute the theoretical predictions and characteristic signatures of the model and confront those predictions with the experimental bounds to select the model parameter space that is consistent with observations. To this end, the past decade has seen the development of a huge variety of software – a trend mapped and encouraged by the TOOLS conference series, initiated by Fawzi Boudjema (LAPTh Annecy) in 1999, which has brought the community together every couple of years since.

Models connecting dark matter with collider experiments are becoming ever more optimised to the needs of users

Three continuously tested codes currently dominate generic BSM dark-matter model computations. Each allows for the computation of relic density from freeze-out and predictions for direct and indirect detection, often up to next-to-leading corrections. Agreement between them is kept below the percentage level. “micrOMEGAs” is by far the most used code, and is capable of predicting observables for any generic model of WIMPs, including those with multiple dark-matter candidates. “DarkSUSY” is more oriented towards supersymmetric theories, but it can be used for generic models as the code has a very convenient modular structure. Finally, “MadDM” can compute WIMP observables for any BSM model from MeV to hundreds of TeV. As MadDM is a plugin of MadGraph, it inherits unique features such as its automatic computation of new dark-matter observables, including indirect-detection processes with an arbitrary number of final-state particles and loop-induced processes. This is essential for analysing sharp spectral features in indirect-detection gamma-ray measurements that cannot be mimicked by any known astrophysical background.

Both micrOMEGAs and MadDM permit the user to confront theories with recast experimental likelihoods for several direct and indirect detection experiments. Jan Heisig (UCLouvain) reported that this is a work in progress, with many more experimental data sets to be included shortly. Torsten Bringmann (University of Oslo) noted that a strength of DarkSUSY is the modelling of qualitatively different production mechanisms in the early universe. Alongside the standard freeze-out mechanism, several new scenarios can arise, such as freeze-in (FIMP models, as chemical and kinetic equilibrium cannot be achieved), dark freeze-out, reannihilation and “cannibalism”, to name just a few. Freeze-in is now supported by micrOMEGAs.

Models connecting dark matter with collider experiments are becoming ever more optimised to the needs of users. For example, micrOMEGAs interfaces with SModelS, which is capable of quickly applying all possible LHC-relevant supersymmetric searches. The software also includes long-lived particles, as commonly found in FIMP models. As MadDM is embedded in MadGraph, noted Benjamin Fuks (LPTHE Paris), tools such as MadAnalysis may be used to recast CMS and ATLAS searches. Celine Degrande (UCLouvain) described another nice tool, FeynRules, which produces model files in both the MadDM and micrOMEGAs formats given the Lagrangian for the BSM model, providing a very useful automatised chain from the model directly to the dark-matter observables, high-energy predictions and comparisons with experimental results. Meanwhile, MadDump expands MadGraph’s predictions and detector simulations from the high-energy collider limits to fixed-target experiments such as NA62. To complete a vibrant landscape of development efforts, Tomas Gonzalo (Monash) presented the GAMBIT collaboration’s work to provide tools for global fits to generic dark-matter models.

A phenomenologist’s dream

Huge efforts are underway to develop a computational platform to study new directions in experimental searches for dark matter, and TOOLS 2020 showed that we are already very close to the phenomenologist’s dream for WIMPs. TOOLS 2020 wasn’t just about dark matter either – it also covered developments in Higgs and flavour physics, precision tests and general fitting, and other tools. Interested parties are welcome to join in the next TOOLS conference due to take place in Annecy in 2022.

The post Tooling up to hunt dark matter appeared first on CERN Courier.

The Standard Model (SM) cannot be the complete theory of particle physics. Neutrino masses evade it. No viable dark-matter candidate is contained within it. And under its auspices the electric dipole moment of the neutron, experimentally compatible with zero, requires the cancellation of two non-vanishing SM parameters that are seemingly unrelated – the strong-CP problem. The physics explaining these mysteries may well originate from new phenomena at energy scales inaccessible to any collider in the foreseeable future. Fortunately, models involving such scales can be probed today and in the next decade by a series of experiments dedicated to searching for very weakly interacting slim particles (WISPs).

WISPs are pseudo Nambu–Goldstone bosons (pNGBs) that arise automatically in extensions of the SM from global symmetries which are broken both spontaneously and explicitly. NGBs are best known for being “eaten” by the longitudinal degrees of freedom of the W and Z bosons in electroweak gauge-symmetry breaking, which underpins the Higgs mechanism, but theorists have also postulated a bevy of pNGBs that get their tiny masses by explicit symmetry breaking and are potentially discoverable as physical particles. Typical examples arising in theoretically well-motivated grand-unified theories are axions, flavons and majorons. Axions arise from a broken “Peccei–Quinn” symmetry and could potentially explain the strong-CP problem, while flavons and majorons arise from broken flavour and lepton symmetries.

Being light and very weakly interacting, WISPs would be non-thermally produced in the early universe and thus remain non-relativistic during structure formation. Such particles would inevitably contribute to the dark matter of the universe. WISPs are now the target of a growing number and type of experimental searches that are complementary to new-physics searches at colliders.

Among theorists and experimentalists alike, the axion is probably the most popular WISP. Recently, massive efforts have been undertaken to improve the calculations of model-dependent relic-axion production in the early universe. This has led to a considerable broadening of the mass range compatible with the explanation of dark matter by axions. The axion could make up all of the dark matter in the universe for a symmetry-breaking scale f_a between roughly 10⁸ and 10¹⁹ GeV (the lower limit being imposed by astrophysical arguments, the upper one by the Planck scale), corresponding to axion masses from 10^–13 eV to 10 meV. For other light pNGBs, generically dubbed axion-like particles (ALPs), the parameter range is even broader. With many plausible relic-ALP-production mechanisms proposed by theorists, experimentalists need to cover as much of the unexplored parameter range as possible.

Although the strengths of the interactions between axions or ALPs and SM particles are very weak, being inversely proportional to f_a, several strategies for observing them are available. Limits and projected sensitivities span several orders of magnitude in the mass-coupling plane (see “The field of play” figure).

IAXO’s design profited greatly from experience with the ATLAS toroid

Since axions or ALPs can usually decay to two photons, an external static magnetic field can substitute one of the two photons and induce axion-to-photon conversion. Originally proposed by Pierre Sikivie, this inverse Primakoff effect can classically be described by adding source terms proportional to B and E to Maxwell’s equations. Practically, this means that inside a static homogeneous magnetic field the presence of an axion or ALP field induces electric-field oscillations – an effect readily exploited by many experiments searching for WISPs. Other processes exploited in some experimental searches and suspected to lead to axion production are their interactions with electrons, leading to axion bremsstrahlung, and their interactions with nucleons or nuclei, leading to nucleon-axion bremsstrahlung or oscillations of the electric dipole moment of the nuclei or nucleons.

The potential to make fundamental discoveries from small-scale experiments is a significant appeal of experimental WISP physics, however the most solidly theoretically motivated WISP parameter regions and physics questions require setups that go well beyond “table-top” dimensions. They target WISPs that flow through the galactic halo, shine from the Sun, or spring into existence when lasers pass through strong magnetic fields in the laboratory.

Dark-matter halo

Haloscopes target the detection of dark-matter WISPs in the halo of our galaxy, where non-relativistic cold-dark-matter axions or ALPs induce electric field oscillations as they pass through a magnetic field. The frequency of the oscillations corresponds to the axion mass, and the amplitude to B/f_a. When limits or projections are given for these kinds of experiments, it is assumed that the particle under scrutiny homogeneously makes up all of the dark matter in the universe, introducing significant cosmological model dependence.

The furthest developed currently operating haloscopes are based on resonant enhancement of the axion-induced electric-field oscillations in tunable resonant cavities. Using this method, the presently running ADMX project at the University of Washington has the sensitivity to discover dark-matter axions with masses of a few µeV. Nuclear resonance methods could be sensitive to halo dark-matter axions with mass below 1 neV and “fuzzy” dark-matter ALPs down to 10^–22 eV within the next decade, for example at the CASPEr experiments being developed at the University of Mainz and Boston University. Meanwhile, experiments based on classical LC circuits, such as ABRACADABRA at MIT, are being designed to measure ALP- or axion-induced magnetic field oscillations in the centre of a toroidal magnet. These could be sensitive in a mass range between 10 neV and 1 µeV.

ALPS II is the first laser-based setup to fully exploit resonance techniques

For dark-matter axions with masses up to approximately 50 µeV, promising developments in cavity technologies such as multiple matched cavities and superconducting or diel­ectric cavities are ongoing at several locations, including at CAPP in South Korea, the University of Western Australia, INFN Legnaro and the RADES detector, which has taken data as part of the CAST experiment at CERN. Above ~40 µeV, however, the cavity concept becomes more and more challenging, as sensitivity scales with the volume of the resonant cavity, which decreases dramatically with increasing mass (as roughly 1/m_a³). To reach sensitivity at higher masses, in the region of a few hundred µeV, a novel “dielectric haloscope” is being developed by the MADMAX (Magnetized Disk and Mirror Axion experiment) collaboration for potential installation at DESY. It exploits the fact that static magnetic-field boundaries between media with different dielectric constants lead to tiny power emissions that compensate the discontinuity in the axion-induced electric fields in neighbouring media. If multiple surfaces are stacked in front of each other, this should lead to constructive interference, boosting the emitted power from the expected axion dark matter in the desired mass range to detectable levels. Other novel haloscope concepts, based on meta-materials (“plasma haloscopes”, for example) and topological insulators, are also currently being developed. These could have sensitivity to even higher axion masses, up to a few meV.

Staying in tune

In principle, axion-dark-matter detection should be relatively simple, given the very high number density of particles – approximately 3 × 10¹³ axions/cm³ for an axion mass of 10 µeV – and the well-established technique of resonant axion-to-photon conversion. But, as the axion mass is unknown, the experiments must be painstakingly tuned to each possible mass value in turn. After about 15 years of steady progress, the ADMX experiment has reached QCD-axion dark-matter sensitivity in the mass regime of a few µeV.

ADMX uses tunable microwave resonators inside a strong solenoidal magnetic field, and modern quantum sensors for readout. Unfortunately, however, this technology is not scalable to the higher axion-mass regions as preferred, for example, by cosmological models where Peccei–Quinn symmetry breaking happened after an inflationary phase of the universe. That’s where MADMAX comes in. The collaboration is working on the dielectric-haloscope concept – initiated and led by scientists at the Max Planck Institute for Physics in Munich – to investigate the mass region around 100 µeV.

Astrophysical hints

Weakly interacting slim particles (WISPs) could be produced in hot astrophysical plasmas and transport energy out of stars, including the Sun, stellar remnants and other dense sources. Observed lifetimes and energy-loss rates can therefore probe their existence. For the axion, or an axion-like particle (ALP) with sub-MeV mass that couples to nucleons, the most stringent limit, f_a > ~10⁸ GeV, stems from the duration of the neutrino signal from the progenitor neutron star of Supernova 1987A.

Tantalisingly, there are stellar hints from observations of red giants, helium-burning stars, white dwarfs and pulsars that seem to indicate energy losses with slight excesses with respect to those expected from standard energy emission by neutrinos. These hints may be explained by axions with masses below 100 meV or sub-keV-mass ALPs with a coupling to both electrons and photons.

Other observations suggest that TeV photons from distant blazars are less absorbed than expected by standard interactions with extragalactic background light – the so-called transparency hint. This could be explained by the conversion of photons into ALPs in the magnetic field of the source, and back to photons in astrophysical magnetic fields. Interestingly, these would have about the same ALP–photon coupling strength as indicated by the observed stellar anomalies, though with a mass that is incompatible with both ALPs which can explain dark matter and with QCD axions (see “The field of play” figure).

MADMAX will use a huge ~9 T superconducting dipole magnet with a bore of about 1.35 m and a stored energy of roughly 480 MJ. Such a magnet has never been built before. The MADMAX collaboration teamed up with CEA-IRFU and Bilfinger-Noell and successfully worked out a conceptual design. First steps towards qualifying the conductor are under way. The plan is for the magnet to be installed at DESY inside the old iron yoke of the former HERA experiment H1. DESY is already preparing the required infrastructure, including the liquid-helium supply necessary to cool the magnet. R&D for the dielectric booster, with up to 80 adjustable 1.25 m² disks, is in full swing.

A first prototype, containing a more modest 20 discs of 30 cm diameter, will be tested in the “Morpurgo” magnet at CERN during future accelerator shutdowns (see “Haloscope home” figure). With a peak field strength of 1.6 T, its dipole field will allow new ALP-dark-matter parameter regions to be probed, though the main purpose of the prototype is to demonstrate the operation of the booster system in cryogenic surroundings inside a magnetic field. The MADMAX collaboration is extremely happy to have found a suitable magnet at CERN for such tests. If sufficient funds can be acquired within the next two to three years for magnet construction, and provided that the prototype efforts at CERN are successful, MADMAX could start data taking at DESY in 2028.

While direct dark-matter search experiments like ADMX and MADMAX offer by far the highest sensitivity for axion searches, this is based on the assumption that the dark matter problem is solved by axions, and if no signal is discovered any claim of an exclusion limit must rely on specific cosmological assumptions. Therefore, other less model-dependent experiments, such as helioscopes or light shining through a wall (LSW) experiments, are extremely beneficial in addition to direct dark-matter searches.

Solar axions

In contrast to dark-matter axions or ALPs, those produced in the Sun or in the laboratory should have considerable momentum. Indeed, solar axions or ALPs should have energies of a few keV, corresponding to the temperature at which they are produced. These could be detected by helioscopes, which seek to use the inverse Primakoff effect to convert solar axions or ALPs into X-rays in a magnet pointed towards the Sun, as at the CERN Axion Solar Telescope (CAST) experiment. Helioscopes could cover the mass range compatible with the simplest axion models, in the vicinity of 10 meV, and could be sensitive to ALPs with masses below 1 eV without any tuning at all.

The CAST helioscope, which reused an LHC prototype dipole magnet, has driven this field in the past decade, and provides the most sensitive exclusion limits to date. Going beyond CAST calls for a much larger magnet. For the next-generation International Axion Observatory (IAXO) helioscope, CERN members of the international collaboration worked out a conceptual design for a 20 m-long toroidal magnet with eight 60 cm-diameter bores. IAXO’s design profited greatly from experience with the ATLAS toroid.

In the past three years, the collaboration, led by the University of Zaragoza, has been concentrating its activities on the BabyIAXO prototype in order to finesse the magnet concept, the X-ray telescopes necessary to focus photons from solar axion conversion and the low-background detectors. BabyIAXO will increase the signal-to-noise ratio of CAST by two orders of magnitude; IAXO by a further two orders of magnitude.

In December 2020 the directorates of CERN and DESY signed a collaboration agreement regarding BabyIAXO: CERN will provide the detailed design of the prototype magnet including its cryostat, while DESY will design and prepare the movable platform and infrastructure (see “Prototype” figure). BabyIAXO will be located at DESY in Hamburg. The collaboration hopes to attract the remaining funds for BabyIAXO so construction can begin in 2021 and first science runs could take place in 2025. The timeline for IAXO will depend strongly on experiences during the construction and operation of BabyIAXO, with first light potentially possible in 2028.

Light shining through a wall

In contrast to haloscopes, helioscopes do not rely on the assumption that all dark matter is made up by axions. But light-shining-through-wall (LSW) experiments are even less model dependent with respect to ALP production. Here, intense laser light could be converted to axions or ALPs inside a strong magnetic field by the Primakoff effect. Behind a light-impenetrable wall they would be re-converted to photons and detected at the same wavelength as the laser light. The disadvantage of LSW experiments is that they only reach sensitivity to ALPs with a mass up to a few hundred µeV with comparably high coupling to photons. However, this is sensitive enough to test the parameter range consistent with the transparency hint and parts of the mass range consistent with the stellar hints (see “Astrophysical hints” panel).

The Any Light Particle Search (ALPS II) at DESY follows this approach. By seeking to observe light shining through a wall, any ALPs would be generated in the experiment itself, removing the need to make assumptions about their production. ALPS II is based on 24 modified superconducting dipole magnets that have been straightened by brute-force deformation, following their former existence in the proton accelerator of the HERA complex. With the help of two 124 m-long high-finesse optical resonators, encompassed by the magnets on both sides of the wall, ALPS II is also the first laser-based setup to fully exploit resonance techniques. Two readout systems capable of measuring a 1064 nm photon flux down to a rate of 2 × 10^–5 s^–1 have been developed by the collaboration. Compared to the present best LSW limits provided by OSQAR at CERN, the signal-to-noise ratio will rise by no less than 12 orders of magnitude at ALPS II. Nevertheless, MADMAX would surpass ALPS II in the sensitivity for the axion-photon coupling strength by more than three orders of magnitude. This is the price to pay for a model-independent experiment – however, ALPS II principally targets not dark-matter candidates but ALPs indicated by astrophysical phenomena.

Tunelling ahead

The installation of the 24 dipole magnets in a straight section of the HERA tunnel was completed in 2020. Three clean rooms at both ends and in the centre of the experiment were also installed, and optics commissioning is under way. A first science run is expected for autumn 2021.

In the overlapping mass region up to 0.1 meV, the sensitivities of ALPS II and BabyIAXO are roughly equal. In the event of a discovery, this would provide a unique opportunity to study the new WISP. Excitingly, a similar case might be realised for IAXO: combining the optics and detectors of ALPS II with simplified versions of the dipole magnets being studied for FCC-hh would provide an LSW experiment with “IAXO sensitivity” regarding the axion-photon coupling, albeit in a reduced mass range. This has been outlined as the putative JURA (Joint Undertaking on Research for Axions) experiment in the context of the CERN-led Physics Beyond Colliders study.

The past decade has delivered significant developments in axion and ALP theory and phenomenology. This has been complemented by progress in experimental methods to cover a large fraction of the interesting axion and ALP parameter range. In close collaboration with universities and institutes across the globe, CERN, DESY and the Max Planck society will together pave the road to the exciting results that are expected this decade.

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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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At the LHC, a mediator may be produced and decay into a pair of stable WIMPs, which then escape the detector unseen – an undetectable process, unless the mediator is produced, for example, in association with a high-p_T gluon radiated from one of the incoming protons. This would provide a clear signature: a high-p_T jet and significant missing transverse momentum (MET). A first “monojet” analysis sought events with MET in excess of 200 GeV, recoiling against a jet with p_T > 150 GeV, with up to three additional jets and no leptons or photons. The leading background arises from events wherein a Z boson decays to neutrinos – a process experimentally indistinguishable from WIMP production. The predictions of this and other backgrounds benefitted from stateof- the-art theoretical calculations, detailed groundwork on particle reconstruction in ATLAS, and the use of data-control regions rich in W and Z boson decays. No significant excess was observed with respect to the Standard Model (SM) (figure 1).

As invisible Higgs-boson decays have a branching fraction of just 10^–3, any signal would indicate new physics

Dijet analysis

A second “dijet” WIMP analysis searches for invisible decays of Higgs bosons produced via vector-boson fusion. Though accounting for just 10% as many Higgs bosons as the dominant gluon-fusion process at the LHC, the topology’s clear signature, with two widely separated jets in pseudorapidity, lends itself to searching for MET, as the jets tend to be close together in the transverse plane when recoiling against a Higgs boson with p_T > 200 GeV. The art of this analysis is again in the precise modelling of SM backgrounds – a feat accomplished here with extrapolations from control regions and the use of jet kinematics to separate signal events from Z-boson decays to neutrinos, and W decays with an undetected charged lepton. As invisible Higgs-boson decays in the SM (chiefly H → ZZ* → 4ν) have a branching fraction of just 10^–3, any significant signal would indicate new physics. No deviation from the SM was observed, allowing a 95% confidence upper limit to be placed on the branching fraction for invisible Higgs-boson decays of 13% – a factor two improvement in sensitivity compared to the previous analysis, despite the increase in pileup – or 9% when combining with other ATLAS Higgs-boson measurements. The results are complementary to direct-detection experiments looking for relic WIMPs with deep underground detectors, as they plumb lower WIMP masses than direct-detection experiments can currently access (figure 2).

These results also translate into limits on alternative dark-matter-related theories such as axion-like-particles (ALPs) and large extra-dimensions, and into model-independent limits on new phenomena. ATLAS will continue to explore the parameter space of dark-sector models such at ALPs, dark photons, dark scalars and heavy neutral leptons, complementing the results of dedicated smaller-scale experiments.

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Assuming DM is a material substance comprised of particles – not an illusion resulting from an imperfect understanding of gravity – there are three independent ways to search for it. One is to directly measure the production of DM particles in a high-energy collider such as the LHC. Another is to infer the presence of DM particles via their scattering off nuclei, as investigated by large underground detectors such as XENON1T and LUX. A third, similarly indirect, strategy is to search for the annihilation or decay of DM particles into ordinary (anti) particles such as positrons or antinuclei – as employed by the AMS experiment on board the International Space Station and in balloon-borne experiments such as GAPS. Low-energy light antinuclei, such as antideuterons and antihelium, are particularly promising signals for such indirect DM searches, since the background stemming from ordinary collisions between cosmic rays and the interstellar medium is expected to be low with respect to the DM signal.

ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei

The ability to interpret any future observation of antinuclei in our galaxy – especially when trying to identify their creation in exotic processes like DM annihilations – requires a quantitative understanding of light antinuclei production and annihilation mechanisms within the interstellar medium. However, the production of light antinuclei in hadronic collisions between cosmic rays and the interstellar medium is still not fully understood: different models compete to explain how these loosely bound objects can be formed in such high-energy collisions. Furthermore, the inelastic annihilation cross section of light antinuclei with matter is completely unknown in the kinematic region relevant for indirect DM searches, forcing current estimates to rely on extrapolations and modelling.

Fortunately, both the antinuclei production mechanism and the interactions between antinuclei and ordinary matter can be studied on Earth using large accelerators. The main contributions so far have come from the LHC at CERN and from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Thanks to its unique low-material-budget tracker, which provides excellent tracking and particle-identification performance for low-momentum particles, ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei.

Antinucleosynthesis in the lab
While antinuclei can also be produced at lower collision energies, only at the LHC are matter and antimatter generated in equal abundances in the region transverse to the beam direction. The most abundant non-trivial antinucleus produced is the antideuteron, which consists of an antineutron and an antiproton. At low momentum, deuterons and antideuterons can be clearly identified thanks to their high energy loss in the ALICE detector’s time-projection chamber. At larger momenta, a clean identification of antideuterons is possible using the ALICE time-of-flight detector. This information, combined with the measured track length and the particle momentum, provide a precise determination of the particle mass. Using these and other techniques, the ALICE collaboration has recently measured the production of (anti)deuterons in proton–proton collisions, as well as in other colliding systems, and set tight constraints on the production models of (anti)nuclei.

There are two main ways to model the production mechanism of (anti)nuclei. Coalescence models assume that primary (anti)neutrons and (anti)protons can bind if they are close enough in phase space. Statistical hadronisation models, on the other hand, assume that hadrons and (anti) nuclei emerge when the collision system reaches thermodynamical equilibrium, making the temperature and the volume of the system the key parameters. Measurements of nuclei-to-proton ratios in various colliding systems have recently enabled the ALICE collaboration to compare the two model approaches in detail (see “Competing models” figure). As can be seen, the two models give different predictions for the evolution of the nuclei-to-proton ratio versus particle multiplicity, with the latest ALICE measurements slightly favouring the coalescence approach.

Similar conclusions about the two models can be drawn using heavier antinuclei, like ³He and ⁴He, which were already measured by ALICE in p–Pb and Pb–Pb collisions. The achievable precision of the measurement is limited by the available data: the antinuclei production rate in pp collisions goes down by a factor of about 1000 for every additional antinucleon in the antinucleus.

The precision of the measurements from proton–proton collisions places strong constraints on the production models, which can then be used to predict the antinuclei fluxes in space. Indeed, the ALICE measurements combined with different coalescence models have already been employed to estimate the antideuteron and antihelium flux from cosmic-ray interactions measurable by the AMS and GAPS experiments. These predictions will allow correct interpretations of the eventual antinuclei signal that might be observed in the future by the two collaborations.

Further helping clarify the results of indirect DM searches, ALICE has recently performed the first measurement of the antideuteron inelastic cross section in the momentum range 0.3 < p < 4 GeV/c – significantly extending our knowledge about this cross section from previous measurements at momenta of 13 and 25 GeV/c at the Serpukhov accelerator complex in Russia in the early 1970s. The collaboration took advantage of the ability of antideuterons produced at the LHC to interact inelastically with the detector material. To quantify this process, ALICE has employed a novel approach based on the antideuteron- to-deuteron ratio reconstructed in collisions of protons and heavy ions at a centre-of-mass energy per nucleon–nucleon pair of 5.02 TeV. Such a ratio depends on both of the inelastic cross sections of deuterons and antideuterons. The former has been measured in various previous experiments at different momenta; the latter can be constrained from the ALICE data by comparing the measured ratio with detailed Monte Carlo simulations.

The resulting antideuteron inelastic cross section is shown (see “Interaction probability” figure), where the two panels correspond to the different sub-detectors employed in the analysis and therefore to different average material crossed by (anti)deuterons – corresponding to a difference of about a factor two in average mass number. The inelastic cross sections include all possible inelastic antideuteron processes such as break-up, annihilation or charge exchange, and the analysis procedure was validated by demonstrating consistency with existing antiproton results from traditional scattering experiments.

The momentum range covered is of particular importance to evaluate the signal predictions for indirect dark-matter searches

The momentum range covered in this latest analysis is of particular importance to evaluate the signal predictions for indirect dark-matter searches. Additionally, these measurements can help researchers to understand the low-energy antideuteron inelastic processes and to model better the inelastic antideuteron cross sections in widely-used toolkits such as Geant4. Together with the proper modelling of antinuclei formation, the obtained results will impact the antideuteron flux expectations at low momentum for ongoing and future satellite- and balloon-borne experiments.

The heavier, the better
ALICE is studying the full range of antinuclei physics with unprecedented precision. These results, which have started to emerge only since 2015, are contributing significantly to our understanding of antinuclei formation and annihilation processes, with important ramifications for DM searches. Both the statistical hadronisation and coalescence models can describe antideuteron production at the LHC, while the detector material can be used as an absorber to study the antinuclei inelastic cross section at low energies relevant for the astrophysical applications.

For the foreseeable future, ALICE will continue to provide an essential reference for the interpretation of astrophysics measurements of antinuclei in space. With the increased integrated luminosity that will be acquired by ALICE during LHC Run 3 from early 2022, it will be possible to extend the current analyses to heavier (anti)nuclei, such as ³He and ⁴He, with even better precision than the currently available measurements for (anti)deuterons. This will allow the collaboration to perform fundamental tests of the production and annihilation mechanisms with heavier, doubly-charged antinuclei, which are more easily identified by satellite-borne experiments and thus expected to provide an even clearer DM signature.

The post ALICE’s dark side appeared first on CERN Courier.

The possibility that dark-matter particles may interact via an unknown force, felt only feebly by Standard Model (SM) particles, has motivated substantial efforts to search for dark forces. The force-carrying particle for such hypothesised interactions is often referred to as a dark photon, in analogy with the ordinary photon that mediates the electromagnetic interaction.

In the minimal dark-photon scenario, the dark photon does not couple directly to SM particles; however, quantum-mechanical mixing between the photon and dark-photon fields can generate a small interaction, providing a portal through which dark photons may be produced and through which they might decay into visible final states.

Hidden-valley scenarios exhibit confinement in the dark sector, similarly to how the strong nuclear force confines quarks

While the minimal dark-photon model is both compelling and simple, it is not the only viable dark-sector scenario. Many other well-motivated dark-sector models exist, and some of these would have avoided detection in all previous experimental searches. Fully exploring the space of dark sectors is vital given the lack of signals observed thus far in the simplest scenarios. For example, so-called hidden-valley (HV) scenarios exhibit confinement in the dark sector, similarly to how the strong nuclear force confines SM quarks, would produce a high multiplicity of light hidden hadrons from showering processes in a similar way to jet production in the SM. These hidden hadrons would typically decay displaced from the proton–proton collision, thus failing the criteria employed in previous dark-photon searches to suppress backgrounds due to heavy-flavour quarks. Therefore, it is desirable to perform experimental searches for dark sectors that are less model dependent, by not focusing solely on the minimal dark-photon scenario.

Using its Run-2 data sample, LHCb recently performed searches for both short-lived and long-lived exotic bosons that decay into the dimuon final state. These searches explored the invariant mass range from near the dimuon threshold up to 60 GeV. None of the searches found evidence for a signal and exclusion limits were placed on the X → μ⁺μ^– cross sections, each with minimal model dependence.

For many types of dark-sector models, these limits are the most stringent to date. This is especially true for the HV scenario (see figure), for which LHCb has placed the first such constraints on physically relevant HV mixing strengths in this mass range.

These results demonstrate the unique sensitivity of the LHCb experiment to dark sectors. Looking forward to Run 3, the trigger will be upgraded, greatly increasing the efficiency to low-mass dark sectors, and the luminosity will be higher. Taken together, these improvements will further expand LHCb’s world-leading dark-sector programme.

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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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A new experiment at Karlsruhe Institute of Technology (KIT) called FUNK – Finding U(1)s of a Novel Kind – has reported its first results in the search for ultralight dark matter. Using a large spherical mirror as an electromagnetic dark-matter antenna, the FUNK team has set an improved limit on the existence of hidden photons as candidates for dark matter with masses in the eV range.

Despite overwhelming astronomical evidence for the existence of dark matter, direct searches for dark-matter particles at colliders and dedicated nuclear-recoil experiments have so far come up empty handed. With these searches being mostly sensitive to heavy dark-matter particles, namely weakly interacting massive particles (WIMPs), the search for alternative light dark-matter candidates is growing in momentum. Hidden photons, a cold, ultralight dark-matter candidate, arise in extensions of the Standard Model which contain a new U(1) gauge symmetry and are expected to couple very weakly to charged particles via kinetic mixing with regular photons. Laboratory experiments that are sensitive to such hidden or dark photons include helioscopes such as the CAST experiment at CERN, and “light-shining-through-a-wall” methods such as ALPS experiment at DESY.

FUNK exploits a novel “dish-antenna” method first proposed in 2012, whereby a hidden photon crossing a metallic spherical mirror surface would cause faint electromagnetic waves to be emitted almost perpendicularly to the mirror surface, and be focused on the radius point. The experiment was conceived in 2013 at a workshop at DESY when it was realised that there was a perfectly suited mirror — a prototype for the Pierre Auger Observatory with a surface area of 14 m² – in the basement of KIT. Various photodetectors placed at the radius point allow FUNK to search for a signal in different wavelength ranges, corresponding to different hidden-photon masses. The dark-matter nature of a possible signal can then be verified by observing small daily and seasonal movements of the spot around the radius point as Earth moves through the dark-matter field. The broadband dish-antenna technique is able to scan hidden photons over a large parameter space.

The mass range of viable hidden-photon dark matter is huge

Joerg Jaeckel

Completed in 2018, the experiment took data during last year in several month-long runs using low-noise PMTs. In the mass range 2.5 – 7 eV, the data exclude a hidden-photon coupling stronger than 10⁻¹² in kinetic mixing. “This is competitive with limits derived from astrophysical results and partially exceeds those from other existing direct-detection experiments,” says FUNK principal investigator Ralph Engel of KIT. So far two other experiments of this type have reported search results for hidden photons in this energy range — the dish-antenna at the University of Tokyo and the SHUKET experiment at Paris-Saclay – though FUNK’s factor-of-ten larger mirror surface brings a greater experimental sensitivity, says the team. Other experiments, such as NA64 at CERN which employs missing-energy techniques, are setting stringent bounds on the strength of dark-photon couplings for masses in the MeV range and above.

“The mass range of viable hidden-photon dark matter is huge,” says FUNK collaborator Joerg Jaeckel of Heidelberg University. “For this reason, techniques which can scan over a large parameter space are especially useful even if they cannot explore couplings as small as is possible with some other dedicated methods. A future exploitation of the setup in other wavelength ranges is possible, and FUNK therefore carries an enormous physics potential.”

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The Belle II collaboration at the SuperKEKB collider in Japan has published its first physics analysis: a search for Z′ bosons, which are hypothesised to couple the Standard Model (SM) with the dark sector. The team scoured four months of data from a pilot run in 2018 for evidence of invisibly decaying Z′ bosons in the process e⁺e⁻→μ⁺μ⁻Z′, and for  lepton-flavour violating Z′ bosons in e⁺e⁻→e^±μ^∓Z′, by looking for missing energy recoiling against two clean lepton tracks. “This is the first ever search for the process e⁺e⁻→μ⁺μ⁻Z′ where the Z′ decays invisibly,” says Belle II spokesperson Toru Iijima of Nagoya University.

The team did not find any excess of events, yielding preliminary sensitivity to the coupling g′ in the so-called L_μ−L_τ extension of the SM, wherein the Z′ couples only to muon and tau-lepton flavoured SM particles and the dark sector. This model also has the potential to explain anomalies in b → sμ⁺μ⁻ decays reported by LHCb and the longstanding muon g-2 anomaly, claims the team.

The results come a little over a year since the first collisions were recorded in the fully instrumented Belle II detector on 25 March 2019. Following in the footsteps of Belle at the KEKB facility, the new SuperKEKB b-factory plans to achieve a 40-fold increase on the luminosity of its predecessor, which ran from 1999 to 2010. First turns were achieved in February 2016, and first collisions between its asymmetric-energy electron and positron beams were achieved in April 2018. The machine has now reached a luminosity of 1.4 × 10³⁴ cm^-2 s^-1 and is currently integrating around 0.7 fb^-1 each day, exceeding the peak luminosity of the former PEP-II/BaBar facility at SLAC, notes Iijima.

By summer the team aims to exceed the Belle/KEKB record of 2.1 × 10³⁴ cm^-2 s^-1 by implementing a nonlinear “crab waist” focusing scheme. First used at the electron-positron collider DAΦNE at INFN Frascati, and not to be confused with the crab-crossing technology used to boost the luminosity at KEKB and planned for the high-luminosity LHC, the scheme stabilises e⁺e^– beam-beam blowup using carefully tuned sextupole magnets located symmetrically on either side of the interaction point. “The 100 fb^-1 sample which we plan to integrate by summer will allow us to provide our first interesting results in B physics,” says Tom Browder of the University of Hawaii, who was Belle II spokesperson until last year.

Flavour debut

Belle II will make its debut in flavour physics at a vibrant moment, complementing  efforts to resolve hints of anomalies seen at the LHC, such as the recent test of lepton-flavour universality in beauty-baryon decays by the LHCb collaboration.

We will then look for the star attraction of the dark sector, the dark photon

Tom Browder

As well as updating searches for  invisible decays of the Z′ with one to two orders of magnitude more data, Belle II will now conduct further dark-sector studies including a search for axion-like particles decaying to two photons, the Z′ decaying to visible final states and dark-Higgstrahlung with a μ⁺μ^– pair and missing energy, explains Browder. “We will then look for the star attraction of the dark sector, the dark photon, with the difficult signature of e⁺e^– to a photon and nothing else.”

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One of the best strategies for searching for new physics in the TeV regime is to look for the decays of new particles. The CMS collaboration has searched in the dilepton channel for particles with masses above a few hundred GeV since the start of LHC data taking. Thanks to newly developed triggers, the searches are now being extended to the more difficult lower range of masses. A promising possible addition to the Standard Model (SM) that could exist in this mass range is the dark photon (Z_D). Its coupling with SM particles and production rate depend on the value of a kinetic mixing coefficient ε, and the resulting strength of the interaction of the Z_D with ordinary matter may be several orders of magnitude weaker than the electroweak interaction.

The CMS collaboration has recently presented results of a search for a narrow resonance decaying to a pair of muons in the mass range from 11.5 to 200 GeV. This search looks for a strikingly sharp peak on top of a smooth dimuon mass spectrum that arises mainly from the Drell–Yan process. At masses below approximately 40 GeV, conventional triggers are the main limitation for this analysis as the thresholds on the muon transverse momenta (p_T), which are applied online to reduce the rate of events saved for offline analysis, introduce a significant kinematic acceptance loss, as evident from the red curve in figure 1.

A dedicated set of high-rate dimuon “scouting” triggers, with some additional kinematic constraints on the dimuon system and significantly lower muon p_T thresholds, was deployed during Run 2 to overcome this limitation. Only a minimal amount of high-level information from the online reconstruction is stored for the selected events. The reduced event size allows significantly higher trigger rates, up to two orders of magnitude higher than the standard muon triggers. The green curve in figure 1 shows the dimuon invariant mass distribution obtained from data collected with the scouting triggers. The increase in kinematic acceptance for low masses can be well appreciated.

The full data sets collected with the muon scouting and standard dimuon triggers during Run 2 are used to probe masses below 45 GeV, and between 45 and 200 GeV, respectively, excluding the mass range from 75 to 110 GeV where Z-boson production dominates. No significant resonant peaks are observed, and limits are set on ε² at 90% confidence as a function of the Z_D mass (figure 2). These are among the world’s most stringent constraints on dark photons in this mass range.

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Axions are extremely light, spinless bosons that were originally proposed in the 1970s to resolve the strong charge–parity problem of quantum chromodynamics and, later, were predicted by theories beyond the Standard Model. Being stable, axions produced during the Big Bang would still be present throughout the universe, possibly accounting for dark matter or some portion of it. In this case, Earth would experience a “wind” of gravitationally interacting dark-matter particles that would couple to matter and antimatter and periodically modulate their fundamental properties, such as their magnetic moment. However, no evidence of such an effect has so far been seen in laboratory experiments with ordinary matter, setting stringent limits on the microscopic properties of cosmic axion-like particles.

Our ALP–antiproton coupling limits are much more stringent than limits derived from astrophysical observations

Stefan Ulmer

The BASE team has now searched for the phenomenon in antimatter via measurements of the precession frequency of the antiproton’s magnetic moment, which it is able to determine with a fractional precision of 1.5×10^-9. The technique relies on single-particle spin-transition spectroscopy – comparable to performing NMR with a single antiproton – whereby individual antiprotons stored in a Penning trap are spin-flipped from one state to another (CERN Courier March 2018 p25). An observed variation in the precession frequency over time could provide evidence for the nature of dark matter and, if antiprotons have a stronger coupling to these particles than protons do, such a matter–antimatter asymmetric coupling could provide a link between dark matter and the baryon asymmetry in the universe.

“We’ve interpreted these data in the framework of the axion wind model where light axion like particles (ALP’s) oscillate through the galaxy, at frequencies defined by the ALP mass,” explains lead author and BASE co-spokesperson Christian Smorra of RIKEN in Japan. “The particles couple to the spins of Standard Model particles, which would induce frequency modulations of the Larmor precession frequency.”

Accruing around 1000 measurements over a three-month period, the team determined a time-averaged frequency of the antiproton’s precession of around 80 MHz with an uncertainty of 120 mHz. No signs of regular variations were found, producing the first laboratory constraints on the existence of an interaction between antimatter and a dark-matter candidate. The BASE data constrain the axion-antiproton interaction parameter (a factor in the matrix element inversely proportional to the postulated coupling between axions and antiprotons) to be above 0.1 GeV for an axion mass of 2×10⁻²³ and above 0.6 GeV for an axion mass of 4×10⁻¹⁷ eV, at 95% confidence. For comparison, similar experiments using matter instead of antimatter achieve limits of above 10 and 1000 TeV for the same mass range – demonstrating that a major violation of established charge-party-time symmetry would be implied by any signal given the current BASE sensitivity. The collaboration also derived limits on six combinations of previously unconstrained Lorentz- and CPT-violating coefficients of the non-minimal Standard Model extension.

“We have not observed any oscillatory signature, however, our ALP–antiproton coupling limits are much more stringent than limits derived from astrophysical observations,” says BASE spokesperson Stefan Ulmer of RIKEN, who is optimistic that BASE will be able to improve the sensitivity of its axion search. “Future studies, with a ten-fold improved frequency stability, longer experimental campaigns and broader spectral scans at higher frequency resolution, will allow us to increase the detection bandwidth.”

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The EHT is a network of eight radio dishes in Antarctica, Chile, Mexico, Hawaii, Arizona and Spain that creates an Earth-sized interferometer. Its ultra-high angular resolution images of radio emission from a supermassive black hole at the heart of galaxy M87* opened a new window on black holes and other phenomena. Recently, a team at Brookhaven National Laboratory used the EHT image to disfavour “fuzzy” models of ultra-light boson dark matter.

Also announced were six New Horizons Prizes worth $100,000 each, which recognize early-career achievements in physics and mathematics. In physics, Jo Dunkley (Princeton); Samaya Nissanke (University of Amsterdam) and Kendrick Smith (Perimeter Institute) were awarded for the development of novel techniques to extract fundamental physics from astronomical data. Simon Caron-Huot (McGill University) and Pedro Vieira (Perimeter Institute) were recognized for their “profound contributions to the understanding of quantum field theory”.

The Breakthrough Prize was founded in 2012 by former physicist and entrepreneur Yuri Milner, with sponsors including Google’s Sergey Brin and Facebook’s Mark Zuckerberg. In August, a Special Breakthrough Prize in Fundamental physics was awarded to Sergio Ferrara, Daniel Freedman and Peter van Nieuwenhuizen for the discovery of supergravity.

All prize recipients, along winners in mathematics and biology, will receive their awards at a ceremony in California on 3 November.

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Hooman Davoudiasl and Peter Denton of Brookhaven National Laboratory have used the recent Event Horizon Telescope image of supermassive black hole M87* to disfavour “fuzzy” models of ultra-light boson dark matter with masses of the order of a few 10^-21 eV (Phys. Rev. Lett. 123 021102). The inferred mass, spin and age of the black hole are incompatible with the existence of such fuzzy dark matter given the principle of superradiance, whereby quantum fluctuations deplete the angular momentum of a rotating black hole by populating a cloud of bosons around it. The effect depends only on the bosons’ mass, and does not presuppose any non-gravitational interactions. Future measurements of M87* and other spinning supermassive black holes have the potential to exclude the entire parameter space for fuzzy dark matter.

An intriguing alternative to cold dark matter, fuzzy dark matter could address the “core-cusp problem”, wherein observations of an approximately constant dark matter density in the inner parts of galaxies conflict with the steep power-law-like behaviour of cosmological simulations. The particles’ long de Broglie wavelengths, of the order of a kiloparsec, would suppress structure at this scale.

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Following the discovery of gravitational waves by the LIGO and Virgo collaborations, there is great interest in observing other parts of the gravitational-wave spectrum and seeing what they can tell us about astrophysics, particle physics and cosmology. The European Space Agency (ESA) has approved the LISA space experiment that is designed to observe gravitational waves in a lower frequency band than LIGO and Virgo, while the KAGRA experiment in Japan, the INDIGO experiment in India and the proposed Einstein Telescope (ET) will reinforce LIGO and Virgo. However, there is a gap in observational capability in the intermediate-frequency band where there may be signals from the mergers of massive black holes weighing between 100 and 100,000 solar masses, and from a first-order phase transition or cosmic strings in the early universe.

This was the motivation for a workshop held at CERN on 22 and 23 July that brought experts from the cold-atom community together with particle physicists and representatives of the gravitational-wave community. Experiments using cold atoms as clocks and in interferometers offer interesting prospects for detecting some candidates for ultralight dark matter as well as gravitational waves in the mid-frequency gap. In particular, a possible space experiment called AEDGE could complement the observations by LIGO, Virgo, LISA and other approved experiments.

The workshop shared information about long-baseline terrestrial cold-atom experiments that are already funded and under construction, such as MAGIS in the US, MIGA in France and ZAIGA in China, as well as ideas for future terrestrial experiments such as MAGIA-advanced in Italy, AION in the UK and ELGAR in France. Delegates also heard about space – CACES (China) and CAL (NASA) – and sounding-rocket experiments – MAIUS (Germany) – using cold atoms in space and microgravity.

A suggestion for an atom interferometer using a pair of satellites is being put forward by the AEDGE team

ESA has recently issued a call for white papers for its Voyage 2050 long-term science programme, and a suggestion for an atom interferometer using a pair of satellites is being put forward by the AEDGE team (in parallel with a related suggestion called STE-QUEST) to build upon the experience with prior experiments. AEDGE was the focus of the CERN workshop, and would have unique capabilities to probe the assembly of the supermassive black holes known to power active galactic nuclei, physics beyond the Standard Model in the early universe and ultralight dark matter. AEDGE would be a uniquely interdisciplinary space mission, harnessing cold-atom technologies to address key issues in fundamental physics, astrophysics and cosmology.

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It is 20 years since the discovery that the expansion of the universe is accelerating, yet physicists still know precious little about the underlying cause. In a classical universe with no quantum effects, the cosmic acceleration can be explained by a constant that appears in Einstein’s equations of general relativity, albeit one with a vanishingly small value. But clearly our universe obeys quantum mechanics, and the ability of particles to fluctuate in and out of existence at all points in space leads to a prediction for Einstein’s cosmological constant that is 120 orders of magnitude larger than observed. “It implies that at least one, and likely both, of general relativity and quantum mechanics must be fundamentally modified,” says Clare Burrage, a theorist at the University of Nottingham in the UK.

With no clear alternative theory available, all attempts to explain the cosmic acceleration introduce a new entity called dark energy (DE) that makes up 70% of the total mass-energy content of the universe. It is not clear whether DE is due to a new scalar particle or a modification of gravity, or whether it is constant or dynamic. It’s not even clear whether it interacts with other fundamental particles or not, says Burrage. Since DE affects the expansion of space–time, however, its effects are imprinted on astronomical observables such as the cosmic microwave background and the growth rate of galaxies, and the main approach to detecting DE involves looking for possible deviations from general relativity on cosmological scales.

Unique environment

Collider experiments offer a unique environment in which to search for the direct production of DE particles, since they are sensitive to a multitude of signatures and therefore to a wider array of possible DE interactions with matter. Like other signals of new physics, DE (if accessible at small scales) could manifest itself in high-energy particle collisions either through direct production or via modifications of electroweak observables induced by virtual DE particles.

Last year, the ATLAS collaboration at the LHC carried out a first collider search for light scalar particles that could contribute to the accelerating expansion of the universe. The results demonstrate the ability of collider experiments to access new regions of parameter space and provide complementary information to cosmological probes.

Unlike dark matter, for which there exists many new-physics models to guide searches at collider experiments, few such frameworks exist that describe the interaction between DE and Standard Model (SM) particles. However, theorists have made progress by allowing the properties of the prospective DE particle and the strength of the force that it transmits to vary with the environment. This effective-field-theory approach integrates out the unknown microscopic dynamics of the DE interactions.

The new ATLAS search was motivated by a 2016 model by Philippe Brax of the Université Paris-Saclay, Burrage, Christoph Englert of the University of Glasgow, and Michael Spannowsky of Durham University. The model provides the most general framework for describing DE theories with a scalar field and contains as subsets many well-known specific DE models – such as quintessence, galileon, chameleon and symmetron. It extends the SM lagrangian with a set of higher dimensional operators encoding the different couplings between DE and SM particles. These operators are suppressed by a characteristic energy scale, and the goal of experiments is to pinpoint this energy for the different DE–SM couplings. Two representative operators predict that DE couples preferentially to either very massive particles like the top quark (“conformal” coupling) or to final states with high-momentum transfers, such as those involving high-energy jets (“disformal” coupling).

Signatures

“In a big class of these operators the DE particle cannot decay inside the detector, therefore leaving a missing energy signature,” explains Spyridon Argyropoulos of the University of Iowa, who is a member of the ATLAS team that carried out the analysis. “Two possible signatures for the detection of DE are therefore the production of a pair of top-anti­top quarks or the production of high-energy jets, associated with large missing energy. Such signatures are similar to the ones expected by the production of supersymmetric top quarks (“stops”), where the missing energy would be due to the neutralinos from the stop decays or from the production of SM particles in association with dark-matter particles, which also leave a missing energy signature in the detector.”

The ATLAS analysis, which was based on 13 TeV LHC data corresponding to an integrated luminosity of 36.1 fb^–1, re-interprets the result of recent ATLAS searches for stop quarks and dark matter produced in association with jets. No significant excess over the predicted background was observed, setting the most stringent constraints on the suppression scale of conformal and disformal couplings of DE to normal matter in the context of an effective field theory of DE. The results show that the characteristic energy scale must be higher than approximately 300 GeV for the conformal coupling and above 1.2 TeV for the disformal coupling.

The search for DE at colliders is only at the beginning, says Argyropoulos. “The limits on the disformal coupling are several orders of magnitudes higher than the limits obtained from other laboratory experiments and cosmological probes, proving that colliders can provide crucial information for understanding the nature of DE. More experimental signatures and more types of coupling between DE and normal matter have to be explored and more optimal search strategies could be developed.”

With this pioneering interpretation of a collider search in terms of dark-energy models, ATLAS has become the first experiment to probe all forms of matter in the observable universe, opening a new avenue of research at the interface of particle physics and cosmology. A complementary laboratory measurement is also being pursued by CERN’s CAST experiment, which studies a particular incarnation of DE (chameleon) produced via interactions of DE with photons.

But DE is not going to give up its secrets easily, cautions theoretical cosmologist Dragan Huterer at the University of Michigan in the US. “Dark energy is normally considered a very large-scale phenomenon, but you may justifiably ask how the study of small systems in a collider can say anything about DE. Perhaps it can, but in a fairly model-dependent way. If ATLAS finds a signal that departs from the SM prediction it would be very exciting. But linking it firmly to DE would require follow-up work and measurements – all of which would be very exciting to see happen.”

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A report from the ATLAS experiment

In our current understanding of the energy content of the universe, there are two major unknowns: the nature of a non-luminous component of matter (dark matter) and the origin of the accelerating expansion of the universe (dark energy). Both are supported by astrophysical and cosmological measurements but their nature remains unknown. This has motivated a myriad of theoretical models, most of which assume dark matter to be a weakly interacting massive particle (WIMP).

WIMPs may be produced in high-energy proton collisions at the LHC, and are therefore intensively searched for by the LHC experiments. Since dark matter is not expected to interact with the detectors, its production leaves a signature of missing transverse momentum (E_T^miss). It can be detected if the dark-matter particles recoil against a visible particle X, which could be a quark or gluon, a photon, or a W, Z or Higgs boson. These are commonly known as X + E_T^miss signatures. To interpret these searches, a variety of simplified models are used that describe dark-matter production kinematics with a minimal number of free parameters. These models introduce new spin-0 or spin-1 mediator particles that propagate the interaction between the visible and the dark sectors. Because the mediators must couple to Standard Model (SM) particles in order to be produced in the proton–proton collisions, the mediators can also be directly searched for through their decays to jets, top-quark pairs and potentially even leptons. For certain model parameters, these direct searches can be more sensitive than the X + E_T^miss ones.

However, simplified models are not full theories like, for example, supersymmetry. Recent theoretical work has therefore focused on developing more complete, renormalisable models of dark matter, such as two-Higgs doublet models (2HDM) with an additional mediator particle. These models introduce a larger number of free parameters, allowing for a richer phenomenology.

Similarly, for dark energy, effective field theory implementations may introduce a stable and non-interacting scalar field that universally couples to matter. This also leads to a characteristic E_T^miss signature at the LHC.

ATLAS has recently released a summary gathering the results from more than 20 experimental searches for dark matter and a first collider search for dark energy. The wide range of analyses gives good coverage for the different dark-matter models studied. For new models, such as 2HDM with an additional pseudoscalar mediator, multiple regions of the parameter space are explored to probe the interplay between the masses, mixing angles and vacuum expectation values. For the 2HDM with an additional vector mediator, the resulting exclusion limits are further improved by combining the E_T^miss + Higgs analyses where the Higgs boson decays to a pair of photons or b-quarks. For the dark-energy models, two operators at the lowest order effective Lagrangian allow for interactions between SM particles and the new scalar particles. These operators are proportional to the mass or momenta of the SM particles, making them most sensitive to the E_T^miss + top–antitop or the E_T^miss + jet final states.

To date, no significant excess over the SM backgrounds has been observed in any of the ATLAS searches for dark matter or dark energy. Limits on the simplified models are set on the mediator-versus- dark-matter masses (figure 1), which can also be compared to those obtained by direct detection experiments. For the 2HDM with a pseudoscalar mediator, limits are placed on the heavy pseudoscalar versus the mediator masses, highlighting the complementarity of different channels in different regions of the parameter space (figure 2). Finally, collider limits on the scalar dark energy model (see Colliders join the hunt for dark energy) are also set and for the models studied improve over the limits obtained from astronomical observation and lab measurements by several orders of magnitude. With the full dataset of LHC collisions collected by ATLAS during Run 2, the sensitivity to these models will continue to improve.

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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.

The post The deepest clean lab in the world appeared first on CERN Courier.

Compelling cosmological and astrophysical evidence for the existence of dark matter suggests that there is a new world beyond the Standard Model of particle physics still to be discovered and explored. Yet, despite decades of effort, direct searches for dark matter at particle accelerators and underground laboratories alike have so far come up empty handed. This calls for new and improved methods to spot the mysterious substance thought to make up most of the matter in the universe.

Dark-matter searches using detectors based on liquefied noble gases such as xenon and argon have long demonstrated great discovery potential and continue to play a major role in the field. Such experiments use a large volume of material in which nuclei struck by a dark-matter particle would create a tiny burst of scintillation light, and the very low expected event rate requires that backgrounds are kept to a minimum. Searches employing argon detectors have a particular advantage because they can significantly reduce events from background sources, such as background from the abundant radioactive decays from detector materials and from electron scattering by solar neutrinos. That will leave the low-rate nuclear recoils induced by coherent scattering of atmospheric neutrinos as the sole residual background – the so-called “neutrino floor”.

Enter the Global Argon Dark Matter Collaboration (GADMC), which was formed in September 2017. Comprising more than 300 scientists from 15 countries and 60 institutions involved in four first-generation dark-matter experiments – ArDM at Laboratorio Subterráneo de Canfranc in Spain, DarkSide-50 at INFN’s Laboratori Nazionali del Gran Sasso (LNGS) in Italy, DEAP-3600 and MiniCLEAN at SNOLAB in Canada – GADMC is working towards the immediate deployment of a dark-matter detector called DarkSide-20k. The experiment would accumulate an exposure of 100 tonne × year and be followed by a much larger detector to collect more than 1000 tonne × year, both potentially with no instrumental background. These experiments promise the most complete exploration of the mass/parameter range of the present dark-matter paradigm.

Direct detection with liquid argon

One well-considered form of dark matter that matches astronomical measurements is weakly interacting massive particles (WIMPs), which would exist in our galaxy with defined numbers and velocities. In a dark-matter experiment employing a liquid-argon detector, such particles would collide with argon nuclei, causing them to recoil. These nuclear recoils produce ionised and excited argon atoms which, after a series of reactions, form short-lived argon dimers (weakly bonded molecules) that decay and emit scintillation light. The time profile of the scintillation light is significantly different from that created by argon-ionising events associated with radioactivity in the detector material, and has been shown to enable a strong rejection of background sources through a technique known as pulse-shape discrimination.

Located at LNGS, DarkSide-50 is the first physics detector of the DarkSide programme for dark-matter detection, with a fiducial mass of 50 kg. The experiment produced its first WIMP search results in December 2014 using argon harvested from the atmosphere and, in October the following year, reported the first ever WIMP search results using lower-radioactivity underground argon.

DarkSide-50 uses a detection scheme based on a dual-phase time projection chamber (TPC), which contains a small region of gaseous argon above a larger region of liquid argon (figure 1, left). In this configuration, secondary scintillation light, generated by ionisation electrons that drift up through the liquid region and are accelerated into the gaseous one, are used together with the primary scintillation light to look for a signal. Compared to single-phase detectors using only the pulse-shape discrimination technique, this search method requires even greater care in restricting the radioactive background through detector design and fabrication but provides excellent position resolution. For low-mass (<10 GeV/c²) WIMPs, the primary scintillation light is nearly absent, but the detectors remain sensitive to dark matter through the observation of the secondary scintillation light.

Argon-based dark-matter searches have had a number of successes in the past two years (figure 2). DarkSide-50 established the availability of an underground source of argon strongly depleted in the radioactive isotope ³⁹Ar, while DEAP-3600 (figure 3), the largest (3.3 tonnes) single-phase liquid-argon running experiment, provided the best value to date on the precision of pulse-shape discrimination for scintillation light, better than 1 part in 10⁹. In terms of measurements, DarkSide-50 released results from a 500-day detector exposure completely free of instrumental background and set the best exclusion limit yet for interactions of WIMPs with masses between 1.8 and 6 GeV/c². Similar results to those from Darkside-50 for the mass region above 40 GeV/c² were reported in the first paper from DEAP-3600, and results from a one-year exposure of DEAP-3600 with a fiducial mass of about 1000 kg are expected to be released in the near future.

High-sensitivity searches for WIMPs using noble-gas dual-phase TPC detectors are complementary to searches conducted at the Large Hadron Collider (LHC) in the mass region accessible at the current LHC energy of 13 TeV (which is limited to masses of a few TeV/c²) and can reach masses of 100 TeV/c² and beyond with very good sensitivity.

Leading limits

The best limits to date on high-mass WIMPs have been provided by xenon-based dual-phase TPCs – the leading result given by the recently released XENON1T exposure of 1 tonne × year (figure 2). In spite of a small residual background, they were able to exclude WIMP-nucleon spin-independent elastic-scatter cross-sections above 4.1 × 10^–47 cm² at 30 GeV/c² at 90% confidence level (CERN Courier July/August 2018 p9). Larger xenon detectors (XENONnT and DARWIN) are also planned by the same collaboration (CERN Courier March 2017 p35).

The next generation of xenon and argon detectors have the potential to extend the present sensitivity by about a factor of 10. But there is still a further factor of 10 to be increased before one reaches the neutrino floor – the ultimate level at which interactions of solar and atmospheric neutrinos with the detector material become the limiting background. This is where the GADMC liquid-argon detectors, which are designed to have pulse-shape discrimination capable of eliminating the background from electron scatters of solar neutrinos and internal radioactive decays, can provide an advantage.

GADMC envisages a two-step programme to explore high-mass dark matter. The first step, DarkSide-20k, has been approved for construction at LNGS by Italy’s National Institute for Nuclear Physics (INFN) and by the US National Science Foundation, with present and potentially future funding from Canada. Also a recognised experiment at CERN called RE-37, DarkSide-20k is designed to collect an exposure of 100 tonne × year in a period of five years (to be possibly extended to 200 tonne × year in 10 years), completely free of any instrumental background. The start of data taking is foreseen for 2022–2023. The second step of the programme will involve building an argon detector that is able to collect an exposure of more than 1000 tonne × year. SNOLAB in Canada is a strong candidate to host this second-stage experiment.

Argon can deliver the ultimate background-free search for dark matter, but that comes with extensive technological development. First and foremost, researchers need to extract and distill large volumes of the gas from underground deposits, as argon in the Earth’s atmosphere is unsuitable owing to its high content of the radioactive isotope ³⁹Ar. Second, the scintillation light has to be efficiently detected, requiring innovative photodetector R&D.

Sourcing pure argon

Focusing on the first need, atmospheric argon has a radioactivity of 1 Bq/kg, which is entirely caused by the activation of ⁴⁰Ar by cosmic rays. Given that the drift time of ionisation electrons over a length of 1 m is 1 ms, a dual-phase TPC detector reaches a complete pile-up condition (i.e. when the event rate exceeds the detector’s ability to read out the information), at a mass of 1 tonne. Scintillation-only detectors do not fare much better, and given that the scintillation lifetime is 10 μs, they are limited to detectors with a fiducial mass of a few tonnes. The argon road to dark matter has thus required early concentration on solving the problem of procuring large batches of argon that are much more depleted in ³⁹Ar than atmospheric argon is. The solution came through an unlikely path: the discovery that underground sources of CO₂ originating from Earth’s mantle carry sizable quantities of noble gases, in reservoirs where secondary production of ³⁹Ar is significantly suppressed.

As part of a project called Urania, funded by INFN, GADMC will soon deploy a plant that is able to extract underground argon at a rate of 250 kg per day from the same site in Colorado, US, where argon for DarkSide-50 was extracted. Argon from this underground source is more depleted in ³⁹Ar than atmospheric argon by a factor of at least 1400, making detectors of hundreds of tonnes possible for high-mass WIMP searches.

Not content with this gift of nature, another project called ARIA, also funded by INFN, by the Italian Ministry of University and Research (MIUR), and by the local government of the Sardinia region, is developing a further innovative plant to actively increase the depletion in ³⁹Ar. The plant will consist of a 350 m-tall cryogenic-distillation tower called Seruci-I, which is under construction in the Monte Sinni coal mine in Sardinia operated by the Carbosulcis mining company. Seruci-I will study the active depletion of ³⁹Ar by cryogenic distillation, which exploits the tiny dependence of the vapour pressure upon the atomic number. Seruci-I is expected to reach a production capacity of 10 kg of argon per day with a factor of 10 of ³⁹Ar depletion per pass. This is more than sufficient to deliver – starting from the gas extracted with the Urania underground source – a one-tonne ultra-depleted-argon target that could enable a leading programme of searches for low-mass dark matter. Seruci-I is also expected to perform strong chemical purification at the rate of several tonnes per day and will be used to perform the final stage of purification for the 50 tonne underground argon batch for DarkSide-20k as well as for GADMC’s final detector.

CERN plays an important role in DarkSide-20k by carrying out vacuum tests of the 30 modules for the Seruci-I column (figure 4) and by hosting the construction of the cryogenics for DarkSide-20k. At the time of its approval in 2017, DarkSide-20k was set to be deployed within a very efficient system of neutron and cosmic-ray rejection, based on that used for DarkSide-50 and featuring a large organic liquid scintillator detector hosted within a tank of ultrapure deionised water. But with the deployment of new organic scintillator detectors now discouraged at LNGS due to tightening environmental regulations, GADMC is completing the design of a large, and more environmentally friendly, liquid-argon detector for neutron and cosmic-ray rejection based on the cryostat technology developed at CERN to support prototype detector modules for the future Deep Underground Neutrino Experiment (DUNE) in the US.

Turning now to the second need of a background-free search for dark matter – the efficient detection of the scintillation light – researchers are focusing on perfecting existing technology to make low-radioactivity silicon photomultipliers (SiPMs) and using them to build large-area photosensors that are capable of replacing the traditional 3″ cryogenic photomultipliers. Plans for DarkSide-20k settled on the use of so-called NUV-HD-TripleDose SiPMs, designed by Fondazione Bruno Kessler of Trento, Italy, and produced by LFoundry of Avezzano, also in Italy. In the meantime, researchers at LNGS and other institutions succeeded in overcoming the huge capacitance per unit surface (50 pF/mm²) required to build photosensors that have an area of 25 cm² and deliver a signal-to-noise ratio of 15 or larger. A new INFN facility, the Nuova Officina Assergi, was designed to enable the high-throughput production of SiPMs to make such photosensors for DarkSide-20k and future detectors, and it is now under construction.

GADMC’s programme is complemented by a world-class effort to calibrate noble-liquid detectors for low-energy nuclear recoils created by low-mass dark matter. On the heels of the SCENE programme that took place at the University of Notre Dame Tandem accelerator in 2013–2015, the R&D programme, developed at the University of Naples Federico II and now installed at the INFN Laboratori Nazionali del Sud, plans to improve the characterisation of the argon response to nuclear recoils. Of special interest is the extension of measurements to 1 keV, in support of searches for low-mass dark matter, and the verification of the possible dependence of the nuclear-recoil signals upon the direction of the initial recoil momentum relative to the drift electric field, which would enable measurements below the neutrino floor. Directionality in argon has already been established for alpha particles, protons and deuterons, and its presence for nuclear recoils was hinted at by the last results of the SCENE experiment.

Although only recently established, GADMC is enthusiastically pursuing this long-term, staged approach to dark-matter detection in a background-free mode, which has great discovery potential extending all the way to the neutrino floor and perhaps beyond.

The post Defeating the background in the search for dark matter appeared first on CERN Courier.

A report from the CMS experiment

Dark energy and dark matter together make up about 95% of the universe, yet we do not know the origin, constituents, or dynamics (apart from gravity) of these substances. Various extensions of the Standard Model (SM) of particle physics predict the existence of new particles as dark-matter candidates. One such model posits the existence of “dark quarks” that are charged under a new QCD-like force. Like normal SM quarks, dark quarks are only found in bound states (such as the dark proton, a stable dark-matter candidate resembling the ordinary proton) and they can only interact with SM quarks via a mediator particle. The similarity between the mechanisms of hadron production in dark and SM QCD would provide a natural explanation for the puzzling closeness of the observed energy densities of dark and baryonic matter.

In an attempt to explain the nature of dark matter, the existence of dark quarks was recently investigated by the CMS collaboration. If dark-QCD mediators were produced in pairs in the CMS detector, their signature would be striking: each mediator particle would decay into one dark quark and one SM quark, both of which hadronise and produce multiple dark and SM pions, respectively. Dark pions can travel sizable distances in the detector before decaying into detectable SM particles. Therefore, the signature would be two ordinary jets originating from the proton–proton collision, and two “emerging jets” composed of multiple neutral particles that decay at a significant distance away from their origin. Signal events could exhibit large missing transverse momentum from decays beyond the acceptance of the CMS detector.

To identify emerging jets, the CMS analysis relies on two discriminants that quantify the displacement of a jet’s constituents from the collision point. One is based on the impact parameters of the tracks associated to the jet; the other is the fraction of a jet’s energy carried by tracks compatible with the primary vertex. Figure 1 shows an event display for an emerging-jet candidate, with two jets containing multiple displaced vertices and consequently tagged by the discriminants. Substantial background is expected from the decays of B mesons and baryons, whose lifetime makes them more likely to pass the discriminating criteria. To model this background, the analysis derives flavour-dependent misidentification probabilities for jets.

This first dedicated search for the emerging jet signature explores a broad dark-QCD parameter space with mediator masses between 0.4 and 2 TeV, dark-pion masses between 1 and 10 GeV, and dark-pion proper decay lengths between 1 mm and 100 cm. The observed number of events in the CMS data is consistent with the background-only expectation, excluding mediator particles with masses of 400–1250 GeV for dark-pion proper decay lengths between 5 and 225 mm (figure 2). While new data are being collected, the quest for dark matter at the LHC is broadening its scope towards new signatures.

The post CMS looks into the dark appeared first on CERN Courier.

Understanding the nature of dark matter is one of the most pressing problems in physics. This strangely nonreactive material is estimated, from astronomical observations, to make up 85% of all matter in the universe. The known particles of the Standard Model (SM) of particle physics, on the other hand, account for a paltry 15%.

Physicists have proposed many dark-matter candidates. Two in particular stand out because they arise in extensions of the SM that solve other fundamental puzzles, and because there are a variety of experimental opportunities to search for them. The first is the neutralino, which is the lightest supersymmetric partner of the SM neutral bosons. The second is the axion, postulated 40 years ago to solve the strong CP problem in quantum chromodynamics (QCD). While the neutralino belongs to the category of weakly interacting massive particles (WIMPs), the axion is the prime example of a very weakly interacting sub-eV particle (WISP).

Neutralinos as WIMPs have dominated the search for cold dark matter since the mid-1980s, when it was realised that massive particles with a cross section of the order of the weak interaction would result in precisely the right density to explain dark matter. There have been tremendous efforts to hunt for WIMPs both at hadron colliders, especially now at CERN’s Large Hadron Collider (LHC), and in large underground detectors, such as CDMS, CRESST, DARKSIDE, LUX, PandaX and XENON. However, up to now, no WIMP has been observed (CERN Courier July/August 2018 p9).

Very light bosons as WISPs are a firm prediction of models that solve problems of the SM by the postulation of a new symmetry which is broken spontaneously in the vacuum. Such extensions contain an additional scalar field with a potential shaped like a Mexican hat – similar to the Higgs potential in the SM (figure 1). This leads to spontaneous breaking of symmetry at a scale corresponding to the radius of the trough of the hat: excitations in the direction along the trough correspond to a light Nambu–Goldstone (NG) boson, while the excitation in the radial direction perpendicular to the trough corresponds to a heavy particle with a mass determined by the symmetry-breaking scale. The strengths of the interactions between such light bosons and regular SM particles are inversely proportional to the symmetry-breaking energy scale and are therefore very weak. Being light, very weakly interacting and cold due to their non-thermal production history, these particles qualify as natural WISP cold dark-matter candidates.

Primordial production

In fact, WISP dark matter is inevitably produced in the early universe. When the temperature in the primordial plasma drops below the symmetry-breaking scale, the boson fields are frozen at a random initial value in each causally-connected region. Later, they relax towards the minimum of their potential at zero fields and oscillate around it. Since there is no significant damping of these field oscillations via decays or interactions, the bosons behave as a very cold dark-matter fluid. If symmetry breaking occurs after the likely inflationary-expansion epoch of the universe (corresponding to a post-inflationary symmetry-breaking scenario), WISP dark matter would also be produced by the decay of topological defects from the realignment of patches of the universe with random initial conditions. A huge region in parameter space spanned by WISP masses and their symmetry-breaking scales can give rise to the observed dark-matter distribution.

The axion is a particularly well-motivated example of a WISP. It was proposed to explain the results of searches for a static electric dipole moment of the neutron, which would constitute a CP-violating effect of QCD. The size of this CP-violation, parameterised by the angle θ, is predicted to have an arbitrary value between –π and π, yet experiments show its absolute value to be less than 10^–10. If θ is replaced by a dynamical field, θ(x), as proposed by Peccei and Quinn in 1977, QCD dynamics ensures that the low-energy effective potential of the axion field has an absolute minimum at θ = 0. Therefore, in vacuum, the CP violating effects due to the θ angle in QCD disappear – providing an elegant solution to the strong CP problem. The axion is the inevitable particle excitation of θ(x), and its mass is determined by the unknown breaking scale of the global symmetry.

Lattice-QCD calculations performed last year precisely determined the temperature and corresponding time after the Big Bang when axion cold dark-matter could have formed as a function of the axion mass. It was found that, in the post-inflationary symmetry breaking scenario, the axion mass has to exceed 28 μeV; otherwise, the predicted amount of dark matter overshoots the observed amount. Taking into account the additional production of axion dark-matter from the decay of topological defects, an axion with a mass between 30 μeV and 10 meV may account for all of the dark matter in the universe. In the pre-inflationary symmetry breaking scenario, smaller masses are also possible.

Axions are not the only WISP species that could account for dark matter. There could be axion-like particles (ALPs), which are very similar to axions but do not solve the CP problem of QCD, or lightweight, weakly interacting, so-called hidden photons, for example. String theory suggests a plenitude of ALPs, which could have couplings to photons, leptons or light quarks.

Due to their tiny masses, WISPs might also be produced inside stars or alter the propagation of photons in the universe. Observations of stellar evolutions hint at such signals: red giants, helium-burning stars and white dwarfs seem to be experiencing unseen energy losses exceeding those expected from neutrino emission. Intriguingly, these anomalies can be explained in a unified manner by the existence of a sub-keV-mass axion or ALP with a coupling both to electrons and photons. Additionally, observations suggest that the propagation of TeV photons in the universe suffers less than expected from interactions with the extragalactic background light. This, in turn, could be explained by the conversion of photons into ALPs and back in astrophysical magnetic fields, interestingly with about the same axion–photon coupling strength as indicated by the observed stellar anomalies. Both effects have been known for almost 10 years. They are scientifically disputed, but a WISP explanation has not yet been excluded.

Experimental landscape

Most experiments searching for WISPs exploit their possible mixing with photons. Given the small masses and feeble interactions of axions and ALPs, however, building experiments that are sensitive enough to detect them is a considerable challenge. In the 1980s, Pierre Sikivie of the University of Florida in the US suggested a way forward based on the conversion of axions to photons: in a static magnetic field, the axion can “borrow” a virtual photon from the field and turn into a real photon (figure 2). Most experiments search for axions and ALPs in this way, with three main approaches being pursued: haloscopes, which look directly for dark-matter WISPs in the galactic halo of our Milky Way; helioscopes, which search for ALPs or axions emitted by the Sun; and laboratory experiments, which aim to generate and detect ALPs in a single setup.

Direct axion dark-matter searches differ in two aspects from WIMP dark-matter searches. First, axion dark matter would convert to photons, while WIMPs are scattered off matter. Second, the particle-number density for axion dark-matter, due to its low mass, is about 15 orders of magnitude larger than it is for WIMP dark matter. In fact, cold dark-matter axions and ALPs behave like a highly degenerate Bose–Einstein condensate with a de Broglie wavelength of the order of metres or kilometres for μeV and neV masses, respectively. Dark-matter axions and ALPs are thus much better pictured as a classical-field oscillation. In a magnetic field, they induce tiny electric-field oscillations with a frequency determined by the axion mass. If the de Broglie wavelength of the dark-matter axion is larger than the experimental setup, the tiny oscillations are spatially coherent in the experiment and can, in principle, be “easily” detected using a resonant microwave cavity tuned to the correct but unknown frequency. The sensitivity of such an experiment increases with the magnetic field strength squared, the volume of the cavity and its quality factor. Unfortunately, since the range of axion mass predicted by theories is huge, methods are required to tune the cavity to the frequency range corresponding to the respective axion masses.

This cavity approach has been the basis of most searches for axion dark-matter in the past decades, in particular the Axion Dark Matter Experiment (ADMX) at the University of Washington, US. Using a tuning rod inside the cavity to change the resonance frequency and, recently, by reducing noise in its detector system, the ADMX team has shown that it can reach axion dark-matter sensitivity. ADMX, which has been pioneering the field for two decades, is currently taking data and could find dark-matter axions at any time, provided the axion mass lies in the range 2–10 μeV. Meanwhile, the HAYSTAC collaboration at Yale University has very recently demonstrated that the same experimental approach can be expanded up to an axion mass of around 30 μeV. Since smaller-volume cavities (usually with lower quality factors) are needed to probe higher frequencies, however, the single-cavity approach is limited to axion masses below about 40 μeV. One novel method to probe higher masses is to use multiple matched cavities, as for example followed by the ADMX and the South Korean Center for Axion and Precision Physics.

Transitions

A different way to exploit the tiny electric-field oscillations from dark-matter axions in a strong magnetic field is to use transitions between materials with different dielectric constants: at surfaces, the axion-induced electromagnetic oscillations have a discontinuity, which is to be balanced by radiation from the surface. For a mirror with a surface area of 1 m² in a 10 T field, this would lead to an undetectable emission of around 10^–27 W if axions make up all of the dark matter. Furthermore, the emission power does not depend on the axion mass. In principle, if a parabolic mirror with a surface area of 10,000 m² could be magnetised with a 10 T field, the predicted radiation power (10^–23 W) could be focused and detected using state-of-the-art amplification techniques, but such an experiment seems impractical at present.

Alternatively, many magnetised dielectric discs in parallel can be placed in front of a mirror (figure 3): since the emission from all surfaces is coherent, constructive interference can boost the signal sensitivity for a given frequency range determined by the spacing between the discs. First studies performed in the past years at the Max Planck Institute for Physics in Munich have revealed that, for axion masses around 100 μeV, the sensitivity could be good enough to cover the predicted dark-matter axion mass range. The MADMAX (Magnetized Disc and Mirror Axion Experiment) collaboration, formed in October 2017, aims to use this approach to close the sensitivity gap in the well-motivated range for dark-matter axions with masses around 100 μeV. First design studies indicate that it is feasible to build a dipole magnet with the required properties using established niobium-titanium superconductor technology. As a first step, a prototype experiment is planned consisting of a booster with a reduced number of discs installed inside a prototype magnet. The experiment will be located at DESY in Hamburg, and first measurements sensitive to new ALPs parameter ranges are planned within the next few years.

Model independent searches

These direct searches for axion dark matter are very promising, but they are hampered at present by the unknown axion mass and rely on cosmological assumptions. Other, less-model dependent, experiments are required to further probe the existence of ALPs.

ALPs with energies of the order of a few keV could be produced in the solar centre, and could be detected on Earth by pointing a strong dipole magnet at the Sun: axions entering the magnet could be converted into photons in the same way they are in cavity experiments. The difference is that the Sun would emit relativistic axions with an energy spectrum very similar to the thermal spectrum in its core, so experiments need to detect X-ray photons and are sensitive to axion masses up to a maximum depending on the length of the apparatus (figure 4, top). This helioscope technique was brought to the fore by the CERN Axion Solar Telescope (CAST), shown in figure 5, which began operations in 2002 and has excluded axion masses above 0.02 eV. As a successor, the International Axion Observatory (IAXO) was formally founded in July 2017 and received an advanced grant from the European Research Council earlier this year. The near-term goal of the collaboration is to build a scaled-down prototype version of the experiment, called babyIAXO, which is under discussion for possible location at DESY.

The third, laboratory-based, approach to search for WISPs also aims to generate and detect ALPs without any model assumption. In the first section of such an experiment, laser light is sent through a strong magnetic field so that ALPs might be generated via interactions of optical photons with the magnetic field. The second section is separated from the first one by a light-tight wall that can only be penetrated by ALPs. These would stream through a strong magnetic field behind the wall, allowing them to be re-converted into photons and giving the impression of light shining through a wall (figure 4, bottom).

Such experiments have been performed since the early 1990s, but no hint for any ALP has shown up. Today, the most advanced project in this laboratory-based category is ALPS II, currently being set up at DESY (figure 6). This experiment will use two optical resonators implemented into the apparatus to “recycle” the light before and increase the re-conversion probability of ALPs into photons behind the wall, allowing ALPS II to reach sensitivities beyond ALP–photon coupling limits from helioscopes. It also plans to use 20 dipoles from the former HERA collider, each of which has to be mechanically straightened, to generate the magnetic field.

Gaining momentum

Searches for very lightweight axions and ALPs, potentially explaining all of the dark matter around us, are strongly gaining momentum. CERN has been supporting such activities in the past (with solar-axion and dark-matter searches at CAST, and the OSQAR and CROWS experiments using the shining-light-through-walls approach) and is also involved in the R&D phase for next-generation experiments such as IAXO (CERN Courier September 2014 p17). With the new initiatives of MADMAX and IAXO, both of which could be located at DESY, and the ALPS II experiment under construction there, experimental axion physics in Europe is set to probe a large fraction of a well-motivated parameter space (figure 7). In addition to complementary experiments worldwide, the next 10 years or so should shine a bright light on WISPs as the solution to the dark-matter riddle, with thrilling data runs expected to start in the early 2020s.

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On 28 May, the world’s largest and most sensitive detector for direct searches of dark matter in the form of weakly interacting massive particles (WIMPs) released its latest results. XENON1T, a 3D-imaging liquid-xenon time projection chamber located at Gran Sasso National Laboratory in Italy, reported its first results last year (CERN Courier July/August 2017 p10). Now, the 165-strong international collaboration has presented the results from an unprecedentedly large exposure of approximately one tonne × year.

The results are based on 1300 kg out of the total 2000 kg active xenon target and 279 days of data-taking, improving the sensitivity by almost four orders of magnitude compared to XENON10 (the first detector of the XENON dark-matter project, which has been hosted at Gran Sasso since 2005). The data are consistent with background expectations, and place the most stringent limit yet on spin-independent interactions of WIMPs with ordinary matter for a WIMP mass higher than 6 GeV/c².

XENON1T spokesperson Elena Aprile of Columbia University in the US describes the result as a milestone in dark-matter exploration. “Showing the result after a one tonne × year exposure was important in a field that moves fast,” she explains. “It is also clear from the new result that we will win faster with a yet-larger mass and lower radon background, which is why we are now pushing the XENONnT upgrade.”

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The origin of solar flares, powerful bursts of radiation appearing as sudden flashes of light, has puzzled astrophysicists for more than a century. The temperature of the Sun’s corona, measuring several hundred times hotter than its surface, is also a long-standing enigma.

A new study suggests that the solution to these solar mysteries is linked to a local action of dark matter (DM). If true, it would challenge the traditional picture of DM as being made of weakly interacting massive particles (WIMPs) or axions, and suggest that DM is not uniformly distributed in space, as is traditionally thought.

The study is not based on new experimental data. Rather, lead author Sergio Bertolucci, a former CERN research director, and collaborators base their conclusions on freely available data recorded over a period of decades by geosynchronous satellites. The paper presents a statistical analysis of the occurrences of around 6500 solar flares in the period 1976–2015 and of the continuous solar emission in the extreme ultraviolet (EUV) in the period 1999–2015. The temporal distribution of these phenomena, finds the team, is correlated with the positions of the Earth and two of its neighbouring planets: Mercury and Venus. Statistically significant (above 5σ) excesses of the number of flares with respect to randomly distributed occurrences are observed when one or more of the three planets find themselves in a slice of the ecliptic plane with heliocentric longitudes of 230°–300°. Similar excesses are observed in the same range of longitudes when the solar irradiance in the EUV region is plotted as a function of the positions of the planets.

If true, our findings will provide a totally different view about dark matter

Konstantin Zioutas

These results suggest that active-Sun phenomena are not randomly distributed, but instead are modulated by the positions of the Earth, Venus and Mercury. One possible explanation, says the team, is the existence of a stream of massive DM particles with a preferred direction, coplanar to the ecliptic plane, that is gravitationally focused by the planets towards the Sun when one or more of the planets enter the stream. Such particles would need to have a wide velocity spectrum centred around 300 km s^–1 and interact with ordinary matter much more strongly than typical DM candidates such as WIMPs. The non-relativistic velocities of such DM candidates make planetary gravitational lensing more efficient and can enhance the flux of the particles by up to a factor of 10⁶, according to the team.

Co-author Konstantin Zioutas, spokesperson for the CAST experiment at CERN, accepts that this interpretation of the solar and planetary data is speculative – particularly regarding the mechanism by which a temporarily increased influx of DM actually triggers solar activity. However, he says, the long persisting failure to detect the ubiquitous DM might be due to the widely assumed small cross-section of its constituents with ordinary matter, or to erroneous DM modelling. “Hence, the so-far-adopted direct-detection concepts can lead us towards a dead end, and we might find that we have overlooked a continuous communication between the dark and the visible sector.”

Models of massive DM streaming particles that interact strongly with normal matter are few and far between, although the authors suggest that “antiquark nuggets” are best suited to explain their results. “In a few words, there is a large ‘hidden’ energy in the form of the nuggets,” says Ariel Zhitnitsky, who first proposed the quark-nugget dark-matter model in 2003. “In my model, this energy can be precisely released in the form of the EUV radiation when the anti-nuggets enter the solar corona and get easily annihilated by the light elements present in such a highly ionised environment.”

The study calls for further investigation, says researchers. “It seems that the statistical analysis of the paper is accurate and the obtained results are rather intriguing,” says Rita Bernabei, spokesperson of the DAMA experiment, which for the first time in 1998 claimed to have detected dark matter in the form of WIMPs on the basis of an observed seasonal modulation of a signal in their scintillation detector. “However, the paper appears to be mostly hypothetical in terms of this new type of dark matter.”

The team now plans to produce a full simulation of planetary lensing taking into account the simultaneous effect of all the planets in the solar system, and to extend the analysis to include sunspots, nano-flares and other solar observables. CAST, the axion solar telescope at CERN, will also dedicate a special data-taking period to the search for streaming DM axions.

“If true, our findings will provide a totally different view about dark matter, with far-reaching implications in particle and astroparticle physics,” says Zioutas. “Perhaps the demystification of the Sun could lead to a dark-matter solution also.”

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A report from the CMS experiment

The quest to find dark matter (DM) has inspired new searches at CMS, specifically looking for interactions between DM and quarks mediated by particles of previously unexplored mass and width. If the DM mediator is a leptophobic vector resonance coupling only to quarks with a universal coupling g′_q, for instance, its decay also produces a dijet resonance (see bottom figure, left) and the value of g′_q determines the width of the mediator.

CMS has traditionally searched for peaks from narrow resonances on the steeply falling dijet invariant mass spectrum predicted by QCD. This search has been updated with the full 2016 data set and limits set on a DM mediator, constraining g′_q for resonances with a mass between 0.6 and 3.7 TeV and width less than 10% of the resonance mass. Two additional dijet searches have now been released: a boosted-dijet search sensitive to lower mediator masses, and an angular-distribution search sensitive to larger couplings and widths.

The first search gets round the limitations of the narrow-resonance search, which only applies above a minimum mass that satisfies the dijet trigger requirements, by requiring resonance production in association with a jet (bottom figure, middle). In such events the resonance is highly boosted and by analysing the jet substructure the QCD background can be highly suppressed, making the search sensitive in a lower mass range. The mass spectrum of the single jet was used to search for resonances over a mass range of 50–300 GeV, and the corresponding constraints on g′_q and the mediator width from boosted dijets explore the lowest mediator masses.

For large couplings and widths, the sensitivity of searches for dijet resonance peaks is strongly reduced. However, a search for a very wide resonance can be performed by studying dijet angular distributions such as the scattering angle between the incoming and outgoing partons. These distributions differ significantly, depending on whether a new particle is produced in the s-channel or from the QCD dijet background, which is dominated by t-channel production (bottom figure, right). Being sensitive to both large-width resonances and non-resonant signatures, this search also sets lower limits on the scale of contact interactions that may arise from quark compositeness in the range 6–22 TeV, as well as signatures of large extra dimensions and quantum black holes. The same search, when interpreted in the context of a vector mediator coupling to DM, excludes values of g′_q greater than 0.6, corresponding to widths higher than 20% of the resonance mass, and extending to mediator masses as high as 5 TeV.

Using these three complementary techniques, CMS has now explored a large range in mass, coupling and width, extending the scope of searches for DM mediators. The expected volume of data from the LHC in upcoming years will allow CMS to extend this reach even further, with the study of three-jet topologies allowing the uncovered mass range of 300–600 GeV to be explored.

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Researchers from the XENON1T dark-matter experiment at Gran Sasso National Laboratory in Italy reported their first results at the 13th Patras Workshop on Axions, WIMPs and WISPs, held in Thessaloniki from 15–19 May (see “Exploring axions and WIMPs in Greece” in Faces & Places). XENON1T is the first tonne-scale detector of its kind and is designed to search for WIMP dark matter by measuring nuclear recoils from WIMP–nucleus scattering. Continuing the programme of the previous XENON10 and XENON100 detectors, the new apparatus contains 3200 kg of ultra-pure liquid xenon (LXe) – 20 times more than its predecessor – in a dual-phase xenon time projection chamber (TPC) to detect nuclear recoils. The TPC encloses about 2000 kg of LXe, while another 1200 kg provides additional shielding.

The experiment started collecting data in November 2016. A blind search based on 34.2 live days of data acquired until January 2017, when earthquakes in the region temporarily suspended the run, revealed the data to be consistent with the background-only hypothesis. This allowed the collaboration to derive the most stringent exclusion limits on the spin-independent WIMP–nucleon interaction cross-section for WIMP masses above 10 GeV/c², with a minimum of 7.7 × 10^–47 cm² for 35 GeV/c² WIMPs at 90% confidence level.

These first results demonstrate that XENON1T has the lowest low-energy background level ever achieved by a dark-matter experiment, with the intrinsic background from krypton and radon reduced to unprecedented low levels. The sensitivity of XENON1T will continue to improve as the experiment records data until the end of 2018, when the collaboration plans to upgrade to a larger TPC due to come online by 2019. Several other experiments, such as PANDA-X and LUX-ZEPLIN, are also competing for the first WIMP detection.

“With our experiment working so beautifully, even exceeding our expectations, it is really exciting to have data in hand to further explore one of the most exciting secrets we have in physics: the nature of dark matter,” says XENON spokesperson Elena Aprile of Columbia University in the US.

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In a paper published in Nature Physics, the CERN Axion Solar Telescope (CAST) has reported important new exclusion limits on coupling of axions to photons. Axions are hypothetical particles that interact very weakly with ordinary matter and therefore are candidates to explain dark matter. They were postulated decades ago to solve the “strong CP” problem in the Standard Model (SM), which concerns an unexpected time-reversal symmetry of the nuclear forces. Axion-like particles, unrelated to the strong-CP problem but still viable dark-matter candidates, are also predicted by several theories of physics beyond the SM, notably string theory.

A variety of Earth- and space-based observatories are searching possible locations where axions could be produced, ranging from the inner Earth to the galactic centre and right back to the Big Bang. CAST looks for solar axions using a “helioscope” constructed from a test magnet originally built for the Large Hadron Collider. The 10 m-long superconducting magnet acts like a viewing tube and is pointed directly at the Sun: solar axions entering the tube would be converted by its strong magnetic field into X-ray photons, which would be detected at either end of the magnet. Starting in 2003, the CAST helioscope, mounted on a movable platform and aligned with the Sun with a precision of about 1/100th of a degree, has tracked the movement of the Sun for an hour and a half at dawn and an hour and a half at dusk, over several months each year.

In the latest work, based on data recorded between 2012 and 2015, CAST finds no evidence for solar axions. This has allowed the collaboration to set the best limits to date on the strength of the coupling between axions and photons for all possible axion masses to which CAST is sensitive. The limits concern a part of the axion parameter space that is still favoured by current theoretical predictions and is very difficult to explore experimentally, allowing CAST to encroach on more restrictive constraints set by astrophysical observations. “Even though we have not been able to observe the ubiquitous axion yet, CAST has surpassed even the sensitivity originally expected, thanks to CERN’s support and unrelenting work by CASTers,” says CAST spokesperson Konstantin Zioutas. “CAST’s results are still a point of reference in our field.”

The experience gained by CAST over the past 15 years will help physicists to define the detection technologies suitable for a proposed, much larger, next-generation axion helioscope called IAXO. Since 2015, CAST has also broadened its research at the low-energy frontier to include searches for dark-matter axions and candidates for dark energy, such as solar chameleons.

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Gravitational-lensing measurements indicate that clusters of galaxies are surrounded by large halos of dark matter. By studying the distribution and colour of galaxies inside galaxy clusters using data from the Sloan Digital Sky Survey (SDSS), researchers have now measured a new feature of the shape of these halos. The results show that the density of dark matter in a halo does not gradually fall off with distance, as might be expected, but instead exhibits a sharp edge.

According to the standard cosmological model, dark-matter halos are the result of small perturbations in the density of the early universe. Over time, and under the influence of gravity, these perturbations grew into large dense clumps that affect surrounding matter: galaxies in the vicinity of a halo will initially all move away due to the expansion of the universe, but gravity eventually causes the matter to fall towards and then orbit the halo. Studying the movements of the matter inside halos therefore provides an indirect measurement of the interaction between normal and dark matter, allowing researchers to probe new physics such as dark-matter interactions, dark energy and modifications to gravity.

Using the SDSS galaxy survey, Bhuvnesh Jain and Eric Baxter from the University of Pennsylvania and colleagues at other institutes report new evidence for an edge-like feature in the density profile of galaxies within a halo. The large amount of SDSS data available allowed a joint analysis of thousands of galaxy clusters each containing thousands of galaxies, revealing an edge inside clusters in agreement with simulations based on “splash-back” models. The edge is associated with newly accreted matter which, after falling into the halo, slows down as it reaches the extremity of its elliptical orbit before falling back towards the halo centre. As the matter “splashes back” it slows down, which leads to a build-up of matter at the edge of the halo and a steep fall-off in the amount of matter right outside this radius.

The authors found additional evidence for the edge by studying the colour of the galaxies. Since new stars that formed in hydrogen-rich regions are more bright in the blue part of the spectrum, galaxies with large amounts of new-star formation are more blue than those with little star formation. As a galaxy travels through a cluster, different mechanisms can strip it of the gasses required to form new blue stars, reducing star formation and making the galaxy appear more red. Models therefore predict galaxies still in the process of falling into the halo to be more blue, while those which already passed the edge and are in orbit have started to become red – exactly as data from the SDSS galaxy survey showed.

A range of ongoing and new galaxy surveys – such as Hyper Suprime-Cam, Dark Energy Survey, Kilo-Degree Survey and the Large Synoptic Survey Telescope – will measure the galaxy clusters in more detail. Using additional information on the shape of the clusters, says the team, it is possible to study both the standard physics of how galaxies interact with the cluster and the possible unknown physics of what the nature of dark matter and gravity is.

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New observations using ESO’s Very Large Telescope (VLT) in Chile indicate that massive, star-forming galaxies in the early universe were dominated by normal, baryonic matter. This is in stark contrast to present-day galaxies, where the effects of dark matter on the rotational velocity of spiral galaxies seem to be much greater. The surprising result, published in Nature by an international team of astronomers led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics in Germany, suggests that dark matter was less influential in the early universe than it is today.

Whereas normal matter in the cosmos can be viewed as brightly shining stars, glowing gas and clouds of dust, dark matter does not emit, absorb or reflect light. This elusive, transparent matter can only be observed via its gravitational effects, one of which is a higher speed of rotation in the outer parts of spiral galaxies. The disc of a spiral galaxy rotates with a velocity of hundreds of kilometres per second, making a full revolution in a period of hundreds of millions of years. If a galaxy’s mass consisted entirely of normal matter, the sparser outer regions should rotate more slowly than the dense regions at the centre. But observations of nearby spiral galaxies show that their inner and outer parts actually rotate at approximately the same speed.

It is widely accepted that the observed “flat rotation curves” indicate that spiral galaxies contain large amounts of non-luminous matter in a halo surrounding the galactic disc. This traditional view is based on observations of numerous galaxies in the local universe, but is now challenged by the latest observations of galaxies in the distant universe. The rotation curve of six massive, star-forming galaxies at the peak of galaxy formation, 10 billion years ago, was measured with the KMOS and SINFONI instruments on the VLT, and the results are intriguing. Unlike local spiral galaxies, the outer regions of these distant galaxies seem to be rotating more slowly than regions closer to the core – suggesting they contain less dark matter than expected. The same decreasing velocity trend away from the centres of the galaxies is also found in a composite rotation curve that combines data from around 100 other distant galaxies, which have too weak a signal for an individual analysis.

Genzel and collaborators identify two probable causes for the unexpected result. Besides a stronger dominance of normal matter with the dark matter playing a much smaller role, they also suggest that early disc galaxies were much more turbulent than the spiral galaxies we see in our cosmic neighbourhood. Both effects seem to become more marked as astronomers look further back in time into the early universe. This suggests that three to four billion years after the Big Bang, the gas in galaxies had already efficiently condensed into flat, rotating discs, while the dark-matter halos surrounding them were much larger and more spread out. Apparently it took billions of years longer for dark matter to condense as well, so its dominating effect is only seen on the rotation velocities of galaxy discs today.

This explanation is consistent with observations showing that early galaxies were much more gas-rich and compact than today’s galaxies. Embedded in a wider dark-matter halo, their rotation curves would be only weakly influenced by its gravity. It would be therefore interesting to explore whether the suggestion of a slow condensation of dark-matter halos could help shed light on this mysterious component of the universe.

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Based on the current lambda-cold-dark-matter (ΛCDM) model of cosmology – which has only two ingredients: general relativity with a nonzero cosmological constant and cold dark matter – we identify at this time three dominant components of the universe: normal baryonic matter, which makes up only 5% of the total energy density; dark matter (27%); and dark energy (68%). This model is extremely successful in fitting observations, such as the Planck mission’s measurements of the cosmic microwave background, but it gives no clues about the nature of the dark-matter or dark-energy components. It should also be noted that the assumption of a nonzero cosmological constant, implying a nonzero vacuum energy density, leads to what has been called the worst prediction ever made in physics: its value as measured by astronomers falls short of what is predicted by the Standard Model for particle physics by well over 100 orders of magnitude.

It is only by combining several complementary probes that the source of the acceleration of the universe can be understood.

Depending on what form it takes, dark energy changes the dynamical evolution during the expansion history of the universe as predicted by cosmological models. Specifically, dark energy modifies the expansion rate as well as the processes by which cosmic structures form. Whether the acceleration is produced by a new scalar field or by modified laws of gravity will impact differently on these observables, and the two effects can be decoupled using several complementary cosmological probes. Type 1a supernovae and baryon acoustic oscillations (BAO) are very good probes of the expansion rate, for instance, while gravitational lensing and peculiar velocities of galaxies (as revealed by their redshift) are very good probes of gravity and the growth rate of structures (see panel “The geometry of the universe” below). It is only by combining several complementary probes that the source of the acceleration of the universe can be understood. The changes are extremely small and are currently undetectable at the level of individual galaxies, but by observing many galaxies and treating them statistically it is possible to accurately track the evolution and therefore get a handle on what dark energy physically is. This demands new observing facilities capable of both measuring individual galaxies with high precision and surveying large regions of the sky to cover all cosmological scales.

Euclid science parameters

Euclid is a new space-borne telescope under development by the European Space Agency (ESA). It is a medium-class mission of ESA’s Cosmic Vision programme and was selected in October 2011 as the first-priority cosmology mission of the next decade. Euclid will be launched at the end of 2020 and will measure the accelerating expansion of our universe from the time it kicked in around 10 billion years ago to our present epoch, using four cosmological probes that can explore both dark-energy and modified-gravity models. It will capture a 3D picture of the distribution of the dark and baryonic matter from which the acceleration will be measured to per-cent-level accuracy, and measure possible variations in the acceleration to 10% accuracy, improving our present knowledge of these parameters by a factor 20–60. Euclid will observe the dynamical evolution of the universe and the formation of its cosmic structures over a sky area covering more than 30% of the celestial sphere, corresponding to about five per cent of the volume of the observable universe.

The dark-matter distribution will be probed via weak gravitational-lensing effects on galaxies. Gravitational lensing by foreground objects slightly modifies the shape of distant background galaxies, producing a distortion that directly reveals the distribution of dark matter (see panel “Tracking cosmic structure” below). The way such lensing changes as a function of look-back time, due to the continuing growth of cosmic structure from dark matter, strongly depends on the accelerating expansion of the universe and turns out to be a clear signature of the amount and nature of dark energy. Spectroscopic measurements, meanwhile, will enable us to determine tiny local deviations of the redshift of galaxies from their expected value derived from the general cosmic expansion alone (see image below). These deviations are signatures of peculiar velocities of galaxies produced by the local gravitational fields of surrounding massive structures, and therefore represent a unique test of gravity. Spectroscopy will also reveal the 3D clustering properties of galaxies, in particular baryon acoustic oscillations.

Together, weak-lensing and spectroscopy data will reveal signatures of the physical processes responsible for the expansion and the hierarchical formation of structures and galaxies in the presence of dark energy. A cosmological constant, a new dark-energy component or deviations to general relativity will produce different signatures. Since these differences are expected to be very small, however, the Euclid mission is extremely demanding scientifically and also represents considerable technical, observational and data-processing challenges.

By further analysing the Euclid data in terms of power spectra of galaxies and dark matter and a description of massive nonlinear structures like clusters of galaxies, Euclid can address cosmological questions beyond the accelerating expansion. Indeed, we will be able to address any topic related to power spectra or non-Gaussian properties of galaxies and dark-matter distributions. The relationship between the light- and dark-matter distributions of galaxies, for instance, can be derived by comparing the galaxy power spectrum as derived from spectroscopy with the dark-matter power spectrum as derived from gravitational lensing. The physics of inflation can then be explored by combining the non-Gaussian features observed in the dark-matter distribution in Euclid data with the Planck data. Likewise, since Euclid will map the dark-matter distribution with unprecedented accuracy, it will be sensitive to subtle features produced by neutrinos and thereby help to constrain the sum of the neutrino masses. On these and other topics, Euclid will provide important information to constrain models.

Euclid’s science objectives translate into stringent performance requirements.

The definition of Euclid’s science cases, the development of the scientific instruments and the processing and exploitation of the data are under the responsibility of the Euclid Consortium (EC) and carried out in collaboration with ESA. The EC brings together about 1500 scientists and engineers in theoretical physics, particle physics, astrophysics and space astronomy from around 200 laboratories in 14 European countries, Canada and the US. Euclid’s science objectives translate into stringent performance requirements. Mathematical models and detailed complete simulations of the mission were used to derive the full set of requirements for the spacecraft pointing and stability, the telescope, scientific instruments, data-processing algorithms, the sky survey and the system calibrations. Euclid’s performance requirements can be broadly grouped into three categories: image quality, radiometric and spectroscopic performance. The spectroscopic performance in particular puts stringent demands on the ground-processing algorithms and demands a high level of control over cleanliness during assembly and launch.

Dark-energy payload

The Euclid satellite consists of a service module (SVM) and a payload module (PLM), developed by ESA’s industrial contractors Thales Alenia Space of Turin and Airbus Defence and Space of Toulouse, respectively. The two modules are substantially thermally and structurally decoupled to ensure that the extremely rigid and cold (around 130 K) optical bench located in the PLM is not disturbed by the warmer (290 K±20 K) and more flexible SVM. The SVM comprises all the conventional spacecraft subsystems and also hosts the instrument’s warm electronics units. The Euclid image-quality requirements demand very precise pointing and minimal “jitter”, while the survey requirements call for fast and accurate movements of the satellite from one field to another. The attitude and orbit control system consists of several sensors to provide sub-arc-second stability during an exposure time, and cold gas thrusters with micronewton resolution are used to actuate the fine pointing. Three star trackers provide the absolute inertial attitude accuracy. Since the trackers are mounted on the SVM, which is separate from the telescope structure and thus subject to thermo-elastic deformation, the fine guidance system is located on the same focal plane of the telescope and endowed with absolute pointing capabilities based on a reference star catalogue.

The PLM is designed to provide an extremely stable detection system enabling the sharpest possible images of the sky. The size of the point spread function (PSF), which is the image of a point source such as an unresolved star, closely resembles the Airy disc, the theoretical limit of the optical system. The PSF of Euclid images is comparable to those of the Hubble space telescope’s, considering Euclid’s smaller primary mirror, and is more than three times smaller compared with what can be achieved by the best ground-based survey telescopes under optimum viewing conditions. The telescope is composed of a 1.2 m-diameter three-mirror “anastigmatic Korsch” arrangement that feeds two instruments: a wide-field visible imager (VIS) for the shape measurement of galaxies, and a near-infrared spectrometer and photometer (NISP) for their spectroscopic and photometric redshift measurements. An important PLM design driver is to maintain a high and stable image quality over a large field of view. Building on the heritage of previous European high-stability telescopes such as Gaia, which is mapping the stars of the Milky Way with high precision, all mirrors, the telescope truss and the optical bench are made of silicon carbide, a ceramic material that combines extreme stiffness with very good thermal conduction. The PLM structure is passively cooled to a stable temperature of around 130 K, and a secondary mirror mechanism will be employed to refocus the telescope image on the VIS detector plane after launch and cool down.

The VIS instrument receives light in one broad visible band covering the wavelength range 0.55–0.90 μm. To avoid additional image distortions, it has no imaging optics of its own and is equipped with a camera made up of 36 4 k × 4 k-pixel CCDs with a pixel scale of 0.1 arc second that must be aligned to a precision better than 15 μm over a distance of 30 cm. Pixel-wise, the VIS camera is the second largest camera that will be flown in space after Gaia’s and will produce the largest images ever generated in space. Unlike Gaia, VIS will compress and transmit all raw scientific images to Earth for further data processing. The instrument is capable of measuring the shapes of about 55,000 galaxies per image field of 0.5 square degrees. The NISP instrument, on the other hand, provides near-infrared photometry in the wavelength range 0.92–2.0 μm and has a slit-less spectroscopy mode equipped with three identical grisms (grating prisms) covering the wavelength range 1.25–1.85 μm. The grisms are mounted in different orientations to separate overlapping spectra of neighbouring objects, and the NISP device is capable of delivering redshifts for more than 900 galaxies per image field. The NISP focal plane is equipped with 16 near infrared HgCdTe detector arrays of 2 k × 2 k pixels with 0.3 arcsec pixels, which represents the largest near-infrared focal plane ever built for a space mission.

The exquisite accuracy and stability of Euclid’s instruments will provide certainty that any observed galaxy-shape distortions are caused by gravitational lensing and are not a result of artefacts in the optics. The telescope will deliver a field of view of more than 0.5 square degrees, which is an area comparable to two full Moons, and the flat focal plane of the Korsch configuration places no extra requirements on the surface shape of the sensors in the instruments. As the VIS and NISP instruments share the same field of view, Euclid observations can be carried out through both channels in parallel. Besides the Euclid satellite data, the Euclid mission will combine the photometry of the VIS and NISP instruments with complementary ground-based observations from several existing and new telescopes equipped with wide-field imaging or spectroscopic instruments (such as CFHT, ESO/VLT, Keck, Blanco, JST and LSST). These combined data will be used to derive an estimate of redshift for the two billion galaxies used for weak lensing, and to decouple coherent weak gravitational-lensing patterns from intrinsic alignments of galaxies. Organising the ground-based observations over both hemispheres and making these data compatible with the Euclid data turns out to be a very complex operation that involves a huge data volume, even bigger than the Euclid satellite data volume.

Ground control

One Euclid field of 0.5 square degrees will generate 520 Gb/day of VIS compressed data and 240 Gb/day of NISP compressed data, and one such field is obtained in an observing period lasting about 1 hour and 15 minutes. All raw science data are transmitted to the ground via a high-density link. Even though the nominal mission will last for six years, mapping out the 36% of the sky at the required sensitivity and accuracy within this time involves large amounts of data to be transmitted at a rate of around 850 Gb/day during just four hours of contact with the ground station. The complete processing pipeline from Euclid’s raw data to the final data products is a large IT project involving a few hundred software engineers and scientists, and has been broken down into functions handled by almost a dozen separate expert groups. A highly varied collection of data sets must be homogenised for subsequent combination: data from different ground and space-based telescopes, visible and near-infrared data, and slit-less spectroscopy. Very precise and accurate shapes of galaxies are measured, giving two orders of magnitude improvement with respect to current analyses.

Based on the current knowledge of the Euclid mission and the present ground-station development, no showstoppers have been identified. Euclid should meet its performance requirements at all levels, including the design of the mission (a survey of 15,000 square degrees in less than six years) and for the space and ground segments. This is very encouraging and most promising, taking into account the multiplicity of challenges that Euclid presents.

On the scientific side, the Euclid mission meets the precision and accuracy requested to characterise the source of the accelerating expansion of the universe and decisively reveal its nature. On the technical side, there are difficult challenges to be met in achieving the required precision and accuracy of galaxy-shape, photometric and spectroscopic redshift measurements. Our current knowledge of the mission provides a high degree of confidence that we can overcome all of these challenges in time for launch.

The geometry of the universe

The evolution of structure is seeded by quantum fluctuations in the very early universe, which were amplified by inflation. These seeds grew to create the cosmic microwave background (CMB) anisotropies after approximately 100,000 years and eventually the dark-matter distribution of today. In the same way that supernovae provide a standard candle for astronomical observations, periodic fluctuations in the density of the visible matter called baryon acoustic oscillations (BAO) provide a standard cosmological length scale that can be used to understand the impact of dark energy. By comparing the distance of a supernova or structure with its measured redshift, the geometry of the universe can be obtained.

Hydrodynamical cosmological simulations of a ΛCDM universe at three different epochs (left-to-right, image left), corresponding to redshift z = 6, z = 2 and our present epoch. Each white point represents the concentration of dark matter, gas and stars, the brightest regions being the densest. The simulation shows the growth rate of structure and the formation of galaxies, clusters of galaxies, filaments and large-scale structures over cosmic time. Euclid uses the large-scale structures made out of matter and dark matter as a standard yardstick: starting from the CMB, we assume that the typical scale of structures (or the peak in the spatial power spectrum) increases proportionally with the expansion of the universe. Euclid will determine the typical scale as a function of redshift by analysing power spectra at several redshifts from the statistical analysis of the dark-matter structures (using the weak lensing probe) or the ordinary matter structures based on the spectroscopic redshifts from the BAO probe. The structures will evolve with redshift also due to the properties of gravity. Information on the growth of structure at different scales in addition to different redshifts is needed to discriminate between models of dark energy and modified gravity

Tracking cosmic structure

Gravitational-lensing effects produced by cosmic structures on distant galaxies (right). Numerical simulations (below) show the distribution of dark matter (filaments and clumps with brightness proportional to their mass density) over a line of sight of one billion light-years. The yellow lines show how light beams emitted by distant galaxies are deflected by mass concentrations located along the line of sight. Each deflection slightly modifies the original shape of the lensed galaxies, increasing their original intrinsic ellipticity by a small amount.

Since all distant galaxies are lensed, all galaxies eventually show a coherent ellipticity pattern projected on the sky that directly reveals the projected distribution of dark matter and its power spectrum. The 3D distribution of dark matter can then be reconstructed by slicing the universe into redshift bins and recovering the ellipticity pattern at each redshift. The growth rate of cosmic structures derived from this inversion process strongly depends on the nature of dark energy and gravity, and will be detected by the outstanding image quality of Euclid’s VIS instrument.

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Astronomers have long contemplated the possibility that there may be forms of matter in the universe that are imperceptible, either because they are too far away, too dim or intrinsically invisible. Lord Kelvin was perhaps the first, in 1904, to attempt a dynamical estimate of the amount of dark matter in the universe. His argument was simple yet powerful: if stars in the Milky Way can be described as a gas of particles acting under the influence of gravity, one can establish a relationship between the size of the system and the velocity dispersion of the stars. Henri Poincaré was impressed by Kelvin’s results, and in 1906 he argued that since the velocity dispersion predicted in Kelvin’s estimate is of the same order of magnitude as that observed, “there is no dark matter, or at least not so much as there is of shining matter”.

The Swiss–US astronomer Fritz Zwicky is arguably the most famous and widely cited pioneer in the field of dark matter. In 1933, he studied the redshifts of various galaxy clusters and noticed a large scatter in the apparent velocities of eight galaxies within the Coma Cluster. Zwicky applied the so-called virial theorem – which establishes a relationship between the kinetic and potential energies of a system of particles – to estimate the cluster’s mass. In contrast to what would be expected from a structure of this scale – a velocity dispersion of around 80 km/s – the observed average velocity dispersion along the line of sight was approximately 1000 km/s. From this comparison, Zwicky concluded: “If this would be confirmed, we would get the surprising result that dark matter is present in much greater amount than luminous matter.”

In the 1950s and 1960s, most astronomers did not ask whether the universe had a significant abundance of invisible or missing mass. Although observations from this era would later be seen as evidence for dark matter, back then there was no consensus that the observations required much, or even any, such hidden material, and certainly there was not yet any sense of crisis in the field. It was in 1970 that the first explicit statements began to appear arguing that additional mass was needed in the outer parts of some galaxies, based on comparisons between predicted and measured rotation curves. The appendix of a seminal paper published by Ken Freeman in 1970, prompted by discussions with radio-astronomer Mort Roberts, concluded that: “If [the data] are correct, then there must be in these galaxies additional matter which is undetected, either optically or at 21 cm. Its mass must be at least as large as the mass of the detected galaxy, and its distribution must be quite different from the exponential distribution which holds for the optical galaxy.” (Figure 1 below.)

Several other lines of evidence began to appear that supported the same conclusion. In 1974, two influential papers (by Jaan Einasto, Ants Kaasik and Enn Saar, and by Jerry Ostriker, Jim Peebles and Amos Yahil) argued that a common solution existed for the mass discrepancies observed in clusters and in galaxies, and made the strong claim that the mass of galaxies had been until then underestimated by a factor of about 10.

By the end of the decade, opinion among many cosmologists and astronomers had crystallised: dark matter was indeed abundant in the universe. Although the same conclusion was reached by many groups of scientists with different subcultures and disciples, many individuals found different lines of evidence to be compelling during this period. Some astronomers were largely persuaded by new and more reliable measurements of rotation curves, such as those by Albert Bosma, Vera Rubin and others. Others were swayed by observations of galaxy clusters, arguments pertaining to the stability of disc galaxies, or even cosmological considerations. Despite disagreements regarding the strengths and weaknesses of these various observations and arguments, a consensus nonetheless began to emerge by the end of the 1970s in favour of dark-matter’s existence.

Enter the particle physicists

From our contemporary perspective, it can be easy to imagine that scientists in the 1970s had in mind halos of weakly interacting particles when they thought about dark matter. In reality, they did not. Instead, most astronomers had much less exotic ideas in the form of comparatively low-luminosity versions of otherwise ordinary stars and gas. Over time, however, an increasing number of particle physicists became aware of and interested in the problem of dark matter. This transformation was not just driven by new scientific results, but also by sociological changes in science that had been taking place for some time.

Half a century ago, cosmology was widely viewed as something of a fringe science, with little predictive power or testability. Particle physicists and astrophysicists did not often study or pursue research in each other’s fields, and it was not obvious what their respective communities might have to offer one another. More than any other problem in science, it was dark matter that brought particle physicists and astronomers together.

As astrophysical alternatives were gradually ruled out one by one, the view that dark matter is likely to consist of one or more yet undiscovered species of subatomic particle came to be held almost universally among both particle physicists and astrophysicists alike.

Perhaps unsurprisingly, the first widely studied particle dark-matter candidates were neutrinos. Unlike all other known particle species, neutrinos are stable and do not experience electromagnetic or strong interactions – which are essential characteristics for almost any viable dark-matter candidate. The earliest discussion of the role of neutrinos in cosmology appeared in a 1966 paper by Soviet physicists Gershtein and Zeldovich, and several years later the topic began to appear in the West, beginning in 1972 with a paper by Ram Cowsik and J McClelland. Despite the very interesting and important results of these and other papers, it is notable that most of them did not address or even acknowledge the possibility that neutrinos could account for the missing mass that had been observed by astronomers on galactic and cluster scales. An exception included the 1977 paper by Lee and Weinberg, whose final sentence reads: “Of course, if a stable heavy neutral lepton were discovered with a mass of order 1–15 GeV, the gravitational field of these heavy neutrinos would provide a plausible mechanism for closing the universe.”

While this is still a long way from acknowledging the dynamical evidence for dark matter, it was an indication that physicists were beginning to realise that weakly interacting particles could be very abundant in our universe, and may have had an observable impact on its evolution. In 1980, the possibility that neutrinos might make up the dark matter received a considerable boost when a group studying tritium beta decay reported that they had measured the mass of the electron antineutrino to be approximately 30 eV – similar to the value needed for neutrinos to account for the majority of dark matter. Although this “discovery” was eventually refuted, it motivated many particle physicists to consider the cosmological implications of their research.

Although we know today that dark matter in the form of Standard Model neutrinos would be unable to account for the observed large-scale structure of the universe, neutrinos provided an important template for the class of hypothetical species that would later be known as weakly interacting massive particles (WIMPs). Astrophysicists and particle physicists alike began to experiment with a variety of other, more viable, dark-matter candidates.

Cold dark-matter paradigm

The idea of neutrino dark matter was killed off in the mid-1980s with the arrival of numerical simulations. These could predict how large numbers of dark-matter particles would evolve under the force of gravity in an expanding universe, and therefore allow astronomers to assess the impact of dark matter on the formation of large-scale structure. In fact, by comparing the results of these simulations with those of galaxy surveys, it was soon realised that no relativistic particle could account for dark matter. Instead, the paradigm of cold dark matter – i.e. made of particles that were non-relativistic at the epoch of structure formation – was well on its way to becoming firmly established.

Meanwhile, in 1982, Jim Peebles pointed out that the observed characteristics of the cosmic microwave background (CMB) also seemed to require the existence of dark matter. If just baryons existed, then one could only explain the observed degree of large-scale structure if the universe started in a fairly anisotropic or “clumpy” state. But by this time, the available data already set an upper limit on CMB anisotropies at a level of 10^–4 – too meagre to account for the universe’s structure. Peebles argued that this problem would be relieved if the universe was instead dominated by massive weakly interacting particles whose density fluctuations begin to grow prior to the decoupling of matter and radiation during which the CMB was born. Among other papers, this received enormous attention within the scientific community and helped establish cold dark matter as the leading paradigm to describe the structure and evolution of the universe at all scales.

Solutions beyond the Standard Model

Neutrinos might be the only known particles that are stable, electrically neutral and not strongly interacting, but the imagination of particle physicists did not remain confined to the Standard Model for long. Instead, papers started to appear that openly contemplated many speculative and yet undiscovered particles that might account for dark matter. In particular, particle physicists began to find new candidates for dark matter within the framework of a newly proposed space–time symmetry called supersymmetry. The cosmological implications of supersymmetry were discussed as early as the late 1970s. In Piet Hut’s 1977 paper on the cosmological constraints on the masses of neutrinos, he wrote that the dark-matter argument was not limited to neutrinos or even to weakly interacting particles. The abstract of his paper mentions another possibility made within the context of the supersymmetric partner of the graviton, the spin-3/2 gravitino: “Similar, but much more severe, restrictions follow for particles that interact only gravitationally. This seems of importance with respect to supersymmetric theories,” wrote Hut.

In their 1982 paper, Heinz Pagels and Joel Primack also considered the cosmological implications of gravitinos. But unlike Hut’s paper, or the other preceding papers that had discussed neutrinos as a cosmological relic, Pagels and Primack were keenly aware of the dark-matter problem and explicitly proposed that gravitinos could provide the solution by making up the missing mass. In many ways, their paper reads like a modern manuscript on supersymmetric dark matter, motivating supersymmetry by its various attractive features and then discussing both the missing mass in galaxies and the role that dark matter could play in the formation of large-scale structure. Around the same time, supersymmetry was being further developed into its more modern form, leading to the introduction of R-parity and constructions such as the minimal supersymmetric standard model (MSSM). Such supersymmetric models included not only the gravitino as a dark-matter candidate, but also neutralinos – electrically neutral mixtures of the superpartners of the photon, Z and Higgs bosons.

Over the past 35 years, neutralinos have remained the single most studied candidate for dark matter and have been the subject of many thousand scientific publications. Papers discussing the cosmological implications of stable neutralinos began to appear in 1983. In the first two of these, Weinberg and Haim Goldberg independently discussed the case of a photino (a neutralino whose composition is dominated by the superpartner of the photon) and derived a lower bound of 1.8 GeV on its mass by requiring that the density of such particles does not overclose the universe. A few months later, a longer paper by John Ellis and colleagues considered a wider range of neutralinos as cosmological relics. In Goldberg’s paper there is no mention of the phrase “dark matter” or of any missing mass problem, and Ellis et al. took a largely similar approach by simply requiring only that the cosmological abundance of neutralinos not be so large as to overly slow or reverse the universe’s expansion rate. Although most of the papers on stable cosmological relics written around this time did not yet fully embrace the need to solve the dark-matter problem, occasional sentences could be found that reflected the gradual emergence of a new perspective.

During the years that followed, an increasing number of particle physicists would further motivate proposals for physics beyond the Standard Model by showing that their theories could account for the universe’s dark matter. In 1983, for instance, John Preskill, Mark Wise and Frank Wilczek showed that the axion, originally proposed to solve the strong CP problem in quantum chromodynamics, could account for all of the dark matter in the universe. In 1993, Scott Dodelson and Lawrence Widrow proposed a scenario in which an additional, sterile neutrino species that did not experience electroweak interactions could be produced in the early universe and realistically make up the dark matter. Both the axion and the sterile neutrino are still considered as well-motivated dark-matter candidates, and are actively searched for with a variety of particle and astroparticle experiments.

The triumph of particle dark matter

In the early 1980s there was still nothing resembling a consensus about whether dark matter was made of particles at all, with other possibilities including planets, brown dwarfs, red dwarfs, white dwarfs, neutron stars and black holes. Kim Griest would later coin the term “MACHOs” – short for massive astrophysical compact halo objects – to denote this class of dark-matter candidates, in response to the alternative of WIMPs. There is a consensus today, based on searches using gravitational microlensing surveys and determinations of the cosmic baryon density based on measurements of the primordial light-element abundances and the CMB, that MACHOs do not constitute a large fraction of the dark matter.

An alternative explanation for particle dark matter is to assume that there is no dark matter in the first place, and that instead our theory of gravity needs to be modified. This simple idea, which was put forward in 1982 by Mordehai Milgrom, is known as modified Newtonian dynamics (MOND) and has far-reaching consequences. At the heart of MOND is the suggestion that the force due to gravity does not obey Newton’s second law, F = ma. If instead gravity scaled as F = ma²/a₀ in the limit of very low accelerations (a << a₀ ~ 1.2 × 10⁻¹⁰ m/s²), then it would be possible to account for the observed motions of stars and gas within galaxies without postulating the presence of any dark matter.

In 2006, a group of astronomers including Douglas Clowe transformed the debate between dark matter and MOND with the publication of an article entitled: “A direct empirical proof of the existence of dark matter”. In this paper, the authors described the observations of a pair of merging clusters collectively known as the Bullet Cluster (image above left). As a result of the clusters’ recent collision, the distribution of stars and galaxies is spatially separated from the hot X-ray-emitting gas (which constitutes the majority of the baryonic mass in this system). A comparison of the weak lensing and X-ray maps of the bullet cluster clearly reveals that the mass in this system does not trace the distribution of baryons. Another source of gravitational potential, such as that provided by dark matter, must instead dominate the mass of this system.

Following these observations of the bullet cluster and similar systems, many researchers expected that this would effectively bring the MOND hypothesis to an end. This did not happen, although the bullet cluster and other increasingly precise cosmological measurements on the scale of galaxy clusters, as well as the observed properties of the CMB, have been difficult to reconcile with all proposed versions of MOND. It is currently unclear whether other theories of modified gravity, in some yet-unknown form, might be compatible with these observations. Until we have a conclusive detection of dark-matter particles, however, the possibility that dark matter is a manifestation of a new theory of gravity remains open.

Today, the idea that most of the mass in the universe is made up of cold and non-baryonic particles is not only the leading paradigm, but is largely accepted among astrophysicists and particle physicists alike. Although dark-matter’s particle nature continues to elude us, a rich and active experimental programme is striving to detect and characterise dark-matter’s non-gravitational interactions, ultimately allowing us to learn the identity of this mysterious substance. It has been more than a century since the first pioneering attempts to measure the amount of dark matter in the universe. Perhaps it will not be too many more years before we come to understand what that matter is.

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What is wrong with the theory of gravity we have?

The current description of gravity in terms of general relativity has various shortcomings. Perhaps the most important is that we cannot simply apply Einstein’s laws at a subatomic level without generating notorious infinities. There are also conceptual puzzles related to the physics of black holes that indicate that general relativity is not the final answer to gravity, and important lessons learnt from string theory suggesting gravity is emergent. Besides these theoretical issues, there is also a strong experimental motivation to rethink our understanding of gravity. The first is the observation that our universe is experiencing accelerated expansion, suggesting it contains an enormous amount of additional energy. The second is dark matter: additional gravitating but non-luminous mass that explains anomalous galaxy dynamics. Together these entities account for 95 per cent of all the energy in the universe.

Isn’t the evidence for dark matter overwhelming?

It depends who you ask. There is a lot of evidence that general relativity works very well at length scales that are long compared to the Planck scale, but when we apply general relativity at galactic and cosmological scales we see deviations. Most physicists regard this as evidence that there exists an additional form of invisible matter that gravitates in the same way as normal matter, but this assumes that gravity itself is still described by general relativity. Furthermore, although the most direct evidence for the existence of dark matter comes from the study of galaxies and clusters, not all astronomers are convinced that what they observe is due to particle dark matter – for example, there appears to be a strong correlation between the amount of ordinary baryonic matter and galactic rotation velocities that is hard to explain with particle dark matter. On the other hand, the physicists who are carrying out numerical work on particle dark matter are trying to explain these correlations by including complicated baryonic feedback mechanisms and tweaking the parameters that go into their models. Finally, there is a large community of experimental physicists who simply take the evidence for dark matter as a given.

Is your theory a modification of general relativity, or a rewrite?

The aim of emergent gravity is to derive the equations that govern gravity from a microscopic quantum, while using ingredients from quantum-information theory. One of the main ideas is that different parts of space–time are glued together via quantum entanglement. This is due to van Raamsdonk and has been extended and popularised by Maldacena and Susskind with the slogan “EPR = ER”, where EPR is a reference to Einstein–Podolsky–Rosen and ER refers to the Einstein–Rosen bridge: a “wormhole” that connects the two parts of the black-hole geometry on opposite sides of the horizon. These ideas are being developed by many theorists, in particular in the context of the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence. The goal is then to derive the Einstein equations from this microscopic-quantum perspective. The first step in this programme was already made before my work, but until now most results were derived for AdS space, which describes a universe with a negative cosmological constant and therefore differs from our own. In my recent paper [arXiv:1611.02269] I extended these ideas to de Sitter space, which contains a positive dark energy and has a cosmological horizon. My insight has been that, due to the presence of positive dark energy, the derivation of the Einstein equations breaks down precisely in the circumstances where we observe the effects of “dark matter”.

How did the idea emerge?

The idea of emergent gravity from thermodynamics has been lingering around since the discovery by Hawking and Bekenstein of black-hole entropy and the laws of black-hole thermodynamics in the 1970s. Ted Jacobson made an important step in 1996 by deriving the Einstein equations from assuming the Bekenstein–Hawking formula, which expresses the microscopic entropy in terms of the area of the horizon measured in Planck units. In my 2010 paper [arXiv:1001.0785] I clarified the origin of the inertia force and its relation to the microscopic entropy in space, assuming that this is given by the area of an artificial horizon. After this work I started thinking about cosmology, and learnt about the observations that indicate a close connection between the acceleration scale in galaxies and the acceleration at the cosmological horizon, which is determined by the Hubble parameter. I immediately realised that this implied a relation between the observed phenomena associated with dark matter and the presence of dark energy.

Your paper is 50 pages long. Can you summarise it here?

The idea is that gravity emerges by applying an analogue of the laws of thermodynamics to the entanglement entropy in the vacuum. Just like the normal laws of thermodynamics can be understood from the statistical treatment of microscopic molecules, gravity can be derived from the microscopic units that make up space–time. These “space–time molecules” are units of quantum information (qubits) that are entangled with each other, with the amount of entanglement measured by the entanglement entropy. I realised that in a universe with a positive dark energy, there is a contribution to the entanglement entropy that grows in proportion to area. This leads to an additional force on top of the usual gravity law, because the dark energy “pushes back” like an elastic medium and results in the phenomena that we currently attribute to dark matter. In short, the laws of gravity differ in the low-acceleration regime that occurs in galaxies and other cosmological structures.

How did the community react to the paper?

Submitting work that goes against a widely supported theory requires some courage, and the fact that I have already demonstrated serious work in string theory helped. Nevertheless, I do experience some resistance – mainly from researchers who have been involved in particle dark-matter research. Some string theorists find my work interesting and exciting, but most of them take a “wait and see” attitude. I am dealing with a number of different communities with different attitudes and scientific backgrounds. A lot of it is driven by sociology and past investments.

How often do you work on the idea?

Emergent gravity from quantum entanglement is now an active field worldwide, and I have worked on the idea for a number of years. I mostly work in the evening for around three hours and perhaps one hour in the morning. I also discuss these ideas with my PhD students, colleagues and visitors. In the Netherlands we have quite a large community working on gravity and quantum entanglement, and recently we received a grant together with theorists from the universities of Groningen, Leiden, Utrecht and Amsterdam, to work on this topic.

Within a month of your paper, Brouwer et al. published results supporting your idea. How significant is this?

My theory predicts that the gravitational force due to a centralised mass exhibits a certain scaling relation. This relation was already known to hold for galaxy rotation curves, but these can only be measured up to distances of about 100 kilo-parsec because there are no visible stars beyond this distance. Brouwer and her collaborators used weak gravitational lensing to determine the gravitational force due to a massive galaxy up to distances of one mega-parsec and confirmed that the same relation still holds. Particle dark-matter models can also explain these observations, but they do so by adjusting a free parameter to fit the data. My prediction has no free parameters and hence I find this more convincing, but more observations are needed before definite conclusions can be drawn.

Is there a single result that would rule your theory in or out?

If a dark-matter particle would be discovered that possesses all the properties to explain all the observations, then my idea would be proven to be false. Personally I am convinced this will not happen, although I am still developing the theory further to be able to address important dynamical situations such as the Bullet Cluster (see “How dark matter became a particle”) and the acoustic oscillations that explain the power spectrum of the cosmic microwave background. One of the problems is that particle dark-matter models are so flexible and can therefore easily be made consistent with the data. By improving and extending the observations of gravitational phenomena that are currently attributed to dark matter, we can make better comparisons with the theory. I am hopeful that within the next decade the precision of the observations will have improved and the theory will be developed to a level that decisive tests can be performed.

How would emergent gravity affect the rest of physics?

Our perspective on the building blocks of nature would change drastically. We will no longer think in terms of elementary particles and fundamental forces, but units of quantum information. Hence, the gauge forces responsible for the electroweak and strong interactions will also be understood as being emergent, and this is the way that the forces of nature will become unified. In this sense, all of our current laws of nature will be seen as emergent.

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The US Department of Energy (DOE) has formally approved a key construction milestone for the LUX-ZEPLIN (LZ) experiment, propelling the project towards its April 2020 goal for completion. On 9 February the project passed a DOE review and approval stage known as “Critical Decision 3”, which accepts the final design and formally launches construction. The LZ detector, which will be built roughly 1.5 km underground at the Sanford Underground Research Facility in South Dakota and be filled with 10 tonnes of liquid xenon to detect dark-matter interactions, is considered one of the best bets to determine whether dark-matter candidates known as WIMPs exist.

The project stems from the merger of two previous experiments: LUX (Large Underground Xenon) and ZEPLIN (ZonEd Proportional scintillation in LIquid Noble gases). It was first approved in 2014 and currently has about 250 participating scientists in 37 institutions in the US, UK, Portugal, Russia and Korea. The detector is expected to be at least 50 times more sensitive to finding signals from dark-matter particles than its predecessor LUX, and will compete with other liquid-xenon experiments under development worldwide in the race to detect dark matter. A planned upgrade to the current XENON1T experiment (called XENONnT) at Gran Sasso National Laboratory in Italy and China’s plans to advance the PandaX-II detector, for instance, are both expected to have a similar schedule and scale to LZ.

The LZ collaboration plans to release a Technical Design Report later this year. “We will try to go as fast as we can to have everything completed by April 2020,” says LZ project director Murdock Gilchriese. “We got a very strong endorsement to go fast and to be first.”

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Dark photons, are hypothetical low-mass spin-1 particles that couple to dark matter but have vanishing couplings with normal matter. Such a boson, which may be associated with a U(1) gauge symmetry in the dark sector and mix kinetically with the Standard Model photon, offers an explanation for puzzling astrophysical observations such as the positron abundance in cosmic rays reported by the PAMELA satellite. Dark photons have also been invoked as possible explanations to the muon g-2 anomaly.

Based on single-photon events in 53 fb⁻1 of e⁺e^– collision data collected at the PEP-II B factory in SLAC, California, the BaBar collaboration has now completed a thorough search for these particles (Aʹ) via the process e⁺e^– → γ Aʹ. The search was based on the assumption that the dark photon decays almost entirely to dark-matter particles and therefore that no energy would be deposited in the BaBar detector from its decay products. Finding no evidence for such processes, the analysis places 90% confidence-level upper limits on the coupling strength of Aʹ to e⁺e^– for dark photons lighter than 8 GeV. In particular, the BaBar limits exclude values of the Aʹ coupling suggested by the dark-photon interpretation of the muon g-2 anomaly, as well as a broad range of parameters for dark-sector models (see figure).

“This paper is the final word from BaBar on a search where the dark photon decays invisibly,” says BaBar spokesperson Michael Roney. “But we are continuing to search for dark photons and other dark-sector particles that have visible decay modes.”

The BaBar result follows another direct search for sub-GeV dark photons carried out recently by CERN’s NA64 experiment, in which electrons incident on an active target probe the process e⁻ Z → e⁻ Z Aʹ. Again, no evidence for such decays was found, and NA64 was able to exclude dark photons with a mass less than around 0.1 GeV.

“The thing is, there are dark photons and dark photons,” says theorist  Sean Carroll of Caltech, who has worked on dark-photon models. “In contrast to massless dark photons, which are analogous to ordinary photons, this experiment constrains a slightly different idea of dark force-carrying particles that are associated with a broken symmetry, which therefore get a mass and then can decay. They are more like ‘dark Z bosons’ than dark photons.”

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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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Although the night sky appears dark between the stars and galaxies that we can see, a strong background emission is present in other regions of the electromagnetic spectrum. At millimetre wavelengths, the cosmic microwave background (CMB) dominates this emission, while a strong X-ray background peaks at sub-nanometre wavelengths. For the past 50 years it has also been known that a diffuse gamma-ray background at picometre wavelengths also illuminates the sky away from the strong emission of the Milky Way and known extra-galactic sources.

This so-called isotropic gamma-ray background (IGRB) is expected to be uniform on large scales, but can still contain anisotropies on smaller scales. The study of these anisotropies is important for identifying the nature of the unresolved IGRB sources. The best candidates are star-forming galaxies and active galaxies, in particular blazars, which have a relativistic jet pointing towards the Earth. Another possibility to be investigated is whether there is a detectable contribution from the decay or the annihilation of dark-matter particles, as predicted by models of weakly interacting massive particles (WIMPs).

Using NASA’s Fermi Gamma-ray Space Telescope, a team led by Mattia Fornasa from the University of Amsterdam in the Netherlands studied the anisotropies of the IGRB in observations acquired over more than six years. This follows earlier results published in 2012 by the Fermi collaboration and shows that there are two different classes of gamma-ray sources. A specific type of blazar appears to dominate at the highest energies, while at lower frequencies star-forming galaxies or another class of blazar is thought to imprint a steeper spectral slope in the IGRB. A possible additional contribution from WIMP annihilation could not be identified by Fornasa and collaborators.

The constraints on dark matter will improve with new data continuously collected by Fermi

The first step in such an analysis is to exclude the sky area most contaminated by the Milky Way and extra-galactic sources, and then to subtract remaining galactic contributions and the uniform emission of the IGRB. The resulting images include only the IGRB anisotropies, which can be characterised by computing the associated angular power spectrum (APS) similarly to what is done for the CMB anisotropies. The authors do this both for a single image (“auto-APS”) and between images recorded in two different energy regions (“cross-APS”).

The derived auto-APS and cross-APS are found to be consistent with a Poisson distribution, which means they are constant on all angular scales. This absence of scale dependence in gamma-ray anisotropies suggests that the main contribution comes from distant active galactic nuclei. On the other hand, the emission by star-forming galaxies and dark-matter structures would be dominated by their local distribution that is less uniform on the sky and thus would lead to enhanced power at characteristic angular scales. This allowed Fornasa and co-workers to derive exclusion limits on the dark-matter parameter space. Although less stringent than the best limits achieved from the average intensity of the IGRB or from the observation of dwarf spheroidal galaxies, they independently confirm the absence, so far, of a gamma-ray signal from dark matter.

The constraints on dark matter will improve with new data continuously collected by Fermi, but a potentially more promising approach is to complement them at higher gamma-ray energies with data from the future Cherenkov Telescope Array and possibly also with high-energy neutrinos detected by IceCube.

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Dark matter is one of the greatest mysteries of our cosmos. More than 80 years after its postulation in modern form by the Swiss–American astronomer Fritz Zwicky, the existence of a new unseen form of matter in our universe is established beyond doubt. Dark matter is not just the gravitational glue that holds together galaxies, galaxy clusters and structures on the largest cosmological scales. Over the past few decades it has become clear that dark matter is also vital to explain the observed fluctuations in cosmic-microwave-background radiation and the growth of structures that began from these primordial density fluctuations in the early universe. Yet despite overwhelming evidence, its existence is inferred only indirectly via its gravitational pull on luminous matter. As of today, we lack the answer to the most fundamental questions: what is dark matter made of and what is its true nature?

DARWIN, the ultimate dark-matter detector using the noble element xenon in liquid form, will be in a unique position to address these fundamental questions. Currently in the design and R&D phase, DARWIN will be constructed at the Gran Sasso National Laboratory (LNGS) in Italy and is scheduled to carry out its first physics runs from 2024. The DARWIN consortium is growing, and currently consists of about 150 scientists from 26 institutions in 11 countries.

Worldwide search

The particles described by the Standard Model of particle physics are unable to account for dark matter. Although neutrinos, the only elementary particles that do not interact with photons, would be ideal candidates, they are much too light and do not form the observed large-scale structures. Dark matter could, however, be made of new elementary particles that were born in the young and energetic universe. Such particles would carry no electric or colour charge, would be either stable or very long-lived and, similar to neutrinos, would interact only feebly (if at all) with known matter via new fundamental forces. Theories beyond the Standard Model predict a wealth of viable dark-matter candidates. The most popular class has the generic name of weakly interactive massive particles (WIMPs), while a different class is axions, or more generally axion-like particles (ALPs).

Worldwide, more than a dozen experiments are preparing to observe low-energy nuclear recoils induced by galactic WIMPs in ultra-sensitive, low-background detectors. Since the predicted WIMP masses and scattering cross-sections are model-dependent and essentially unknown, searches must cover a vast parameter space. Among the most promising detectors are those based on liquefied noble-gas targets such as liquid xenon (LXe) or liquid argon (LAr) – a well-established technology that can be scaled up to tonne-scale target masses and take data over periods lasting several years.

DARWIN, which will operate a multi-tonne liquid-xenon time projection chamber (TPC), follows in the footsteps of its predecessors XENON, ZEPLIN, LUX and PandaX. The technology employed by these experiments is very similar and, in addition, the entire XENON collaboration is now a part of the DARWIN collaboration. Since December 2016, an upgraded detector called XENON1T has been recording its first dark-matter data at LNGS using two tonnes of liquid xenon as the WIMP target (the total mass of xenon in the detector is 3.3 tonnes). It will probe WIMP–nucleon cross-sections down to as little as 1.6 × 10^–47 cm² at a mass of 50 GeV/c² (for comparison, the scattering cross-section of low-energy ⁷Be solar neutrinos on electrons is about 6 × 10^–45 cm²). A further planned upgrade called XENONnT with seven tonnes of LXe will increase the WIMP sensitivity by one order of magnitude.

The goals of DARWIN are even more ambitious, promising an unprecedented sensitivity of 2.5 × 10^–49 cm² at a WIMP mass of 40 GeV/c². Such a reach would allow us to explore the entire experimentally accessible parameter space for WIMPs, to the point where the WIMP signal becomes indistinguishable from background processes from coherent neutrino-nucleus scattering events.

Rich physics programme

DARWIN will not only search for WIMP dark matter. Because of its ultra-low background level, it will be sensitive to additional, hypothetical particles that are expected to have non-vanishing couplings to electrons. These include solar axions, galactic ALPs and bosonic super-weakly interacting massive particles called superWIMPs, which have masses at the keV scale and are candidates for warm dark matter. It will also detect low-energy solar neutrinos produced by proton–proton fusion reactions in the Sun (so-called pp neutrinos) with high statistics, and therefore address one of the remaining observational challenges in the field of solar neutrinos: a precise comparison of the Sun’s neutrino and photon luminosities. Capable of providing a statistical precision of less than one per cent on this comparison with just five years of data, the high-statistics measurement of the pp-neutrino flux would provide a stringent test of the solar model, as well as neutrino properties, because non-standard neutrino interactions could modify the survival probability of electron neutrinos at these low energies.

The DARWIN observatory will also observe coherent neutrino-nucleus interactions from ⁸B solar neutrinos and be sensitive to neutrinos of all flavours from core-collapse supernovae: it would see about 800 events, or 20 events/tonne, from a supernova with 27 solar masses at a distance of 10 kpc, for example. By looking at the time evolution of the event rate from a nearby supernova, DARWIN could possibly even distinguish between different supernova models. Finally, DARWIN would search for the neutrinoless double beta (0νββ) decay of ¹³⁶Xe, which has a natural abundance of 8.9 per cent in xenon. The observation of this ultra-rare nuclear decay would directly prove that neutrinos are Majorana particles, and that lepton number is violated in nature (CERN Courier July/August 2016 p34).

One common feature of these exciting questions in contemporary particle and astroparticle physics is the exceedingly low expected interaction rates in the detector, corresponding to less than one event per tonne of target material and year. In addition, these searches – with the exception of the 0νββ decay – require an energy threshold that is as low as possible (a few keV), while the 0νββ decay, superWIMP and axion searches will profit from the very good energy resolution of the detector. A multi-tonne liquid-xenon observatory such as DARWIN can address the combination of an ultra-low background level, a low-energy threshold and a good energy resolution within a single, large, monolithic detector.

The WIMP landscape

The current best sensitivity to WIMP searches for masses above 6 GeV/c² is provided by detectors using LXe as a target, and the majority of existing (XENON1T, LUX, PandaX) and planned (LZ, XENONnT) LXe dark-matter detectors employ dual-phase TPCs (figure 1). These detectors maintain xenon at a constant temperature of about –100 °C and detect two distinct signals (the prompt scintillation light and the ionisation electrons) via arrays of photosensors operated in the liquid and vapour phase. The observation of both signals delivers information about the type of interaction and its energy, as well as the 3D position and timing of an event. WIMP collisions and coherent neutrino scatters will produce nuclear recoils, while pp neutrinos, axions, superWIMPs and double beta decays, along with the majority of background events, will cause electronic recoils. Fast neutrons from materials or induced through cosmic-ray muons will also give rise to nuclear recoils, but WIMPs and neutrinos will scatter only once in a given detector, while neutrons can scatter multiple times in large detectors such as DARWIN.

Since the primary intent of DARWIN is to investigate dark-matter interactions, it is vital that background processes are understood. The observatory can exploit the full discovery potential of the liquefied xenon technique with a 40 tonne LXe TPC that allows all known sources of background to be considered. These stem from several sources: the residual radioactivity of detector-construction materials (γ radiation, neutrons); β decays of the anthropogenic ⁸⁵Kr present in the atmosphere due to nuclear fuel reprocessing, weapons tests and accidents such as that at Fukushima nuclear plant in Japan; and the progenies of ²²²Rn in the LXe target. Two-neutrino double beta decays (2νββ) of ¹³⁶Xe and interactions of low-energy solar neutrinos (pp, ⁷Be) are another source of background, as are higher-energy neutrino interactions with xenon nuclei in coherent neutrino-nucleus scattering.

In the standard WIMP-scattering scenario, the leading interactions between a dark-matter particle and a nucleon are due to two subtly different processes: spin-dependent couplings and isospin-conserving, spin-independent couplings. Since LXe contains nuclei with and without spin, DARWIN can probe both types of interactions. Assuming an exposure of 200 tonnes × years (500 tonnes × years), a spin-independent WIMP sensitivity of 2.5 × 10^–49 cm² (1.5 × 10^–49 cm²) can be reached at a WIMP mass of 40 GeV/c². For spin-dependent WIMP–neutron couplings and WIMP masses up to about 1 TeV, the searches conducted by DARWIN will be complementary to those of the LHC and High-Luminosity LHC at a centre-of-mass energy of 14 TeV. Natural xenon includes two isotopes with nonzero total nuclear angular momentum, ¹²⁹Xe and ¹³¹Xe, at a combined abundance of about 50%. If the WIMP–nucleus interaction is indeed spin-dependent, DARWIN will also probe inelastic WIMP–nucleus scattering, where these two nuclei are excited into low-lying states at 40 keV and 80 keV, respectively, with subsequent prompt de-excitation. The discovery of such a signature would be a clear indication for an axial-vector coupling of WIMPs to nuclei.

Ultimate detector

Should dark-matter particles be discovered by one of the running (XENON1T, DEAP-3600) or near-future (LZ, XENONnT) detectors, DARWIN would be able to reconstruct the mass and scattering cross-section from the measured nuclear recoil spectra. With an exposure of 200 tonnes × years, 152, 224 and 60 events would be observed for the three WIMP masses, respectively (figure 2). DARWIN may therefore be the ultimate liquid-xenon dark-matter detector, capable of probing the WIMP paradigm and thus detect or exclude WIMPs with masses above 6 GeV/c², down to the extremely low cross-sections of 1.5 × 10^–49 cm².

Should WIMPs not be observed in the DARWIN detector, the WIMP paradigm would be under very strong pressure. With its large, uniform target mass, low-energy threshold, and ultra-low background level, the observatory will also open up a unique opportunity for other rare event searches such as axions and other weakly interacting light particles. It will address open questions in neutrino physics, which is one of the most promising areas in which to search for physics beyond the Standard Model. At its lowest energies, the DARWIN detector will observe coherent neutrino-nucleus interactions from solar ⁸B neutrinos, thus precisely testing the standard-solar-model flux prediction, and may detect neutrinos from galactic supernovae.

The DARWIN observatory was approved for an initial funding period, via ASPERA, in 2010. It is included in the European Roadmap for Astroparticle Physics and in various other programs, for example by the Swiss State Secretariat for Education, Research and Innovation and the Strategic Plan for Astroparticle Physics in the Netherlands. The current phase will culminate with a technical design report in 2019, followed by engineering studies in 2020 and 2021, with the construction at LNGS and first physics runs scheduled to start in 2022 and 2024, respectively. The experiment will operate for at least 10 years and may write a new chapter in the exciting story of dark matter.

DARWIN scales LXe technology to new heights

The DARWIN observatory will operate a large amount (50 tonnes) of liquid xenon in a low-background cryostat surrounded by concentric shielding structures (diagram right). The heart of the experiment is the dual-phase TPC, containing 40 tonnes of instrumented xenon mass (diagram below). The high density of liquid xenon (3 kg/l) results in a short radiation length and allows for a compact detector geometry with efficient self-shielding. A drift field of the order 0.5 kV cm–1 across the liquid target will cause the electrons to drift away from the interaction vertex towards the liquid–gas interface. Large field-shaping rings made from oxygen-free, high-conductivity copper will ensure the homogeneity of the field. The TPC will mostly be constructed from copper as a conductor and polytetrafluoroethylene as an insulator, with the latter being an efficient reflector for vacuum ultra-violet scintillation light. It will be housed in a double-walled cryostat, and all the detector materials will first be selected for ultra-low intrinsic radioactivity using dedicated, high-purity germanium (HPGe) detector screening facilities. In the baseline scenario, the prompt and proportional scintillation signals will be recorded by two arrays of photomultiplier tubes (PMTs) installed above and below the xenon target. These will have a diameter of 3″ or 4″ and feature a very low intrinsic radioactivity, high quantum efficiency of 35% at 178 nm, a gain of around 5 × 106 and a very low dark count rate at –100 °C. Albeit a proven and reliable technology, PMTs are bulky, expensive and generate a significant fraction of the radioactive background in a dark-matter detector, especially concerning nuclear recoils produced by neutrons from (alpha, n) reactions. Several alternative light read-out schemes are thus being considered by the collaboration in small R&D set-ups. Among these are arrays of silicon photomultipliers (with a potential scheme where the TPC is fully surrounded by photosensors), gaseous photomultipliers and hybrid photosensors. A novel concept of liquid-hole multipliers could allow for charge and light read-out in a single-phase TPC, and potentially result in a significant improvement in light yield and thus a lower energy threshold.

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Black holes are arguably humankind’s most intriguing intellectual construction. Featuring a curvature singularity where space–time “ends” and tidal forces are infinite, black-hole interiors cannot be properly understood without a quantum theory of gravity. They are defined by an event horizon – a surface beyond which nothing escapes to the outside – and an exterior region called a photosphere, which is able to trap light rays. These uncommon properties explain why black holes were basically ignored for half a century, considered little more than a bizarre mathematical solution of Einstein’s equations but one without counterpart in nature.

LIGO’s discovery of gravitational waves provides the strongest evidence to date for the existence of black holes, but these tiny distortions of space–time have much more to tell us. Gravitational waves offer a unique way to test the basic tenets of general relativity, some of which have been taken for granted without observations. Are black holes the simplest possible macroscopic objects? Do event horizons and black holes really exist, or is their formation halted by some as-yet unknown mechanism? In addition, gravitational waves can tell us if gravitons are massless and if extra-light degrees of freedom fill the universe, as predicted in the 1970s by Peccei and Quinn in an attempt to explain the smallness of the neutron electric-dipole moment, and more recently by string theory. Ultralight fields affect the evolution of black holes and their gravitational-wave emission in a dramatic way that should be testable with upcoming gravitational-wave observatories.

The existence of black holes

The standard criterion with which to identify a black hole is straightforward: if an object is dark, massive and compact, it’s a black hole. But are there other objects which could satisfy the same criteria? Ordinary stars are bright, while neutron stars have at most three solar masses and therefore neither is able to explain observations of very massive dark objects. In recent years, however, unknown physics and quantum effects in particular have been invoked that change the structure of the horizon, replacing it by a hard surface. In this scenario, the exterior region – including the photosphere – would remain unchanged, but black holes would be replaced by very compact, dark stars. These stars could be made of normal matter under extraordinary quantum conditions or of exotic matter such as new scalar particles that may form “boson stars”.

Unfortunately, the formation of objects invoking poorly understood quantum effects is difficult to study. The collapse of scalar fields, on the other hand, can theoretically allow boson stars to form, and these may become more compact and massive through mergers. Interestingly, there is mounting evidence that compact objects without horizons but with a photosphere are unstable, ruling out entire classes of alternatives that have been put forward.

Gravitational waves might soon provide a definite answer to such questions. Although current gravitational-wave detections are not proof for the existence of black holes, they are a strong indicator that photospheres exist. Whereas observations of electromagnetic processes in the vicinities of black holes only probe the region outside of the photosphere, gravitational waves are sensitive to the entire space–time and are our best probe of strong-field regions.

A typical gravitational-wave signal generated by a small star falling head-on into a massive black hole looks like that in figure 1. As the star crosses the photosphere, a burst of radiation is emitted and a sequence of pulses dubbed  “quasinormal ringing” follow, determined by the characteristic modes of the black hole. But if the star falls into a quantum-corrected or exotic compact object with no horizon, part of the burst generated during the crossing of the photosphere reflects back at the object surface. The resulting signal in a detector would thus initially look the same, but be followed by lower amplitude “echoes” trapped between the photosphere and the surface of the object (figure 1, lower panel). These echoes, although tricky to dig out in noisy data, would be a smoking gun for new physics. With increasing sensitivity in detectors such as LIGO and Virgo, observations will be pushing back the object’s surface closer to the horizon, perhaps even to the point where we can detect the echo of quantum effects.

Dark questions

Understanding strong-field gravity with gravitational waves can also test the nature of dark matter. Although dark matter may interact very feebly with Standard Model particles, according to Einstein’s equivalence principle it must fall just like any other particle. If dark matter is composed of ultralight fields, as recent studies argue, then black holes may serve as excellent dark-matter detectors. You might ask how a monstrous, supermassive black hole could ever be sensitive to ultralight fields. The answer lies in superradiant resonances. When black holes rotate, as most do, they display an interesting effect discovered in the 1970s called superradiance: if one shines a low-frequency lamp on a rotating black hole, the scattered beam is brighter. This happens at the expense of the hole’s kinetic energy, causing the spin of the black-hole to decrease.

Not only electromagnetic waves, but also gravitational waves and any other bosonic field can be amplified by a rotating black hole. In addition, if the field is massive, low-energy fluctuations are trapped near the horizon and are forced to interact repeatedly with the black hole, producing an instability. This instability extracts rotational energy and transfers it to the field, which grows exponentially in amplitude and forms a rotating cloud around the black hole. For a one-million solar-mass black hole and a scalar field with a mass of 10^–16 eV, the timescale for this to take place is less than two minutes. Therefore, the very existence of ultralight fields is constrained by the observation of spinning black holes. With this technique, one can place unprecedented bounds on the mass of axion-like particles, another popular candidate for dark matter. For example, we know from current astrophysical observations that the mass of dark photons must be smaller than 10^–20 eV, which is 100 times better than accelerator bounds. The technique relies only on measurements of the mass and spin of black holes, which will be known with unprecedented precision with future gravitational-wave observations.

Superradiance, together with current electromagnetic observations of spinning black holes, can also be used to constrain the mass of the graviton, since any massive boson would trigger superradiant instabilities. Spin measurements of the supermassive black hole in galaxy Fairall 9 requires the mass of the graviton to be lighter than 5 × 10^–23 eV – an impressive number which is even more stringent than the bound recently placed by LIGO.

Gravitational lighthouses

Furthermore, numerical simulations suggest that the superradiant instability mechanism eventually causes a slowly evolving and non-symmetric cloud to form around the black hole, emitting periodic gravitational waves like a gravitational “lighthouse”. This would not only mean that black holes are not as simple as we thought, but lead to a definite prediction: some black holes should be emitting nearly monochromatic gravitational waves whose frequency is dictated only by the field’s mass. This raises terrific opportunities for gravitational-wave science: not only can gravitational waves provide the first direct evidence of ultralight fields and of possible new effects near the horizon, but they also carry detailed information about the black-hole mass and spin. If light fields exist, the observation of a few hundred black holes should show “gaps” in the mass-spin plane corresponding to regions where spinning black holes are too unstable to exist.

This is a surprising application of gravitational science, which can be used to investigate the existence of new particles such as those possibly contributing to the dark matter. The idea of using observations of supermassive black holes to provide new insights not accessible in laboratory experiments would certainly be exciting. Perhaps these new frontiers in gravitational-wave astrophysics, in addition to probing the most extreme objects, will also give us a clearer understanding of the microscopic universe.

The post Linking waves to particles appeared first on CERN Courier.

In the first half of the 20th century, many of the most important discoveries of new particles were made by cosmic-ray experiments. Examples include antimatter, the muon, pion, kaon and other hadrons, which opened up the field of high-energy physics and set in motion our modern understanding of elementary particles. This came about because cosmic-ray interactions with nuclei in the upper atmosphere are among the highest-energy events known, surpassing anything that could be produced in laboratories at the time – and even in collisions at the LHC today.

However, around the middle of the century the balance of power in particle physics shifted to accelerator experiments. By generating high-energy interactions in the laboratory under controlled conditions, accelerators offered new possibilities for precise measurements and thus for the study of rare particles and phenomena. These experiments helped to flush out the quark model and also the fundamental force-carrying bosons, leading to the establishment of the Standard Model (SM) – whose success was crowned by the discovery of the Higgs boson at the LHC in 2012.

Today, thanks to its unique position on the International Space Station, the AMS experiment combines the best of both worlds as a highly sensitive particle detector that is free from the complicated environment of the atmosphere (see “Cosmic rays continue to confound“). Collecting data since 2011, AMS has initiated a new epoch of precision cosmic-ray experiments that help to address basic puzzles in particle physics such as the nature of dark matter. The experiment’s latest round of data continues to throw up surprises. Arriving at the correct interpretation of events due to particles produced far away in the universe, however, still presents challenges for physicists trying to understand dark matter and the cosmological asymmetry between matter and antimatter.

Best of both worlds

The emphasis in particle physics now is on the search for physics beyond the SM, for which many motivations come from astrophysics and cosmology. Examples include dark matter, which contributes many times more to the overall density of matter in the universe than does the conventional matter described by the SM, and the origin of matter itself. Many physicists think that dark matter may be composed of particles that could be detected at the LHC, or might reveal themselves in astrophysical experiments such as AMS. As for the origin of matter, the big question has been whether it is due to an intrinsic difference between the properties of matter and antimatter particles, or whether the dominance of matter over antimatter in the universe around us is merely a local phenomenon. Although it is unlikely that there exist other regions of the observable universe where antimatter dominates, there is limited direct experimental evidence against it.

The AMS approach to cosmic-ray physics is based on decades of experience in high-statistics, high-precision accelerator experiments. It has a strong focus on measurements of antiparticle spectra that allows it to search indirectly for possible dark-matter particles, which would produce antiparticles if they annihilated with each other, as well as for possible harbingers of astrophysical concentrations of antimatter. In parallel, AMS is able to make measurements of the energy spectra of many different nuclear species, posing challenges for models of the origin of cosmic rays – a mystery that has stood ever since their discovery in 1912.

Unconventional physics?

The latest AMS results on the cosmic-ray electron and positron fluxes provide very accurate measurements of the very different spectra of these particles. Numerous previous experiments had discovered an increase in the positron-to-electron ratio at increasing energies, although with considerable scatter. AMS has now confirmed this trend with greater precision, but it also indicates that the positron-to-electron ratio may decrease again at energies above about 300 GeV. The differences between the electron and positron fluxes mean that different mechanisms must be dominating their production. The natural question is whether some exotic mechanism is contributing to positron production.

One possibility is the annihilation of dark-matter particles, but a more conventional possibility is production by electromagnetic processes around one or more nearby pulsars. In both cases, one might expect the positron spectrum to turn down at higher energies, being constrained by either the mass of the dark-matter particle or by the strength of the acceleration mechanism around the pulsar(s). In the latter case, one would also expect the positron flux to be non-isotropic, but no significant effect has been seen so far. It will be interesting to see whether the high-energy decrease in the positron-to-electron ratio is confirmed by future AMS data, and whether this can be used to discriminate between exotic and conventional models for positron production.

A more sensitive probe of unconventional physics could be provided by the AMS measurement of the spectrum of antiprotons. These cannot be produced in the electromagnetic processes around pulsars, but would be produced as “secondaries” in the collisions between primary-matter cosmic rays and ordinary-matter particles. It is striking, for instance, that the antiproton-to-proton ratio measured by AMS is almost constant at energies of about 10 GeV. The ratio is significantly higher than some earlier calculations of secondary antiproton production, although recent calculations (which account more completely for the theoretical uncertainties) indicate that the antiproton-to-proton ratio may be somewhat higher – possibly even consistent with the AMS measurements. As with the case for positron production, extending the measurements to higher energies will be crucial for distinguishing between exotic and conventional mechanisms for antiproton production.

AMS has also released interesting data concerning the fluxes of protons, helium and lithium nuclei. Intriguingly, all three spectra show strong indications of breaks in the spectra at rigidities of around 200 GV. The higher-energy portions of the spectra lie significantly above simple power-law extrapolations of the lower-energy data. It seems that some additional acceleration mechanism might be playing a role at higher energies, providing food-for-thought for astrophysical models of cosmic-ray acceleration. In particular, the unexpected shape of the spectrum of primary protons in the cosmic rays may also need to be taken into account when calculating the secondary antiproton spectrum.

The AMS data on the boron-to-carbon ratio also provide interesting information for models of the propagation of cosmic rays. In the most general picture, cosmic rays can be considered as a relativistic gas diffusing through a magnetised plasma. This leads to a boron-to-carbon ratio that decreases as a power, Δ, of the rigidity, with different models yielding values of Δ between –1/2 and –1/3. The latest AMS data constrain this power law with very high precision: Δ = –0.333±0.015, in excellent agreement with the simplest Kolmogorov model of diffusion.

The AMS collaboration has already collected data on the production of many heavier nuclei, and it would be interesting if the team could extract information about unstable nuclear isotopes that might have been produced by a recent nearby supernova explosion. Such events might already have had an effect on Earth: analyses of deep-ocean sediments have recently confirmed previous reports of a layer of iron-60 that was presumably deposited by a supernova explosion within about 100 parsecs about 2.5 million years ago, and there is evidence of iron-60 also in lunar rock samples and cosmic rays. Other unstable isotopes of potential interest include beryllium-10, aluminium-26, chlorine-39, manganese-53 and nickel-59.

Promising prospects

What else may we expect from AMS in the future? The prospective gains from measuring the spectra of positrons and antiprotons to higher energies have already been mentioned. Since these antiparticles can also be produced by other processes, such as pulsars and primary-matter cosmic rays, they may not provide smoking guns for antimatter production via dark-matter annihilation, or for concentrations of antimatter in the universe. However, searches for antinuclei in cosmic rays present interesting prospects in either or both of these directions. The production of antideuterons in dark-matter annihilations may be visible above the background of secondary production by primary-matter cosmic rays, for example. On the other hand, the production of heavier antinuclei in both dark-matter annihilations and cosmic-ray collisions is expected to be very small. The search for such antinuclei has always been one of the main scientific objectives of AMS, and the community looks forward to hearing whatever data they may acquire on their possible (non-)appearance.

As this brief survey has indicated, AMS has already provided much information of great interest for particle physicists studying scenarios for dark matter, for astrophysicists and for the cosmic-ray community. Moreover, there are good prospects for further qualitative advances in future years of data-taking. The success of AMS is another example of the fruitful marriage of particle physics and astrophysics, in this case via the deployment in space of a state-of-the-art particle spectrometer. We look forward to seeing the future progeny of this happy marriage.

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A next-generation dark-matter detector in the US called LUX-ZEPLIN (LZ), which will be at least 100 times more sensitive than its predecessor, is on schedule to begin its deep-underground hunt for WIMPs in 2020. In August, LZ received a US Department of Energy approval (“Critical Decision 2 and 3b”) concerning the project’s overall scope, cost and schedule. The latest approval step sets in motion the building of major components and the preparation of its nearly mile-deep cavern at the Sanford Underground Research Facility (SURF) in Lead, South Dakota.

The experiment, which is supported by a collaboration of more than 30 institutions and about 200 scientists worldwide, is designed to search for dark-matter signals from within a chamber filled with 10 tonnes of purified liquid xenon. LZ is named for the merger of two dark-matter-detection experiments: the Large Underground Xenon experiment (LUX) and the UK-based ZonEd Proportional scintillation in LIquid Noble gases (ZEPLIN) experiment. LUX, a smaller liquid-xenon-based underground experiment at SURF that earlier this year ruled out a significant region of WIMP parameter space, will be dismantled to make way for the new project.

“Nobody looking for dark-matter interactions with matter has so far convincingly seen anything, anywhere, which makes LZ more important than ever,” says LZ project-director Murdock Gilchriese of the University of California at Berkeley.

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The Large Underground Xenon (LUX) experiment located at Sanford Underground Research Facility (SURF) in South Dakota, US, has released its latest results in the search for dark matter. Following the completion of its final 20 month-long run, during which the detector amassed a data set that is four times larger than before, no signal was found and the results were consistent with background expectations.

LUX is based on a 370 kg liquid-xenon dual-phase time-projection chamber that offers a high sensitivity to spin-independent nuclear-recoil interactions. It entered operation in 2013 to search directly for WIMPs in addition to other dark-matter candidates. The experiment’s latest results, which were presented on 21 July at the 2016 International Dark Matter conference in Sheffield, UK, carve out previously un-probed parameter space and exclude spin-independent WIMPS at the level of 0.22 zeptobarns. The collaboration plans further analyses of other dark-matter candidates, including axions.

Researchers are looking ahead to the next-generation LUX-ZEPLIN (LZ) detector, also located at SURF, which is scheduled to start operations by the end of the decade. With an active mass of seven tonnes, LZ will be sensitive to WIMP masses ranging from a few GeV to hundreds of TeV, and will therefore probe even deeper into dark-matter parameter space.

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The largest 3D map of distant galaxies ever made has allowed one of the most precise measurements yet of dark energy, which is currently driving the accelerating expansion of the universe. The new measurements, which were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) programme of the Sloan Digital Sky Survey-III, took five years to make and include 1.2 million galaxies over one quarter of the sky – equating to a volume of 650 cubic billion light-years.

BOSS measures the expansion rate by determining the size of baryonic acoustic oscillations, which are remnants of primordial acoustic waves. “We see a dramatic connection between the sound-wave imprints seen in the cosmic microwave background to the clustering of galaxies 7–12 billion years later,” says co-leader of the BOSS galaxy-clustering working group Rita Tojeiro. “The ability to observe a single well-modelled physical effect from recombination until today is a great boon for cosmology.”

The map shows galaxies being pulled towards each other by dark matter, while on much larger scales it reveals the effect of dark energy ripping the universe apart. It also reveals the coherent movement of galaxies toward regions of the universe with more matter, with the observed amount of in-fall explained well by general relativity. The results have been submitted to the Monthly Notices of the Royal Astronomical Society.

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Astrophysics and cosmology have established that about 80% of the mass in the universe consists of dark matter. Dark matter and normal matter interact gravitationally, and they may also interact weakly, raising the possibility that collisions at the LHC may produce pairs of dark-matter particles.

With low interaction strength, dark-matter particles would escape the LHC detectors unseen, accompanied by Standard Model particles. These particles, such as single jets, photons, or W, Z or Higgs bosons, could either be produced in the interaction with the dark matter or radiated from the colliding partons. One result would be “mono-X” signals, named because the Standard Model particle, X, would appear alone, without other visible particles balancing their momentum in the transverse plane of the detector.

During Run 1 of the LHC, ATLAS developed a broad programme of searches for mono-X signals. Now, new results from the ATLAS collaboration in the mono-jet and mono-photon channels are the first of these searches in the proton–proton collision data collected in 2015 after increasing the LHC collision energy to 13 TeV. With only 3.2 fb^–1 of collisions, six times fewer than studied in Run 1, these first Run 2 results already achieve comparable sensitivity to beyond-the-Standard-Model phenomena. In each search, the data with large missing transverse momentum are compared with data-driven estimates of Standard Model backgrounds. As an example, the background to the mono-jet search is known to 4–12%, an estimate nearly as precise as that obtained in the final Run 1 analysis. ATLAS has also released preliminary Run 2 results in the mono-Z, mono-W and mono-H channels.

If dark-matter production is observed, ATLAS has the potential to characterise the interaction itself. To produce dark matter in LHC collisions, the interaction must involve the constituent partons within the proton. If the interaction is mediated by s-channel exchange of a new boson, a decay back to the Standard Model partons could also occur.

The ATLAS collaboration has also released new results from the dijet search channel, where new phenomena could modify the smooth dijet invariant mass distribution. With 3.6 fb^–1 of data, the search already surpasses the sensitivity of Run 1 dijet searches for many kinds of signals. The dijet results are presented on a simplified model of dark-matter production, where the dark boson has axial-vector couplings to quarks and Dirac dark matter.

The results of the mono-photon, mono-jet and dijet searches are shown in figure 1, assuming a version of the axial-vector dark boson whose couplings to dark matter are four times stronger than those to Standard Model quarks. In this scenario, ATLAS dijet results exclude the existence of mediating particles with masses from about 600 GeV to 2 TeV. The mono-jet and mono-photon channels exclude the parameter space at lower mediator and dark-matter masses. For even larger ratios of the dark-matter-to-quark coupling values, dijet constraints quickly weaken, and mono-X searches play a more powerful role.

On the verge of new data-taking in 2016, with the LHC expected to deliver an order of magnitude more luminosity, mono-X and direct mediator searches at ATLAS are set to probe this and other models with unprecedented sensitivity.

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An excess of gamma rays at energies of a few GeV was found to be a good candidate for a dark-matter signal. Two years later, a pair of research articles refute this interpretation by showing that the excess photons detected by the Fermi Gamma-ray Space Telescope are not smoothly distributed as expected for dark-matter annihilation. Their clustering reveals instead a population of unresolved point sources, likely millisecond pulsars.

The Milky Way is thought to be embedded in a dark-matter halo with a density gradient increasing towards the galactic centre. The central region of our Galaxy is therefore a prime target to find an electromagnetic signal from dark-matter annihilation. If dark matter is made of weakly interacting massive particles (WIMPs) heavier than protons, such a signal would naturally be in the GeV energy band. A diffuse gamma-ray emission detected by the Fermi satellite and having properties compatible with a dark-matter origin created hope in recent years of finally detecting this elusive form of matter more directly than only through gravitational effects.

Two independent studies published in Physical Review Letters are now disproving this interpretation. Using different statistical-analysis methods, the two research teams found that the gamma rays of the excess emission at the galactic centre are not distributed as expected from dark matter. They both find evidence for a population of unresolved point sources instead of a smooth distribution.

The study, led by Richard Bartels of the University of Amsterdam, the Netherlands, uses a wavelet transformation of the Fermi gamma-ray images. The technique consists of a convolution of the photon count map with a wavelet kernel shaped like a Mexican hat, with a width tuned near the Fermi angular resolution of 0.4° in the relevant energy band of 1–4 GeV. The intensity distribution of the derived wavelet peaks is found to be inconsistent with that expected from a truly diffuse origin of the emission. The distribution suggests instead that the entire excess emission is due to a population of mostly undetected point sources with characteristics matching those of millisecond pulsars.

In the coming decade, new facilities at radio frequencies will be able to detect hundreds of new millisecond pulsars in the central region of the Milky Way.

These results are corroborated by another study led by Samuel Lee of the Broad Institute in Cambridge and Princeton University. This US team used a new statistical method – called a non-Poissonian template fit – to estimate the contribution of unresolved point sources to the gamma-ray excess emission at the galactic centre. The team’s results predict a new population of hundreds of point sources hiding below the detection threshold of Fermi. The possibility of detecting the brightest ones in the years to come with ongoing observations would confirm this prediction.

In the coming decade, new facilities at radio frequencies will be able to detect hundreds of new millisecond pulsars in the central region of the Milky Way. This would definitively rule out the dark-matter interpretation of the GeV excess seen by Fermi. In the meantime, the quest towards identifying the nature of dark matter will go on, but little by little the possibilities are narrowing down.

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On 17 December, the Chinese Academy of Sciences (CAS) successfully launched the DArk Matter Particle Explorer (DAMPE) satellite from the Jiuquan Satellite Launch Center in northwest China, marking the entrance of a new player in the global hunt for dark matter.

The nature of dark matter is one of the most fundamental questions of modern science, and many experiments have been set up to unravel this mystery, using either large underground detectors or at colliders (for example at the LHC), or with space missions (for example, AMS, CERN Courier November 2014 p6, or CALET, CERN Courier November 2015 p11).

DAMPE is the first science satellite launched by CAS. Built with advanced particle-detection technologies, DAMPE will extend the dark-matter search in space into the multi-TeV region. It will measure electrons and photons in the 5 GeV–10 TeV range with unprecedented energy resolution (1.5% at 100 GeV), to find dark-matter annihilation in these channels. It will also measure precisely the flux of nuclei up to above 100 TeV, which will bring new insights into the origin and propagation of high-energy cosmic rays. With its excellent photon-detection capability, the DAMPE mission is also well placed for new discoveries in high-energy γ-ray astronomy. The DAMPE collaboration consists of Chinese (Purple Mountain Observatory, University of Science and Technology, Institute of High Energy Physics, Institute of Modern Physics, Lanzhou, National Space Science Center) and European (University of Geneva, INFN Perugia, Bari and Lecce) institutes.

The DAMPE detector weighs 1.4 tonnes and consumes 400 W. It consists of, from top to bottom, a plastic scintillator detector (PSD) that serves as an anti-coincidence detector, a silicon-tungsten tracker-converter (STK), a BGO imaging calorimeter of about 31 radiation lengths, and a neutron detector (NUD). The STK, which improves the tracking and photon detection capability of DAMPE greatly, was proposed and designed by the European team and was constructed in Europe, in collaboration with IHEP, in a record time of two years. DAMPE became a CERN-recognised experiment in March 2014 and has profited greatly from the CERN test-beam facilities, both in the Proton Synchrotron and the Super Proton Synchrotron. In fact, CERN provided more than 60 days of beam from July 2012 to December 2015, allowing DAMPE to calibrate its detector extensively with various types of particles, with energy raging from 1 to 400 GeV.

Three days after the launch, on 20 December, the STK was powered on, and four days later, the high voltage of the calorimeter was also turned on. To the satisfaction of the collaboration, all of the detector sub-systems functioned very well, and in-orbit commissioning is now well under way to tune the detector to optimal condition for the three-year observation period. A great deal of data collection, process and analysis lie ahead, but thanks to CERN, we can look forward to a well-callibrated DAMPE detector to produce exciting new measurements in the very near future.

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In recent years, evidence for the existence of dark matter from astrophysical observations has become indisputable. Although the nature of dark matter remains unknown, many theoretically motivated candidates have been proposed. Among them, the most popular ones are Weakly Interacting Massive Particles (WIMPs) with predicted masses in the range from a few GeV/c² to TeV/c² and with interaction strengths roughly on the weak scale.

WIMPs are being searched for using three complementary techniques: indirectly, by detecting the secondary products of WIMP annihilation or decay in celestial bodies; by producing WIMPs at colliders, foremost the LHC; and by direct detection, by measuring the energy of recoiling nuclei produced by collisions with WIMPs in low-background detectors.

On 11 November 2015, the most sensitive detector for the direct detection of WIMPs, XENON1T, was inaugurated at the Italian Laboratori Nazionali del Gran Sasso (LNGS) – the largest underground laboratory in the world. XENON1T, led by Elena Aprile of Columbia University, was built and is operated by a collaboration of 21 research groups from France, Germany, Italy, Israel, the Netherlands, Portugal, Sweden, Switzerland, the United Arabic Emirates and the US. In total, about 130 physicists are involved.

XENON1T is the current culmination of the XENON programme of dark matter direct-detection experiments. Starting with the 25 kg XENON10 detector about 10 years ago, the second phase of the experiment, XENON100 (CERN Courier October 2013 p13) with 161 kg, has been tremendously successful: in the summer of 2012, the XENON collaboration published results from a search for spin-independent WIMP–nucleon interactions that provided the most stringent constraints on WIMP dark matter, until superseded by the LUX experiment (CERN Courier December 2013 p8) with a larger target.

XENON100 has since then also provided a series of other important results, such as constraints on the spin-dependent WIMP nucleon cross-section, constraints on solar axions and galactic axion-like particles and, more recently, searches for annual rate modulations, which exclude WIMP–electron scattering that could have provided a dark-matter explanation of the signal observed by DAMA/LIBRA (CERN Courier November 2015 p10).

Low background is key

The new XENON1T detector has an estimated sensitivity that is a factor of 100 better than XENON100. This will be reached after about two years of data taking. With only one week of data-taking, XENON1T will be able to reach the current LUX limit, opening up a new phase in the search for dark matter in early 2016.

The XENON detectors are dual-phase time-projection chambers (TPCs) filled with liquid xenon (LXe) as the target material. Interactions of particles in the liquefied xenon give rise to prompt scintillation light and ionisation. The ionised electrons are drifted in a strong electric field and extracted into the gas above the liquid where a secondary scintillation signal is produced. Both scintillation signals are read out by arrays of photomultiplier tubes (PMTs) placed above and below the target volume. The position of the interaction vertex can be reconstructed in 3D by using the hit pattern on the upper PMT array and the time delay between the prompt and secondary scintillation signal. The position reconstruction facilitates self-shielding by only selecting events that interact with the inner “fiducial” volume of the detector. Because of their small cross-section, WIMPs will interact only once in the detector, so the background (e.g. from neutrons) can be reduced further by selecting single-scatter interactions. Beta and gamma backgrounds are reduced by selecting events with a ratio of secondary-to-prompt signal that is typical for nuclear recoils.

The XENON1T detector is filled with about 3.5 tonnes of liquid xenon in total. Its TPC – 1 m high and 1 m in diameter in a cylindrical shape, laterally defined by highly reflective Teflon – is the largest liquid-xenon TPC ever built. Specially designed copper field-shaping electrodes ensure the uniformity of the drift field for the desired field strength of 1 kV/cm. The TPC’s active volume contains 2 tonnes of LXe viewed by two arrays of 3 inch PMTs – 121 at the bottom immersed in LXe and 127 on the top in the gaseous phase. The xenon gas is liquefied and kept at a temperature of about –95 °C by a system of pulse-tube refrigerators. The xenon gas is stored and can be recovered in liquid phase in a custom-designed stainless-steel sphere that can hold up to 7.6 tonnes of xenon in high-purity conditions. Figure 3 shows the XENON1T detector and service building situated in Hall B at LNGS. Figure 1 shows XENON collaborators active in assembling the TPC in a clean room above ground at LNGS.

The expected WIMP–nucleon interaction rate is less than 10 events in 1 tonne of xenon per year. Background rejection is therefore the key to success for direct-detection experiments. Externally induced backgrounds can be minimised by exploiting the self-shielding capabilities. In addition, the detector is surrounded by a cylindrical water vessel 10 m high and 9.6 m in diameter. It is equipped with PMTs to tag muons that could induce neutrons, with an efficiency of 99.9%.

For a detector the size of XENON1T, radioactive impurities in the detector materials and the xenon itself become the biggest challenge for background reduction. Extensive radiation-screening campaigns, using some of the world’s most sensitive germanium detectors, have been conducted, and high-purity PMTs have been specially developed by Hamamatsu in co-operation with the collaboration. Contamination of the xenon by radioactive radon (mainly ²²²Rn) and krypton (⁸⁵Kr), which dominate the target-intrinsic background, led to the development of cryogenic-distillation techniques to suppress the abundance of these isotopes to unprecedented low levels.

The best scenario

After about two years of data taking, XENON1T will be able to probe spin-independent WIMP–nucleon cross-sections of 1.6 × 10^–47 cm² (at a WIMP mass of 50 GeV/c²), see figure 2. In popular scenarios involving supersymmetry, XENON1T will either discover WIMPs or will exclude most of the theoretically relevant parameter space. Following the inauguration, the first physics run is envisaged to start early this year.

Most of the infrastructure, for example the outer cryostat, the Cherenkov muon veto, the xenon cryogenics, the purification and storage systems and the data-acquisition system, has been dimensioned for a larger experiment, named XENONnT, which is designed to contain more than 7 tonnes of LXe. A new TPC, about 40% larger in diameter and height and equipped with about 400 PMTs, will replace the XENON1T TPC. The goal for XENONnT is to achieve another order of magnitude improvement in sensitivity within a few years of data taking. XENONnT is scheduled to start data taking in 2018.

• For further details, see www.xenon1t.org/.

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Our star has been the target of human investigation since the beginning of science. However, a plethora of observations are not yet understood. A good example is the unnaturally hot solar corona, the temperature of which spans 1–10 MK. This anomaly has been studied since 1939 but, in spite of a tremendous number of observations, no real progress in understanding its origin has been made. We also know that a significant fraction of the Sun’s total luminosity, about 4%, can escape as some form of radiation that we do not yet know, without being in conflict with the constraints imposed by the evolution of the Sun. In this framework, physicists have hypothesised the existence of exotic particles, including axions and chameleons. Other particles, such as the celebrated WIMPs, also point to the Sun as a target for relevant investigations. Indeed, over cosmic time periods, WIMPs can be gravitationally trapped inside the solar core. There, they condense, allowing their mutual annihilation into known particles, including escaping high-energy neutrinos.

A breakthrough discovery in the so-called “dark sector” could pop up at any time. The question is when this will happen and where: in an Earth-bound laboratory or in a space-bound one. It is worth stressing that it is not at all obvious whether the extreme conditions in the Sun can be completely duplicated on Earth.

Benchmark for axion searches

For many days in recent years, CAST – the CERN Axion Solar Telescope (CERN Courier April 2010 p22) – has pointed its antenna towards the Sun for about 100 minutes during sunrise and sunset. Its aim was to detect solar axions through the Primakoff effect (1950), a classic detection scheme from particle physics. This solar-axion search was completed in November 2015 (CERN Bulletin, https://cds.cern.ch/journal/CERNBulletin/2015/39/News%20Articles/2053133?ln=en), and even though CAST has not observed an axion signature, it provides world-best limits on the axion interaction strength with normal matter in the form of the magnetic field present inside the CAST magnet bores.

The results of the CAST scientific programme were also achieved thanks to the X-ray telescope (XRT) recovered from the ABRIXAS German space mission and installed downstream on one of the magnet bores. The telescope works as a lens focusing the photon flux onto the detector. Any increase in the signal-to-noise ratio would be a signature of axions. This unique technique, borrowed from astrophysics, allowed the collaboration to simultaneously measure signal and background. Given its success, a second X-ray telescope was added in 2014.

Very accurate tracking of the Sun is crucial to the experiment’s data analysis. To provide this, CERN surveyors pinpoint exactly where the telescope lies and where it is pointing to, relative to a reference in time and space. However, to be absolutely certain, twice a year, when the Sun is visible through a window in the CAST experimental hall, the magnet tracks the Sun with a camera mounted and aligned to point exactly along its axis. This process of “Sun filming” has confirmed that CAST is pointing at the centre of the Sun with sufficient precision.

Up to now, CAST has been looking for exotica that the Sun might have produced some 10 minutes earlier. However, thanks to a continuous upgrade programme for the detectors and the development of new ideas, the collaboration is now extending its horizons, back in time closer to the Big Bang and into the dark sector. In its 119th meeting, the CERN SPS and PS experiments Committee (SPSC) recommended the new CAST physics programme for approval, which includes searches for relic axions and chameleons.

Axions from the Big Bang

Due to their extremely long lifetime (longer than the age of the universe), axions produced during the Big Bang could still be detected today. These relic particles have been searched for with instruments using a resonant cavity immersed in a strong magnetic field where axions are expected to convert into photons (with a probability that depends quadratically on the magnetic-field intensity). The signal is further enhanced when the cavity is at resonance with the photon frequency. In particular, the signal strength depends on the cavity “quality factor”, defined as the ratio between the cavity fundamental frequency and the resonance line width.

However, the inherent problem of axion searches is the unknown rest mass, although the cosmologically preferred mass range for the so-called cold dark-matter axions lies between μeV/c² and meV/c², with a favoured region around 0.1 meV. The photon energy is equal to the axion rest mass, because its kinetic energy is negligibly small. To scan the regions of interest, the cavity resonant frequency is varied over a certain axion-mass range, basically determined by cavity size and shape.

Dipole magnets, such as the CAST magnet, can be transformed into relic axion antennas by means of new resonant microwave cavities. These cavities, designed and built by the Korean Centre for Axion and Precision Physics (CAPP) in collaboration with CERN, will be inserted inside the dipole magnetic field within the 1.7 K cold bores to search for microwave photons converted from cosmological axions, which would be direct messengers from the Big Bang era. In addition, a second microwave sensor will be inserted in the other bore. With its new set-up currently under construction, CAST should have access to an axion-mass range up to 100 μeV/c². At these relatively high mass values, detection becomes much harder, but the hope is that this region, which is critical for the dark-matter conundrum, will also be explored.

Chameleons come on stage

As may be imagined, detecting chameleons – new scalar particles that are possible candidates for the unknown dark energy – is not a trivial matter. The CAST collaboration plans to do it by exploiting two different couplings: Primakoff coupling to photons and direct coupling to matter.

The expected energy spectrum of solar chameleons has a peak at about 600 eV, making it even harder to detect them through their Primakoff coupling than the axions. Therefore, sub-keV threshold, low-background photon detectors are required. To tackle this problem, the CAST collaboration decided to start with a Silicon Drift Detector (SDD), becoming, with recently published results, the first chameleon helioscope. The new InGRID detector, based on the MicroMegas concept and having on-board read-out electronics, replaced the CCD camera in the XRT focal plane in 2014, improving the overall expected performance of CAST for solar chameleons.

Chameleon particles are theorised to have amazing properties: they can freely traverse thick slabs of dense matter if they impinge on them normally (i.e. perpendicular to), or they can bounce off nanometre-thin membranes, not much denser than ordinary glass, when approaching them at a grazing incidence angle of just a few degrees. In doing so, they exert a minute force, much like grains of sand hitting the palm of a hand. If detected, this tiny force is the signature of the direct interaction of chameleons with matter.

Forces are experienced in everyday life, so there may seem to be nothing special about detecting them. However, sensing exceedingly tiny forces requires advanced skills and techniques. The KWISP opto-mechanical force sensor is able to instantaneously feel forces of 10^–14 N – that is, the weight of a single bacterium. It uses a Si₃N₄ membrane, just 100 nm thick, to intercept the flux of solar chameleons. Being as elastic as a drumhead, it flexes under their collective force (pressure) by an amount less than the size of an atomic nucleus. The membrane sits inside a Fabry–Pérot optical resonator, made of two high-reflectivity super mirrors facing each other, where a standing wave from an IR laser beam is trapped. As the membrane flexes, the characteristic frequency of this wave changes, generating the signal. The power of the KWISP sensor comes from the combined response of two high-Q resonators, the optical (Fabry–Perot) and the mechanical (membrane).

In addition to KWISP, a further ingredient is necessary in the search for chameleons: a time-dependent amplitude modulation on the chameleon flux in such a way as to beat the drum at its eigenfrequency for maximum effect. To solve this problem, the authors have invented the chameleon chopper, which is basically a rotating optically flat surface, applying the principle of chameleon optics: transmission at normal incidence, reflection at grazing incidence. Surprisingly, phase-locking techniques can also exploit this angular variation to obtain additional information on chameleon physics.

According to theory, the internal surfaces of the ABRIXAS telescope, designed to reflect X-rays impinging at grazing incidence, would also reflect and focus chameleons. This increases their flux by a factor larger than 100, which is further enhanced by the exposure time gained from Sun-tracking. This unplanned ability of the X-ray telescope is one of those lucky events by which nature sometimes smiles at scientists, allowing them to explore its secrets.

The KWISP prototype is currently taking data at INFN Trieste (Italy) and a clone is being commissioned at CERN to take advantage of the CAST infrastructure. As mentioned also by the SPSC referees, with the force-sensor KWISP, it should be possible to address more fundamental physics questions, such as quantum gravity or the validity of Newton’s 1/R² law at short distances. We plan, with colleagues from the Technical University in Darmstadt (Germany), Freiburg University (Germany) and CAPP (Korea), to develop an advanced KWISP design, aKWISP, and we welcome the interest of additional collaborators.

While it remains one of the lowest-cost astroparticle physics experiments, CAST is preparing to leap further into the dark sector. As history teaches us (see table 1), the Sun may be the key to this, although as our understanding of the Sun deepens, we will most probably uncover more mysteries about the star that gives us life.

• For more information, see https://cds.cern.ch/record/2022893.

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Nearly 400 days of data taken by the XENON collaboration were used to look for the telltale signature of dark matter, an event rate that varies periodically over the course of a year.

The null result of this search – the first of its kind using a liquid-xenon detector – strongly challenges dark-matter interpretations of the annual modulation observed by the DAMA/LIBRA experiments. Both subterranean experiments are operated at the Laboratori Nazionali del Gran Sasso (LNGS).

An annually varying flux of dark matter through the Earth is expected due to the Earth’s orbital motion around the Sun, which results in a change of relative velocity between the Earth and the dark-matter halo thought to encompass the Milky Way. The observation of such an annual modulation is considered to be a crucial aspect of the direct detection of dark matter.

The DAMA/LIBRA experiments have observed an annual modulation of the residual rate in their sodium-iodide detectors since 1998. However, previous null results from several experiments searching for dark-matter-induced nuclear recoils, including XENON100, have challenged such an interpretation of the DAMA/LIBRA signal.

An alternative explanation, that the DAMA/LIBRA signal is instead due to dark-matter interactions with electrons, is challenged strongly by the new results from XENON100. In studies recently published in Science and Physical Review Letters, three models that predict dark-matter interactions with electrons were considered. The very low rate of electronic recoils in XENON100 allowed these models to be ruled out with high probability.

The studies highlight the overall stability and low background of XENON100, a landmark performance achieved with this type of technology so far. Liquid-xenon detectors continue to lead the field of direct dark-matter detection in terms of their sensitivity to these rare processes. The commissioning of the next generation of XENON experiments at the underground site in LNGS is nearing completion. The detector, XENON1T, is expected to be 100 times more sensitive than its predecessor, and will hopefully shed more light on the elusive nature of dark matter.

Weblink

• arxiv.org/abs/1507.07748

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The search for particles that could constitute dark matter in the universe relies on detecting their interplay with the Standard Model particles through a three-pronged approach: via direct-detection experiments, via indirect-detection experiments, and with hermetic detectors at colliders, covering the full 4π-phase space. Because dark matter behaves as a weakly interacting neutral particle, it escapes the detectors without interacting, so in collider experiments its production is inferred by measuring the imbalance in transverse momentum left in the detector. At the LHC, a search for the pair production of dark-matter particles can be performed by looking for events with a large momentum imbalance in association with initial-state radiation of either a jet or a photon – the “monojet” or “monophoton” searches.

The CMS collaboration now has results based on proton–proton collision data collected at a centre-of-mass energy of 8 TeV, amounting to 20 fb^–1 of integrated luminosity. In the analysis, both monojet and monophoton searches employ a “cut-and-count” approach. A set of cuts is applied to select potential dark-matter events and, at the same time, to reduce the contamination from Standard Model processes.

One of the dominant and irreducible backgrounds for both searches is the decay of the Z boson into neutrinos, which accounts for roughly 60–70% of the total monojet/monophoton events. The searches look for an excess of events above those expected from the Standard Model processes. In the absence of an excess, limits can be placed on the pair production of dark-matter particles. The results are presented within the framework of an effective field theory where a contact interaction is assumed between the dark-matter and Standard Model particles. Because the effective field theory is not valid for the full parameter space probed at the LHC, the searches are also interpreted in the context of a simplified model with an s-channel mediator. Both assumptions are depicted in the Feynman diagrams in figure 1.

The results (see figure 2) show that CMS extends the sensitivity to spin-independent dark-matter–Standard Model interactions including a vector operator to dark-matter masses that are lower (below 5 GeV) than is currently accessible to the direct-detection experiments. For spin-dependent interactions that include an axial-vector operator, the sensitivity of CMS (not shown here) extends down to dark-matter–nucleon cross-sections of 10^–41 cm². If the particle mediating the dark-matter–Standard Model interaction is accessible at LHC energies, CMS has the opportunity to search for the mediator itself. Figure 3 shows the constraints placed on the mass and coupling strengths of vector-mediator interactions in the monophoton analysis.

The LHC plays a significant role in the search for dark matter and complements well the searches by the direct-detection experiments. The CMS collaboration is now looking forward to intensifying the search with data at 13 TeV and opening up a completely new energy regime to spot hints of dark-matter particles.

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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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The first volume in the Peking University–World Scientific Advance Physics Series, this book introduces the current state of research on dark energy. The first part deals with preliminary knowledge, including general relativity, modern cosmology, etc. The second part reviews major theoretical ideas and models of dark energy, and the third part reviews some observational and numerical work. It will be useful for graduate students and researchers who are interested in dark energy.

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Because radiation from the colliding partons is most likely a jet, the “monojet” search is a powerful search for dark matter. The ATLAS collaboration now has a new result in this channel and, while it does not show evidence for dark-matter production at the LHC, it does set significantly improved limits on the possible rate for a variety of interactions. The reach of this analysis depends strongly on a precise determination of the background from Z bosons decaying to neutrinos at large-boson transverse-momentum. By deriving this background from data samples of W and Z bosons decaying to charged leptons, the analysis achieves a total background uncertainty in the result of 3–14%, depending on the transverse momentum.

To compare with non-collider searches for weakly interacting massive particle (WIMP) dark matter, the limits from this analysis have been translated via an effective field theory into upper limits on WIMP–nucleon scattering or on WIMP annihilation cross-sections. When the WIMP mass is much smaller than several hundred giga-electron-volts – the kinematic and trigger thresholds used in the analysis – the collider results are approximately independent of the WIMP mass. Therefore, the results play an important role in constraining light dark matter for several types of spin-independent scattering interactions (see figure). Moreover, collider results are insensitive to the Lorentz structure of the interaction. The results shown on spin-dependent interactions are comparable to the spin-independent results and significantly stronger than those of other types of experiments.

The effective theory is a useful and general way to relate collider results to other dark-matter experiments, but it cannot always be employed safely. One advantage of the searches at the LHC is that partons can collide with enough energy to resolve the mediating interaction directly, opening complementary ways to study it. In this situation, the effective theory breaks down, and simplified models specifying an explicit mediating particle are more appropriate.

The new ATLAS monojet result is sensitive to dark-matter production rates where both effective theory and simplified-model viewpoints are worthwhile. In general, for large couplings of the mediating particles to dark matter and quarks, the mediators are heavy enough to employ the effective theory, whereas for couplings of order unity the mediating particles are too light and the effective theory is an incomplete description of the interaction. The figures use two types of dashed lines to depict the separate ATLAS limits calculated for these two cases. In both, the calculation removes the portion of the signal cross-section that depends on the internal structure of the mediator, recovering a well-defined and general but conservative limit from the effective theory. In addition, the new result presents constraints on dark-matter production within one possible simplified model, where the mediator of the interaction is a Z’-like boson.

While the monojet analysis is generally the most powerful search when the accompanying Standard Model particle is radiated from the colliding partons, ATLAS has also employed other Standard Model particles in similar searches. They are especially important when these particles arise from the dark-matter interaction itself. Taken together, ATLAS has established a broad and robust programme of dark-matter searches that will continue to grow with the upcoming data-taking.

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The US Department of Energy Office of High Energy Physics and the National Science Foundation Physics Division have announced their joint programme for second-generation dark-matter experiments, aiming at direct detection of the elusive dark-matter particles in Earth-based detectors. It will include ADMX-Gen2 – a microwave cavity searching for axions – and the LUX-Zeplin (LZ) and SuperCDMS-SNOLAB experiments targeted at weakly interacting massive particles (WIMPs). These selections were partially in response to recommendations of the P5 subpanel of the US High-Energy Physics Advisory Panel for a broad second-generation dark-matter direct-detection programme at a funding level significantly above that originally planned.

While ADMX-Gen2 consists mainly of an upgrade of the existing apparatus to reach a lower operation temperature of around 100 mK, and is rather inexpensive, the two WIMP projects are significantly larger. SuperCDMS will initially operate around 50 kg of ultra-pure germanium and silicon crystals at the SNOLAB laboratory in Ontario, for a search focused on WIMPs with low masses, below 10 GeV/c². The detectors will be optimized for low-energy thresholds and for very good particle discrimination. The experiment will be designed such that up to 400 kg of crystals can be installed at a later stage. The massive LZ experiment will employ about 7 tonnes of liquid xenon as a dark-matter target in a dual-phase time-projection chamber (TPC), installed at the Sanford Underground Research Facility in South Dakota. It is targeted mainly towards WIMPs with masses above 10 GeV/ c². The timescale for these experiments foresees that the detector construction will start in 2016, with commissioning in 2018. All three experiments need to run for several years to reach their design sensitivities.

Meanwhile, other projects are operational and taking data, and several new second-generation experiments, with target masses beyond the tonne scale, are fully funded and currently being installed. The Canadian–UK project DEAP-3600, installed at SNOLAB, should take its first data with a 3.6-tonne single-phase liquid-argon detector by the end of this year. Its sensitivity goal is a factor 10–25 beyond the current best limit, depending on the WIMP mass. XENON1T, a joint effort by US, European, Swiss and Israeli groups, aims to surpass this goal using 3 tonnes of liquid xenon, of which 2 tonnes will be inside a dual-phase TPC. Construction is progressing fast at the Gran Sasso National Laboratory, and first data are expected by 2015. These experiments and their upgrades, the newly funded US projects, and other efforts around the globe, should open up a bright future for direct-dark-matter searches in the years to come.

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The recent discovery of a Higgs boson at CERN appears to represent the summit in the successful experimental verification of the Standard Model of particle physics. However, although essentially all of the data from particle accelerators are so far in perfect agreement with the model’s predictions, a number of important theoretical and observational considerations point to the necessity of physics beyond the Standard Model. An especially powerful argument comes from cosmology. The currently accepted cosmological model invokes two exotic ingredients – dark matter and dark energy – which pervade the universe. In particular, the observational evidence for dark matter (via its gravitational effects on visible matter) is now overwhelming, even though the particle-physics nature of both dark matter and dark energy remains a mystery.

At the same time, the theoretical foundations of the Standard Model have shortcomings that prompt theorists to propose and explore hypothetical ways to extend it. Supersymmetry is one such hypothesis, which also naturally provides particles as candidates for dark matter, known as weakly interacting massive particles (WIMPs). Other extensions to the Standard Model predict particles that could lie hidden at the low-energy frontier, of which the axion is the prototype. The fact that supersymmetry has not yet been observed at the LHC, and that no clear signal of WIMPs has appeared in dark-matter experiments, has increased the community’s interest in searching for axions. However, there are independent and powerful motivations for axions, and dark matter composed of both WIMPs and axions is viable, implying that they should not be considered as alternative, exclusive solutions to the same problem.

After more than a decade of searching for solar axions, CAST has put the strongest limits yet on axion–photon coupling

Axions appear in Standard Model extensions that include the Peccei–Quinn mechanism, which provides the most promising solution so far to one of the problems of the Standard Model: why do strong interactions seem not to violate charge–parity symmetry, while according to QCD, the standard theory of strong interactions, they should do? Unlike many particles predicted by theories that go beyond the Standard Model, axions should be light, and it might seem that they should have been detected already. Nevertheless, they could exist and remain unnoticed because they naturally couple only weakly with Standard Model particles.

A generic property of axions is that they couple with photons in a way that axion–photon conversion (and vice versa) can occur in the presence of strong magnetic or electric fields. This phenomenon is the basis of axion production in the stars, as well as of most strategies for detecting axions. Magnets are therefore at the core of any axion experiment, as is the case for axion helioscopes, which look for axions from the Sun. This is the strategy followed by the CERN Axion Solar Telescope (CAST), which uses a decommissioned LHC test magnet (CERN Courier April 2010 p22). After more than a decade of searching for solar axions, CAST has put the strongest limits yet on axion–photon coupling across a range of axion masses, surpassing previous astrophysical limits for the first time and probing relevant axion models of sub-electron-volt mass. However, to improve these results and go deep into unexplored axion parameter space requires a completely new experiment.

The International Axion Observatory (IAXO) aims for a signal-to-noise ratio 10⁵ better than CAST. Such an improvement is possible only by building a large magnet, together with optics and detectors that optimize the axion helioscope’s figure of merit, while building on experience and concepts of the pioneering CAST project.

The central component of IAXO is a superconducting toroid magnet. The detector relies on a high magnetic field distributed across a large volume to convert solar axions to detectable X-ray photons. The magnet’s figure of merit is proportional to the square of the product of magnetic field and length, multiplied by the cross-sectional area filled with the magnetic field. This consideration leads to a 25-m-long and 5.2-m-diameter toroid assembled from eight coils, generating 2.5 T in eight bores of 600 mm diameter, thereby having a figure of merit that is 300 times better than the CAST magnet. The toroid’s stored energy is 500 MJ.

The design is inspired by the barrel and endcap toroids of the ATLAS experiment at the LHC, which has the largest superconducting toroids ever built and currently in operation at CERN. The superconductor used is a NbTi/Cu-based Rutherford cable co-extruded with aluminum – a successful technology common to most modern detector magnets. The IAXO detector needs to track the Sun for the longest possible period, so to allow rotation around the two axes, the 250-tonne magnet is supported at its centre of mass by a system used for large telescopes (figure 1). The necessary services for vacuum, helium supply, current and controls rotate together with the magnet.

Each of the eight magnet bores will be equipped with X-ray focusing optics that rely on the fact that at X-ray energies the index of refraction is less than unity for most materials. By working at shallow (or grazing) incident angles, it is possible to make mirrors with high reflectivity. Mirrors are commonly used at synchrotrons and free-electron lasers to condition or focus the intense X-ray beams for user experiments, but IAXO requires optics with much larger apertures. For nearly 50 years, the X-ray astronomy and astrophysics community has been building telescopes following the design principle of Hans Wolter, employing two conic-shaped mirrors to provide true-imaging optics. This class of optics allows “nesting” – that is, placing concentric co-focal X-ray mirrors inside one another to achieve high throughput.

IAXO will use CERN’s expertise efficiently to venture deep into unexplored axion parameter space

The IAXO collaboration envisions using optics similar to those used on NASA’s NuSTAR – an X-ray astrophysics satellite with two focusing telescopes that operate in the 3–79 keV band. NuSTAR’s optics consist of thousands of thermally formed glass substrates deposited with multilayer coatings to enhance the reflectivity above 10 keV (figure 2). For IAXO, the multilayer coatings will be designed to match the softer 1–10 keV solar-axion spectrum.

At the focal plane in each of the optics, IAXO will have small time-projection chambers read by pixelized planes of Micromegas. These detectors (figure 2) have been developed extensively within the CAST collaboration and show promise for detecting X-rays with a record background level of 10^–8–10^–7 counts/keV/cm²/s. This is achieved by the use of radiopure detector components, appropriate shielding, and offline discrimination algorithms on the 3D event topology in the gas registered by the pixelized read-out.

Beyond the baseline described above, additional enhancements are being considered to explore extensions of the physics case for IAXO. Because a high magnetic field in a large volume is an essential component in any axion experiment, IAXO could evolve into a generic “axion facility” and facilitate various detection techniques. Most intriguing is the possibility of hosting microwave cavities and antennas to search for dark-matter axions in mass ranges that are complementary to those in previous searches.

The growing IAXO collaboration has recently finished the conceptual design of the experiment, and last year a Letter of Intent was submitted to the SPS and PS Experiments Committee of CERN. The committee acknowledged the physics goals of IAXO and recommended proceeding with the next stage – the creation of the Technical Design Report. These are the first steps towards the realization of the most ambitious axion experiment so far.

After more than three decades, the axion hypothesis remains one of the most compelling portals to new physics beyond the Standard Model, and must be considered seriously. IAXO will use CERN’s expertise efficiently to venture deep into unexplored axion parameter space. Complementing the successful high-energy frontier at the LHC, the IAXO facility would open a new window on the dark universe.

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Until recently, the main experimental method to look for dark-matter particles was to exploit their elastic scattering on nuclei inside sensitive detectors, working typically at low temperatures. These direct-detection experiments aim to observe the scattering by measuring the momentum of the recoiling nucleus. While interesting hints for dark-matter detection in various mass ranges have been reported by some of these experiments, none of these hints have been confirmed by later, more precise measurements.

Several years ago, a new idea appeared: to look for the production of pairs of dark-matter particles in high-energy particle collisions, like those at the LHC, via a process described by the same Feynman diagram as the scattering of the dark-matter particles on quarks inside the nuclei, but “rotated” by 90°. While such direct-detection experiments look for the process q_χ → q_χ, experiments at the LHC can look for qq → χχ. The challenge is to trigger on these events, because dark-matter particles would leave no trace in the detector. One possibility is to search for a more complicated process, where an additional particle, for example a gluon or a photon, is produced together with the dark matter.

The CMS experiment has performed a number of such searches, which are referred to collectively as mono-X searches, because they look for a single object, X, recoiling against the invisible particles. Recently, these searches have been extended to more complicated signatures, for example the production of dark matter in association with a pair of top quarks, which are produced in abundance at the LHC. The new analysis looks for top-quark pairs that are recoiling against a large amount of “missing” transverse momentum, carried away by dark-matter particles.

As figure 1 shows, a new measurement by CMS of the production of top-quark pairs in association with missing transverse momentum sets stringent limits in the plane of the dark-matter particle mass M_χ vs an effective interaction energy scale, M* (CMS Collaboration 2014a). The interaction of dark matter with the known particles is usually assumed to be carried by new “messenger” particles. If the messengers are heavy – which would be a good reason why they have not yet been seen – the interaction can be approximated via a point-like interaction with an effective energy scale of M*. This is similar to Enrico Fermi’s effective theory of muon decay, where the messenger – a W boson – is much heavier than the muon.

Another interesting way to look for dark matter is based on precision measurements of the properties of the Higgs boson. If the mass of the dark-matter particle is less than roughly half of the Higgs boson mass, for instance, M_χ < 60 GeV, then it is possible to look for a direct decay of the Higgs boson into a χχ pair. This decay is called “invisible” because its products are not detected.

The CMS collaboration recently published a search for such invisible Higgs-boson decays, where the production of the Higgs is tagged either by the presence of a Z boson (associated ZH production), or by the presence of two forward jets, characteristic of vector-boson fusion (CMS Collaboration 2014b). The upper limits set on the invisible branching fraction of the Higgs boson are 51% and 58% at a 90% and 95% confidence level, respectively. The former limit can be translated to limits on the mass of the dark-matter particle vs its interaction cross-section with a nucleon, which allows for a direct comparison with the limits coming from various direct-detection experiments, as figure 2 shows. The limits are set for various types of dark-matter particle: scalar, vector, or a Majorana fermion. They are significantly more stringent than the direct-detection limits for low masses for dark matter, emphasizing the complementarity of the searches by the LHC and the direct-detection experiments.

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At the end of February, the Cryogenic Dark Matter Search collaboration announced new results, obtained with the SuperCDMS detector. They expanded their search down to a previously untested dark-matter particle-mass range of 4–6 GeV/c² and a dark-matter nucleon cross-section range of 1 × 10^–40–1 × 10^–41 cm². Their exclusion results contradict recent hints of dark-matter detection by another experiment, CoGeNT, which uses particle detectors made of germanium – the same material used by SuperCDMS.

For their new results, CDMS employed a redesigned cryogenic detector known as iZIP that has ionization and phonon sensors interleaved on both sides of the germanium crystals. This substantially improves rejection of surface events from residual radioactivity, which have limited dark-matter sensitivity in previous searches. The collaboration operated these detectors 0.7 km underground in the Soudan mine in northern Minnesota, to shield them from cosmic-ray backgrounds.

There have been several recent hints for low-mass dark-matter particle detection, from previous data using silicon instead of germanium detectors in CDMS, and from three other experiments—DAMA, CoGeNT and CRESST—all finding their data compatible with the existence of dark-matter particles between 5 and 20 GeV/c². But such light dark-matter particles are hard to pin down. The lower the mass of the dark-matter particles, the less energy they leave in detectors, and the more likely it is that background noise will drown out any signals.

The new CDMS iZIP detectors, with their improved background rejection, are continuing this search at Soudan, and hopefully soon in the lower background environment at SNOLAB. Confirmation of a signal of the direct detection of dark matter, and understanding of the interaction of dark matter with normal matter, is likely to require spotting these particles with different target nuclei in at least two different experiments.

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The collaboration that built and runs the Large Underground Xenon (LUX) experiment, operating in the Sanford Underground Research Laboratory, has released its first results in the search for weakly interacting massive particles (WIMPs) – a favoured candidate for dark matter.

The LUX detector holds 370 kg of liquid xenon, with 250 kg actively monitored in a dual-phase (liquid–gas) time-projection chamber measuring 47 cm in diameter and 48 cm in height (cathode-to-gate). If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons and electrons. The electrons are drawn upwards by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons. Light detectors in the top and bottom of the tank can detect single photons and so the two photon signals – one at the interaction point, the other at the top of the tank – can be pinpointed to within a few millimetres. The energy of the interaction can be measured precisely from the brightness of the signals.

The detector was filled with liquid xenon in February and the first results, for data taken during April to August, represent the analysis of 85.3 live days of data with a fiducial volume of 118 kg. The data are consistent with a background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP–nucleon elastic scattering with a minimum upper limit on the cross-section of 7.6 ×10^–46 cm² at a WIMP mass of 33 GeV/c². The data are in strong disagreement with low-mass WIMP signal interpretations of the results from several recent direct-detection experiments.

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Numerous astronomical observations indicate that about one quarter of the energy content of the universe is made up of a mysterious substance known as dark matter. The Planck collaboration recently measured this to the precise percentage of 26.8%, which is slightly greater than the previous value from nine years of observations by the Wilkinson Microwave Anisotropy Probe (WMAP). Dark matter, which is five times more abundant than baryonic matter, provides compelling evidence for new physics and could be made of a new particle not present in the Standard Model. Theories beyond the Standard Model, such as supersymmetric models or theories with extra dimensions, suggest promising candidates and naturally predict so-called weakly interacting massive particles (WIMPs), which are stable or have lifetimes longer than the age of the universe.

There are several complementary strategies to detect dark matter. The ATLAS and CMS experiments at the LHC search for such particles produced in proton–proton collisions. Indirect searches, for example by the AMS-02 or IceCube detectors, aim at detecting the products of dark-matter annihilation in cosmic rays.

Because dark-matter particles are expected to be abundant in the Galaxy, with an energy density of about 0.3 GeV/c²/cm³ at the location of the Sun, the most direct strategy is to look for their interactions in laboratory-based detectors. In general, it is possible to study spin-independent WIMP–nucleon interactions – which scale with the square of the target’s mass number, A – or spin-dependent couplings to unpaired nucleons in the target nucleus. Because of their nonrelativistic Maxwellian velocity distribution with a typical speed of around 220 km/s and because the WIMPs interact significantly only with nuclei (and not with the electrons), the expected signal is a featureless exponential nuclear-recoil spectrum. The recoil energies depend on the mass of the WIMP and on the target material and are typically of the order of a few tens of kilo-electron-volts.

Because the expected interaction rates are small, a sensitive WIMP detector needs to feature a large target mass, an ultralow background and a low energy threshold. In addition, it should allow the distinction of the nuclear-recoil signal (from WIMPs and also from background neutrons) from the overabundant electronic-recoil background from γ and β radiation.

The most sensitive dark-matter detector to date is XENON100, which is operated by the XENON collaboration and situated at the Italian Laboratori Nazionali del Gran Sasso (LNGS), under about 1.3 km of rock that provides a natural shield from cosmic rays. The experiment searches for WIMP interactions in a target of 62 kg of liquid xenon. The noble gas xenon is cooled to around –90°C to bring it to the liquid state with a density of around 3 g/cm³. Its high mass number, A, of around 130 makes it one of the heaviest of all target materials for dark-matter detection.

The detector was built from materials selected for their low intrinsic radioactivity

XENON100 is operated as a dual-phase time-projection chamber (TPC), as figure 1 illustrates. Particle interactions excite the liquid xenon, leading to prompt scintillation light, and also ionize the target atoms. A uniform electric field causes the ionization electrons to drift away from the interaction site to the top of the TPC. Here a strong electric field extracts them into the xenon-gas phase above the liquid. Subsequent scattering on the gas atoms leads to signal amplification and a secondary scintillation signal, which is directly proportional to the ionization extracted. Both the prompt and secondary scintillation light are detected by two arrays of low-radioactivity photomultipliers (PMTs), which are installed above and below the cylindrical target of around 30 cm height and 30 cm diameter (figure 2). The PMTs are immersed in the liquid and gaseous xenon to achieve the highest-possible light-detection efficiency and therefore the lowest threshold. The 3D position of the interaction vertex is obtained by combining the time difference between the prompt and the secondary scintillation signal with the hit pattern of the localized secondary signal on the array of 98 PMTs above the target. The number of secondary signals defines the event multiplicity.

The detector was built from materials selected for their low intrinsic radioactivity. Thanks to its novel detector design – placing most radioactive components outside of a massive passive shield – and the self-shielding provided by the liquid xenon, XENON100 features the lowest published background of all dark-matter experiments. The self-shielding is exploited by selecting only events that interact with the inner part of the detector (“fiducialization”) and by rejecting all events that exhibit a coincident signal in the active veto, which is made of 99 kg of liquid xenon that surrounds the target. Because of their small cross-section, WIMPs will interact only once in the detector, so background can be reduced further by selecting single-scatter interactions with a charge-to-light ratio typical for the expected nuclear-recoil events.

In the summer of 2012, the XENON collaboration published results from a search for spin-independent WIMP–nucleon interactions based on 225 live days of data (XENON collaboration 2012). No indication for dark matter was found but the derived upper limits are the most stringent to date for WIMP masses above 7 GeV/c². The same data have now been interpreted in terms of spin-dependent interactions and the results published recently (XENON collaboration 2013). This latest analysis requires knowledge of the axial-vector coupling and the nuclear structure of the two xenon isotopes with unpaired nucleons, ¹²⁹Xe and ¹³¹Xe. Improved calculations were employed here, which are based on chiral-effective field-theory currents. Compared with older calculations, these yield superior agreement between calculated and predicted nuclear energy-spectra (Menendez et al. 2012).

The specific nuclear structure of the relevant xenon isotopes leads to different sensitivities for the two extreme cases that are usually considered. For the case where WIMPs are assumed to couple to protons only, the new XENON100 limit is competitive with other results (figure 3). Indirect dark-matter searches looking for signals from the annihilation of WIMPs trapped in the Sun (which mainly consists of protons) are particularly sensitive to this channel. For the neutron-only coupling, XENON100 sets a new best limit for most masses, improving the previous constraints by more than an order of magnitude (figure 3).

The aim is to reach a dark-matter sensitivity two orders of magnitude better than the current best value

While XENON100 continues to take science data at LNGS, the development of a larger liquid-xenon detector is well under way. XENON1T will be about 35 times larger than XENON100, with a TPC of around 100 cm in height and diameter. The aim is to reach a dark-matter sensitivity two orders of magnitude better than the current best value. This will probe a significant part of the theoretically favoured WIMP parameter space but will require the radioactive background of the new instrument to be 100 times lower than that of XENON100. The greatly increased liquid xenon target mass of more than two tonnes helps to achieve this goal.

The largest background challenge comes from uniformly distributed traces of radioactive radon (mainly ²²²Rn) and krypton (⁸⁵Kr, present in natural krypton at a fraction of about 10^–11) dissolved in the xenon, because the background from these isotopes cannot be reduced by target fiducialization. To achieve the background goals for XENON1T, the contamination of radon and krypton in the xenon filling will be reduced to below a level of parts per 10¹² by careful material selection and surface treatment and by cryogenic distillation, respectively. Additionally, all of the construction materials for the detector are being carefully selected based on their intrinsic radioactivity using ultrasensitive germanium detectors. A few of the world’s most sensitive detectors are owned and operated by institutions in the XENON collaboration.

The XENON1T detector will be placed inside a large water shield to protect it from environmental radioactivity (figure 4). The water will be equipped with PMTs to tag muons via emission of Cherenkov light, because muon-induced neutrons could mimic WIMP signals. The construction of the water tank is underway in Hall B of LNGS and will be finished by the end of 2013. Together with the XENON1T service building, it will be the first visible landmark of the experiment underground. The other XENON1T systems – from detector and cryogenics to massive facilities for the storage and purification of xenon – are currently being designed, built, commissioned and tested at the various collaborating institutions. In particular, the challenges associated with building a TPC of 100 cm drift length, which will be the longest liquid xenon-based TPC ever, are being addressed with dedicated R&D set-ups.

Once the main underground facilities are erected, the XENON1T low-background cryostat – to contain the TPC and more than three tonnes of xenon – will be installed inside the water shield. The infrastructure for the storage, purification and liquefaction have been designed to handle more than double the amount of xenon initially used in XENON1T. Their commissioning underground is expected to be completed by the summer of 2014. The timeline foresees commissioning of the full XENON1T experiment by the end of 2014 and the first data by early 2015. After two years of data-taking, XENON1T will reach a sensitivity of 2 × 10^–47 cm² for spin-independent WIMP-nucleon cross-sections at a WIMP mass of 100 GeV/c². This is a factor 100 better than the current best WIMP result from XENON100.

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Possible hints of a particle that may be a candidate for dark matter have already begun to appear in the direct-detection experiments; most recently the CDMS-II collaboration reported the observation of three candidate events in its silicon detectors with an estimated background of 0.7 events. This result points to low masses, below 10 GeV/c², as a region that should be particularly interesting to search. This mass region is where the direct-detection experiments start to lose sensitivity because they rely on measuring the recoil energy imparted to a nucleus by collisions with the dark-matter particles. For a low-mass χ, the kinetic energy transferred to the nucleus in the collision is small, and the detection sensitivity drops as a result.

The CMS collaboration has searched for hints of these elusive particles in “monojet” events, where the dark-matter particles escape undetected, yielding only “missing momentum” in the event. A jet of initial-state radiation can accompany the production of the dark-matter particles, so a search is conducted for an excess of these visible companions compared with the expectation from Standard Model processes. The results are then interpreted within the framework of a simple “effective” theory for their production, where the particle mediating the interaction is assumed to have high mass. An important aspect of the search by CMS is that there is no fall in sensitivity for low masses.

The monojet search requires at least one jet with more than 110 GeV of energy and has the best sensitivity if there is more than 400 GeV of missing momentum. Events with additional leptons or multiple jets are vetoed. After event selection, 3677 events were found in the recent analysis, with an expectation from Standard Model processes of 3663 ± 196 events. The contribution from electroweak processes dominate this expectation, either from pp → Z+jets with the Z decaying to two neutrinos or from pp → W+jets, where the W decays into a lepton and neutrino, while the lepton escapes detection.

With no significant deviation from the expectation from the Standard Model, CMS has set limits on the production of dark matter, as shown in the figures of the χ–nucleon cross-section versus χ mass. The limits show that CMS has good sensitivity in the low-mass regions of interest, for both spin-dependent and spin-independent interactions.

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In November the Baryon Oscillation Spectroscopic Survey (BOSS) released its second major result of 2012, using 48,000 quasars with redshifts (z) up to 3.5 as backlights to map intergalactic hydrogen gas in the early universe for the first time, as far back as 11,500 million years ago.

As the light from each quasar passes through clouds of gas on its way to Earth, its spectrum accumulates a thicket of hydrogen absorption lines, the “Lyman-alpha forest”, whose redshifts and prominence reveal the varying density of the gas along the line of sight. BOSS collected enough close-together quasars to map the distribution of the gas in 3D over a wide expanse of sky.

The largest component of the third Sloan Digital Sky Survey, BOSS measures baryon acoustic oscillations (BAO) – recurring peaks of matter density that are most evident in net-like strands of galaxies. Initially imprinted in the cosmic microwave background radiation, BAO provide a ruler for measuring the universe’s expansion history and probing the nature of dark energy.

In March 2012, BOSS released its first results on more than 350,000 galaxies up to z = 0.7, or 7000 million years ago. However, only quasars are bright enough to probe the gravity-dominated early universe when expansion was slowing, well before the transition to the present, where dark energy dominates and expansion is accelerating. When complete, BOSS will have surveyed 1.5 million galaxies and 160,000 quasars.

To resolve the nature of dark energy will need even greater precision. The BigBOSS collaboration, which, like BOSS, is led by scientists at Lawrence Berkeley National Laboratory (LBNL), proposes to modify the 4-m Mayall Telescope to survey 24 million galaxies to z = 1.7, plus two million quasars to z = 3.5. The Gordon and Betty Moore Foundation recently awarded a grant of $2.1 million to help fund the spectrograph and corrector optics, two key BigBOSS technologies.

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The 4% Universe is, as you might gather from the title, an account of how the scientific community has come to the idea that only (a little over) 4% of the universe seems to be made of the same stuff as you and me. In other words, normal matter is only a tiny percentage of all that there is, with the remainder being about 23% dark matter holding galaxies together and 73% being dark energy, which drives the acceleration of cosmic expansion.

This account is unusual, written more like a thriller than in the style of many popularizations. There is a great emphasis on not only describing the sequences of events leading to the discoveries of dark matter and dark energy but also of the people involved. Personalities, co-operations, disagreements, collaboration and individualism all take a large part of the stage, making the book lively and readable. I had originally planned to read it in chunks over a few days but found myself taking it all in during a single sitting, somewhat later into the night than I had planned!

This book would be a nice gift for anyone with a genuine interest in science but, oddly enough, it may be a hard read for someone without at least some background knowledge. At the same time, it is short on details (no equations, graphs, plots or photographs) for a practising physicist who is not so interested in the personal dramas involved. If you’re looking for a book about dark matter and dark energy per se, then this may not be the best choice. While the science is probably more than 4% of the book, the bulk is about sociology, history and politics.

Nevertheless, technical terms are well explained, down to footnotes for those who need to know what the Kelvin scale or a megaparsec is. The physics is pretty good, too, but not perfect in all places. For example, the discussion on the Casimir effect seems not quite to get that the energy density between the plates is negative with respect to the region outside.

The emphasis is very much on astronomy and astronomical observation and how data are collected and presented. Particle physicists should not expect much about the direct search for particles that could make up dark matter. The LHC merits a brief mention but without further discussion. Axions and neutralinos are introduced as dark-matter candidates but without any explanation of the ideas that gave rise to them.

Apart from the insights into the sociology of how “big astronomy” is done, I think that the book’s greatest merit is to drive home how much our view of the universe has changed in the past 100 or so years – from a rather simple, static universe to an expanding, even accelerating one, with far more stars and galaxies than had ever been imagined and, now, the realization that all of that visible matter may be only a few per cent of all that is. That, as well as to show how cosmology has made the giant step from being little different from theology to being a real scientific discipline.

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By professional astronomy standards, the 2.5 m telescope at Apache Point Observatory is quite small. More than 50 research telescopes are larger and many are located at much better sites. Apache Point Observatory is also a little too close to city lights – the atmospheric turbulence that dominates the sharpness of focus is about two times worse than at the best sites on Earth – and summer monsoons shut down the observatory for two months each year.

Yet, the Sloan Digital Sky Survey (SDSS), using this telescope, has produced the most highly cited data set in the history of astronomy (Trimble and Ceja 2008; Madrid and Macchetto 2009). Its success is rooted in the combination of high-quality, multipurpose data and open access for everyone: SDSS has obtained 5-filter images of about a quarter of the sky, spectra of 2.4 million objects and has made them publicly available on a yearly basis, even as the survey continues.

SDSS-III launched its ninth data release (DR9) on 31 July. This is the first release to include data from the upgraded spectrographs of the Baryon Oscillation Spectroscopic Survey (BOSS) – the largest of the four subsurveys of SDSS-III. By measuring more distant galaxies, these spectra probe a larger volume of the universe than all previous surveys combined.

BOSS has already published its flagship measurement of baryon acoustic oscillations (BAO) to constrain dark energy using these data (Anderson et al. 2012). BAO are the leftover imprint of primordial matter-density fluctuations that froze out as the universe expanded, leaving correlations in the distances between galaxies. The size scale of these correlations acts as a “standard ruler” to measure the expansion of the universe, complementing the “standard candles” of Type Ia supernovae that led to the discovery of the accelerating expansion of the universe.

Another major BOSS analysis using these data is still in progress. In principle, BAO can also be measured by using bright, distant quasars as backlights and measuring the “Lyman alpha forest” absorption in the spectra as intervening neutral hydrogen absorbs the quasars’ light. The wavelength of the absorption traces the red shift of the hydrogen and the amount of absorption traces its density. Thus, this also measures the structure of matter – including BAO – but at much further distances than is possible with galaxies. BOSS has the first data set with enough quasars to make this measurement and the collaboration is nearing completion of this analysis. However, the final results are not yet published and now the data are public for anyone else to try this.

Are there any surprises in the results? Not yet. BOSS has the most accurate BAO measurements yet, with distances measured to 1.7%, but the results are consistent with the “ΛCDM” cosmological standard model, which includes a dark-energy cosmological constant (Λ) and cold dark matter (CDM). But DR9 contains only about a third of the full BOSS survey and BOSS has already finished observations for data release 10 (DR10), due to be released in July 2013. DR10 will also include the first data from APOGEE, another SDSS-III subsurvey that probes the dynamical structure and chemical history of the Milky Way.

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XENON100 is an ultrasensitive device. It uses 62 kg of ultrapure liquid xenon as a WIMP target and simultaneously measures ionization and scintillation signals that are expected from rare collisions between WIMPs and the nuclei of xenon atoms. The detector is operated deep underground at the Gran Sasso National Laboratory, to shield it from cosmic rays. To avoid false events occurring from residual radiation from the detector’s surroundings, only data from the inner 34 kg of liquid xenon are taken as candidate events. In addition, the detector is shielded by specially designed layers of copper, polyethylene, lead and water to reduce the background noise even further.

In 2011, the XENON100 collaboration published results from 100 days of data-taking. The achieved sensitivity already pushed the limits for WIMPs by a factor 5 to 10 compared with results from the earlier XENON10 experiment. During the new run, a total of 225 live days of data were accumulated in 2011 and 2012, with lower background and hence improved sensitivity. Again, no signal was found.

The two events observed are statistically consistent with the expected background of one event. The new data improve the bounds to 2.0 × 10^–45 cm² for the elastic interaction of a WIMP mass of 50 GeV. This is another factor of 3.5 compared with the earlier results and cuts significantly into the expected WIMP parameter region. Measurements are continuing with XENON100 and a still more sensitive, 100-tonne experiment, XENON1T, is currently under construction.

The XENON collaboration consists of scientists from 15 institutions in China, France, Germany, Israel, Italy, the Netherlands, Portugal, Switzerland and the US.

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A programme of experiments based on innovative detectors aims to take dark-matter detection to a new level of sensitivity.

Dark energy and dark matter together present one of the most challenging mysteries of the universe. While explaining the first seems to be within the reach of only cosmologists and astrophysicists, the latter appears to be accessible also to particle physicists. One of the most recent and innovative experiments designed for the direct detection of dark-matter particles is DarkSide, a prototype for which – DarkSide 10 – is currently being tested in the Gran Sasso National Laboratory in central Italy. The first detector for physics – DarkSide 50 – is scheduled for commissioning underground in December this year.

Astronomical observations suggest that dark matter is made of a new species of non-baryonic particle, which must lie outside the Standard Model. These particles must also be neutral, quite massive, stable and weakly interacting – hence the acronym WIMPs, for weakly interacting massive particles. One of the most promising candidates for a dark-matter particle is the neutralino, the lightest particle that is predicted in theories based on supersymmetry. However, constraints from recent measurements by experiments at CERN’s LHC suggest that WIMPs may have a different origin.

Several potential background sources can mimic the interaction between dark-matter particles and nuclei.

A powerful way of detecting WIMPs directly in the local galactic halo is to look for the nuclear recoils produced when they collide with ordinary matter in a sensitive detector. However, WIMP-induced nuclear recoils are difficult to detect. Theory indicates that they would be extremely rare, with some 10 events expected per year in 100 kg of liquid argon for a WIMP mass of 50 GeV/c² and a WIMP–nucleon cross-section of 10^–45 cm². They would also produce energy deposits below the order of 100 keV. Moreover, there are several potential background sources that can mimic the interaction between dark-matter particles and nuclei.

Sources of background

In a typical target, there are three main sources of background at energies up to tens of kilo-electron-volts: natural β and γ radioactivity, which induces electron recoils; α decays on the surface of the target in which the daughter nucleus recoils into the target and the α particle remains undetected; and nuclear recoils produced by the elastic scattering of background neutrons. This latter process is nearly indistinguishable from the signals expected for WIMPs and requires an efficient neutron veto in the apparatus.

DarkSide is a new experiment that uses novel techniques to suppress background sources as much as possible, while also understanding them well. The programme centres on a series of detectors of increasing mass, each making possible a convincing claim for the detection of dark matter based on the observation of a few well characterized nuclear-recoil events in an exposure of several years. The design concept involves a two-phase, liquid-argon time-projection chamber (LAr-TPC) in which the energy released in WIMP-induced nuclear recoils can produce both scintillation and ionization. Arrays of photomultiplier tubes at the bottom and top of the cylindrical active volume detect the scintillation light. A pair of novel transparent high-voltage electrodes and a field cage provide a uniform drift field of about 1 kV/cm to extract the ionization produced. A reflective, wavelength-shifting lining renders the scintillation light from the argon (wavelength 128 nm) visible to the photomultipliers.

In a two-phase argon TPC, rejection of background comes from three independent discrimination parameters: pulse-shape analysis of the direct liquid-argon scintillation signal (S1); the ratio of ionization produced in an event to scintillation, where the former is read out by extracting ionization electrons from the liquid into the gaseous argon phase, where they are accelerated and emit light through electroluminescence (S2); and reconstruction of the event’s location in 3D using the TPC. The z co-ordinates for the event are determined by the time delay between S2 and S1, while the transverse co-ordinates are determined through the distribution of the S2 light across the layer of photomultiplier tubes.

As in other experiments searching for rare events, DarkSide’s detectors will be constructed using materials with low intrinsic radioactivity. In particular, the experiment uses underground argon with extremely low quantities of ³⁹Ar, which is present in atmospheric argon at levels of about 1Bq/kg as a result of the interaction of cosmic rays, primarily with ⁴⁰Ar. The DarkSide collaboration has developed processes to extract argon from underground gas wells, where the proportion of ³⁹Ar is low. A particularly good source of underground argon is in the Kinder Morgan Doe Canyon Complex in Colorado. The CO₂ natural gas extracted there contains about 600 ppm of argon. The DarkSide collaboration has operated an extraction facility at the Kinder Morgan site since February 2010; it has to date extracted some 90 kg of underground depleted argon and subsequently distilled 23 kg to about 99.99% purity. (The throughput is about 1 kg/day, with 99% efficiency.) Studies of the residual ³⁹Ar content of the distilled gas with a low-background detector at the Kimballton Underground Research Facility, Virginia, give an upper limit for the ³⁹Ar content equivalent to 0.6% of the ³⁹Ar in atmospheric argon.

It is not only the argon that has to have low intrinsic radioactivity. Nuclear recoils produced by energetic neutrons that scatter only once in the active volume form a background that is, on an event-by-event basis, indistinguishable from dark-matter interactions. Neutrons capable of producing these recoil backgrounds are created by radiogenic processes in the detector material. In detectors made from clean materials, the dominant source of the radiogenic neutrons is typically the photodetectors, so ultralow background photodetectors are another important goal for DarkSide. A long-term collaboration with the Hamamatsu Corporation has resulted in the commercialization of 3-inch photomultiplier tubes with a total γ activity of around only 60 mBq per tube, with a further 10-fold reduction foreseen in the near future.

To measure and exclude neutron background produced by cosmic-ray muons, the DarkSide TPC will be deployed within an active neutron veto based on liquid scintillator, which will in turn be deployed within 1000 m³ of water in a tank 10 m high and 11 m in diameter, which was previously used in the Borexino Counting Test Facility at Gran Sasso. The liquid-scintillator neutron veto is a unique feature of the DarkSide design and is filled with ultrapure, boron-loaded organic scintillator, which has been distilled using the purification system of the Borexino experiment. The water serves as a Cherenkov detector to veto muons. Monte Carlo simulations suggest that with this combined veto system, the number of neutron events generated by cosmic-rays at the depth of the Gran Sasso Laboratory should be negligible, even for exposures of the order of tonne-years.

The DarkSide programme will follow a staged approach. The collaboration has been operating DarkSide 10, a prototype detector with a 10 kg active mass, in the underground laboratory at Gran Sasso since September 2011. This has been a valuable test bed during the construction of the veto system. It has allowed the light-collection, high-voltage and TPC field structures – and the data-acquisition and particle-discrimination analysis systems – to be optimized using γ and americium-beryllium sources. The first physics detector in the programme, DarkSide 50, should be deployed inside the completed veto system in the Gran Sasso Laboratory by the end of 2012. Looking forward to the second generation, upgrades to the underground argon plants are planned, and the nearly completed veto system has been designed to accommodate a DarkSide-G2 detector, which will have a fiducial mass of 3.5 tonnes.

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Dark-matter particles produced at the LHC would presumably escape undetected, yielding “missing momentum” in the event. However, they could be accompanied by a jet or a photon, or some other particle. CMS has looked for evidence of these visible companions by studying “monojet” and “monophoton” data. Within the framework of a simple model for the production of dark matter, the CMS analysis significantly extends the sensitivity of direct searches, which look for tiny interactions of dark-matter particles in very sensitive detectors.

The way that the dark-matter particles (χ) are produced and interact depends on their spin. With respect to direct searches, CMS is sensitive in the low-mass region below 3.5 GeV if the spin of the produced particles is ignored, and it can set the world’s best limits at all masses in the spin-dependent case.

The monophoton search looks for single, isolated photons (γ) with transverse energy greater than 145 GeV and more than 130 GeV of missing transverse energy. Events with excessive hadronic activity (jets) are vetoed. After the application of selection criteria, 73 events remain, where 71.9 ± 9.1 would be expected in the absence of dark-matter particles. Standard Model background-events are expected mainly from pp → Zγ – where the Z decays to two neutrinos – and from events with misidentified jets or electrons, or from instrumental sources.

The monojet search requires at least one jet with transverse momentum greater than 110 GeV and more than 350 GeV of missing transverse momentum. Events with isolated leptons or more than two jets are vetoed. After event selection, 1142 events are found in data with an expectation from Standard Model processes of 1224 ± 101 events. Again, a contribution from “invisible” decays to neutrinos dominate this expectation, either from pp → Z+jets with the Z decaying to two neutrinos, or from pp → W+jets where the W escapes detection. There seem to be no signs of a new production mechanism for the two “mono-object” signatures analysed, so CMS can use the null results to place limits on the cross-section for dark matter. The limits depend on the presumed mass of the dark-matter particles and are presented as regions in the plane of cross section vs mass in the figures.

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Paradoxically, work on “light candles” led to the discovery that the universe is much darker than anyone thought. Arnaud Marsollier caught up with Saul Perlmutter recently to find out more about this Nobel breakthrough.

Saul Perlmutter admits that measuring an acceleration of the expansion of the universe – work for which he was awarded the 2011 Nobel Prize in Physics together with Brian Schmidt and Adam Riess – came as a complete surprise. Indeed, it is exactly the opposite of what Perlmutter’s team was trying to measure: the decelerating expansion of the universe. “My very first reaction was the reaction of any physicist in such a situation: I wondered which part of the chain of the analysis needed a new calibration,” he recalls. After the team had checked and rechecked over several weeks, Perlmutter, who is based at Lawrence Berkeley National Laboratory and the University of California, Berkeley, still wondered what could be wrong: “If we were going to present this, then we would have to make sure that everybody understood each of the checks.” Then, after a few months, the team began to make public its result in the autumn of 1997, inviting scrutiny from the broader cosmology community.

Despite great astonishment, acceptance of the result was swift. “Maybe in science’s history, it’s the fastest acceptance of a big surprise,” says Perlmutter. In a colloquium that he presented in November 1997, he remembers how cosmologist Joel Primack stood up and instead of talking to Perlmutter he addressed the audience, declaring: “You may not realize this, but this is a very big problem. This is an outstanding result you should be worried about.” Of course, some colleagues were sceptical at first. “There must be something wrong, it is just too crazy to have such a small cosmological constant,” said cosmologist Rocky Kolb in a later conference in early 1998.

According to Perlmutter, one of the main reasons for the quick acceptance by the community of the accelerating expansion of the universe is that two teams reported the same result at almost the same time: Perlmutter’s Supernova Cosmology Project and the High-z Supernova Search Team of Schmidt and Riess. Thus, there was no need to wait a long time for confirmation from another team. “It was known that the two teams were furious competitors and that each of them would be very glad to prove the other one wrong,” he adds. By the spring of 1998, a symposium was organized at Fermilab that gathered many cosmologists and particle physicists specifically to look at these results. At the end of the meeting, after subjecting the two teams to hard questioning, some three quarters of the people in the room raised their hands in a vote to say that they believed the results.

What could be responsible for such an acceleration of the expanding universe? Dark energy, a hypothetical “repulsive energy” present throughout the universe, was the prime suspect. The concept of dark energy was also welcomed because it solves some delicate theoretical problems. “There were questions in cosmology that did not work so well, but with a cosmological constant they are solved,” explains Perlmutter. Albert Einstein had at first included a cosmological constant in his equations of general relativity. The aim was to introduce a counterpart to gravity in order to have a model describing a static universe. However, with evidence for the expansion of the universe and the Big Bang theory, the cosmological constant had been abandoned by most cosmologists. According to George Gamow, even Einstein thought that it was his “biggest blunder” (Gamow 1970). Today, with the discovery of the acceleration of the expansion of the universe, the cosmological constant “is back”.

Since the discovery, other kinds of measurements – for example on the cosmic microwave background radiation (CMB), first by the MAXIMA and BOOMERANG balloon experiments, and then by the Wilkinson Microwave Anisotropy Probe satellite – have proved consistent with, and even made stronger, the idea of an accelerating expansion of the universe. However, it all leads to a big question: what could be the nature of dark energy? In the 20th century, physicists were already busy with dark matter, the mysterious invisible matter that can only be inferred through observations of its gravitational effects on other structures in the universe. Although they still do not know what dark matter is, physicists are increasingly confident that they are close to finding out, with many different kinds of experiments that can shed light on it, from telescopes to underground experiments to the LHC. In the case of dark energy, however, the community is far from agreeing on a consistent explanation.

When asked what dark energy could be, Perlmutter’s eyes light up and his broad smile shows how excited he is by this challenging question. “Theorists have been doing a very good job and we have a whole landscape of possibilities. Over the past 12 years there was an average of one paper a day from the theorists. This is remarkable,” he says. Indeed, this question has now become really important as it seems that physicists know about a mere 5% of the whole mass-energy of the universe, the rest being in the form of dark matter or, in the case of more than 70%, the enigmatic, repulsive stuff known as dark energy or a vacuum energy density.

Including a cosmological constant in Einstein’s equations of general relativity is a simple solution to explain the acceleration of the expansion of the universe. However, there are other possibilities. For example, a decaying scalar field of the kind that could have caused the first acceleration at the beginning of the universe or the existence of extra dimensions could save the standard cosmological model. “We might even have to modify Einstein’s general relativity,” Perlmutter says. Indeed, all that is known is that the expansion of the universe is accelerating, but there is no clue as to why. The ball is in the court of experimentalists, who will have to provide theorists with more data and refined measurements to show precisely how the expansion rate changes over time. New observations by different means will be crucial, as they could show the way forward and decide between the different available theoretical models.

“We have improved the supernova technique and we know what we need to make a measurement that is 20 times more accurate,” he says. There are also two other precision techniques currently being developed to probe dark energy either in space or from the ground. One uses baryon acoustic-oscillations, which can be seen as “standard rulers” in the same way that supernovae are used as standard candles (see box, previous page). These oscillations leave imprints on the structure of the universe at all ages. By studying these imprints relative to the CMB, the earliest “picture of the universe” available, it is possible to measure the rate at which the expansion of the universe is accelerating. The second technique is based on gravitational lensing, a deflection of light by massive structures, which allows cosmologists to study the history of the clumping of matter in the universe, with the attraction of gravity contesting with the accelerating expansion. “We think we can use all of these techniques together,” says Perlmutter. Among the projects he mentions, are the US-led ground-based experiments BigBOSS and the Large Synoptic Survey Telescope and ESA’s Euclid satellite, all of which are under preparation.

However, the answer to this obscure mystery – or at least part of it – could come from elsewhere. The full results from ESA’s Planck satellite, for instance, are eagerly awaited because they should provide unprecedented precision on measurements of the CMB. “The Planck satellite is an ingredient in all of these analyses,” explains Perlmutter. In addition, cosmology and particle physics are increasingly linked. In particular, the LHC could bring some input into the story quite soon. “It is an exciting time for physics,” he says. “If we just get one of these breakthroughs through the LHC, it would help a lot. We are really hoping that we will see the Higgs and maybe we will see some supersymmetric particles. If we are able to pin down the nature of dark matter, that can help a lot as well.” Not that Perlmutter thinks that the mystery of dark energy is related to dark matter, considering that they are two separate sectors of physics, but as he says, “until you find out, it is still possible”.

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Introducing the relevant theoretical ideas, observational methods and results, this textbook is ideally suited to graduate courses on dark energy, as well as supplement advanced cosmology courses. It covers the cosmological constant, quintessence, k-essence, perfect fluid models, extra-dimensional models and modified gravity. Observational research is reviewed, from the cosmic microwave background to baryon acoustic oscillations, weak lensing and cluster abundances.

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The INFN’s Gran Sasso National Laboratory provides the world’s largest underground infrastructure for astroparticle physics. It currently hosts four operational dark-matter experiments – CRESST, DAMA-LIBRA, WArP and XENON – and was therefore a fitting venue for WONDER, the Workshop On Next Dark-matter Experimental Research. Designed to generate fruitful discussions about the future of the exciting field of dark-matter physics, the workshop was held on 22–23 March and attracted around 100 participants.

As is well known, “dark matter” is the name given to 23% of the “inventory” of the universe, the existence of which is indicated by several experimental facts, the first and most famous being the anomalous behaviour of the radial velocity of galaxies. Although some alternative models still survive to explain these unexpected effects, the most fascinating explanation – at least for particle physicists – is the existence of stable, massive particles that interact only weakly with ordinary matter and permeate all galaxies, including ours. Supersymmetry provides a nice theoretical framework for such an explanation, and the lightest supersymmetric particle, the neutralino, could be a viable candidate for dark matter. First, however, someone has to observe some experimental evidence to pin down the characteristics of the “dark” particles, which are often referred to as “WIMPs” – weakly interacting massive particles. The question is: how to identify the particles?

One way is to look for the production of WIMPs in collisions at the LHC at CERN. Other “indirect” techniques look for likely signatures of annihilations of WIMPs occurring in the Sun, Earth or galactic halo; these could appear, for example, as anomalous neutrino or gamma-ray fluxes. A third method is to observe the direct interactions of WIMPs with ordinary matter. Underground laboratories are the ideal place to carry out this quest. Anywhere else on the surface of the Earth, the overwhelming cosmic radiation would drown out the tiny signal (if it exists), making the search as hopeless as trying to spot a distant star in daylight.

Even amid the “cosmic silence” at the heart of a mountain (as at Gran Sasso), dark-matter experiments struggle to attain the best sensitivity with elaborate techniques and, above all, by trying to reduce the residual gamma and neutron backgrounds to unprecedentedly low levels.

DAMA-LIBRA, one of the first experiments at Gran Sasso, does in fact observe a significant modulation signal in its scintillators of high-purity sodium iodine, which is identical to the one that the motion of the Earth through the dark-matter halo is supposed to cause. The DAMA collaboration presents this signal as evidence for the discovery of dark matter and the scientific community waits for a confirmation, possibly with new, different techniques. The problem is that, until to now, the other experiments seem to rule out DAMA-LIBRA’s result, although the comparison between different techniques is far from straightforward. Theoretical models still survive that reconcile all current experimental results with a positive discovery by DAMA-LIBRA.

Among today’s technologies, detectors employing cryogenic noble liquids occupy a pre-eminent position. These seem to allow for excellent signal-to-background discrimination, coupled with the possibility to build massive detectors. The Gran Sasso National Laboratory provided a natural location to discuss the future of these searches because it hosts three experiments, other than DAMA-LIBRA, that are competing for the discovery of dark matter, namely CRESST, WArP and XENON. The race is particularly interesting between the latter two of these because they use the same “double-phase” technique, but with different targets. XENON employs 160 kg of its homonymous noble element in liquid form, while WArP has a similar amount of liquid argon, a medium with which research groups at INFN have considerable expertise.

Carlo Rubbia, the spokesperson of the WIMP Argon Programme (WArP), opened the workshop with an excellent and comprehensive overview of the experimental landscape. This was followed by theoretical talks that helped to set up the general framework of the field. With regard to experimental activities, preliminary results from the XENON100 detector provided a highlight of the workshop. About 11 days of data have been analysed and were presented by the XENON spokesperson Elena Aprile, from Columbia University. The data show an extremely low background – the lowest ever reached – and raise even stronger expectations for future results.

Claudio Montanari of INFN presented the status of WArP, which has just started data-taking, while Wolfgang Seidel of the Max Planck Institute talked about interesting results from CRESST, a detector made from scintillating calcium-tungstate crystals. Activities beyond Gran Sasso were also discussed. Masaki Yamashita of Kamioka/Tokyo presented Xmass, a particularly promising detector based on liquid xenon, which is close to its commissioning phase in the Kamioka mine in Japan. Newer techniques also seem to be interesting and promising. These include the directional detectors that Neil Spooner of Sheffield University described and, in particular, the bubble chambers COUPP and PICASSO, which Nigel Smith of SNOLAB discussed in his extensive overview of dark-matter activities around the world.

Two stimulating talks were dedicated to the problem of backgrounds, especially from neutrons. Frank Calaprice of Princeton University and Vitaly Kudryavtsev of Sheffield University described these issues. The final session covered, in depth and in a critical manner, the issues of backgrounds, sensitivity and stability for each group of techniques.

Overall, the workshop revealed an extremely lively field, with existing detectors producing new results, others about to enter their commissioning phase, advanced projects being proposed for new underground facilities and intense theoretical activity. We all “wonder” if a discovery is just round the corner.

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– Pierre Sikivie of University of Florida.

Axions are one of the favoured candidates for the mysterious dark matter created in the early universe. A variety of observatories located on Earth and in outer space form a quasi-network that can target specific places in the search for these particles, such as the galactic centre, the inner Earth and the Sun’s hot core. The CERN Axion Solar Telescope (CAST) points at the Sun – its aim being the direct detection of axions or other exotic particles with similar properties.

While relic axions from the early universe should propagate with a velocity of about one thousandth of the speed of light, solar axions – with a broad spectral shape of around 4–5 keV kinetic energy – are relativistic. The open window for the axion rest mass is currently in the micro-electron-volt to electron-volt range. The several orders of magnitude difference in kinetic energy associated with the two origins make for different experimental search techniques: microwave cavities for relic axions versus X-ray detectors for solar axions. However, both techniques use a magnetic field as the catalyst that allows axions to become photons

Accelerator laboratories, with their powerful magnets are natural locations for axion helioscopes – the instruments used to search for axions from the Sun. The first experiment to look at the Sun, which incorporated a 2.2-m iron-core magnet, was set up by a Rochester-Brookhaven-Fermilab (RBF) collaboration in the early 1990s. It was followed by the Sumico experiment based on a 2.3-m long superconducting magnet at the University of Tokyo, which is still in operation. The CAST helioscope at CERN uses a decommissioned LHC-dipole test magnet, with a field of 9 T and two tubes – originally designed to house the beam pipes – that are 9.2 m long and have an aperture of 43 mm. The dipole is one of four original prototypes and was rescued at the last minute before it was about to be scrapped along with the others. A comparison of CAST’s performance with its two predecessors in Brookhaven and Tokyo shows that the LHC magnet was good choice.

The possibility that a bending magnet could be used to make visible the “dark” Sun was – and still is – inspiring and motivating. To transform the multi-tonne superconducting, superfluid-helium-cooled magnet from a static LHC prototype dipole into a helioscope that can track the Sun with millimetre precision involved delicate engineering work and cryo-expertise. Thankfully, Louis Walckiers in the Accelerator Technology Division supported the idea, even though we had both just failed to prove with the same magnet that the biomechanics of cell-structure formation becomes confused in a 9 T environment.

Recycling space technology

Position-sensitive X-ray detectors of the MicroMegas type, invented by Georges Charpak and Ioannis Giomataris at CERN, now cover three of the ends of the tubes through the magnet, making CAST the only axion helioscope to have implemented such technology. For the fourth exit, together with Dieter Hoffmann and Joachim Jacoby of TU Darmstadt we were able to recover an excellent X-ray imaging telescope from the German space programme, which was delivered by Heinrich Bräuninger from the Max Planck Institute for Extraterrestrial Physics in Garching. With state-of-the-art X-ray optics and low-noise X-ray pixel detectors at the focal plane, this not only improves the signal-to-noise ratio substantially but also allows for the unambiguous identification of the axion signal. Its CCD imaging camera simultaneously measures the expected solar-axion signal spot and the surrounding background. This is an important feature that makes CAST unique as an axion helioscope. With most of the components located, CAST received formal approval at CERN in April 2000.

In the same way that much of the CAST equipment was recycled from particle physics so, too, was its working principle: the Primakoff effect, known since 1951, which regards the production of neutral pions by the interaction of high-energy photons with the high electric field of the nucleus as the reverse of the decay into two photons. The expectation is that the quasi-stable axion should “decay” in the presence of a magnetic field into a photon emitted exactly along the axion’s trajectory. In principle this allows for a perfect axion telescope thanks to the spatial resolution of the X-ray telescope.

The Primakoff effect deserves to be a textbook example of macroscopic quantum-mechanical coherence, which, in astrophysical magnetic fields, can extend over kiloparsecs – although only for very small axion rest masses. For CAST, coherence holds over the whole length of the magnet, around 9 m, provided that the particle rest mass is below 0.02 eV/c² when the two pipes are vacuum-pumped. To extend the detection sensitivity to higher masses, adding a certain amount of helium as a refractive gas to the 1.8 K cold magnetic pipes restores coherence for a rest mass up to around 1 eV/c² from a few millimetres up to 9 m but for a narrow range in solar axion rest mass. With this adaptation, suggested in 1988 by two collaboration members Karl van Bibber and Georg Raffelt, and implemented during 2005 and 2006, CAST has become a scanning experiment. The rest-mass range for solar axions that will be scanned by the end of 2010 fits the cosmologically derived upper limit of about 1 eV/c², from the Wilkinson Microwave Anisotropy Probe (WMAP) data, and the lower limit around 1 μeV/c², which arises because axions with lower rest mass would be produced earlier in the early universe, with a total mass exceeding that of the critical density (“overclosure”).

The precise pressure settings for the helium gas and controlled changes in the very cold magnet pipes are highly demanding and are not without risk. CAST has benefited greatly from CERN’s world-class cryogenic expertise in this respect, with its reliable user-friendly gas system designed by Tapio Niinikoski and his PhD student Nuno Elias. At present an extensive thermodynamic simulation is being performed with the aim of reconstructing the changing conditions of the helium gas as the magnet tracks the Sun. For example, to achieve the homogeneity in gas density necessary to keep coherence, the temperature variations along the 9-m long pipes should be in the milli-kelvin range; this is made possible by the surrounding bath of superfluid liquid helium at about 1.8 K.

CAST is also a “special” experiment when compared with others because its highly sensitive magnet and low-background detectors must operate while in motion, even though the speed of about 2 m an hour is almost imperceptible. In addition, CAST’s equipment must withstand quenches of the superconducting magnet. After each quench the gas control system must cope with extreme conditions within seconds. However, during 15,000 hours of operation with the magnet on, and more than 2000 hours of solar tracking, CAST has survived potentially catastrophic events because its safety features have – thanks to the careful work of CERN’s Martyn Davenport – never failed simultaneously.

Scientific return

While CAST has failed so far to find direct evidence for solar axions, it has been able to provide new robust limits on the interaction of solar axions with a magnetic field, i.e. the sea of virtual photons (figure 1). Its experimentally derived limit dominates the relevant phase space and competes with the best astrophysically derived lower value for the coupling constant, g_aγ. CAST is now moving into a theoretically motivated region, having almost fulfilled the original expectations set a decade ago with all of the input uncertainties at that time.

Moving beyond the initial proposal, CAST has in parallel explored – for the first time for a solar axion search – the region of high-energy solar axions, following the proposal of collaboration member Juan Collar. It has also made the first measurements below 1 keV, covering so far the range of around 1–3 eV. Moving to energies above this is possible; however, it will require larger energy steps and some new state-of-the-art detector technology to explore this interesting energy region that covers of most Sun’s puzzling X-ray activity.

Without detecting any solar-axion signature so far, the question arises: what is the scientific return from CAST? Certainly, the first benefit is educational, with students completing some 10 PhD theses and an equal number of diploma theses. There have also been several CAST summer students at CERN. On the research side, CAST has helped to revive axion activities around the world, fitting between pure axion searches in the laboratory and a variety of astrophysical/cosmological observatories that usually did not have axions in their original list of objectives. The state-of-the-art detectors in these observatories cover photon energies from micro-electron-volts upwards. With CAST, the implementation of X-ray optics in axion helioscopy has become widely accepted as a necessary ingredient for future scaled-up versions.

While CAST’s results have became a reference in the relevant field, they have also been used by other teams to search, for example, for “paraphotons” – sterile massive photons from the “hidden sector”. Furthermore, two members of the CAST collaboration, Milica Krĉmar and Biljana Lakić, have used the experiment’s results to explore theories of large extra dimensions, which predict “massive” axions of the Kaluza-Klein type. Interestingly, such massive exotica could be gravitationally trapped in the Sun and could build a bright halo, as a result of their spontaneous decay, as we have suggested with Luigi Di Lella of CERN.

The axion signal that the CAST collaboration aims to observe while tracking the Sun consists of excess X-rays emerging from the magnet tubes. Interestingly, there is abundant solar X-ray emission of otherwise unknown origin, which is further enhanced just above the magnetized photosphere. For more than 70 years, known physics has failed to explain this intriguing behaviour, which could, however, arise from the conversion or decay of axions or other similar exotica near the Sun’s restless surface. The outermost solar layers, i.e. the photosphere, might act occasionally as scaled-up and highly effective catalysts of axions or similar particles, emitting large numbers of X-rays (like a fine-tuned CAST might do one day). Then, extending Sikivie’s original idea, the otherwise mysterious solar surface makes these axions visible as X-rays. New X-ray observatories in space are already providing more and more exciting evidence that something new and interesting is going on in the Sun’s outer layers. The complete axion scheme may make the Sun even more special than it already is.

Such a solar scenario might eventually point to a “superCAST”, which in 5 to 10 years may well make the present CAST look like an old fashioned miniature device – provided that Sikivie’s pioneering idea behind CAST is not replaced by a novel conceptual design. For example, together with Andrzej Siemko of CERN we have proposed using a quadrupole magnet as a potentially better axion catalyst than the dipole magnets used at present in almost all axion experiments. This idea, which was also discussed theoretically by Eduardo Guendelman in 2008, is motivated observationally because otherwise puzzling solar X-ray activity correlates not only with magnetic fields but even more with places of varying field vector.

Alvaro De Rújula commented in 1998 that “axion searches are mandatory, fun, creative – and proceeding”. His words are just as true today, as the CAST project continues into its second decade.

• I am very grateful to all members of the CAST collaboration, to CERN for its hospitality and support, including the librarians, and to my colleagues at the University of Patras for their real help.

This article is dedicated to the memory of the following members of the CAST collaboration who have sadly passed away since the project’s inception: Engin Abat, Engin Arik, Fatma Senel Boydag, Ozgen Berkol Dogan, Angel Morales and Julio Morales.

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AMS-02, the experiment that will seek dark matter, missing matter and antimatter in space aboard the International Space Station (ISS), has recently received the green light to be part of the STS-134 NASA mission in 2010.

NASA has announced that the last or last-but-one mission of the space shuttle programme will be the one that is to deliver the Alpha Magnetic Spectrometer (AMS) to the ISS. The space shuttle Discovery is due to lift off in July 2010 and its mission will include the installation of AMS to the exterior of the space station, using arms on both the shuttle and station. Last year both the US House of Representatives and the Senate unanimously approved a bill requesting NASA to install AMS on the ISS, which was signed by president George W Bush a month later.

AMS is a cosmic-ray detector based on technologies developed at CERN, where it is currently based. The installation of the detector to the right side of the space station’s truss will be a delicate operation. It will be lifted out by the shuttle’s robotic arm and handed on to the station’s robotic arm, which will then install AMS in its location.

The astronauts selected for this flight include the European astronaut Roberto Vittori, a colonel in the Italian air force with a degree in physics. He will come to CERN in October with the rest of the crew to learn more about the experiment. The data collected by AMS will be transmitted instantly from the ISS to the Marshall Space Flight Center in Huntsville, Alabama, and finally to CERN, where all of the detector controls and physics analyses will be performed.

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Recent measurements of cosmic-ray leptons – electrons and positrons – have generated a buzz because they might point to unknown astrophysical or exotic cosmic phenomena. A new measurement of the cosmic-ray positron fraction, e⁺/(e^–+e⁺), by the satellite-borne PAMELA detector shows an unambiguous rise between 10 GeV and 100 GeV. This confirms previous claims by the High-Energy Antimatter Telescope (HEAT) and AMS-01 collaborations (figure 1). At the same time, the Advanced Thin Ionization Calorimeter (ATIC), Fermi Gamma-Ray Telescope and HESS collaborations have published new results on the sum e^–+e⁺ at higher energies, up to a few tera-electron-volts. Although there are still discrepancies between these three experiments they could indicate the presence of a feature in the energy spectrum of e^–+e⁺ between 600 GeV and 1 TeV. Whether it is a bold peak, as ATIC claims, or a more shy bump, as the Fermi data indicate, is still unclear (figure 2). Further work and crosschecks are necessary to reach a definite answer. Another issue concerns whether this feature arises from electrons only or from both electrons and positrons.

There is nevertheless the hint of a signal in this energy range, which is quite challenging to reproduce with conventional cosmic-ray models. A workshop held in Paris in May, “Testing Astroparticle with the New GeV/TeV Observations. Positrons And electRons: Identifying the Sources (TANGO in PARIS)”, provided the opportunity to discuss and confront the possible interpretations of these results.

Conventional cosmic-ray production

The current understanding is that most cosmic rays are produced in the remnants of supernovae – what is left after the cataclysmic ends to the lives of many stars. Some cosmic-ray species (positrons, antiprotons, boron etc.) do not exist in stars but are instead produced by the spallation reaction of other cosmic rays with the interstellar medium. Once made, cosmic rays diffuse in the galactic magnetic field; they lose energy, are convected and eventually reach Earth.

Even taking into account the uncertainties underlying the state-of-the-art cosmic-ray transport modelling it is not possible to reproduce the PAMELA data, as figure 1 shows (T Delahaye et al. 2009). One solution is that the model for standard astrophysical positrons is mistaken in some way. For instance, the source distribution in the galaxy might be more complex than generally believed and positron production by spallating proton cosmic-rays on interstellar matter might be higher than expected. Such an effect could arise from a local over-density of proton sources (the spiral arms) or of interstellar matter around supernova remnants. However, in these models, it is difficult at the same time not to over-produce other cosmic-ray species, such as antiprotons or boron.

Another solution is that supernovae and spallating cosmic rays are not alone in the significant production of high-energy charged particles, so that other astrophysical objects also contribute. As electrons and positrons lose a lot of energy as they propagate in the galaxy, one single nearby source could explain the observed feature. Pulsars seem to be a good candidate for such an effect because they may produce electrons and positrons evenly, thus enriching the surrounding positron fraction. Unfortunately, the way that pulsars could produce electron–positron pairs and release them in the galaxy is not yet clear – making predictions difficult. Nevertheless, recent observations from Fermi have revealed that pulsars are more numerous than expected, so there is a high chance that we are missing many of them. Hence explaining the PAMELA/ATIC feature with pulsars is feasible.

The most exciting solution would be that these excesses arise from the effects of dark matter, so allowing a first insight into physics beyond the Standard Model. Indeed, in such a scenario, the mass of our galaxy would be dominated by new non-standard particles, which would annihilate or decay into standard particles, contributing to the cosmic-ray flux.

While it is extremely appealing, the dark-matter solution is puzzling. The natural way to agree with constraints from cosmology (freeze-out of the dark-matter particles in the early universe) is to have a new particle with mass and couplings of the order of the electroweak scale. If this particle could annihilate or decay into Standard Model particles then the corresponding cosmic-ray production rate would be small, which would not allow the reproduction of features as significant as the ones seen by PAMELA, ATIC, Fermi and HESS. To account for them, the dark-matter signal must be magnified with respect to the standard picture in some way, by a factor ranging from 100 to 1000, depending on the model. This is a well known fact – which the models make possible either with some particle-physics effect (for dark-matter particles of masses typically larger than a few tera-electron-volts or so) or as a consequence of local enhancements of the signal caused by dark-matter substructures.

Trouble appears when confronting this interpretation with channels where corresponding excesses should appear, such as cosmic antiprotons and photons. PAMELA recently published fresh measurements of the antiproton flux up to 100 GeV (figure 3), which show no specific feature. Antiprotons are interesting because the theoretical uncertainty associated with the background estimate is lower than for that of positrons – and most models with new physics expect annihilations or decays of dark matter to produce antiprotons. It is therefore possible to put an upper limit to the signal enhancement necessary to explain the leptonic data (Donato et al. 2009). It eventually appears that the antiproton data are incompatible with the large enhancements that are required by leptons for conventional dark-matter candidates.

The only way out is to have either a very heavy particle (of mass larger than 10 TeV) or to suppress the hadronic annihilation or decay modes of the dark-matter particle. In the first case, an excess of antiprotons should appear in future higher-energy data; in the second, no hadrons are produced by this so-called “leptophilic” dark matter. In both cases the properties of the new particle are different from those usually expected. Within minimal supersymmetric dark-matter models, for instance, large masses imply a loss of naturalness and direct electron/positron production in the annihilation is suppressed. In addition, when confronting models that survive the antiproton constraints to photon observations, the net tightens even more. Indeed, all of these electrons and positrons should also be produced in places where large magnetic fields are present (e.g. at the galactic centre) and consequently produce sizable radio emission, which is in general above the measured values (at least in the most standard galactic models).

The previous considerations assume a particle-physics type enhancement – i.e. an overall enhancement of the production of exotic cosmic leptons – regardless of the location in the galaxy. However, one could ask if these cosmic-ray features are the same everywhere in the galaxy. An interesting possibility is that a nearby clump of dark matter is responsible for some local excesses (Brun et al. 2009). In this case, the antiproton constraints may be less stringent and the ones from photon observations are totally avoided. The main feature responsible for the local lepton anomalies would then be a nearby (closer than a few kiloparsecs), bright clump. (As electrons and positrons do not propagate over large distances, just one massive clump could contribute sufficiently). In fact, dark-matter haloes are expected to form by successive mergers of small structures. Large haloes, such as the one of our galaxy, should contain a lot of smaller subhaloes (up to 20% of the total halo mass). Large numerical simulations can model the formation of these structures and calculate the probability of finding a configuration that fulfils the requirements to account for the lepton excesses in a halo of the size of the Milky Way. Unfortunately, this probability is found to be extremely low; usually fewer than 1% of the simulations exhibit such a favourable scenario. If such a clump does exist, however, the gamma-ray satellite Fermi has enough sensitivity to detect the associated gamma-ray emission.

Epilogue?

It is definitely possible to reproduce the observed cosmic-ray data with the help of dark-matter signals. Within this hypothesis, however, there will always be some tension between the different channels and observables or quite a high level of fine tuning. It could be that we are circling the properties of dark-matter particles but it is more likely that the bulk of the observed leptons come from a nearby astrophysical source that produces a large fraction of electron–positron pairs. In this case, the signal would constitute an additional background for indirect searches for dark matter through lepton channels that had not previously been accounted for.

A big step forward will be the measurement of the small anisotropy in the arrival directions of the cosmic-ray leptons, if any. If it is observed and points towards a known pulsar, then the conclusion will be clear. It is also urgent to separate electrons from positrons at higher energies and to increase statistics in all channels. Future results from PAMELA, and especially AMS-02, on leptons and also on fluxes of all nuclei will be of great help in feeding the cosmic-ray propagation models. The indirect searches for dark matter through charged channels can then continue, in particular looking for fine structure in the spectra. It will then be interesting (and challenging) to interpret future data and weigh them against results from the LHC and direct-detection experiments.

Whatever the nature of the source, we might be witnessing the first direct observation of a nearby source of cosmic rays with energies in the range of giga- to tera-electron-volts. These are exciting times and we might have to wait a little longer for the solution to this cosmic puzzle. The answer(s) will certainly come from a convergence of information from different messengers. Thanks to its large field of view, the Fermi telescope should reveal something about a nearby source, should it be a pulsar or something more exotic. Eventually, future large neutrino and gamma-ray observatories (such as KM3NeT and the Cherenkov Telescope Array) will certainly offer a great opportunity to take a deeper look into this brainteaser.

• The presentations slides and videos the TANGO talks are available at http://irfu.cea.fr/Meetings/TANGOinPARIS.

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The conference started with an overview of searches for supersymmetry at the LHC and dark matter by Elisabetta Barberio of the University of Melbourne. To date, the only evidence for cold dark matter from underground detectors is from the DAMA/LIBRA experiment in the Gran Sasso National Laboratory, as Pierluigi Belli from the collaboration explained. This experiment, which looks for an expected seasonal modulation of the signal for weakly interacting massive particles (WIMPs), now has a significance of 8.4 σ. Unfortunately, all other direct searches for dark matter do not currently have the statistics to look for this signal. Nevertheless, Jason Kumar from Hawaii described how testing the DAMA/LIBRA result at the Super-Kamiokande detector might prove interesting.

Later sessions covered other searches for dark matter. Tarek Saab from Florida gave an overview of ongoing direct searches in underground laboratories, including recent results from the Cryogenic Dark Matter Search experiment in the Soudan mine, and Nigel Smith of the UK’s Rutherford Appleton Laboratory presented results from the ZEPLIN III experiment in the Boulby mine. Irina Krivosheina of Heidelberg and Nishnij Novgorod discussed the potential offered by using bare germanium detectors in liquid nitrogen or argon for dark-matter searches, on the basis of the results from the GENIUS-Test-Facility in the Gran Sasso National Laboratory. Chung-Lin Shan of Seoul National University reported on how precisely WIMPs can be identified in experimental searches in a model-independent way.

Searching for signals from dark-matter annihilation in X-rays and weighing supermassive black holes with X-ray emitting gas were subjects for Tesla Jeltema of the University of California Observatories/Lick Observatory and David Buote of the University of California, Irvine. Stefano Profumo of the University of California, Santa Cruz, provided an overview of fundamental physics with giga-electron- volt gamma rays. Iris Gebauer of Karlsruhe addressed the excess of cosmic positrons indicated by the Energetic Gamma Ray Experiment Telescope, which are still under discussion, as well as the new anomalies observed by the Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics (PAMELA, PAMELA finds an anomalous cosmic positron abundance ) satellite experiment and the Advanced Thin Ionization Calorimeter (ATIC) balloon experiment. These results and the limits that they set on some annihilating dark matter (neutralino or gravitino) models were also discussed by Kazunori Nakayama of Tokyo and Koji Ishiwata of Tohoku.

Other presentations outlined results and prospects for the AMANDA, IceCube and ANTARES experiments, which study cosmic neutrinos – though there is still a long way to go before they have conclusive results. Emmanuel Moulin of the Commissariat à l’énergie Atomique/Saclay presented results from imaging atmospheric Cherenkov telescopes, in particular the recent measurements from HESS, which exploited the fact that dwarf spheroidal galaxies, such as Canis Major, are highly enriched in dark matter and are therefore good candidates for its detection. Unfortunately, the results do not yet have the sensitivity of the Wilkinson Microwave Anisotropy Probe in restricting either the minimal supersymmetric Standard Model or Kaluza–Klein scenarios.

Leszek Roszkowski of Sheffield gave an overview of supersymmetric particles (neutralinos) as cold dark matter, while scenarios of gravitino dark matter and their cosmological and particle-physics implications were presented by Gilbert Moultaka of the University of Montpellier and Yudi Santoso of the Institute for Particle Physics Phenomenology, Durham. Dharam Vir Ahluwalia of the University of Canterbury put the case for the existence of a local fermionic dark-matter candidate with mass-dimension one, on the basis of non-standard Wigner classes. However, as the proposed fields, as outlined in detail by Ben Martin of Canterbury, do not fit into Steven Weinberg’s formalism of quantum-field theory, this suggestion led to dispute between other experts. An interesting candidate for dark matter was presented by Norma Susanna Mankoc-Borstnik of the University of Ljubljana, who proposed a fifth family as candidates for forming dark matter.

Dark energy and the cosmos

Dark energy was a major topic at the conference. Chris Blake of Swinburn University of Technology in Melbourne presented the prospects for the WiggleZ survey at the Anglo-Australian Telescope, the most sensitive experiment of this kind, and Matt Visser of Victoria University in Wellington gave a cosmographic analysis of dark energy. On the theoretical side there are diverging approaches to dark energy, including attempts to explain it in a “radically conservative way without dark energy”, as David Wiltshire of Canterbury University, Christchurch, explained.

A particular highlight was the presentation by Terry Goldman of Los Alamos, which discussed a possible connection between sterile fermion mass and dark energy. His conclusion was that a neutrino with mass of 0.3 eV could solve the problem of dark energy. This possibility was qualitatively supported by results of non-extensive statistics in astroparticle physics that Manfred Leubner of the University of Innsbruck presented, in the sense that dark energy is expected to behave like an ordinary gas. Goldman’s suggestion is also of interest with respect to the final result of the Heidelberg–Moscow double-beta-decay experiment, reported by Hans Klapdor-Kleingrothaus, which predicts a Majorana neutrino mass of 0.2–0.3 eV.

Danny Marfatia of the University of Kansas discussed mass-varying neutrinos in his presentation about phase transition in the fine structure constant. He proposed that the coupling of neutrinos to a light scalar field might explain why Ω_{dark energy} is of the same order as Ω_matter. Possible connections between dark matter and dark energy with models of warped extra dimensions and the hierarchy problem were outlined by Ishwaree Neupane of the University of Canterbury and Yong Min Cho of Seoul National University.

Dark mass and the centre of the galaxy was the topic of a special session in which Andreas Eckart of the University of Cologne presented recent results on the luminous accretion onto the dark mass at the centre of the Milky Way. Patrick Scott of Stockholm University discussed dark stars at the galactic centre, while Benoit Famaey of the Université Libre de Bruxelles and Felix Stoehr of the Space Telescope European Coordinating Facility/ESO in Garching discussed the distribution of dark and baryonic matter in galaxies. Primordial molecules and the first structures in the universe were the topics addressed by Denis Puy of the Univesité Montpellier II. Youssef Sobouti of the Institute of Advanced Studies on Basic Science in Zanjan, Iran, presented a theorem on a “natural” connection between baryonic dark matter and its dark companion, while Matthias Buckley of the California Institute of Technology put forward ideas about dark matter and “dark radiation”.

Gravity also came under scrutiny. David Rapetti of SLAC explored the potential of constraining gravity with the growth of structure in X-ray galaxy clusters, while Agnieszka Jacholkowska of IN2P3/Centre National de la Recherche Scientifique gave an experimental view of probing quantum-gravity effects with astrophysical sources. In a special session on general relativity, Roy Patrick Kerr of Canterbury University gave an interesting historical lecture entitled “Cracking the Einstein Code”.

To conclude, the lively and highly stimulating atmosphere of Dark 2009 reflected a splendid future for research in the field of dark matter in the universe and for particle physics beyond the Standard Model. The proceedings will be published by World Scientific.

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There is overwhelming evidence that the universe contains dark matter made from unknown elementary particles. Astronomers discovered more than 75 years ago that spiral galaxies, such as the Milky Way, spin faster than allowed by the gravity of known kinds of matter. Since then there have been many more observations that point to the existence of this dark matter.

Gravitational lensing, for example, provides a unique probe of the distribution of luminous-plus-dark matter in individual galaxies, in clusters of galaxies and in the large-scale structure of the universe. The deflection of  gravitational light depends only on the gravitational field between the emitter and the observer, and it is independent of the nature and state of the matter producing the gravitational field, so it yields by far the most precise determinations of mass in extragalactic astronomy. Gravitational lensing has established that, like spiral galaxies, elliptical galaxies are dominated by dark matter.

Strong evidence for the fact that most of the dark matter has a non-baryonic nature comes from the observed heights of the acoustic peaks in the angular power spectrum of the cosmic microwave background measured by the Wilkinson Microwave Anisotropy Probe, because the peaks are sensitive to the fraction of mass in the baryons. It turns out that only about 4% of the mass of the universe is in baryons, whereas about 20% is in non- baryonic dark matter – a finding that is also in line with inferences from primordial nucleosynthesis.

A host of candidates

This leaves some pressing questions. What is the microscopic nature of this non- baryonic dark matter? Why is its mass fraction today about 20%? How dark is it? How cold is it? How stable is it?

Progress in finding the answers to such questions provided the focus for the 2008 DESY b  Theory Workshop, which was held on 29 September – 2 October.

Organized by Manuel Drees of Bonn, it sought to combine results from a range of experiments and confront them with theoretical predictions. It is clear that the investigation of the microscopic nature of dark matter has recently entered a decisive phase. Experiments are being carried out around the globe to try to identify traces of the  mysterious dark-matter particles. Since the different theoretical candidates appear to have quite distinctive signatures, there are good reasons to expect that from a combination of all of these efforts a common picture will materialize within the next decade.

Theoretical particle physicists have proposed a whole host of candidates for the constituents of non-baryonic dark matter, with fancy names such as axions, axinos, gravitinos, neutralinos and lightest Kaluza–Klein partners. The best-motivated of these occur in extensions of the Standard Model that have been proposed to solve other problems besides the dark-matter puzzle. The axion, for example, arose in extensions that aim to solve the strong CP problem. It later turned out to be a viable dark- matter candidate if its mass is in the micro-electron-volt range. Gravitinos and neutralinos, on the other hand, are the superpartners of the graviton and the neutral bosons, respectively. They arise in supersymmetric extensions of the Standard Model, which aim at a solution of the hierarchy problem and at a grand unification of the strong and electroweak interactions. In fact, neutralinos are natural candidates for dark matter because they have cross-sections of the order of electroweak interactions and their masses are expected to be of the order of the weak scale (i.e. 100 GeV). This leads to the fact that their relic density resulting from freeze-out in the early universe is just right to account for the observed amount of dark matter.

Neutralinos belong to the class of weakly interacting massive particles (WIMPs). Such particles seem to be more or less generic in extensions of the Standard Model at the tera-electron-volt scale, but their stability (or a long enough lifetime) has to be imposed. This is not necessary for super-weakly interacting massive particles (superWIMPs), such as sterile neutrinos, gravitinos, hidden sector gauge bosons (gauginos) and the axino. For example, unstable but long-lived gravitinos in the 5–300 GeV mass range are viable candidates for dark matter and provide a consistent thermal history of the universe, including successful Big Bang nucleosynthesis.

Detecting dark matter

Owing to their relatively large elastic cross-sections with atomic nuclei, WIMPs such as neutralinos are good candidates for direct detection in the laboratory, yielding up to one event per day, per 100 kg of target material. The expected WIMP signatures are nuclear recoils, which should occur uniformly throughout the detector volume at a rate that shows an annual flux modulation by a few per cent. Intriguingly, the DAMA experiment in the Gran Sasso National Laboratory has seen evidence for such an annual modulation.

However, there is some tension with other direct-detection experiments. Theoretical studies have revealed that interpretation in terms of a low-mass (5–50 GeV) WIMP is marginally compatible with the current limits from other experiments. In contrast to DAMA, which looks just for scintillation light, most of the latter exploit at least two observables out of the set (phonons, charge, light) to reconstruct the nuclear recoil-energy.

Many different techniques based on cryogenic detectors (e.g. the Cryogenic Dark Matter Search), noble liquids (e.g. the XENON Dark Matter Project) or even bubble chambers, are currently employed to search for WIMPs via direct detection. Detectors with directional sensitivity (e.g. the Directional Recoil Identification From Tracks experiment) may not only have a better signal-to-background discrimination but may also be capable of measuring the local dark-matter, phase-space distribution. In summary, these direct experiments are currently probing some of the theoretically interesting regions for WIMP candidates. The next generation of experiments may enter the era of WIMP (astro) physics.

The axion is another dark-matter candidate for which there are ongoing direct- detection experiments. Both the Axion Dark Matter Experiment (ADMX) in the US and the Cosmic Axion Research with Rydberg Atoms in a Resonant Cavity (CARRACK) experiment in Japan exploit a cooled cavity inside a strong magnetic field to search for the stimulation of a cavity resonance from a dark-matter axion–photon conversion in the microwave frequency region, corresponding to the expected axion mass. While they differ in their detector technology – ADMX uses microwave telescope technology whereas CARRACK employs Rydberg atom technology – both experiments are designed to cover the 1–10 μeV mass range. Indeed, if dark matter consists just of axions then it should soon be found in these experiments. The CERN Axion Solar Telescope, meanwhile, is looking for axions produced in the Sun.

There are also of course possibilities for indirect detection. Dark matter may not be absolutely dark. In fact, in regions where the dark-matter density is high (e.g. in the Earth, in the Sun, near the galactic centre, in external galaxies), neutralinos or other WIMPs may annihilate to visible particle–antiparticle pairs and lead to signatures in gamma-ray, neutrino, positron and antiproton spectra. Moreover, superWIMPs (e.g. gravitinos), may also leave their traces in cosmic-ray spectra if they are not absolutely stable.

Interestingly, the Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics (PAMELA) satellite experiment recently observed an unexpected rise in the fraction of positrons at energies of 10–100 GeV, thereby confirming earlier observations by the High Energy Antimatter Telescope balloon experiment. In addition, the Advanced Thin Ionization Chamber balloon experiment has reported a further anomaly in the electron-plus positron flux, which can be interpreted as the continuation of the PAMELA excess to about 800 GeV. The quantification of these excesses is still quite uncertain, not least because of relatively large systematic uncertainties. It is well established that they cannot be explained by the standard mechanism, namely the secondary production of positrons arising from collisions between cosmic-ray protons and the interstellar medium within our galaxy. However, a very conventional astrophysical source for them could be nearby pulsars.

On a more speculative level, these observations have inspired theorists to search for pure particle-physics models that accommodate all results. Generically, interpretations in terms of WIMP annihilation seem to be disfavoured, because they require a huge clumpiness of the Milky Way dark-matter halo, which is at variance with recent numerical simulations of the latter. This constraint is relaxed in superWIMP scenarios, where the positrons may be produced in the decay of dark-matter particles (e.g. gravitinos).

It is clear that one of the keys to understanding the origin of the excess in the positron fraction is the accurate, separate measurement of positron and electron fluxes, which can be done with further PAMELA data and with the Alpha Magnetic Spectrometer satellite experiment. Furthermore, distinguishing different interpretations of the observed excesses requires a multimessenger approach (i.e. to search for signatures in the radio range, synchrotron radiation, neutrinos, antiprotons and gamma rays).

Fortunately the Fermi Gamma-Ray Space Telescope is in orbit and taking data. Together with other cosmic-ray experiments it will probe interesting regions of parameter space in WIMP and superWIMP scenarios of dark matter.

Dark matter at colliders

Clearly, at colliders the existence of a dark-matter candidate can be inferred only indirectly from the apparent missing energy, associated with the dark-matter particles, in the final state of the collision. However, such a measurement can be made with precision and under controlled conditions. To extract the properties, such as the mass, of dark-matter particles, these final-state measurements have to be compared with predictions from theoretical models. In a supersymmetric extension of the Standard Model, for example, with the neutralino as the lightest superpartner, experiments at the LHC would search for signatures from the cascade decay of gluinos and squarks into gluons, quarks, leptons and neutralinos. This would show up as large missing transverse-energy in events with some jets and leptons. The endpoints in kinematic distributions could then be used for the determination of the dark-matter candidate’s mass, which could be compared with the mass determined eventually by measurements of recoil energy in direct-detection experiments.

This complementarity between direct, indirect and collider searches for dark matter is essential. Although collider experiments might identify a dark-matter candidate and precisely measure its properties, they will not be able to distinguish a cosmologically stable particle from one that is long-lived but unstable. In turn, direct detection cannot tell definitely what kind of WIMP has been observed. Moreover, in many superWIMP dark matter scenarios a direct detection is impossible, while detection at the LHC may be feasible. For example, if the lightest superpartner is a gravitino (or hidden gaugino) and the next-to-lightest is a charged lepton, experiments at the LHC may search for the striking signature of a displaced vertex plus an ionizing track.

In many cases, however, precision measurements from a future electron–positron collider seem to be necessary to exploit fully the collider–cosmology–astrophysics synergy. In addition “low-energy photon- collider” experiments – such as the Axion-Like Particle Search at DESY, the GammeV experiment at Fermilab and the Optical Search for QED magnetic birefringence, axions and photo regeneration at CERN, where the interactions of intense laser beams with strong electromagnetic fields are probed – may give viable insight into the existence of very lightweight, axion-like, dark-matter candidates.

In summary, there is evidence for non-baryonic dark matter that is not made of any known elementary particle. We are today in the exploratory stage to figure out its microscopic nature. Many ideas are currently being explored in theories and in experiments, and more will come. Nature has given us a few clues that we need to exploit. The data coming soon from accelerators, and from direct and indirect detection experiments, will be the final arbiter.

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In the search for answers to these questions, more than 250 participants, ranging from senior experts to young students, attended the 3rd Biennial Leopoldina Conference on Dark Energy held on 7–11 October 2008 at the Ludwig Maximilians University (LMU) in Munich. The meeting was organized jointly by the Bonn-Heidelberg-Munich Transregional Research Centre “The Dark Universe” and the German Academy of Sciences Leopoldina, with support from the Munich-based Excellence Cluster “Origin and Structure of the Universe”. The goal of the international symposium was to gain a better understanding of the nature of dark energy by bringing together observers, modellers and theoreticians from particle physics, astrophysics and cosmology to present and discuss their latest results and to explore possible future routes in the rapidly expanding field of dark-energy research.

Around 60 plenary talks at the conference were held in the central auditorium (Aula) of LMU Munich, with lively discussions following in poster sessions (where almost 100 posters were displayed) and during the breaks in the inner court of the university. There were fruitful exchanges between physicists engaged in a range of observations, from ground-based studies of supernovae to satellite probes of the cosmic microwave background (CMB), and theorists in search of possible explanations for the accelerated expansion of the universe, which was first reported in 1998. This acceleration has occurred in recent cosmic history, corresponding to redshifts of about z ≤ 1.

An accelerating expansion

Brian Schmidt of the Australian National University in Canberra gave the observational keynote speech. He led the High-z Supernova Search Team that presented the first convincing evidence for the existence of dark energy – which works against gravity to boost the expansion of the universe – almost simultaneously with the Supernova Cosmology Project led by Saul Perlmutter of the Lawrence Berkeley National Laboratory and the University of California at Berkeley. Adam Riess, a member of the High-z team, presented constraints on dark energy from the latest supernovae data, including those from the Hubble Space Telescope at redshift z > 1. This is where the acceleration becomes a deceleration, owing to the lessening impact of dark energy at earlier times (figure 1).

Both teams independently discovered the accelerating expansion of the universe by studying distant type Ia supernovae. They found that the light from these events is fainter than expected for a given expansion velocity, indicating that the supernovae are farther away than predicted (figure 2, p18). This implies that the expansion is not slowing under the influence of gravity – as might be expected – but is instead accelerating because of some uniformly distributed, gravitationally repulsive substance accounting for more than 70% of the mass-energy content of the universe – now known as dark energy.

Type Ia supernovae arise from runaway thermonuclear explosions following accretion on a carbon/oxygen white dwarf star and after calibration have an almost uniform brightness. This makes them “standard candles”, suitable as tools for the precise measurement of astronomical distances. Wolfgang Hillebrandt of the Munich Max-Planck Institute for Astrophysics presented 3D simulations of type Ia supernova explosions. It is still a matter of debate how standard these so-called “standard candles” really are. Their colour–luminosity relationship is inconsistent with Milky Way-type dust and, as Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics mentioned, the role of dust is generally underestimated. Future supernova observations in the near infrared hold promise because, at these wavelengths, the extinction by dust is five times lower. Bruno Leibundgut of ESO said that infrared observations using the future James Webb Space Telescope will be crucial in solving the problem of reddening from dust.

As Schmidt pointed out, and others detailed in subsequent talks, measurements of the temperature fluctuations in the CMB provide independent support for the theory of an accelerating universe. These were first observed by the Cosmic Background Explorer in 1991 and subsequently in 2000 by the Boomerang and MAXIMA balloon experiments. Since 2003 the Wilkinson Microwave Anisotropy Probe (WMAP) has observed the full-sky CMB with high resolution. Additional evidence came from the Sloan Digital Sky Survey and 2-degree Field Survey. In 2005 they measured ripples in the distribution of galaxies that were imprinted in acoustic oscillations of the plasma when matter and radiation decoupled as protons and electrons combined to form hydrogen atoms, 380,000 years after the Big Bang. These are the “baryonic acoustic oscillations” (BAOs).

Dark-energy candidates

Eiichiro Komatsu of the Department of Astronomy at the University of Texas in Austin, lead author of WMAP’s paper on the cosmological interpretation of the five-year data, said that anything that can explain the observed luminosity distances of type Ia supernovae, as well as the angular-diameter distances in the CMB and BAO data, is “qualified for being called dark energy” (figure 3). Candidates include energy, modified gravity and an extreme inhomogeneity of space.

Although the latter approach was presented in several talks, the impression prevailed that the effects of dark energy are too large to be accounted for through spatial inhomogeneities and an accordingly adapted averaging procedure in general relativity. Komatsu – and many other speakers – clearly favours the Lambda-cold-dark-matter (ΛCDM) model, with a small cosmological constant Λ to account for the accelerated expansion. The dark-energy equation of state is usually taken to be w = p/ρ= –0.94 ± 0.1(stat.) ± 0.1 (syst.) with a negative pressure, p; a varying w is not currently favoured by the data. Several speakers presented various versions of modified gravity. Roy Maartens of the University of Portsmouth in the UK acknowledged that ΛCDM is currently the best model. As an alternative he presented a braneworld scenario in which the vacuum energy does not gravitate and the acceleration arises from 5D effects. This scenario is, however, challenged by both geometric and structure-formation data.

Theoretical keynote-speaker Christof Wetterich of Heidelberg University emphasized that the physical origin, the smallness and the present-day importance of the cosmological constant are poorly understood. In 1988, almost simultaneously with but independently from Bharat Ratra and James Peebles, he proposed the existence of a time-dependent scalar field, which gives rise to the concept of a dynamical dark energy and time-dependent fundamental “constants”, such as the fine-structure constant. Although observations may eventually decide between dynamical or static dark energy, this is not yet possible from the available data.

Yet another indication for the accelerated expansion comes from the investigation of the weak-lensing effect, as Matthias Bartelmann of Heidelberg University and others explained. This method of placing constraints on dark energy through its effect on the growth of structure in the universe relies on coherent distortions in the shapes of background galaxies by foreground mass structures, which include dark matter. The NASA-DOE Joint Dark Energy Mission (JDEM) is a space probe that will make use of this effect, in addition to taking BAO observations and distance and redshift measurements of more than 2000 type Ia supernovae a year. The project is now in the conceptual-design phase and has a target launch date of 2016. ESA’s corresponding project – the Dark UNiverse Explorer – is part of the planned Euclid mission, scheduled for launch in 2017. There were presentations on both missions.

The first major scientific results from the 10 m South Pole Telescope (SPT) initial survey were the highlight of the report by John Carlstrom, principal investigator for the project. The telescope is one of the first microwave telescopes that can take large-sky surveys with precision. It will be possible to use the resulting size-distribution pattern together with information from other telescopes to determine the strength of dark energy.

Carlstrom described the detection of four distant, massive clusters of galaxies in an initial analysis of SPT survey data – a first step towards a catalogue of thousands of galaxy clusters. The number of clusters as a function of time depends on the expansion rate, which leads back to dark energy. Three of the detected galaxy clusters were previously unknown systems. They are the first clusters detected in a Sunyaev–Zel’dovich (SZ) effect survey, and are the most significant SZ detections from a subset of the ongoing SPT survey. This shows that SZ surveys, and the SPT in particular, can be an effective means of finding galaxy clusters. The hope is for a catalogue of several thousand galaxy clusters in the southern sky by the end of 2011 – enough to rival the constraints on dark energy that are expected from the Euclid Mission and NASA’s JDEM.

The conference was lively and social activities enabled discussions outside the conference auditorium, particularly during the lunch breaks in nearby Munich restaurants. The presentations and discussions all demonstrated that the search for definite signatures and possible sources of the accelerated expansion of the universe continues to flourish and has an exciting future ahead. The results on supernovae and the CMB have led the way, but there is still much to learn. In his conference summary, Michael Turner of the University of Chicago emphasized that “cosmology has entered an era with large quantities of high-quality data”, and that the quest to understand dark energy will remain a grand scientific adventure. Future observational facilities – such as the Planck probe of the CMB, which is scheduled for launch around Easter 2009, the all-sky galaxy-cluster X-ray mission eROSITA, ESA’s Euclid and NASA’s JDEM – are all designed to produce unprecedented high-precision cosmology results that will shed new light on dark energy.

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This progress in science, triggered by the seemingly pedestrian Sun, seems set to continue, as a variety of solar phenomena still defy theoretical understanding. It may be that one answer lies in astroparticle physics and the curious hypothetical particle known as the axion. Neutral, light, and very weakly interacting, this particle was proposed more than 25 years ago to explain the absence of charge-parity (CP) symmetry violation in the strong interaction.

So what are the problems with the Sun? These lie, perhaps surprisingly, with the more visible, outermost layers, which have been observed for hundreds, if not thousands, of years.

First, why is the corona – the Sun’s atmosphere with a density of only a few nanograms per cubic metre – so hot, with a temperature of millions of degrees? This question has challenged astronomers since Walter Grotrian, of the Astrophysikalisches Observatorium in Potsdam, discovered the corona in the 1930s. Within a few hundred kilometres, the temperature rises to be about 500 times that of the underlying chromosphere, instead of continuing to fall to the temperature of empty space (2.7 K). While the flux of extreme ultraviolet photons and X-rays from the higher layers is some five orders of magnitude less than the flux from the photosphere (the visible surface), it is nevertheless surprisingly high and inconsistent with the spectrum from a black body with the temperature of the photosphere (figure 1). Thus, some unconventional physics must be at work, since heat cannot run spontaneously from cooler to hotter places. In short, everything above the photosphere should not be there at all.

Another question is how does the corona continuously accelerate the solar wind of some thousand million tonnes of gas per second at speeds as high as 800 km/s? The same puzzle holds for the transient but dramatic coronal mass ejections (CMEs). How and where is the required energy stored, and how are the ejections triggered? This question is probably related to the mystery of coronal heating. And what is it that triggers solar flares, which heat the solar atmosphere locally up to about 10 to 30 million degrees, similar to the high temperature of the core, some 700,000 km beneath? These unpredictable events appear to be like violent “explosions” occurring near sunspots in the lower corona. This suggests magnetic energy as their main energy source, but how is the energy stored and how is it released so rapidly and efficiently within seconds? Even though many details are known, new observations call into question the 40-year-old standard model for solar flares, which 150 years after their discovery still remain a major enigma.

On the Sun’s surface, what is it that causes the 11-year solar cycle of sunspots and solar activity? This seems to be the biggest of all solar mysteries, since it involves the oscillation of the huge “magnets” of a few kilogauss on the face of the Sun, ranging from 300 to 100,000 km in size. The origin of sunspots has been one of the great puzzles of astrophysics since Galileo Galilei first observed them in the early 1600s. Their rhythmic comings and goings, first measured by the apothecary Samuel Heinrich Schwabe in 1826, could be the key to understanding the unpredictable Sun, since everything in the solar atmosphere varies in step with this magnetic cycle.

Beneath the Sun’s surface, the contradiction between solar spectroscopy and the refined solar interior models provided by helioseismology has revived the question about the heavy-element composition of the Sun, with new abundances some 25 to 35% lower than before. Abundances vary from place to place and from time to time in the Sun, and are enhanced near flares, showing an intriguing dependence on the square of the magnetic intensity in these regions. The so-called “solar oxygen crisis” or “solar model problem” is thus pointing at some non-standard physical process or processes that occur only in the solar atmosphere, and with some built-in magnetic sensor.

These are just some of the most striking solar mysteries, each crying out for an explanation. So can astroparticle physics help? The answer could be “yes”, using a scenario in which axions, or particles like axions, are created and converted to photons in regions of high magnetic fields or by their spontaneous decay.

The expectation from particle physics is that axions should couple to electromagnetic fields, just as neutral pions do in the Primakoff effect known since 1951, which regards the production of pions by high-energy photons as the reverse of the decay into two photons. Interestingly, axions could even couple coherently to macroscopic magnetic fields, giving rise to axion–photon oscillation, as the axions produce photons and vice versa. The process is further enhanced in a suitably dense plasma, which can increase the coherence length. This means that the huge solar magnetic fields could provide regions for efficient axion–photon mutation, leading to the sudden appearance of photons from axions streaming out from the Sun’s interior. The photosphere and solar atmosphere near sunspots are the most likely magnetic regions for this process to become “visible”, as the material above is transparent to emerging photons.

According to this scenario, the Sun should be emitting axions, or axion-like particles, with energies reflecting the temperature of the source. Thus one or more extended sources of new low-energy particles (below around 1 keV), and the ubiquitous solar magnetic fields of strengths varying from around 0.5 T, as measured at the surface, up to 100 T or much more in the interior, might together give rise to the apparently enigmatic behaviour of a star like the Sun.

Conventional solar axion models, inspired by QCD, have one small source of particles in the solar core, with an energy spectrum that peaks at 4 to 5 keV. They therefore exclude the low energies where the solar mysteries predominantly occur. This immediately suggests an extended axion “horizon”. Experiments to detect solar axions – axion helioscopes such as the CERN Solar Axion Telescope (CAST) – should widen their dynamic range towards lower energies, in order to enter this new territory.

The revised solar axion scenario must also accommodate two components of photon emission, namely, a continuous inward emission together, occasionally, with an outward radiation pressure. Massive and light axion-like particles, both of which have been proposed, can provide these thermodynamically unexpected inward and outward photons respectively. They offer an exotic but still simple solution, given the Sun’s complexity.

The emerging picture is that the transition region (TR) between the chromosphere and the corona (which is only about 100 km thick and only some 2000 km above the solar surface) is the manifestation of a space and time dependent balance between the two photon emissions. However, the almost equally probable disappearance of photons into axion-like particles in a magnetic environment must also be taken into account in understanding the solar puzzles. The TR could be the most spectacular place in the Sun, since it is where the mysterious temperature inversion appears, while flares, CMEs and other violent phenomena originate near the TR.

Astrophysicists generally consider the ubiquitous solar magnetism to be the key to understanding the Sun. The magnetic field appears to play a crucial role in heating up the corona, but the process by which it is converted into heat and other forms of energy remains an unsolved problem. In the new scenario, the generally accepted properties of the radiative decay of particles like axions and their coupling to magnetic fields are the device to resolve the problem – in effect, a real “απó μηχανηζ θεóζ” (the deus ex machina of Greek tragedy). The magnetic field is no longer the energy source, but is just the catalyst for the axions to become photons, and vice versa.

The precise mechanism for enhancing axion–photon mutation in the Sun that this picture requires remains elusive and challenging. One aim is to reproduce it in axion experiments. CAST, for example, seeks to detect photons created by the conversion of solar axions in the 9 T field of a prototype superconducting LHC dipole. However, the process depends on the unknown mass of the axion. Every day the CAST experiment changes the density of the gas inside the two tubes in the magnet in an attempt to match the velocity of the solar axion with that of the emerging photon propagating in the refractive gas.

It is reasonable to assume that fine tuning of this kind in relation to the axion mass might also occur in the restless magnetic Sun. If the energy corresponding to the plasma frequency equals the axion rest mass, the axion-to-photon coherent interaction will increase steeply with the product of the square of the coherence length and the transverse magnetic field strength. Since solar plasma densities and/or magnetic fields change continuously, such a “resonance crossing” could result in an otherwise unexpected photon excess or deficit, manifesting itself in a variety of ways, for example, locally as a hot or cold plasma. Only a quantum electrodynamics that incorporates an axion-like field can accommodate such transient brightening as well as dimming (among many other unexpected observations).

These ideas also have implications for the better tuning not only of CAST, but also of orbiting telescopes such as the Japanese satellite Hinode (formerly Solar B), NASA’s Reuven Ramaty High Energy Solar Spectroscopic Imager and the NASA–ESA Solar and Heliospheric Observatory, which have been transformed recently to promising axion helioscopes, following suggestions by CERN’s Luigi di Lella among others. The joint Japan–US–UK mission Yohkoh has also joined the axion hunt, even though it ceased operation in 2001, by making its data freely available.

The revised axion scenario therefore seems to fit as an explanation for most (if not all) solar mysteries. Such effects can provide signatures for new physics as direct and as significant as those from laboratory experiments, even though they are generally considered as indirect; the history of solar neutrinos is the best example of this kind.

Following these ideas and others on millicharged particles, paraphotons or any other weakly interacting sub-electron-volt particles, axion-like exotica will mean that the Sun’s visible surface – and probably not its core – holds the key to its secrets. As in neutrino physics, the multifaceted Sun, from its deep interior to the outer corona and the solar wind, could be the best laboratory for axion physics and the like. The Sun, the most powerful accelerator in the solar system, whose working principle is not yet understood, has not been as active as it is now for some 11,000 years. Is this an opportunity not to be missed?

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Such a modulation would be the consequence of the Earth’s rotation around the Sun. There would be different detection rates for dark-matter particles when the Earth goes in the same direction as the flux from the galactic halo compared with when it goes against the flux, six months later.

The current experiment, DAMA/LIBRA, has been taking data at the Gran Sasso National Laboratory in Italy since March 2003. Located at almost 1 km deep, so as to be shielded against the cosmic-ray background, the experiment uses 25 crystals of sodium iodide, each with a mass of 9.7 kg and extremely high radiopurity. If a dark-matter particle collides in one of these, it should produce a faint flash of light, which is measured.

Taking the new data together with those from the previous results gives a total exposure of 0.82 tonne-years, and a result that suggests the presence of dark-matter particles in the galactic halo at a confidence level of 8.2 σ (Bernabei et al. 2008). The effect observed is independent of the various theoretical models of dark matter, such as weakly interacting massive particles or axions. Currently, it remains that no other dark-matter experiment has detected the modulation, and so the hunt continues.

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The COUPP experiment is an intriguing new application of the bubble chamber technique, located 100 m underground in the tunnel for the Neutrinos at the Main Injector project at Fermilab. It uses a small quartz vessel at room temperature filled with 1.5 kg of superheated iodotrifluoromethane (CF₃I), a refrigerant that is often used in fire extinguishers. Two effects reveal the formation of bubbles in the chamber: the sound and pressure rise caused by their growth, and the changes in their appearance monitored by two CCD cameras. Once they reach a millimetre or so in size, they trigger the system to record photographs of the chamber. The chamber then goes through a cycle of compression and decompression to bring it back to a bubble-free, superheated state.

The innovative detector offers several advantages in the search for WIMPs, the most important being that the superheated liquid can be tuned to respond only to particles with large stopping power. This means that it can be set up in such a way that muons, gamma-rays, X-rays, and other kinds of common background, deposit too little energy to form bubbles. When the detector is searching for WIMPs, the threshold for bubble nucleation is typically above 50 keV/μm.

The team operated the chamber continuously from December 2005 to December 2006, with around 100 s between expansions. Although the chamber is only a prototype, the first results from COUPP, combined with the findings of other dark-matter searches, contradict the claims for the observation of WIMPs by the Dark Matter experiment (DAMA) in Italy. Previous experiments had already constrained the possibility that the DAMA observations result from dark-matter, spin-independent interactions and COUPP has now ruled out the last region of parameter space allowed for a spin-dependent explanation (Behnke et al. 2008). If the DAMA result had been due to spin-dependent WIMPs, then COUPP should have found hundreds of examples, but instead it found none above background.

The COUPP team now aims to improve the sensitivity of the experiment by increasing the amount of liquid in the detector from 1 l to 30 l, and it expects to start testing the larger chamber soon. The experiment could move to a deeper tunnel to reduce the background from cosmic radiation even further.

Meanwhile, the CDMS collaboration announced the world’s most stringent limits on how often dark-matter particles interact with ordinary matter and on how heavy they are, in particular in the theoretically favoured mass range of more than 40 times the proton mass. The CDMS experiment, situated in the Soudan Underground Laboratory, Minnesota, is now running with all of its detectors – 19 germanium detectors of 250 g each and 11 silicon detectors of 100 g. The detectors operate at 50 mK and each consists of a disc some 7.5 cm in diameter and 1 cm thick.

The new results, announced at the Dark Matter 2008 conference in Marina del Ray, California, are based on the analysis of data collected from 15 germanium detectors between October 2006 and July 2007 (Ahmed et al. 2008). The analysis resulted in no dark-matter events and excludes the parameter space for WIMPs with masses above 42 GeV/c².

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If you are a particle physicist interested in cosmology, this book is for you. It makes a broad, clear and precise overview of our current understanding of dark matter and dark energy – the invisible actors governing the fate of the universe.

It is a challenge to try to make these apparently obscure concepts familiar to any motivated reader without a scientific background. But the author, Iain Nicolson, has been entirely successful in his enterprise. With a pleasant balance between text and colourful illustrations, he guides the reader through a fascinating, invisible and mysterious world that manifests its presence by shaping galaxies and the universe itself.

The book starts with an introduction to key concepts in astrophysics and the development of classical cosmology. It then describes the observational evidence for dark matter in galaxies and clusters of galaxies, showing that massive extremely dim celestial bodies cannot account for the missing mass. Particle physics is not neglected, with a description of our understanding of ordinary “baryonic” matter and the quest for detecting exotic weakly interacting massive particles (WIMPs). An entire chapter is also devoted to the idea that modified Newtonian dynamics (MOND) could be an alternative to the existence of dark matter. The second half of the book is devoted to cosmological observations and arguments that suggest the existence of dark energy – an even more mysterious ingredient of the universe. The pieces assemble through these chapters to reveal a universe that is flattened out by inflation and that is essentially made of cold dark matter, with dark energy acting as a cosmological constant.

This new cosmology is generally accepted as the standard model and gives the full measure of the dark side of the universe. The visible matter studied by astronomers so far appears to be just the tip of the iceberg (less than 1%) and even baryonic matter studied so far by physicists is only about 5% of the mass–energy content of the universe. The remaining 95% is unknown territory, which the book invites us to explore using all techniques available. This will be the major challenge for physics in the 21st century.

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In 2006, the PVLAS collaboration reported a small rotation of the polarization plane of laser light passing through a strong rotating magnetic dipole field. Though small, the detected rotation was around four orders of magnitude larger than predicted by QED (Zavattini et al. 2006). One possible interpretation involves ALPs produced via the coupling of photons to the magnetic field.

Combining the PVLAS result with upper limits achieved 13 years earlier by the BFRT experiment at Brookhaven National Laboratory (Cameron et al. 1993) yields values of the ALP’s mass and its coupling strength to photons of roughly 1 MeV and 2 × 10^-6 GeV^-1, respectively (Ahlers et al. 2006). If the PVLAS result is verified, these two values challenge theory because a standard QCD-motivated axion with a mass of 1 MeV should have a coupling constant seven orders of magnitude smaller. Another challenge to the particle interpretation of the PVLAS result comes from the upper limit measured recently at CERN with the axion solar helioscope CAST, which should have clearly seen such ALPs. However this apparent contradiction holds true only if such particles are produced in the Sun and can escape to reach the Earth.

So far there is no direct experimental evidence for conventional axions. The first sensitive limits were derived about two decades ago from astrophysics data (mainly from the evolution of stars, where axions produced via the Primakoff effect would open a new energy-loss channel so stars would appear older than they are), and also from experiments searching for axions of astrophysical origin (cavity experiments and CAST for example) and accelerator-based experiments. The conclusions were that QCD-motivated axions with masses in the micro-electron-volt to milli-electron-volt range seem to be most likely – if they exist at all.

The combined PVLAS–BFRT result would fit well into these expectations if the coupling constant were not too large by orders of magnitude. Theoreticians have tried to deal with this problem and develop models in line with the ALP interpretation of the PVLAS data and astrophysical observations. There may be some possibilities involving “specifically designed” ALP properties. However, to the authors’ understanding, such attempts fail if the conclusion announced at the workshop persists: according to preliminary new PVLAS results, the new particle is a scalar, whereas conventional axions are pseudoscalars. Consequently either the interpretation of the data or the experimental results must be reconsidered.

Although the PVLAS collaboration has measured the Coutton–Mouton effect – birefringence of a gas in a dipole magnetic field – for various gases with unprecedented sensitivity, the workshop openly considered possible systematic uncertainties. While experimental tests rule out many uncertainties, others are still to be checked. For example, the relatively large scatter of individual PVLAS measurements and the influence of the indirect effects of magnetic fringe fields remain to be understood. The PVLAS collaboration is therefore planning further detailed analyses.

In search of ALPs

One clear conclusion is the need for more experimental data. A “smoking gun” proof of the PVLAS particle interpretation would be the production and detection of ALPs in the laboratory. In principle the BFRT collaboration has already attempted this in an approach called “light shining through a wall”. In the first part of such an experiment, light passes through a magnetic dipole field in which ALPs would be generated; a “wall” then blocks the light. Only the ALPs can pass this barrier to enter a second identical dipole magnet, in which some of them would convert back into photons (figure 1). Detection of these reconverted photons would then give the impression of light shining through a wall. The intensity of the light would depend on the fourth power of the magnetic field strength and the orientation of the light polarization plane with respect to the magnetic dipole field.

The PVLAS collaboration and other groups are planning a direct experimental verification of the ALP hypothesis. Table 1 provides an overview of some of the approaches presented at the workshop. Besides PVLAS the ALPS, BMV and LIPSS experiments should take data in 2007. BMV and OSQAR (as well as the Taiwanese experiment Q&A) will confirm directly the rotation of the light polarization plane that PVLAS claims. The BMV collaboration aims for such a measurement in late 2007.

Research during the coming year should therefore clarify the PVLAS claim in much greater detail. The measurement of a new axion-like particle would be revolutionary for particle physics and probably also for our understanding of the constituents of the universe. However, considering the theoretical difficulties described above, a different scenario might emerge. Within a year from now we might be confronted both with an independent confirmation of the PVLAS result on the rotation of the light polarization plane, and simultaneously with only upper limits on ALP production by the light shining through a wall approaches. This situation would require new theoretical models.

The planned experiments listed in Table 1 do not have the sensitivity to probe conventional QCD-inspired axions. In the near future, CAST will be the only set-up to touch the predictions for solar-axion production. The workshop in Princeton, however, heard about other promising experimental efforts to search directly for axions or other unknown bosons with similar properties. These studies use state-of-the-art microwave cavities – for example, as in ADMX in the US, which is looking for dark-matter axions – or pendulums to search for macroscopic forces mediated by ALPs.

On the theoretical side, as we mentioned above, attempts to interpret the PVLAS result have generated some doubts on the existence of a new ALP. Perhaps micro-charged particles inspired by string theory might provide a more natural explanation of the PVLAS result. Researchers are thus discussing novel ideas of how to turn experimental test benches for accelerator cavity development into sensitive set-ups to test for micro-charged particles. However, as Ed Witten explained in the workshop summary talk, string theories also predict many ALPs, so perhaps we are on the cusp of discovering an entire new sector of pseudoscalar particles.

In summary, it is clear that small-scale non-accelerator-based particle-physics experiments can have a remarkable input to particle physics. Stay tuned for further developments.
The authors wish to thank the Princeton Institute for Advanced Study for the warm hospitality, and especially Raul Rabadan and Kris Sigurdson for their perfect organization of the workshop.

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While dark matter has become the focus of a range of research, from cosmology to particle physics, it has proved difficult to rule out the alternative scenario in which gravity is slightly altered from the standard 1/r² force law. The new study, however, has discovered a system in which the inferred dark matter is not coincident with the observable matter, and the difference in position is too great to be accounted for by modifying gravity. This, the team says, provides direct empirical proof for dark matter.

The team from the universities of Arizona and Florida, the Kavli Institute for Particle Astrophysics and Cosmology, and the Harvard-Smithsonian-Center for Astrophysics has combined observations from various telescopes to build a picture of what is happening in the galaxy cluster 1E0657-558. This cluster is particularly interesting because it shows evidence that a smaller cluster has at some stage ripped through a larger cluster, creating a bow-shaped shock wave.

Using images from the Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope and the Magellan telescope to provide information on gravitational lensing of more-distant galaxies, the team has created a map of the gravitational potential across the cluster 1E0657-558. This reveals two regions in which the mass is concentrated.

The team has also observed the cluster with NASA’s Chandra X-ray Observatory to measure the positions of the two clouds of hot gas that are associated with the merging galaxies. It finds that these two clouds of X-ray emitting plasma of normal baryonic matter are not coincident with the two central locations of the gravitational mass, which in fact are further apart. This suggests that the plasma clouds have slowed as they passed through each other and interacted, while dark matter in the two clusters has not interacted.

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The idea of the axion has been around for some 30 years, proposed as a solution to the strong charge-parity (CP) problem in quantum chromodynamics (QCD), the theory of strong interactions. According to the basic field equations of QCD, strong interactions should violate CP symmetry, rather as weak interactions do. However, strong interactions show no sign of CP violation. In 1977, Roberto Peccei and Helen Quinn suggested that to restore CP conservation in strong interactions, a new symmetry must be present, compensating the original CP-violating term in QCD almost exactly – to at least one part in 10¹⁰. The breakdown of this gives rise to the so-called axion field proposed by Steven Weinberg and Frank Wilczek, and the associated pseudo-scalar particle – the axion. Appropriately, Peccei, from the University of California Los Angeles, gave the first lecture of the workshop series and described the theoretical raison d’être of the Peccei-Quinn symmetry.

Evidence for strong CP violation should in particular appear in an electron dipole moment (EDM) for the neutron, but this has not yet been detected. Instead, we know from a high-precision measurement using polarized ultracold neutrons at the Institut Laue Langevin (ILL) in Grenoble that the neutron EDM is at least some 10 orders of magnitude below expectation. Peter Geltenbort of ILL presented the recently announced limit of 3 × 10^-26 e cm. This is part of a series of experiments started by Nobel laureates Norman Ramsey and Edward Purcell in the 1950s, which continues today with the ambitious goal of reaching 10-28 e cm by the end of the decade. Other proposed neutron EDM experiments include those at the Paul Scherrer Institut and at the Spallation Neutron Source in Oak Ridge with goals of 10^-27 e cm and 10^-28 e cm, respectively. A new technique with the deuteron may provide the route for the next sensitivity scale, reaching 10^-29 e cm, as Yannis Semertzidis of Brookhaven explained.

Stars and dark matter

CP violation seems to be necessary to explain the survival of matter at the expense of antimatter after the Big Bang. Thus the creation of relic axions shortly after the dawn of time could have been enormous, perhaps amounting to some six times more in mass than ordinary matter. In addition to the scenario of relic axions, Georg Raffelt, an axion pioneer from the Max Planck Institute, introduced the connections between astrophysics and axions, with the stars as axion sources as his central topic. The effect of such an energy-loss channel on stellar physics provides constraints on the interaction strength of axions with ordinary particles. The Sun, our best known star, should be a strong axion source in the sky, allowing a direct search for these almost-invisible particles.

This is precisely the objective of the CAST helioscope at CERN, which searches for solar axions using a recycled LHC test dipole magnet pointing at the Sun for some three hours a day. The signal of solar axions will be an excess of X-rays detected during solar tracking. While the relic axions are expected to move slowly at about 300 km/s, those escaping from the solar core must be super relativistic, despite their assumed kinetic energy of only about 4 keV. CAST is the first helioscope ever built with an imaging X-ray optical system, whose working principle was explained by Peter Friedrich from Max-Planck-Institut für extraterrestrische Physik and Regina Soufli from Lawrence Livermore National Laboratory (LLNL) in their lectures on X-ray optics. For axion detection, the X-ray optics act as a concentrator to enhance the signal-to-noise ratio by focusing the converted solar X-rays into a small spot on a CCD chip or a micromesh gaseous structure (Micromegas), as developed by Yannis Giomataris and Georges Charpak. CAST has been taking data since the end of 2002 and has already published first results.

The possible existence of axions in the universe means that they are a candidate for (very) cold dark matter, as another axion pioneer, Pierre Sikivie, from the University of Florida explained. He also described the technique that he invented in 1983 for detecting axions. The idea is that axions in the galactic halo may be resonantly converted to microwave photons in a cavity permeated by a strong magnetic field. The expected signals are extremely weak, measured in yoctowatts, or 10^-24 W. The same holds also for the solar axions inside the CAST magnet, whose energies of a few kilo-electron-volts (keV) are several orders of magnitude higher. The process depends on various parameters, such as the magnetic-field vector and size, the plasma density, the (unpredictable) axion rest mass and the photon polarization – all of which provide the multiparameter space in which axion hunters search for their quarry.

Sikivie also described the search for relic axions at LLNL, the topic of the CERN seminar at the start of the first workshop, presented by Karl van Bibber from LLNL. The Axion Dark Matter eXperiment (ADMX), which uses a microwave cavity to look for axionic dark matter as proposed by Sikivie, has been taking data for a decade. It is now undergoing an upgrade to use near-quantum-limited SQUID amplifiers. In his review, van Bibber also described CARRACK, a similar experiment in Kyoto, which uses a Rydberg-atom single-quantum detector as the back-end of the experiment.

The axion, together with the Higgs boson – another so-far undetected particle required by theory – may contribute not only to dark matter but also to dark energy, as Metin Arik from Istanbul explained. This leads to the question of why the dark-energy density is so small.

Light polarization

Giovanni Cantatore presented the Polarizzazione del Vuoto con LASer (PVLAS) experiment at the INFN Legnaro National Laboratory, which has recently caused a stir in the axion community. In a recent paper in Physical Review Letters, the PVLAS collaboration reports that a magnetic field can be used to rotate the polarization of light in a vacuum. The detected rotation is extremely small, about 0.00001°. The slight twist in the polarization, the result of photons of a given polarization disappearing from the beam, could suggest the existence of a light, new neutral boson, as the signal strength observed by PVLAS is much larger than would be expected on the basis of quantum electrodynamics alone.

The particle suggested by PVLAS is not exactly the expected axion; its coupling to two photons is so strong that experiments searching for axions, such as CAST, should have seen many of them coming from astrophysical sources. It would need peculiar properties not to conflict with the current astrophysical observations, but there is no fundamental reason barring it from having such properties. Eduard Masso from the University of Barcelona reviewed the theoretical motivation for axions and the importance of an axion-like coupling to photons, and addressed the apparent conflict between the PVLAS results and CAST and the astrophysically derived bounds.

Andreas Ringwald from DESY pointed out that the possible interpretation of the PVLAS anomaly in terms of the production of an axion-like particle has triggered a revisit of astrophysical considerations. Models exist in which the production of axion-like particles in stars is suppressed compared with the production in a vacuum. In these models, the bounds derived from the age of stars or from CAST may be relaxed by some orders of magnitude. The workshop participants agreed unanimously that the PVLAS result needs direct confirmation of the particle hypothesis with laboratory-based experiments.

Semertzidis spoke about a PVLAS-type experiment that was performed at Brookhaven more than 15 years ago, with most of the PVLAS collaborators as major players. They also observed large signals, which they attributed however to the laser light motion at the magnet frequency. He went on to suggest that laser motion at the magnet rotation frequency might also produce signals at the second harmonic that would look like axion signals. The PVLAS collaboration has spent five years looking for a systematic artifact that might explain their observations, and plans to attempt to settle the question in a new photon-regeneration experiment. Here, any particles produced from photons in a first magnet, would propagate into a second magnet blocked to photons, where they would convert back into photons.

The solar-axion energy range less than 0.5-1 KeV remains a challenging new territory

Detection of such regenerated photons would provide a very robust confirmation of the particle interpretation of the PVLAS result, and similar regeneration experiments are in preparation elsewhere. Keith Baker presented the plans by the Hampton University-
Jefferson Lab collaboration to use the world’s highest-power tunable free-electron laser (FEL), in the LIght Pseudoscalar-Scalar Particle Search (LIPSS) experiment, which will run during the coming months. As Ringwald pointed out, there are a number of experiments based either on photon polarization or on photon regeneration measurements that should soon exceed the sensitivity of PVLAS. At DESY, there is a proposal to exploit the photon beam from the Free-electron LASer in Hamburg (FLASH) for the Axion Production at the FEL (APFEL) experiment, which will take advantage of unique properties of the FLASH beam. The available photon energies (around 40 eV) are just in the range where photon regeneration is most sensitive to masses in the milli-electron-volt range. In addition, the tuning possibilities of FLASH will allow a mass determination, and the pulsed nature of the photon beam allows noise reduction by timing.

Two linked experiments to search for axions proposed by a team from CERN and several other institutes are also well advanced. These were presented by Pierre Pugnat from CERN, who explained how this approach allows for simultaneous investigations of the magneto-optical properties of the quantum vacuum and of photon regeneration. The team could start next year to check the PVLAS result. The two experiments are integrated in the same LHC superconducting dipole magnet and so can provide solid results via mutual cross-checks.

Carlo Rizzo from Université Paul Sabatier/Toulouse presented a different detection concept in the Biréfringence Magnétique du Vide experiment at the Laboratoire National des Champs Magnétiques Pulsées in Toulouse. The goal is to study quantum vacuum magnetism and the experiment will be in operation this summer to test the PVLAS result.

Frank Avignone from South Carolina reviewed possibilities that go beyond the current experimental searches for axions, such as the use of coherent Bragg-Primakoff conversion in single crystals, coherence issues in vacuum and gas-filled magnetic helioscopes, and novel proposals to detect hadronic axions with suppressed electromagnetic couplings. Emmanuel Paschos of the University of Dortmund addressed possible coherence phenomena in low-energy axion scattering and its potential use for axion detection. This could be an important application of light-sensitive detectors used in underground dark-matter experiments, where they may allow the first low-energy axion searches, as reported by Klemens Rottler from the University of Tübingen and the CRESST dark-matter experiment. After all, the solar-axion energy range less than 0.5-1 keV remains a challenging new territory.

From the Sun and beyond

The signatures of axions are not confined to the solar system, and there were a number of interesting presentations on searches for axions or axion-like particles with telescopes on the ground or in orbit. A cosmologically interesting topic concerns axion-photon conversion induced by intergalactic magnetic fields, which offers an alternative explanation for the dimming of distant supernovae, without the need for cosmic acceleration. However, the same mechanism would cause excessive spectral distortion of the cosmic microwave background (CMB). Alessandro Mirizzi of Bari concludes that owing to the spectral shape of the CMB, photon-axion oscillation can play only a relatively minor role in supernova dimming. Nevertheless, a combined analysis of all the observables affected by the photon-axion oscillations would be required to give a final verdict on this model.

In related work, Damien Hutsemékers from the University of Liège has investigated the potential for photon-axion conversion within a magnetic field over cosmological distances, as it can affect the polarization of light from distant objects such as quasars. He reported on the remarkable observation, using the ESO telescopes in Chile, of alignments of quasar polarization vectors that might be due to axion-like particles along the line of sight.

Rizzo also discussed potential axion signatures in astrophysical observations, presenting an impressive movie. He reported that axion and quantum vacuum effects have been studied in the double neutron-star system J0737-3039. Astrophysical observations of such effects will be possible in 2007 with the ESA XMM/Newton and NASA GLAST telescopes in orbit.

Coming nearer to Earth, Hooman Davoudiasl from the University of Wisconsin-Madison showed that solar axion conversion to photons in the Earth’s magnetosphere can produce an X-ray flux, with average energy about 4 keV, which is measurable on the dark side of the Earth. (The low strength of the Earth’s magnetic field is compensated for by a large magnetized volume.) The signal has distinct features: a flux of X-rays coming from the dark Earth, pointing back to the core of the Sun, with a thermal distribution characteristic of the solar core, and orbital as well as annual modulations. For axion masses less than 10^-4 eV, a low-Earth-orbit X-ray telescope could probe the axion-photon coupling well below the current laboratory bounds, with a few days of data-taking. Also, the question was discussed as to whether axion-photon oscillations occur inside solar magnetic fields, sufficient to give the enhanced X-ray emission from places such as in sunspots.

Another possibility is the detection of the radiative decay of massive axions, predicted in extra dimensional models, which change drastically their mass, lifetime and detection, as Emilian Dudas from Ecole Polytechnique argued. In this context, Juhani Huovelin from Helsinki Observatory presented space-borne X-ray observations of the Sun and the sky background with ESA’s SMART-1, the first European mission to the Moon, which began operation in 2004 and will continue data-taking until September 2006. The important instruments onboard for axion research are an X-ray camera from CCLRC Rutherford Appleton Laboratory in the UK, and the X-ray Solar Monitor (XSM) from the University of Helsinki. The XSM measures solar X-ray spectra with high time resolution in the 1-20 keV energy range.

Extensive data have already been accumulated, including a series of lengthy observations of the X-ray Sun during quiescence and flares, as well as various observations of the background sky. Preliminary analysis of the data indicates possible residual emission at several intervals in the 2-10 keV range after fitting known solar and sky-background emission components. A future more-refined analysis will show whether the residual emission is statistically significant, and possibly related to X-rays from the decay of gravitationally trapped massive axions. The NASA solar mission RHESSI has also entered this kind of research, with the aim of detecting the same sort of particles near the surface of the Sun, as we published with Luigi di Lella at CERN five years ago. SMART and RHESSI use the Moon and Sun respectively to block out the background sky, thereby creating a large fiducial volume to search for axion radiative decay. The 1 m³ DRIFT detector operating in the Boulby Mine in the UK provides a similar capability in an underground experiment, as Eirini Tziaferi and Neil Spooner from Sheffield explained.

The friendly atmosphere of the two workshops saw plenty of fruitful discussions in which new ideas could emerge. For example, Ringwald has recently suggested a laboratory photon-regeneration experiment with X-rays. It seems that the ESRF at Grenoble offers one of the best opportunities worldwide for such an experiment, with photon energies in the 3-70 keV range. Also, as Sikivie highlighted, there is strong scientific interest in building a next-generation microwave cavity embedded in a large-bore superconducting solenoid to detect galactic-halo axions. CERN, together with several collaborating institutes, for example, could build a microwave cavity of around 1 m³ integrated inside a 8-10 T magnetic field.

The workshop participants unanimously concluded with a call to CERN to become a focal point for axion physics. There will be more ideas and new results by the next workshop in June 2007 in Patras.

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Dark matter was so named by the Swiss scientist Fritz Zwicky. In studying the movements of the galaxies in the Coma cluster in the 1930s, he discovered that there must be much more matter than is visible. Later, the rotation speeds of gases and stars in spiral galaxies revealed that practically every galaxy has a halo of dark matter surrounding it. This dark matter must be much more widely distributed than the visible matter, since the rotation speeds do not fall off like 1⁄√r, as expected from the visible matter in the centre, but stay more or less constant.

The fact that the dark matter is distributed over large distances implies that it undergoes little energy loss, so any interactions it has must be weak. Therefore, dark-matter particles are generically called WIMPs, for weakly interacting massive particles. These WIMPs must, however, be able to annihilate if they were produced in thermal equilibrium with all other particles in the early universe. At that time the number densities of different particles were all of the same order of magnitude and just as the baryon/photon ratio was reduced by 10 orders of magnitude by baryon annihilation, the WIMP number density, which is of the same order of magnitude as the baryon number density, can only have been reduced by annihilation, assuming the WIMPs are stable. (If they are not stable they must have a lifetime of the order of the lifetime of the universe, otherwise they would no longer exist.)

The gamma rays play a very special role as they point straight back to the source

If WIMPs in our galaxy collide and annihilate into quark pairs, these in turn will produce stable particles including gamma rays. The gamma rays play a very special role as they point straight back to the source, in contrast to charged particles, which change their direction in galactic magnetic fields; moreover, as they hardly interact they can be easily observed from across the galaxy. Gamma rays therefore offer a perfect means for reconstructing the distribution or halo profile of dark matter though observations in different sky directions.

Of course this assumes that gamma rays from dark-matter annihilation can be differentiated from the background, but this is indeed possible, since the spectral shapes are very different, as can be understood as follows. WIMPs have almost no kinetic energy, so after their annihilation into quark pairs the WIMP mass is converted into the energy of the quarks. The gamma rays produced in the fragmentation of such mono-energetic quarks have been well studied at CERN’s Large Electron Positron collider; they originate mainly from the decay of the copiously produced π⁰ mesons. The background, on the other hand, originates predominantly from the decay of π⁰ mesons produced by cosmic rays (mainly protons) scattering inelastically on the gas of the galactic disc, and so corresponds to the spectrum of gamma rays produced in fixed-target experiments with proton-proton collisions. In this case the gamma-ray spectrum can be calculated from the known cosmic-ray spectrum.

Clearly the steep power-law spectrum of cosmic rays will yield a spectrum of gamma rays that differs from that of the mono-energetic quarks produced in dark-matter annihilation. These different shapes can therefore be fitted to the data with free normalization factors, which then determine the relative contributions from dark-matter annihilation and background. Fitting the shapes has the advantage that the amount of background is determined from the data itself in each sky direction, so there is no need to rely on complicated galactic propagation models to obtain absolute background fluxes.

So what can be seen in the gamma-ray sky? A very detailed gamma-ray distribution over the whole sky was obtained by the Energetic Gamma Ray Emission Telescope (EGRET) on NASA’s Compton Gamma Ray Observatory, which collected data from 1991 to 2000. The EGRET telescope was carefully calibrated at SLAC in a quasi-monochromatic photon beam in the energy range 0.02 to 10 GeV. In 1997 the EGRET collaboration published their findings on a diffuse component of the gamma rays that cannot be described by the background: they observed an excess as large as a factor of two above the background for gamma-ray energies above 1 GeV. Recently, at the University of Karlsruhe, we have shown that this apparent excess traces the distribution of dark matter, since knowing the distribution of both the visible and dark matter allows us to reconstruct the rotation curve of our galaxy, especially its peculiar non-flat shape, which can be explained by the EGRET excess.

Mapping the flux

Figure 1 shows the excess for the flux from the galactic centre. The curve through the data points corresponds to the two-parameter fit, where the parameters are the normalization factors for the two known spectral shapes of signal and background, as discussed above; the red and yellow areas indicate the contributions from the dark-matter annihilation signal and the background, respectively. The WIMP mass was taken to be 60 GeV, which gives an excellent fit, although WIMP masses between 50 and 100 GeV are allowed, if extremes of the background shapes are allowed. The fit was repeated for 180 independent sky directions. In every direction the excess was observed and in every direction an excellent fit could be obtained for a WIMP mass of 60 GeV, if the contribution from the extragalactic background was also taken into account towards the galactic poles.

Such a detailed mapping of the flux of dark-matter annihilation in the sky allows a reconstruction of the distribution of dark matter in our galaxy. The result is surprising: it yields a pseudo-isothermal profile, as observed from the rotation curves in many galaxies, but with a substructure in the galactic plane in the form of doughnut-shaped rings at radii of 4 and 14 kpc. The position of our solar system at a distance of 8 kpc from the centre is located between this inner and outer ring. The enhanced gamma radiation at 14 kpc was also discussed in the original paper by Hunter et al. in 1997 and called the “cosmic enhancement factor”.

The ring structures in the dark-matter halo are expected to have a significant influence. A star inside the outer ring will feel an inward gravitational force from the galactic centre and an outward force from the outer ring, so the total gravitational force is reduced. This means that fast stars will go out of orbit inside the outer ring, thus causing a minimum in the rotation curve for radii within the outer ring. Outside the ring the gravitational forces from the centre and the ring add together, thus providing a maximum in the rotation curve. These effects are indeed observed, as shown in figure 2, indicating that the EGRET excess really does trace the dark matter in our galaxy.

The origin of these substructures in the dark-matter distribution is thought to be the hierarchical clustering of dark matter into galaxies: small clumps of dark matter grow from the quantum fluctuations appearing after inflation in the early universe and these clumps combine to form galaxies. That the outer ring originates from the infall of a dwarf galaxy is supported by the fact that hundreds of millions of old, mostly burned-out stars have recently been discovered in this region (Newberg et al. 2002, Ibata et al. 2003 and Crane et al. 2003). The small velocity dispersion and large-scale height perpendicular to the galactic disc of these stars proves that they cannot be part of the disc.

The position and shape of the inner ring coincides with a ring of molecular hydrogen. Molecules form from atomic hydrogen in the presence of dust or heavy nuclei, so a ring of neutral hydrogen suggests an attractive gravitational potential well in which the dust can settle. The significant contribution of the inner ring to the rotation curve is also indicated in figure 2.

The perfect WIMP

The conclusion that the EGRET excess traces dark matter makes no assumption about the nature of the dark matter, except that its annihilation produces hard gamma rays consistent with the fragmentation of mono-energetic quarks between 50 and 100 GeV. Supersymmetry, which presupposes a symmetry between particles with even spin (bosons) and odd spin (fermions), provides a good WIMP candidate. This symmetry requires a doubling of the particle species in the Standard Model: each boson obtains a fermion as superpartner and vice versa. These superpartners are still to be found, but the lightest is expected to be stable, neutral, massive and barely interacting with normal matter, i.e. it is the perfect WIMP.

Although the present data cannot prove the supersymmetric nature of dark matter, it is intriguing that the WIMP mass and WIMP annihilation cross-section (which can be calculated from the present WIMP density) are perfectly compatible with supersymmetry, including all constraints from electroweak precision experiments and limits from direct searches for Higgs bosons and supersymmetric particles, at least if the spin-0 superpartners are in the tera-electron-volt range. Figure 3 shows the allowed range of masses for spin-0 and spin-½ superpartners, assuming mass unification at the grand unification scale, i.e. common masses m₀ (m½) for the spin-0 (½) supersymmetric particles.

The allowed region in figure 3 is within reach of the Large Hadron Collider, so finding the predicted spectrum of light spin-½ and heavy spin-0 superpartners would prove the supersymmetric nature of the WIMP, especially if the lightest superpartner is stable and has the same mass as the WIMP mass deduced from the EGRET data. The lightest superpartner has properties akin to a spin-½ photon for the allowed region of figure 3, in which case the dark matter could be considered the supersymmetric partner of the cosmic microwave background, if supersymmetry is discovered. It is interesting to note that this region of parameter space yields perfect unification of the gauge couplings without any free parameters. In our first analysis in 1991, the scale of the supersymmetric masses had to be treated as a free parameter (Amaldi et al. 1991).

The statistical significance of the EGRET excess is at least 10 s and alternative models without dark matter do not yield good fits if all sky directions are considered. Furthermore, alternative models do not explain the peculiar shape of the rotation curve, or the occurrence of the hydrogen rings at 4 and 14 kpc and the high density of old stars at 14 kpc. Therefore, we conclude that the EGRET excess provides an intriguing hint that dark matter is not so dark, but is visible by flashes of typically 30-40 gamma rays for each annihilation.

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In 1998, observations of distant supernovae provided detailed information about the expansion rate of the universe, demonstrating that it is accelerating. This can be interpreted as evidence of “dark energy”, a new component of the universe, representing some 70% of its total mass. (Of the rest, about 25% appears to be another mysterious component, dark matter, while only about 5% consists of the ordinary “baryonic” matter.) Other explanations include a modification of gravity at large distances and more exotic ideas, such as the presence of a dynamic scalar field referred to as “quintessence”.

Although the hypothesis of dark energy is fascinating and more appealing than the other explanations, it faces a serious problem. Attempts to calculate the amount of dark energy give answers much larger than its measured magnitude: more than 100 orders of magnitude larger, in fact.

Kolb and colleagues offer an alternative explanation, which they say is rather conservative. They propose no new ingredient for the universe; instead, their explanation is firmly rooted in inflation, an essential concept of modern cosmology, according to which the universe experienced an incredibly rapid expansion at a very early stage.

The new explanation, which the researchers refer to as the Super-Hubble Cold Dark Matter (SHCDM) model, considers what would happen if there were cosmological perturbations with very long wavelengths (“super-Hubble”) larger than the size of the observable universe. They show that a local observer would infer an expansion history of the universe that would depend on the time evolution of the perturbations, which in certain cases would lead to the observation of accelerated expansion. The origin of the long-wavelength perturbations is inflation, as, effectively, the visible universe is only a tiny part of the pre-inflation-era universe. The accelerating universe is therefore simply an impression due to our inability to see the full picture.

Of course, observation is the ultimate arbiter between theories. The SHCDM model predicts a different relationship between luminosity-distance and redshift than the dark-energy models do. While the two models are indistinguishable within current experimental precision, more precise cosmological observations in the future should be able to distinguish between them.

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Axions were theorized more than 25 years ago to explain the absence of charge-parity (CP) symmetry violation in the strong interaction. These neutral, very light particles (in the mass range 10^–5 – 10 eV/c²) interact so weakly with ordinary matter that they could have survived until now from their birth at the very beginning of the universe, so could contribute to dark matter. However, axions could also be created today, for example near the strong electric field inside the hot plasma core of the Sun, where thermal X-rays could be efficiently converted into axions. These axions would stream out freely and arrive on Earth in quantities larger than solar neutrinos.

CAST, currently the world’s only working “axion helioscope”, is a prototype superconducting magnet for the Large Hadron Collider that has been refurbished and fitted with X-ray detectors, plus a focusing mirror system for X-rays that was recovered from the German space programme. The 9 T field in the magnet can convert solar axions passing through CAST into X-rays, with the highest efficiency for such a detector to date.

The first results from CAST show that the axion-photon coupling constant is g_aγγ< 1.16 x 10^–10 GeV^–1 for axion masses below 0.02 eV (Zioutas et al. 2004). This new limit is five times smaller than the previous best laboratory measurements, from the Tokyo axion helioscope experiment (Moriyama et al. 1998). However, CAST’s new result is comparable, in the mass range studied, to the best limit derived from stellar energy-loss arguments. It also excludes an important part of the parameter space that is not excluded by solar-age considerations, which allow an axion-photon coupling somewhat larger than the Tokyo limit.

So far CAST has covered the low end of the axion rest-mass range, m_a< 0.02 eV/c². The group is currently remodelling the telescope; filling the two tubes of the magnet with helium gas will keep the X-rays and axions in phase over the magnet’s entire length of 9.26 m. This will allow a search for axions of higher mass, covering more of the range expected from theory and not excluded by astrophysical and cosmological observations.

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With the first data from their underground observatory in northern Minnesota, the scientists of the Cryogenic Dark Matter Search (CDMS) have peered with greater sensitivity than ever before into the suspected realm of WIMPs, or weakly interacting massive particles. The results show, however, that if they do exist WIMPs are still staying out of sight.

WIMPs are of interest for the two extremes of the very large and the very small. There is strong evidence for large amounts of nonluminous dark matter in the universe, which cannot consist of normal matter (baryons) but seems likely to consist of WIMPs. At the opposite end of the scale supersymmetry yields a range of massive new particles, but the lightest – such as the neutralino – could be stable and therefore a good candidate to be a WIMP.

The CDMS II experiment, which is run by a collaboration of 48 scientists from 13 institutions, plus 28 engineering, technical and administrative staff, is located nearly 780 m below ground in a former iron mine in Soudan, Minnesota. The experiment uses four 250 g germanium detectors and two 100 g silicon detectors, which are cooled to less than 50 mK so that molecular motion becomes negligible. Substantial shielding and the 780 m of rock together reduce the background due to cosmic rays and radioactivity.

The detectors simultaneously measure the ionization and vibration (phonons) produced by particle interactions within the crystals. WIMPs should reveal their presence by creating less ionization than other particles for the same amount of vibration. This is because the WIMPs will scatter from nuclei in the detectors while other particles are more likely to scatter from electrons, and recoiling electrons create more ionization than recoiling nuclei. The timing of the phonons also provides a means of distinguishing between WIMPs and other particles.

The CDMS II results show with 90% certainty that the interaction cross-section for a WIMP with a mass of 60 GeV must be less than 4 x 10^-43 cm², or about one interaction every 25 days per kilogram of germanium (Akerib et al. 2004). This measurement is at least four times more sensitive than the best previous measurement offered by the EDELWEISS experiment in the Fréjus Underground Laboratory in France.

The results, which are described in a paper submitted to Physical Review Letters, were presented at the April Meeting of the American Physical Society on 1-4 May in Denver. The data set the world’s lowest exclusion limits on the cross-section for coherent WIMP-nucleon scalar interactions for all WIMP masses above 15 GeV. They thereby rule out a significant range of neutralino supersymmetric models.

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New developments in extensions of the Standard Model, through supergravity, superstrings and extra dimensions, were among the highlights of “Beyond the Desert 03 – Accelerator, Non-accelerator and Space Approaches”, which was held last year in Castle Ringberg in Tegernsee, Germany. Supergravity had recently celebrated its 20th birthday and two of its “inventors” – Pran Nath and Richard Arnowitt – were among the participants at the conference.

Nath, of Northeastern University, Boston, summarized the developments of minimal supergravity grand unification (mSUGRA) and its extensions since the formulation of these models in 1982, while Arnowitt, from Texas A&M, highlighted the connection to dark matter and the value of g-2 of the muon. Focusing on quantum gravity, Alon Faraggi of Oxford argued that the experimental data of the past decade suggest that the quantum-gravity vacuum should possess two key ingredients – the existence of three generations and their embedding into SO(10) representations. He explained that the Z1 x Z2 orbifold of the heterotic string provides examples of vacua that accommodate these properties. He also showed that three generations require a non-perturbative breaking of the grand unification gauge group, and in this context examined the issue of mass and mixing in the neutrino versus the quark systems.

Fundamental physics, including fundamental symmetries, formed another important aspect of the meeting. Peter Herczeg from Los Alamos reviewed CPT-invariant, and CP- and P-violating electron-quark interactions in extensions of the Standard Model. Turning to fundamental constants, Harald Fritzsch of Munich discussed astrophysical indications that the fine structure constant has undergone a small time variation during the cosmological evolution, within the framework of the Standard Model and grand unification. The case where the variation is caused by a time variation of the unification scale is particularly interesting.

Interferometry

The potential of neutron interferometry for tests of fundamental physics was outlined by Helmut Rauch of Vienna. Recent experiments in neutron interferometry, based on post-selection methods, have renewed the discussion about quantum non-locality and the quantum measuring process. It has been shown that interference phenomena can be revived when the overall interference pattern has lost its contrast. This indicates a persistent coupling in phase space, even in cases of spatially separated Schroedinger-cat-like situations.

Interesting developments in general relativity and aspects of special relativity were also discussed at the conference. Mayeul Arminjon of Grenoble presented a new “scalar ether theory” of gravitation. One of the motivations for trying such an alternative approach is to solve problems that occur in general relativity and in most extensions of it – namely the existence of singularities and the interpretation of the gauge condition. Arminjon showed that this scalar theory fits nicely with observations on binary pulsars. Lorenzo Iorio of Bari reported on new perspectives in testing the general relativistic Lense-Thirring effect. Turning to experiment, the present status of the search for gravitational waves was outlined by Peter Aufmuth of Hannover. Only astrophysical events, such as supernovae, or compact objects, for example, black holes and neutron stars, produce detectable gravitational wave amplitudes. The current generation of resonant-mass antennas and laser interferometers has reached the sensitivity necessary to detect gravitational waves from sources in the Milky Way. Within a few years the next generation of detectors will open the field of gravitational astronomy.

Cosmological connections

Talks about the early universe included cosmological, quantum-gravitational and other possible violations of CPT symmetry. Nick Mavromatos of King’s College, London, discussed the various ways in which CPT symmetry may be violated, and reviewed their phenomenology in current or near-future experimental facilities, both terrestrial and astrophysical. First he outlined violations of CPT symmetry due to the impossibility of defining a scattering matrix as a consequence of the existence of microscopic or macroscopic space-time boundaries, such as Planck-scale black-hole event horizons or cosmological horizons due to the presence of a positive cosmological constant in the universe. Second he discussed CPT violation due to the breaking of Lorentz symmetry, which may characterize certain approaches to quantum gravity. He stressed that although most of the Lorentz-violating cases of CPT breaking are already excluded by experiment, there are some (stringy) models that can evade these constraints.

Trans-Planckian physics was discussed by Ulf Danielsson of Uppsala, who outlined how the cosmic microwave background radiation might probe physics at or near the Planck scale. Danielsson reviewed a potential modulation of the power spectrum of primordial density fluctuations generated through trans-Planckian (maybe stringy) effects during inflation.

Margarida Rebelo of Lisbon discussed CP violation in the leptonic sector at both low and high energies in the framework of the “seesaw” mechanism. She pointed out that leptogenesis is a possible and likely explanation for the observed baryon asymmetry of the universe. It seems to be one of the most promising scenarios, in view of the fact that several other alternative proposals are on the verge of being ruled out. The leptogenesis scenario implies constraints on both light and heavy neutrino masses, which, as she showed, are consistent with the present value obtained from the double beta decay of ⁷⁶Ge.

Cosmoparticle physics was another major theme of the conference. Maxim Khlopov of Rome and Moscow gave a broad overview of the topic, calling it the “Challenge for the Millennium”, and results linking particle-physics experiments with cosmological problems, and vice versa, were among the experimental highlights.

The existence of dark matter in the universe has for many years been an intriguing problem. Rita Bernabei of Rome presented the final results of the DAMA dark-matter experiment, which confirm their first indications for the observation of cold dark matter at a 6 σ level. Measurements of the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe (WMAP), which are revealing the proportions of dark matter – and dark energy – in the universe, were presented by Eiichiro Komatsu of Princeton. Neutrino parameters are also deducible from this experiment, as well as from current large-scale galaxy surveys, as Steen Hannestad of Odense described. However, the cosmic microwave background experiments cannot at present differentiate between the different neutrino-mass scenarios.

Neutrino highlights

Moving on to ground-based studies of neutrino properties, the Heidelberg-Moscow double beta decay experiment in the Gran Sasso Laboratory has results for the period 1990-2003, which were presented by Hans Volker Klapdor-Kleingrothaus of MPI Heidelberg. With three additional years of data included in this analysis, the evidence for neutrinoless double beta decay has now improved to a 4.2 σ level. For 10 years this experiment has been the most sensitive double beta experiment worldwide, and with the statistics now reached, it has essentially already achieved scientifically what was expected from the larger GENIUS project proposed in 1997. The conclusion from this result is that the total lepton number is not conserved (neutrino oscillations reveal only the violation of family lepton number). This has fundamental consequences for the early universe. Furthermore, according to the Schechter-Valle theorem, the existence of neutrinoless double beta decay implies that the neutrino is a Majorana particle. (The announcement of the start of the GENIUS Test Facility in Gran Sasso, in May 2003, was now of most interest in the context of the search for dark matter. The goal of the GENIUS Test Facility is to confirm the DAMA result by looking for the seasonal modulation signal.)

On the theoretical side Mariana Kirchbach of San Luis Potosi in Mexico stressed the importance of double beta decay for fixing the absolute scale of the neutrino mass spectrum. She showed that in the case of Majorana neutrinos, in single beta decay the mass might lead to unexpected results. In this scenario a sensitive tritium decay experiment should see no mass if the neutrino is a Majorana particle, while the dependence of the neutrinoless double beta decay rate. Ernest Ma of Irvine outlined how a rather precise knowledge of neutrino oscillation parameters, i.e. the correct form of the 3 x 3 neutrino mass matrix, may be obtained from symmetry principles. He showed that the latter predict three nearly degenerate Majorana neutrinos with masses in the 0.2 eV range. This theoretical result is of great interest, in view of the results from double beta decay, WMAP, etc.

Contributions to fundamental physics, obtained using Penning traps, were outlined by one of the pioneers of the field, Ingmar Bergstrom of Stockholm. A Penning trap is a storage device in which frequency measurements can be used to determine the mass of electrons and ions, as well as g-factors of electrons and positrons, with extremely high accuracy. Bergstrom has recently measured, for example, the Q value of the double beta decay of ⁷⁶Ge with unprecedented precision.

Other experimental highlights on neutrinos included the results obtained for solar neutrinos by the Sudbury Neutrino Observatory (SNO). As George Ewan of Kingston, Canada, described, SNO now has strong evidence at a 5.3 σ level, and independently of the details of solar models, that neutrinos change flavour on their way from the Sun to the Earth. These results, together with those of other neutrino experiments, among them the Japanese 250 km long-baseline experiment that was presented by Takashi Kobayashi of KEK, mean that our knowledge of neutrino properties has improved considerably over the past few years. In this context, Oliver Manuel of Missouri gave a highly interesting, non-mainstream view of the structure of the solar core.

Supernova and relic neutrinos were the topic of another session. Irina Vladimirovna Krivosheina of Heidelberg and Nishnij-Novgorod, who was a member of the Baksan group that was one of three groups which observed neutrinos from the supernova SN1987A, gave a retrospective view of this exciting event and some insider details of its discovery. Mark Vagins of Irvine and Shinichiro Ando of Tokyo discussed further the observation of relic and supernova neutrinos, one of the future tasks of the Super-Kamiokande experiment in Japan.

Accelerator approaches

Turning to the physics of nuclei, results on superheavy elements have reached an exciting level. Dieter Ackermann showed that elements 107-112 have been synthesized and unambiguously identified at GSI, Darmstadt. The observation of elements 112, 116 and 118 by the Oganessian group at Dubna was also announced by Vladimir Utyonkov. At the interface between nuclear physics and particle physics, the status of the search for a phase transition between hadronic matter and a quark-gluon plasma at Brookhaven’s Relativistic Heavy Ion Collider was outlined by Raimond Snellings of Amsterdam, and compared with measurements at CERN’s Super Proton Synchrotron.

Several sessions were devoted to the search for new physics with colliders. The final analyses of the search for Higgs bosons, R-parity violation, leptoquarks and exotic couplings at CERN and Fermilab, presented by Rosy Nikolaidou of CEA Saclay, Silvia Costantini of Rome “La Sapienza”, Stefan Soeldner-Remboldt of Manchester and others, show no indication of physics beyond the Standard Model. This reinforces the observation that the only new physics to emerge recently is from underground experiments.

Particles from space

Nearly a century after the discovery of cosmic rays, their origins are still unknown. Eckart Lorenz of Munich reviewed the status and perspectives of ground-based gamma-ray astronomy, where new telescopes under construction, such as MAGIC, should lead to a big step in sensitivity. At gamma-ray energies of around 10-30 GeV the universe becomes basically transparent, so gamma-emitting objects as far as red-shifts of more than three should become visible, that is, up to a time where star and galaxy formation has been particularly strong. New projects like MAGIC will allow the gap to be closed between satellite-borne instruments and previous, ground-based telescopes. Exciting results from the CANGAROO experiment, an array of four imaging Cherenkov telescopes in Australia, were presented by Ken’ichi Tsuchiya of Tokyo. The team has observed TeV gamma rays from SNR SN1006 and from new types of objects, such as gamma rays from a normal spiral galaxy showing starburst activity, NGC253. This is the first detection of gamma rays from an extragalactic object other than active galactic nuclei, and is the largest structure ever detected.

The Auger Observatory is under construction and will look for cosmic rays at the highest energies. It will be the largest cosmic-ray detector ever built, covering 3000 square kilometres in both the southern and northern hemispheres in its final configuration. Johannes Bluemer of Karlsruhe described the present status of the construction at the southern site in Argentina, which began in 1999.

The highest cosmic energies, beyond the Greisen-Kuzmin-Zatsepin limit, find an interesting theoretical explanation in the Z-burst scenario, in which a large fraction of the cosmic rays are decay products of Z-bosons produced in the scattering of ultra-high-energy neutrinos on cosmological relic neutrinos. This was discussed by Daniel Fargion of Rome and Sandor Katz of DESY and Budapest. Interestingly, they find that neutrinos should have a mass in the range of 0.1-1 eV – which is consistent with the result of the HEIDELBERG-MOSCOW experiment – in order to make this explanation work properly.

Hunting for antimatter

The search for antimatter (and dark matter) with the Alpha Mass Spectrometer, which is planned to be installed on the International Space Station in 2005/2006 for a three-year mission, was discussed by Frank Raupach of Aachen. The existence of large domains of antimatter in the universe is still an open question. The observed uniformity of the cosmic microwave background indicates that no voids exist at all between matter and antimatter worlds, hence annihilation processes should be inevitable and the resulting diffuse gamma-ray spectrum might be observable.

Returning to neutrinos, but this time from space, Christian Spiering of Zeuthen gave an overview of results from AMANDA, the neutrino telescope at the South Pole, and Jan-Arys Dzhilkibaev reviewed the status and perspectives of the Baikal Neutrino Project. Finally, Yoshitaka Kuno from Osaka outlined the goals of future neutrino and muon factories. A neutrino factory would have great potential for examining the mass hierarchy of neutrinos, the matter effects, and CP violation in the neutrino sector. A rich physics programme would also be possible with a high-intensity muon beam at a muon factory, ranging from searches for muon processes that violate lepton flavour (such as µ^– to e^– conversion) and the muon electric dipole moment to further precision measurements of the muon magnetic moment (g-2). Lepton flavour violation in the charged sector will be studied also by the muon to electron conversion experiment, MECO, presented by Michael Herbert of Irvine.

In summary, the lively and highly stimulating atmosphere during this Beyond meeting reflected a splendid scientific future for particle physics. The proceedings of Beyond 03 are now available as a book, Beyond the Desert 2003, Springer Proceedings in Physics, vol 92.

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Dark energy was first discovered in 1998 by two groups using supernovae as markers of cosmological distance as a function of time – the Supernova Cosmology Project led by Saul Perlmutter at Lawrence Berkeley National Laboratory and the High-z Supernova Search Team led by Brian Schmidt at Australian National University. Measurements indicated that distant supernovae were dimmer than expected from the cosmological inverse square law in a universe dominated by matter (S Perlmutter et al. 1999, A Riess et al. 1998). That is, they appeared to be further away than expected from the expansion rate of the universe if gravitation due to the matter contents were the main force. Some form of dark energy was required at the 99% confidence level, and in amounts sufficient to counteract, on cosmic scales, the gravitational attraction from the clustered matter.

Since then, deeper and more precise supernova measurements and further lines of evidence confirm this conclusion (J Tonry et al. 2003, R Knop et al. 2003, D Spergel et al. 2003). Detailed measurements of the cosmic microwave background power spectrum, by the Wilkinson Microwave Anisotropy Probe satellite and by ground-based experiments, imply the presence of dark energy too. They also show that the spatial geometry of the universe is consistent with the flatness prediction of inflation. But observations of galaxy clusters tell us that the matter contribution to the total energy density can amount to only 20-30% of the needed critical density. Any two of the three lines of evidence imply that the dark energy composes roughly three-quarters of the energy density of the universe, while the third method provides a crosscheck. Such an amount of dark energy acts to accelerate the cosmic expansion.

The nature of dark energy

While gravitation due to matter or radiation is attractive, a sufficiently negative pressure p would offset a positive energy density ρ to give repulsive gravity under Einstein’s equations (the gravitating density depends on ρ+3p), pulling on space to accelerate the expansion of the universe. Researchers often discuss this in terms of the equation of state ratio of the pressure to energy density: w = p/ρ.

Negative pressures are not a wholly exotic phenomenon. After all, one of the equations of expansion of the universe, the Friedmann equation, looks remarkably similar to the first law of thermodynamics: d(ρV) = -pdV, where V is the volume considered. Negative pressure leads to an overall plus sign, turning this equation into something that looks like the tension in a spring or rubber band. Such a “springiness” of space was postulated soon after Einstein developed the general theory of relativity in his cosmological constant term, and Hermann Weyl attempted to link such a background energy to the quantum vacuum. If the vacuum is a true ground state then all observers must agree on its form. But the only Lorentz invariant energy-momentum tensor is the diagonal Minkowski tensor that has negative pressure equal and opposite to its energy density, that is, the cosmological constant has equation of state ratio w = -1. This would cause an accelerating universe.

So why are cosmologists not satisfied with identifying the cosmological constant with dark energy? In The Hunting of the Snark – the poem by Lewis Carroll, who was in fact Charles Dodgson, a mathematician at Oxford – when the explorers set sail to find the mysterious snark, the captain “had bought a large map representing the sea, without the least vestige of land: and the crew were much pleased when they found it to be a map they could all understand.” The cosmological constant term is such a featureless sea, but there are two problems with using it to describe our universe. The expected sea level for the quantum vacuum is much higher than we observe: naively one should indeed have a featureless universe, with matter drowned by 120 orders of magnitude below the energy density of the cosmological constant. But the cosmic concordance measures only a factor of a few difference. Furthermore, the matter and radiation we see in the universe evolves with the expansion of the universe, while the cosmological constant does not. Even an order of magnitude equality between them occurs in only one characteristic timescale (e-folding), out of the 23 in the expansion of the universe since the well-understood epoch of nuclei formation in the early universe. (See S Weinberg 1989 and S Carroll 2001 for more on these fine-tuning and coincidence puzzles.)

Hunting the dark energy

Researchers are thus driven to consider other explanations for the dark energy. Models with dynamical high-energy physics fields, often called “quintessence” when involving a simple scalar field, go some way toward alleviating the timing or coincidence puzzle, though there is still no clear underlying theory explaining the current effective energy density. Such a field would need an effective mass of 10^-33 eV, that is, with a Compton wavelength of the order of the radius of the universe. However, there are rich attempts at phenomenology stretching back two decades (longer if scalar-tensor theories of gravitation are included). An early high-energy physics model was proposed by Andrei Linde in 1986, demonstrating how a linear potential could give rise to accelerating expansion. On the cosmology side, Robert Wagoner in 1986 examined how a general equation of state component would not only affect the expansion, but could be observationally probed with cosmological distance and age measurements.

Both the modelling of and the investigation of the observational consequences of dark energy are now active industries within research in cosmology, covering a wide variety of the physics of the early and late universe. In general, the dark-energy equation of state will vary with time and so needs to be probed with observations over a range of epochs, or astronomical redshifts z (the fractional difference in the scale of the universe today relative to an earlier time). The major challenge over the next few years in cosmology will be to characterize the equation of state function w (z ). On the phenomenology front, one might hope for a natural, robust model to emerge, but the theorists’ prolificness seems too great for this to settle the question. Indeed, models beyond scalar fields involving modifications of general relativity, extra dimensions, or quantum-phase transitions have also been proposed. Fortunately these can be written in terms of an effective w (z ) (E Linder and A Jenkins 2003) and subjected to cosmological measurements.

Three main routes to probing dark energy exist in cosmology. The first, and currently most favoured, involves mapping the expansion history of the universe. The second seeks to measure the growth rate of the formation of large-scale structures such as clusters of galaxies. The third involves the cosmic microwave background radiation – looking not for the time variation of the dark energy (since the cosmic microwave background photons effectively all come from the same redshift), but for the subtle spatial fluctuations in the dark-energy distribution on cosmic scales. Observations of Type Ia supernovae, which first discovered the dark energy in 1998, fall in the first category and seem the most promising. The second and third approaches are likely to run into limits imposed, respectively, by uncertainties involving entangled astrophysics and cosmic variance (intrinsic uncertainty due to observing only one universe). However, new methods and cross correlations between probes may eventually be practical.

In mapping the expansion history, cosmologists probe the deceleration due to the gravitation of matter and the acceleration due to dark energy at various epochs. Variations in the growth of distances reveal a picture of the cosmic environment, and hence the dynamic influence of dark energy, in the way that the width of tree rings indicates the Earth’s climatic environment over time. Type Ia supernovae can be seen to great distances and calibrated in luminosity (made “standard candles” through detailed observations). Thus the measurement of the received flux directly indicates their distance, and hence the time in the past they exploded, while the redshift of the photons is simply the ratio of the size of the universe now relative to then. Together these give the exact expansion history.

Future endeavours

The best current supernova data extend out only to redshift z ≅ 1 (when the scale of the universe was 1/(1+z ) = 1/2 its current size) with any reasonable statistics, but they already constrain the averaged equation of state ratio to w = -1.05^+0.15_-0.20 (R Knop et al. 2003) or w = -1.0^+0.14_-0.24 (J Tonry et al. 2003). Clues to the underlying physical theory, however, reside in the dynamics, the time-varying function w (z ). A dedicated dark-energy mission, the Supernova/Acceleration Probe (SNAP) satellite, is being designed to determine the present value w₀ to 7% and derivative w ‘ = dw/dz to ±0.15 (1 σ, including both statistical and systematic uncertainties). Led by Michael Levi and Saul Perlmutter of the Lawrence Berkeley National Laboratory, the project involves over 100 scientists and engineers from more than 15 institutions, including France and Sweden. Launch is proposed for 2010.

Meanwhile, an intense research effort continues. One example is the European Dark Energy Network (EDEN), a proposed European Union research training network of 13 nodes (including CERN, led by Gabriele Veneziano), coordinated by Pedro Ferreira of Oxford. Models attempt to link dark energy to dark matter, extra dimensions, modifications of gravity and a zoo of simple and non-minimally coupled scalar fields. These predict a range of values for the equation of state ratio w₀, within the current constraints, and a wholly open variety of w ‘, both positive and negative. Some even lead to an eventual reversal of the acceleration and a collapse of the universe. It is amusing that the first dark-energy model, the linear potential, possesses this quality. Future data will constrain the allowed parameters of classes of high-energy physics models and the fate of the universe, including how long we have left until a cosmic doomsday! (See R Kallosh et al. 2003 for the linear potential case leading to a Big Crunch and R Caldwell et al. 2003 for a Big Rip.)

Can signs of the nature of dark energy be uncovered at particle accelerators? It is difficult to see how. The energy scale of the physics is presumably of the order 10¹⁶ GeV, and by its “dark” nature the coupling to matter is vanishingly small. On scales smaller than the universe, the dynamical effect of dark energy is negligible. The entire dark-energy content within the solar system equals that of three hours of solar luminosity. Perhaps if the physics involves the modification of gravity or extra dimensions, a precise laboratory test could see a signature (see E Adelberger et al. 2003 for a current experiment). But the true hunting grounds for the nature of dark energy and the physics causing the acceleration of the universe lie in cosmology. Just as advances have been made in the past two decades in theory and observations beyond the simplistic view of early universe inflation as a pure deSitter phase – “sea without the least vestige of land” – so too will dark-energy studies delve deeper into fundamental physics. Instruments now being designed could tell us within the next decade whether we must come to grips with a minuscule but finite cosmological constant or some exciting new dynamical physics.

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On 28 April, the UK minister for science and innovation, Lord Sainsbury, opened a new research cavern at the Boulby Underground Laboratory for Dark Matter Research, at Boulby in North Yorkshire, UK. The Boulby lab is situated in a working salt and potash mine and houses experiments to detect weakly interacting massive particles (WIMPs), a prime candidate for dark matter in the universe. The laboratory has recently benefited from a £3.1 million Joint Infrastructure Award (JIF) from the UK Particle Physics and Astronomy Research Council, which has provided new enhanced underground laboratories and complementary surface facilities.

Situated more than 1 km beneath the Earth’s surface within a salt and potash mine, the laboratory is isolated from interference from cosmic rays and benefits from an environment with low natural radioactivity. The laboratory operates on behalf of the UK Dark Matter Consortium – the University of Sheffield, the Rutherford Appleton Laboratory, the Imperial College of Science, Technology and Medicine, and the University of Edinburgh.

The Boulby lab currently houses three experiments to detect dark matter – NAIAD (NaI Advanced Array Detector), ZEPLIN I (from ZonEd Proportional scintillation in LIquid Noble gases, now operating with liquid xenon), and DRIFT (Directional Recoil Identification From Tracks). DRIFT is the first experiment to be installed in the new area of the laboratory and is unique because its aim is not only to detect WIMPS, but also to determine which direction they come from.

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The universe around us is nothing like it looks. The stars make up less than 1% of the matter in the universe; while all the gas and other forms of baryonic matter account for less than 5%. We know little about the other 95% except that it is probably divided into 35% cold dark matter and about 60% dark energy.

Dark energy is detected by the recent acceleration of the universe and is observed by the study of type 1a supernova sources. A series of symposia have been organized in Southern California for the past 8 years to hear the latest in the developments in this field of particle cosmology. It was at the 1998 meeting that the two teams that have observed the accelerating universe first made a joint announcement of these important results.

The particle physics of dark matter is perhaps the most advanced in our understanding of these phenomena. Perhaps the best motivated and best understood form of particle dark matter comes from supersymmetry (SUSY).

This theory gives a “semi-natural” explanation of the amount of dark matter in the universe, which would take the form of weakly interacting massive particles (WIMPs) – the parameters are constrained by data from CERN’s LEP experiments and elsewhere. The strong interplay between proposed dark matter detectors and the direct observation of SUSY particles at CERN’s forthcoming Large Hadron Collider (LHC) reveals a strong connection between collider particle physics and astroparticle physics.

There was a complete discussion of the current search for SUSY dark matter and future detectors at the meeting. The DAMA experiment at Italy’s Gran Sasso underground laboratory continues to claim a signal for SUSY due to an observed annual variation. However, there are now three experiments – Edelweiss at Modane in France, ZEPLIN I at Boulby in the UK, and CDMS I at the Stanford Linear Accelerator Center in the US – that cut deeply into the region allowed by DAMA. These experiments all use some form of background discrimination.

A joint analysis of the CDMS I data at DAMA was claimed to exclude the DAMA signal from a WIMP source to 98% confidence level, even assuming all of the CDMS I events are not neutron-induced. The DAMA group disputes this claim, however. The DAMA experiment is being upgraded and hopefully this dispute will be resolved soon. The current predictions for the rate of SUSY WIMP detectors are nearly all well below the DAMA sensitivity, as was discussed extensively at the meeting.

Bigger machines

It was generally agreed that a new generation of much larger detectors will be needed to provide a clean detection of the SUSY WIMP signal. There are several discriminating detectors in the 10-30 kg mass range being constructed or upgraded such as CDMS II, Edelweiss and ZEPLIN II. To provide a clear study of the WIMP signal, detectors of the target mass of 1 tonne will be needed, and there are preliminary studies of possible detectors for this mass range. It is truly remarkable that detectors of this great sensitivity are being developed.

Dark energy

The issue of the origin of dark energy is more complex and possibly much more obscure. After the pioneering work of the two teams working on type 1a supernovae, there are projects for two impressive detectors that will to try to identify the equation of state of the dark energy.

The SNAP satellite would observe type 1a supernovae out to a redshift of around z = 1.5. The other possibility is to study type 1a supernovae from the ground using a large “dark matter” telescope in Chile called the Large Synoptic Survey Telescope (LSST). It may be that both of these methods will be needed to unravel the equation of state and demonstrate that the effect is either due to a cosmological constant or some other elementary particle-like source.

In one of the most interesting talks at the meeting, Paul Stenhardt of Princeton discussed the impact of an accelerating universe on the old question of whether the universe may be cyclic in time. It is possible that an accelerating universe could wipe out the entropy of the universe over a long time and then if the equation of state of the dark energy complies, the universe might contract to a “big crunch”. According to this viewpoint, the accelerating state of the universe is actually required rather than being a bizarre add-on to a Friedmann universe as currently held belief would prefer.

There was considerable discussion of the possibility of self-interacting, warm and hot dark matter (in light of recent claims for the observation of double-beta decay). None of these issues was clarified at the meeting.

During the course of the Southern California meetings, a much clearer picture of the bulk of components of the universe has emerged, but we have yet to find any evidence of what this stuff really is. Hopefully this will change as the new WIMP detectors underground and new detectors in space start taking data and the LHC is turned on. The next symposium will be held in February 2004 in Marina del Rey.

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Astrophysics, particle physics, cosmology and fundamental physics in space have much in common to bring their practitioners together. CERN and the European Southern Observatory (ESO) have already held several joint symposia, and in 2000 a workshop organized by CERN and the European Space Agency (ESA) was held in Geneva. In March this year, some 200 scientists travelled to ESO’s headquarters near Munich for the first symposium to be hosted by all three organizations.

Following an introduction from ESO’s director-general, Catherine Cesarsky, the global properties and evolution of the universe took centre stage. These have been studied in terms of a few parameters whose values have been broadly established in a remarkably short time. The universe is flat with a critical density (W = 1), but baryons constitute only about 5%. Dark non-baryonic matter accounts for 25% of the overall density, and about 70% is “dark energy” with a negative pressure accelerating the expansion of the universe. All these contributions to the overall density should be precisely known within a decade. The apparent concordance of the parameters describing the universe obtained through very different measurements is already impressive, and leads to the question of why these parameters have the values they do.

Some speakers were even tempted to raise the anthropic principle, although this tenacious myth is neither quantitative nor falsifiable, and does not teach us anything new. Nevertheless, it has gathered new momentum within a framework where many different universes could have been born, or even within a single universe where widely differing domains could exist and where we happen to live in the one domain providing for our needs.

Global properties

The global properties of the universe were covered in the opening sessions. Neil Turok of Cambridge spoke about the very early universe – past and future – since the universe he proposed has a succession of Big Bangs and Big Crunches. After discussing the different contributions to W, Turok stressed that each measurement has little value on its own unless it is assessed within a particular theoretical framework, and that we should keep challenging the framework.

An explanation for many observed features is found in the standard inflationary universe scenario, which Turok challenged with a model of colliding branes. He called for an open-minded approach to such ideas, and for further tests of inflation such as the polarization of the cosmic microwave background (CMB) and the observation of a background of long-wavelength gravitational waves.

Paolo de Bernardis of the University of Rome reviewed CMB properties from an experimental point of view. The key result is the flatness of the universe, but the observation of peaks in the angular analysis of the CMB are what have allowed the baryonic density to be pinned down to 5%, and shown that temperature fluctuations are scale-independent. De Bernardis stressed, however, that measurements are currently restricted to a very limited coverage of the sky. This will be much extended with NASA’s MAP and ESA’s Planck missions, which will also measure CMB polarization.

A key feature is the current acceleration of the universe’s expansion, direct evidence for which comes from observations of supernovae. ESO’s Bruno Leibundgut showed how the observation of 27 low-redshift type 1a supernovae has built confidence that they are reliable standard candles. This has allowed observations of 54 high-redshift ones to be interpreted as fainter than expected from standard Hubble expansion. Taken at face value, this could result from a vacuum energy density of 0.7, which accelerates expansion today but would have led to a deceleration in the past when the matter density was higher. Leibundgut added a cautionary note, however, saying that much remains to be understood about the systematic uncertainties of type 1a supernovae. Much larger statistics, probing to higher redshift, are needed, as is a better understanding of the their explosion. Yannick Mellier of the Observatoire de Paris reviewed the complementary determination of the matter density via weak gravitational lensing. There is good agreement on the matter and vacuum density between six different teams. When combined with the analysis of the CMB, the matter density is pinned down to around 0.3 and the vacuum density to around 0.7. It is puzzling that the vacuum energy density should be so tiny compared with the Planck scale or electroweak breaking scale.

Dark matter

Direct searches for dark matter were reviewed by Charling Tao of Marseille. After recalling the first hints provided by the rotation curves of galaxies, interpreted as being due to a halo of baryonic matter, she explained how massive compact halo objects identified through gravitational lensing are too few to account for the effect. Underground experiments have looked for weakly interacting massive particles, so far to no avail. Concluding the discussion of exotic dark matter candidates was Georg Raffelt of the Max Planck Institute (MPI) in Munich, who described the CERN axion solar telescope, CAST.

With their tiny but non-zero masses, neutrinos provide the first clear departure from the Standard Model of particle physics, but are no longer expected to provide an appreciable contribution to dark matter. Pilar Hernandez of CERN reviewed the status of neutrino physics. Oscillations with mass square differences of the order of 10^-3-10^-5 eV² and maximal mixing are now favoured.

Reviews of the Standard Model of particle physics and the exciting prospects for research at CERN’s forthcoming Large Hadron Collider were the subject of many presentations. Antonio Pich of Valencia spoke of low-energy Standard Model tests, while CERN’s Fabiola Gianotti covered high-energy tests. John Ellis of CERN went beyond the Standard Model, raising the possibility of extra dimensions at short distances as a rival to supersymmetry.

Studying the extreme

Edward van den Heuvel of Amsterdam reviewed gamma-ray bursts (GRBs). A beautiful example of serendipity, GRBs were discovered while looking for something else – nuclear tests in the atmosphere. Now known as the most powerful cosmic explosions, they occur at the level of one per day with energy output reaching that of up to a million supernovae. They last from seconds to minutes and appear at random in the sky, which is to be expected, since their X-ray and visible afterglows are associated with highly redshifted host galaxies. Many efforts are underway to observe and study GRBs with redshifts up to z = 12.

Remaining with the extreme, Heinrich Völk of Heidelberg discussed very-high-energy gamma rays, and Alan Watson of Leeds talked about the highest-energy cosmic rays. The observation of cosmic rays above about 10¹¹ GeV is a puzzle, since as Greisen, Zatsepin and Kuzmin pointed out, the CMB should make space opaque to them. Understanding their origin will rely on observations with the Auger observatory and later with ESA’s Extreme Universe Space Observatory.

Francis Halzen from the University of Wisconsin-Madison talked about high-energy neutrinos from astrophysical sources. Detection at rates appropriate for meaningful study demands very large detectors such as the 1 km³ Icecube detector being installed deep under Antarctic ice, and underwater detectors such as ANTARES.

One session was devoted to massive objects. It now seems likely that massive black holes at the centre of galaxies fuel the massive energy output of quasars, equivalent to 10¹²-10¹⁵ suns, and that all galaxies were once active. Quasar density peaks at a redshift of z = 2. In closer galaxies, the presence of a relatively quiet black hole of millions of solar masses is inferred from the swift motion of stars around a dark centre. The central part of our Milky Way, for example, has been observed to within 3 light years of the centre, where stars circle at velocities up to 1500 km/s. Interpretations more exotic than the presence of a giant black hole are not expected to hold.

Gravitational waves

Bernhard Schutz of the MPI in Potsdam gave a review of gravitational wave sources, while Karsten Danzmann of the MPI in Hannover discussed experimental searches. Gravitational waves carry huge energies but interact very feebly, crossing the universe almost unperturbed. Ground-based detectors, sensitive to frequencies above 10 Hz, are complementary to detectors in space, which will look for frequencies below 0.1 Hz. Both should be sensitive to amplitudes below 10²². In his talk, Stephano Vitale of Trento discussed several ESA fundamental physics missions including SMART-2, a test mission for the ambitious LISA gravitational wave interferometer, which should fly in 2006.

Other forthcoming space experiments include the ESA-NASA STEP mission, which will test the equivalence principle to six orders of magnitude better than the present limit. Roberto Battiston of Perugia described Alpha Magnetic Spectrometer (AMS) findings on properties of the cosmic-ray flux, and showed how future AMS missions could bring down the anti-helium to helium ratio from 10^-6 to around 10^-9.

Planetary systems

Ewine van Dishoeck of Leiden discussed the formation of star and planetary systems from large clouds of gas and dust, saying that around 15% of stars have a disk from which planets could form. Solar system formation would take some 100 million years. This field will soon see major developments with new tools such as the Atacama Large Millimetre Array, ESA’s Infrared Space Observatory, NASA’s Space Infra Red Telescope Facility and the Next Generation Space Telescope.

Michel Mayor of Geneva recalled that more than 80 extra solar planets have already been seen, some of them with masses as low as 50 times the mass of the Earth. He discussed how planets are found through radial velocity surveys and planetary transit, the latter giving direct evidence for gaseous giants like Jupiter. The diversity of planets observed was not anticipated – some have very short periods, elongated orbits or very large masses up to 10 times the mass of Jupiter. Michael Perryman of ESA showed how missions under study could make the search for Earth-like planets possible, saying these may be as numerous as one per thousand stars. Many conditions, however, would need to be satisfied to make life possible. Perryman outlined a “habitable zone” requiring the presence of a Jupiter-type planet as a protection against meteorites, and stressed that it would need to exist over billions of years.

The meeting drew to a close with presentations about future directions at CERN, ESA and ESO. Exciting projects are being completed, are under construction or are at the planning stage in all three organizations. The closing lecture was given by Martin Rees of Cambridge, who brought the symposium to a brilliant finale with the conclusion that we live in exciting times.

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A recent meeting underlined the continual freshness of studying the ancient history of the universe. Cosmology, which studies the structure of the universe, has been developing rapidly in the past few decades. The number of articles on cosmology, astrophysics and even astronomy in recent issues of CERN Courier testifies to this. Improved observational means and an increased understanding of particle phenomena in the early universe have transformed cosmology from a speculative philosophy into an exact science, yielding diversified knowledge about the laws of nature.

In 1997 particle physicists interested in cosmology recognized the need for a regular workshop dedicated to particle astrophysics and particle cosmology. Particle astrophysics includes, in broad terms, studies of and searches for relic particles and other remnants from the early universe constituting dark matter, as well as neutrino astrophysics, which gives important information about neutrino properties. Particle cosmology deals with particle aspects of the physics of the very early universe: inflation, reheating, cosmological aspects of Grand Unified Theories and strings, baryogenesis, phase transitions and other aspects of symmetry breaking. To complete the picture, particle physicists also need to make contact with purely gravitational issues, with the possible need to rewrite general relativity and with purely astronomical work leading to the determination of cosmological parameters.

The first workshop in the series, COSMO-97, was held in the Lake District, England; COSMO-98 was held in Asilomar, California; COSMO-99 was held in Trieste, Italy; and COSMO-2000 was held in Cheju Island, Korea. This year’s meeting, COSMO-01, was held in Rovaniemi, Finland, right on the Arctic Circle, on 30 August – 4 September. In Finland, research in cosmology started in the early 1980s and today it is one of the most fruitful research fields in the physics department of the University of Helsinki and in the Helsinki Institute of Physics. New impetus and new resources have arrived since Finland joined the European PLANCK project, in which Finnish theoreticians and instrument builders have specific responsibilities.

The Rovaniemi programme comprised 33 invited plenary talks and 63 contributed talks in two parallel sessions. The plenary speakers treated aspects of inflationary cosmology, quintessence cosmology, string cosmology, extra dimensions, the ekpyrotic universe, baryogenesis, Big Bang nucleosynthesis, phase transitions, the angular power spectrum of the cosmic microwave background, large-scale structure, magnetic fields, dark matter, cosmological parameters, neutrino astrophysics and ultrahigh-energy cosmic rays.

The proceedings of the workshop will be published in the SLAC electronic conference proceedings archive and in the Los Alamos arXive astro-ph.

The next workshop in this series will be held on 18-21 September 2002 in Chicago.

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The CERN Solar Axion Telescope, CAST, aims to shed light on a 30-year-old riddle of particle physics by detecting axions originating from the 15 million degree plasma in the Sun’s core. Axions were proposed as an extension to the Standard Model of particle physics to explain why CP violation – a phenomenon linked to the dominance of matter over antimatter in the universe – is observed in weak but not strong interactions – the so-called strong-CP problem.

One of the most striking consequences of this is the neutron electric dipole moment, which, due to a CP-violating term in the standard equations, is calculated to be ten orders of magnitude larger than its measured upper limit. This can be overcome by introducing a further symmetry, the spontaneous breaking of which yields the axion – a neutral pion-like particle that interacts very feebly. Owing to their potential abundance in the early universe, axions are also leading candidates for the invisible dark matter of the universe.

Searches for solar axions began a decade ago when the US Brookhaven Laboratory first pointed an axion telescope at the Sun – a highly useful source of weakly interacting particles for fundamental research, as the solar neutrino anomaly amply demonstrates. Axions would be produced in the Sun through the scattering of photons from electric charges – the Primakoff effect – and their numbers could equal those of solar neutrinos. The idea behind the Brookhaven experiment, first proposed by Pierre Sikivie, was to put the Primakoff effect to work in reverse, using a magnetic field to catalyse the conversion of solar axions back into X-ray photons of a few kilo-electronvolts.

The Brookhaven telescope was later joined by another in Tokyo, while other experiments continued the search in different ways. Experiments at Brookhaven, the Lawrence Livermore Laboratory and Kyoto, for example, search for relic axions from the early universe. CERN’s NOMAD experiment joined the hunt, looking for axion production in a neutrino beam. Searches based on axion Bragg scattering have been performed by the SOLAX collaboration using a 1 kg single crystal of germanium in an underground laboratory in Argentina, while optical detection techniques are employed by Italy’s INFN experiment, PVLAS.

This list is not complete, but, taken together, earlier experiments have scanned the kinetic energy range from 10^-11 eV up to 10¹¹ eV, so far without success. CAST, however, could make a difference because of the length and strength of the magnetic field that it will have available by using a prototype magnet for CERN’s LHC collider.

The conversion efficiency for axions increases as the square of the product of the transverse magnetic field component and its length. This makes a 9 tesla, 10 m LHC prototype dipole magnet with straight beam pipes ideal for the task, giving a conversion efficiency exceeding that of the two earlier telescopes by a factor of almost 100.

CAST’s LHC magnet will be mounted on a moving platform with X-ray detectors on either end, allowing it to observe the Sun for half an hour at sunrise and half an hour at sunset. The rest of the day will be devoted to background measurements and, through the Earth’s motion, observations of a large portion of the sky. CAST’s X-ray detectors are under development, with the collaboration looking at gas-filled and solid-state options. A chamber using the “micromegas” principle has been tested.

The aperture of the LHC magnet’s beam pipes is around five times the predicted solar axion source size, so its X-ray detectors must be correspondingly large, implying a high level of noise. To overcome this problem, the CAST collaboration is considering using X-ray lenses to focus the converted X-rays emerging parallel from the 50 mm magnet aperture to a submillimetre spot. This will bring a vast signal-to-noise improvement over the original CAST proposal and the earlier solar axion telescopes. An option to recover mirrors constructed for the German orbiting X-ray telescope ABRIXAS is being pursued.

CAST is a new departure for CERN, relying not on the lab’s expertise in accelerators but on its know-how in X-ray detection, magnets and cryogenics. With a discovery potential for axions extending beyond that dictated by astrophysical considerations, the experiment leaves room for surprises and could open up a new field of terrestrial axion astrophysics. CAST should be ready to begin its search this autumn.

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In July 1994 Swedish musical personality Martin Engstrom launched the Verbier Festival and Academy in Valais, Switzerland, which has gone on to become a regular feature of the late-July arts calendar. This festival has attracted prominent figures from the musical and theatrical world, such as Zubin Mehta, Isaac Stern and Isabelle Huppert. It is now a valuable step on the ladder for aspiring young artists.

CERN physicist André Martin and his wife Schu knew Aspen, in the Rocky Mountains, where there is a very successful annual summer symbiosis of music, mountains and physics, with the famous Music Festival on one hand and the Aspen Center for Physics on the other. It was tempting to propose scientific activities in conjunction with the Verbier Music Festival.

In the summer of 2000 this was realized for the first time through a conference entitled CAPP (Cosmology and Astroparticle Physics) 2000, organized by Ruth Durrer, Juan Garcia-Bellido, André Martin and Misha Shaposhnikov. About 100 participants came from as far afield as Australia and Korea, to Verbier’s “Centre Culturel du Hameau”.

Prestigious lecturers also came from all over the world. The programme covered both theoretical and experimental physics. One focus was the extremely accurate measurements of the structure of cosmic microwave background radiation by the balloon experiments Boomerang and Maxima at the South and North poles, respectively. This, combined with new measurements of the Hubble galactic recession parameter, leads to a picture of the universe which is asymptotically flat (W = 1), with an accelerating expansion, a non-vanishing cosmological constant and an age of between 14 and 18 billion years, fitting most inflationary models.

W = 1 is the result of W = 0.3 for matter and 0.7 for the vacuum. The former retains a need for invisible “dark matter”, also needed to explain the observed rotation of galaxies. Although definite cases of gravitational lensing have been seen (see Not enough stellar mass objects to fill the galactic halo?), their interpretation does not seem to fit with the Massive Astronomical Compact Halo Object (MACHO) picture.

On the other hand the Weakly Interacting Massive Particle (WIMP) interpretation of dark matter is still possible, which would also be an indication in favour of supersymmetry.

Among the projects for the future, more refined detectors of the cosmic microwave background such as the Planck mission and the Virgo project for detecting gravitational waves were described. Tremendous progress has been made in recent years thanks to the new instruments, and this looks set to continue. In particular the continued detailed analysis of fluctuations in the cosmic microwave background radiation will lead to a further confirmation of the inflationary models.

Returning to the music festival, a public lecture “L’Univers, passé, présent et futur” given in “Café Schubert” (where musicians attending the festival are habitually interviewed), was well received by an audience including Swiss Federal Councillor Pascal Couchepin.

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Physics was born with ancient feet firmly on the ground, but late in the 19th century the term “astrophysics” crept into use to define the newer quest to understand extra-terrestrial mechanisms as well as terrestrial ones.

At the turn of the millennium a new dictionary term, “cosmophysics”, might have been coined to describe the quest to understand the universe at large as well as its individual components.

In the past 20 years, as the mechanisms of the Big Bang have become increasingly understood, particle physics and cosmology have become inextricably linked. At the same time, new developments in space technology have enabled new experiments, such as AMS and GLAST, to be carried aloft, high above the stifling blanket of the Earth’s atmosphere. These provide new observations and measurements that have increased our understanding considerably.

As well as the physics involved, these studies call for a range of technological expertise to mount precision experiments under harsh conditions.

This development was underlined in a workshop entitled “Fundamental Physics in Space”, which was organized jointly by CERN and the European Space Agency (ESA), and held at CERN on 5-7 April. Although laboratory and space physics are developing along several parallel avenues, the meeting provided a valuable but rare opportunity for laboratory and space physicists to compare notes and discuss topics of common interest.

The workshop, which was initiated bythe Board of the Joint Astrophysics Division of the European Physical Society and the European Astronomical Society, followed on from the May 1998 decision by both ESA and CERN to create working groups to study joint activities.

Opening the summary talks at the meeting, CERN director-general Luciano Maiani pointed to the growing overlap between particle and space physics. Recently, both subjects have underlined the important role played by the most invisible aspect of the universe – the vacuum.

ESA director-general Antonio Rodotà recalled the pioneer work carried out in the 1970s that pointed out the need for opening up the physics exploration of space, particularly for precision measurements and the deeper exploration of gravity, which now provide cornerstone missions for the new millennium.

Cosmology is flourishing

What Chandrasekhar once called “the graveyard of astronomy” is now a flourishing field, commented Lodewijk Woltjer of the St Michel Observatory and former ESO director-general, as he commenced his summary of the cosmology sessions. Indeed, to hear the wealth of science presented and the number of new instruments in the pipeline, it looks like its future is bright.

Type 1A supernovae have long been used as “standard candles” to measure distances in space. A measure of their apparent luminosity gives a measure of their distance. However, the method is prone to many errors and different teams can get very different results. Gustav Tammann of Basle explained how, with new corrections for decline rate and colour, the Hubble constant becomes 59 ± 7, corresponding to the universe being 17 billion years old. “Photometry with the Hubble Space Telescope is working at the limits of what is possible, the main problem being the background,” he said.

Another cosmological parameter is omega, the ratio of matter in the universe to the critical level needed to halt the expansion of the universe. The inflation model of the Big Bang predicts that omega should be exactly equal to one – that the universe is “flat”.

That the universe is expanding forever seems to have a certain philosophical appeal for some people. However, I have never really understood this, because our fate won’t be very much different!

Lodewijk Woltjer

Woltjer summarized current results that suggest that contributions of both radiating and dark matter to the density of the universe give an omega of around one-third. A non-zero cosmological constant or some new form of energy would be needed to make up the difference.

Sidney Bludman from DESY and Penn State showed how quintessence, or negative pressure, could solve this problem. A consequence would be to increase the expansion of the universe with time – an accelerating universe. A non-zero cosmological constant has also been suggested by studies of distant supernovae.

This picture was confirmed by Jean-Loup Puget of Orsay in his round-up of observations of the cosmic microwave background radiation. Results from the Boomerang experiment announced earlier this year give omega as one (±0.3) and suggest a non-zero cosmological constant. Puget looked forward to results from ESA’s Planck satellite, which is due to be launched in 2007.

“That the universe is expanding forever seems to have a certain philosophical appeal for some people,” said Woltjer. “However, I have never really understood this, because our fate won’t be very much different!”

Imaging dark matter

The most exciting cosmology news was that gravitational lensing has now really come of age. Cosmic shear raised a significant amount of interest. Peter Schneider of Bonn showed how gravitational weak lensing can reveal the invisible. His team has discovered a “dark clump” of 10¹⁵solar masses (assuming a redshift of 0.8) with no optical counterpart, which he believes is the first-ever lensing-detected dark matter cluster.

Schneider was waiting for the results of infrared observations of the region. If confirmed, this technique will have enormous implications for cosmology. “The future is very encouraging,” he said. Indeed, he announced another area where gravitational weak lensing is showing results – measuring the effect of lensing across a large field can help to map the dark matter making up so-called galactic halos. Observations by the Sloan Digital Survey have shown no sign of halo truncation at distances of up to 150 kpc. In fact, says Schneider, galaxies probably don’t really have halos of dark matter at all; what is seen is just a correlation between the galaxies’ positions and the overall large-scale dark matter distribution.

This view is supported by Carlos Frenk of Durham. With the Virgo Consortium, he has carried out simulations of the evolution of matter and dark matter in the universe. His modelling shows dark matter evolving in enormous filaments with galaxies forming at high-density nodes.

Woltjer reminded participants that a lot of assumptions are made before carrying out such simulations, in particular regarding the relationship between gas, dust and stellar objects. “We are still a long way from constructing the universe from first principles,” he commented.

Frenk was optimistic about the future. “Enormous progress has been made in instrumentation over recent years. If the 1980s belonged to the theorists, then the late 1990s most certainly belonged to the experimentalists,” he said.

Future telescopes

The next-generation space telescope (NGST), still on the drawingboard, should contribute. At redshifts of greater than 5, only 5% of stars have formed. However, “this is a very interesting fraction of stars,” said Frenk. He believes that the NGST will detect primeval galaxies at redshifts of up to 10.

Peter Shaver of ESO reviewed the recent progress in detection techniques, in particular for observations of the first galaxies and quasars. The discovery of the Lyman alpha break in the spectrum of high-redshift objects has caused a revolution over the last five years or so, enabling more and more high-redshift galaxies to be recognized. “We are closing in on the reionization epoch,” he said. In his opinion, the furthest galaxy discovered to date is at a redshift of 5.74. He believes that claims for galaxies at a redshift of 6.68 are yet to be proven.

With the NGST it will be interesting to look at the evolution of galaxies at high redshift, and also the quasar epoch around redshift 2. NGST will be launched in around 2010. Another useful tool for studying early galaxies, which is to be launched in 2007, is ESA’s Far Infrared and Submillimetre Telescope (FIRST), explained Reinhard Genzel of Garching. These space observations will be paralleled by the ALMA ground-based infrared array.

Gravitation

Moving on to another type of radiation altogether, Martin Huber from ESTEC summarized the session on gravitational wave astronomy. Gravity waves are ideal probes of the universe because they interact very weakly and carry huge energies. Their existence has long been confirmed by measurements of the energy loss from binary star systems. However, they have never been detected directly.

Besides the classic resonance detectors, current ground-based detectors include the GEO 600 and TAMA interferometers. The next generation of ground-based detectors, Virgo and Ligo, will improve in accuracy by a factor of 10. “I am confident that we will detect gravity waves within the next decade,” said Huber. “However, it will be very difficult to pinpoint the sources.”

Most of the sources within the frequency range of the next detectors will be transient. Bernard Schutz of the Einstein Institute, Potsdam explained that the ideal sources are compact, such as black holes, and repeating, such as rotating binary systems. Ground detectors can only observe at frequencies above 1 Hz because the Earth’s background noise cannot be screened. Events in this frequency range are rare or weak, such as supernova collapses and compact binary spindown.

The future is the ESA cornerstone mission, LISA, which is to be built jointly with NASA. This interferometer in space will observe in the low-frequency window below 1 Hz, where emission occurs from many known strong sources, such as massive black holes and compact binary star systems.

An afternoon gravitation session served as a public presentation of the mission. Karsten Danzmann of Hannover gave a taster of the physics to come. “More than 90% of the universe is dark,” he said. “If part of the dark matter clumps, then gravitational wave detectors may be the only way to see it directly.”

Another exciting area is the stochastic gravitational wave background. “Just as the cosmic microwave background radiation shows us the universe when it was 300 000 years old, a gravitational wave background would be a picture of the Big Bang itself – when the universe was perhaps just 10-24 s old,” said Danzmann. The planned LISA launch date is in 2010. “It is a completely new field,” said Huber. “We should expect the unexpected.”

The other session on gravitation showed how space experiments could really test the physics of gravity. In particular, Pierre Touboul of ONERA and Nicholas Lockerbie of Glasgow talked about two new satellite experiments that are planned to test the equivalence principle, or the universality of free fall. The French team is working on mscope, which is to be launched in around 2003. It hopes to test the equivalence principle to 1 part in 10¹⁵– an improvement of three orders of magnitude on current experiments. The ESA/NASA STEP mission could be launched in around 2005 and will test to 1 part in 10¹⁸. “String theory gives a natural explanation of why gravity is dynamic without assuming it,” said Thibault Damour of IHES, Bures-sur-Yvette. “In theory, not only is space not rigid but there are also coupling constants that imply a violation of the equivalence principle.”

Accelerators in the sky

There is apparently no end to the mysteries of the heavens – our lifelong acquaintance with puny, everyday mechanisms makes us ill-equipped to understand the mighty forces at work in the depths of the universe.

New telescopes peering into the depths of space from fresh vantage points reveal sourcespumping out energy at unimaginable  rates. Many of these, whatever they emit and however they are seen, are poorly understood and can be conveniently grouped under the heading “extreme sources”. In his summary, P L Biermann of Bonn said: “the sky contains all this and a lot more”.

Jewels in the intense source crown are the mysterious gamma-ray bursts – now an everyday occurrence. Attempts to explain how so much energy can be released focus on extremely relativistic fireballs. Other fireballs – active galactic nuclei, black holes, etc – are also held to be responsible for X- and gamma-ray fireworks.

While electromagnetic radiation points back to its source, cosmic rays, tangled by intergalactic magnetic fields, do not reveal where they come from. The tip of the mystery cosmic-ray iceberg is now 24 cosmic-ray events that, in principle, should never be seen – their energy is beyond that “allowed” by interactions with the all-pervading cosmic microwave background. How can such extreme energies be produced and how can they elude the all-pervading background radiation?

Cosmic rays – once the point of entry for particle physics – are now a new point of departure. The universe has to contain “radiogalaxy hot spots” – cosmic accelerators larger than a typical galaxy, to whirl charged particles to such “astronomical” velocities.

Dark matter

That most of the universe is composed of invisible dark matter is perhaps the ultimate physics paradox. Attempts to uncover dark matter and to resolve this paradox are a major theme in astrophysics research, both theoretical and experimental.

At the CERN/ESA meeting, Alvaro de Rujula of CERN summarized the dark matter sessions, where direct searches for exotic particles, such as axions (“aaxions”, according to de Rujula), have yet to turn up positive evidence. More promising is the area of gravitational lensing. Objects can be invisible but still exert a gravitational pull, which can disturb visible light in transit.

One specialist area is gravitational microlensing, which is looking for the effects of otherwise invisible objects as they cross the line of sight of a more distant luminous object. Interpreting this mass of results is still difficult, but de Rujula suggested that, while dark matter massive astronomical compact halo objects (MACHOs) are out of favour, weakly interacting massive particles (WIMPS) are coming in.

The DAMA (sodium iodide) detector at Gran Sasso has reported an annual signal variation that has been interpreted as possible evidence for galactic WIMP particles. Such a signal is not seen by the Cryogenic Dark Matter Search (CDMS) experiment at Stanford using silicon and germanium sensors.

This part of the programme also covered neutrino astronomy. As well as providing a new window on the universe, neutrino astrophysics has offered evidence for neutrino mixing, and therefore for non-zero neutrino mass. A new understanding of neutrinos would provide fresh light on the basic interactions of nature.

The limited seasonal and diurnal variation in solar neutrino signals provides important limits on neutrino-mixing mechanisms. The big Superkamiokande detector in Japan dominates the world data on extra-terrestrial neutrinos and has now intercepted 17 terrestrial neutrinos fired from the KEK laboratory, some 250 km distant – the first time that terrestrial neutrinos have been tracked over such a long path.

Extra-terrestrial neutrino physics “has a long past and a brilliant future”, ventured de Rujula.

In particle physics the continual demands to handle and analyse increased data rates and to attain greater precision provides a fertile ground for detector innovation.

Michel Spiro of Saclay, chairman of CERN’s LEP
Experiments Committee, summarized the session covering the
use and potential use in space experiments of instrumentation
developed for high-energy
physics.

Innovations in instrumentation

Detectors in space “see” X-ray and gamma radiation before it is absorbed by the atmosphere. Highly sensitive cryogenic X-ray detectors will be a useful new addition to the sensor armoury. The massive R&D programmes for the major experiments at CERN’s future LHC collider have already yielded an impressive array of techniques – pixel detectors as “eyes” and scintillators for energy measurement – which could go on to provide useful opportunities. Time projection chambers are another means of providing remarkable images of physics beyond the atmosphere.

As well as the detectors, read-out mechanisms too are developing quickly. Sensors and chips can be dissociated and exploit complementary technologies. Photomultiplier technology has received considerable impetus from experiments studying neutrinos.

The LHC experiments are also blazing new trails in data acquisition and handling (see “Grid” feature) and in semiconductor technology.

Spiro highlighted several new flagship space-borne experiments exploiting particle physics know-how – the AMS detector for the Space Station and the GLAST telescope, which is due for launch in 2005, while the Supernova Accelerator Probe (SNAP) and Extreme Universe Space Observatory (EUSO) proposals could continue this tradition.

Gert Viertel of ETH Zurich summarized the current instrumentation of space. Here the requirement for very high timing accuracy has driven the development of precise atomic clocks. Pixel detectors already have a distinguished track record of astronomical measurements. Superconducting tunnel junctions are poised to begin a new chapter of space research.

Away from the detectors, the highly successful GEANT simulation software developed for particle physics is finding increasing use in astrophysics and astronomy.

While particle physics is a fertile breeding ground for new detector technology, it is not the only variable in the equation. Space borne experiments, requiring years of fruitful operation with minimal or no manual maintenance and intervention, have their special requirements.

This new contact between particle physicists and cosmophysicists is already paying dividends on the instrumentation front. CERN’s “recognized experiment” status now covers a range of studies that do not use accelerator beams, but ensure that the laboratory remains a focal point of this physics. At the start of the millennium, the rapidly maturing field of cosmophysics is poised to make a major contribution to our knowledge and understanding of the universe.

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The mass of our galaxy (the Milky Way) can be computed from the dynamics of its rotation and of the motion of its satellites. It can also be evaluated by adding up its visible components, primarily stars. That these two estimates disagree by a factor of 5-10 constitutes the problem of galactic dark matter.

Either the Newtonian/Einsteinian laws of dynamics are wrong at the galactic scale, or there exists some form of galactic matter that does not emit or absorb enough electromagnetic radiation to be directly “visible”. Studies of many other spiral galaxies confirm that this problem is not unique to the Milky Way.

Originally proposed in 1986 by B Paczynski of Princeton, gravitational microlensing is a novel and indirect way to search for galactic dark matter through the deflection and magnification of the light of extragalactic stars. The search for microlensing has recently shed new light on the galactic dark matter puzzle.

Microlensing

In 1990, three groups began the search for gravitational microlensing. The main problem was the inherent large scale of such surveys. To produce a detectable magnification of the light of a distant star, an intervening compact massive object has to come closer to the star’s line of sight than one milli-arcsecond, or five nanoradians (the angle subtended on Earth by an Apollo mission lunar jeep); the tighter the alignment, the larger the magnification.

This happens so seldom that one expects less than one star in a million to be affected significantly at any given time, hence the necessity to survey some 10 million stars over many years. In contrast, variable stars are more than a thousand times as frequent and constitute a serious experimental background.

The shape of microlensing magnification is predictable and does not depend on the wavelength, contrary to most variable stars. The phenomenon is transient, because of the motion of the dark lensing object with respect to the distant star. Its duration scales as the square root of the lensing object mass, and this can be used to estimate these masses.

To simplify, one could say that two of the groups, EROS (Expérience de Recherche d’Objets Sombres – an experiment to search for dark objects) and MACHO (Massive Astronomical Compact Halo Objects), were most concerned with the dark matter problem. To probe the content of the galactic halo, they chose to monitor stars in the Magellanic Clouds – two irregular dwarf galaxies, satellites of the Milky Way, that lie close to the celestial South Pole.

The third group, OGLE (Optical Gravitational Lensing Experiment), chose to look first for microlensing where it was bound to find some – the centre of our galaxy. The microlensing rate there owing to known low-mass stars was expected to be about one in a million per year. In contrast, the rate towards the Magellanic Clouds could be anywherebetween zero, if there are no compact  dark objects in the galactic halo, and about a thousand times the galactic centre rate, if the halo is swarming with lunar mass dark bodies.

The main goal of EROS and MACHO was to detect a few microlensing events caused by brown dwarfs – would-be stars not massive enough to burn via thermonuclear reactions. These objects, between a tenth and a hundredth the size of the Sun, would give rise to a microlensing rate of the order of the expected galactic centre rate.

Divergence

In September 1993 the discovery of the first microlensing candidates by the three groups aroused high hopes that the galactic dark matter problem was about to be solved. However, in the following years a gradual divergence appeared between the EROS and MACHO results. Based on two years of Large Magellanic Cloud (LMC) CCD camera images, the MACHO group presented its result as pointing to a galactic halo half-full of 0.5 ± 0.2 solar mass objects, and compatible with being totally comprised of such objects.

In contrast the EROS group observed such a small number of candidate microlensing events that it published only upper limits, first based on a photographic plate LMC survey (1990-4), and then on a survey, started in 1996, of the Small Magellanic Cloud (SMC), which uses two large CCD cameras. The EROS limits excluded, in particular, a halo full of 0.5 solar mass objects.

Despite these somewhat inconsistent results, agreement was reached on one important point: because all microlensing candidates observed by MACHO and EROS lasted longer than a month, the halo could contain no more than 10-20% of dark objects in the wide mass range between the mass of planet Mercury and one-tenth that of the Sun, that is from 10^-7 to 0.1 solar masses. This excluded brown dwarfs – the dark matter candidate that had been the main motivation for this search (figure 1).

Reconciliation

Fortunately the past months have witnessed a reconciliation of the MACHO and EROS findings. The latter presented results from the first two years of an ongoing six-year survey of 17 million LMC stars that produced a meagre crop of two new microlensing candidates. Combined with their previous limits, this enables them to exclude a galactic halo fully comprised of objects of up to four solar masses – quite a respectable mass for a stellar object. (For a halo of 0.5 solar mass objects, the upper limit is now near 30%; figure 1.)

MACHO has analysed almost six years of LMC images out of seven-and-a-half years of surveying, and it has now stopped taking data. From its 13-17 observed microlensing candidates, it now favours a 20% contribution of 0.5 solar mass objects to the halo mass budget, but these are compatible with halo mass fractions ranging from 8% to 50%.

The duration of the candidates is similar, so that both results are compatible. However, the two groups interpret them differently. The MACHO collaboration favours an interpretation in terms of galactic halo objects. The distribution of stellar luminosities of its microlensing candidates agrees with that of LMC stars, which is to be expected given that the dark lensing objects do not choose the LMC star that they will lens for. The distribution of magnifications is also compatible with a random distance of the lens to the star’s line of sight.

These two tests could have revealed a possible contamination of the sample by intrinsic variable stars, but they do not shed light on the position of the dark lenses. This can be achieved by studying the spatial distribution of microlensing candidates, which should follow that of LMC stars in the case of halo lenses, or be more peaked towards the LMC centre if the lenses are low-mass LMC stars.

The MACHO group finds that the observed distribution favours halo lenses, but that it cannot completely exclude LMC lenses. The two options are, of course, very different in terms of the galactic dark matter composition.

With three to four microlensing candidates towards the Magellanic Clouds over eight years, EROS has a harder time comparing measured and expected distributions. However, it makes the following observations: compared with MACHO, EROS has chosen to monitor less frequently more stars, spread over a three times wider solid angle. Thus the smaller EROS lensing rate could be interpreted as a spatial dependence of the event rate, favouring the LMC-lens hypothesis. Moreover, while MACHO seems rather confident that its sample is background free, no such claim is heard from EROS.

Small Magellanic Cloud

Finally, there is the question of the SMC, where one candidate was seen by both groups in 1997. This event is longer than those of all EROS or MACHO LMC candidates, which does not favour its interpretation as a halo lens: as the Magellanic Clouds are separated by only 20° in the sky and are at comparabledistances from us, one would expect the characteristics of (halo) microlensing events towards both clouds to be very similar.

More quantitatively, the probability of this event being compatible with the LMC event durations is only 3%. On the contrary, as stellar velocities in the SMC are smaller than in those the LMC, it would be natural for SMC microlensing events to last longer if the lenses belong to the Magellanic Clouds.

In addition, the SMC event lasted long enough that the Earth had time to complete three-quarters of its orbit around the Sun during the magnification. This could have led to observable microlensing deformations. Such effects are not seen in EROS and MACHO data, implying that the lens is either a low-mass SMC star or a few solar-mass halo object. In the latter case its mass would not be compatible with that deduced from lenses towards the LMC.

Thus EROS concludes that this particular lens lies in the SMC. Much is expected from the comparison of LMC and SMC events but, because there are only a fifth the number of stars in the latter, no definitive conclusion can yet be reached. Nevertheless, EROS expects to be able to make a statement with an analysis of four years of data. The MACHO analysis of SMC images is also eagerly awaited.

If the MACHO interpretation is correct and there are plenty of half a solar mass objects in the galactic halo, the next challenge is to find out what they are. They cannot be ordinary stars, because these would be bright enough to be visible. One exotic scenario is primordial black holes made in the early universe at the time of the quark-hadron transition. Old white dwarf stars are another possibility: there are counter-arguments to their abundant presence in the halo, but they have the advantage that they could be detected by looking for nearby, dim high-velocity objects. Some groups are conducting such searches, including EROS. One group, led by R Ibata (Max Planck, Heidelberg), has claimed the detection of a few halo white dwarfs, but their interpretation as halo objects is unconfirmed.

Valuable results

Whatever the future developments involving galactic dark matter, microlensing surveys have already provided concrete results. The lensing probability towards the centre of the galaxy was found by the OGLE and MACHO groups to be three times as large as expected, so that microlensing can teach us much about galactic structure.

The surveys have also yielded many variable stars. This has allowed, for example, studies with unprecedented statistics of Magellanic Cloud Cepheids, a pivotal cosmic yardstick, as well as the discovery of new types of variable stars.

The monitoring of long microlensing events of bright stars provides a novel way to look for planets around the lenses

Finally, the monitoring of long microlensing events of bright stars provides a novel way to look for planets around the lenses. Compared with the current, highly successful searches that use precise measurements of stellar radial velocities, microlensing should be sensitive to lower-mass planets orbiting more distant and more typical stars.

There is a good chance that, after reconciling their results, EROS and MACHO will soon agree. A conclusion that can already be drawn is that the largest part of galactic dark matter does not comprise of dark astronomical objects lighter than a few solar masses.

As far as microlensing is concerned, the search should now be extended to longer events corresponding to heavier lensing objects. In parallel, the groups looking for other dark matter candidates, such as Weakly Interacting Massive Particles, with underground or underwater detectors or at large particle accelerators, will certainly be encouraged by the recent microlensing results.

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A large part of the dark matter that seems to dominate the universe is expected to be in the form of relic particles. This “dark universe” could be a unique window through which we will be able to look for new physics. The DAMA experiment at the INFN Gran Sasso National Laboratory is investigating this new frontier and sees an annual variation, which is suggestive of the Earth’s motion against a background “wind” of particles.

Particles from the dark universe

Measurements of luminous matter (stars) lead to the conclusion that the universe does not contain enough stellar matter to halt the residual Big Bang expansion. Without enough such gravitational braking, the universe will continue to expand forever. However, many experimental observations suggest that luminous matter is not the end of the story. To account for the observed motion in the cosmos, gravitational fields much stronger than those attributable to luminous matter are required ­more than about 90% of the mass in the universe should be the result of invisible dark matter.

This conclusion is further supported by simulations using cosmological models that point out the necessity for large numbers of relic particles ­ weakly interacting massive particles (WIMPs) ­ from the early universe.

This scenario implies that our galaxy should be completely embedded in a large WIMP halo. Our solar system, which is moving with a velocity of about 232 km/s with respect to the galactic system, feels a continuous WIMP “wind”.

The quantitative study of this wind would provide information on the evolution of the universe and investigate new physics possibilities. The lightest neutral particle (the neutralino) expected by the supersymmetric extension of the Standard Model is the best WIMP candidate.

How to catch a WIMP?

Direct detection of WIMPs is very difficult because they rarely interact. WIMP searches should be shielded from cosmic rays and operate in an environment of very low radioactivity. The detectors should be built using low-radioactive materials.

DAMA’s home is deep underground in the INFN Gran Sasso National Laboratory in Italy. The collaboration (involving the University and INFN-Roma2, University and INFN-Roma, IHEP-Beijing) is mainly devoted to the search for WIMPs in the same mass and cross-section region as accelerator experiments, and several results have already emerged.

Thus high-atomic-number target nuclei, such as iodine (in the form of NaI(Tl)) and xenon (in the form of a liquid xenon scintillator) are used. The search mainly focuses on WIMP­nucleus elastic scattering from the target-nuclei part of the detector, which would show up via nuclear recoil energies in the kilo-electronvolt range.

To help to isolate a possible WIMP signal from the background, the main feature of the WIMP wind is its annual modulation. As the Earth rotates around the Sun, it would be crossed by a higher WIMP flux in June (when its rotational velocity is in the same direction as that of the solar system with respect to the galaxy) and by a smaller one in December (when the two velocities are subtracted). The fractional difference of the rate is some 7%.

To see such modulation requires heavy, stable detectors with appropriate features and stability control. The 100 kg highly radiopure NaI(Tl) DAMA set-up is an example. Clear signatures overcome the difficulties of comparing different experiments and techniques.

The 100 kg DAMA NaI(Tl) set-up

DAMA uses highly radiopure NaI(Tl) scintillators that are produced in collaboration with the CRISMATEC company. All of the materials and the crystal growth and handling procedures have been studied carefully. A major effort has gone into optimizing the detectors and the electronics to give a relatively high number of photoelectrons per kilo-electronvolt and a low noise level.

The low background photomultiplier tubes employed in the experiment (two for each detector, working in coincidence at a single photoelectron threshold) have been developed by Electron Tubes Ltd. The materials were preselected by the company and their radioactivity was measured deep underground.

The other main parts of the experiment are the passive shield (to exclude environmental contributions to the counting rate), which surrounds the copper box housing the detectors, and the glovebox placed on the top of the shield for calibration. The whole of the apparatus is kept in highly pure nitrogen with slight overpressure with respect to the atmosphere in order to keep out radioactive radon gas.

The materials of the shield have been selected and monitored for low radioactivity. The upper glovebox is used to insert radioactive sources to calibrate the detectors in the same experimental conditions as those occurring during the production measurements.

This set-up is mainly devoted to studies of WIMP annual modulation, therefore particular care has been taken in the stability and monitoring of the running condition parameters, such as the operating temperature, the high-purity nitrogen flux, the glovebox overpressure, the total and single hardware rates above single photoelectron threshold, the environmental radon level and so on. All of the information related to these parameters is continuously recorded with actual data.

The experiment is taking data from a single photoelectron threshold to several mega-electronvolts, although the hardware conditions are obviously optimized for the lowest energy region. Pulse shape information is recorded over a period of 3250 ns for the lowest energy events.

Searching for annual modulation

Any annual modulation of the WIMP­nucleus differential energy spectrum should have all of the following features attributable to WIMP interactions:

• a modulation varying as a cosine function;
• a period of exactly one year;
• a proper summer-winter phase;
• only seen in a defined low-energy region;
• single “hit” events;
• a modulated amplitude below 7%.

Single-hit events with only one detector firing are the ones of interest in WIMP search, the probability of a WIMP interacting in more than one detector being negligible.

After checking the monitored parameters, the time-dependent component of the rate is extracted from the collected data by grouping the events in cells of one day, 1 keV and one detector. The number of events in each cell is then compared by applying a maximum likelihood analysis with the expectation from the standard WIMP model. The limit on the neutralino mass (the most favoured WIMP candidate) achieved at accelerators is taken into account.

The analysis of a first dataset suggested the possible presence of a signal compatible with the features of a neutralino with dominant spin-independent interaction. A second year of data taking with larger statistics has underlined this possibility. The signal modulation is shown in figure 1 and the neutralino interpretation in figure 2.

The combined analysis of these two years of data (total statistics: 19511 kg/day) provides a confidence level of 99.6% for a neutralino mass of 59 GeV (+17/­14) and a proton cross-section of 7.0 (+0.4 /­1.2) 10^-6 pb in the frame of the standard WIMP model.

Possible systematic effects and alternative explanations have been investigated, as discussed for example at the 3K Cosmology International Conference in Rome last October. None of the effects considered could simulate all six of the criteria for the annual modulation signature and provide the observed modulation.

Despite this the collaboration has been very cautious ­ mindful of the difficulties of dealing with rare events ­ and has increased its efforts to investigate all aspects of this intriguing result.

The region singled out by DAMA is consistent with the hypothesis of a relic neutralino as a dominant component of the cold dark matter in the galactic halo, as has been pointed out by a Turin group, which also says that some properties of the relevant supersymmetric particles should be accessible at present accelerators and in WIMP indirect searches.

The inclusion of these relic neutralinos in supergravity models hs also been considered by American physicists. A group from Rome and Moscow suggested that the effect could be the result of a heavy neutrino. Finally, the effect of the uncertainties on the dark halo local density and on the WIMP velocity distribution has been examined recently with the conclusion that the relic neutralinos possibly involved in the annual modulation effect would have a mass in the 30­130 GeV range with an upper bound extending to some 180 GeV when possible bulk rotation of the dark matter halo is introduced.

Perspectives and plans

The analysis of further statistics is in progress as well as further data taking and an upgrade of the apparatus. If new research for the improved radiopurification of NaI(Tl) is successful, the active mass could be increased to 250 kg.

A final result would mean reproducing the effect over several annual cycles, including all tof he consistency checks.

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The invisible dark matter of the universe weighs heavily on cosmology. However, whatever and wherever this invisible material is, it must be made of something, and the most plausible candidates are relic particles from the early phase of the universe. The search for dark matter, mostly using non-accelerator experiments, has become an established part of particle physics.

These questions were examined when physicists from all over the world met in Heidelberg for the Second International Conference on Dark Matter in Astro- and Particle Physics (DARK98). The goal was to shed light on theoretical backgrounds from particle physics and cosmology, to discuss the results of dark matter detection experiments and to examine future projects.

The most compelling evidence for both baryonic (nuclear) and non-baryonic dark matter comes from observations of the rotation curves of galaxies. In particular, the rotation curves of dwarf spirals are completely dark matter dominated, pointed out Andreas Burkert (Heidelberg). The rotation curve of one of the best measured dwarf spirals can only be fitted to theoretical predictions if both an outer cold dark matter halo and an inner spherical distribution of massive compact baryonic objects (MACHOs) is assumed.

The search for MACHOs in the halo of our own galaxy ­ in the form of planets, white and brown dwarfs or primordial black holes ­ exploits the gravitational microlensing effect ­ the temporary brightening of a background star as an unseen object passes close to the line of sight. For several years a number of groups have been monitoring the brightness of millions of stars in the Magellanic clouds, as Kim Griest (San Diego) and Marc Moniez (Orsay) explained.

MACHOs or WIMPs?

Several candidates have already been detected and if interpreted as dark matter would make up half of the amount needed in the galactic halo. However, no stellar candidate seems to be able to explain the observations. MACHOs could be an exotic form of baryonic matter, like primordial black holes, or they could be located outside the halo of our galaxy.

The leading non-baryonic dark matter candidates are the so-called weakly interacting massive particles (WIMPs). If WIMPs populate the halo of our galaxy, they could be detected directly in laboratory experiments, or indirectly through their annihilation products in the halo ­ the centre of the Sun or Earth.

Blas Cabrera (Stanford) gave an overview of the direct detection experiments. The goal is to look for the elastic scattering of WIMPs off nuclei in a low-background target detector. The Stanford Cold Dark Matter Search (CDMS) experiment, he explained, uses detectors of ultrapure germanium and silicon operated at a temperature of 20 mK. The simultaneous measurement of both ionization and phonon signals allows nuclear recoil events to be differentiated from electron interactions ­ a very effective background suppression method. For the moment, the experiment is located at the Stanford Underground Facility, 10.6 m below ground, but the goal is to operate the detector in the deep Soudan mine in Minnesota.

The DAMA experiment, presented by Rita Bernabei (Rome), is running 115.5 kg of sodium iodide detectors in the Gran Sasso underground laboratory near Rome. Its high statistics open the possibility of looking for WIMPs via a variation in the event rate owing to the movement of the Sun in the galactic halo and the Earth’s rotation around the Sun. The analysis of about 13 kg/yr reveals a positive WIMP annual modulation signal, which meanwhile has been confirmed with higher statistics from 54 kg/yr. However, a further confirmation by DAMA and by other experiments must be awaited.

The Heidelberg group reported on the two most sensitive germanium experiments ­ the Heidelberg­Moscow experiment and Heidelberg Dark Matter Search (HDMS) ­ both of which are located in the Gran Sasso Laboratory. The Heidelberg­Moscow experiment, which also searches for neutrinoless double beta decay in enriched germanium-76, currently gives the most stringent limits on WIMP­nucleon scattering for raw data.

HDMS, a dedicated dark matter experiment, aims to improve this limit by one order of magnitude. Like the Heidelberg­Moscow experiment, it looks for a small ionization signal inside a high-purity germanium crystal.

With the expected sensitivity, HDMS will be able to test, like CDMS, the complete DAMA evidence region. The new project of the Heidelberg group, GENIUS, presented by Laura Baudis, aims for a sensitivity that is a thousand times as good as that of present experiments. GENIUS will operate in its dark matter version 40 “naked” germanium crystals (100 kg) in a 12 x 12 m tank of liquid nitrogen. Reaching the target sensitivity, it could test almost the complete parameter space predicted for certain supersymmetric particles, thus deciding whether WIMPs make up the dominant part of our galactic halo.

Terrestrial indirect detection experiments search for high-energetic neutrinos as annihilation products of WIMPs in the centre of the Earth or the Sun. The MACRO experiment in Gran Sasso looks for an excess of neutrino induced upward-going muons, explained Teresa Montaruli (Bari). No WIMP annihilation signal has been found, but the sensitivity of the experiment sets stringent upper limits on the flux of upward-going muons and thus excludes significant portions of the parameter space predicted for the supersymmetric particles.

An alternative indirect signature for dark matter particles would be a distorted spectrum of secondary antiprotons owing to the pair annihilation of neutralinos in the halo. Pierre Salati (Annecy) compared the measured low-energetic antiproton flux by the BESS balloon experiment with theoretical predicted fluxes. While there is some room left for a possible signal of exotic origin, this cannot be seen as evidence for a supersymmetry induced signal, he claimed. To disentangle such a signal from the secondary antiproton flux much more sensitive detectors, like the Alpha Magnetic Spectrometer (AMS), are needed.

Superheavy dark matter

Recently a new class of dark matter candidates ­ superheavy dark matter ­ have emerged. If one gives up the assumption that the particle was in thermal equilibrium in the early universe, explained Edward Kolb (Chicago), then its present abundance is no longer determined by annihilation and much heavier particles ­ the formidable sounding WIMPZILLAs ­ are allowed. There are two necessary conditions for WIMPZILLAs: they must be stable, or at least have a lifetime much greater than the age of the universe; and their interaction rate must be sufficiently weak that thermal equilibrium with the primordial plasma was never obtained. Kolb presented a number of ways in which such a particle could have been created, like gravitational production during the transition between an inflationary and a matter- or radiation-dominated universe, and during the defrosting phase after inflation.

Like the new millennium, dark matter could be just around the corner. The next meeting ­ DARK2000 ­ will take place in Heidelberg. DARK98 was organized by H V Klapdor-Kleingrothaus (with Laura Baudis as scientific secretary) from the Max Planck Institut für Kernphysik, Heidelberg.

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While cosmology is one of the oldest of sciences, it is only this century that it has become truly quantitative, with measurements from ground-based detectors extending beyond the traditional visible window and, more recently, with data from an impressive array of space-borne instrumentation. Underlining the new maturity of the science are the emerging values for the basic parameters of the cosmological equations.

A latter-day Copernican revolution came when Edwin Hubble discovered in the 1920s that the universe is still expanding, subsequently understood to be the aftermath of the initial Big Bang. Ever since, observational cosmology has tried to pin down how this expansion has evolved. The “Hubble constant” ­ the apparent ratio between expansion velocity and distance ­ has long been controversial. One typical “result” was the paradox that the universe appeared to be younger than its oldest stars ­ the “old wine in new bottles” dilemma.

Thanks to new data, including parallax measurements from the Hipparcos satellite, the Hubble constant and the age of far-flung objects in the universe are now more compatible. The oldest stars are of the same vintage as our universe.

Talks at the CERN meeting, covering observations from the Hubble Space Telescope and other satellites and from systematic supernova searches, showed that the “world average” Hubble constant now looks to be about 67, with a likely age of the universe about 14 gigayears.

Wendy Freedman of the Hubble Space Telescope team showed that the spectrum of the Hubble flow looks remarkably smooth (with the local “infall” drift towards Virgo subtracted). With reliable new data, statistical fluctuations have largely gone away, and the emphasis turns instead to systematic effects.

Observations of distant supernovae, which exploded when the universe was still young, reveal how the universe has since expanded. For the supernova search teams, Saul Perlmutter and Robert Kirshner demonstrated how the subtle effects now being seen at these extreme distances cannot be fitted by a single Hubble constant, and the idea of a “cosmological constant” ­ an anti-gravity repulsion ­ has made a comeback.

According to the basic Big Bang/Hubble picture, the further away an object is, the faster it appears to recede, with the expansion of the universe inexorably slowing as gravity steadily applies the brakes. However, the data from supernovae suggest this is an oversimplified picture, with an anti-gravity effect assisting the expansion, so that the Big Bang can sometimes appear to accelerate.

This reopens the debate on whether the universe is “open”, continuing to expand for ever, or “closed”, ultimately to disappear in a final “Big Crunch”. Neither is yet excluded.

At the CERN workshop, inflation pioneer Andrei Linde showed how an infant universe born in a quantum fluctuation supposedly attained its present proportions due to a brief initial flash of “inflation” which transformed a quantum bubble into a living universe. The incredible rate of this explosion strongly suggests total reconciliation with gravity, so that what we now see should be “flat”, neither continually expanding nor destined to recombine.

Achieving a flat universe with the new cosmological data is not ruled out, but the cosmological constant plays an important role. Flatness is not achieved by conventional gravitational pull alone.

Dark matter

Although inflation practically dictates a flat universe, there is not enough visible matter out there to accomplish the task, and invisible “dark matter” is invoked to provide the extra gravitational pull needed to close the universe. A continuing challenge is to find this missing matter ­ material we cannot see but which has to be there to explain the gravitational behaviour we do see. However, the arrival of a non-zero cosmological constant provides an additional gravitational effect to help close the universe using less dark matter.

One dark matter candidate is MACHOS ­ Massive Astrophysical Compact Halo Objects. At the CERN workshop, Michel Spiro summarized the search for MACHOs using gravitational lensing, in which otherwise invisible intervening matter can affect the image of more distant objects as they move across the sky.

One MACHO-seeking collaboration, itself called MACHO, now has 14 candidates in the direction of the Large Magellanic Cloud (LMC), whose durations range from 15 to 90 days. Another collaboration ­ EROS ­ has two, each lasting about four weeks. MACHO covers most of the LMC, but with low efficiency, while the complementary EROS search covers a restricted area containing some 150,000 stars with high efficiency. Taken together, these results imply that planetary mass objects account for less than 10% of the halo. Their attention is now also extended to the Small Magellanic Cloud, while other dark-matter searches have also joined the hunt.

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