Neutrino physics has come a long way since the discovery of neutrino oscillations in 1998. Experiments now measure oscillation parameters with a precision of a few per cent. At NuFact 2025, the IceCube collaboration reported new oscillation measurements using atmospheric neutrinos from 11 years of observations at the South Pole. The measurements achieve world-leading sensitivity on neutrino mixing angles, alongside new constraints on the unitarity of the neutrino mixing matrix. Meanwhile, the JUNO experiment in China celebrated the start of data-taking with its liquid-scintillator detector (see “JUNO takes aim at neutrino-mass hierarchy”). JUNO will determine the neutrino mass ordering by observing the fine oscillation patterns of antineutrinos produced in nuclear reactors.

Neutrino scattering

Beyond oscillations, a major theme of the conference was neutrino scattering. Although neutrinos are the most abundant massive particles in the universe, their interactions with matter remain poorly understood. Measuring and modelling these processes is essential: they probe nuclear structure and hadronic physics in a novel way, while also providing the foundation for oscillation analyses in current and next-generation experiments. Exciting advances were reported across the field. The SBND experiment at Fermilab announced the collection of around three million neutrino interactions using the Booster Neutrino Beam. ICARUS presented its first neutrino–argon cross-section measurement. MicroBooNE, MINERvA and T2K showcased new results on neutrino–nucleus interaction and compared them with theoretical models. The e4ν collaboration highlighted electron beams as potential sources of data to refine neutrino-scattering models, supporting efforts to achieve the detailed interaction picture needed for the coming precision era of oscillation physics. At higher energies, FASER and SND@LHC showcased their LHC neutrino observations with both emulsion and electronic detectors.

Neutrino physics is one of the most vibrant and global areas of particle physics today

CERN’s role in neutrino physics was on display throughout the conference. Beyond the results from ICARUS, FASER and SND@LHC, other contributions included the first observation of neutrinos in the ProtoDUNE detectors, the status of the MUonE experiment – aimed at measuring the hadronic contribution to the muon anomalous magnetic moment – and the latest results from NA61. The role of CERN’s Neutrino Platform was also highlighted in contributions about the T2K ND280 near-detector upgrade and the WAGASCI–BabyMIND detector, both of which were largely assembled and tested at CERN. Discussions featured the results of the Water Cherenkov Test Experiment, which operated in the T9 beamline to prototype technology for Hyper-Kamio­kande, and other novel CERN-based ideas, such as nuSCOPE – a proposal for a short-baseline experiment that would “tag” individual neutrinos at production, formed from the merging of ENUBET and NuTag. Building on a proof-of-principle result from NA62, which identified a neutrino candidate via its parent kaon decay, this technique could represent a paradigm shift in neutrino beam characterisation.

NuFact 2025 reinforced the importance of diversity and inclusion in science. The IDEEO working group led discussions on how varied perspectives and equitable participation strengthen collaboration, improve problem solving and attract the next generation of researchers. Dedicated sessions on education and outreach also highlighted innovative efforts to engage wider communities and ensure that the future of neutrino physics is both scientifically robust and socially inclusive. From precision oscillation measurements to ambitious new proposals, NuFact 2025 demonstrated that neutrino physics is one of the most vibrant and global areas of particle physics today.

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CERN’s current NA62 effort was nevertheless present in force. Eight presentations spanned its wide-ranging programme, from precision studies of rare kaon decays to searches for lepton-flavour and lepton-number violation, and explorations beyond the Standard Model (SM). Complementary perspectives came from Japan’s KOTO experiment at J-PARC, from multipurpose facilities such as KLOE-2, Belle II and CERN’s LHCb experiment, as well as from a large and engaged theoretical community. Together, these contributions underscored the vitality of kaon physics: a field that continues to test the SM at the highest levels of precision, with a strong potential to uncover new physics.

NA62 reported a big success on the so-called “golden mode” ultra-rare decay K⁺ → π⁺νν, a process that is highly sensitive to new physics (CERN Courier July/August 2024 p30). NA62 has already delivered remarkable progress in this domain: by analysing data up to 2022, the collaboration more than doubled its sample from 20 to 51 candidate events, achieving the first 5σ observation of the decay (CERN Courier November/December 2024 p11). This is the smallest branching fraction ever measured, and, intriguingly, shows a mild 1.7σ tension with the Standard Model prediction, which itself is known with a 2% theoretical uncertainty. With the experiment continuing to collect data until CERN’s next long shutdown (LS3), NA62’s final dataset is expected to triple the current statistics, sharpening what is already one of the most stringent tests of the SM.

Another major theme was the study of rare B-meson decays where kaons often appear in the final state, for example B → K^* (→ Kπ) ℓ⁺ℓ^–. Such processes are central to the long-debated “B anomalies,” in which certain branching fractions of rare semileptonic B decays show persistent tensions between experimental results and SM predictions (CERN Courier January/February 2025 p14). On the experimental front, CERN’s LHCb experiment continues to lead the field, delivering branching-fraction measurements with unprecedented precision. Progress is also being made on the theoretical side, though significant challenges remain in matching this precision. The conference highlighted new approaches reducing uncertainties and biases, based both on phenomenological techniques and lattice QCD.

Kaon physics is in a particularly dynamic phase. Theoretical predictions are reaching unprecedented precision, and two dedicated experiments are pushing the frontiers of rare kaon decays. At CERN, NA62 continues to deliver impactful results, even though plans for a next-stage European successor did not advance this year. Momentum is building in Japan, where the proposed KOTO-II upgrade, if approved, would secure the long-term future of the programme. Just after the conference, the KOTO-II collaboration held its first in-person meeting, bringing together members from both KOTO and NA62 – a promising sign for continued cross-fertilisation. Looking ahead, sustaining two complementary experimental efforts remains highly desirable: independent cross-checks and diversified systematics. Both will be essential to fully exploit the discovery potential of rare kaon decays.

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While the ICFA is neither a decision-making body nor a representation of funding agencies, its mandate assigns to the committee the important task of promoting international collaboration and coordination in all phases of the construction and exploitation of very-high-energy accelerators. This role is especially relevant in today’s context of strategic planning and upcoming decisions – with the ongoing European Strategy update, the Chinese decision process on CEPC in full swing, and the new perspectives emerging on the US–American side with the recent National Academy of Sciences report (CERN Courier September/October 2025 p10).

Consequently, the ICFA heard presentations on these important topics and discussed priorities and timelines. In addition, the theme of “physics beyond colliders” – and with it, the question of maintaining scientific diversity in an era of potentially vast and costly flagship projects – featured prominently. In this context, the importance of national laboratories capable of carrying out mid-sized particle-physics experiments was underlined. This also featured in the usual ICFA regional reports.

An important part of the work of the committee is carried out by the ICFA panels – groups of experts in specific fields of high relevance. The ICFA heard reports from the various panel chairs at the Wisconsin meeting, with a focus on the Instrumentation, Innovation and Development panel, where Stefan Söldner-Rembold (Imperial College London) recently took over as chair, succeeding the late Ian Shipsey. Among other things, the panel organises several schools and training events, such as the EDIT schools, as well as prizes that increase recognition for senior and early-career researchers working in the field of instrumentation.

Maintaining scientific diversity in an era of potentially vast and costly flagship projects  featured prominently

Another focus was the recent work of the Data Lifecycle panel chaired by Kati Lassila-Perini (University of Helsinki). This panel, together with numerous expert stakeholders in the field, recently published recommendations for best practices for data preservation and open science in HEP, advocating the application of the FAIR principles of findability, accessibility, interoperability and reusability at all levels of particle-physics research. The document provides guidance for researchers, experimental collaborations and organisations on implementing best-practice routines. It will now be distributed as broadly as possible and will hopefully contribute to the establishment of open and FAIR science practices.

Formally, the ICFA is a working group of the International Union for Pure and Applied Physics (IUPAP) and is linked to Commission C11, Particles and Fields. IUPAP has recently begun a “rejuvenation” effort that also involves rethinking the role of its working groups. Reflecting the continuity and importance of the ICFA’s work, Marcelo Gameiro Munhoz, chair of C11, presented a proposal to transform the ICFA into a standing committee under C11 – a new type of entity within IUPAP. This would allow ICFA to overcome its transient nature as a working group.

Finally, there were discussions on plans for a new set of ICFA seminars – triennial events in different world regions that assemble up to 250 leaders in the field. Following the 13th ICFA Seminar on Future Perspectives in High-Energy Physics, hosted by DESY in Hamburg in late 2023, the baton has now passed to Japan, which is finalising the location and date for the next edition, scheduled for late 2026.

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Marzia Bordone (University of Zurich) highlighted central questions in flavour physics, such as the tensions in the determinations of quark flavour-mixing parameters and the anomalies in leptonic and semileptonic B-meson decays (CERN Courier January/February 2025 p14). She showed that new bosons beyond the Standard Model that primarily interact with the heaviest quarks are theoretically well motivated and could be responsible for these flavour anomalies. Bordone emphasised that collaboration between experiment and theory, as well as data from future colliders like FCC-ee, will be essential to understand whether these effects are genuine signs of new physics.

Lina Necib (MIT) shared impressive new results on the distribution of galactic dark matter. Though invisible, dark matter interacts gravitationally and is present in all galaxies across the universe. Her team used exquisite data from the ESA Gaia satellite to track stellar trajectories in the Milky Way and determine the local dark-matter distribution to within 20–30% precision – which means about 300,000 dark-matter particles per cubic metre assuming they have mass similar to that of the proton. This is a huge improvement over what could be done just one decade ago, and will aid experiments in their direct search for dark matter in laboratories worldwide.

The most quoted dark-matter candidates at Invisibles25 were probably axions: particles once postulated to explain why the strong interactions that bind protons and neutrons behave in the same way for particles and antiparticles. Nicole Righi (King’s College London) discussed how these particles are ubiquitous in string theory. According to Righi, their detection may imply a hot Big Bang, with a rather late thermal stage, or hint at some special feature of the geometry of ultracompact dimensions related to quantum gravity.

The most intriguing talk was perhaps the CERN colloquium given by the 2011 Nobel laureate Adam Riess (Johns Hopkins University). By setting up an impressive system of distance measurements to extragalactic systems, Riess and his team have measured the expansion rate of the universe – the Hubble constant – with per cent accuracy. Their results indicate a value about 10% higher than that inferred from the cosmic microwave background within the standard ΛCDM model, a discrepancy known as the “Hubble tension”. After more than a decade of scrutiny, no single systematic error appears sufficient to account for it, and theoretical explanations remain tightly constrained (CERN Courier March/April 2025 p28). In this regard, Julien Lesgourgues (RWTH Aachen University) pointed out that, despite the thousands of papers written on the Hubble tension, there is no compelling extension of ΛCDM that could truly accommodate it.

While 95% of the universe’s energy density is invisible, the community studying it is very real. Invisibles now has a long history and is based on three innovative training networks funded by the European Union, as well as two Marie Curie exchange networks. The network includes more than 100 researchers and 50 PhD students spread across key beneficiaries in Europe, as well as America, Asia and Africa – CERN being one of their long-term partners. The energy and enthusiasm of the participants at this conference were palpable, as nature continues to offer deep mysteries that the Invisibles community strives to unravel.

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One of the highlights concerned the Higgs boson’s coupling to the charm quark, with the CMS collaboration presenting a new search using Higgs production in association with a top–antitop pair. The analysis, targeting Higgs decays into charm–quark pairs, reached a sensitivity comparable to the best existing direct constraints on this elusive interaction. New ATLAS analyses showcased the impact of the large Run 3 dataset, hinting at great potential for Higgs physics in the years to come – for example, Run 3 data has reduced the uncertainties on the coupling of the Higgs boson to muons and Zγ by 30% and 38%, respectively. On the di-Higgs front, the expected upper limit on the signal-strength modifier, measured in the bbγγ final state only, has now surpassed in sensitivity the combination of all Run 2 HH channels (see “A step towards the Higgs self-coupling”). The sensitivity to di-Higgs production is expected to improve significantly during Run 3, raising hopes of seeing a signal before the next long shutdown, from mid-2026 to the end of 2029.

Juan Rojo (Vrije Universiteit Amsterdam) discussed parton distribution functions for Higgs processes at the LHC, while Thomas Gehrmann (University of Zurich) reviewed recent developments in general Higgs theory. Mathieu Pellen (University of Freiburg) provided a review of vector-boson fusion, Jose Santiago Perez (University of Granada) summarised the effective field theory framework and Oleksii Matsedonskyi (University of Cambridge) reviewed progress on electroweak phase transitions. In his “vision” talk, Alfredo Urbano (INFN Rome) discussed the interplay between Higgs physics and early-universe cosmology. Finally, Benjamin Fuks (LPTHE, Sorbonne University) presented a toponium model, bringing the elusive romance of top–quark pairs back into the spotlight (CERN Courier September/October 2025 p9).

After a cruise on the Seine in the light of the Olympic Cauldron, participants were propelled toward the future during the European Strategy for Particle Physics session. The ESPPU secretary Karl Jakobs (University of Freiburg) and various session speakers set the stage for spirited and vigorous discussions of the options before the community – in particular, the scenarios to pursue should the FCC programme, the clear plan A, not be realised. The next Higgs Hunting workshop will be held in Orsay and Paris from 16 to 18 September 2026.

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The first Workshop on the Impact of Higgs Studies on New Theories of Fundamental Interactions, which took place on the Island of Capri, Italy, from 6 to 10 October 2025, gathered around 40 experimentalists and theorists to explore the pivotal role of the Higgs boson in exploring BSM physics. Participants discussed the implications of extended scalar sectors and the latest ATLAS and CMS searches, including current potential anomalies in LHC data.

“The Higgs boson has moved from the realm of being just a new particle to becoming a tool for searches for BSM particles,” said Greg Landsberg (Brown University) in an opening talk.

An extended scalar sector can address several mysteries in the SM. For example, it could serve as a mediator to a hidden sector that includes dark-matter particles, or play a role in generating the observed matter–antimatter asymmetry during an electroweak phase transition. Modified or extended Higgs sectors also arise in supersymmetric and other BSM models that address why the 125 GeV Higgs boson is so light compared to the Planck mass – despite quantum corrections that should drive it to much higher scales – and might shed light on the perplexing pattern of fermion masses and flavours.

One way to look for new physics in the scalar sector is modifications in the decay rates, coupling strengths and CP-properties of the Higgs boson. Another is to look for signs of additional neutral or charged scalar bosons, such as those predicted in longstanding two-Higgs-doublet or Higgs-triplet models. The workshop saw ATLAS and CMS researchers present their latest limits on extended Higgs sectors, which are based on an increasing number of model-independent or signature-based searches. While the data so far are consistent with the SM, a few mild excesses have attracted the attention of some theorists.

In diphoton final states, a slight excess of events persists in CMS data at a mass of 95 GeV. Hints of a small excess at a mass of 152 GeV are also present in ATLAS data, while a previously reported excess at 650 GeV has faded after full examination of Run 2 data. Workshop participants also heard suggestions that the Brout–Englert–Higgs potential could allow for a second resonance at 690 GeV.

The High-Luminosity LHC will enable us to explore the scalar sector in detail

“We haven’t seen concrete evidence for extended Higgs sectors, but intriguing features appear in various mass scales,” said CMS collaborator Sezen Sekmen (Kyungpook National University). “Run 3 ATLAS and CMS searches are in full swing, with improved triggering, object reconstruction and analysis techniques.”

Di-Higgs production, the rate of which depends on the strength of the Higgs boson’s self-coupling, offers a direct probe of the shape of the Brout–Englert–Higgs potential and is a key target of the LHC Higgs programme. Multiple SM extensions predict measurable effects on the di-Higgs production rate. In addition to non-resonant searches in di-Higgs production, ATLAS and CMS are pursuing a number of searches for BSM resonances decaying into a pair of Higgs bosons, which were shown during the workshop.

Rich exchanges between experimentalists and theorists in an informal setting gave rise to several new lines of attack for physicists to explore further. Moreover, the critical role of the High-Luminosity LHC to probe the scalar sector of the SM at the TeV scale was made clear.

“Much discussed during this workshop was the concern that people in the field are becoming demotivated by the lack of discoveries at the LHC since the Higgs, and that we have to wait for a future collider to make the next advance,” says organiser Andreas Crivellin (University of Zurich). “Nothing could be further from the truth: the scalar sector is not only the least explored of the SM and the one with the greatest potential to conceal new phenomena, but one that the High-Luminosity LHC will enable us to explore in detail.”

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The 2025 European Physical Society Conference on High Energy Physics (EPS-HEP), held in Marseille from 7 to 11 July, took centre stage in this pivotal year for high-energy physics as the community prepares to make critical decisions on the next flagship collider at CERN to enable major leaps at the high-precision and high-energy frontiers. The meeting showcased the remarkable creativity and innovation in both experiment and theory, driving progress across all scales of fundamental physics. It also highlighted the growing interplay between particle, nuclear, astroparticle physics and cosmology.

Advancing the field relies on the ability to design, build and operate increasingly complex instruments that push technological boundaries. This requires sustained investment from funding agencies, laboratories, universities and the broader community to support careers and recognise leadership in detectors, software and computing. Such support must extend across construction, commissioning and operation, and include strategic and basic R&D. The implementation of detector R&D (DRD) collaborations, as outlined in the 2021 ECFA roadmap, is an important step in this direction.

Physics thrives on precision, and a prime example this year came from the Muon g–2 collaboration at Fermilab, which released its final result combining all six data runs, achieving an impressive 127 parts-per-billion precision on the muon anomalous magnetic moment (CERN Courier July/August 2025 p7). The result agrees with the latest lattice–QCD predictions for the leading hadronic–vacuum-polarisation term, albeit within a four times larger theoretical uncertainty than the experimental one. Continued improvements to lattice QCD and to the traditional dispersion-relation method based on low-energy e⁺e^– and τ data are expected in the coming years.

Runaway success

After the remarkable success of LHC Run 2, Run 3 has now surpassed it in delivered luminosity. Using the full available Run-2 and Run-3 datasets, ATLAS reported 3.4σ evidence for the rare Higgs decay to a muon pair, and a new result on the quantum-loop mediated decay into a Z boson and a photon, now more consistent with the Standard Model prediction than the earlier ATLAS and CMS Run-2 combination (see “Mapping rare Higgs-boson decays”). ATLAS also presented an updated study of Higgs pair production with decays into two b-quarks and two photons, whose sensitivity was increased beyond statistical gains thanks to improved reconstruction and analysis. CMS released a new Run-2 search for Higgs decays to charm quarks in events produced with a top-quark pair, reaching sensitivity comparable to the traditional weak-boson-associated production. Both collaborations also released new combinations of nearly all their Higgs analyses from Run 2, providing a wide set of measurements. While ATLAS sees overall agreement with predictions, CMS observes some non-significant tensions.

Advancing the field relies on the ability to design, build and operate increasingly complex instruments that push technological boundaries

A highlight in top-quark physics this year was the observation by CMS of an excess in top-pair production near threshold, confirmed at the conference by ATLAS (see “ATLAS confirms top–antitop excess”). The physics of the strong interaction predicts highly compact, colour-singlet, quasi-bound pseudoscalar top–antitop state effects arising from gluon exchange. Unlike bottomonium or charmonium, no proper bound state is formed due to the rapid weak decay of the top quark (see “Memories of quarkonia”). This “toponium” effect can be modelled with the use of non-relativistic QCD. Both experiments observed a cross section about 100 times smaller than for inclusive top-quark pair production. The subtle signal and complex threshold modelling make the analysis challenging, and warrant further theoretical and experimental investigation.

A major outcome of LHC Run 2 is the lack of compelling evidence for physics beyond the Standard Model. In Run 3, ATLAS and CMS continue their searches, aided by improved triggers, reconstruction and analysis techniques, as well as a dataset more than twice as large, enabling a more sensitive exploration of rare or suppressed signals. The experiments are also revisiting excesses seen in Run 2, for example, a CMS hint of a new resonance decaying into a Higgs and another scalar was not confirmed by a new ATLAS analysis including Run-3 data.

Hadron spectroscopy has seen a renaissance since Belle’s 2003 discovery of the exotic X(3872), with landmark advances at the LHC, particularly by LHCb. CMS recently reported three new four-charm-quark states decaying into J/ψ pairs between 6.6 and 7.1 GeV. Spin-parity analysis suggests they are tightly bound tetraquarks rather than loosely bound molecular states (CERN Courier November/December 2024 p33).

Rare observations

Flavour physics continues to test the Standard Model with high sensitivity. Belle-II and LHCb reported new CP violation measurements in the charm sector, confirming the expected small effects. LHCb observed, for the first time, CP violation in the baryon sector via Λ_b decays, a milestone in CP violation history. NA62 at CERN’s SPS achieved the first observation of the ultra-rare kaon decay K⁺ → π⁺νν with a branching ratio of 1.3 × 10^–10, matching the Standard Model prediction. MEG-II at PSI set the most stringent limit to date on the lepton-flavour-violating decay μ → eγ, excluding branching fractions above 1.5 × 10^–13. Both experiments continue data taking until 2026.

Heavy-ion collisions at the LHC provide a rich environment to study the quark–gluon plasma, a hot, dense state of deconfined quarks and gluons, forming a collective medium that flows as a relativistic fluid with an exceptionally low viscosity-to-entropy ratio. Flow in lead–lead collisions, quantified by Fourier harmonics of spatial momentum anisotropies, is well described by hydrodynamic models for light hadrons. Hadrons containing heavier charm and bottom quarks show weaker collectivity, likely due to longer thermalisation times, while baryons exhibit stronger flow than mesons due to quark coalescence. ALICE reported the first LHC measurement of charm–baryon flow, consistent with these effects.

Spin-parity analysis suggests the states are tightly bound tetraquarks

Neutrino physics has made major strides since oscillations were confirmed 27 years ago, with flavour mixing parameters now known to a few percent.  Crucial questions still remain: are neutrinos their own antiparticles (Majorana fermions)? What is the mass ordering – normal or inverted? What is the absolute mass scale and how is it generated? Does CP violation occur? What are the properties of the right-handed neutrinos? These and other questions have wide-ranging implications for particle physics, astrophysics and cosmology.

Neutrinoless double-beta decay, if observed, would confirm that neutrinos are Majorana particles. Experiments using xenon and germanium are beginning to constrain the inverted mass ordering, which predicts higher decay rates. Recent combined data from the long-baseline experiments T2K and NOvA show no clear preference for either ordering, but exclude vanishing CP violation at over 3σ in the inverted scenario. The KM3NeT detector in the Mediterranean, with its ORCA and ARCA components, has delivered its first competitive oscillation results, and detected a striking ~220 PeV muon neutrino, possibly from a blazar (CERN Courier March/April 2025 p7). The next-generation large-scale neutrino experiments JUNO (China), Hyper-Kamiokande (Japan) and LBNF/DUNE (USA) are progressing in construction, with data-taking expected to begin in 2025, 2028 and 2031, respectively. LBNF/DUNE is best positioned to determine the neutrino mass ordering, while Hyper-Kamiokande will be the most sensitive to CP violation. All three will also search for proton decay, a possible messenger of grand unification.

There is compelling evidence for dark matter from gravitational effects across cosmic times and scales, as well as indications that it is of particle origin. Its possible forms span a vast mass range, up to the ~100 TeV unitarity limit for a thermal relic, and may involve a complex, structured “dark sector”. The wide complementarity among the search strategies gives the field a unifying character. Direct detection experiments looking for tiny, elastic nuclear recoils, such as XENONnT (Italy), LZ (USA) and PandaX-4T (China), have set world-leading constraints on weakly interacting massive particles. XENONnT and PandaX-4T have also reported first signals from boron-8 solar neutrinos, part of the so-called “neutrino fog” that will challenge future searches. Axions, introduced theoretically to suppress CP violation in strong interactions, could be viable dark-matter candidates. They would be produced in the early universe with enormous number density, behaving, on galactic scales, as a classical, nonrelativistic, coherently oscillating bosonic field, effectively equivalent to cold dark matter. Axions can be detected via their conversion into photons in strong magnetic fields. Experiments using microwave cavities have begun to probe the relevant μeV mass range of relic QCD axions, but the detection becomes harder at higher masses. New concepts, using dielectric disks or wire-based plasmonic resonance, are under development to overcome these challenges.

Cosmological constraints

Cosmology featured prominently at EPS-HEP, driven by new results from the analysis of DESI DR2 baryon acoustic oscillation (BAO) data, which include 14 million redshifts. Like the cosmic microwave background (CMB), BAO also provides a “standard ruler” to trace the universe’s expansion history – much like supernovae (SNe) do as standard candles. Cosmological surveys are typically interpreted within the ΛCDM model, a six-parameter framework that remarkably accounts for 13.8 billion years of cosmic evolution, from inflation and structure formation to today’s energy content, despite offering no insight into the nature of dark matter, dark energy or the inflationary mechanism. Recent BAO data, when combined with CMB and SNe surveys, show a preference for a form of dark energy that weakens over time. Tensions also persist in the Hubble expansion rate derived from early-universe (CMB and BAO) and late-universe (SN type-Ia) measurements (CERN Courier March/April 2025 p28). However, anchoring SN Ia distances in redshift remains challenging, and further work is needed before drawing firm conclusions.

Cosmological fits also constrain the sum of neutrino masses. The latest CMB and BAO-based results within ΛCDM appear inconsistent with the lower limit implied by oscillation data for inverted mass ordering. However, firm conclusions are premature, as the result may reflect limitations in ΛCDM itself. Upcoming surveys from the Euclid satellite and the Vera C. Rubin Observatory (LSST) are expected to significantly improve cosmological constraints.

Cristinel Diaconu and Thomas Strebler, chairs of the local organising committee, together with all committee members and many volunteers, succeeded in delivering a flawlessly organised and engaging conference in the beautiful setting of the Palais du Pharo overlooking Marseille’s old port. They closed the event with a memorial phrase of British cyclist Tom Simpson: “There is no mountain too high.”

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The international conference Dark Matter and Stars: Multi-Messenger Probes of Dark Matter and Modified Gravity (ICDMS) was held at Queen’s University in Kingston, Ontario, Canada, from 14 to 16 July. The meeting brought together around 70 researchers from across astrophysics, cosmology, particle physics and gravitational theory. The goal was to foster interdisciplinary dialogue on how observations of stellar systems, gravitational waves and cosmological data can help shed light on the dark sector. The conference was specifically dedicated to exploring how astrophysical and cosmological systems can be used to probe the nature of dark matter.

The first day centred on compact objects as natural laboratories for dark-matter physics. Giorgio Busoni (University of Adelaide) opened with a comprehensive overview of recent theoretical progress on dark-matter accumulation in neutron stars and white dwarfs, highlighting refinements in the treatment of relativistic effects, optical depth, Fermi degeneracy and light mediators – all of which have shaped the field in recent years. Melissa Diamond (Queen’s University) followed with a striking talk with a nod to Dr. Strangelove, exploring how accumulated dark matter might trigger thermonuclear instability in white dwarfs. Sandra Robles (Fermilab) shifted the perspective from neutron stars to white dwarfs, showing how they constrain dark-matter properties. One of the authors highlighted postmerger gravitational-wave observations as a tool to distinguish neutron stars from low-mass black holes, offering a promising avenue for probing exotic remnants potentially linked to dark matter. Axions featured prominently throughout the day, alongside extensive discussions of the different ways in which dark matter affects neutron stars and their mergers.

ICDMS continues to strengthen the interface between fundamental physics and astrophysical observations

On the second day, attention turned to the broader stellar population and planetary systems as indirect detectors. Isabelle John (University of Turin) questioned whether the anomalously long lifetimes of stars near the galactic centre might be explained by dark-matter accumulation. Other talks revisited stellar systems – white dwarfs, red giants and even speculative dark stars – with a focus on modelling dark-matter transport and its effects on stellar heat flow. Complementary detection strategies also took the stage, including neutrino emission, stochastic gravitational waves and gravitational lensing, all offering potential access to otherwise elusive energy scales and interaction strengths.

The final day shifted toward galactic structure and the increasingly close interplay between theory and observation. Lina Necib (MIT) shared stellar kinematics data used to map the Milky Way’s dark-matter distribution, while other speakers examined the reliability of stellar stream analyses and subtle anomalies in galactic rotation curves. The connection to terrestrial experiments grew stronger, with talks tying dark matter to underground detectors, atomic-precision tools and cosmological observables such as the Lyman-alpha forest and baryon acoustic oscillations. Early-career researchers contributed actively across all sessions, underscoring the field’s growing vitality and introducing a fresh influx of ideas that is expanding its scope.

The ICDMS series is now in its third edition. It began in 2018 at Instituto Superior Técnico, Portugal, and is poised to become an annual event. The next conference will take place at the University of Southampton, UK, in 2026, followed by the Massachusetts Institute of Technology in the US in 2027. With increasing participation and growing international interest, the ICDMS series continues to strengthen the interface between fundamental physics and astrophysical observations in the quest to understand the nature of dark matter.

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As higher experimental precision relies on new technologies, new theory results require better algorithms, both from the mathematical and computer-algebraic side, and new techniques in quantum field theory. The central software package for perturbative calculations, FORM, now has a new major release, FORM 5. Progress has also been achieved in integration-by-parts reduction, which is of central importance for reducing to a much smaller set of master integrals. New developments were also reported in analytic and numerical Feynman-diagram integration using Mellin–Barnes techniques, new compact function classes such as Feynman–Fox integrals, and modern summation technologies and methods to establish and solve gigantic recursions and differential equations of degree 4000 and order 100. The latest results on elliptic integrals and progress on the correct treatment of the γ₅-problem in real dimensions were also presented. These technologies allow the calculation of processes up to five loops and in the presence of more scales at two- and three-loop order. New results for single-scale quantities like quark condensates and the ρ-parameter were also reported.

In the loop

Measurements at future colliders will depend on the precise knowledge of parton distribution functions, the strong coupling constant α_s(M_Z) and the heavy-quark masses. Experience suggests that going from one loop order to the next in the massless and massive cases takes 15 years or more, as new technologies must be developed. By now, most of the space-like four-loop splitting functions governing scaling violations are known with a good precision, as well as new results for the three-loop time-like splitting functions. The massive three-loop Wilson coefficients for deep-inelastic scattering are now complete, requiring far larger and different integral spaces compared with the massless case. Related to this are the Wilson coefficients of semi-inclusive deep-inelastic scattering at next-to-next-to leading order (NNLO), which will be important to tag individual flavours at the EIC. For the α_s(M_Z) measurement at low-scale processes, the correct treatment of renormalon contributions is necessary. Collisions at high energies also allow the detailed study of scattering processes in the forward region of QCD. Other long-term projects concern NNLO corrections for jet-production at e⁺e^– and hadron colliders, and other related processes like Higgs-boson and top-quark production, in some cases with a large number of partons in the final state. This also includes the use of effective Lagrangians.

Many more steps lie ahead if we are to match the precision of measurements at high-luminosity colliders

The complete calculation of difficult processes at NNLO and beyond always drives the development of term-reduction algorithms and analytic or numerical integration technologies. Many more steps lie ahead in the coming years if we are to match the precision of measurements at high-luminosity colliders. Some of these will doubtless be reported at Loopsummit-3 in summer 2027.

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The 39th edition of the International Cosmic Ray Conference (ICRC), a key biennial conference in astroparticle physics, was held in Geneva from 15 to 24 July. Plenary talks covered solar, galactic and ultra-high-energy cosmic rays. A strong multi-messenger perspective combined measurements of charged particles, neutrinos, gamma rays and gravitational waves. Talks were informed by limits from the LHC and elsewhere on dark-matter particles and primordial black-holes. The bundle of constraints has improved very significantly over the past few years, allowing more meaningful and stringent tests.

Solar modelling

The Sun and its heliosphere, where the solar wind offers insights into magnetic reconnection, shock acceleration and diffusion, are now studied in situ thanks to the Solar Orbiter and Parker Solar Probe spacecraft. Long-term PAMELA and AMS data, spanning over an 11-year solar cycle, allow precise modelling of solar modulation of cosmic-ray fluxes below a few tens of GeV. AMS solar proton data show a 27-day periodicity up to 20 GV, caused by corotating interaction regions where fast solar wind overtakes slower wind, creating shocks. AMS has recorded 46 solar energetic particle (SEP) events, the most extreme reaching a few GV, from magnetic-reconnection flares or fast coronal mass ejections. While isotope data once suggested such extreme events occur every 1500 years, Kepler observations of Sun-like stars indicate they may happen every 100 years, releasing more than 10³⁴ erg, often during weak solar minima, and linked to intense X-ray flares.

The spectrum of galactic cosmic rays, studied with high-precision measurements from satellites (DAMPE) and ISS-based experiments (AMS-02, CALET, ISS-CREAM), is not a single power law but shows breaks and slope changes, signatures of diffusion or source effects. A hardening at about 500 GV, common to all primaries, and a softening at 10 TV, are observed in protons and He spectra by all experiments – and for the first time also in DAMPE’s O and C. As the hardening is detected in primary spectra scaling at the same rigidity (charge, not mass) as in secondary-to-primary ratios, they are attributed to propagation in the galaxy and not to source-related effects. This is supported by secondary (Li, Be, B) spectra with breaks about twice as strong as primaries (He, C, O). A second hardening at 150 TV was reported by ISS-CREAM (p) and DAMPE (p + He) for the first time, broadly consistent – within large hadronic-model and statistical uncertainties – with indirect ground-based results from GRAPES and LHAASO.

A strong multi-messenger perspective combined measurements of charged particles, neutrinos, gamma rays and gravitational waves

Ratios of secondary over primary species versus rigidity R (energy per unit charge) probe the ratio of the galactic halo size H to the energy-dependent diffusion coefficient D(R), and so measure the “grammage” of material through which cosmic rays propagate. Unstable/stable secondary isotope ratios probe the escape times of cosmic rays from the halo (H²/D(R)), so from both measurements H and D(R) can be derived. The flattening evidenced by the highest energy point at 10 to 12 GeV/nucleon of the ¹⁰Be/⁹Be ratio as a function of energy, hints at a possibly larger halo than previously believed beyond 5 kpc, to be tested by HELIX. AMS-02 spectra of single elements will soon allow separation of the primary and secondary fractions for each nucleus, also based on spallation cross-sections. Anomalies remain, such as a flattening at ~7 TeV/nucleon in Li/C and B/C, possibly indicating reacceleration or source grammage. AMS-02’s ⁷Li/⁶Li ratio disagrees with pure secondary models, but cross-section uncertainties preclude firm conclusions on a possible Li primary component, which would be produced by a new population of sources.

The muon puzzle

The dependency of ground-based cosmic-ray measurements on hadronic models has been widely discussed by Boyd and Pierog, highlighting the need for more measurements at CERN, such as the recent proton-O run being analysed by LHCf. The EPOS–LHC model, based on the core–corona approach, shows reduced muon discrepancies, producing more muons and a heavier composition, namely deeper shower maxima (+20 g/cm²) than earlier models. This clarifies the muon puzzle raised by Pierre Auger a few years ago of a larger muon content in atmospheric showers than simulations. A fork-like structure remains in the knee region of the proton spectrum, where the new measurements presented by LHAASO are in agreement with IceTop/IceCube, and could lead to a higher content of protons beyond the knee than hinted at by KASCADE and the first results of GRAPES. Despite the higher proton fluxes, a dominance of He above the knee is observed, which requires a special kind of close-by source to be hypothesised.

Multi-messenger approaches

Gamma-ray and neutrino astrophysics were widely discussed at the conference, highlighting the relevance of multi-messenger approaches. LHAASO produced impressive results on UHE astrophysics, revealing a new class of pevatrons: microquasars alongside young massive clusters, pulsar wind nebulae (PWNe) and supernova remnants.

Microquasars are gamma-ray binaries containing a stellar-mass black hole that drives relativistic jets while accreting matter from their companion stars. Outstanding examples include Cyg X-3, a potential PeV microquasar, from which the flux of PeV photons is 5–10 times higher than in the rest of the Cygnus bubble.

Five other microquasars are observed beyond 100 TeV: SS 433, V4641 Sgr, GRS 1915 + 105, MAXI J1820 + 070 and Cygnus X-1. SS 433 is a microquasar with two gamma-ray emitting jets nearly perpendicular to our line of sight, terminated at 40 pc from the black hole (BH) identified by HESS and LHAASO beyond 10 TeV. Due to the Klein–Nishina effect, the inverse Compton flux above ~10 TeV is gradually suppressed, and an additional spectral component is needed to explain the flux around 100 TeV.

Gamma-ray and neutrino astrophysics were widely discussed at the conference

Beyond 100 TeV, LHAASO also identifies a source coincident with a giant molecular cloud; this component may be due to protons accelerated close to the BH or in the lobes. These results demonstrate the ability to resolve the morphology of extended galactic sources. Similarly, ALMA has discovered two hotspots, both at 0.28° (about 50 pc) from GRS 1915 + 105 in opposite directions from its BH. These may be interpreted as two lobes, or the extended nature of the LHAASO source may instead be due to the spatial distribution of the surrounding gas, if the emission from GRS 1915 + 105 is dominated by hadronic processes.

Further discussions addressed pulsar halos and PWNe as unique laboratories for studying the diffusion of electrons and mysterious as-yet-unidentified pevatrons, such as MGRO J1908 + 06, coincident with a SNR (favoured) and a PSR. One of these sources may finally reveal an excess in KM3NeT or IceCube neutrinos, proving their cosmic-ray accelerator nature directly.

The identification and subtraction of source fluxes on the galactic plane is also important for the measurement of the galactic plane neutrino flux by IceCube. This currently assumes a fixed spectral index of E^–2.7, while authors like Grasso et al. presented a spectrum becoming as soft as E^–2.4, closer to the galactic centre. The precise measurements of gamma-ray source fluxes and the diffuse emission from galactic cosmic rays interacting in the interstellar matter lead to better constraints on neutrino observations and on cosmic ray fluxes around the knee.

Cosmogenic origins

KM3NeT presented a neutrino of energy well beyond the diffuse cosmic neutrino flux of IceCube, which does not extend beyond 10 PeV (CERN Courier March/April 2025 p7). Its origin was widely discussed at the conference. The large error on its estimated energy – 220 PeV, within a 1σ confidence interval of 110 to 790 PeV – makes it nevertheless compatible with the flux observed by IceCube, for which a 30 TeV break was first hypothesised at this conference. If events of this kind are confirmed, they could have transient or dark-matter origins, but a cosmogenic origin is improbable due to the IceCube and Pierre Auger limits on the cosmogenic neutrino flux.

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Reconciling general relativity and quantum mechanics remains a central problem in fundamental physics. Though successful in their own domains, the two theories resist unification and offer incompatible views of space, time and matter. The field of quantum gravity, which has sought to resolve this tension for nearly a century, is still plagued by conceptual challenges, limited experimental guidance and a crowded landscape of competing approaches. Now in its third instalment, the “Quantum Gravity” conference series addresses this fragmentation by promoting open dialogue across communities. Organised under the auspices of the International Society for Quantum Gravity (ISQG), the 2025 edition took place from 21 to 25 July at Penn State University. The event gathered researchers working across a variety of frameworks – from random geometry and loop quantum gravity to string theory, holography and quantum information. At its core was the recognition that, regardless of specific research lines or affiliations, what matters is solving the puzzle.

One step to get there requires understanding the origin of dark energy, which drives the accelerated expansion of the universe and is typically modelled by a cosmological constant Λ. Yasaman K Yazdi (Dublin Institute for Advanced Studies) presented a case for causal set theory, reducing spacetime to a discrete collection of events, partially ordered to capture cause–effect relationships. In this context, like a quantum particle’s position and momentum, the cosmological constant and the spacetime volume are conjugate variables. This leads to the so-called “ever-present Λ” models, where fluctuations in the former scale as the inverse square root of the latter, decreasing over time but never vanishing. The intriguing agreement between the predicted size of these fluctuations and the observed amount of dark energy, while far from resolving quantum cosmology, stands as a compelling motivation for pursuing the approach.

In the spirit of John Wheeler’s “it from bit” proposal, Jakub Mielczarek (Jagiellonian University) suggested that our universe may itself evolve by computing – or at least admit a description in terms of quantum information processing. In loop quantum gravity, space is built from granular graphs known as spin networks, which capture the quantum properties of geometry. Drawing on ideas from tensor networks and holography, Mielczarek proposed that these structures can be reinterpreted as quantum circuits, with their combinatorial patterns reflected in the logic of algorithms. This dictionary offers a natural route to simulating quantum geometry, and could help clarify quantum theories that, like general relativity, do not rely on a fixed background.

Quantum clues

What would a genuine quantum theory of spacetime achieve, though? According to Esteban Castro Ruiz (IQOQI), it may have to recognise that reference frames, which are idealised physical systems used to define spatio-temporal distances, must themselves be treated as quantum objects. In the framework of quantum reference frames, notions such as entanglement, localisation and superposition become observer-dependent. This leads to a perspective-neutral formulation of quantum mechanics, which may offer clues for describing physics when spacetime is not only dynamical, but quantum.

The conference’s inclusive vocation came through most clearly in the thema­tic discussion sessions, including one on the infamous black-hole information problem chaired by Steve Giddings (UC Santa Barbara). A straightforward reading of Stephen Hawking’s 1974 result suggests that black holes radiate, shrink and ultimately destroy information – a process that is incompatible with standard quantum mechanics. Any proposed resolution must face sharp trade-offs: allowing information to escape challenges locality, losing it breaks unitarity and storing it in long-lived remnants undermines theoretical control. Giddings described a mild violation of locality as the lesser evil, but the controversy is far from settled. Still, there is growing consensus that dissolving the paradox may require new physics to appear well before the Planck scale, where quantum-gravity effects are expected to dominate.

Once the domain of pure theory, quantum gravity has become eager to engage with experiment

Among the few points of near-universal agreement in the quantum-gravity community has long been the virtual impossibility of detecting a graviton, the hypothetical quantum of the gravitational field. According to Igor Pikovski (Stockholm University), things may be less bleak than once thought. While the probability of seeing graviton-induced atomic transitions is negligible due to the weakness of gravity, the situation is different for massive systems. By cooling a macroscopic object close to absolute zero, Pikovski suggested, the effect could be amplified enough, with current interferometers simultaneously monitoring gravitational waves in the correct frequency window. Such a signal would not amount to a definitive proof of gravity’s quantisation, just as the photoelectric effect could not definitely establish the existence of photons, nor would it single out a specific ultraviolet model. However, it could constrain concrete predictions and put semiclassical theories under pressure. Giulia Gubitosi (University of Naples Federico II) tackled phenomenology from a different angle, exploring possible deviations from special relativity in models where spacetime becomes non-commutative. There, coordinates are treated like quantum operators, leading to effects like decoherence, modified particle speeds and soft departures from locality. Although such signals tend to be faint, they could be enhanced by high-energy astrophysical sources: observations of neutrinos corresponding to gamma-ray bursts are now starting to close in on these scenarios. Both talks reflected a broader, cultural shift: quantum gravity, once the domain of pure theory, has become eager to engage with experiment.

Quantum Gravity 2025 offered a wide snapshot of a field still far from closure, yet increasingly shaped by common goals, the convergence of approaches and cross-pollination. As intended, no single framework took centre stage, with a dialogue-based format keeping focus on the central, pressing issue at hand: understanding the quantum nature of spacetime. With limited experimental guidance, open exchange remains key to clarifying assumptions and avoiding duplication of efforts. Building on previous editions, the meeting pointed toward a future where quantum-gravity researchers will recognise themselves as part of a single, coherent scientific community.

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UPC studies have expanded significantly since the first UPC workshop in Mexico in December 2023. The opportunity to study scattering processes in a clean photon–nucleus environment at collider energies has inspired experimentalists to examine both inclusive and exclusive scattering processes, and to look for signals of collectivity and even the formation of quark–gluon plasma (QGP) in this unique environment.

For many years, experimental activity in UPCs was mainly focused on exclusive processes and QED phenomena including photon–photon scattering. This year, fresh inclusive particle-production measurements gained significant attention, as well as various signatures of QGP-like behaviour observed by different experiments at RHIC and at the LHC. The importance of having complementing experiments to perform similar measurements was also highlighted. In particular, the ATLAS experiment joined the ongoing activities to measure exclusive vector–meson photoproduction, finding a cross section that disagrees with the previous ALICE measurements by almost 50%. After long and detailed discussions, it was agreed that different experimental groups need to work together closely to resolve this tension before the next UPC workshop.

Experimental and theoretical developments very effectively guide each other in the field of UPCs. This includes physics within and beyond the Standard Model (BSM), such as nuclear modifications to the partonic structure of protons and neutrons, gluon-saturation phenomena predicted by QCD (CERN Courier January/February 2025 p31), and precision tests for BSM physics in photon–photon collisions. The expanding activity in the field of UPCs, together with the construction of the Electron Ion Collider (EIC) at Brookhaven National Laboratory in the US, has also made it crucial to develop modern Monte Carlo event generators to the level where they can accurately describe various aspects of photon–photon and photon–nucleus scatterings.

As a photon collider, the LHC complements the EIC. While the centre-of-mass energy at the EIC will be lower, there is some overlap between the kinematic regions probed by these two very different collider projects thanks to the varying energy spectra of the photons. This allows the theoretical models needed for the EIC to be tested against UPC data, thereby reducing theoretical uncertainty on the predictions that guide the detector designs. This complementarity will enable precision studies of QCD phenomena and BSM physics in the 2030s.

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Perhaps the most intriguing open question surrounding neutron stars is what is actually inside them. Clearly they are primarily composed of neutrons, but many theories suggest that other forms of matter should appear in the highest density regions near the centre of the star, including free quarks, hyperons and kaon or pion condensates. Diverse data can constrain these hypotheses, including astronomical inferences of the masses and radii of neutron stars, observations of the mergers of neutron stars by LIGO, and baryon production patterns and correlations in heavy-ion collisions at the LHC. Theoretical consistency is critical here. Several talks highlighted the importance of low-energy nuclear data to understand the behaviour of nuclear matter at low densities, though also emphasising that at very high densities and energies any description should fall within the realm of QCD – a theory that beautifully describes the dynamics of quarks and gluons at the LHC.

Another key question for neutron stars is how fast they cool. This depends critically on their composition. Quarks, hyperons, nuclear resonances, pions or muons would each lead to different channels to cool the neutron star. Measurements of the temperatures and ages of neutron stars might thereby be used to learn about their composition.

Research into neutron stars has progressed so rapidly in recent years that it allows key tests of fundamental physics

The workshop revealed that research into neutron stars has progressed so rapidly in recent years that it allows key tests of fundamental physics including tests of particles beyond the Standard Model, including the axion: a very light and weakly coupled dark-matter candidate that was initially postulated to explain the “strong CP problem” of why strong interactions are identical for particles and antiparticles. The workshop allowed particle theorists to appreciate the various possible uncertainties in their theoretical predictions and propagate them into new channels that may allow sharper tests of axions and other weakly interacting particles. An intriguing question that the workshop left open is whether the canonical QCD axion could condense inside neutron stars.

While many uncertainties remain, the workshop revealed that the field is open and exciting, and that upcoming observations of neutron stars, including neutron-star mergers or the next galactic supernova, hold unique opportunities to understand fundamental questions from the nature of dark matter to the strong CP problem.

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Alfred D Stone (Yale University) called upon participants to challenge the folklore surrounding quantum theory’s birth. Philosopher Elise Crull (City College of New York) drew overdue attention to Grete Hermann, who hinted at entanglement before it had a name and anticipated Bell in identifying a flaw in von Neumann’s no-go theorem, which had been taken as proof that hidden-variable theories are impossible. Science writer Philip Ball questioned Heisenberg’s epiphany itself: he didn’t invent matrix mechanics in a flash, claims Ball, nor immediately grasp its relevance, and it took months, and others, to see his contribution for what it was (see “Lend me your ears” image).

Building on a strong base

A clear takeaway from Helgoland 2025 was that the foundations of quantum mechanics, though strongly built on Helgoland 100 years ago, nevertheless remain open to interpretation, and any future progress will depend on excavating them directly (see “Four ways to interpret quantum mechanics“).

Does the quantum wavefunction represent an objective element of reality or merely an observer’s state of knowledge? On this question, Helgoland 2025 could scarcely have been more diverse. Christopher Fuchs (UMass Boston) passionately defended quantum Bayesianism, which recasts the Born probability rule as a consistency condition for rational agents updating their beliefs. Wojciech Zurek (Los Alamos National Laboratory) presented the Darwinist perspective, for which classical objectivity emerges from redundant quantum information encoded across the environment. Although Zurek himself maintains a more agnostic stance, his decoherence-based framework is now widely embraced by proponents of many-worlds quantum mechanics (see “The minimalism of many worlds“).

The foundations of quantum mechanics remain open to interpretation, and any future progress will depend on excavating them directly

Markus Aspelmeyer (University of Vienna) made the case that a signature of gravity’s long-speculated quantum nature may soon be within experimental reach. Building on the “gravitational Schrödinger’s cat” thought experiment proposed by Feynman in the 1950s, he described how placing a massive object in a spatial superposition could entangle a nearby test mass through their gravitational interaction. Such a scenario would produce correlations that are inexplicable by classical general relativity alone, offering direct empirical evidence that gravity must be described quantum-mechanically. Realising this type of experiment requires ultra-low pressures and cryogenic temperatures to suppress decoherence, alongside extremely low-noise measurements of gravitational effects at short distances. Recent advances in optical and opto­mechanical techniques for levitating and controlling nanoparticles suggest a path forward – one that could bring evidence for quantum gravity not from black holes or the early universe, but from laboratories on Earth.

Information insights

Quantum information was never far from the conversation. Isaac Chuang (MIT) offered a reconstruction of how Heisenberg might have arrived at the principles of quantum information, had his inspiration come from Shannon’s Mathematical Theory of Communication. He recast his original insights into three broad principles: observations act on systems; local and global perspectives are in tension; and the order of measurements matters. Starting from these ingredients, one could in principle recover the structure of the qubit and the foundations of quantum computation. Taking the analogy one step further, he suggested that similar tensions between memorisation and generalisation – or robustness and adaptability – may one day give rise to a quantum theory of learning.

Helgoland 2025 illustrated just how much quantum mechanics has diversified since its early days. No longer just a framework for explaining atomic spectra, the photoelectric effect and black-body radiation, it is at once a formalism describing high-energy particle scattering, a handbook for controlling the most exotic states of matter, the foundation for information technologies now driving national investment plans, and a source of philosophical conundrums that, after decades at the margins, has once again taken centre stage in theoretical physics.

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The conference mixed updates on theoretical developments in Higgs-boson pair production, searches for new physics in the scalar sector, and the most recent results from Run 2 and Run 3 of the LHC. Among the highlights was the first Run 3 analysis released by ATLAS on the search for di-Higgs production in the bbγγ final state – a particularly sensitive channel for probing the Higgs self-coupling. This result builds on earlier Run 2 analyses and demonstrates significantly improved sensitivity, now comparable to the full Run 2 combination of all channels. These gains were driven by the use of new b-tagging algorithms, improved mass resolution through updated analysis techniques, and the availability of nearly twice the dataset.

Complementing this, CMS presented the first search for ttHH production – a rare process that would provide additional sensitivity to the Higgs self-coupling and Higgs–top interactions. Alongside this, ATLAS presented first experimental searches for triple Higgs boson production (HHH), one of the rarest processes predicted by the SM. Work on more traditional final states such as bbττ and bbbb is ongoing at both experiments, and continues to benefit from improved reconstruction techniques and larger datasets. 

Beyond current data, the workshop featured discussions of the latest combined projection study by ATLAS and CMS, prepared as part of the input to the upcoming European Strategy Update. It extrapolates results of the Run 2 analyses to expected conditions of the High-Luminosity LHC (HL-LHC), estimating future sensitivities to the Higgs self-coupling and di-Higgs cross-section in scenarios with vastly higher luminosity and upgraded detectors. Under these assumptions, the combined sensitivity of ATLAS and CMS to di-Higgs production is projected to reach a significance of 7.6σ, firmly establishing the process. 

These projections provide crucial input for analysis strategy planning and detector design for the next phase of operations at the HL-LHC. Beyond the HL-LHC, efforts are already underway to design experiments at future colliders that will enhance sensitivity to the production of Higgs pairs, and offer new insights into electroweak symmetry breaking.

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The workshop’s scientific programme included field theoretical approaches to QCD, the behaviour of hadronic and quark matter in astrophysical contexts, hadronic structure and decays, lattice QCD calculations, recent experimental developments in relativistic heavy-ion collisions, and the interplay of strong and electroweak forces within the Standard Model.

Fernanda Steffens (University of Bonn) explained how deep-inelastic-scattering experiments and theoretical developments are revealing the internal structure of the proton. Kenji Fukushima (University of Tokyo) addressed the theoretical framework and phase structure of strongly interacting matter, with particular emphasis on the QCD phase diagram and its relevance to heavy-ion collisions and neutron stars. Chun Shen (Wayne State University) presented a comprehensive overview of the state-of-the-art techniques used to extract the transport properties of quark–gluon plasma from heavy-ion collision data, emphasising the role of Bayesian inference and machine learning in constraining theoretical models. Li-Sheng Geng (Beihang University) explored exotic hadrons through the lens of hadronic molecules, highlighting symmetry multiplets such as pentaquarks, the formation of multi-hadron states and the role of femtoscopy in studying unstable particle interactions.

This edition of Hadrons was dedicated to the memory of two individuals who left a profound mark on the Brazilian hadronic-physics community: Yogiro Hama, a distinguished senior researcher and educator whose decades-long contributions were foundational to the development of the field in Brazil, and Kau Marquez, an early-career physicist whose passion for science remained steadfast despite her courageous battle with spinal muscular atrophy. Both were remembered with deep admiration and respect, not only for their scientific dedication but also for their personal strength and impact on the community.

Its mission is to cultivate a vibrant and inclusive scientific environment

Since its creation in 1988, the Hadrons workshop has played a central role in developing Brazil’s scientific capacity in particle and nuclear physics. Its structure facilitates close interaction between master’s and doctoral students, and senior researchers, thus enhancing both technical training and academic exchange. This model continues to strengthen the foundations of research and collaboration throughout the Brazilian scientific community.

This is the main event for the Brazilian particle- and nuclear-physics communities, reflecting a commitment to advancing research in this highly interactive field. By circulating the venue across multiple regions of Brazil, each edition further renews its mission to cultivate a vibrant and inclusive scientific environment. This edition was closed by a public lecture on QCD by Tereza Mendes (University of São Paolo), who engaged local students with the foundational questions of strong-interaction physics.

The next edition of the Hadrons series will take place in Bahia in 2028.

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The highlight of the conference was new results on the muon magnetic anomaly. The Muon g-2 experiment at Fermilab released its final measurement of a_μ = (g-2)/2 on 3 June, while the conference was in progress, reaching a precision of 127 ppb on the published value. This uncertainty is more than four times smaller than that reported by the previous experiment. One week earlier, on 27 May, the Muon g-2 Theory Initiative published their second calculation of the same quantity, following that published in summer 2020. A major difference between the two calculations is that the earlier one used experimental data and the dispersion integral to evaluate the hadronic contribution to a_μ, whereas the update uses a purely theoretical approach based on lattice QCD. The strong tension with the experiment of the earlier calculation is no longer present, with the new calculation compatible with experimental results. Thus, no new physics discovery can be claimed, though the reason for the difference between the two approaches must be understood (see “Fermilab’s final word on muon g-2“). 

The MEG II collaboration presented an important update to their limit on the branching fraction for the lepton-flavour-violating decay μ → eγ. Their new upper bound of 1.5 × 10^–13 is determined from data collected in 2021 and 2022. The experiment recorded additional data from 2023 to 2024 and expects to continue data taking for two more years. These data will be sensitive to a branching fraction four to five times smaller than the current limit.

LHCb, Belle II, BESIII and NA62 all discussed recent results in quark flavour physics. Highlights include the first measurement of CP violation in a baryon decay by LHCb and improved limits on CP violation in D-meson decay to two pions by Belle II. With more data, the latter measurements could potentially show that the observed CP violation in charm is from a non-Standard-Model source. 

The Belle II collaboration now plans to collect a sample between 5 to 10 ab^–1 by the early 2030s before undergoing an upgrade to collect a 30 to 50 ab^–1 sample by the early 2040s. LHCb plan to run to the end of the High-Luminosity LHC and collect 300 fb^–1. LHCb recorded almost 10 fb^–1 of data last year – more than in all their previous running, and now with a fully software-based trigger with much higher efficiency than the previous hardware-based first-level trigger. Future results from Belle II and the LHCb upgrade are eagerly anticipated.

The 24th FPCP conference will be held from 18 to 22 May 2026 in Bad Honnef, Germany. 

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CERN Director-General Fabiola Gianotti estimated the integral FCC programme to offer unparalleled opportunities to explore physics at the shortest distances, and noted growing support and enthusiasm for the programme within the community. That enthusiasm is reflected in the growing collaboration: the FCC collaboration now includes 162 institutes from 38 countries, with 28 new Memoranda of Understanding signed in the past year. These include new partnerships in Latin America, Asia and Ukraine, as well as Statements of Intent from the US and Canada. The FCC vision has also gained visibility in high-level policy dialogues, including the Draghi report on European competitiveness. Scientific plenaries and parallel sessions highlighted updates on simulation tools, rare-process searches and strategies to probe beyond the Standard Model. Detector R&D has progressed significantly, with prototyping, software development and AI-driven simulations advancing rapidly.

In accelerator design, developments included updated lattice and optics concepts involving global “head-on” compensation (using opposing beam interactions) and local chromaticity corrections (to the dependence of beam optics on particle energy). Refinements were also presented to injection schemes, beam collimation and the mitigation of collective effects. A central tool in these efforts is the Xsuite simulation platform, whose capabilities now include spin tracking and modelling based on real collider environments such as SuperKEKB.

Technical innovations also came to the fore. The superconducting RF system for FCC-ee includes 400 MHz Nb/Cu cavities for low-energy operation and 800 MHz Nb cavities for higher-energy modes. The introduction of reverse-phase operation and new RF source concepts – such as the tristron, with energy efficiencies above 90% (CERN Courier May/June 2025 p30) – represent major design advances.

Design developments

Vacuum technologies based on ultrathin NEG coating and discrete photon stops, as well as industrialisation strategies for cost control, are under active development. For FCC-hh, high-field magnet R&D continues on both Nb₃Sn prototypes and high-temperature superconductors.

Sessions on technical infrastructure explored everything from grid design, cryogenics and RF power to heat recovery, robotics and safety systems. Sustainability concepts, including renewable energy integration and hydrogen storage, showcased the project’s interdisciplinary scope and long-term environmental planning.

FCC Week 2025 extended well beyond the conference venue, turning Vienna into a vibrant hub for public science outreach

The Early Career Researchers forum drew nearly 100 participants for discussions on sustainability, governance and societal impact. The session culminated in a commitment to inclusive collaboration, echoed by the quote from Austrian-born artist, architect and environmentalist Friedensreich Hundertwasser (1928–2000): “Those who do not honour the past lose the future. Those who destroy their roots cannot grow.”

This spirit of openness and public connection also defined the week’s city-wide engagement. FCC Week 2025 extended well beyond the conference venue, turning Vienna into a vibrant hub for public science outreach. In particular, the “Big Science, Big Impact” session – co-organised with the Austrian Federal Economic Chamber (WKO) – highlighted CERN’s broader role in economic development. Daniel Pawel Zawarczynski (WKO) shared examples of small and medium enterprise growth and technology transfer, noting that CERN participation can open new markets, from tunnelling to aerospace. Economist Gabriel Felbermayr referred to a recent WIFO analysis indicating a benefit-to-cost ratio for the FCC greater than 1.2 under conservative assumptions. The FCC is not only a tool for discovery, observed Johannes Gutleber (CERN), but also a platform enabling technology development, open software innovation and workforce training.

The FCC awards celebrate the creativity, rigour and passion that early-career researchers bring to the programme. This year, Tsz Hong Kwok (University of Zürich) and Audrey Piccini (CERN) won poster prizes, Sara Aumiller (TU München) and Elaf Musa (DESY) received innovation awards, and Ivan Karpov (CERN) and Nicolas Vallis (PSI) were honoured with paper prizes sponsored by Physical Review Accelerators and Beams. As CERN Council President Costas Fountas reminded participants, the FCC is not only about pushing the frontiers of knowledge, but also about enabling a new generation of ideas, collaborations and societal progress.

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The 31st Quark Matter conference took place from 6 to 12 April at Goethe University in Frankfurt, Germany. This edition of the world’s flagship conference for ultra-relativistic heavy-ion physics was the best attended in the series’ history, with more than 1000 participants.

A host of experimental measurements and theoretical calculations targeted fundamental questions in many-body QCD. These included the search for a critical point along the QCD phase diagram, the extraction of the properties of the deconfined quark–gluon plasma (QGP) medium created in heavy-ion collisions, and the search for signatures of the formation of this deconfined medium in smaller collision systems.

Probing thermalisation

New results highlighted the ability of the strong force to thermalise the out-of-equilibrium QCD matter produced during the collisions. Thermalisation can be probed by taking advantage of spatial anisotropies in the initial collision geometry which, due to the rapid onset of strong interactions at early times, result in pressure gradients across the system. These pressure gradients in turn translate into a momentum-space anisotropy of produced particles in the bulk, which can be experimentally measured by taking a Fourier transform of the azimuthal distribution of final-state particles with respect to a reference event axis.

An area of active experimental and theoretical interest is to quantify the degree to which heavy quarks, such as charm and beauty, participate in this collective behaviour, which informs on the diffusion properties of the medium. The ALICE collaboration presented the first measurement of the second-order coefficient of the momentum anisotropy of charm baryons in Pb–Pb collisions, showing significant collective behaviour and suggesting that charm quarks undergo some degree of thermalisation. This collective behaviour appears to be stronger in charm baryons than charm mesons, following similar observations for light flavour.

A host of measurements and calculations targeted fundamental questions in many-body QCD

Due to the nature of thermalisation and the long hydrodynamic phase of the medium in Pb–Pb collisions, signatures of the microscopic dynamics giving rise to the thermalisation are often washed out in bulk observables. However, local excitations of the hydrodynamic medium, caused by the propagation of a high-energy jet through the QGP, can offer a window into such dynamics. Due to coupling to the coloured medium, the jet loses energy to the QGP, which in turn re-excites the thermalised medium. These excited states quickly decay and dissipate, and the local perturbation can partially thermalise. This results in a correlated response of the medium in the direction of the propagating jet, the distribution of which allows measurement of the thermalisation properties of the medium in a more controlled manner.

In this direction, the CMS collaboration presented the first measurement of an event-wise two-point energy–energy correlator, for events containing a Z boson, in both pp and Pb–Pb collisions. The two-point correlator represents the energy-weighted cross section of the angle between particle pairs in the event and can separate out QCD effects at different scales, as these populate different regions in angular phase space. In particular, the correlated response of the medium is expected to appear at large angles in the correlator in Pb–Pb collisions.

The use of a colourless Z boson, which does not interact in the QGP, allows CMS to compare events with similar initial virtuality scales in pp and Pb–Pb collisions, without incurring biases due to energy loss in the QCD probes. The collaboration showed modifications in the two-point correlator at large angles, from pp to Pb–Pb collisions, alluding to a possible signature of the correlated response of the medium to the traversing jets. Such measurements can help guide models into capturing the relevant physical processes underpinning the diffusion of colour information in the medium.

Looking to the future

The next addition of this conference series will take place in 2027 in Jeju, South Korea, and the new results presented there should notably contain the latest complement of results from the upgraded Run 3 detectors at the LHC and the newly commissioned sPHENIX detector at RHIC. New collision systems like O–O at the LHC will help shed light on many of the properties of the QGP, including its thermalisation, by varying the lifetime of the pre-equilibrium and hydrodynamic phases in the collision evolution.

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On 16 January, physicists and statisticians met in the CERN Council Chamber to celebrate 25 years of the PhyStat series of conferences, workshops and seminars, which bring together physicists, statisticians and scientists from related fields to discuss, develop and disseminate methods for statistical data analysis and machine learning.

The special symposium heard from the founder and primary organiser of the PhyStat series Louis Lyons (Imperial College London and University of Oxford), who together with Fred James and Yves Perrin initiated the movement with the “Workshop on Confidence Limits” in January 2000. According to Lyons, the series was to bring together physicists and statisticians, a philosophy that has been followed and extended throughout the 22 PhyStat workshops and conferences, as well as numerous seminars and “informal reviews”. Speakers called attention to recognition from the Royal Statistical Society’s pictorial timeline of statistics, starting with the use of averages by Hippias of Elis in 450 BC and culminating with the 2012 discovery of the Higgs boson with 5σ significance.

Lyons and Bob Cousins (UCLA) offered their views on the evolution of statistical practice in high-energy physics, starting in the 1960s bubble-chamber era, strongly influenced by the 1971 book Statistical Methods in Experimental Physics by W T Eadie et al., its 2006 second edition by symposium participant Fred James (CERN), as well as Statistics for Nuclear and Particle Physics (1985) by Louis Lyons – reportedly the most stolen book from the CERN library. Both Lyons and Cousins noted the interest of the PhyStat community not only in practical solutions to concrete problems but also in foundational questions in statistics, with the focus on frequentist methods setting high-energy physics somewhat apart from the Bayesian approach more widely used in astrophysics.

Giving his view of the PhyStat era, ATLAS physicist and director of the University of Wisconsin Data Science Institute Kyle Cranmer emphasised the enormous impact that PhyStat has had on the field, noting important milestones such as the ability to publish full likelihood models through the statistical package RooStats, the treatment of systematic uncertainties with profile-likelihood ratio analyses, methods for combining analyses, and the reuse of published analyses to place constraints on new physics models. In regards to the next 25 years, Cranmer predicted the increasing use of methods that have emerged from PhyStat, such as simulation-based inference, and pointed out that artificial intelligence (the elephant in the room) could drastically alter how we use statistics.

Statistician Mikael Kuusela (CMU) noted that Phystat workshops have provided important two-way communication between the physics and statistics communities, citing simulation-based inference as an example where many key ideas were first developed in physics and later adopted by statisticians. In his view, the use of statistics in particle physics has emerged as “phystatistics”, a proper subfield with distinct problems and methods.

Another important feature of the PhyStat movement has been to encourage active participation and leadership by younger members of the community.  With its 25th anniversary, the torch is now passed from Louis Lyons to Olaf Behnke (DESY), Lydia Brenner (NIKHEF) and a younger team, who will guide Phystat into the next 25 years and beyond.

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Thirty-two students attended the first DRD1 Gaseous Detectors School at CERN last November. The EP-DT Gas Detectors Development (GDD) lab hosted academic lectures and varied hands-on laboratory exercises. Students assembled their own detectors, learnt about their operating characteristics and explored radiation-imaging methods with state-of-the-art readout approaches – all under the instruction of more than 40 distinguished lecturers and tutors, including renowned scientists, pioneers of innovative technologies and emerging experts.

DRD1 is a new worldwide collaborative framework of more than 170 institutes focused on R&D for gaseous detectors. The collaboration focuses on knowledge sharing and scientific exchange, in addition to the development of novel gaseous detector technologies to address the needs of future experiments. This instrumentation school, initiated in DRD1’s first year, marks the start of a series of regular training events for young researchers that will also serve to exchange ideas between research groups and encourage collaboration.

The school will take place annually, with future editions hosted at different DRD1 member institutes to reach students from a number of regions and communities.

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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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Everything started a century ago, when scientists like Niels Bohr, Max Planck and Wolfgang Pauli, but also Albert Einstein, Erwin Schrödinger and many others, came up with ideas that would revolutionise our description of the subatomic world. This is when physics transitioned from being a deterministic discipline to a mostly probabilistic one, at least when we look at subatomic scales. Brave predictions of weird behaviours started to attract the attention of an increasingly larger part of the scientific community, and continued to appear decade after decade. The most popular ones being: particle entanglement, the superposition of states and the tunnelling effect. These are also some of the most impactful quantum effects, in terms of the technologies that emerged from them.

One hundred years on, and the scientific community is somewhat acclimatised to observing and measuring the probabilistic nature of particles and quanta. Lasers, MRI and even sliding doors would not exist without the pioneering studies on quantum mechanics. However, it’s common opinion that today we are on the edge of a second quantum revolution.

“International years” are proclaimed to raise awareness, focus global attention, encourage cooperation and mobilise resources towards a certain topic or research domain. The International Year of Quantum also aims to reverse-engineer the approach taken with artificial intelligence (AI), a technology that came along faster than any attempt to educate and prepare the layperson for its adoption. As we know, this is creating a lot of scepticism towards AI, which is often felt to be too complex and designed to generate a loss of control in its users.

The second quantum revolution has begun and we are at the dawn of future powerful applications

The second quantum revolution has begun in recent years and, while we are rapidly moving from simply using the properties of the quantum world to controlling individual quantum systems, we are still at the dawn of future powerful applications. Some quantum sensors are already being used, and quantum cryptography is quite well understood. However, quantum bits need further studies and the exploration of other quantum fields has not even started yet.

Unlike AI, we still have time to push for a more inclusive approach to the development of new technology. During the international year, hundreds of events, workshops and initiatives will emphasise the role of global collaboration in the development of accessible quantum technologies. Through initiatives like the Quantum Technology Initiative (QTI) and the Open Quantum Institute (OQI), CERN is actively contributing not only to scientific research but also to promoting the advancement of its applications for the benefit of society.

The IYQ inaugural event was organised at UNESCO Headquarters in Paris in February 2025. At CERN, this year’s public event season is devoted to the quantum year, and will present talks, performances, a film festival and more. The full programme is available at visit.cern/events.

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The most recent edition attracted more than 100 participants to the historic Hotel Shanker in Kathmandu, Nepal, from 9 to 13 December 2024. The conference aimed to facilitate interactions between researchers from BCVSPIN countries and the broader international community, covering topics such as collider physics, cosmology, gravitational waves, dark matter, neutrino physics, particle astrophysics, physics beyond the Standard Model and machine learning. Participants ranged from renowned professors from across the globe to aspiring students.

Speaking of aspiring students, the main event was preceded by the BCVSPIN-2024 Masterclass in Particle Physics and Workshop in Machine Learning, hosted at Tribhuvan University from 4 to 6 December. The workshop provided 34 undergraduate and graduate students from around Nepal with a comprehensive introduction to particle physics, high-energy physics (HEP) experiments and machine learning. In addition to lectures, the workshop engaged students in hands-on sessions, allowing them to experience real research by exploring core concepts and applying machine-learning techniques to data from the ATLAS experiment. The students’ enthusiasm was palpable as they delved into the intricacies of particle physics and machine learning. The interactive sessions were particularly engaging, with students eagerly participating in discussions and practical exercises. Highlights included a special talk on artificial intelligence (AI) and a career development session focused on crafting CVs, applications and research statements. These sessions ensured participants were equipped with both academic insights and practical guidance. The impact on students was profound, as they gained valuable skills and networking opportunities, preparing them for future careers in HEP.

The BCVSPIN conference officially started the following Monday. In the spirit of BCVSPIN, the first plenary session featured an insightful talk on the status and prospects of HEP in Nepal, providing valuable insights for both locals and newcomers to the initiative. Then, the latest and the near-future physics highlights of experiments such as ATLAS, ALICE, CMS, as well as Belle, DUNE and IceCube, were showcased. From physics performance such as ATLAS nailing b-tagging with graph neural networks, to the most elaborate mass measurement of the W boson mass by CMS, not to mention ProtoDUNE’s runs exceeding expectations, the audience were offered comprehensive reviews of the recent breakthroughs on the experimental side. The younger physicists willing to continue or start hardware efforts surely appreciated the overview and schedule of the different upgrade programmes. The theory talks covered, among others, dark-matter models, our dear friend the neutrino and the interactions between the two. A special talk on AI invited the audience to reflect on what AI really is and how – in the midst of the ongoing revolution – it impacts the fields of physics and physicists themselves. Overviews of long-term future endeavours such as the Electron–Ion Collider and the Future Circular Collider concluded the programme.

BCVSPIN offers younger scientists precious connections with physicists from the international community

A special highlight of the conference was a public lecture “Oscillating Neutrinos” by the 2015 Nobel Laureate Takaaki Kajita. The event was held near the historical landmark of Patan Durbar Square, in the packed auditorium of the Rato Bangala School. This centre of excellence is known for its innovative teaching methods and quality instruction. More than half the room was filled with excited students from schools and universities, eager to listen to the keynote speaker. After a very pedagogical introduction explaining the “problem of solar neutrinos”, Kajita shared his insights on the discovery of neutrino oscillations and its implications for our understanding of the universe. His presentation included historical photographs of the experiments in Kamioka, Japan, as well as his participation at BCVSPIN in 1994. After encouraging the students to become scientists and answering as many questions as time allowed, he was swept up in a crowd of passionate Nepali youth, thrilled to be in the presence of such a renowned physicist.

The BCVSPIN initiative has changed the landscape of HEP in South and Southeast Asia. With participation made affordable for students, it is a stepping stone for the younger generation of scientists, offering them precious connections with physicists from the international community.

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The workshop opened with a review of accelerator operations, supported by input from December’s Joint Accelerator Performance Workshop. Maintaining current performance levels requires an extraordinary effort across all the facilities. Performance data from the ongoing Run 3 shows steady improvements in availability and beam delivery. These results are driven by dedicated efforts from system experts, operations teams and accelerator physicists, all working to ensure excellent performance and high availability across the complex.

Electron clouds parting

Attention is now turning to Run 4 and the High-Luminosity LHC (HL-LHC) era. Several challenges have been identified, including the demand for high-intensity beams, radiofrequency (RF) power limitations and electron-cloud effects. In the latter case, synchrotron-radiation photons strike the beam-pipe walls, releasing electrons which are then accelerated by proton bunches, triggering a cascading electron-cloud buildup. Measures to address these issues will be implemented during Long Shutdown 3 (LS3), ensuring CERN’s accelerators continue to meet the demands of its diverse physics community.

LS3 will be a crucial period for CERN. In addition to the deployment of the HL-LHC and major upgrades to the ATLAS and CMS experiments, it will see a widespread programme of consolidation, maintenance and improvements across the accelerator complex to secure future exploitation over the coming decades.

Progress on the HL-LHC upgrade was reviewed in detail, with a focus on key systems – magnets, cryogenics and beam instrumentation – and on the construction of critical components such as crab cavities. The next two years will be decisive, with significant system testing scheduled to ensure that these technologies meet ambitious performance targets.

Planning for LS3 is already well advan­ced. Coordination between all stakeholders has been key to aligning complex interdependencies, and the experienced teams are making strong progress in shaping a resource-loaded plan. The scale of LS3 will require meticulous coordination, but it also represents a unique opportunity to build a more robust and adaptable accelerator complex for the future. Looking beyond LS3, CERN’s unique accelerator complex is well positioned to support an increasingly diverse physics programme. This diversity is one of CERN’s greatest strengths, offering complementary opportunities across a wide range of fields.

The high demand for beam time at ISOLDE, n_TOF, AD-ELENA and the North and East Areas underscores the need for a well-balanced approach that supports a broad range of physics. The discussions highlighted the importance of balancing these demands while ensuring that the full potential of the accelerator complex is realised.

Future opportunities such as those highlighted by the Physics Beyond Colliders study will be shaped by discussions being held as part of the update of the European Strategy for Particle Physics (ESPP). Defining the next generation of physics programmes entails striking a careful balance between continuity and innovation, and the accelerator community will play a central role in setting the priorities.

A forward-looking session at the workshop focused on the Future Circular Collider (FCC) Feasibility Study and the next steps. The physics case was presented alongside updates on territorial implementation and civil-engineering investigations and plans. How the FCC-ee injector complex would fit into the broader strategic picture was examined in detail, along with the goals and deliverables of the pre-technical design report (pre-TDR) phase that is planned to follow the Feasibility Study’s conclusion.

While the FCC remains a central focus, other future projects were also discussed in the context of the ESPP update. These include mature linear-collider proposals, the potential of a muon collider and plasma wakefield acceleration. Development of key technologies, such as high-field magnets and superconducting RF systems, will underpin the realisation of future accelerator-based facilities.

The next steps – preparing for Run 4, implementing the LS3 upgrade programmes and laying the groundwork for future projects – are ambitious but essential. CERN’s future will be shaped by how well we seize these opportunities.

The shared expertise and dedication of CERN’s personnel, combined with a clear strategic vision, provide a solid foundation for success. The path ahead is challenging, but with careful planning, collaboration and innovation, CERN’s accelerator complex will remain at the heart of discovery for decades to come.

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Triggering systems play a crucial role in filtering the vast amounts of data generated by modern collider experiments. A good trigger design selects features in the event sample that greatly enrich the proportion of the desired physics processes in the recorded data. The key considerations are timing and selectivity. Timing has long been at the core of experiment design – detectors must capture data at the appropriate time to record an event. Selectivity has been a feature of triggering for almost as long. Recording an event makes demands on running time and data-acquisition bandwidth, both of which are limited.

Evolving architecture

Thanks to detector upgrades and major changes in the cost and availability of fast data links and storage, the past 10 years have seen an evolution in LHC triggers away from hardware-based decisions using coarse-grain information.

Detector upgrades mean higher granularity and better time resolution, improving the precision of the trigger algorithms and the ability to resolve the problem of having multiple events in a single LHC bunch crossing (“pileup”). Such upgrades allow more precise initial-level hardware triggering, bringing the event rate down to a level where events can be reconstructed for further selection via high-level trigger (HLT) systems.

To take advantage of modern computer architecture more fully, HLTs use both graphics processing units (GPUs) and central processing units (CPUs) to process events. In ALICE and LHCb this leads to essentially triggerless access to all events, while in ATLAS and CMS hardware selections are still important. All HLTs now use machine learning (ML) algorithms, with the ATLAS and CMS experiments even considering their use at the first hardware level.

ATLAS and CMS are primarily designed to search for new physics. At the end of Run 3, upgrades to both experiments will significantly enhance granularity and time resolution to handle the high-luminosity environment of the HL-LHC, which will deliver up to 200 interactions per LHC bunch crossing. Both experiments achieved efficient triggering in Run 3, but higher luminosities, difficult-to-distinguish physics signatures, upgraded detectors and increasingly ambitious physics goals call for advanced new techniques. The step change will be significant. At HL-LHC, the first-level hardware trigger rate will increase from the current 100 kHz to 1 MHz in ATLAS and 760 kHz in CMS. The price to pay is increasing the latency – the time delay between input and output – to 10 µsec in ATLAS and 12.5 µsec in CMS.

The proposed trigger systems for ATLAS and CMS are predominantly FPGA-based, employing highly parallelised processing to crunch huge data streams efficiently in real time. Both will be two-level triggers: a hardware trigger followed by a software-based HLT. The ATLAS hardware trigger will utilise full-granularity calorimeter and muon signals in the global-trigger-event processor, using advanced ML techniques for real-time event selection. In addition to calorimeter and muon data, CMS will introduce a global track trigger, enabling real-time tracking at the first trigger level. All information will be integrated within the global-correlator trigger, which will extensively utilise ML to enhance event selection and background suppression.

Substantial upgrades

The other two big LHC experiments already implemented substantial trigger upgrades at the beginning of Run 3. The ALICE experiment is dedicated to studying the strong interactions of the quark–gluon plasma – a state of matter in which quarks and gluons are not confined in hadrons. The detector was upgraded significantly for Run 3, including the trigger and data-acquisition systems. The ALICE continuous readout can cope with 50 kHz for lead ion–lead ion (PbPb) collisions and several MHz for proton–proton (pp) collisions. In PbPb collisions the full data is continuously recorded and stored for offline analysis, while for pp collisions the data is filtered.

Unlike in Run 2, where the hardware trigger reduced the data rate to several kHz, Run 3 uses an online software trigger that is a natural part of the common online–offline computing framework. The raw data from detectors is streamed continuously and processed in real time using high-performance FPGAs and GPUs. ML plays a crucial role in the heavy-flavour software trigger, which is one of the main physics interests. Boosted decision trees are used to identify displaced vertices from heavy quark decays. The full chain from saving raw data in a 100 PB buffer to selecting events of interest and removing the original raw data takes about three weeks and was fully employed last year.

The third edition of TDHEP suggests that innovation in this field is only set to accelerate

The LHCb experiment focuses on precision measurements in heavy-flavour physics. A typical example is measuring the probability of a particle decaying into a certain decay channel. In Run 2 the hardware trigger tended to saturate in many hadronic channels when the luminosity was instantaneously increased. To solve this issue for Run 3 a high-level software trigger was developed that can handle 30 MHz event readout with 4 TB/s data flow. A GPU-based partial event reconstruction and primary selection of displaced tracks and vertices (HLT1) reduces the output data rate to 1 MHz. The calibration and detector alignment (embedded into the trigger system) are calculated during data taking just after HLT1 and feed full-event reconstruction (HLT2), which reduces the output rate to 20 kHz. This represents 10 GB/s written to disk for later analysis.

Away from the LHC, trigger requirements differ considerably. Contributions from other areas covered heavy-ion physics at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC), fixed-target physics at CERN and future experiments at the Facility for Antiproton and Ion Research at GSI Darmstadt and Brookhaven’s Electron–Ion Collider (EIC). NA62 at CERN and STAR at RHIC both use conventional trigger strategies to arrive at their final event samples. The forthcoming CBM experiment at FAIR and the ePIC experiment at the EIC deal with high intensities but aim for “triggerless” operation.

Requirements were reported to be even more diverse in astroparticle physics. The Pierre Auger Observatory combines local and global trigger decisions at three levels to manage the problem of trigger distribution and data collection over 3000 km² of fluorescence and Cherenkov detectors.

These diverse requirements will lead to new approaches being taken, and evolution as the experiments are finalised. The third edition of TDHEP suggests that innovation in this field is only set to accelerate.

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The four main LHC experiments played a prominent role at the conference, presenting a large set of newly published results from studies performed on data collected during LHC Run 2, as well as several new preliminary results performed on the new data samples from Run 3.

Jet modifications

A number of significant results on the modification of jets in heavy-ion collisions were presented. Splitting functions characterising the evolution of parton showers are expected to be modified in the presence of the QGP, providing experimental access to the medium properties. A more differential look at these modifications was presented through a correlated measurement of the shared momentum fraction and opening angle of the first splitting satisfying the “soft drop” condition in jets. Additionally, energy–energy correlators have recently emerged as promising observables where the properties of jet modification in the medium might be imprinted at different scales on the observable.

The first measurements of the two-particle energy–energy correlators in p–Pb and Pb–Pb collisions were presented, showing modifications in both the small- and large-angle correlations for both systems compared to pp collisions. A long-sought after effect of energy exchanges between the jet and the medium is a correlated response of the medium in the jet direction. For the first time, measurements of hadron–boson correlations in events containing photons or Z bosons showed a clear depletion of the bulk medium in the direction of the Z boson, providing direct evidence of a medium response correlated to the propagating back-to-back jet. In pp collisions, the first direct measurement of the dead cone of beauty quarks, using novel machine-learning methods to reconstruct the beauty hadron from partial decay information, was also shown.

Several new results from studies of particle production in ultraperipheral heavy-ion collisions were discussed. These studies allow us to investigate the possible onset of gluon saturation at low Bjorken-x values. In this context, new results of charm photoproduction, with measurements of incoherent and coherent J/ψ mesons, as well as of D⁰ mesons, were released. Photonuclear production cross-sections of di-jets, covering a large interval of photon energies to scan over different regions of Bjorken-x, were also presented. These measurements pave the way for setting constraints on the gluon component of nuclear parton distribution functions at low Bjorken-x values, over a wide Q² range, in the absence of significant final-state effects.

New experiments will explore higher-density regions of the QCD–matter phase diagram

During the last few years, a significant enhancement of charm and beauty-baryon production in proton–proton collisions was observed, compared to measurements in e⁺e^– and ep collisions. These observations have challenged the assumption of the universality of heavy-quark fragmentation across different collision systems. Several intriguing measurements on this topic were released at the conference. In addition to an extended set of charm meson-to-meson and baryon-to-meson production yield ratios, the first measurements of the production of Σ_c^0,++(2520) relative to Σ_c^0,++(2455) at the LHC, obtained exploiting the new Run 3 data samples, were discussed. New insights on the structure of the exotic χ_c1(3872) state and its hadronisation mechanism were garnered by measuring the ratio of its production yield to that of ψ(2S) mesons in hadronic collisions.

Additionally, strange-to-non-strange production-yield ratios for charm and beauty mesons as a function of the collision multiplicity were released, pointing toward an enhanced strangeness production in a higher colour-density environment. Several theoretical approaches implementing modified hadronisation mechanisms with respect to in-vacuum fragmentation have proven to be able to reproduce at least part of the measurements, but a comprehensive description of the heavy-quark hadronisation, in particular for the baryonic sector, is still to be reached.

A glimpse into the future of the experimental opportunities in this field was also provided. A new and intriguing set of physics observables for a complete characterisation of the QGP with hard probes will become accessible with the planned upgrades of the ALICE, ATLAS, CMS and LHCb detectors, both during the next long LHC shutdown and in the more distant future. New experiments at CERN, such as NA60+, or in other facilities like the Electron–Ion Collider in the US and J-PARC-HI in Japan, will explore higher-density regions of the QCD–matter phase diagram.

The next edition of this conference series is scheduled to be held in Nashville, US, from 1 to 5 June 2026.

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The International Muon Collider Collaboration (IMCC), supported by the EU MuCol study, is working to assess the potential of a muon collider as a future facility, along with the R&D needed to make it a reality. European engagement in this effort crystalised following the 2020 update to the European Strategy for Particle Physics (ESPPU), which identified the development of bright muon beams as a high-priority initiative. Worldwide interest in a muon collider is quickly growing: the 2023 Particle Physics Project Prioritization Panel (P5) recently identified it as an important future possibility for the US particle-physics community; Japanese colleagues have proposed a muon-collider concept, muTRISTAN (CERN Courier July/August 2024 p8); and Chinese colleagues have actively contributed to IMCC efforts as collaboration members.

Lighting the way

The workshop focused on reviewing the scope and design progress of a muon-cooling demonstrator facility, identifying potential host sites and timelines, and exploring science programmes that could be developed alongside it. Diktys Stratakis (Fermilab) began by reviewing the requirements and challenges of muon cooling. Delivering a high-brightness muon beam will be essential to achieving the luminosity needed for a muon collider. The technique proposed for this is ionisation cooling, wherein the phase-space volume of the muon beam decreases as it traverses a sequence of cells, each containing an energy- absorbing mat­erial and accelerating radiofrequency (RF) cavities.

Roberto Losito (CERN) called for a careful balance between ambition and practicality – the programme must be executed in a timely way if a muon collider is to be a viable next-generation facility. The Muon Cooling Demonstrator programme was conceived to prove that this technology can be developed, built and reliably operated. This is a critical step for any muon-collider programme, as highlighted in the ESPPU–LDG Accelerator R&D Roadmap published in 2022. The plan is to pursue a staged approach, starting with the development of the magnet, RF and absorber technology, and demonstrating the robust operation of high-gradient RF cavities in high magnetic fields. The components will then be integrated into a prototype cooling cell. The programme will conclude with a demonstration of the operation of a multi-cell cooling system with a beam, building on the cooling proof of principle made by the Muon Ionisation Cooling Experiment.

Chris Rogers (STFC RAL) summarised an emerging consensus that it is critical to demonstrate the reliable operation of a cooling lattice formed of multiple cells. While the technological complexity of the cooling-cell prototype will undergo further review, the preliminary choice presents a moderately challenging performance that could be achieved within five to seven years with reasonable investment. The target cooling performance of a whole cooling lattice remains to be established and depends on future funding levels. However, delegates agreed that a timely demonstration is more important than an ambitious cooling target.

Worldwide interest in a muon collider is quickly growing

The workshop also provided an opportunity to assess progress in designing the cooling-cell prototype. Given that the muon beam originates from hadron decays and is initially the size of a watermelon, solenoid magnets were chosen as they can contain large beams in a compact lattice and provide focusing in both horizontal and vertical planes simultaneously. Marco Statera (INFN LASA) presented preliminary solutions for the solenoid coil configuration based on high-temperature superconductors operating at 20 K: the challenge is to deliver the target magnetic field profile given axial forces, coil stresses and compact integration.

In ionisation cooling, low-Z absorbers are used to reduce the transverse momenta of the muons while keeping the multiple scattering at manageable levels. Candidate materials are lithium hydride and liquid hydrogen. Chris Rogers discussed the need to test absorbers and containment windows at the highest intensities. The potential for performance tests using muons or intensity tests using another particle species such as protons was considered to verify understanding of the collective interaction between the beam and the absorber. RF cavities are required to replace longitudinal energy lost in the absorbers.  Dario Giove (INFN LASA) introduced the prototype of an RF structure based on three coupled 704 MHz cavities and presented a proposal to use existing INFN capabilities to carry out a test programme for materials and cavities in magnetic fields. The use of cavity windows was also discussed, as it would enable greater accelerating gradients, though at the cost of beam degradation, increased thermal loads and possible cavity detuning. The first steps in integ­rating these latest hardware designs into a compact cooling cell were presented by Lucio Rossi (INFN LASA and UMIL). Future work needs to address the management of the axial forces and cryogenic heat loads, Rossi observed.

Many institutes presented a strong interest in contributing to the programme, both in the hardware R&D and hosting the eventual demonstrator. The final sessions of the workshop focused on potential host laboratories.

The event underscored the critical need for sustained innovation, timely implementation and global cooperation

At CERN, two potential sites were discussed, with ongoing studies focusing on the TT7 tunnel, where a moderate-power 10 kW proton beam from the Proton Synchrotron could be used for muon production. Preliminary beam physics studies of muon beam production and transport are already underway. Lukasz Krzempek (CERN) and Paul Jurj (Imperial College London) presented the first integration and beam-physics studies of the demonstrator facility in the TT7 tunnel, highlighting civil engineering and beamline design requirements, logistical challenges and safety considerations, finding no apparent showstoppers.

Jeff Eldred (Fermilab) gave an overview of Fermilab’s broad range of candidate sites and proton-beam energies. While further feasibility studies are required, Eldred highlighted that using 8 GeV protons from the Booster is an attractive option due to the favourable existing infrastructure and its alignment with Fermilab’s muon-collider scenario, which envisions a proton driver based on the same Booster proton energy.

The Fermilab workshop represented a significant milestone in advancing the Muon Cooling Demonstrator, highlighting enthusiasm from the US community to join forces with the IMCC and growing interest in Asia. As Mark Palmer (BNL) observed in his closing remarks, the event underscored the critical need for sustained innovation, timely implementation and global cooperation to make the muon collider a reality.

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The portrait of the Higgs boson painted by experimental data is becoming more and more precise. Many new Run 2 and first Run 3 results have developed the picture this year. Highlights included the latest di-Higgs combinations with cross-section upper limits reaching down to 2.5 times the Standard Model (SM) expectations. A few excesses seen in various analyses were also discussed. The CMS collaboration reported a brand new excess of top–antitop events near the top–antitop production threshold, with a local significance of more than 5σ above the background described by perturbative quantum chromodynamics (QCD) only, that could be due to a pseudoscalar top–antitop bound state. A new W-boson mass measurement by the CMS collaboration – a subject deeply connected to electroweak symmetry breaking – was also presented, reporting a value consistent with the SM prediction with a very accurate precision of 9.9 MeV (CERN Courier November/December 2024 p7).

Parton shower event generators were in the spotlight. Historical talks by Torbjörn Sjöstrand (Lund University) and Bryan Webber (University of Cambridge) described the evolution of the PYTHIA and HERWIG generators, the crucial role they played in the discovery of the Higgs boson, and the role they now play in the LHC’s physics programme. Differences in the modelling of the parton–shower systematics by the ATLAS and CMS collaborations led to lively discussions!

The vision talk was given by Lance Dixon (SLAC) about the reconstruction of scattering amplitudes directly from analytic properties, as a complementary approach to Lagrangians and Feynman diagrams. Oliver Bruning (CERN) conveyed the message that the HL-LHC accelerator project is well on track, and Patricia McBride (Fermilab) reached a similar conclusion regarding ATLAS and CMS’s Phase-2 upgrades, enjoining new and young people to join the effort, to ensure they are ready and commissioned for the start of Run 4.

The next Higgs Hunting workshop will be held in Orsay and Paris from 15 to 17 July 2025, following EPS-HEP in Marseille from 7 to 11 July.

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IUPAP was established in 1922 in the aftermath of World War I to coordinate international efforts in physics. Its logo is recognisable from conferences and proceedings, but its mission is less widely understood. IUPAP is the only worldwide organisation dedicated to the advancement of all fields of physics. Its goals include promoting global development and cooperation in physics by sponsoring international meetings; strengthening physics education, especially in developing countries; increasing diversity and inclusion in physics; advancing the participation and recognition of women and of people from under-represented groups; enhancing the visibility of early-career talents; and promoting international agreements on symbols, units, nomenclature and standards. At the 33rd assembly, 300 physicists were elected to the executive council and specialised commissions for a period of three years.

Global scientific initiatives were highlighted, including the International Year of Quantum Science and Technology (IYQ2025) and the International Decade on Science for Sustainable Development (IDSSD) from 2024 to 2033, which was adopted by the United Nations General Assembly in August 2023. A key session addressed the importance of industry partnerships, with delegates exploring strategies to engage companies in IYQ2025 and IDSSD to further IUPAP’s mission of using physics to drive societal progress. Nobel laureate Giorgio Parisi discussed the role of physics in promoting a sustainable future, and public lectures by fellow laureates Barry Barish, Takaaki Kajita and Samuel Ting filled the 1820-seat Oriental Universal Theater with enthusiastic students.

A key focus of the meeting was visa-related issues affecting international conferences. Delegates reaffirmed the union’s commitment to scientists’ freedom of movement. IUPAP stands against any discrimination in physics and will continue to sponsor events only in locations that uphold this value – a stance that is orthogonal to the policy of countries imposing sanctions on scientists affiliated with specific institutions.

A joint session with the fall meeting of the Chinese Physical Society celebrated the 25th anniversary of the IUPAP working group “Women in Physics” and emphasised diversity, equity and inclusion in the field. Since 2002, IUPAP has established precise guidelines for the sponsorship of conferences to ensure that women are fairly represented among participants, speakers and committee members, and has actively monitored the data ever since. This has contributed to a significant change in the participation of women in IUPAP-sponsored conferences. IUPAP is now building on this still-necessary work on gender by focusing on discrimination on the grounds of disability and ethnicity.

The closing ceremony brought together the themes of continuity and change. Incoming president Silvina Ponce Dawson (University of Buenos Aires) and president-designate Sunil Gupta (Tata Institute) outlined their joint commitment to maintaining an open dialogue among all physicists in an increasingly fragmented world, and to promoting physics as an essential tool for development and sustainability. Outgoing leaders Michel Spiro (CNRS) and Bruce McKellar (University of Melbourne) were honoured for their contributions, and the ceremonial handover symbolised a smooth transition of leadership.

As the general assembly concluded, there was a palpable sense of momentum. From strategic modernisation to deeper engagement with global issues, IUPAP is well-positioned to make physics more relevant and accessible. The resounding message was one of unity and purpose: the physics community is dedicated to leveraging science for a brighter, more sustainable future.

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Out of more than 30 presentations, a significant number featured pulsed, high-peak-power RF sources working at frequencies above 3 GHz in the S, C and X bands. These involve high-efficiency klystrons that are being designed, built and tested for the KEK e^–/e⁺ Injector, the new EuPRAXIA@SPARC_LAB linac, the CLIC testing facilities, muon collider R&D, the CEPC injector linac and the C³ project. Reported increases in beam-to-RF power efficiency range from 15 percentage points for the retro­fit prototype for CLIC to more than 25 points (expected) for a new greenfield klystron design that can be used across most new projects.

A very dynamic area for R&D is the search of efficient sources for the continuous wave (CW) and long-pulse RF needed for circular accelerators. Typically working in the L-band, existing devices deliver less than 3 MW in peak power. Solid-state amplifiers, inductive output tubes, klystrons, magnetrons, triodes and exotic newly rediscovered vacuum tubes called “tristrons” compete in this arena. Successful prototypes have been built for the High-Luminosity LHC and CEPC with power efficiency gains of 10 to 20 points. In the case of the LHC, this will allow 15% more power without an impact on the electricity bill; in the case of a circular Higgs factory, this will allow a 30% reduction. CERN and SLAC are also investigating very-high-efficiency vacuum tubes for the Future Circular Collider with a potential reduction of close to 50% on the final electricity bill. A collaboration between academia and industry would certainly be required to bring this exciting new technology to light.

Besides the astounding advances in vacuum-tube technology, solid-state amplifiers based on cheap transistors are undergoing a major transformation thanks to the adoption of gallium-nitride technology. Commercial amplifiers are now capable of delivering kilowatts of power at low duty cycles with a power efficiency of 80%, while Uppsala University and the European Spallation Source have demonstrated the same efficiency for combined systems working in CW.

The search for energy efficiency does not stop at designing and building more efficient RF sources. All aspects of operation, power combination and using permanent magnets and efficient modulators need to be folded in, as described by many concrete examples during the workshop. The field is thriving.

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The committee heard extensive reports from the leading HEP laboratories and various world regions on their recent activities and plans, including a presentation by Paris Sphicas, the chair of the European Committee for Future Accelerators (ECFA), on the process for the update of the European strategy for particle physics (ESPP). Launched by CERN Council in March 2024, the ESPP update is charged with recommending the next collider project at CERN after HL-LHC operation.

A global task

The ESPP update is also of high interest to non-European institutions and projects. Consequently, in addition to the expected inputs to the strategy from European HEP communities, those from non-European HEP communities are also welcome. Moreover, the recent US P5 report and the Chinese plans for CEPC, with a potential positive decision in 2025/2026, and discussions about the ILC project in Japan, will be important elements of the work to be carried out in the context of the ESPP update. They also emphasise the global nature of high-energy physics.

An integral part of the work of ICFA is carried out within its panels, which have been very active. Presentations were given from the new panel on the Data Lifecycle (chair Kati Lassila-Perini, Helsinki), the Beam Dynamics panel (new chair Yuan He, IMPCAS) and the Advanced and Novel Accelerators panel (new chair Patric Muggli, Max Planck Munich, proxied at the meeting by Brigitte Cros, Paris-Saclay). The Instrumentation and Innovation Development panel (chair Ian Shipsey, Oxford) is setting an example with its numerous schools, the ICFA instrumentation awards and centrally sponsored instrumentation studentships for early-career researchers from underserved world regions. Finally, the chair of the ILC International Development Team panel (Tatsuya Nakada, EPFL) summarised the latest status of the ILC Technological Network, and the proposed ILC collider project in Japan.

ICFA noted interesting structural developments in the global organisation of HEP

A special session was devoted to the sustainability of HEP accelerator infrastructures, considering the need to invest efforts into guidelines that enable better comparison of the environmental reports of labs and infrastructures, in particular for future facilities. It was therefore natural for ICFA to also hear reports not only from the panel on Sustainable Accelerators and Colliders led by Thomas Roser (BNL), but also from the European Lab Directors Working Group on Sustainability. This group, chaired by Caterina Bloise (INFN) and Maxim Titov (CEA), is mandated to develop a set of key indicators and a methodology for the reporting on future HEP projects, to be delivered in time for the ESPP update.

Finally, ICFA noted some very interesting structural developments in the global organisation of HEP. In the Asia-Oceania region, ACFA-HEP was recently formed as a sub-panel under the Asian Committee for Future Accelerators (ACFA), aiming for a better coordination of HEP activities in this particular region of the world. Hopefully, this will encourage other world regions to organise themselves in a similar way in order to strengthen their voice in the global HEP community – for example in Latin America. Here, a meeting was organised in August by the Latin American Association for High Energy, Cosmology and Astroparticle Physics (LAA-HECAP) to bring together scientists, institutions and funding agencies from across Latin America to coordinate actions for jointly funding research projects across the continent.

The next in-person ICFA meeting will be held during the Lepton–Photon conference in Madison, Wisconsin (USA), in August 2025.

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Achieving a theoretical uncertainty of only a few per cent in the measurement of physical observables is a vastly challenging task in the complex environment of hadronic collisions. To keep pace with experimental observations at the LHC and elsewhere, precision computing has had to develop rapidly in recent years – efforts that have been monitored and driven by the biennial High Precision for Hard Processes (HP2) conference for almost two decades now. The latest edition attracted 120 participants to the University of Torino from 10 to 13 September 2024.

All speakers addressed the same basic question: how can we achieve the most precise theoretical description for a wide variety of scattering processes at colliders?

The recipe for precise prediction involves many ingredients, so the talks in Torino probed several research directions. Advanced methods for the calculation of scattering amplitudes were discussed, among others, by Stephen Jones (IPPP Durham). These methods can be applied to detailed high-order phenomenological calculations for QCD, electroweak processes and BSM physics, as illustrated by Ramona Groeber (Padua) and Eleni Vryonidou (Manchester). Progress in parton showers – a crucial tool to bridge amplitude calculations and experimental results – was presented by Silvia Ferrario Ravasio (CERN). Dedicated methods to deal with the delicate issue of infrared divergences in high-order cross-section calculations were reviewed by Chiara Signorile-Signorile (Max Planck Institute, Munich).

The Torino conference was dedicated to the memory of Stefano Catani, a towering figure in the field of high-energy physics, who suddenly passed away at the beginning of this year. Starting from the early 1980s, and for the whole of his career, Catani made groundbreaking contributions in every facet of HP2. He was an inspiration to a whole generation of physicists working in high-energy phenomenology. We remember him as a generous and kind person, and a scientist of great rigour and vision. He will be sorely missed.

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The meeting highlighted the urgent need to address stroke as a pressing health challenge in Europe. Each year, more than one million acute stroke cases occur in Europe, with nearly 10 million survivors facing long-term consequences. In 2017, the economic burden of stroke treatments was estimated to be €60 billion – a figure that continues to grow. UMBRELLA’s partners outlined their collective ambition to translate a vast and fragmented stroke data set into actionable care innovations through standardisation and integration.

UMBRELLA will utilise advanced digital technologies to develop AI-powered predictive models for stroke management. By standardising real-world stroke data and leveraging tools like imaging technologies, wearable devices and virtual rehabilitation platforms, UMBRELLA aims to refine every stage of care – from diagnosis to recovery. Based on post-stroke data, AI-driven insights will empower clinicians to uncover root causes of strokes, improve treatment precision and predict patient outcomes, reshaping how stroke care is delivered.

Central to this effort is the integration of CERN’s federated-learning platform, CAFEIN. A decentralised approach to training machine-learning algorithms without exchanging data, it was initiated thanks to seed funding from CERN’s knowledge transfer budget for the benefit of medical applications: now CAFEIN promises to enhance diagnosis, treatment and prevention strategies for stroke victims, ultimately saving countless lives. A main topic of the kickoff meeting was the development of the “U-platform” – a federated data ecosystem co-designed by Siemens Healthineers and CERN. Based on CAFEIN, the infrastructure will enable the secure and privacy preserving training of advanced AI algorithms for personalised stroke diagnostics, risk prediction and treatment decisions without sharing sensitive patient data between institutions. Building on CERN’s expertise, including its success in federated AI modelling for brain pathologies under the EU TRUST­roke project, the CAFEIN team is poised to handle the increasing complexity and scale of data sets required by UMBRELLA.

Beyond technological advancements, the UMBRELLA consortium discussed a plan to establish standardised protocols for acute stroke management, with an emphasis on integrating these protocols into European healthcare guidelines. By improving data collection and facilitating outcome predictions, these standards will particularly benefit patients in remote and underserved regions. The project also aims to advance research into the causes of strokes, a quarter of which remain undetermined – a statistic UMBRELLA seeks to change.

This ambitious initiative not only showcases CERN’s role in pioneering federated-learning technologies but also underscores the broader societal benefits brought by basic science. By pushing technologies beyond the state-of-the-art, CERN and other particle-physics laboratories have fuelled innovations that have an impact on our everyday lives. As UMBRELLA begins its journey, its success holds the potential to redefine stroke care, delivering life-saving advancements to millions and paving the way for a healthier, more equitable future.

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Changing flavour

The discussion on rare B decays continued in the session on flavour-changing neutral-currents. With new lattice-QCD results pinning down short-distance local hadronic contributions, the discussion focused on understanding the long-distance contributions arising from hadronic resonances and charm rescattering. Arianna Tinari (Zurich) and Martin Hoferichter (Bern) judged the latter not to be dramatic in magnitude. Lakshan Madhan (Cambridge) presented a new amplitude analysis in which the long and short-distance contributions are separated via the kinematic dependence of the decay amplitudes. New theo­retical analyses of the nonlocal form factors for B → K^(*)μ⁺μ^– and B → K^(*)e⁺e^– were representative of the workshop as a whole: truly the bee’s knees.

Another challenge to accurate theory predictions for rare decays, the widths of vector final states, snuck its way into the flavour-changing charged-currents session, where Luka Leskovec (Ljubljana) presented a comprehensive overview of lattice methods for decays to resonances. Leskovec’s optimistic outlook for semileptonic decays with two mesons in the final state stood in contrast to prospects for applying lattice methods to D-D mixing: such studies are currently limited to the SU(3)-flavour symmetric point of equal light-quark masses, explained Felix Erben (CERN), though he offered a glimmer of hope in the form of spectral reconstruction methods currently under development.

LHCb’s beauty and charm physics programme reported substantial progress. Novel techniques have been implemented in the most recent CP-violation studies, potentially leading to an impressive uncertainty of just 1° in future measurements of the CKM angle gamma. LHCb has recently placed a special emphasis on beauty and charm baryons, where the experiment offers unique capabilities to perform many interesting measurements ranging from CP violation to searches for very rare decays and their form factors. Going from three quarks to four and five, the spectroscopy session illustrated the rich and complex debate around tetraquark and pentaquark states with a big open discussion on the underlying structure of the 20 or so discovered at LHCb: which are bound states of quarks and which are simply meson molecules? (CERN Courier November/December 2024 p26 and p33.)

LHCb’s ability to do unique physics was further highlighted in the QCD, electroweak (EW) and exotica session, where the collaboration has shown the most recent publicly available measurement of the weak-mixing angle in conjunction with W/Z-boson production cross-sections and other EW observables. LHCb have put an emphasis on combined QCD + QED and effective-field-theory calculations, and the interplay between EW precision observables and new-physics effects in couplings to the third generation. By studying phase space inaccessible to any other experiment, a study of hypothetical dark photons decaying to electrons showed the LHCb experiment to be a unique environment for direct searches for long-lived and low-mass particles.

Attendees left the workshop with a fresh perspective

Parallel to Implications 2024, the inaugural LHCb Open Data and Ntuple Wizard Workshop, took place on 22 October as a satellite event, providing theorists and phenomenologists with a first look at a novel software application for on-demand access to custom ntuples from the experiment’s open data. The LHCb Ntupling Service will offer a step-by-step wizard for requesting custom ntuples and a dashboard to monitor the status of requests, communicate with the LHCb open data team and retrieve data. The beta version was released at the workshop in advance of the anticipated public release of the application in 2025, which promises open access to LHCb’s Run 2 dataset for the first time.

A recurring satellite event features lectures by theorists on topics following LHCb’s scientific output. This year, Simon Kuberski (CERN) and Saša Prelovšek (Ljubljana) took the audience on a guided tour through lattice QCD and spectroscopy.

With LHCb’s integrated luminosity in 2024 exceeding all previous years combined, excitement was heightened. Attendees left the workshop with a fresh perspective on how to approach the challenges faced by our community.

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EOSC’s sixth symposium attracted 450 delegates to Berlin from 21 to 23 October 2024, with a further 900 participating online. Since its launch in 2017, EOSC activities have focused on conceptualisation, prototyping and planning. In order to develop a trusted federation of research data and services for research and innovation, EOSC is being deployed as a network of nodes. With the launch during the symposium of the EOSC EU node, this year marked a transition from design to deployment.

While EOSC is a flagship science initiative of the European Commission, FAIR concerns researchers and stakeholders globally. Via the multiple projects under the wings of EOSC that collaborate with software and data institutes around the world, a pan-European effort can be made to ensure a research landscape that encourages knowledge sharing while recognising work and training the next generation in best practices in research. The EU node – funded by the European Commission, and the first to be implemented – will serve as a reference for roughly 10 additional nodes to be deployed in a first wave, with more to follow. They are accessible using any institutional credentials based on GÉANT’s MyAccess or with an EU login. A first operational implementation of the EOSC Federation is expected by the end of 2025.

A thematic focus of this year’s symposium was the need for clear guidelines on the adaption of FAIR governance for artificial intelligence (AI), which relies on the accessibility of large and high-quality datasets. It is often the case that AI models are trained with synthetic data, large-scale simulations and first-principles mathematical models, although these may only provide an incomplete description of complex and highly nonlinear real-world phenomena. Once AI models are calibrated against experimental data, their predictions become increasingly accurate. Adopting FAIR principles for the production, collection and curation of scientific datasets will streamline the design, training, validation and testing of AI models (see, for example, Y Chen et al. 2021 arXiv:2108.02214).

EOSC includes five science clusters, from natural sciences to social sciences, with a dedicated cluster for particle physics and astronomy called ESCAPE: the European Science Cluster of Astronomy and Particle Physics. The future deployment of the ESCAPE Virtual Research Environment across multiple nodes will provide users with tools to bring together diverse experimental results, for example, in the search for evidence of dark matter, and to perform new analyses incorporating data from complementary searches.

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Particle-physics experiments typically produce large amounts of highly complex data. Extracting information about the properties of fundamental physics interactions from these data is a non-trivial task. The general availability of simulation frameworks makes it relatively straightforward to model the forward process of data analysis: to go from an analytically formulated theory of nature to a sample of simulated events that describe the observation of that theory for a given particle collider and detector in minute detail. The inverse process – to infer from a set of observed data what is learned about a theory – is much harder as the predictions at the detector level are only available as “point clouds” of simulated events, rather than as the analytically formulated distributions that are needed by most statistical-inference methods.

Traditionally, statistical techniques have found a variety of ways to deal with this problem, mostly centered on simplifying the data via summary statistics that can be modelled empirically in an analytical form. A wide range of ML algorithms, ranging from neural networks to boosted decision trees trained to classify events as signal- or background-like, have been used in the past 25 years to construct such summary statistics.

The broader field of ML has experienced a very rapid development in recent years, moving from relatively straightforward models capable of describing a handful of observable quantities, to neural models with advanced architectures such as normalising flows, diffusion models and transformers. These boast millions to billions of parameters that are potentially capable of describing hundreds to thousands of observables – and can now extract features from the data with an order-of-magnitude better performance than traditional approaches. 

New generation

These advances are driven by newly available computation strategies that not only calculate the learned functions, but also their analytical derivatives with respect to all model parameters, greatly speeding up training times, in particular in combination with modern computing hardware with graphics processing units (GPUs) that facilitate massively parallel calculations. This new generation of ML models offers great potential for novel uses in physics data analyses, but have not yet found their way to the mainstream of published physics results on a large scale. Nevertheless, significant progress has been made in the particle-physics community in learning the technology needed, and many new developments using this technology were shown at the workshop.

This new generation of machine-learning models offers great potential for novel uses in physics data analyses

Many of these ML developments showcase the ability of modern ML architectures to learn multidimensional distributions from point-cloud training samples to a very good approximation, even when the number of dimensions is large, for example between 20 and 100. 

A prime use-case of such ML models is an emerging statistical analysis strategy known as simulation-based inference (SBI), where learned approximations of the probability density of signal and background over the full high-dimensional observables space are used, dispensing with the notion of summary statistics to simplify the data. Many examples were shown at the workshop, with applications ranging from particle physics to astronomy, pointing to significant improvements in sensitivity. Work is ongoing on procedures to model systematic uncertainties, and no published results in particle physics exist to date. Examples from astronomy showed that SBI can give results of comparable precision to the default Markov chain Monte Carlo approach for Bayesian computations, but with orders of magnitude faster computation times.

Beyond binning

A commonly used alternative approach to the full-fledged theory parameter inference from observed data is known as deconvolution or unfolding. Here the goal is publishing intermediate results in a form where the detector response has been taken out, but stopping short of interpreting this result in a particular theory framework. The classical approach to unfolding requires estimating a response matrix that captures the smearing effect of the detector on a particular observable, and applying the inverse of that to obtain an estimate of a theory-level distribution – however, this approach is challenging and limited in scope, as the inversion is numerically unstable, and requires a low dimensionality binning of the data. Results on several ML-based approaches were presented, which either learn the response matrix from modelling distributions outright (the generative approach) or learn classifiers that reweight simulated samples (the discriminative approach). Both approaches show very promising results that do not have the limitations on the binning and dimensionality of the distribution of the classical response-inversion approach.

A third domain where ML is facilitating great progress is that of anomaly searches, where an anomaly can either be a single observation that doesn’t fit the distribution (mostly in astronomy), or a collection of events that together don’t fit the distribution (mostly in particle physics). Several analyses highlighted both the power of ML models in such searches and the bounds from statistical theory: it is impossible to optimise sensitivity for single-event anomalies without knowing the outlier distribution, and unsupervised anomaly detectors require a semi-supervised statistical model to interpret ensembles of outliers.

A final application of machine-learned distributions that was much discussed is data augmentation – sampling a new, larger data sample from a learned distribution. If the synthetic data is significantly larger than the training sample, its statistical power will be greater, but will derive this statistical power from the smooth interpolation of the model, potentially generating so-called inductive bias. The validity of the assumed smoothness depends on its realism in a particular setting, for which there is no generic validation strategy. The use of a generative model amounts to a tradeoff between bias and variance.

Interpretable and explainable

Beyond the various novel applications of ML, there were lively discussions on the more fundamental aspects of artificial intelligence (AI), notably on the notion of and need for AI to be interpretable or explainable. Explainable AI aims to elucidate what input information was used, and its relative importance, but this goal has no unambiguous definition. The discussion on the need for explainability centres to a large extent on trust: would you trust a discovery if it is unclear what information the model used and how it was used? Can you convince peers of the validity of your result? The notion of interpretable AI goes beyond that. It is an often-desired quality by scientists, as human knowledge resulting from AI-based science is generally desired to be interpretable, for example in the form of theories based on symmetries, or structures that are simple, or “low-rank”. However, interpretability has no formal criteria, which makes it an impractical requirement. Beyond practicality, there is also a fundamental point: why should nature be simple? Why should models that describe it be restricted to being interpretable? The almost philosophical nature of this question made the discussion on interpretability one of the liveliest ones in the workshop, but for now without conclusion.

Human knowledge resulting from AI-based science is generally desired to be interpretable

For the longer-term future there are several interesting developments in the pipeline. In the design and training of new neural models, two techniques were shown to have great promise. The first one is the concept of foundation models, which are very large models that are pre-trained by very large datasets to learn generic features of the data. When these pre-trained generic models are retrained to perform a specific task, they are shown to outperform purpose-trained models for that same task. The second is on encoding domain knowledge in the network. Networks that have known symmetry principles encoded in the model can significantly outperform models that are generically trained on the same data.

The evaluation of systematic effects is still mostly taken care of in the statistical post-processing step. Future ML techniques may more fully integrate systematic uncertainties, for example by reducing the sensitivity to these uncertainties through adversarial training or pivoting methods. Beyond that, future methods may also integrate the currently separate step of propagating systematic uncertainties (“learning the profiling”) into the training of the procedure. A truly global end-to-end optimisation of the full analysis chain may ultimately become feasible and computationally tractable for models that provide analytical derivatives.

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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.

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SLAC and LBNL directors John Sarrao and Mike Witherell opened the meeting by emphasising the vital roles of international collaboration between national laboratories in advancing scientific discovery. Sarrao highlighted SLAC’s historical contributions to high-energy physics and expressed enthusiasm for the FCC’s scientific potential. Witherell reflected on the legacy of particle accelerators in fundamental science and the importance of continued innovation.

CERN Director-General Fabiola Gianotti identified three pillars of her vision for the laboratory: flagship projects like the LHC; a diverse complementary scientific programme; and preparations for future projects. She identified the FCC as the best future match for this vision, asserting that it has unparalleled potential for discovering new physics and can accommodate a large and diverse scientific community. “It is crucial to design a facility that offers a broad scientific programme, many experiments and exciting physics to attract young talents,” she said.

International collaboration, especially with the US, is important in ensuring the project’s success

FCC-ee would operate at several centre-of-mass energies corresponding to the Z-boson pole, W-boson pair-production, Higgs-boson pole or top-quark pair production. The beam current at each of these points would be determined by the design value of 50 MW synchrotron-radiation power per beam. At lower energies, the machine could accommodate more bunches, achieving 1.3 amperes and a luminosity in excess of 10³⁶ cm^–2 s^–1 at the Z pole. Measurements of electroweak observables and Higgs-boson couplings would be improved by a factor of between 10 and 50. Remarkably, FCC-ee would also provide 10 times the ambitious design statistics of SuperKEKB/Belle II for bottom and charm quarks, making it the world-leading machine at the intensity frontier. Along with other measurements of electroweak observables, FCC-ee will indirectly probe energies up to 70 TeV for weakly interacting particles. Unlike at proposed linear colliders, four interaction points would increase scientific robustness, reduce systematic uncertainties and allow for specialised experiments, maximising the collider’s physics output.

For FCC-hh, two approaches are being pursued for the necessary high-field superconducting magnets. The first involves advancing niobium–tin technology, which is currently mastered at 11–12 T for the High-Luminosity LHC, with the goal of reaching operational fields of 14 T. The second focuses on high-temperature superconductors (HTS) such as REBCO and iron-based superconductors (IBS). REBCO comes mainly in tape form (CERN Courier May/June 2023 p37), whereas IBS comes in both tape and wire form. With niobium-tin, 14 T would allow proton–proton collision energies of 80 TeV in a 90 km ring. HTS-based magnets could potentially reach fields up to 20 T, and centre-of-mass energies proportionally higher, in the vicinity of 120 TeV. If HTS magnets prove technically feasible, they could greatly decrease the cryogenic power. The development of such technologies also holds great promise beyond fundamental research, for example in transportation and electricity transmission.

FCC study leader Michael Benedikt (CERN) outlined the status of the ongoing feasibility study, which is set to be completed by March 2025. No technical showstoppers have yet been found, paving the way for the next phase of detailed technical and environmental impact studies and critical site investigations. Benedikt stressed the importance of international collaboration, especially with the US, in ensuring the project’s success.

The next step for the FCC project is to provide information to the CERN Council, via the upcoming update of the European strategy for particle physics, to facilitate a decision on whether to pursue the FCC by the end of 2027 or in early 2028. This includes further developing the civil engineering and technical design of major systems and components to present a more detailed cost estimate, continuing technical R&D activities, and working with CERN’s host states on regional implementation development and authorisation processes along with the launch of an environmental impact study. FCC would intersect 31 municipalities in France and 10 in Switzerland. Detailed work is ongoing to identify and reserve plots of land for surface sites, address site-specific design aspects, and explore socio-economic and ecological opportunities such as waste-heat utilisation.

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The committee heard extensive reports from the leading HEP laboratories and various world regions on their recent activities and plans, including a presentation by Paris Sphicas, the chair of the European Committee for Future Accelerators (ECFA), on the process for the update of the European strategy for particle physics (ESPP). Launched by CERN Council in March 2024, the ESPP update is charged with recommending the next collider project at CERN after HL-LHC operation.

A global task

The ESPP update is also of high interest to non-European institutions and projects. Consequently, in addition to the expected inputs to the strategy from European HEP communities, those from non-European HEP communities are also welcome. Moreover, the recent US P5 report and the Chinese plans for CEPC, with a potential positive decision in 2025/2026, and discussions about the ILC project in Japan, will be important elements of the work to be carried out in the context of the ESPP update. They also emphasise the global nature of high-energy physics.

An integral part of the work of ICFA is carried out within its panels, which have been very active. Presentations were given from the new panel on the Data Lifecycle (chair Kati Lassila-Perini, Helsinki), the Beam Dynamics panel (new chair Yuan He, IMPCAS) and the Advanced and Novel Accelerators panel (new chair Patric Muggli, Max Planck Munich, proxied at the meeting by Brigitte Cros, Paris-Saclay). The Instrumentation and Innovation Development panel (chair Ian Shipsey, Oxford) is setting an example with its numerous schools, the ICFA instrumentation awards and centrally sponsored instrumentation studentships for early-career researchers from underserved world regions. Finally, the chair of the ILC International Development Team panel (Tatsuya Nakada, EPFL) summarised the latest status of the ILC Technological Network, and the proposed ILC collider project in Japan.

ICFA noted interesting structural developments in the global organisation of HEP

A special session was devoted to the sustainability of HEP accelerator infrastructures, considering the need to invest efforts into guidelines that enable better comparison of the environmental reports of labs and infrastructures, in particular for future facilities. It was therefore natural for ICFA to also hear reports not only from the panel on Sustainable Accelerators and Colliders led by Thomas Roser (BNL), but also from the European Lab Directors Working Group on Sustainability. This group, chaired by Caterina Bloise (INFN) and Maxim Titov (CEA), is mandated to develop a set of key indicators and a methodology for the reporting on future HEP projects, to be delivered in time for the ESPP update.

Finally, ICFA noted some very interesting structural developments in the global organisation of HEP. In the Asia-Oceania region, ACFA-HEP was recently formed as a sub-panel under the Asian Committee for Future Accelerators (ACFA), aiming for a better coordination of HEP activities in this particular region of the world. Hopefully, this will encourage other world regions to organise themselves in a similar way in order to strengthen their voice in the global HEP community – for example in Latin America. Here, a meeting was organised in August by the Latin American Association for High Energy, Cosmology and Astroparticle Physics (LAA-HECAP) to bring together scientists, institutions and funding agencies from across Latin America to coordinate actions for jointly funding research projects across the continent.

The next in-person ICFA meeting will be held during the Lepton–Photon conference in Madison, Wisconsin (USA), in August 2025.

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The Thessaloniki School on Field Theory and Applications in HEP was the first school in the third cycle of the programme. Fifty-four students from 16 countries were joined by a number of online participants in a programme of lectures and tutorials.

We are now approaching 110 years since the general theory of relativity was founded and the theoretical prediction of the existence of black holes. There is subsequently at least half a century of developments related to the quantum aspects of black holes. At the Thessaloniki School, Tarek Anous (Queen Mary) delivered a pivotal series of lectures on the thermal properties of black holes, entanglement and the information paradox, which continues to be unresolved.

Nikolay Bobev (KU Leuven) summarised the ideas behind holography; Daniel Grumiller (TU Vienna) addressed the application of the holographic principle in flat spacetimes, including Carrollian/celestial holography; Slava Rychkov (Paris-Saclay) gave an introduction to conformal field theory in various dimensions; while Vassilis Spanos (NKU Athens) provided an introduction to modern cosmology. The programme was completed by Kostas Skenderis (Southampton), who addressed renormalisation in conformal field theory, anti-de Sitter and de Sitter spacetimes.

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HEP solutions

There is a scientific consensus that the Earth has been warming consistently since the industrial revolution, with the Earth’s surface temperature now about 1.2 °C warmer than in the late 1800s. The Paris Agreement of 2015 aims to limit this increase to 1.5 °C, requiring a 50% cut in emissions by 2030. However, the current rise in greenhouse-gas emissions far exceeds this target. The relevance of a 1.5 °C limit is underscored by the fact that the difference between now and the last ice age (12,000 years ago) is only about 5 °C, explained Veronique Boisvert (Royal Holloway) in her riveting talk on the intersection of HEP and climate solutions. If temperatures rise by 4 °C in the next 50 years, as predicted by the Intergovernmental Panel on Climate Change’s high-emissions scenario, it could cause disruptions beyond what our civilisation can handle. Intensifying heat waves and extreme weather events are already causing significant casualties and socio-economic disruptions, with 2023 the warmest year on record since 1850.

Masakazu Yoshioka (KEK) and Ben Shepherd (Daresbury) delved deeply into sustainable accelerator practices. Cement production for facility construction releases significant CO₂, prompting research in material sciences to reduce these emissions. Accelerator systems consume significant energy, and if powered by electricity grids coming from grid fossil fuels, they increase the carbon footprint. Energy-saving measures include reducing power consumption and recovering and reusing thermal energy, as demonstrated by CERN’s initiative to use LHC cooling water to heat homes in Ferney-Voltaire. Efforts should also focus on increasing CO₂ absorption and fixation in accelerator regions. Such measures can be effective – Yoshioka estimated that Japan’s Ichinoseki forest can absorb more CO₂ annually than the construction emissions of the proposed ILC accelerator over a decade.

Suzanne Evans (ARUP) explained how to perform lifecycle assessments of carbon emissions to evaluate environmental impacts. Sustainability efforts at C3, CEPC, CERN, DESY and ISIS-II were all presented. Thomas Roser (BNL) presented the ICFA strategy for sustainable accelerators, and Jorgen D’Hondt (Vrije Universiteit Brussel) outlined the Horizon Europe project Innovate for Sustainable Accelerating Systems (CERN Courier July/August 2024 p20).

Gaseous detectors contribute significantly to emissions through particle detection, cooling and insulation. Ongoing research to develop eco-friendly gas mixtures for Cherenkov detectors, resistive plate chambers and other detectors were discussed at length – alongside an emphasis from delegates on the need for more efficient and leak-free recirculating systems. On the subject of greener computing solutions, Loïc Lannelongue (Cambridge) emphasised the high-energy consumption of servers, storage and cooling. Collaborative efforts from grassroots movements, funding bodies and industry will be essential for progress.

Stopping global warming is an urgent task for humanity

Thijs Bouman (Groningen) delivered an engaging talk on the psychological aspects of sustainable energy transitions, emphasising the importance of understanding societal perceptions and behaviours. Ayan Paul (DESY) advocated for optimising scientific endeavours to reduce environmental impact, urging a balance between scientific advancement and ecological preservation. The workshop concluded with an interactive session on the “Know Your Footprint” tool by the Young High Energy Physicists (yHEP) Association, facilitated by Naman Bhalla (Freiburg), to calculate individual carbon impacts (CERN Courier May/June 2024 p66). The workshop also sparked dynamic discussions on reducing flight emissions, addressing travel culture and the high cost of public transport. Key questions included the effectiveness of lobbying and the need for more virtual meetings.

Jyoti Parikh, a recipient of the Nobel Peace Prize awarded to Intergovernmental Panel on Climate Change authors in 2007 and member of India’s former Prime Minister’s Council on Climate Change, presented the keynote lecture on global energy system and technology choices. While many countries aim to decarbonise their electricity grids, challenges remain. Green sources like solar and wind have low operating costs but unpredictable availability, necessitating better storage and digital technologies. Parikh emphasised that economic development with lower emissions is possible, but posed the critical question: “Can we do it in time?”

Stopping global warming is an urgent task for humanity. We must aim to reduce greenhouse-gas emissions to nearly zero by 2050. While collaboration within local communities and industries is imperative; and individual efforts may seem small, every action is one step toward global efforts for our collective benefit. Sustainable HEP 2024 showcased innovative ideas, practical solutions and collaborative efforts to reduce the environmental impact of HEP. The event highlighted the community’s commitment to sustainability while advancing scientific knowledge.

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Since its inception in the mid-1980s, the Strings conference has sought to summarise the latest developments in the interconnected fields of quantum gravity and quantum field theory, all under the overarching framework of string theory. As one of the most anticipated gatherings in theoretical physics, the conference serves as a platform for exchanging knowledge, fostering new collaborations and pushing the boundaries of our understanding of the fundamental aspects of the physical laws of nature. The most recent edition, Strings 2024, attracted about 400 in-person participants to CERN in June, with several hundred more scientists following on-line.

One way to view string theory is as a model of fundamental interactions that provides a unification of particle physics with gravity. While generic features of the Standard Model and gravity arise naturally in string theory, it has lacked concrete experimental predictions so far. In recent years, the strategy has shifted from concrete model building to more systematically understanding the universal features that models of particle physics must satisfy when coupled to quantum gravity.

Into the swamp

Remarkably, there are very subtle consistency conditions that are invisible in ordinary particle physics, as they involve indirect arguments such as whether black holes can evaporate in a consistent manner. This has led to the notion of the “Swampland”, which encompasses the set of otherwise well-behaved quantum field theories that fail these subtle quantum-gravity consistency conditions. This may lead to concrete implications for particle physics and cosmology.

An important question addressed during the conference was whether these low-energy consistency conditions always point back to string theory as the only consistent “UV completion” (fundamental realisation at distance scales shorter than can be probed at low energies) of quantum gravity, as suggested by numerous investigations. Whether there is any other possible UV completion involving a version of quantum gravity unrelated to string theory remains an important open question, so it is no surprise that significant research efforts are focused in this direction.

Attempts at explicit model construction were also discussed, together with a joint discussion on cosmology, particle physics and their connections to string theory. Among other topics, recent progress on realising accelerating cosmologies in string theory was reported, as well as a stringy model for dark energy.

A different viewpoint, shared by many researchers, is to employ string theory rather as a framework or tool to study quantum gravity, without any special emphasis on its unification with particle physics. It has long been known that there is a fundamental tension when trying to combine gravity with quantum mechanics, which many regard as one of the most important, open conceptual problems in theoretical physics. This becomes most evident when one zooms in on quantum black holes. It was in this context that the holographic nature of quantum gravity was discovered – the idea that all the information contained within a volume of space can be described by data on its boundary, suggesting that the universe’s fundamental degrees of freedom can be thought of as living on a holographic screen. This may not only hold the key for understanding the decay of black holes via Hawking radiation, but can also teach us important lessons about quantum cosmology.

Strings serves as a platform for pushing the boundaries of our understanding of the fundamental aspects of the physical laws of nature

Thousands of papers have been written on this subject within the last decades, and indeed holographic quantum gravity continues to be one of string theory’s most active subfields. Recent breakthroughs include the exact or approximate solution of quantum gravity in low-dimensional toy models in anti-de Sitter space, the extension to de-Sitter space, an improved understanding of the nature of microstates of black holes, the precise way they decay, discovering connections between emergent geometry and quantum information theory, and developing powerful tools for investigating these phenomena, such as bootstrap methods.

Other developments that were reviewed include the use of novel kinds of generalised symmetries and string field theory. Strings 2024 also gave a voice to more tangentially related areas such as scattering amplitudes, non-perturbative quantum field theory, particle phenomenology and cosmology. Many of these topics are interconnected to the core areas mentioned in this article and with each other, both technically and/or conceptually. It is this intricate web of highly non-trivial consistent interconnections between subfields that generates meaning beyond the sum of its parts, and forms the unifying umbrella called string theory.

The conference concluded with a novel “future vision” session, which considered 100 crowd-sourced open questions in string theory that might plausibly be answered in the next 10 years. These 100 questions provide a glimpse of where string theory may head in the near future.

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Hadronic contributions

One of the highest profile topics in lattice QCD is the evaluation of hadronic contributions to the magnetic moment of the muon. For many years, the experimental measurements from Brookhaven and Fermilab have appeared to be in tension with the Standard Model (SM), based on theoretical predictions that rely on data from e⁺e^– annihilation to hadrons. Intense work on the lattice by multiple groups is now maturing rapidly and providing a valuable cross-check for data-driven SM calculations.

At the lowest order in quantum electrodynamics, the Dirac equation accounts for precisely two Bohr magnetons in the muon’s magnetic moment (g = 2) – a contribution arising purely from the muon interacting with a single real external photon representing the magnetic field. At higher orders in QED, virtual Standard Model particles modify that value, leading to a so-called anomalous magnetic moment g–2. The Schwinger term adds a virtual photon and a contribution to g-2 of approximately 0.2%. Adding individual virtual W, Z or Higgs bosons adds a well defined contribution a factor of a million or so smaller. The remaining relevant contributions are from hadronic vacuum polarisation (HVP) and hadronic light-by-light (HLBL) scattering. HVP and HLBL both add hadronic contributions integrated to all orders in the strong coupling constant to interactions between the muon and the external electric field, which also feature additional virtual photons. Though their contributions to g-2 are in the ballpark of the small electroweak contribution, they are more difficult to calculate, and dominate the error budget for the SM prediction of the muon’s g-2.

Christine Davies (University of Glasgow) gave a comprehensive survey of muon g–2 that stressed several high-level points: the small HLBL contribution looks to be settled, and is unlikely to be a key piece to the puzzle; recent tensions among the e⁺e^– experiments for HVP have emerged and need to be better understood; and in the most contentious region, all eight recent lattice–QCD calculations agree with each other and with the very recent e⁺e^– → hadrons experiment CMD 3 (2024 Phys. Rev. Lett. 132 231903), though not so much with earlier experiments. Thus, lattice QCD and CMD 3 suggest there is “almost certainly less new physics in muon g–2 than previously hoped, and perhaps none,” said Davies. We shall see: many groups are preparing results for the full HVP, targeting a new whitepaper from the Muon g–2 Theory Initiative by the end of this year, in anticipation of the final measurement from the Fermilab experiment sometime in 2025.

New directions

While the main focus of Lattice calculations is the study of QCD, lattice methods have been applied beyond that. There is a small but active community investigating systems that could be relevant to physics beyond the Standard Model, including composite Higgs models, supersymmetry and dark matter. These studies often inspire formal “theoretical” developments that are of interest beyond the lattice community. Particularly exciting directions this year were the development on emergent phases, non-invertible symmetries and their possible application to formulate chiral gauge theories, one of the outstanding theoretical issues in lattice gauge theories.

The lattice QCD community is one of the main users of high-performance computing resources

The lattice QCD community is one of the main users of high-performance computing resources, with its simulation efforts generating petabytes of Monte Carlo data. For more than 20 years, a community wide effort, the international lattice data grid (ILDG), has allowed this data to be shared. Since its inception, ILDG implemented the FAIR principles – data should be findable, accessible, interoperable and reusable – almost fully. The lattice QCD community is now discussing Open Science. Ed Bennett (Swansea) led a panel discussion that explored the benefits of ILDG embracing open science, such as higher credibility for published results, and not least the means to fulfill the expectations of funding bodies. Sustainably maintaining the infrastructure and employing the personnel required calls for national or even international community efforts to convince the funding agencies to provide corresponding funding lines, but also the researchers of the benefits of open science.

The Kenneth G. Wilson Award for Excellence in Lattice Field Theory was awarded to Michael Wagman (Fermi­lab) for his lattice-QCD studies of noise reduction in nuclear systems, the structure of nuclei and transverse-momentum-dependent hadronic structure functions. Fifty years on from Wilson’s seminal paper, two of the field’s earliest contributors, John Kogut (US Department of Energy) and Jan Smit (University of Amsterdam), reminisced about the birth of the lattice in a special session chaired by Liverpool pioneer Chris Michael. Both speakers gave fascinating insights into a time where physics was extracted from a handful of small-volume gauge configurations, compared to hundreds of thousands today.

Lattice 2025 will take place at the Tata Institute of Fundamental Research in Mumbai, India, from 3 to 8 November 2025.

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Today, around 5000 accelerator scientists and engineers work in more than 50 countries, collaborating with a pool of technical experts three to four times that size. While most are deeply involved in operations and upgrades, their careers also include designing and constructing new facilities, beam-physics research, developing critical technical components, and project leadership. Their work often involves technology transfer, industrial applications, education and training of future experts, and public and academic outreach.

A global field

The need for regular meetings of the entire field has long been recognised. Historically, regional conferences like the biannual particle-accelerator conferences (PACs) in the US (1965–2009), the biannual EPACs in Europe (1988–2008) and the triannual APACs in Asia (1998–2007) served this purpose. These gatherings covered all types of accelerators, particles and use-cases. As the field became truly global, leaders established the series of international PACs (IPACs), which rotate through the regions in a three-year cycle, convening about 1500 attendees. The 15th IPAC took place from 19 to 24 May in Nashville, Tennessee, with almost 200 registrants from Asia, more than 400 from Europe and nearly 700 from the US.

The “beef” of the conference was in the reports from facilities, but no one person can summarise all the progress, and I must restrict myself to personal highlights in fields that are close to my heart. Fascinating progress was reported on energy-recovery linacs (ERLs) and associated technologies such as superconducting RF and fixed-field-alternating-gradient accelerators, following the recent success of the CBETA accelerator test facility at Cornell. Another hot topic in my eyes was design work and experimental studies towards strong hadron cooling for the Electron–Ion Collider. This year’s progress in industrial and medical accelerators is also impressive, with noteworthy presentations on radioisotope production and radiotherapy (Oliver Kester, TRIUMF and Michael Galonska, GSI), light sources for semiconductor manufacturing (Bruce Dunham, SLAC), accelerator-driven fusion (Richard Magee, TAE Technologies), and 96 exhibitions from companies and institutions worldwide.

CERN’s FCC-ee project was discussed in several sessions. Nuria Catalan-Lasheras (CERN) gave a memorable talk demonstrating impressive progress on high-power klystrons (RF sources). At present, klystrons have about 55% efficiency – RF power divided by wall-plug power – but she noted that they have the potential to go to as high as about 85% efficiency. The path is clear: increase voltage and decrease current, thereby reducing the “microperveance” of the klystrons. This will be crucial at FCC-ee, which must continuously replenish 100 MW of synchrotron radiation losses with 100 MW of RF power. The klystron efficiency improvement alone can save more than 60 MW – fully a third of the current power consumption of the CERN accelerator complex.

The “beef” of the conference was in the reports from facilities, but no one person can summarise all the progress

Muon colliders were presented as a unique opportunity to achieve a substantial energy increase compared to hadrons (Diktys Stratakis, Fermilab). Due to the point-like nature of the muon, the full centre-of-mass energy is available for probing new physics processes in every collision. Therefore, a 10 TeV muon collider can provide comparable high-energy-physics breakthroughs to a 100 TeV proton–proton collider, where colliding partons only carry a fraction of the proton’s energy. Due to its compactness, the cost of a 10 TeV muon collider compares to that of the FCC-ee and is likely to be many times lower than any other alternative concept that can achieve 10 pCM (parton centre-of-mass) energies (T Roser et al. 2023 JINST 18 P05018). The challenge lies in developing technologies for muon production, cooling and acceleration in the next two decades. In the upcoming 19 to 25 years it should be technically feasible for the accelerator community to demonstrate the technologies of a) high-intensity and short proton bunches; b) high-power proton targets; c) muon cooling; d) fast muon acceleration; e) 10 to 12 T superconducting magnets lined with tungsten inserts to protect coils from the muon decay products, and; f) effective spreading of the narrow cones of ultra-high-energy neutrinos by wiggling the beams, to avoid damage caused by the chargeless neutrinos when the muons decay.

In the conference’s closing talk, I reviewed three dozen future-collider proposals, analysed the ultimate energies potentially attainable in all types of colliding beams and accelerators within reasonable cost and power consumption limits, and laid out arguments that energies beyond a PeV (thousands of TeV) can be achieved, concluding that muons are the particles of the future for high-energy physics.

I can attest to IPAC’s success in fostering real-life interactions in the global accelerator landscape

The prize session was a highlight, with acceptance speeches from KEK’s Kaoru Yokoya (APS Wilson Prize) and SLAC’s Gennady Stupakov (IEEE NPSS PAST Award). Yokoya outlined his participation in various electron–positron machines and proposals such as the TRISTAN e⁺e^– collider and the ILC. Stupakov emphasised the importance of beam-dynamics theory in the age of computer modelling and simulations.

Ever since the first edition in Kyoto in 2010, I can attest to IPAC’s success in fostering real-life interactions in the global accelerator landscape. After the conference, I counted more than a hundred encounters of 5 minutes or more – something that would be difficult to achieve at a smaller or more specialised conference. It was pleasing to see many Chinese colleagues attend this US-based conference, but I did not identify any participants from Russia – a concerning development for our science’s international spirit. I hope political barriers will not interfere with next year’s IPAC’25 in Taiwan.

On a personal note, I would like to thank the organisers for putting together great scientific and social programmes, and the dedicated Joint Accelerator Conferences Website team, whose tireless efforts ensured that virtually all conference proceedings – papers, talks and posters – were available online by the final day, setting a standard that other fields of high-energy physics could greatly benefit from.

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Immense effort

Analysing the performance of CERN’s accelerator complex, speakers noted the impressive progress to date, examined limitations in the LHC and injectors and discussed improvements for optimal performance in upcoming runs. It’s difficult to do justice to the immense technical effort made by all systems, operations and technical infrastructure teams that underpins the exploitation of the complex. Machine availability emerged as a crucial theme, recognised as critical for both maximising the potential of existing facilities and ensuring the success of the HL-LHC. Fault tracking, dedicated maintenance efforts and targeted infrastructure improvements across the complex were highlighted as key contributors to achieving and maintaining optimal uptime.

As the HL-LHC project moves into full series production, the technical challenges associated with magnets, cold powering and crab cavities are being addressed (CERN Courier January/February 2024 p37). Looking beyond Long Shutdown 3 (LS3), potential limitations are already being targeted now, with, for example, electron-cloud mitigation measures planned to be deployed in LS3. The transition to the high-luminosity era will involve a huge programme of work that requires meticulous preparation and a well-coordinated effort across the complex during LS3, which will see the deployment of the HL-LHC, a widespread consolidation effort, and other upgrades such as that planned for the ECN3 cavern at CERN’s North Area.

The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective

The breadth and depth of the physics being performed at CERN facilities is quite remarkable, and the Chamonix workshop reconfirmed the high demand from experimentalists across the board. The unique capabilities of ISOLDE, n_TOF, AD-ELENA, and the East and North Areas were recognised. The North Area, for example, provides protons, hadrons, electrons and ion beams for detector R&D, experiments, the CERN neutrino platform, irradiation facilities and counts more than 2000 users. The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective. The potential for long-term exploitation and leveraging fully the capabilities of the LHC and other facilities is considerable, demanding continued support and development.

In the longer term, CERN is exploring the potential construction of the FCC via a dedicated feasibility study that has just delivered a mid-term report – a summary of which was presented at Chamonix. The initiative is accompanied by R&D on key accelerator technologies. The physics case for FCC-ee was well made for an audience of mostly non-particle physicists, concluding that the FCC is the only proposed collider that covers each key area in the field – electroweak, QCD, flavour, Higgs and searches for phenomena beyond the Standard Model – in paradigm-shifting depth.

Environmental consciousness

Sustainability was another focus of the Chamonix workshop. Building and operating future facilities with environmental consciousness is a top priority, and full life-cycle analyses will be performed for any options to help ensure a low-carbon future.

Interesting times, lots to do. To quote former CERN Director-General Herwig Schopper from 1983: “It is therefore clear that, for some time to come, there will be interesting work to do and I doubt whether accelerator experts will find themselves without a job.”

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More than 150 German particle physicists gathered at Bonn University for a community event on a future collider at CERN. More precisely, the focus set for this meeting was to discuss the opportunities that the FCC-ee would offer should this collider be built at CERN. The event was organised by the German committee for particle physics, KET, and took place from 22 to 24 May. Representatives from almost all German institutes and groups active in particle physics were present, an attendance that shows the large interest in the collider to be built at CERN after the successful completion of the HL-LHC programme.

The main workshop was preceded by a dedicated session with more than 80 early-career scientists, organised by the Young High Energy Physicists Association, yHEP, to bring the generation that will benefit most from a future collider at CERN up to speed on the workshop topics. It included a presentation by former ECFA chair Karl Jakobs (Freiburg University) “From Strategy Discussions to Decision-Taking for Large Projects”, explaining the mechanisms and bodies involved in setting a project like the FCC-ee on track.

The opening session of the main workshop featured a fresh view on “The physics case for an e⁺e^– collider at CERN” by Margarete Mühlleitner (KIT Karlsruhe), who spread excitement about the strong and comprehensive physics case from super-precise measurements of the properties of the Z boson, the W boson and the top quark to what most people associate with a future e⁺e^– collider: precision measurements of the Higgs boson and insights about its connection to many of the still open questions of particle physics like dark matter or the matter–antimatter asymmetry. Markus Klute (KIT Karlsruhe) gave an in-depth review of the FCC-ee project. The midterm results of the FCC feasibility study indicate that no showstoppers were found in all the aspects studied so far and that the integrated FCC programme offers unparalleled exploration potential through precision measurements and direct searches. The picture was rounded off by a presentation from Jenny List (DESY, Hamburg) who talked about alternative options to realise an e⁺e^– Higgs factory at CERN, and the perspective of the early-career researchers was highlighted by Michael Lupberger (Bonn University). While all these presentations concentrated on the science and technology of the FCC-ee or alternatives, Eckart Lilienthal, representing the German Ministry of Education and Research, BMBF, reminded the audience that a future collider project at CERN needs an affordable financial plan and that – given the large uncertainties at present – this requires the community to prepare for different scenarios including one without the FCC-ee. Lilienthal confirmed that the future of CERN remains of the highest priority to BMBF.

The event was an important step in building consensus in the German community for a future collider project at CERN

The workshop went on to review many aspects of the FCC-ee and possible alternatives in more detail: accelerator R&D, detector concepts and technologies, computing and software, theory challenges as well as sustainability. The workshop witnessed the first meetings of the newly established German detector R&D consortia on silicon detectors, gaseous detectors and calorimetry. They will receive BMBF funding for the next three years and will allow German groups to strongly participate in the recently formed international DRD consortia in the context of the ECFA detector roadmap.

The path ahead

The workshop concluded with discussion sessions on the future collider scenarios for CERN, the engagement of the German community and a path to prepare the German input to the update of the European Strategy for Particle Physics. A series of three additional community workshops will be held in Germany before this input is due in March 2025.

The Bonn event was an important step in building consensus in the German community for a future collider project at CERN. The FCC-ee project generated a lot of interest and many groups plan to embark more strongly on this project. Contributions concerning the physics case, theory challenges, detector design and development, software, computing, and accelerator development were discussed. Alternative options for a future collider project at CERN need to be kept open to address the unanswered fundamental questions of particle physics in case the FCC-ee is not built at CERN. This event was clear evidence that a bright future for CERN remains of highest priority for the German particle-physics community and funding agency.

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Crossing frontiers

85% of the matter in the universe at most minimally interacts with the electromagnetic force but provided the gravitational seed for large-scale structure formation in the early universe. Hugh Lippincott (University of California, Santa Barbara) summarised cross-frontier searches. Pursuing all possible scenarios via direct detection will require scaling up existing technology and developing new technologies such as quantum sensors to probe lighter dark-matter candidates. On the one hand, the 60 to 80 tonne “XLZD” liquid xenon detector will merge the expertise of the XENONnT, LUX-ZEPLIN and DARWIN collaborations; on the low-mass side, Reina Maruyama (Yale) discussed the ALPHA and HAYSTAC haloscopes, which seek to convert axions into photons in highly tuned resonant cavities. Indirect detection and collider experiments will also play an important role in closing in on minimal dark-matter models.

Delegates expressed a sense of urgency to probe higher energies. Cari Cesarotti (MIT) advocated R&D towards a future muon collider, arguing that muons offer a clean and power-efficient route to the 10 TeV scale and above. Recently, experts have estimated that challenges due to the finite muon lifetime could be overcome on a 20-year technically limited timeline. Both CERN and China have proposed building 100 km-circumference tunnels, initially hosting an electron–positron collider followed by a 100 TeV hadron machine, however, the timeline suggests that almost all of the conference attendees would be retired before hadron collisions come online. Elliot Lipeles (Pennsylvania) proposed skipping the electron-positron stage and immediately pursuing an intermediate-energy hadron collider: existing magnets in a 100 km tunnel could produce 37 TeV collisions, advancing measurements of the Higgs self-coupling and electroweak phase transition, dark matter and its mediators, and naturalness.

The energy, intensity and cosmic frontiers of particle physics target deeply connected questions

Neutrinos were discussed at length. Georgia Karagiorgi (Columbia University) argued that three short-baseline anomalies remain, potentially hinting at additional sterile neutrinos or dark-sector portals. Julieta Gruszko (North Carolina at Chapel Hill) presented an exciting future for experiments that seek to discern the fundamental nature of neutrinos. A new tonne-scale generation of detectors comprising LEGEND1000, nEXO and CUPID may succeed in confirming the Majorana nature of the neutrino if they observe neutrinoless double beta decay.

Talks on the importance of science communication and education provoked a great deal of discussion. Ethan Siegal, host of popular podcast “Starts with a Bang” spoke on public outreach, Kevin Pedro (Fermilab) on advocacy with policy­makers in Washington, DC, and Roger Freedman (University of California Santa Barbara) on educating the next generation of physicists. In public programming, Nausheen Shah (Wayne State) was the guest speaker at a screening of Hidden Figures, the inspiring true story of the black women who helped the US win the space race, and Philip Chang (University of California San Diego) lectured on “An Invitation to Imagine Something from Nothing”.

The energy, intensity and cosmic frontiers of particle physics target deeply connected questions. Dark matter, dark energy, cosmic inflation and baryogenesis have remained unexplained for decades, and the structure of the Standard Model itself provokes questions, not least in relation to the Higgs boson and neutrinos. Innovative and complementary experiments are needed across all areas of particle physics. Judging from the 2024 Aspen Winter Conference, the future of the field is in good hands.

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The seventh workshop on Medical Applications of Spectroscopic X-ray Detectors was held at CERN from 15 to 18 April. This year’s workshop brought together more than 100 experts in medical imaging, radiology, physics and engineering. The workshop focused on the latest advancements in spectroscopic X-ray detectors and their applications in medical diagnostics and treatment. Such detectors, whose origins are found in detector R&D for high-energy physics, are now experiencing a breakthrough moment in medical practice.

Spectroscopic X-ray detectors represent a significant advancement in medical imaging. Unlike traditional X-ray detectors that measure only the intensity of X-rays, these advanced detectors can differentiate the energies of X-ray photons. This enables enhanced tissue differentiation, improved tumour detection and advanced material characterisation, which may lead in certain cases to functional imaging without the need for radioactive tracers.

The technology has its roots in the 1980s and 1990s when the high-energy-physics community centred around CERN developed a combination of segmented silicon sensors and very large-scale integration (VLSI) readout circuits to enable precision measurements at unprecedented event rates, leading to the development of hybrid pixel detectors (see p37). In the context of the Medipix Collaborations, CERN has coordinated research on spectroscopic X-ray detectors including the development of photon-counting detectors and new semiconductor materials that offer higher sensitivity and energy resolution. By the late 1990s, several groups had proofs of concept, and by 2008, pre-clinical spectral photon-counting computed-tomography (CT) systems were under investigation.

Spectroscopic X-ray detectors offer unparalleled diagnostic capabilities, enabling more detailed imaging and earlier and precise disease detection

In 2011, leading researchers in the field decided to bring together engineers, physicists and clinicians to help address the scientific, medical and engineering challenges associated with guiding the technology toward clinical adoption. In 2021, the FDA approval of Siemens Healthineers’ photon-counting CT scanner marked a significant milestone in the field of medical imaging, validating the clinical benefits of spectroscopic X-ray detectors. The mobile CT scanner, OmniTom Elite from NeuroLogica, approved in March 2022, also integrates photon counting detector (PCD) technology. The 3D colour X-ray scanner developed by MARS Bioimaging, in collaboration with CERN based on Medipix3 technology, has already shown significant promise in pre-clinical and clinical trials. Clinical trials of MARS scanners demonstrated its applications for detecting acute fractures, evaluation of fracture healing and assessment of osseous integration at the bone–metal interface for fracture fixations and joint replacements. With more than 300 million CT scans being performed annually around the world, the potential impact for spectroscopic X-ray imaging is enormous, but technical and medical challenges remain, and the need for this highly specialised workshop continues.

The scientific presentations in the 2024 workshop covered the integration of spectroscopic CT in clinical workflows, addressed technical challenges in photon counting detector technology and explored new semiconductor materials for X-ray detectors. The technical sessions on detector physics and technology discussed new methodologies for manufacturing high-purity cadmium–zinc–tellurium semiconductor crystals and techniques to enhance the quantum efficiency of current detectors. Sessions on clinical applications and imaging techniques included case studies demonstrating the benefits of multi-energy CT in cardiology and neurology, and advances in using spectroscopic detectors for enhanced contrast agent differentiation. The sessions on computational methods and data processing covered the implementation of AI algorithms to improve image reconstruction and analysis, and efficient storage and retrieval systems for large-scale spectral imaging datasets. The sessions on regulatory and safety aspects focused on the regulatory pathway for new spectroscopic X-ray detectors, ensuring patient and operator safety with high-energy X-ray systems.

Enhancing patient outcomes

The field of spectroscopic X-ray detectors is rapidly evolving. Continued research, collaboration and innovation to enhance medical diagnostics and treatment outcomes will be essential. Spectroscopic X-ray detectors offer unparalleled diagnostic capabilities, enabling more detailed imaging and earlier and precise disease detection, which improves patient outcomes. To stay competitive and meet the demand for precision medicine, medical institutions are increasingly adopting advanced imaging technologies. Continued collaboration among researchers, physicists and industry leaders will drive innovation, benefiting patients, healthcare providers and research institutions.

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Packed sessions, more than 100 talks and lively discussions at Rencontres de Moriond electroweak, held from 24 to 31 March in La Thuile, Italy, captured the latest thinking in the field. The Standard Model (SM) emerged intact, while new paths of enquiry were illuminated.

Twelve years after the discovery of the Higgs boson, H, a wide variety of analyses by ATLAS and CMS are bringing the new scalar into sharper focus. This includes its mass, for which CMS has reported the most precise single measurement using the H → ZZ → 4ℓ channel: 125.04 ± 0.11 (stat) ± 0.05 (syst) GeV. A Run 2 legacy mass measurement combining ATLAS and CMS results is under way, while projections for the HL-LHC indicate that an uncertainty at the 10–20 MeV level is attainable. For the H width, which is potentially highly sensitive to new physics but notoriously difficult to measure at a hadron collider, the experiments constrain its value to be less than three times the SM width at 95% confidence level using an indirect method with reasonable assumptions. A precision of about 20% is expected from the full HL-LHC dataset.

New generation

The measured H cross sections in all channels continue to support the simplest incarnation of the SM H sector, with a new result from CMS testing the bbH production mode in the ττ and WW channels. Now that the H couplings to the most massive particles are well established, the focus is moving to the second-generation fermions. Directly probing the shape of the Brout–Englert–Higgs potential, and sensitive to new-physics contributions, the H self-coupling is another key target. HH production has yet to be observed at the LHC due to its very low cross section (the combined ATLAS and CMS limit is currently 2.5–3 times the SM value), but an extensive measurement programme utilising multiple channels is under way and Moriond saw new results presented based on HH → bbbb and HH → γγττ decays (see “Homing in on the Higgs self-interaction“).

Searches for exotic H decays, or for additional low-mass scalar bosons as predicted by two-Higgs-doublet extensions to the SM, were a Moriond highlight. A wide scope of new H-boson (a, A) searches have been released by ATLAS and CMS, including a new search for H → aa → muons by CMS in the mass range 0.2–60 GeV and, on the higher mass side, new limits on H/A → tt by ATLAS and A → ZH → ℓℓ tt by CMS. Although none show significant deviations from the SM, most of the searches are statistically limited and there remains a large amount of phase space available for extended H sectors. Generating much conversation in the corridors was a new-physics interpretation of ATLAS and CMS data in terms of a Higgs-triplet model, based on results  in the HH → γγ channel and top-quark differential distributions.

The LHC experiments are making stunning progress in precision electroweak measurements, as exemplified by a new measurement by CMS of the effective leptonic electroweak mixing angle sin²θ^ℓ_eff= 0.23157 ± 0.00031, the first LHC measurement of the W-boson width by ATLAS, and precise measurements of the W and Z cross sections at 13.6 TeV. ATLAS announced at Moriond the most precise single-experiment test of lepton-flavour universality in comparisons between W-boson decays to muons and electrons. A wide-ranging presentation of electroweak results based on two-photon collisions at the LHC described recent attempts by CMS to extract the anomalous magnetic moment of the tau lepton. And LHCb showcased its capabilities in providing an independent measurement of the W-boson mass and the Z-boson cross section. Participants heard about the increasing relevance of lattice QCD in precision electroweak measurements, for example in determining the running of alpha and the weak mixing angle. A tension between the predictions from lattice QCD and from more traditional dispersive approaches exists, with a similar origin to that for the anomalous magnetic moment of the muon.

Following the recent observation of entanglement in top-quark pairs by ATLAS and CMS, a presentation addressing the intriguing ability of colliders to carry out fundamental tests of quantum mechanics generated much discussion. Offering full access to spin information, collider experiments can study quantum correlations, wavefunction collapse and decoherence at unprecedented energies, possibly enabling a Bell measurement at the HL-LHC and the first observation of toponium.

Seeking signals from beyond

Searches for long-lived particles by ATLAS, CMS and LHCb – including the first at LHC Run 3 by CMS – were high on the Moriond agenda. Heavy gauge and scalar bosons, left–right gauge boson masses and heavy neutral leptons are among other new-physics scenarios being constrained. Casting the net as wide as possible, the LHC experiments are developing AI anomaly-detection algorithms, while the power of effective field theory (EFT) in parameterising the effect of heavy new particles on LHC measurements continues to grow via a diverse range of analyses. Even at O(6) in the SMEFT, no fewer than 59 Wilson coefficients, each related to different underlying physics processes, need to be to measured.

Neutrinoless double-beta decay, which would be an unambiguous sign of new physics, continues to be hunted by a host of experiments

Tensions between theory and experiment remain in some processes involving b → s or b → c quark transitions. Moriond saw much discussion on such processes, including new results from Belle II on the branching ratio of the highly suppressed decay B → Kνν. Participants heard about the need for theory progress, as has been the case recently with impressive calculations of b → sγ. Predictions for b → sμμ – which show a tension with experiment and that are independent of the R(K) parameters clocking the relative rates of B → Kμ⁺μ^– and B → Ke⁺e^– – are excellent ways to probe new physics. Concerning b → c transitions, updates on R(D^*) from Belle II and on R(D^*) and R(D) from LHCb based on the muonic decay of the tau lepton take the world-average tension to 3.17σ. The stability of the SM prediction of R(D^*) was also questioned.

New flavours

The flavour sector is awash with new results. LHCb presented fresh analyses exploring mixing and CP violation in the charm sector – a unique gateway to the flavour structure of up-type quarks – while CMS presented a new measurement of CP violation in B_s → J/ψ K⁺K^– decays. In ultra-rare kaon decays, KOTO presented a new upper limit on the branching ratio of K⁰_L → πνν (< 2 × 10^–9 at 90% confidence level) and projects a sensitivity < 10^–13 with the proposed KOTO II upgrade. NA62 presented a preliminary measurement of the branching ratio of the very rare decay π⁰ → e⁺e^– (5.86 ± 0.37 × 10^–8), in agreement with the SM, and results for K⁺ → πγγ, the latter offering the first evidence that second-order terms must be included in chiral perturbation theory. Belle and Belle II showed new radiative and electroweak penguin results concerning processes such as B⁰ → γγ, and BESIII presented a precise measurement of the CKM matrix element V_cs. A sweeping theory perspective on the mysterious flavour structure of the SM introduced participants to “flavour modular symmetries” – a promising new game in town for a potential theory of flavour based on modular forms, which are well known in mathematics and were used in the proof of Fermat’s last theorem.

The final sessions of Moriond electro­weak turned to neutrinos, dark matter and astroparticle physics. KATRIN is soon to release an update on the neutrino mass limit based on six times more data, with an expected uncertainty of m_ν < 0.5 eV, and is undertaking R&D towards a proposed upgrade (KATRIN++) that would use new technology to push the mass limit down further. The collaboration is also stepping up its search for new physics via high-precision spectroscopy and is working towards an upgrade called TRISTAN that will soon zone in on the sterile neutrino hypothesis.

In Japan, the T2K facility has undergone an extensive renewal period including its first operation with the near-detector ND280 upgrade in August 2023, which increased the acceptance. Designed to explore neutrino mass ordering and leptonic CP violation, T2K data so far show a slight preference for the “normal” mass ordering while admitting a CP-conserving phase at the level of 2σ. However, a joint analysis between T2K and NOvA, a neutrino oscillation experiment in the US with a longer baseline and complementary sensitivity, prefers a more degenerate parameter space where either CP conservation or the inverted ordering are acceptable solutions. The combined data place a strong constraint on Δm₃₂.

Neutrinoless double-beta decay (NDBD), which would reveal the neutrino to be a Majorana particle and be an unambiguous sign of new physics, continues to be hunted by a host of experiments. LEGEND-200’s first physics data was shown, setting up an ultimate goal of placing a lower limit on the NDBD half-life of 10²⁸ years for ⁷⁶Ge. Also located at Gran Sasso, CUORE, which has been collecting data since 2019, will operate for one more year before an upgrade is planned. In parallel, designs for a next-generation tonne-scale upgrade, CUPID, are being finalised. Neutrino aficionados were also treated to scotogenic three-loop models, in which neutrinos gain a Dirac mass term from radiative corrections, and to the latest results from FASER at the LHC, including the first emulsion-detector measurements of the ν_e and ν_μ cross sections at TeV energies, and a search for axion-like particles.

IceCube, which studies resonant disappearance of antineutrinos due to matter effects, showed intriguing results that delve into new-physics territory. Adding sterile neutrinos improves global fits by 7σ, participants heard, but brings inconsistencies too. Generating much interest, the global p-value for the null hypothesis of the sterile neutrino in the muon disappearance channel is 3.1%, in tension with MINOS. The Deep Core IceCube upgrade will increase the number of strings in the observatory, while the more significant Gen-2 upgrade will expand its overall area. A theory overview of the status of sterile neutrinos, taking into account recent results from MiniBooNE, MicroBooNE, PROSPECT, STEREO, GALEX, SAGE, BEST and others, concluded that experimental evidence for such a fourth neutrino state is fading but not excluded. The so-called reactor anomaly is probably explained by smaller uranium contribution than previously accounted for, while the upgraded Neutrino-4 experiment will shed light on tensions with PROSPECT and STEREO.

Cosmological constraints

The status of dark photons was also reviewed. Constraints are being placed from many sources, including colliders, astrophysical and cosmological bounds, haloscopes, and most recently radio telescopes, the James Webb Space Telescope and beam-dump experiments. PandaX-4T, which seeks to constrain WIMP dark matter and NDBD, is about to restart data-taking. LZ, another large liquid-xenon detector, has placed record limits on dark matter based on its first 60 days of data-taking. Results from the first observing run of a novel kind of laser-interferometric detector, LIDA, to observe axion-like particles in the galactic halo are promising.

No particle-physics conference would be complete without the anomalous magnetic moment of the muon

The latest supersymmetry and dark-matter searches at ATLAS and CMS were also presented, including a new result on R-parity violating supersymmetry and fresh limits on the chargino mass. BESIII reported on exotic searches for massive dark photons, muon-philic particles, glueballs and the QCD axion. Searches for axion-like particles are multiplying in many shapes and forms. In terms of flavour probes of axions, the strongest bounds come from NA62. Less conventionally, probing ultralight dark matter by searching for oscillatory behaviour in gravitational waves is gaining traction. Recent NanoGrav data show no signs of such a signal.

All eyes on the muon

No contemporary particle-physics conference would be complete without the anomalous magnetic moment of the muon – a powerful quantity that takes into account all known and unknown particles, for which the measured value is in significant tension with the SM prediction. As the Fermilab Muon g-2 experiment continues to improve the experimental precision (currently 0.2 ppm), all eyes are on how the SM calculation is performed – specifically the systematic uncertainty associated with a process called hadronic vacuum polarisation. A huge amount of work is going into understanding this quantity, both in terms of the calculational machinery and underlying data used. When computed using lattice QCD, the tension between experiment and theory is significantly reduced. However, the calculations are so complex that few groups have been able to execute them. That is set to change this year, Moriond participants heard, as new lattice calculations are unblinded ahead of the Lattice 2024 meeting in August, followed by a decision on whether to include such results in the official SM prediction at the seventh plenary workshop of the Muon g-2 Theory Initiative at KEK in September.

Experimentally and theoretically, all tools are being thrown at the SM in an attempt to find an explanation for dark matter, the cosmological baryon asymmetry, neutrino masses and other outstanding mysteries. The many high-quality talks at this year’s Moriond electroweak session, including an impressive batch of flash talks in dedicated young-researcher sessions, covered all aspects of the adventure and set the standard for future analyses. An incredible interplay between astrophysical, cosmological, collider and other experimental measurements is rapidly eating into the available parameter space for new physics. Ten years ago, the Moriond theory-summary speaker remarked “new physics must be around the corner, but we see no corner”. While the same could be said today, physicists have a much clearer view of the road ahead.

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The study of the Higgs boson remains central to the LHC programme. ATLAS reported a new result on Standard Model (SM) Higgs-boson production with decays to tau leptons, achieving the most precise single-channel measurement of the vector-boson-fusion production mode to date. Determining the production modes of the Higgs boson precisely may shed light on the existence of new physics that would be observed as deviations from the SM predictions.

Beyond single Higgs production, the di-Higgs production (HH) search is one of the most exciting and fundamental topics for LHC physics in the coming years as it directly probes the Higgs potential (see “Homing in on the Higgs self-interaction“). ATLAS has combined results for HH production in multiple final states, providing the best-expected sensitivity to the HH production cross-section and Higgs-boson self-coupling, allowing κ_λ (the Higgs self-coupling with respect to the SM value) to be within the range –1.2 < κ_λ < 7.2.

The search for beyond-the-SM (BSM) physics to explain the many unresolved questions about our universe is being conducted with innovative ideas and methods. CMS has presented new searches involving signatures with two tau leptons, examining the hypotheses of an excited tau lepton and a heavy neutral spin-1 gauge boson (Z′) produced via Drell-Yan and, for the first time, via vector boson fusion. These results set stringent constraints on BSM models with enhanced couplings to third-generation fermions.

Other new-physics theoretical models propose additional BSM Higgs bosons. ATLAS presented a search for such particles being produced in association with top quarks, setting limits on their cross-section that significantly improve upon previous ATLAS  results. Additional BSM Higgs bosons could explain puzzles such as dark matter, neutrino oscillations and the observed matter–antimatter asymmetry in the universe.

The dark side

Some BSM models imply that dark-matter particles could arise as composite mesons or baryons of a new strongly-coupled theory that is an extension of the SM. ATLAS investigated this dark sector through searches for high-multiplicity hadronic final states, providing the first direct collider constraints on this model to complement direct dark-matter-detection experimental results.

CMS have used low-pileup inelastic proton–proton collisions to measure event-shape variables related to the overall distribution of charged particles. These measurements showed the particle distribution to be more isotropic than predicted by theoretical models.

The LHC experiments also presented multiple analyses of proton–lead (p–Pb) and pp collisions, exploring the potential production of quark–gluon plasma (QGP) – a hot and dense phase of deconfined quarks and gluons found in the early universe that is frequently studied in heavy-ion Pb–Pb collisions, among others, at the LHC. Whether it can be created in smaller collision systems is still inconclusive.

ALICE reported a high-precision measurement of the elliptic flow of anti-helium-3 in QGP using the first Run-3 Pb–Pb run. The much larger data sample compared to the previous Run 2 measurement allowed ALICE to distinguish production models for these rarely produced particles for the first time. ALICE also reported the first measurement of an impact-parameter-dependent angular anisotropy in the decay of coherently photo-produced ρ⁰ mesons in ultra-peripheral Pb–Pb collisions. In these collisions, quantum interference effects cause a decay asymmetry that is inversely proportional to the impact parameter.

CMS reported its first measurement of the complete set of optimised CP-averaged observables from the process B⁰ → K^*0μ⁺μ^–. These measurements are significant because they could reveal indirect signs of new physics or subtle effects induced by low-energy strong interactions. By matching the current best experimental precision, CMS contributes to the ongoing investigation of this process.

LHCb presented measurements of the local and non-local contributions across the full invariant-mass spectrum of B^0* → K^*0μ⁺μ^–, tests of lepton flavour universality in semileptonic b decays, and mixing and CP violation in D → Kπ decays.

The future of the field was discussed in a well-attended panel session, which emphasised exploring the full potential of the HL-LHC and engaging younger generations

From a theoretical perspective, progress in precision calculations has exceeded expectations. Many processes are now known to next-to-next-to-leading order or even next-to-next-to-next-to-leading order (N³LO) accuracy. The first parton distribution functions approximating N³LO accuracy have been released and reported at LHCP, and modern parton showers have set new standards in perturbative accuracy.

In addition to these advances, several new ideas and observables are being proposed. Jet substructure, for instance, is becoming a precision science and valuable tool due to its excellent theoretical properties. Effective field theory (EFT) methods are continuously refined and automated, serving as crucial bridges to new theories as many ultraviolet theories share the same EFT operators. Synergies between flavour physics, electroweak effects and high-transverse-momentum processes at colliders are particularly evident within this framework. The use of the LHC as a photon collider showcases the extraordinary versatility of LHC experiments and their synergy with theoretical advancements.

Discovery machine

The HL-LHC upgrade was thoroughly discussed, with several speakers highlighting the importance and uniqueness of its physics programme. This includes fundamental insights into the Higgs potential, vector-boson scattering, and precise measurements of the Higgs boson and other SM parameters. Thanks to the endless efforts by the four collaborations to improve their performances, the LHC already rivals historic lepton colliders for electroweak precision in many channels, despite the cleaner signatures of lepton collisions. The HL-LHC will be capable of providing extraordinarily precise measurements while also serving as a discovery machine for many years to come.

The future of the field was discussed in a well-attended panel session, which emphasised exploring the full potential of the HL-LHC and engaging younger generations. Preserving the unique expertise and knowledge cultivated within the CERN community is imperative. Next year’s LHCP conference will be held at National Taiwan University in Taipei from 5 to 10 June.

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Going green

Based on state-of-the-art technology, the portfolio of current and future accelerator-driven RIs in Europe could develop to consume up to 1% of Germany’s annual electricity demand. With the ambition to maintain the attractiveness and competitiveness of European RIs, and enable Europe’s Green Deal, the Innovate for Sustainable Accelerating Systems (iSAS) project has been approved by Horizon Europe. Its aim is to establish an enhanced collaboration in the field to broaden, expedite and amplify the development and impact of novel energy-saving technologies to accelerate particles.

In general terms, a particle accelerator has a system to create the particles to be accelerated, a system preparing beams with these particles, an accelerating system that effectively accelerates the particle beams, a magnet system to steer the beam, an experimental facility using the particles, and finally a beam dump. In linear accelerating structures, most of the electrical power taken from the grid to operate the accelerator is used by the accelerating system itself.

The core of an accelerating system is a series of cavities that can deliver a high-gradient electric field. For many modern accelerators, these cavities are superconducting and therefore cryogenically cooled to about 2 K. They are powered with radio frequency (RF) power generators to deliver the field at a specific frequency and accordingly to provide energy to the particle beams as they traverse. These superconducting RF (SRF) systems are the enabling technology for frontier accelerators, but are energy-intensive devices where only a fraction of the power extracted from the grid is effectively transmitted to the accelerated particles. In addition, the beam energy is radiated by recirculating beams and ultimately dumped and lost. As an example, the European XFEL’s superconducting RF system uses 5–6 MW for 0.1 MW of average beam power, leading to a power conversion of less than 3%.

The objective of iSAS is to innovate those technologies that have been identified as being a common core of SRF accelerating systems and that have the largest leverage for energy savings with a view to minimising the intrinsic energy consumption in all phases of operation. In the landscape of accelerator-driven RIs, solutions are being developed to reuse the waste heat produced, develop energy-efficient magnets and RF power generators, and operate facilities on opportunistic schedules when energy is available on the grid. The iSAS project has a complementary focus on the energy efficiency of the SRF accelerating technologies themselves. This will contribute to the vital transition to sustain the tremendous 20th-century applications of accelerator technology in an energy-conscious 21st century.

Interconnected technologies

Based on a recently established European R&D roadmap for accelerator technology and based on a collaboration between leading European research institutions and industry, several interconnected technologies will be developed, prototyped and tested, each enabling significant energy savings on their own in accelerating particles. The collection of energy-saving technologies will be developed with a portfolio of forthcoming applications in mind, and to explore energy-saving improvements in accelerator-driven RIs. Considering the developments realised, the new technologies will be coherently integrated into the parametric design of a new accelerating system, a linac SRF cryomodule, optimised to achieve high beam-power in accelerators with an energy consumption that is as low as reasonably possible. This new cryomodule design will enable Europe to develop and build future energy-sustainable accelerators and particle colliders.

iSAS has been approved by Horizon Europe to help develop novel energy-saving technologies to accelerate particles

On 15 and 16 April, the iSAS kick-off meeting was organised at IJCLab (Orsay, France) with around 100 participants. Each of the working groups enthusiastically presented their impactful R&D plans and, in all cases, concrete work has begun. To save energy from RF power systems, novel fast-reacting tuners are being developed to compensate rapidly for detuning of the cavity’s frequency caused by mechanical vibrations, and methods are being invented to integrate them into smart digital control systems. To save energy from the cryogenics, and based on the ongoing Horizon Europe I.FAST project, superconducting cavities with thin films of Nb₃Sn are being further developed to operate with high performance at 4.2 K instead of 2 K, thereby reducing the grid-power to operate the cryogenic system. The cryogenic system requires three times less cooling power to maintain a 4.2 K bath at 4.2 K when heat is dissipated in the bath compared to maintaining a 2 K bath at 2 K. Finally, to save energy from the accelerated particle beam itself, the technology of energy recovery linacs (ERLs) is being improved to operate efficiently with high-current beams by developing novel higher-order mode dampers that significantly avoid heat loads in the cavities.

To address the engineering challenges related to the integration of the new energy-saving technologies, an existing ESS cryovessel will be equipped with new cavities and novel dampers, and the resulting linac SRF cryomodule will be tested in operation in the PERLE accelerator at IJCLab (Orsay, France). PERLE is a growing international collaboration to demonstrate the performance of ERLs with high-power beams that would enable applications in future particle colliders. Its first phase is being implemented at IJCLab with the objective to have initial beams in 2028.

The timescale to innovate, prototype and test new accelerator technologies is inherently long, in some cases longer than the typical duration of R&D projects. It is therefore essential to continue to collaborate and enhance the R&D process so that energy-sustainable technologies can be implemented without delay, to avoid hampering the scientific and industrial progress enabled by accelerators. Accordingly, iSAS plans co-development with industrial partners to jointly achieve a technology readiness level that will be sufficient to enter the large-scale production phase of these new technologies.

Empowering industry

While the readiness of several energy-saving technologies will be prepared towards industrialisation with impact on current RIs, iSAS is also a pathfinder for sustainable future SRF particle accelerators and colliders. Through inter- and multidisciplinary research that delivers and combines various technologies, it is the long-term ambition of iSAS to reduce the energy footprint of SRF accelerators in future RIs by half, and even more when the systems are integrated in ERLs. Accordingly, iSAS will help maintain Europe’s leadership for breakthroughs in fundamental sciences and enable high-energy collider technology to go beyond the current frontiers of energy and intensity in an energy-sustainable way. In parallel, the new sustainable technologies will empower and stimulate European industry to conceive a portfolio of new applications and take a leading role in, for example, the semiconductor, particle therapy, security and environmental sectors.

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A highlight of the conference was the evening plenary poster session. Luis Alberto Rodriguez Chacon (The Cyprus Institute), Cornelis Mommers (Mainz University) and Sotiris Pitelis (Mainz) were recognised with the prestigious EPS poster prize, and presented their work on the calculation of the gluon momentum fraction in mesons through lattice QCD simulations, exotic atoms, and the X17 discovery potential from γD → e⁺e^–pn with neutron tagging. This edition of EINN also hosted topical workshops on the QCD analysis of nucleon structure and experimental opportunities at the Electron-Ion Collider. Preceding the conference, a two-day meeting on careers in photonuclear physics was tailored to be a platform for PhD students and postdoctoral researchers to establish professional networks.

With QCD taking a central role in contemporary physics research worldwide, the EINN conference is poised to maintain its crucial role as an international forum for the field.

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A workshop at CERN from 4 to 8 December 2023 leveraged impressive experimental progress in a range of fields. Attended by nearly 100 international scientists – a noteworthy increase from the 40 experts who attended the first workshop at ICTP Trieste in 2019 – the workshop showcased the field’s expanded research interest and collaborative efforts. Concretely, about 10 novel detector concepts have been developed since the first workshop.

One can look for GWs in a few different ways: observing changes in the space between detector components, exciting vibrations in detectors, and converting GWs into electromagnetic radiation in strong magnetic fields. Substantial progress has been made in all three experimental directions.

Levitating concepts

The leading concepts for the first approach involve optically levitated sensors such as high-aspect-ratio sodium–cyttrium–fluoride prisms, and semi-levitated sensors such as thin silicon or silicon–nitride nanomembranes in long optical resonators. These technologies are currently under study by various groups in the Levitated Sensor Detectors collaboration and at DESY.

For the second approach, the main focus is on millimetre-scale quartz cavities similar to those used in precision clocks. A network of such detectors, known as GOLDEN, is being planned, involving collaborations among UC Davis, University College London and Northwestern University. Superconducting radio-frequency cavities also present a promising technology. A joint effort between Fermilab and DESY is leveraging the existing MAGO prototype to gain insights and design further optimised cavities.

Regarding the third approach, a prominent example is optical high-precision interferometry, combined with a series of accelerator dipole magnets similar to those used in the light-shining-through-a-wall axion-search experiment, ALPS II (Any Light Particle Search II) or the axion helioscope CAST and its planned successor IAXO. In fact, ALPS II is anticipated to commence a dedicated GW search in 2028. Additionally, other notable concepts inspired by axion dark-matter searches involve toroidal magnets, exemplified by experiments like ABRACADABRA, or solenoidal magnets such as BASE or MADMAX.

All three approaches stand to benefit from burgeoning advances in quantum sensing, which promise to enhance sensitivities by orders of magnitude. In this landscape, axion dark-matter searches and UHF GW detection are poised to work in close collaboration, leveraging quantum sensing to achieve unprecedented results. Concepts that demonstrate synergies with axion-physics searches are crucial at this stage, and can be facilitated by incremental investments. Such collaboration builds awareness within the scientific community and presents UHF searches as an additional, compelling science case for their construction.

The workshop showcased the field’s expanded research interest and collaborative efforts

Cross-disciplinary research is also crucial to understand cosmological sources and constraints on UHF GWs. For the former, our understanding of primordial black holes has significantly matured, transitioning from preliminary estimates to a robust framework. Additional sources, such as parabolic encounters and exotic compact objects, are also gaining clarity. For the latter, the workshop highlighted how strong magnetic fields in the universe, such as those in extragalactic voids and planetary magnetospheres, can help set limits on the conversion between electromagnetic and gravitational waves.

Despite much progress, the sensitivity needed to detect UHF GWs remains a visionary goal, requiring the constant pursuit of inventive new ideas. To aid this, the community is taking steps to be more inclusive. The living review produced after the first workshop (arXiv:2011.12414) will be revised to be more accessible for people outside our community, breaking down detector concepts into fundamental building blocks for easier understanding. Plans are also underway to establish a comprehensive research repository and standardise data formats. These initiatives are crucial for fostering a culture of open innovation and expanding the potential for future breakthroughs in UHF GW research. Finally, a new, fully customisable and flexible GW plotter including the UHF frequency range is being developed to benefit the entire GW community.

The journey towards detecting UHF GWs is just beginning. While current sensitivities are not yet sufficient, the community’s commitment to developing innovative ideas is unwavering. With the collective efforts of a dedicated scientific community, the next leap in gravitational-wave research is on the horizon. Limits exist to be surpassed!

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This year, the PBC initiative has significantly strengthened CERN’s dark-sector searches, explained Mike Lamont and Joachim Mnich, directors for accelerators and technology, and research and computing, respectively. In particular, the newly approved SHiP proton beam-dump experiment (see SHiP to chart hidden sector) will complement the searches for light dark-sector particles that are presently conducted with NA64’s versatile setup, which is suitable for electron, positron, muon and hadron beams.

First-phase success

The FASER and SND experiments, now taking data in the LHC tunnel, are two of the successes of the PBC initiative’s first phase. Both search for new physics and study high-energy neutrinos along the LHC collision axis. FASER’s successor, FASER2, promises a 10,000-fold increase in sensitivity to beyond-the-Standard Model physics, said Jonathan Feng (UC Irvine). With the potential to detect thousands of TeV-scale neutrinos a day, it could also measure parton distribution functions and thereby enhance the physics reach of the high-luminosity LHC (HL-LHC). FASER2 may form part of the proposed Forward Physics Facility, set to be located 620 m away, along a tangent from the HL-LHC’s interaction point 1. A report on the facility’s technical infrastructure is scheduled for mid-2024, with a letter of intent foreseen in early 2025. By contrast, the CODEX-b and ANUBIS experiments are being designed to search for feebly interacting particles transverse to LHCb and ATLAS, respectively. In all these endeavours, the Feebly Interacting Particle Physics Centre will act as a hub for exchanges between experiment and theory.

Francesco Terranova (Milano-Bicocca) and Marc Andre Jebramcik (CERN) explained how ENUBET and NuTAG have been combined to optimise a “tagged” neutrino beam for cross-section measurements, where the neutrino flavour is known by studying the decay process of its parent hadron. In the realm of quantum chromodynamics, SPS experiments with lead ions (the new NA60+ experiment) and light ions (NA61/SHINE) are aiming to decode the phases of nuclear matter in the non-perturbative regime. Meanwhile, AMBER is proposing to determine the charge radii of kaons and pions, and to perform meson spectroscopy, in particularwith kaons.

The LHCspin collaboration presented a plan to open a new frontier of spin physics at the LHC building upon the successful operation of the SMOG2 gas cell that is upstream of the LHCb detector. Studying collective phenomena at the LHC in this way could probe the structure of the nucleon in a so-far little-explored kinematic domain and make use of new probes such as charm mesons, said Pasquale Di Nezza (INFN Frascati).

Measuring moments

The TWOCRYST collaboration aims to demonstrate the feasibility and the performance of a possible fixed-target experiment in the LHC to measure the electric and magnetic dipole moments (EDMs and MDMs) of charmed baryons, offering a complementary probe of searches for CP violation in the Standard Model. The technique would use two bent crystals: the first to deflect protons from the beam halo onto a target, with the resulting charm baryons then deflected by the second (precession) crystal onto a detector such as LHCb, while at the same time causing their spins to precess in the strong electric and magnetic fields of the deformed crystal lattice, explained Pascal Hermes (CERN).

New ideas ranged from the measurement of molecular electric dipole moments at ISOLDE to measuring the gravitational field of the LHC beam

Several projects to detect axion-like particles were discussed, including a dedicated superconducting cavity for heterodyne detection being jointly developed by PBC and CERN’s Quantum Technology Initiative. Atom interferometry is another subject of common interest, with PBC demonstrating the technical feasibility of installing an atom interferometer with a baseline of 100 m in one of the LHC’s access shafts. Other new ideas ranged from the measurement of molecular EDMs at ISOLDE to measuring the gravitational field of the LHC beam.

With the continued determination to fully exploit the scientific potential of the CERN accelerator complex and infrastructure for projects that are complementary to high-energy-frontier colliders testified by many fruitful discussions, the annual meeting concluded as a resounding success. The PBC community ended the workshop by thanking co-founder Claude Vallée (CPPM Marseille), who retired as a PBC convener after almost a decade of integral work, and welcomed Gunar Schnell (Ikerbasque and UPV/EHU Bilbao), who will take over as convener.

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The conference highlighted the impressive progress and diversity of UPC physics, which goes far beyond the initial studies of exclusive pro­-cesses. UPC23 covered the latest results from experiments at RHIC and the LHC, and prospects for the future Electron-Ion Collider (EIC) at Brookhaven National Laboratory. Discussions delved into the intricacies of inelastic photo-nuclear events, including the exciting programme of open charm that is yet to be explored, and examined how UPCs serve as a novel lens for investigating the quark–gluon plasma and other final-state nuclear effects. Lots of attention was devoted to the physics of low-x parton densities – a fundamental aspect of protons and nuclei that photons can probe in a unique way.

Enriched understanding

Among the conference’s theoretical highlights, Farid Salazar (UCLA) showed how vector–meson photoproduction could be a powerful method to detect gluon saturation across different collision systems, from proton–nucleus to electron–nucleus to UPCs. Zaki Panjsheeri (Virginia) put forth innovative ideas to study double-parton correlations, linking UPC vector–meson studies to generalised parton distributions, enhancing our understanding of the proton’s structure. Ashik Ikbal (Kent State), meanwhile, introduced exciting proposals to investigate quantum entanglement through exclusive J/ψ photoproduction at RHIC.

The conference also provided a platform for discussing the active exploration of light-by-light scattering and two-photon processes for probing fundamental physics and searches for axion-like particles, and for putting constraints on the anomalous magnetic moment of the tau lepton (see CMS closes in on tau g–2).

Energy exploration

Physicists at the LHC have effectively repurposed the world’s most powerful particle accelerator into a high-energy photon collider. This innovative approach, traditionally the domain of electron beams in colliders like LEP and HERA, and anticipated at the EIC, allows the LHC to explore photon-induced interactions at energies never before achieved. David Grund (Czech Technical University in Prague), Georgios Krintiras (Kansas) and Cesar Luiz Da Silva (Los Alamos) shared the latest LHC findings on the energy dependence of UPC J/ψ events. These results are crucial for understanding the onset of gluon saturation – a state where gluons become so dense reaching saturation, the dynamical equilibrium where the emission and recombination occurs. However, the data also align with the nuclear phenomenon known as gluon shadowing, which arises from multiple-scattering processes. David Tlusty (Creighton) presented the latest findings from the STAR Collaboration, which has recently expanded its UPC programme, complementing the energy exploration at the LHC. Klaudia Maj (AGH University of Krakow) presented the latest results on two-photon interactions and photonuclear jets from the ATLAS collaboration, including measurements that may be probing the quark-gluon plasma. 

Delegates discussed the future opportunities for UPC physics with the large integrated luminosity expected for Runs 3 and 4 at the LHC

Carlos Bertulani (Texas A&M) paid tribute to Gerhard Baur, who passed away on June 16 last year. Bertulani and Baur co-authored “Electromagnetic processes in relativistic heavy ion collisions” – a seminal paper with more than 1000 citations. Bertulani invited delegates to consider the untapped potential of UPCs in the study of anti-atoms and exotic atoms.

Delegates also discussed the future opportunities for UPC physics with the large integrated luminosity expected for Run 3 and Run 4 at the LHC, with the planned detector upgrades for Run 4 such as FoCal, the recent upgrades by STAR, the sPHENIX programme and at the EIC. Delegates are expecting event selection and instrumentation close to the beam line, for example using “zero degree” calorimeters, to offer the greatest experimental opportunities in the coming years.

The next edition of the UPC conference will take place in Saariselka, Finland in June 2025.

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“He is a restless person,” noted Albrecht Wagner (DESY), who presented a whistlestop tour of Schopper’s 35 years working in Germany, following his childhood in Bohemia. Whether in Hamburg, Erlangen, Mainz or Karlsruhe, he never missed out on an opportunity to see new places – though always maintaining the Austrian diet to which his children attribute his longevity. On one occasion, Schopper took a sabbatical to work with Lise Meitner in Stockholm’s Royal Institute of Technology. At the time, the great physicist was performing the first nuclear-physics studies in the keV range, said Wagner, and directed Schopper to measure the absorption rate of beta-decay electrons in various materials using radioactive sources and a Geiger–Müller counter. Schopper is one of the last surviving physicists to have worked with her, observed Wagner.

Schopper’s scientific contributions have included playing a major part in the world’s first polarised proton source, Europe’s first R&D programme for superconducting accelerators and the development of hadronic calorimeters as precision instruments, explained Christian Fabjan (TU Vienna/HEPHY). Schopper dubbed the latter the sampling total absorption calorimeter, or STAC, playing on the detector’s stacked design, but the name didn’t stick. In recognition of his contributions, hadronic calorimeters might now be renamed Schopper total absorption calorimeters, joked Fabjan.

As CERN DG from 1981 to 1988, Schopper oversaw the lion’s share of the construction of the LEP, before it began operations in July 1989. To accomplish this, he didn’t shy away from risks, budget cuts or unpopular opinions when the situation called for it, said Chris Llewellyn Smith, who would himself serve as DG from 1994 to 1998. Llewelyn Smith credited Schopper with making decisions that would benefit not only LEP, but also the LHC. “Watching Herwig deal with these reviews was a wonderful apprenticeship, during which I learned a lot about the management of CERN,” he recalled.

After passing CERN’s leadership to Carlo Rubbia, Schopper became a fulltime science diplomat, notably including 20 years in senior roles at UNESCO between 1997 and 2017, and significant contributions to SESAME, the Synchrotron-light for Experimental Science and Applications in the Middle East (see CERN Courier January/Feb­ruary 2023, p28). Khaled Toukan of Jordan’s Atomic Energy Commission, CERN Council president Eliezer Rabinovici and Maciej Nałecz (Polish Academy of Science, formerly of UNESCO) all spoke of Schopper’s skill in helping to develop SESAME as a blueprint for science for peace and development. “Herwig likes building rings,” Toukan fondly recounted.

As with any good birthday party, Herwig received gifts: a first copy of his biography, a NASA hoodie emblazoned with “Failure is not an option” from Sam Ting (MIT), who is closely associated with Schopper since their time together at DESY, and the Heisenberg medal. “You’ve even been in contact with the man himself,” noted Heisenberg Society president Johannes Blümer, referring to several occasions Schopper met Heisenberg at conferences and even once discussed politics with him.

Schopper continues to counsel DGs to this day – and not only on physics. Confessing to occasionally being intimidated by his lifetime of achievements, CERN DG Fabiola Gianotti intimated that they often discuss music. “Herwig likes all composers, but not baroque ones. For him, they are too rational and intellectual.” For this, he will always have physics.

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Among MENU 2023’s highlights on nucleon structure, a preliminary analysis by the NNPDF collaboration suggests that the proton contains more charm than anticharm, with Niccolò Laurenti (Università degli Studi di Milano) showing evidence of a non-vanishing intrinsic valence charm contribution to the proton’s wavefunction. Meanwhile, Michael Kohl (Hampton University) concluded that the proton–radius puzzle is still not resolved. To make progress, form-factor measurements in electron scattering must be scrutinised, and the use of atomic spectroscopy data clarified, he said.

Hadron physics

A large part of this year’s conference was dedicated to hadron spectroscopy, with updates from Belle II, BESIII, GlueX, Jefferson Lab, JPAC, KLOE/KLOE-2 and LHCb, as well as theoretical overviews covering everything from lattice quantum chromodynamics to effective-field theories. Special emphasis was also given to future directions in hadron physics at future facilities such as FAIR, the Electron-Ion Collider and the local Mainz Energy-Recovering Superconducting Accelerator (MESA) facility – a future low-energy but high-intensity electron accelerator that will make it possible to carry out experiments in nuclear astrophysics, dark-sector searches and tests of the SM. Among upgrade plans at Jefferson Lab, Eric Voutier (Paris-Saclay) presented a future experimental programme with positron beams at CEBAF, the institute’s Continuous Electron Beam Accelerator Facility. The upgrade will allow for a rich physics programme covering two-photon exchange, generalised polarisabilities, generalised parton distribution functions and direct dark-matter searches.

Highlights on nucleon structure include a preliminary analysis suggesting that the proton contains more charm than anticharm

Hadron physics is also closely related to searches for new physics, as precision observables of the Standard Model are in many cases limited by the non-perturbative regime of quantum chromodynamics. A prime example is the physics of the anomalous magnetic moment of the muon, for which a puzzling discrepancy between data-driven dispersive and lattice–quantum chromodynamics calculations of hadronic contributions to the Standard Model prediction persists (CERN Courier May/June 2021 p25). The upcoming collaboration meeting of the Muon g-2 Theory Initiative in September 2024 at KEK will provide important new insights from lattice QCD and e⁺e^– experiments. It remains to be seen whether the eventual theoretical consensus will confirm a significant deviation from the experimental value, which is currently being updated by Fermilab’s Muon g-2 experiment using their last three years of data.

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These boundary conditions put large-scale science projects under pressure to reduce CO₂ emissions during construction, operation and potentially decommissioning. For context: given the current French energy mix, CERN’s annual 1.3 TWh electricity consumption (which is mostly used for accelerator operation) corresponds to roughly 50 kt CO₂e global warming potential (GWP), while recent estimates for the construction of tunnels for future colliders are in the multi-100 kt CO₂e GWP range.

Green realisation

To discuss potential ways forward, a Workshop on Sustainability for Future Accelerators (WSFA2023) took place on 25–27 September in Morioka, Japan within the framework of the recently started EU project EAJADE (Europe–America–Japan Accelerator Development and Exchange). Around 50 international experts discussed a slew of topics ranging from life-cycle assessments (LCAs) of accelerator technologies with carbon-reduction potential to funding initiatives towards sustainable accelerator R&D, and local initiatives aimed at the “green” realisation of future colliders. With the workshop being held in Japan, the proposed International Linear Collider (ILC) figured prominently as a reference project – attracting considerable attention from local media.

The general context of discussions was set by Beate Heinemann, DESY director for particle physics, on behalf of the European Laboratory Directors Group (LDG). The LDG recently created a working group to assess the sustainability of accelerators, with a mandate to develop guidelines and a minimum set of key indicators pertaining to the methodology and scope of reporting of sustainability aspects for future high-energy physics projects. Since LCAs are becoming the main tool to estimate GWP, a number of project representatives discussed their take on sustainability and steps towards performing LCAs. Starting with the much-cited ARUP study on linear colliders published in 2023 (edms.cern.ch/document/2917948/1), there were presentations on the ESS in Sweden, the ISIS-II neutron and muon source in the UK, the CERN sustainability forum, the Future Circular Collider, the Cool Copper Collider and other proposed colliders. Also discussed were R&D items for sustainable technologies, including CERN’s High Efficiency Klystron Project, the ZEPTO permanent-magnet project, thin film-coated SRF cavities and others.

A second big block in the workshop agenda was devoted to the “greening” of future accelerators and potential local and general construction measures towards achieving this goal. The focus was on Japanese efforts around the ILC, but numerous results can be re-interpreted in a more general way. Presentations were given on the potential of concrete to turn from a massive carbon source into a carbon sink with net negative CO₂e balance (a topic with huge industrial interest), on large-scale wooden construction (e.g. for experimental halls), and on the ILC connection with the agriculture, forestry and fisheries industries to reduce CO₂ emissions and offset them by increasing CO₂ absorption. The focus was on building an energy recycling society by the time the ILC would become operational.

What have we learnt on our way towards sustainable large-scale research infrastructures? First, that time might be our friend: energy mixes will include increasingly larger carbon-free components, making construction projects and operations more eco-friendly. Also, new and more sustainable technologies will be developed that help achieve global climate goals. Second, we as a community must consider the imprint our research leaves on the globe, along with as many indicators as possible. The GWP can be a beginning, but there are many other factors relating, for example, to rare-earth elements, toxicity and acidity. The LCA methodology provides the accelerator community with guidelines for the planning of more sustainable large-scale projects and needs to be further developed – including end-of-life, decommissioning and recycling steps – in an appropriate manner. Last but not least, it is clear that we need to be proactive in anticipating the changes happening in the energy markets and society with respect to sustainability-driven challenges at all levels.

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The opening talk by Monica Pepe Altarelli underscored LHCb’s diverse physics programme, solidifying its role as a highly versatile forward detector. While celebrating successes, her talk candidly addressed setbacks, notably the new results in tests of lepton-flavour universality. LHCb detector and computing upgrades for Run 3 include a fully software-based trigger using graphics processing units. The collaboration is also working towards an Upgrade II programme for Long Shutdown 4 (2033–2034) that would position LHCb as a potentially unique global flavour facility.

On mixing and CP violation, the October workshop unveiled intriguing insights in both the beauty and charm sectors. In the beauty sector, notable highlights encompass measurements of the mixing parameter ΔΓ_s and of CP-violating phases such as ϕ_s,d, ϕ_s^sss and γ. CP asymmetries were further scrutinised in B → DD decays, accounting for SU(3) breaking and re-scattering effects. In the charm sector, the estimated CP asymmetries considering final-state interactions were found to be small compared to the experimental values related to D⁰ → π^–π⁺ and D⁰ → K^–K⁺ decays. Novel measurements of CP violation in three-body charm hadron decays were also presented.

Unique capabilities

On the theoretical front, discussions delved into the current status of bottom-baryon lifetimes. Recent lattice predictions on the ε_K parameter were also showcased, offering refined constraints on the unitarity triangle. The LHCb experiment’s unique capabilities were discussed in the heavy ions and fixed-target session. Operating in fixed-target mode, LHCb collected data pertaining to proton–ion and lead–ion interactions during LHC Run 2 using the SMOG system. Key highlights included measurements impacting theoretical models of charm hadronisation, global analyses of nuclear parton density functions, and the identification of helium nuclei and deuterons. The first Run 3 data with the SMOG2 upgrade showed promising results in proton–argon and proton–hydrogen collisions, opening a path to measurements with implications for heavy-ion physics and astrophysics.

The session on flavour-changing charged currents unveiled a recent measurement concerning the longitudinal polarisation of D^*− mesons in B⁰ → D^*−τ^–ν_τ decays, aligning with Standard Model (SM) expectations. Discussions delved into lepton-flavour-universality tests that showed a 3.3σ tension with predictions in the combined R(D^(*)) measurement. Noteworthy were new lattice-QCD predictions for charged current decays, especially R(D^(*)), showcasing disparities in the SM prediction across different lattice groups. Updates on the CKM matrix elements |V_ub| and |V_cb| lead to a reduced tension between inclusive and exclusive determinations. The session also discussed the impact of high-energy constraints of Wilson coefficients on charged-current decays and Bayesian inference of form-factor parameters, regulated by unitarity and analyticity. The QCD spectroscopy and exotics session also featured important findings, including the discovery of novel baryon states, notably Ξ_b(6087)⁰ and Ξ_b(6095)⁰. Pentaquark exploration involved diverse charm–hadron combinations, alongside precision measurements of the Ω⁰_c mass and first observations of b-hadron decays with potential exotic-state contributions. Charmonia-associated production provided fresh insights for testing QCD predictions, and an approach based on effective field theory (EFT) interpreting pentaquarks as hadronic molecules was presented. A new model-independent Born–Oppenheimer EFT framework for the interpretation of doubly heavy tetraquarks, utilising lattice QCD predictions, was introduced. Scrutinising charm–tetraquark decays and the interpretation of newly discovered hadron states at the LHC were also discussed.

During the flavour-changing neutral-current session a new analysis of B⁰ → K^*0μ⁺μ^– decays was presented, showing consistency with SM expectations. Stringent limits on branching fractions of rare charm decays and precise differential branching fraction measurements of b-baryon decays were also highlighted. Challenges in SM predictions for b → sℓℓ and rare charm decays were discussed, underscoring the imperative for a deeper comprehension of underlying hadronic processes, particularly leveraging LHCb data. Global analyses of b → dℓℓ and b → sℓℓ decays were presented, alongside future prospects for these decays in Run 3 and beyond. The session also explored strategies to enhance sensitivity to new physics in B^± → π^±μ⁺μ^– decays.

The keynote talk, delivered by Svjetlana Fajfer, offered a comprehensive summary and highlighted existing anomalies that demand further consideration. Tackling these challenges necessitates precise measurements at both low and high energies, with the collaborative efforts of LHCb, Belle II, CMS and ATLAS. Additionally, advancements in lattice QCD and other novel theoretical approaches are needed for precise theoretical predictions in tandem with experimental efforts.

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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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A significant fraction of ICFA’s work is carried out within a set of seven panels, which meet regularly and assemble expertise on more technical or detailed aspects of particular importance to the field. One is devoted to the International Linear Collider (ILC). For more than two decades, ICFA has promoted the realisation of the ILC, for which a global design effort was put in place in 2005. In parallel, an international collaboration under CERN’s leadership had been working on the Compact Linear Collider (CLIC). Recognising the synergies between the two concepts, ICFA established a single coordinating structure, the Linear Collider Collaboration (LCC), in 2012. Also that year, the Japanese high-energy physics community proposed to host the ILC in Japan as a global project.

The LCC mandate came to an end in 2020, when ICFA put in place the ILC International Development Team (IDT) and its working groups. In June 2021 the IDT developed a proposal for the “preparatory laboratory” as a first step towards the realisation of the ILC in Japan.

Evolving landscape

While the IDT is continuing its work, the global Higgs-factory landscape has evolved since the early days of the ILC: more – linear and circular – studies and proposals are on the table, not least as demonstrated by the P5 report in the US. ICFA will soon discuss in what way its discussions and structures need to be adapted to better reflect this evolving landscape.

In November 2023 ICFA established a new panel devoted to the “data lifecycle”, which involves everything from data acquisition, processing, distribution, storage, access, analysis, simulation and preservation, to management, software, workflows, computing and networking. The panel, which replaces two previous ones on related topics, was created in response to the growing importance of data management and open science in recent years. Its membership is currently being put together with the aim to develop ideas and strategies for workforce development and professional recognition mechanisms.

For more than two decades, ICFA has promoted the realisation of the ILC

ICFA’s farthest-reaching and most visible activity is the ICFA Seminar. The 13th ICFA seminar on “Future Perspectives in High-Energy Physics” took place at DESY from 28 November to 1 December 2023. For the first time in six years (the prior ICFA seminar had taken place in 2017 in Ottawa, Canada), this select crowd of scientists, lab directors and funding agency representatives could come together in person for updates and discussions. One highlight was the panel discussion between the directors of KEK, CERN, Fermilab and IHEP, in which views on a future global strategy were discussed. The seminar concluded on a festive note with the formal passing of the ICFA chair baton from Stuart Henderson (JLAB) to Pierluigi Campana (INFN), who will lead ICFA for the next three years.

ICFA is the only global representation of the particle-physics community, and the ideal discussion forum for global strategic developments, especially large international collider projects. In view of the current situation with numerous opportunities for future facilities – not least a future Higgs factory, but also smaller and more diverse projects – the committee and its panels look forward to serving the field of particle physics through continued advocacy, exploration, discussion and facilitation.    

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“As South Africa, we regard this opportunity as a great privilege for us to host this year’s edition of the TIPP conference,” said minister of higher education, science and innovation Blade Nzimande during an opening address. He was followed by speeches from Angus Paterson, deputy CEO of the National Research Foundation, and Makondelele Victor Tshivhase, director of the national research facility iThemba LABS.

The South African CERN (SA–CERN) programme within the National Research Foundation and iThemba LABS supports more than 120 physicists, engineers and students that contribute to the ALICE, ATLAS and ISOLDE experiments, and to theoretical particle physics. The SA–CERN programme identifies technology transfer in particle physics as key to South African society. This aligns symbiotically with the technology innovation platform of iThemba LABS to create a platform for innovation, incubation, industry collaboration and growth. For the first time, TIPP 2023 included a dedicated parallel session on technology transfer, which was chaired by Massimo Caccia (University of Insubria), Paolo Giacomelli (INFN Bologna) and Christophe De La Taille (CNRS/IN2P3).

The scientific programme kicked off with a plenary presentation on the implementation of the ECFA detector R&D roadmap in Europe by Thomas Bergauer (HEPHY). Other plenary presentations included overviews on bolometers for neutrinos, the Square Kilometre Array (SKA), technological advances by the LHC experiments, NaI experiments, advances in instrumentation at iThemba LABS, micro-pattern gaseous detectors, inorganic and liquid scintillator detectors, noble liquid experiments, axion detection, water cherenkov detectors for neutrinos, superconducting technology for future colliders and detectors, and the PAUL facility in South Africa.

A panel discussion between former CERN Director-General Rolf Heuer (DESY), Michel Spiro (IRFU) and Manfred Krammer (CERN), Imraan Patel (deputy director general of the Department of Science and Innovation), Angus Paterson and Rob Adam (SKA) triggered an exchange of insights about international research infrastructures such as CERN and SESAME for particle physics and science diplomacy.

Prior to TIPP2023, 25 graduate students from Botswana, Cameroon, Ghana, South Africa and Zambia participated in a school of instrumentation in particle, nuclear and medical physics held at iThemba LABS, comprising lectures, hands-on demonstrations, and insightful presentations by researchers from CERN, DESY and IJCLAB, which provided a global perspective on instrumentation.

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Experimental highlights focused on the latest results from CMS and ATLAS. CMS presented a new measurement of the associated production of a Higgs boson with top quarks decaying into b quarks, while ATLAS showed a new measurement of the associated production of a vector boson and a boosted Higgs boson in fully hadronic final states. A major highlight was a new CMS measurement of the Higgs-boson mass in the four-lepton decay channel, reaching the highest precision to date in a single decay channel as well as placing indirect constraints on the Higgs-boson width. Precision measurements were also shown in the framework of effective field theory, which allows potential subtle deviations with respect to the Standard Model to be probed. A small number of intriguing excesses observed, for instance, in the search for partners of the Higgs boson decaying into W-boson or photon pairs were also extensively discussed.

Following a historical talk on the “long and winding road” that led particle physicists from LEP to the discovery of the Higgs boson by Steve Myers, who was CERN director of accelerators and technology when the LHC started up, a dedicated session discussed Higgs-physics prospects at colliders beyond the High-Luminosity LHC (HL-LHC). Patrizia Azzi (INFN Padova) presented the experimental prospects at the proposed Future Circular Collider, and Daniel Schulte (CERN) described the status of muon colliders, highlighting the strong interest within the community and leading to a lively discussion.

The latest theory developments related to Higgs physics were discussed in detail, starting with state-of-the-art predictions for the various Higgs-boson production modes by Aude Gehrmann-De Ridder (ETH Zurich). Andrea Wulzer (CERN) overviewed the theory prospects relevant for future collider projects, while Raffaele Tito D’Agnolo (IPhT, Saclay) presented the connections between the properties of the Higgs boson and cosmology and Arttu Rajantie (Imperial College) focused on implications of the Higgs vacuum metastability on new physics. Finally, a “vision” talk by Matthew McCullough (CERN) questioned our common assumption that the Higgs boson discovered at the LHC is really compatible with Standard Model expectations, considering the current precision of the measurements of its properties.

During several experimental sessions, recent results covering a wide range of topics were presented – in particular those related to vector-boson scattering, since their high-energy behaviour is driven by the properties of the Higgs boson. The Higgs-boson self-coupling was another topic of interest. The best precision on this measurement is currently achieved by combining indirect constraints from processes involving a single Higgs boson together with direct searches for the rare production of a Higgs-boson pair. While the Run 3 data set will provide an opportunity to further improve the sensitivity to the latter, its observation is expected to take place towards the end of HL-LHC operations. Finally, Stéphanie Roccia (LPSC) presented the implications of experimental measurements of the neutron electron dipole moment on the CP-violating couplings of the Higgs boson to fermions, absent in the Standard Model. Concluding talks were given by Massimiliano Grazzini (University of Zurich) and Andrea Rizzi (University and INFN Pisa). The next Higgs Hunting workshop will be held in Orsay and Paris from 23 to 25 September 2024.

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The congress started with parallel meetings on LHC physics, astroparticle physics, nuclear physics and theoretical physics. Two extra sessions were held, one covering technology transfer and the other discussing instrumentation R&D aimed at supporting the HL-LHC, future Higgs factories, and other developments in line with the European strategy for particle physics. The opening ceremony was followed by a lecture by Manuel Aguilar (CIEMAT), who gave an overview of the past 50 years of research in high-energy physics in Spain and the IMFP series. The first edition, held in Formigal (Spanish Pyrenees) in February 1973, was of great significance given the withdrawal of Spain from CERN in 1969, which put high-energy physics in Spain in a precarious position. The participation of prestigious foreign scientists in the first and subsequent editions undoubtedly contributed to the return of Spain to CERN in 1983.

LHC physics was one of the central themes of the event, in particular the first results from Run 3 as well as improvements in theoretical precision and Spain’s contribution to the HL-LHC upgrades. Other discussions and presentations focused on the search for new physics and especially dark-matter candidates, as well as new technologies such as quantum sensors. The conference also reviewed the status of studies related to neutrino oscillations and mass measurements, as well as searches for neutrinoless double beta decay and high-energy neutrinos in astrophysics. Results from gamma-ray and gravitational-wave observatories were discussed, as well as prospects for future experiments.

The programme included plenary sessions devoted to nuclear physics (such as the use of quantum computing to study the formation of nuclei), QCD studies in collisions of very high-energy heavy ions and in neutron stars, and nuclear reactions in storage rings. New technologies applied in nuclear and high-energy physics and their most relevant applications, especially in medical physics, complemented the programme alongside an overview of observational cosmology.

Roundtable discussions focused on grants offered by the European Research Council, R&D strategies and, following a clear presentation of the perspectives of future accelerators by ECFA chair Karl Jacobs (University of Freiburg), possible Spanish strategies for future projects with the participation of industry representatives. The congress also covered science policy, with the participation of the national programme manager Pilar Hernández (University of Valencia).

Prior to the opening of the conference, 170 students from various schools in Cantabria were welcomed to take part in an outreach activity “A morning among scientists” organised by IFCA and CPAN, while Álvaro de Rújula (University of Boston) gave a public talk on artificial intelligence. Finally, an excellent presentation by Antonio Pich (University of Valencia) on open questions in high-energy physics brought the conference to a close.

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The SEENET–MTP network has since grown to include 24 institutions from 12 countries, and more than 450 individual members. There are also 13 partner institutions worldwide. During its 20 years of existence, the network has undertaken: more than 20 projects; 30 conferences, workshops and schools; more than 360 researcher and student exchanges and fellowships; and more than 350 joint papers. Following initial support from CERN’s theoretical physics department, a formal collaboration agreement resulted in the joint CERN–SEENET–MTP PhD training programme with at least 150 students taking part in the first two cycles from 2015 to 2022. Significant support also came from the European Physical Society and ICTP Trieste, and the third cycle of the PhD programme will start in June 2024 in Thessaloniki, Greece.

Networking is the most promising auxiliary mechanism to preserve and build local capacity in fundamental physics in the region

Unfortunately, the general focus on (Western) Balkan states has shifted during the past few years to other parts of the world. However, networking is the most natural and promising auxiliary mechanism to preserve and build local capacity in fundamental physics in the region. The central SEENET-MTP event in this anniversary year, the BWXX workshop held in Vrnjačka Banja from 29 to 31 August 2023, marked the endurance of the initiative and offered 30 participants an opportunity to consider topics such as safe supersymmetry breaking (B Bajc, Slovenia), string model building using quantum annealers (I Rizos, Greece), entropy production in open quantum systems (A Isar, Romania), advances in noncommutative field theories and gravity (M Dimitrijević Ćirić, Serbia), and the thermodynamic length for 3D holographic models and optimal processes (T Vetsov, Bulgaria).

A subsequent meeting held during an ICTP workshop on string theory, holography and black holes from 23 to 27 October 2023, partially supported by CERN, invited participants to brainstorm about future SEENET–MTP activities – the perfect setting to trace the directions of this important network’s activity in its third decade.

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The workshop gathered almost 200 scientists and engineers in a hybrid format. Students, including undergraduates, and early-career researchers were strongly represented, as were key industry partners. A strong aim of the conference was to engage scientific communities outside particle physics to develop areas where the tools and techniques from particle physics could be game-changing.

The workshop focused on current and emerging techniques and scientific applications for deep learning and inference acceleration, including novel methods for efficient algorithm design, ultrafast on-detector inference and real-time systems. Acceleration as a service, hardware platforms, coprocessor technologies, distributed learning and hyper-parameter optimisation. The four-day event consisted of three workshop-style days with invited and contributed talks, and a final day dedicated to technical demonstrations and satellite meetings.

The tools and techniques from particle physics could be game-changing

The interdisciplinary nature of the workshop – which encompassed particle physics, free electron lasers, nuclear fusion, astrophysics, computer science and biology – made for a varied and interesting agenda. Attendees heard talks on how fast machine learning is being harnessed to speed up the identification of gravitational waves, and how it is needed to handle the high data rates and fast turnaround of experiments at free-electron lasers. In the medical arena, speakers addressed the need for faster image processing and data analysis for diagnosis and treatment, and the use of fast machine learning in biology to search for known and unknown features in large, heterogeneous datasets. The use of machine learning in control systems and simulations was discussed in the context of laser-driven accelerators and nuclear-fusion experiments, while in theoretical physics the application of machine learning to solve the electron wave equation in condensed matter, working towards a detailed and fundamental understanding of superconductivity, was presented.

Industry partners including AMD, Graphcore, Groq and Intel discussed current- and future-generation hardware platforms and architectures, and facilitated tutorials on their development toolchains. Researchers from Groq and Graphcore presented their latest dedicated chips for artificial-intelligence applications and showed that they have interesting applications to problems in particle physics, weather forecasting, protein folding, fluid dynamics, materials science and solving partial differential equations. AMD and Intel demonstrated the flexibility of their FPGA platforms and explained how to optimise them for scientific machine-learning applications.

A highlight of the social programme was a public lecture from Grammy Award-winning rapper Lupe Fiasco, who discussed his work with Google on large-language models. The workshop will return to the US next year, before landing in Zurich in 2025.

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The first ICFA Beam Dynamics workshop on Cold Copper Accelerator Technology and Applications was held at Cornell University from 31 August to 1 September 2023. Nearly 100 people came together to discuss the technology and explore next directions for R&D. Originally conceived at SLAC as an attractive approach to a linear-collider Higgs factory (dubbed the Cool Copper Collider, C³), interest in the technology has expanded to other areas.

Following opening presentations by Julia Thom-Levy (Cornell associate vice provost for research and innovation) and Jared Maxson (who leads the cold copper programme at Cornell), Emilio Nanni (SLAC) presented an overview of radio-frequency (RF) breakthroughs using cold copper cavities. He described three major advantages over conventional materials such as superconducting niobium: increased material conductivity at cryogenic temperatures (a reduction in resistance by a factor of three), significant reduction in pulsed heating, and improved yield strength and thermal diffusion. Combined, these lead to a high potential acceleration gradient of 70–120 MV/m, and an estimated 8 km footprint for a 550 GeV Higgs factory.

The optimised C-band cavity design enables a novel coupling of RF signals into each of the 40 cells along the cavity. A 9 m-long cryomodule would provide 1 GeV of acceleration. Some challenges identified for future R&D in the coming years are vibration control, meeting linac alignment specifications of 10 microns, and reducing the cost via optimised RF. Other applications of cold-copper technology include an ultra-compact free-electron laser (FEL) with 10–100 fs timing resolution as well as synergies with other proposed colliders such as ILC and FCC, where it could be used for positron production or as an injector, respectively. Walter Wuensch (CERN) summarised the extensive work over the past two decades on high-field limitations to copper performance. Breakdowns, field emission current and pulsed heating are fundamental limitations to performance, along with some practical ones such as limited RF power, conditioning time, small-aperture requirements, wakefields, power feeds and cooling capacity. Wuensch concluded that the community has a reasonably good understanding of copper, but that the demands for higher gradients and more performant cavities require careful optimisation.

The accelerator R&D community has a reasonably good under-standing of copper, but the demands for higher gradients and more performant cavities require careful optimisation

The workshop also delved into the details of cryomodule design, fabrication and damping, as well as the progress of relevant developments at LANL and INFN Frascati. Numerous industry participants gave presentations, including researchers from Radiabeam, Scandinova, Canon, EEC Permanent Magnets and Calabazas Creek.

Day two started with Caterina Vernieri (SLAC) presenting the C³ ambition for a Higgs factory based on extensive, recently published studies. Jamie Rosenzweig (UCLA) presented the design for an ultra-compact FEL and Paul Gueye (Michigan State) provided an overview of a potential high-gradient linac at the Facility for Rare Isotope Beams. Sami Tantawi (SLAC) presented potential medical applications of the technology, aimed at FLASH and very-high-energy-electron treatment modalities. Xi Yang (BNL) reviewed ultrafast electron diffraction devices and how moving from keV to MeV energies using compact copper accelerators could open new research opportunities. A session devoted to sustainability at CERN was covered by Maxim Titov (CEA Saclay), while Sarah Carsen (Cornell) presented the renewable programme at Cornell, which includes lake-source cooling of the campus and CESR accelerator complex, 28 MW of installed solar power, as well as geothermal plans. The successful mini-workshop concluded with a request to complete a report summarising the R&D discussions and post them on the Indico workshop site.

The accelerator R&D community awaits the P5 report (see p7) and the resulting strategies of the Department of Energy and National Science Foundation for accelerator research over the next decade.

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History could have turned out differently, said Haidt, since both CERN and Brookhaven National Laboratory (BNL) were competing in the new era of high-energy neutrino physics: “The CERN beam was a flop initially, allowing BNL to snatch the muon-neutrino discovery in 1962, but a second attempt at CERN was better.” This led André Lagarrigue to dream of a giant bubble chamber, Gargamelle, financed and built by French institutes and operated by CERN with beams from the Proton Synchrotron (PS) from 1970 to 1976. Picking out the neutral-current signal from the neutron-cascade background was a major challenge, and a solution seemed hopeless until Haidt and his collaborators made a breakthrough regarding the meson component of the cascade.

The ten years between the discovery of neutral currents and the W and Z bosons are what took CERN from competent mediocrity to world leader

Lyn Evans

By early July 1973, it was realised that Gargamelle had seen a new effect. Paul Musset presented the results in the CERN auditorium on 19 July, yet by that autumn Gargamelle was “treated with derision” due to conflicting results from a competitor experiment in the US. ‘The Gargamelle claim is the worst thing to happen to CERN,’ Director-General John Adams was said to have remarked. Jack Steinberger even wagered his cellar that it was wrong. Following further cross checks by bombarding the detector with protons, the Gargamelle result stood firm. At the end of Haidt’s presentation, collaboration members who were present in the audience were recognised with a warm round of applause.

From the PS to the SPS
The neutral-current discovery and the subsequent Gargamelle measurement of the weak mixing angle made it clear not only that the electroweak theory was right but that the W and Z were within reach of the technology of the day. Moving from the PS to the SPS, Jean-Pierre Revol (Yonsei University) took the audience to the UA1 and UA2 experiments ten years later. Again, history could have taken a different turn. While CERN was working towards a e⁺e^– collider to find the W and Z, said Revol, Carlo Rubbia proposed the radically different concept of a hadron collider — first to Fermilab, which, luckily for CERN, declined. All the ingredients were presented by Rubbia, Peter McIntyre and David Cline in 1976; the UA1 detector was proposed in 1978 and a second detector, UA2, was proposed by CERN six months later. UA1 was huge by the standards of the day, said Revol. “I was advised not to join, as there were too many people! It was a truly innovative project: the largest wire chamber ever built, with 4π coverage. The central tracker, which allowed online event displays, made UA1 the crucial stepping stone from bubble chambers to modern electronic ones. The DAQ was also revolutionary. It was the beginning of computer clusters, with same power as IBM mainframes.”

First SppS collisions took place on 10 July 1981, and by mid-January 1983 ten candidate W events had been spotted by the two experiments. The W discovery was officially announced at CERN on 25 January 1983. The search for the Z then started to ramp up, with the UA1 team monitoring the “express line” event display around the clock. On 30 April, Marie Noelle Minard called Revol to say she had seen the first Z. Rubbia announced the result at a seminar on 27 May, and UA2 confirmed the discovery on 7 June. “The SppS was a most unlikely project but was a game changer,” said Haidt. “It gave CERN tremendous recognition and paved the way for future collaborations, at LEP then LHC.”

Former UA2 member Pierre Darriulat (Vietnam National Space Centre) concurred: “It was not clear at all at that time if the collider would work, but the machine worked better than expected and the detectors better than we could dream of.” He also spoke powerfully about the competition between UA1 and UA2: “We were happy, but it was spoiled in a way because there was all this talk of who would be ‘first’ to discover. It was so childish, so ridiculous, so unscientific. Our competition with UA1 was intense, but friendly and somewhat fun. We were deeply conscious of our debt toward Carlo and Simon [van der Meer], so we shared their joy when they were awarded the Nobel prize two years later.” Darriulat emphasised the major role of the Intersecting Storage Rings and the input of theorists such as John Ellis and Mary K Gaillard, reserving particular praise for Rubbia. “Carlo did the hard work. We joined at the last moment. We regarded him as the King, even if we were not all in his court, and we enjoyed the rare times when we saw the King naked!”

Our competition with UA1 was intense, but friendly and somewhat fun

Pierre Darriulat

The ten years between the discovery of neutral currents and the W and Z bosons are what took CERN “from competent mediocrity to world leader”, said Lyn Evans in his account of the SppS feat. Simon van der Meer deserved special recognition, not just for his 1972 paper on stochastic cooling, but also his earlier invention of the magnetic horn, which was pivotal in increasing the neutrino flux in Gargamelle. Evans explained the crucial roles of the Initial Cooling Experiment and the Antiproton Accumulator, and the many modifications needed to turn the SPS into a proton-antiproton collider. “All of this knowledge was put into the LHC, which worked from the beginning extremely well and continues to do so. One example was intrabeam scattering. Understanding this is what gives us the very long beam lifetimes at the LHC.”

Long journey
The electroweak adventure began long before CERN existed, pointed out Wolfgang Hollik, with 2023 also marking the 90^th anniversary of Fermi’s four-fermion model. The incorporation of parity violation came in 1957 and the theory itself was constructed in the 1960s by Glashow, Salam, Weinberg and others. But it wasn’t until ‘t Hooft and Veltman showed that the theory is renormalizable in the early 1970s that it became a fully-fledged quantum field theory. This opened the door to precision electroweak physics and the ability to search for new particles, in particular the top quark and Higgs boson, that were not directly accessible to experiments. Electroweak theory also drove a new approach in theoretical particle physics based around working groups and common codes, noted Hollik.

The afternoon session of the symposium took participants deep into the myriad of electroweak measurements at LEP and SLD (Guy Wilkinson, University of Oxford), Tevatron and HERA (Bo Jayatilaka, Fermilab), and finally the LHC (Maarten Boonekamp, Université Paris-Saclay and Elisabetta Manca, UCLA). The challenges of such measurements at a hadron collider, especially of the W-boson mass, were emphasised, as were their synergies with QCD in measurements in improving the precision of parton distribution functions.

The electroweak journey is far from over, however, with the Higgs boson offering the newest exploration tool. Rounding off a day of excellent presentations and personal reflections, Rebeca Gonzalez Suarez (Uppsala University) imagined a symposium 40 years from now when the proposed collider FCC-ee at CERN has been operating for 16 years and physicists have reconstructed nearly 10¹³ W and Z bosons. Such a machine would take the precision of electroweak physics into the keV realm and translate to a factor of seven increase in energy scale. “All of this brings exciting challenges: accelerator R&D, machine-detector interface, detector design, software development, theory calculations,” she said. “If we want to make it happen, now is the time to join and contribute!”

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Kaons, which contain one strange and either a lighter up or down quark, have played a central role in the development of the Standard Model (SM). Augusto Ceccucci (CERN) pointed out that many of the SM’s salient features – including flavour mixing, parity violation, the charm quark and CP violation – were discovered through the study of kaons, leading to the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. The full particle content of the SM was finally experimentally established at CERN with the Higgs-boson discovery in 2012, but many open questions remain.

The kaon’s special role in this context was the central topic of the workshop. The study of rare kaon decays provides a unique sensitivity to new physics, up to  scales higher than those at collider experiments. In the SM, the rare decay of a charged or neutral kaon into a pion plus a pair of charged or neutral leptons is strongly suppressed, even more so than the similar rare B-meson decays. This is due to the absence at tree-level of flavour-changing neutral current interactions (e.g. s → d) in the SM. Such a transition can only proceed at loop level involving the creation of at least one very heavy (virtual) electroweak gauge boson (figure “Decayed”, left). While experimentally this suppression constitutes a formidable challenge in identifying the decay products amongst a variety of background signals, new-physics contributions could leave a significantly measurable imprint through tree-level or virtual contributions. In contrast to rare B decays, the “gold-plated” rare kaon decay channels K⁺→π⁺νν and K_L→π⁰νν do not suffer from large hadronic uncertainties and are experimentally clean due to the limited number of possible decay channels.

The charged-kaon decay is currently being studied at NA62, and a measurement of its branching ratio with a precision of 15% is expected by 2025. However, as highlighted by NA62 physics coordinator Karim Massri (Lancaster University), to improve this measurement and thus significantly increase the  likelihood of a discovery, the experimental precision must be reduced to the level of the theoretical prediction, i.e. 5%. This can only be achieved with a next-generation experiment. The HIKE experiment, a proposed high-intensity kaon factory at CERN currently under approval, would reach the 5% precision goal on the measurement of K⁺→π⁺νν during its first phase of operation. experiment, a future high-intensity kaon factory at CERN currently under approval, will reach the 5% precision goal on the measurement of K⁺→π⁺νν during its first phase of operation. Afterwards, a second phase with a neutral K_L beam aiming at the first observation of the very rare decays K_L→π⁰ℓ⁺ℓ^– is foreseen. With a setup and detectors optimised for the measurement of the most challenging processes, the HIKE programme would be able to achieve unprecedented precision on most K⁺ and K_L decays.

For KOTO, Koji Shimi and Hashime Nanjo reported on the experimental progress on K_L→π⁰ℓ⁺ℓ^– and presented a new bound on its branching ratio. A planned phase two of KOTO, if funded, aims to measure the branching ratio with a precision of 20%. Although principally designed for the study of (rare) bottom-quark decays, LHCb can also provide information about the rare decay of the shorter-lived K_S.Radoslav Marchevski (EPFL Lausanne) presented the status and the prospects for a proposed LHCb-Phase II upgrade.

From the theory perspective, underpinned by impressive new perturbative, lattice QCD and effective-field-theory calculations presented at the workshop, the planned measurement of K⁺→π⁺νν at HIKE clearly has discovery potential, remarked Gino Isidori (University of Zurich). Together with other rare decay channels such as K_L→μ⁺μ^–, K_L→π⁰ℓ⁺ℓ^– and K⁺→π⁺ℓ⁺ℓ^– that would be measured by HIKE, added Giancarlo D’Ambrosio (INFN), the combined global theory analyses of experimental data will allow for discovering new physics if it exists within the reach of the experiment, and for providing solid constraints for new physics.

A decision on HIKE and other proposed experiments in CERN’s North Area will take place in early December.

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The conference provided an excellent review of the status of scientific questions being addressed by experiments in underground labs, including the latest constraints on dark matter from PandaX, LUX-ZEPLIN, SuperCDMS, CRESST and XENONnT. The various techniques for studying dark matter indirectly, for example via cosmic radiation, were reviewed, as well as direct searches at accelerator facilities. The many and diverse efforts ongoing worldwide to understand the nature of neutrinos were covered comprehensively, including the parametrisation of their mixing properties, their absolute mass, whether neutrinos are their own antiparticle, and their role in the early and late universe and in supernova explosions. Two plenary presentations focused on recent highlights in the field: IceCube’s confirmation of neutrinos from the galactic plane, and evidence of a GW background at nanohertz frequencies measured with pulsar timing arrays (CERN Courier September/October 2023 p7). Others summarised the status of cosmology in theory and experiment, cosmic-ray physics and the detection of GWs.

Among participants was Arthur McDonald, co-recipient of the 2015 Nobel Prize in Physics for the discovery of neutrino oscillations, who gave a talk “Using messengers from outer space to understand our universe and its evolution” to a packed audience of all ages in the Festsaal ÖAW. He also celebrated his 80th birthday during the conference, earning a big round of applause.

A total of 110 posters were presented, more than half from early-career scientists. The five winners were: Korbinian Urban (TUM) for “TRISTAN: A novel detector for searching keV-sterile neutrinos at the KATRIN experiment”; Christoph Wiesinger (TUM) for “TAXO – Towards an ultra-low background semiconductor detector for IAXO”; Steffen Turkat (TU Dresden) for “Low-background radioactivity counting at the most sensitive HPGE detector in Germany”; Angelina Kinast (TUM) for “First results on 170 enrichment of CaWO₄ crystals for spin-dependent DM search with CRESST”; and Krystal Alfonso (Virginia Tech) for “Analysis techniques for the search of neutrinoless double-beta decay of Te-130 with CUORE”.

The next edition of TAUP will take place in 2025 in Chengdu, China.

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The participants were treated to many LHC Run 2 legacy results, as well as brand-new ones using freshly analysed Run 3 data. A large chunk of these results comprised precision measurements of the Higgs boson in view of gaining a deeper understanding of the origin of electroweak symmetry breaking. As the Higgs boson is deeply connected to many open questions potentially linked to physics beyond the Standard Model (SM), such as the origin of particle masses and flavour, studying it in the context of effective field theory is a particularly hot topic. A rich potential programme of “simplified” models for Higgs physics that can better quantify the reach of the LHC and offer new observables is also under development.

New frontiers

The ATLAS and CMS collaborations presented no fewer than 37 and 27 new preliminary results, respectively. Besides Higgs-sector physics, the experiments revealed their latest results of searches for physics beyond the SM, including new limits on the existence of supersymmetric and dark-matter particles. At the intensity frontier, the latest search for the ultra-rare decay K⁺ → π⁺e⁺e^–e⁺e^– from the NA62 experiment placed upper limits on dark-boson candidate masses, underlining the powerful complementarity between CERN’s fixed-target and LHC programmes. The Belle II collaboration presented first evidence of the decay B⁺ → K⁺ νν, as well as the result of their R(X) = Br(B → Xτν_τ)/Br(B → Xℓν_ℓ) measurement – the first at a B factory. The LHCb collaboration also presented an update of its recent R(D^*) = Br(B → D^*τν_τ)/Br(B → D^*ℓν_ℓ) measurement. Another highlight was LHCb’s observation of the hypernuclei antihypertriton and hypertriton.

Intense discussions took place on novel and potentially game-changing accelerator concepts

The state of the art in neutrino physics was presented, covering the vast landscape of experiments seeking to shed light on the three-flavour paradigm as well as the origin of the neutrino masses and mixings. So far, analyses by T2K and NOvA show a weak preference for a normal mass ordering, while the inverted mass ordering is not yet ruled out. With a joint analysis between T2K and NOvA in progress, updates are expected next year. At CERN the FASER experiment, which made the first observation of muon neutrinos at a collider earlier this year, presented the first observation of collider electron neutrinos. Looking outwards, a long-awaited discovery of galactic neutrinos was presented by IceCube.

The current FCC feasibility status was presented, along with that of other proposed colliders that could serve as Higgs factories. The overarching need to join forces between the circular- and linear-collider communities and to use all the gained knowledge for getting at least one accelerator approved was reflected during the discussions and many talks, as were the sustainability and energy consumption of detector and accelerator concepts. Intense discussions took place on novel and potentially game-changing accelerator concepts, such as energy recovery technologies or plasma acceleration. While not yet ready to be used on a large scale, they promise to have a big impact on the way accelerators are built in the future. Beyond colliders, the community also looked ahead to the DUNE and Hyper-Kamiokande experiments, and to proposed experiments such as the Einstein Telescope and those searching for axions.

A rich social programme included a public lecture by Andreas Hoecker (CERN) about particle physics at the highest energies, a concert with an introduction to the physics of the organ by Wolfgang Hillert (University of Hamburg), as well as an art exhibition called “High Energy” and a Ukrainian photo exhibition depicting science during times of war.

The next EPS-HEP conference will take place in 2025 in Marseille.

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Over five days of pronouncements, presentations and posters, topics included current and future prospects in detector technologies, advances in theoretical calculations (with a particular focus on effective field theories), and improving diversity and outreach in physics. Results from a large number of experiments were presented, many of which are building excitement for the next generation of measurements that seek to provide even more rigorous tests of the Standard Model (SM) and improved searches for physics beyond it (BSM).

The results presented were too numerous to review comprehensively. However, they tended to skew towards flavour physics, with a particular emphasis on searches for CP- and lepton flavour-violation and tests of lepton-flavour universality (LFU). Overall, tensions between the SM and experimental measurements of LFU remain. In particular, Kazuki Kojima (Nagoya University) presented a measurement of R(D^*), which is a test of LFU performed with B-meson decays, finding the ratio R(D^*) = 0.267 ^+0.041 _–0.039 (stat.) ^+0.028 _–0.033 (syst.). While compatible with the SM, it increases the tension with theory from 3.2σ to 3.3σ when all measurements of R(D) and R(D^*) are combined.

Not to be outdone, the LHC experiments presented a range of precision measurements of SM parameters, further reducing the available parameter space for BSM physics. In particular, Linda Finco (INFN Torino) from ATLAS presented the most precise measurement of the Higgs-boson mass: 125.11 ± 0.09 (stat.) ± 0.06 (syst.) GeV, using the full Run 1 and Run 2 datasets for both the H → ZZ → 4ℓ and H → γγ channels. This is one of the most precisely determined masses of any SM particle, a real achievement of precision physics.

Now that the available parameter space for BSM models is shrinking, more innovative approaches to particle physics are needed. One such approach, presented by Ling Sun (Australian National University), is to use the phenomenon of superradiance to search for ultralight bosons around rapidly rotating black holes. The boson clouds extract angular momentum from the black hole when the superradiance condition is met, producing gravitational radiation that could be measured by current and future gravitational-wave detectors. Such a method provides an avenue to measure particles that interact only through gravity, opening a novel avenue for exploring particles beyond the SM.

On the penultimate evening, Alan Duffy (Swinburne University) and Suzie Sheehy (University of Oxford and University of Melbourne) delivered a public lecture “How to discover a universe” to a mix of conference participants, high-school students and the interested public, stressing that science is cultural as well as technological. The best poster was awarded to Emily Filmer (University of Adelaide) for “Searches for BSM physics using challenging long-lived signatures with the ATLAS detector”, while the “people’s choice” was awarded to Eliot Walton (Monash University) for her poster “The Queer History of Physics”. Australia’s small but growing particle-physics community was extremely well represented, and the exposure of the global community to us made Lepton Photon 2023 a resounding success.

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This year’s conference focused on “Hot topics in particle physics and cosmology: theory facing experimental prospects”. The first day’s plenaries were mainly devoted to machine-learning techniques and to collider physics. Enthusiastic speakers on the former met with some reservation in parts of the audience, which stressed the need for a good balance between new techniques and new physics ideas, while the collider talks emphasised the importance of precision Higgs physics and its prospects at the LHC and HL-LHC for a full understanding of the Brout–Englert–Higgs mechanism. On the theory side, the approach of effective field theory was strongly advocated. A separate session was devoted to flavour physics, in which new ideas on the origin of flavour were presented. A review of rare decays as precision tests of the Standard Model (SM) and as probes of new physics complemented the experimental summary.

The conference was dominated by topics at the interface between particle physics and cosmology. Covered in the many talks were axion couplings and search strategies, axions in rare decays, models of CP violation with nucleon and atomic electric dipole moments with or without the QCD axion, searches for very light and weakly interacting axion-like particles as a complementarity to heavy new particle searches in colliders, and much more.

The conference was very successful in connecting new theoretical ideas with planned experimental programmes

Another issue vividly under consideration was dark matter. Among important theoretical questions is the role of gravity in the production of dark matter. Avoiding overabundance of gravitationally produced dark matter is an important constraint on effective quantum gravity. Similar logic concerns right-handed neutrinos as a dark-matter candidate in simple extensions of the SM. Both were analysed in a number of presentations. Anticipating the results from the Nanograv experiment (CERN Courier September/October 2023 p7), various concrete sources of such signals were reviewed, such as primordial black-hole production, domain walls, cosmic strings and phase transitions in the early universe. Selected theoretical aspects of dark-matter models (such as accidental dark matter with its several realisations) and the analysis of their experimental signatures through new theoretical developments in computing high-energy photon signals from heavy classic WIMPs were presented.

More exotic problems at the interface between particle physics and cosmology were also touched upon. One example is how annihilating dark matter can affect late stellar evolution and the spectrum of black holes, which can be tested with gravitational-wave observations. Another is how the apparent anomaly in the primordial abundance of ⁴He can be linked to a neutrino–antineutrino asymmetry in the early universe that impacts Big Bang nucleosynthesis. Gravitational waves as a probe of beyond-the-SM physics were discussed at length, also including possible new-physics signals from pulsar timing arrays.

The conference was very successful in connecting new theoretical ideas with planned experimental programmes. The next Planck meeting will be held in Lisbon.

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An official partner of UNESCO, Rencontres du Vietnam’s scientific conferences and schools promote collaboration between Vietnamese or Asia-Pacific scientists and colleagues from other parts of the world. ICISE’s ambitious goal is to focus on the development of science and education, helping young Asian students and scientists to grow their knowledge by attending lectures and exchanging ideas with high-level overseas counterparts.

The 2023 event, entitled “Windows on the Universe”, took place from 6 to 12 August and consisted of two joint conferences, one reporting on the progress and developments in particle physics and the other discussing recent developments in astrophysics. The conference featured joint sessions between both communities, as well as separate plenary and parallel sessions for each discipline. The event attracted some 150 participants, including theorist and 1999 Nobel Laureate Gerard ’t Hooft.

In the tradition of ICISE-based conferences, a significant proportion of participants came from Asia, in particular from Vietnam, where the fundamental-research community has grown considerably since the start of ICISE activities. For example, Vietnam is now a member of the T2K experiment. Son Cao (IFIRSE) gave the plenary review on the results from this and other long-baseline neutrino experiments. Many others, including young scientists, presented their latest work during the parallel sessions.

During an extended opening session, some of the very first “Rencontres” participants shared entertaining memories of how it all began. The scientific part of the meeting followed, with keynote talks from a select group of excellent speakers covering most of the activities in particle physics and astrophysics. A highlight was the final day, when different views on future directions in particle physics were discussed, and the latest Fermilab muon g−2 measurement experiment – released just hours beforehand (see Muon g-2 update sets up showdown with theory) – was presented.

Throughout the week, ICISE confirmed its reputation as an excellent venue for conferences in Southeast Asia. At the end of the meeting, a group of some 40 scientists accepted an invitation to spend a day in Hanoi for an audience with Vietnam’s president, Võ Văn Thưởng, who, together with his staff, discussed science and education in the country. This was the final highlight of a very successful celebratory edition of the Rencontres du Vietnam in 2023.

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About 90 physicists attended the sixth plenary workshop of the Muon g-2 Theory Initiative, held in Bern from 4 to 8 September, to discuss the status and strategies for future improvements of the Standard Model (SM) prediction for the anomalous magnetic moment of the muon. The meeting was particularly timely given the recent announcement of the results from runs two and three of the Fermilab g-2 experiment (Muon g-2 update sets up showdown with theory), which reduced the uncertainty of the world average to 0.19 ppm, in dire need of a SM prediction at commensurate precision. The main topics of the workshop were the two hadronic contributions to g-2, hadronic vacuum polarisation (HVP) and hadronic light-by-light scattering (HLbL), evaluated either with a lattice–QCD or data-driven approach.

Hadronic vacuum polarisation

The first one-and-a-half days were devoted to the evaluation of HVP – the largest QCD contribution to g-2, whereby a virtual photon briefly transforms into a hadronic “blob” before being reabsorbed – from e⁺e^– data. The session started with a talk from the CMD-3 collaboration at the VEPP-2000 collider, whose recent measurement of the e⁺e^– → π⁺π^– cross section generated shock waves earlier this year by disagreeing (at the level of 2.5–5σ) with all previous measurements used in the Theory Initiative’s 2020 white paper. The programme also featured a comparison with results from the earlier CMD-2 experiment, and a report from seminars and panel discussions organised by the Theory Initiative in March and July on the details of the CMD-3 result. While concerns remain regarding the estimate of certain systematic effects, no major shortcomings could be identified.

Further presentations from BaBar, Belle II, BESIII, KLOE and SND detailed their plans for new measurements of the 2π channel, which in the case of BaBar and KLOE involve large data samples never analysed before for this measurement. Emphasis was put on the role of radiative corrections, including a recent paper by BaBar on additional radiation in initial-state-radiation events and, in general, the development of higher-order Monte Carlo generators. Intensive discussions reflected a broad programme to clarify the extent to which tensions among the experiments can be due to higher-order radiative effects and structure-dependent corrections. Finally, updated combined fits were presented for the 2π and 3π channels, for the former assessing the level of discrepancy among datasets, and for the latter showing improved determinations of isospin-breaking contributions.

CMD-3 generated shock waves by disagreeing with all previous measurements at the level of 2.5-5σ

Six lattice collaborations (BMW, ETMC, Fermilab/HPQCD/MILC, Mainz, RBC/UKQCD, RC^*) presented updates on the status of their respective HVP programmes. For the intermediate-window quantity (the contribution of the region of Euclidean time between about 0.4–1.0 fm, making up about one third of the total), a consensus has emerged that differs from e⁺e^–-based evaluations (prior to CMD-3) by about 4σ, while the short-distance window comes out in agreement. Plans for improved evaluations of the long-distance window and isospin-breaking corrections were presented, leading to the expectation of new, full computations for the total HVP contribution in addition to the BMW result in 2024. Several talks addressed detailed comparisons between lattice-QCD and data-driven evaluations, which will allow physicists to better isolate the origin of the differences once more results from each method become available. A presentation on possible beyond-SM effects in the context of the HVP contribution showed that it seems quite unlikely that new physics can be invoked to solve the puzzles.

Light-by-light scattering

The fourth day of the workshop was devoted to the HLbL contribution, whereby the interaction of the muon with the magnetic field is mediated by a hadronic blob connected to three virtual photons. In contrast to HVP, here the data-driven and lattice-QCD evaluations agree. However, reducing the uncertainty by a further factor of two is required in view of the final precision expected from the Fermilab experiment. A number of talks discussed the various contributions that feed into improved phenomenological evaluations, including sub-leading contributions such as axial-vector intermediate states as well as short-distance constraints and their implementation. Updates on HLbL from lattice QCD were presented by the Mainz and RBC/UKQCD groups, as were results on the pseudoscalar transition form factor by ETMC and BMW. The latter in particular allow cross checks of the numerically dominant pseudoscalar- pole contributions between lattice QCD and data-driven evaluations.

It is critical that the Theory Initiative work continues beyond the lifespan of the Fermilab experiment

On the final day, the status of alternative methods to determine the HVP contribution were discussed, first from the MUonE experiment at CERN, then from τ data (by Belle, CLEOc, ALEPH and other LEP experiments). First MUonE results could become available at few-percent precision with data taken in 2025, while a competitive measurement would proceed after Long Shutdown 3. For the τ data, new input is expected from the Belle II experiment, but the critical concern continues to be control over isospin-breaking corrections. Progress in this direction from lattice QCD was presented by the RBC/UKQCD collaboration, together with a roadmap showing how, potentially in combination with data-driven methods, τ data could lead to a robust, complementary determination of the HVP contribution.

The workshop concluded with a discussion on how to converge on a recommendation for the SM prediction in time for the final Fermilab result, expected in 2025, including new information expected from lattice QCD, the BaBar 2π analysis and radiative corrections. A final decision for the procedure for an update of the 2020 white paper is planned to be taken at the next plenary meeting in Japan in September 2024. In view of the long-term developments discussed at the workshop – not least the J-PARC Muon g-2/EDM experiment, due to start taking data in 2028 – it is critical that the work by the Theory Initiative continues beyond the lifespan of the Fermilab experiment, to maximise the amount of information on physics beyond the SM that can be inferred from precision measurements of the anomalous magnetic moment of the muon.

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The key focus of the conference series is to review the latest results in heavy-flavour physics and discuss future directions. Heavy-flavour decays, in particular those of hadrons that contain b quarks, offer powerful probes of physics beyond the Standard Model (SM). Beauty 2023 took place 30 years after the opening meeting in the series. A dedicated session was devoted to reflections on the developments in flavour physics over this period, and also celebrating the life of Sheldon Stone, who passed away in October 2021. Sheldon was both an inspirational figure in flavour physics as a whole, a driving force behind the CLEO, BTeV and LHCb experiments, and a long-term supporter of the Beauty conference series.

LHC results
Many important results have emerged from the LHC since the last Beauty conference. One concerns the CP-violating parameter sin2β, for which measurements by the BaBar and Belle experiments at the start of the millennium marked the dawn of the modern flavour-physics era.  LHCb has now measured sin2β with a precision better than any other experiment, to match its achievement for ϕ_s, the analogous parameter in B_s⁰ decays, where ATLAS and CMS have also made a major contribution. Continued improvements in the knowledge of these fundamental parameters will be vital in probing for other sources of CP violation beyond the SM.

Over the past decade, the community has been intrigued by strong hints of the breakdown of lepton-flavour universality, one of the guiding tenets of the SM, in B decays. Following a recent update from LHCb, it seems that lepton universality may remain a good symmetry, at least in the class of electroweak-penguin decays such as B→K^(*)l⁺l^–, where much of the excitement was focused (CERN Courier January/February 2023 p7). Nonetheless, there remain puzzles to be understood in this sector of flavour physics, and anomalies are emerging elsewhere. For example, non-leptonic decays of the kind B_s→ D_s ⁺K^– show intriguing patterns through CP-violation and decay-rate information.

The July conference was noteworthy as being a showcase for the first major results to emerge from the Belle II experiment. Belle II has now collected 362 fb^-1 of integrated luminosity on the Υ(4S) resonance, which constitutes a dataset similar in size to that accumulated by BaBar and the original Belle experiment, and results were shown from early tranches of this sample.  In some cases, these results already match or exceed in sensitivity and precision what was achieved at the first generation of B-factory experiments, or indeed elsewhere. These advances can be attributed to improved instrumentation and analysis techniques. For example, world-leading measurements of the lifetimes of several charm hadrons were presented, including the D⁰, D⁺, D_s⁺ and Λ_c⁺. Belle II and its accelerator, SuperKEKB, will emerge from a year-long shutdown in December with the goal to increase the dataset by a factor of 10-20 in the coming half decade.

Full of promise
The future experimental programme of flavour physics is full of promise. In addition to the upcoming riches expected from Belle II, an upgraded LHCb detector is being commissioned in order to collect significantly larger event samples over the coming decade. Upgrades to ATLAS and CMS will enhance these experiments’ capabilities in flavour physics during the High-Luminosity LHC era, for which a second upgrade to LHCb is also foreseen. Conference participants also learned of the exciting possibilities for flavour physics at the proposed future collider FCC-ee, where samples of several 10¹² Z⁰ decays will open the door to ultra-precise measurements in an analysis environment much cleaner than at the LHC. These projects will be complemented by continued exploration of the kaon sector, and studies at the charm threshold for which a high-luminosity Super Tau Charm Factory is proposed in China.

The scientific programme of Beauty 2023 was complemented by outreach events in the city, including a `Pints of Science’ evening and a public lecture, as well as a variety of social events. These and the stimulating presentations made the conference a huge success, demonstrating that flavour remains a vibrant field and continues to be a key player in the search for new physics beyond the Standard Model.

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The PHYSTAT series of seminars and workshops provides a unique meeting ground for physicists and statisticians. The latest in-person meeting, after previously being postponed due to COVID, covered the field of systematic errors (sometimes known as nuisance parameters), which are becoming increasingly important in particle physics as larger datasets reduce statistical errors in many analysis channels. Taking place from 23 to 28 April at the Banff International Research Station (BIRS) in the Canadian Rockies, the workshop attracted 42 delegates working not only on the LHC experiments but also on neutrino physics, cosmic-ray detectors and astrophysics.

The organisers had assigned half of the time to discussions, and that time was used. Information flowed in both directions: physicists learned about the Wasserstein distance and statisticians learned about jet energy scales. The dialogue was constructive and positive – we have moved on from the “Frequentist versus Bayesian” days and now everyone is happy to use both – and the discussions continued during coffee, dinner and hikes up the nearby snow-covered mountains. 

Our understanding of traditional problems continues to grow. The “signal plus background” problem always has new features to surprise us, unfolding continues to present challenges, and it seems we always have more to learn about simple concepts like errors and significance. There were also ideas that were new to many of us. Optimal transport and the Monge problem provide a range of tools whose use is only beginning to be appreciated, while neural networks and other machine-learning techniques can be used to help find anomalies and understand uncertainties. The similarities and differences between marginalisation and profiling require exploration, and we probably need to go beyond the asymptotic formulae more often than we do in practice.

Another “Banff challenge”, the third in a sequence, was set by Tom Junk of Fermilab. The first two had a big impact on the community and statistical practice. This time Tom provided simulated data for which contestants had to find the signal and background sizes, using samples with several systematic uncertainties – these uncertainties were unspecified, but dark hints were dropped. It’s an open competition and anyone can try for the glory of winning the challenge.

Collaborations were visibly forming during the latest PHYSTAT event, and results will be appearing in the next few months, not only in papers but in practical procedures and software that will be adopted and used in the front line of experimental research.

This and other PHYSTAT activities continue, with frequent seminars and several workshops (zoom, in-person and hybrid) in the planning stage.

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The initial physics results from the Run 3 dataset collected in 2022 by ATLAS and CMS were shown, featuring the first measurement of the Higgs-boson production cross-section by ATLAS at 13.6 TeV. Clearly the Run 2 dataset is still a gold mine for the LHC experiments. The programme of precision measurements of Higgs-boson properties is continuing with improved accuracy from the full Run 2 dataset. In particular, ATLAS and CMS reported a new combined result targeting the rare decay H → Zγ, for which they found evidence at the level of 3.4σ and a measured rate slightly higher but comparable to that predicted by the Standard Model.

Innovative signatures

Searches for physics beyond the Standard Model (SM) remains a very active field of research at the LHC, with many innovative signatures explored, including those of long-lived particles. Some of these searches use new anomaly-detection techniques and explore potential lower-production cross sections. A new search of leptoquarks by CMS exploiting the leptonic tau content of the proton was reported, while ATLAS reported a search for stau production in supersymmetry models with much improved sensitivity. Many other searches were also presented, and while a few low-level excesses exist, more data will be required to check if these are statistical fluctuations or not.

The SM is under intense scrutiny but is still very successful at the high-energy frontier. A recent re-analysis of the W-boson mass by ATLAS with the 7 TeV dataset shows good agreement with SM predictions, unlike the CDF result released in 2022. Validating the model used for the ATLAS W-mass measurement, new precise measurements of the W and Z bosons’ transverse momentum distributions were reported by ATLAS using Run 2 data collected under lower pileup conditions. Vector-boson scattering processes are an important probe of the electroweak symmetry breaking mechanism, and most such processes are now observed at the LHC.

Exploring the top-quark sector, many recent results focused on rare top-production processes. Four-top production was observed recently by ATLAS and CMS. First evidence for the rare tWZ production mode was shown by CMS at LHCP 2023. Some of these rare production modes are seen with rates somewhat higher than predicted, and more data will be required to conclude if the differences are significant. Top production is also used to investigate more exotic scenarios. A new CMS result, measuring the tt– production cross section as a function of sidereal time, was reported. No indication of Lorentz invariance violation is observed.

Presentations covered the broad spectrum of physics at the LHC brilliantly

On the flavour-physics side, LHCb reported a new precise measurement of CP violation in the “golden” B → J/ψ K_s decay, with the most precise extraction of the beta angle of the CKM quark-mixing matrix (see p16). Recent LHCb results on the flavour “anomalies” no longer show an indication for lepton universality violation in B → Ke⁺e^– compared to B → Kμ⁺μ^– decay rates, but some puzzles remain and there is still some tension in the tau-to-muon ratio in the tree-level decays B → B^(*)τ(µ)ν. Lepton-flavour violation is investigated in a new CMS result searching for the forbidden τ → 3µ decays, where an upper limit close to the Belle result was reported.

Characterisation of the quark–gluon plasma is actively studied using PbPb collision data. New results from ALICE regarding investigations of jet-quenching properties as well as charm fragmentation studies were shown at the conference.

The recent detections of collider-produced neutrinos by the new FASER and SND experiments were also presented, marking the start of a new physics programme at the LHC.

Broad spectrum

Several theory presentations highlighted recent progress in SM predictions for a wide range of processes including the electroweak sector, top-quark and Higgs-boson productions, as well as linking LHC physics to lattice QCD computations – work that is vital to fully exploit the physics potential of the LHC. Open questions in the various sectors were summarised and prospects for new-physics searches in Run 3, including those related to the Higgs-boson sector, were discussed. Links between LHC physics and dark matter were also highlighted, with examples of light dark-matter models and feebly interacting particles. Effective field theories, which are key tools to probe new physics in a generic way, were described with emphasis on the complementarity with searches targeting specific models.

Overall, the presentations covered the broad spectrum of physics at the LHC brilliantly. Future data, including from the High-Luminosity LHC phase, should allow physicists to continue to address many of the field’s open questions. Next year’s LHCP conference will be held at Northeastern University in Boston.

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The Forward Physics Facility (FPF) is a proposed new facility to operate concurrently with the High-Luminosity LHC, housing several new experiments on the ATLAS collision axis. The FPF offers a broad, far-reaching physics programme ranging from neutrino, QCD and hadron-structure studies to beyond-the-Standard Model (BSM) searches. The project, which is being studied within the Physics Beyond Colliders initiative, would exploit the pre-existing HL-LHC beams and thus have minimal energy-consumption requirements.

On 8 and 9 June, the 6th workshop on the Forward Physics Facility was held at CERN and online. Attracting about 160 participants, the workshop was organised in sessions focusing on the facility design, the proposed experiments and physics studies, leaving plenty of time for discussion about the next steps.

Groundbreaking

Regarding the facility itself, CERN civil-engineering experts presented its overall design: a 65 m-long, 10 m-high/wide cavern connected to the surface via an 88 m-deep shaft. The facility is located 600 m from the ATLAS collision point, in the SM18 area of CERN. A workshop highlight was the first results from a site investigation study, whereby a 20 cm-diameter core was taken at the proposed location of the FPF shaft to a depth of 100 m. The initial analysis of the core showed that the geological conditions are positive for work in this area. Other encouraging studies towards confirming the FPF feasibility were FLUKA simulations of the expected muon flux in the cavern (the main background for the experiments), the expected radiation level (shown to allow people to enter the cavern during LHC operations with various restrictions), and the possible effect on beam operations of the excavation works. One area where more work is required concerns the possible need to install a sweeper magnet in the LHC tunnel between ATLAS and the FPF to reduce the muon backgrounds.

Currently there are five proposed experiments to be installed in the FPF: FASER2 (to search for decaying long-lived particles); FASERν2 and AdvSND (dedicated neutrino detectors covering complementary rapidity regions); FLArE (a liquid-argon time projection chamber for neutrino physics and light dark-matter searches); and FORMOSA (a scintillator-based detector to search for milli-charged particles). The three neutrino detectors offer complementary designs to exploit the huge number of TeV energy neutrinos of all flavours that would be produced in such a forward-physics configuration. Four of these have smaller pathfinder detectors, FASER(ν), SND@LHC and milliQan that are already operating during LHC Run 3. First results from these pathfinder experiments were presented at the CERN workshop, including the first ever direct observation of collider neutrinos by FASER and SND@LHC, which provide a key proof of principle for the FPF. The latest conceptual design and expected performance of the FPF experiments were presented. Furthermore, first ideas on models to fund these experiments are in place and were discussed at the workshop.

In the past year, much progress has been made in quantifying the physics case of the FPF. It effectively extends the LHC with a “neutrino–ion collider’’ with complementary reach to the Electron–Ion Collider under construction in the US. The large number of high-energy neutrino interactions that will be observed at the FPF allows detailed studies of deep inelastic scattering to constrain proton and nuclear parton distribution functions (PDFs). Dedicated projections of the FPF reveal that uncertainties in light-quark PDFs could be reduced by up to a factor of two or even more compared to current models, leading to improved HL-LHC predictions for key measurements such as the W-boson mass.

In the past year, much progress has been made in quantifying the physics case of the FPF

High-energy electrons and tau neutrinos at the FPF predominantly arise from forward charm production. This is initiated by gluon–gluon scattering involving very low and high momentum fractions, with the former reaching down to Bjorken-x values of 10^–7 – beyond the range of any other experiment. The same FPF measurements of forward charm production are relevant for testing different models of QCD at small-x, which would be instrumental for Higgs production at the proposed Future Circular Collider (FCC-hh). This improved modeling of forward charm production is also essential for understanding the backgrounds to diffuse astrophysics neutrinos at telescopes such as IceCube and KM3NeT. In addition, measurements of the ratio of electron-to-muon neutrinos at the FPF probe forward kaon-to-pion production ratios that could explain the so-called muon puzzle (a deficit in muons in simulations compared to measurements), affecting cosmic-ray experiments.

The FPF experiments would also be able to probe a host of BSM scenarios in uncharted regions of parameter space, such as dark-matter portals, dark Higgs bosons and heavy neutral leptons. Furthermore, experiments at the FPF will be sensitive to the scattering of light dark-matter particles produced in LHC collisions, and the large centre-of-mass energy enables probes of models, such as quirks (long-lived particles that are charged under a hidden-sector gauge interaction), and some inelastic dark-matter candidates, which are inaccessible at fixed-target experiments. On top of that, the FPF experiments will significantly improve the sensitivity of the LHC to probe millicharged particles.

The June workshop confirmed both the unique physics motivation for the FPF and the excellent progress in technical and feasibility studies towards realising it. Motivated by these exciting prospects, the FPF community is now working on a Letter of Intent to submit to the LHC experiments committee as the next step.

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With a wide programme of plenary and parallel sessions, the workshop was a great opportunity for the community to discuss current and future R&D directions, with a focus on sustainability, and was testament to the eagerness of physicists from all over the world to join forces to build the next Higgs factory. More than 200 scientists participated, about 30% of which were early-career researchers and industry partners.

Energy frontiers

As set out by the 2020 update of the European strategy for particle physics and the Energy Frontier report from Snowmass 2021, particle physicists agreed that precision Higgs-boson measurements are the best path toward further progress and to provide insights into potential new-physics interactions. The Higgs boson is central for understanding fundamental particles and interactions beyond the Standard Model. Examples include the nature of dark matter and matter–antimatter asymmetry, which led to the prevalence of matter in our universe. 

Ideally, data-taking at a future e⁺e^–Higgs factory should follow the HL-LHC directly, requiring construction to start by 2030, in parallel with HL-LHC data-taking. Any significant delay will put at risk the availability of essential and unique expertise, and human resources, and endanger the future of the field.

Among the e⁺e^– colliders being evaluated by the community, the International Linear Collider (ILC), based on superconducting RF technology, has the most advanced design. It is currently under consideration for construction in Japan. However, for a long time now, Japan has not initiated a process to host this collider. One alternative approach is to construct a large circular collider – a strategy now being pursued by CERN with the FCC-ee, and by China with the CEPC. Both colliders would require tunnels of about 100 km circumference to limit synchrotron radiation. The FCC-ee machine is foreseen to operate in 2048, seven years after the end of the HL-LHC programme, with a substantial cost in time and resources for the large tunnel. An alternative is to construct a compact linear e⁺e^– collider based on high-gradient acceleration. CERN has a longstanding R&D effort along these lines, CLIC, that would operate at a collision energy of 380 GeV. 

New technologies proposed for higher-energy stages will require decades of R&D

Given the global uncertainties around each proposal, it is prudent to investigate alternative plans based on technologies that could enable compact designs and possibly provide a roadmap to extend the energy reach of future colliders. As also highlighted in the Snowmass Energy Frontier report, consideration should be given to the timely realisation of a Higgs factory in the US as an international effort. For instance, the Cool Copper Collider (C³) is a new and even more compact proposal for a Higgs-producing linear collider. It was developed during Snowmass 2021 and made its debut at LCWS with more than 15 talks and five posters. This proposal would use normal-conducting RF cavities to achieve a collision energy of 500 GeV with an 8 km-long collider, making it significantly smaller and likely more cost-effective than other proposed Higgs factories.

There are many advantages of the linear approach. Among them, linear colliders are able to access energies of 500 GeV and beyond, while for circular e⁺e^– colliders the expected luminosity drops off above centre-of-mass energies of 350–400 GeV. This would allow precision measurements that are crucial for indirect searches for new physics, including measurements of the top-quark mass and electroweak couplings, the top-Higgs coupling, and the cross section for double-Higgs production.

At LCWS 2023, the community showed progress on R&D for both accelerator and detector technologies and outlined how further advances in ILC technology, as well as alternative technologies such as C³ and CLIC, promise lower costs and/or extended energy reach for later stages of this programme. Discoveries at a Higgs factory may point to specific goals for higher energy machines, with quark and lepton collisions at least 10 times the energies of the LHC. New technologies proposed for such higher-energy stages – using pp, muon and e⁺e^– colliders – will require decades of R&D. Construction and operation of a linear Higgs factory would be a key contribution towards this programme by developing an accelerator workforce and providing challenges to train young scientists.

In this regard, a key outcome of the SLAC workshop was a statement supporting the timely realisation of a Higgs factory based on a linear collider to access energies beyond 500 GeV and enable the measurements vital for new physics to the P5 committee, which is currently evaluating priorities in US high-energy physics for the next two decades.

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From 31 May to 2 June, 180 participants came together in the CERN main auditorium (with a further 100 online) to exchange on topics including dark matter, particle astrophysics, cosmology of the early and late universe, and gravitational waves. The programme alternated between invited overview talks from leading scientists and lightning talks by early-career researchers. No fewer than 50 posters reflected the rich diversity of EuCAPT science, with prizes for the best poster and best lightning talks awarded at the end of the conference.

A highlight of the symposium was an interactive session with the members of the different EuCAPT task forces, ranging from outreach, training and community building to funding and many more, which allowed participants to learn more about the work done within the consortium and to join these activities. EuCAPT founding director Gianfranco Bertone (University of Amsterdam), who gave a well-attended public evening talk at CERN and who is due to step down in January 2024, said: “Leading EuCAPT has been an incredible experience. In four years we have grown into a vibrant and diverse community of more than 1600 scientists, based at 130 institutions across Europe. With a solid organisational structure in place, and many ongoing scientific activities, we are now ready to take the next steps.”

With further EuCAPT activities, such as the first EuCAPT school in Valencia this autumn, ongoing throughout the year, the EuCAPT community will continue to grow such that at the next EuCAPT symposium there will be ample new scientific developments and progress to discuss.

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The Future Circular Collider (FCC) offers a multi-stage facility – beginning with an e⁺e^– Higgs and electroweak factory (FCC-ee), followed by an energy-frontier hadron collider (FCC-hh) in the same 91 km tunnel – that would operate until at least the end of the century. Following the recommendation of the 2020 update of the European strategy for particle physics, CERN together with its international partners have launched a feasibility study that is due to be completed in 2025. FCC Week 2023, which took place in London from 5 to 9 June, and attracted about 500 people, offered an excellent opportunity to strengthen the collaboration, discuss the technological and scientific opportunities, and plan the submission of the mid-term review of the FCC feasibility study to the CERN Council later this year.

The FCC study, along with the support of the European Union FCCIS project, aims to build an ecosystem of science and technology involving fundamental research, computing, engineering and skills for the next generation. It was therefore encouraging that around 40% of FCC Week participants were aged under 40.

Working together

In his welcome speech, Mark Thomson (UK STFC executive chair) stressed the importance of a Higgs factory as the next tool in exploring the universe at a fundamental level. Indeed, one of the no-lose theorems of the FCC programme, pointed out by Gavin Salam (University of Oxford), is that it will shed light on the Higgs’ self-interaction, which governs the shape of the Brout–Englert–Higgs potential. In her plenary address, Fabiola Gianotti (CERN Director-General) confirmed that the current schedule for the completion of the FCC feasibility study is on track, and stressed that the FCC is the only facility commensurate with the present size of CERN’s community, providing up to four experimental points, concluding “we need to work together to make it happen”.

Designing a new accelerator infrastructure poses a number of challenges, from civil engineering and geodesy to the development of accelerator technologies and detector concepts to meet the physics goals. One of the major achievements of the feasibility study so far is the development of a new FCC layout and placement scenario, thanks to close collaboration with CERN’s host states and external consultants. As Johannes Gutleber (CERN) reported, the baseline scenario has been communicated with the affected communes in the surrounding area and work has begun to analyse environmental aspects at the surface-site locations. Synergies with the local communities will be strengthened during the next two years, while an authorisation process has been launched to start geophysical investigations next year.

Essential for constructing the FCC tunnel is a robust 3D geological model, for which further input from subsurface investigations into areas of geological uncertainty is needed. On the civil-engin­eering side, two further challenges include alignment and geodesy for the new tunnel. Results from these investigations will be collected and fed into the civil-engineering cost and schedule update of the project. Efforts are also focusing on optimising cavern sizes, tunnel widenings and shaft diameters based on more refined requirements from users.

Transfer lines have been optimised such that existing tunnels can be reused as much as possible and to ensure compatibility between the lepton and hadron FCC phases. Taking CERN’s full experimental programme into account, the option of using the SPS as pre-booster for FCC-ee will be consolidated and compared with the cost with a high-energy linac option.

A new generation of young researchers will need to take the reins to ensure FCC gets delivered and exploit the physics opportunities offered by this visionary research infrastructure

At the heart of the FCC study are sustainability and environmental impact. Profiting from an R&D programme on high-efficiency klystrons initially launched for the proposed Compact Linear Collider, the goal is to increase the FCC-ee klystron efficiency from 57% (as demonstrated in the first prototypes) to 80% – resulting in an energy saving of 300 GWh per year without considering the impact that this development could have beyond particle physics. Other accelerator components where work is ongoing to minimise energy consumption include low-loss magnets, SRF cavities and high-efficiency cryogenic compressors.

The FCC collaboration is also exploring ways in which to reuse large volumes of excavated materials, including the potential for carbon capture. This effort, which builds on the results of the EU-funded “Mining the Future” competition launched in 2020, aims to re-use the excavated material locally for agriculture and reforestation while minimising global nuisances such as transport. Other discussions during FCC Week focused on the development of a renewable energy supply for FCC-ee. 

If approved, a new generation of young researchers will need to take the reins to ensure FCC gets delivered and exploit the physics opportunities offered by this visionary research infrastructure. A dedicated early-career researcher session at FCC Week gave participants the chance to discuss their hopes, fears and experiences so far with the FCC project. A well-attended public event “Giant Experiments, Cosmic Questions” held at the Royal Society and hosted by the BBC’s Robin Ince also reflected the enthusiasm of non-physicists for fundamental exploration.

The highly positive atmosphere of FCC Week 2023 projected a strong sense of momentum within the community. The coming months will keep the FCC team extremely busy, with several new institutes expected to join the collaboration and with the scheduled submission of the feasibility-study mid-term review advancing fast ahead of its completion in 2025.

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Following inspiring opening speeches by Antonio Zoccoli (INFN president) and Alfonso Franciosi (Elettra president) about the important role of particle accelerators in Italy, the scientific programme got under way. It included 87 talks and over 1500 posters covering all particles (electrons, positrons, protons, ions, muons, neutrons, …), all types of accelerators (storage rings, linacs, cyclotrons, plasma accelerators, …), all use-cases (particle physics, photon science, neutron science, medical and industrial applications, material physics, biological and chemical, …) and institutes involved across the world. The extensive programme offered such a wide perspective of excellence and ambition that it is only possible to highlight a short subset of what was presented.

Starting proceedings was a report by Malika Meddahi (CERN) on the successful LHC Injectors Upgrade (LIU) project. This project, with its predominantly female leadership team, was executed on budget and on schedule. It provides the LHC with beams of increased brightness as required by the ongoing luminosity upgrade, as later reported by CERN’s Oliver Brüning. The focus then shifted to advanced X-ray light sources. Emanuel Karantzoulis (Elettra) presented Elettra 2.0 – a new ultra-low emittance light source in construction in Trieste. Axel Brachmann (SLAC) updated participants on the status of LCLS-II, the world´s first CW X-ray free-electron laser (XFEL). While beam commissioning is somewhat delayed, the superconducting RF accelerator structures perform beyond the performance specification and the facility is in excellent condition. The week´s programme included an impressive overview by Dong Wang (Shanghai Advanced Research Institute) on the future of XFELs for which user demand has led to an enormous investment aiming in particular at “high average power”, which will be used to serve many more experiments including those for highly non-linear QED. Gianluca Geloni (European XFEL) showed that user operation for the world`s presently most powerful XFEL has been successfully enhanced with self-seeding. Massimo Ferrario (INFN) described the promise of a novel, high-tech plasma-based FEL being explored by the European EuPRAXIA project.

Jörg Blaurock (FAIR/GSI) presented the status of the €3.3 billion FAIR project. Major obstacles have been overcome and the completed tunnel and many accelerator components are now being prepared for installation, starting in 2024. The European Spallation Source in Sweden is advancing well and the proton linac is approaching full beam commissioning, as presented by Ryoichi Miyamoto (ESS) and Andrea Pisent (INFN). Yuan He from China (IMP, CAS) presented opportunities in accelerator-driven nuclear power, both in safety and in reusing nuclear fuels, and impressed participants with the news on a Chinese facility that is progressing well in terms of up-time and reliability. This theme was also addressed by Ulrich Dorda (Belgian Nuclear Research Centre) who presented the status of the Multi-purpose Hybrid Research Reactor for High-tech Applications (MYRRHA) project. Another impressive moment of the programme was Andrey Zelinsky’s (NSC in Ukraine) presentation on the Ukraine Neutron Source facility at the National Science Center “Kharkov Institute of Physics & Technology” (NSC KIPT). Construction, system checks and integration tests for this new facility have been completed and beam commissioning is being prepared under extremely difficult circumstances, as a result of Russia’s invasion.

Technological highlights included a report by Claire Antoine (CEA) on R&D into thin-film superconducting RF cavities and their potential game-changing role in sustainability. Sustainability was a major discussion topic throughout IPAC23, and several speakers presented the role of accelerators for the development of fusion reactors. The final talk of the conference by Beate Heinemann (DESY) showed that without accelerators, much knowledge in particle physics would still be missing and she argued for new accelerator facilities at the energy frontier to allow further discoveries.

The prize session saw Xingchen Xu (Fermilab), Mikhail Krasilnikov (DESY/Zeuthen) and Katsunobu Oide (KEK) receive the 2023 EPS-AG accelerator prizes. In addition, the Bruno Touschek prize was awarded to Matthew Signorelli (Cornell University), while two student poster prizes went to Sunar Ezgi (Goethe Universität Frankfurt) and Jonathan Christie (University of Liverpool).

IPAC23 included for the first time in Europe an equal opportunity session, which featured talks from Maria Masullo (INFN) and Louise Carvalho (CERN) on gender and STEM, pointing to the need to change the narrative and to move “from talk to targets”. The 300 participants in the session learnt about ways to improve gender balance but also about such important topics as neurodiversity. The very well attended industrial session of IPAC23 brought together projects and industry in a mixed presentation and round-table format.

For the organizers, IPAC23 has been a remarkable and truly rewarding effort, seeing the many delegates, industry colleagues and students from all over the world coming together for a lively, peaceful and collaborative conference. The many outstanding posters and talks promise a bright future for the field of particle accelerators.

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The first presentation was by Roger Penrose, who talked about black holes, singularities and conformal cyclic cosmology. He discussed the cosmology of dark matter and dark energy, and inspired participants with the fascinating idea of one aeon going over to another aeon endlessly with no beginning or end of time and space.

Larry McLerran’s talk “Quarkyonic matter and neutron stars” provided an intuitive understanding of the origin of the equation of state of neutron stars at very high density, followed by Debadesh Bandyopadhyay’s talk on unlocking the mysteries of neutron stars. Jean-Paul Blaziot talked about the emergence of hydrodynamics in expanding quark–gluon plasma, whereas Edward Shuryak discussed the role of sphaleron explosions and baryogengesis in the cosmological electroweak phase transition. Subir Sarkar’s talk “Testing the cosmological principle” was provocative, as usual, and Sunil Mukhi and Aninda Sinha described the prospects for string theory. Sumit Som, Chandana Bhattacharya, Nabanita Naskar and Arup Bandyopadhyay discussed the low- and medium-energy physics possible using cyclotrons at Kolkata.

Moving to extreme nuclear matter, Barbara Jacak talked about experimental studies of transport in dense gluon matter. Jurgen Schukraft, Federico Antinori, Tapan Nayak, Bedangadas Mohanty and Subhasis Chattopadhyay spoke on signatures for the early-universe quark-gluon plasma and described the experimental programme of the ALICE experiment at the LHC, and Dinesh Srivastava focussed on the electromagnetic signatures of quark-gluon plasma.

A carnival of ideas, a mixture of low- to high-energy physics on the one hand and the cosmology of the creation of the universe on the other

Amanda Cooper-Sarkar emphasised the role of parton distribution functions in searches for new physics at colliders such as the LHC. Shoji Nagamiya presented the physics prospects of the J-PARC facility in Japan, Paolo Giubellino described the evolution of the latest FAIR accelerator at GSI, and Horst Stöcker discussed how to observe strangelets using fluctuation tools. In his presentation on the history of CERN, former Director-General Rolf Heuer talked about the marvels of large-scale collaboration capturing the thrill of a big discovery.

The MMAP 2020 conference witnessed a carnival of ideas, a mixture of low- to high-energy physics on the one hand and the cosmology of the creation of the universe on the other.

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

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

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

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

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

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While current facilities are being operated and upgraded, the gravitational-wave community is also focusing on a new generation of GWTs that will provide even better sensitivity. This would be achieved by longer interferometer arms, together with a drastic reduction of noise that might require cryogenic cooling of the mirrors. The two leading studies are the Einstein Telescope (ET) in Europe and the Cosmic Explorer (CE) in the US. The total length of the vacuum vessels envisaged for the ET and CE interferometers is 120 km and 160 km, respectively, with a tube diameter of 1 to 1.2 m. The required operational pressures are typical to those needed for modern accelerators (i.e. in the region of 10^-10 mbar for hydrogen and even lower for other gas species). The next generation of GWTs would therefore represent the largest ultrahigh vacuum systems ever built.

The next generation of gravitational-wave telescopes would represent the largest ultrahigh vacuum systems ever built.

Producing these pressures is not difficult, as present vacuum systems of GWT interferometers have a comparable degree of vacuum. Instead, the challenge is cost. Indeed, if the previous generation solutions were adopted, the vacuum pipe system would amount to half of the estimated cost of CE and not far from one-third of ET, which is dominated by underground civil engineering. Reducing the cost of vacuum systems requires the development of different technical approaches with respect to previous-generation facilities. Developing cheaper technologies is also a key subject for future accelerators and a synergy in terms of manufacturing methods, surface treatments and installation procedures is already visible.

Within an official framework between CERN and the lead institutes of the ET study –  Nikhef in the Netherlands and INFN in Italy – CERN’s TE-VSC and EN-MME groups  are sharing their expertise in vacuum, materials, manufacturing and surface treatments with the gravitational-wave community. The activity started in September 2022 and is expected to conclude at the end of 2025 with a technical design report and a full test of a vacuum-vessel pilot sector. During the workshop “Beampipes for Gravitational Wave Telescopes 2023”, held at CERN from 27 to 29 March, 85 specialists from different communities encompassing accelerator and gravitational-wave technologies and from companies that focus on steel production, pipe manufacturing and vacuum equipment gathered to discuss the latest progress. The event followed a similar one hosted by LIGO Livingston in 2019, which gave important directions for research topics.

Plotting a course
In a series of introductory contributions, the basic theoretical elements regarding vacuum requirements and the status of CE and ET studies were presented, highlighting initiatives in vacuum and material technologies undertaken in Europe and the US. The detailed description of current GWT vacuum systems provided a starting point for the presentations of ongoing developments. To conduct an effective cost analysis and reduction, the entire process must be taken into account — including raw material production and treatment, manufacturing, surface treatment, logistics, installation, and commissioning in the tunnel. Additionally, the interfaces with the experimental areas and other services such as civil engineering, electrical distribution and ventilation are essential to assess the impact of technological choices for the vacuum pipes.

The selection criteria for the structural materials of the pipe were discussed, with steel currently being the material of choice. Ferritic steels would contribute to a significant cost reduction compared to austenitic steel, which is currently used in accelerators, because they do not contain nickel. Furthermore, thanks to their body-centred cubic crystallographic structure, ferritic steels have a much lower content of residual hydrogen – the first enemy for the attainment of ultrahigh vacuum – and thus do not require expensive solid-state degassing treatments. The cheapest ferritic steels are “mild steels” which are common materials in gas pipelines after treatment to fight corrosion. Ferritic stainless steels, which contain more than 12% in weight of dissolved chromium, are also being studied for GWT applications. While first results are encouraging, the magnetic properties of these materials must be considered to avoid anomalous transmission of electromagnetic signals and of the induced mechanical vibrations.

Four solutions regarding the design and manufacturing of the pipes and their support system were discussed at the March workshop. The baseline is a 3 to 4 mm-thick tube similar to the ones operational in Virgo and LIGO, with some modifications to cope with the new tunnel environment and stricter sensitivity requirements. Another option is a 1 to 1.5 mm-thick corrugated vessel that does not require reinforcement and expansion bellows. Additionally, designs based on double-wall pipes were discussed, with the inner wall being thin and easy to heat and the external wall performing the structural role. An insulation vacuum would be generated between the two walls without the cleanliness and pressure requirements imposed on the laser beam vacuum. The forces acting on the inner wall during pressure transients would be minimised by opening axial movement valves, which are not yet fully designed. Finally, a gas-pipeline solution was also considered, which would be produced by a half-inch thick wall made of mild steel. The main advantage of this solution is its relatively low cost, as it is a standard approach used in the oil and gas industry. However, corrosion protection and ultrahigh vacuum needs would require surface treatment on both sides of the pipe walls. These treatments are currently under consideration.  For all types of design, the integration of optical baffles (which provide an intermittent reduction of the pipe aperture to block scattered photons) is a matter of intense study, with options for position, material, surface treatment, and installation reported. The transfer of vibrations from the tunnel structure to the baffle is also another hot topic.

The manufacturing of the pipes directly from metal coils and their surface treatment can be carried out at supplier facilities or directly at the installation site. The former approach would reduce the cost of infrastructure and manpower, while the latter would reduce transport costs and provide an additional degree of freedom to the global logistics as storage area would be minimized. The study of in-situ production was brought to its limit in a conceptual study of a process that from a coil could deliver pipes as long as desired directly in the underground areas: The metal coil arrives in the tunnel; then it is installed in a dedicated machine that unrolls the coil and welds the metallic sheet to form the pipe to any length.

These topics will undergo further development in the coming months, and the results will be incorporated into a comprehensive technical design report. This report will include a detailed cost optimization and will be validated in a pilot sector at CERN. With just under two and a half years of the project remaining, its success will demand a substantial effort and resolute motivation. The optimism instilled by the enthusiasm and collaborative approach demonstrated by all participants at the workshop is therefore highly encouraging.

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On 13 and 14 March CERN hosted an international workshop on atom interferometry and the prospects for future large-scale experiments employing this quantum-sensing technique. The workshop had some 300 registered participants, of whom about half participated in person. As outlined in a keynote introductory colloquium by Mark Kasevich (Stanford), one of the pioneers of the field, this quantum sensing technology holds great promise for making ultra-sensitive measurements in fundamental physics. Like light interferometry, atom interferometry involves measuring interference patterns, but between atomic wave packets rather than light waves. Interactions between coherent waves of ultralight bosonic dark matter and Standard Model particles could induce an observable shift in the interference phase, as could the passage of gravitational waves.

Atom interferometry is a well-established concept that can provide exceptionally high sensitivity, e.g., to inertial/gravitational effects. Experimental designs take advantage of features used by state-of-the-art atomic clocks in combination with established techniques for building inertial sensors. This makes atom interferometry an ideal candidate to hunt for physics beyond the Standard Model such as waves of ultralight bosonic dark matter, or to measure gravitational waves in a frequency range around 1 Hz that is inaccessible to laser interference experiments on Earth, such as LIGO, Virgo and KAGRA, or the upcoming space-borne experiment LISA. As discussed during the workshop, measurements of gravitational waves in this frequency range could reveal mergers of black holes with masses intermediate between those accessible to laser interferometers, casting light on the formation of the supermassive black holes known to inhabit the centres of galaxies. Atom interferometer experiments can also explore the limits of quantum mechanics and its interface with gravity, for example by measuring a gravitational analogue of the Aharonov-Bohm effect.

A deep shaft at Point 4 of the LHC is a promising location for an atom interferometer with a vertical baseline of over 100 m

Although the potential of atom interferometers for fundamental scientific measurements was the principal focus of the meeting, it was also emphasised that technologies based on the same principles also have wide-ranging practical applications. These include gravimetry, geodesy, navigation, time-keeping and Earth observation from space, providing, for example, a novel and sensitive technique for monitoring the effects of climate change through measurements of the Earth’s gravitational field.

Several large atom interferometers with a length of 10m already exist, for example at Stanford University, or are planned, for example in Hanover (VLBAI), Wuhan and at Oxford University (AION). However, many of the proposed physics measurements require next-generation setups with a length of 100m, and such experiments are under construction at Fermilab (MAGIS), in France (MIGA) and in China (ZAIGA). The Atomic Interferometric Observatory and Network (AION) collaboration is evaluating possible sites in the UK and at CERN. In this context, a recent conceptual feasibility study supported by the CERN Physics Beyond Colliders study group concluded that a deep shaft at Point 4 of the LHC is a promising location for an atom interferometer with a vertical baseline of over 100 m. The March workshop provided a forum for discussing such projects, their current status, future plans and prospective sensitivities.

Looking further ahead, participants discussed the prospects for one or more km-scale atom interferometers, which would provide the maximal sensitivity possible with a terrestrial experiment to search for ultralight dark matter and gravitational waves. It was agreed that the global community interested in such experiments would work together towards establishing an informal proto-collaboration that could develop the science case for such facilities, provide a forum for exchanging ideas how to develop the necessary technological advances and develop a roadmap for their realisation.

A highlight of the workshop was a poster session that provided an opportunity for 30 early-career researchers to present their ideas and current work on projects exploiting the quantum properties of cold atoms and related topics. The liveliness of this session showed how this interdisciplinary field at the boundaries between atomic physics, particle physics, astrophysics and cosmology is inspiring the next generation of researchers. These researchers may form the core of the team that will lead atom interferometers to their full potential.

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The workshop programme covered current and future experimental projects to probe the Sakharov conditions: collider measurements of CP violation (LHCb, Belle II, FCC-ee), searches for electric dipole moments (PSI, FNAL), long-baseline neutrino experiments (NOvA, DUNE, T2K, Hyper-Kamiokande, ESSnuSB) and searches for baryon- and lepton-number violating processes such as neutrinoless double beta decay (GERDA, CUORE, CUPID-Mo, KamLAND-Zen, EXO-200) and neutron–antineutron oscillations (ESS). These were put in context with the different theoretical approaches to baryogenesis and leptogenesis.

With the workshop’s aim to provide a discussion forum for junior and senior scientists from various backgrounds, and following the tradition of the Ecole des Houches, a six-hour mini-school took place in parallel with more specialised talks. A first lecture by Julia Harz (University of Mainz) introduced the hypotheses related to baryogenesis, and another by Adam Falkowski (IJCLab) described how CP violation is treated in effective field theory. Each lecture provided both a common theoretical background, and an opportunity to discuss the fundamental motivation driving experimental searches for new sources of CP violation in particle physics.

In his summary talk, Mikhail Shaposhnikov (EPFL Lausanne) explained that it is impossible to identify which mechanism leads to the existing baryon asymmetry in the universe. He added that we live in exciting times and reviewed the vast number of opportunities in experiment and theory lying ahead.

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Natural patterns

While investigating these successively explanatory layers of nature, broad patterns emerge. One of which is known colloquially as “naturalness”. This pattern essentially asserts that in reversing the direction and going from one microscopic theory, “the UV-completion”, to its larger-scale shell, “the IR”, the values of parameters measured in the latter are, essentially, “typical”. Typical, in the sense that they reflect the scales, magnitudes and, perhaps most importantly, the symmetries of the underlying UV completion. As Murray Gell-Mann once said: “everything not forbidden is compulsory”.

So, if some symmetry is broken by a large amount by some interaction in the UV theory, the same symmetry, in whatever guise it may have adopted, will also be broken by a large amount in the IR theory. The only exception to this is accidental fine-tuning, where large UV-breakings can in principle conspire and give contributions to IR-breakings that, in practical terms, accidentally cancel to a high degree, giving a much smaller parameter than expected in the IR theory. This is colloquially known as “unnaturalness”.

There are good examples of both instances. There is no symmetry in QCD that could keep a proton light; unsurprisingly it has mass of the same order as the dominant mass scale in the theory, the QCD scale, m_p ~ Λ_QCD. But there is a symmetry in QCD that keeps the pion light. The only parameters in UV theory that break this symmetry are the light quark masses. Thus, the pion mass-squared is expected to be around m²_π ~ m_q Λ_QCD. Turns out, it is.

There are also examples of unnatural parameters. If you measure enough different physical observables, observations that are unlikely on their own become possible in a large ensemble of measurements – a sort of theoretical “look elsewhere effect”. For example, consider the fact that the Moon almost perfectly obscures the Sun during a lunar eclipse. There is no symmetry which requires that the angular size of the Moon should almost match that of the Sun to an Earth-based observer. Yet, given many planets and many moons, this will of course happen for some planetary systems.

However, if an observation of a parameter returns an apparently unnatural value, can one be sure that it is accidentally small? In other words, can we be confident we have definitively explored all possible phenomena in nature that can give rise to naturally small parameters? 

From 30 January to 3 February, participants of an informal CERN theory institute “Exotic Approaches to Naturalness” sought to answer this question. Drawn from diverse corners of the theorist zoo, more than 130 researchers gathered, both virtually and in person, to discuss questions of naturalness. The invited talks were chosen to expose phenomena in quantum field theory and beyond which challenge the naive naturalness paradigm.

Coincidences and correlations

The first day of the workshop considered how apparent numerical coincidences can lead to unexpectedly small parameters in the IR due to the result of selection rules that do not immediately manifest from a symmetry, known as “natural zeros”. A second set of talks considered how, going beyond quantum field theory, the UV and IR can potentially be unexpectedly correlated, especially in theories containing quantum gravity, and how this correlation can lead to cancellations that are not apparent from a purely quantum field theory perspective.

The second day was far-ranging, with the first talk unveiling some lower dimensional theories of the sort one more readily finds in condensed matter systems, in which “topological” effects lead to constraints on IR parameters. A second discussed how fundamental properties, such as causality, can impose constraints on IR parameters unexpectedly. The last demonstrated how gravitational effective theories, including those describing the gravitational waves emitted in binary black hole inspirals, have their own naturalness puzzles.

The ultimate goal is to now go forth and find new angles of attack on the biggest naturalness questions in fundamental physics

Midweek, alongside an inspirational theory colloquium by Nathaniel Craig (UC Santa Barbara), the potential role of cosmology in naturalness was interrogated. An early example made famous by Steven Weinberg concerns the role of the “anthropic principle” in the presently measured value of the cosmological constant. However, since then, particularly in recent years, theorists have found many possible connections and mechanisms linking naturalness questions to our universe and beyond.

The fourth day focussed on the emerging world of generalised and higher-form symmetries, which are new tools in the arsenal of the quantum field theorist. It was discussed how naturalness in IR parameters may potentially arise as a consequence of these recently uncovered symmetries, but whose naturalness would otherwise be obscured from view within a traditional symmetry perspective. The final day studied connections between string theory, the swampland and naturalness, exploring how the space of theories consistent with string theory leads to restricted values of IR parameters, which potentially links to naturalness. An eloquent summary was delivered by Tim Cohen (CERN).

Grand slam

In some sense the goal of the workshop was to push back the boundaries by equipping model builders with new and more powerful perspectives and theoretical tools linked to questions of naturalness, broadly defined. The workshop was a grand slam in this respect. However, the ultimate goal is to now go forth and use these new tools to find new angles of attack on the biggest naturalness questions in fundamental physics, relating to the cosmological constant and the Higgs mass.

The Standard Model, despite being an eminently marketable logo for mugs and t-shirts, is incomplete. It breaks down at very short distances and thus it is the IR of some more complete, more explanatory UV theory. We don’t know what this UV theory is, however, it apparently makes unnatural predictions for the Higgs mass and cosmological constant. Perhaps nature isn’t unnatural and generalised symmetries are as-yet hidden from our eyes, or perhaps string theory, quantum gravity or cosmology has a hand in things? It’s also possible, of course, that nature has fine-tuned these parameters by accident, however, that would seem – à la Weinberg – to point towards a framework in which such parameters are, in principle, measured in many different universes. All of these possibilities, and more, were discussed and explored to varying degrees.

Perhaps the most radical possibility, the most “exotic approach to naturalness” of all, would be to give up on naturalness altogether. Perhaps, in whatever framework UV completes the Standard Model, parameters such as the Higgs mass are simply incalculable, unpredictable in terms of more fundamental parameters, at any length scale. Shortly before the advent of relativity, quantum mechanics, and all that have followed from them, Lord Kelvin (attribution contested) once declared: “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement”. The breadth of original ideas presented at the “Exotic Approaches to Naturalness” workshop, and the new connections constantly being made between formal theory, cosmology and particle phenomenology, suggest it would be similarly unwise now, as it was then, to make such a wager.

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Opening the event, Masanori Yamauchi, director-general of KEK, summarised the early history of Kobayashi-Maskawa (KM) theory and the ideas to test it as a theory of CP violation. He recalled his time as a member of the Belle collaboration at the KEKB accelerator, including the memorable competition with the BaBar experiment at SLAC during the late 1990s and early 2000s, which finally led to the conclusion that KM theory explains the observed CP violation. Kobayashi and Maskawa shared one half of the 2008 Nobel Prize in Physics “for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature”.

The scientific sessions were initiated by Amarjit Soni (BNL), who summarised various ideas to measure CP violation from cascade decays of B mesons including the celebrated papers by A I Sanda and co-workers in 1980–1981, which gave a strong motivation to build B factories. Stephen Olsen (Chung Ang University), who was one of the leaders of the Belle collaboration, looked back at the situation in the early 1980s when B-meson mixing was first observed, and emphasised the role of the accelerator physicists who achieved the 100-fold increase in luminosity that was necessary to measure CP angles. Adrian Bevan (Queen Mary University of London) added a perspective from the BaBar experiment, while the more recent impressive development by the LHCb experiment was summarised by Patrick Koppenburg (Nikhef).

Theoretical developments remain an integral part of quark-flavour physics. Matthias Neubert (University of Mainz) gave an overview of the theoretical tools developed to understand B-meson decays, which include heavy-quark symmetry, heavy-quark effective field theory, heavy-quark expansion and QCD factorisation, and Zoltan Ligeti (LBNL) summarised concurrent developments of theory and experiment to determine the sides of the CKM triangle. Lattice QCD also played a central role in the determination of the CKM matrix elements by providing precision computation of non-perturbative parameters, as discussed by Aida El-Khadra (University of Illinois).

There are valuable lessons from the KM paper when applied to the search beyond the Standard Model

The B sector is not the only place where CP violation is observed. Indeed, it was first observed in kaon mixings, and important pieces of information have been obtained since then. A number of theoretical ideas dedicated to the study of kaon CP violation were discussed by Andrzej Buras (Technical University of Munich), and experimental projects were overviewed by Taku Yamanaka (Osaka University).

There are still unsolved mysteries around quark-flavour physics. The most notable is the origin of the fermion generations, which may only be understood by accumulating more data to find any discrepancy with the Standard Model. SuperKEKB/Belle II, the successor of KEKB/Belle, plans to accumulate 50 times more data in the coming decades, while LHCb will continue to improve the precision of measurement in hadronic collisions. Nanae Taniguchi (KEK) reported the current status of SuperKEKB/Belle II, which has been in physics operation since 2019 and has already broken peak-luminosity records in e⁺e^– collisions. Gino Isidori (University of Zurich) gave his view on the possible shape of physics to come. “There are valuable lessons from the KM paper, which are still valuable today, when applied to the search beyond the Standard Model,” he concluded. 

As a closing remark, Makoto Kobayashi reminisced about the time when he built the theory as well as the time when the KEKB/Belle experiment was running. “I was able to watch the development of the B factory so closely from the very beginning,” he said. “I am grateful to the colleagues who gave me such a great opportunity.”

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

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

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

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

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

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Neutrinos first
Neutrino masses and mixing provide a unique window on the only new physics so far seen beyond the Standard Model. The measured mass differences and mixing parameters provide a consistent picture suggesting the presence of a new scale potentially at approximately 10¹⁵ GeV. However, to complete this picture two fundamental elements are missing: the absolute mass scale of neutrinos and the determination, via neutrinoless double beta decay, of whether  neutrinos have a Majorana nature. Also of fundamental importance are the mass-squared ordering of  neutrinos, the maximality (or not) of atmospheric mixing, and the measurement of leptonic CP violation. All these questions were addressed by a range of new experimental results, many of which were presented for the first time.

NOvA and T2K presented a very consistent picture of the PMNS framework with a slight preference of the normal over the inverted ordering

The KATRIN collaboration reported an absolute upper limit on the electron-neutrino mass of 800 meV and is expected to reach a limit of 200 meV eventually. With a detailed analysis of their tritium decay spectrum, the team was also able to exclude rapid oscillations of electron neutrinos with potential sterile neutrinos and to set a limit on cosmic-neutrino local over-densities. The KamLandZEN, CUPID-Mo and Majorana Demonstrator experiments showed first results on neutrinoless double-beta decay searches in different systems.  KamLandZEN  had the largest number of radionuclei, providing upper limits on the effective electron neutrino mass between 36 and 156 meV (depending on model assumptions) and is expected to reach 20 meV with more data. CUPID-Mo and Majorana Demonstrator experiments are expected to eventually reach stronger limits down to approximately 10 meV. The latter experiment, based on germanium detectors, also reported interesting bounds on models for wave-function collapse.

The long-baseline ν_μ oscillation experiments NOvA and T2K presented analyses of their latest intermediate dataset, showing a very consistent picture of the PMNS framework with a slight preference (at the one or two standard-deviation level) of the normal over the inverted ordering and the upper over the lower octant for θ₂₃. Both experiments are sensitive to electron-neutrino appearance. NOvA, however, provided the first evidence for electron anti-neutrino appearance and a first long-baseline measurement of sin²θ₂₃, in very good agreement with the reactor neutrino data. Both experiments exclude CP conserving values of δ_CP of 0 or π at 90% confidence. IceCUBE with its DeepCORE extension also presented stunning atmospheric neutrino-oscillation results comparable with SuperKamiokande and long-baseline experiment sensitivities. All these experiments provide strong supporting evidence of the validity of the three neutrino-flavour paradigm.

Longstanding neutrino anomalies were discussed in detail. The reactor-neutrino deficit interpretation in terms of the existence of a sterile neutrino species is incompatible with several short baseline data. The significance of the LSND and MiniBooNE short-baseline low-energy excess was revisited in the light of new backgrounds. The long-standing gallium anomaly was further verified and confirmed by the independent experiment BEST. The BEST observations are, however, also not compatible with a simple sterile-neutrino oscillation pattern. The PROSPECT reactor-neutrino experiment also showed first results excluding the gallium anomaly in terms of an oscillation with a sterile neutrino. Finally, a peaking anomaly, in the range 5-7 MeV, was observed by several experiments (including RENO, DayaBay, NEOS, Chooz and PROSPECT). This anomaly cannot be easily interpreted in terms of fundamental neutrino physics. Instead, nuclear models have been discussed in detail and should be looked at carefully.

Finally, the results of CONUS, a Coherent neutrino scattering experiment based on high precision germanium detectors,  set limits on light vector mediators and the neutrino magnetic moment.

The three-neutrino paradigm is standing tall with some anomalies that  need to be further clarified, in particular the BEST gallium anomaly.

On the theoretical side, it was shown that leptogenesis is possible for any right-handed neutrino masses above about 0.1 GeV, which, if light enough, can be probed by the proposed SHiP experiment at CERN, as well as FCC-ee and HL-LHC. Neutrino experiments such as COHERENT were analysed in the framework of Standard Model Effective Field Theory.

The IceCUBE experiment also showed splendid multi-messenger results from high- and ultrahigh-energy neutrino observations and pointed out their ability to probe the Standard Model with ultrahigh-energy neutrinos that have travelled cosmic distances. These neutrinos are expected to be even mixtures of the three neutrino species; any deviation would be a clear sign of new physics. The cosmic-neutrino data also highlighted the missing data in neutrino-nucleon interactions in the range of a few 100 GeV to 10 TeV. At this year’s Moriond conference, the birth of collider neutrino physics was also presented, with the first results from the FASERν and SND experiments. FASERν showed the first unambiguous observation of neutrinos from proton-proton collisions at LHC point 1.

Overall the three neutrino paradigm is standing tall with some anomalies that still need to be further clarified, in particular the BEST gallium anomaly.

From neutrinos to quarks

From a theoretical point of view, neutrino and heavy-quark physics are two sides of the same coin: they provide information related to the flavour problem, namely the unexplained origin of quark and lepton families, masses and mixings. The fact that in the Standard Model fermion mass hierarchies arise from Yukawa couplings does not make it more satisfactory. The recently observed anomalies in semi-leptonic B decays exhibiting unexpected lepton-flavour patterns have raised numerous speculations and have in particular suggested that the flavour scale might be right around the corner at the TeV scale, motivating models discussed at the conference involving a new Z’ gauge boson or a scalar or a vector leptoquark from a twin Pati-Salam theory of flavour.

However, the recent results from LHCb on the main anomalies have shed new light on the question. LHCb discussed their recent reanalysis of the R(K) and R(K*) ratio of decay rates of B→K^(*)μμ /ee with the inclusion of an additional background from misidentified electrons are now in excellent agreement with the Standard Model. LHCb also presented a new result on the measurement of the R(D*) ratio of decay rates including fully hadronic τ decays and a new combined measurement of the R(D) and R(D*) ratios. With these new measurements from LHCb the R(D*) ratio agrees with the Standard Model predictions. A tension at the 3 standard deviations level is still observed, mostly due to the R(D) ratio.

Alternatively, D-meson decays were extensively discussed as a promising new playground for discovering new physics due to the richness of new data available, and the efficiency of the GIM mechanism for the charm quark and SU(3) flavour symmetry leading to easily verifiable null tests of the Standard Model.

Results of various rare decay and new resonance searches were presented by LHC experiments, with for example the ambitious searches of the extremely rare decay mode of the D meson in two muons, the observation by the CMS experiment of the decay of the η meson to four muons and the search for  states decaying to di-charmonium states as J/ψ/J/ψ or J/ψ/ψ_2S to four muons, which could correspond to four charm tetra-quark states.

Leaving no stone unturned, the LHC experiments have presented a whole host of new results of searches for new phenomena beyond the Standard Model

A highlight of the conference was the strong contribution from the Belle II experiment in all areas of heavy flavour physics, including: several measurements of b→s transitions, including a fully inclusive measurement; several time dependent CP-violation observables, which yield precisions on the CKM parameter sin(2β) on a par with the current world’s best measurements in those channels; as well as new input to the |V_ub| and |V_cb| puzzle (the tension between exclusive and inclusive measurements which suffer from different theoretical uncertainties), with an exclusive measurement in the golden B→πlν mode and an inclusive measurement of the B→D*lν decay.

LHCb presented nice new results in the b→sss transition in the φφ channel showing that no CP- violating effect is seen, with results separated in different polarisation modes. LHCb also presented a new measurement of the CKM angle γ in the B^±→D[K^∓π^±π^∓π^±]h^± (h = π, K) channel and an overall combination yielding a precision of approximately 3.7º.

Finally, a status report was given by the KOTO experiment which is searching for the extremely rare K_L→πνν process. The two first runs (starting in 2015 until 2018) have allowed the collaboration to identify two new backgrounds and provide methods to mitigate them since 2019.  With these improvements the KOTO experiment should reach sensitivities at the 10^-10 level, close to the expected branching fraction in the Standard Model of 3×10^-11. All measurements shown so far are compatible with the CKM paradigm.

Also in the quark sector, the latest measurements and the prospects in measurements of the neutron electric dipole moment were presented, providing strong constraints on new physics scenarios at high energy scales.

Lattice-QCD studies have made remarkable progress in recent years, with hadronic contributions to  muon g-2 being more or less under control, more so in the case of light-by-light contributions, which agree well with other results, and less so regarding the hadronic vacuum polarisation with  errors being driven down by the BMW collaboration, which by itself seems to lead to more consistency with the FNAL and BNL results. However, the BMW results are not yet fully confirmed either by other lattice groups or the R-ratio from experiment, with the recent VEPP data being out of line with previous experiments.

Higher precision from lattice calculations has also led to the so-called Cabibbo anomaly reported at Moriond, whereby the unitarity of the first row of the CKM matrix seems to be violated by 2.7σ. If confirmed by future experiments and lattice calculations, this could be a signal for new physics.

In addition, in the lepton flavour sector Belle II presented their first and already the world’s most precise tau-mass measurement, which agrees with previous measurements. With only approximately half the luminosity accumulated by the Belle experiment, Belle II presented measurements surpassing the Belle precision, thus displaying the excellent performance of the experiment.

Dark searches

A variety of dark-matter candidates were discussed including: primordial black hole with improved limits using 21 cm hydrogen astronomy; weakly interacting massive particles (WIMPs) from new electroweak fermion multiplets with heavier masses; heavy singlet dilaton-like scalars; keV neutrinos from an inverse seesaw model; axions or axion-like particles with an extended window of masses arising from non-standard cosmology; and ultralight dark matter such as dark photons whose interactions with the detector could be simulated by the software package DarkELF. An interesting proposal for axion detectors that can double up as high-frequency gravitational wave detectors was also discussed.

A flurry of results of searches for dark-sector particles at the LHC, Belle II, Babar, NA62, BES and PADME were shown.

The XENONnT collaboration presented new results, unblinded for the occasion, with an exposure of 95.1 days corresponding to 1.1 tonne-year. LZ also presented their latest results with a similar exposure. The two experiments, along with the PandaX xenon-based experiment, are now exploring new territory at low WIMP-nucleon cross sections.

These very low cross sections motivate further searches for the existence of a dark sector with dark photons or axion-like particles. A flurry of results of searches for dark-sector particles at the LHC, Belle II, Babar, NA62, BES and the PADME experiment were shown. PADME, a fixed-target e⁺e^– experiment, also presented their ability to directly probe an anomaly which was also seen in ¹²C and ⁴He.

Theories of new heavy particles were also discussed, ranging from an analysis of the minimal supersymmetric Standard Model which showed that gluinos of 1 TeV and stop squarks of 500 GeV could still have escaped detection, to theories of two Higgs-doublet models plus a Higgs singlet, which might be responsible for the 95 GeV diphoton events, to the observation that vector-like fermions (which come in opposite chirality pairs) have the right properties to avoid a metastable universe.

Electroweak searches at the LHC

The LHC experiments presented results from a host of searches for new phenomena beyond the Standard Model, leaving no stone unturned. These looked for signatures of models motivated by theories addressing the shortcomings of the Standard Model, astrophysical and cosmological observations such as dark matter that could be interpreted as the existence of a fundamental field, and experimental anomalies observed such as in the lepton-flavour or muon g-2 anomalies. These searches place very important limits on the presence of new phenomena up to the few-TeV scale. With 20 times more data, the High-Luminosity LHC (HL-LHC) will provide invaluable opportunities to significantly increase the search domain and bring potential for discoveries.

The LHC experiments also presented a series of new results based on W and Z production, coinciding very well with the 40th anniversary of the W and Z boson discoveries at the CERN SppS. The CMS collaboration showed a measurement of the τ polarisation. This measurement can be directly translated in terms of a measurement of the weak mixing angle with a precision of approximately 10%, which is close to the precision reached by e⁺e^– experiments. The CMS collaboration also presented a measurement of the invisible width of the Z boson that is more precise than the direct invisible-width measurements performed at LEP. ATLAS showed the precise measurement of the Z boson transverse momentum differential cross section integrated over the full phase space of leptons produced in the Z decay, and with it was able to provide the current most precise measurement of α_S with a precision comparable to the current world average or estimates using lattice QCD. ATLAS also presented a new measurement of the W-boson mass using a re-analysis of 7 TeV data collected in 2011, yielding a value slightly lower (by 10 MeV) and with a precision improved to 16 MeV, thus increasing the experimental tension with the recently published CDF measurement.

The LHC results have already obtained precision and sensitivity to processes that were thought to be unreachable prior to the start of operations.

ATLAS and CMS also showed results for more complex and rare processes equally highlighting the remarkable progresses made at the precision frontier. Both experiments showed an observation of the four top quarks production process and ATLAS presented the observation of two new tri-boson production processes, WZγ and Wγγ. ATLAS also presented a new measurement of the associated production of a W boson in association with a pair of top quarks which is a key background to numerous very important processes, as for instant the associated production of a Higgs boson with a pair of top quarks.

The results presented at this year’s Moriond elctroweak session show how LHC results have already obtained precision and sensitivity to processes that were thought to be unreachable prior to the start of operations. An outstanding example discussed in detail was the progress made in the search for di-Higgs production by ATLAS and CMS, a cornerstone of the HL-LHC physics programme to constrain the Higgs boson trilinear self-coupling. These results showed that combined, experiments should reach the sensitivity for the observation of this process at the LHC. Another example which was also discussed is the race to reach sensitivity to the Higgs-boson decays to charm quarks, where new methods based on deep learning techniques are making significant progress.

To further improve on the expected precision reach at the HL-LHC, intermediate goals at Run 3 are extremely important. Both ATLAS and CMS presented new results on measurements of Z boson, top, and Higgs boson production with LHC Run 3 data taken in 2022.

This year’s Moriond conference showed an extraordinary harvest of new results, giving an opportunity to take stock on the open questions and see the remarkable progress made since last year.

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The scientific scope of BPU11 covered the full landscape of physics via 139 lectures (12 plenary and 23 invited) and 150 poster presentations. A novel addition was five roundtables dedicated to high-energy physics (HEP), widening participation, careers in physics, quantum and new technologies, and models of studying physics in European universities with a focus on Balkan countries. The hybrid event attracted about 476 participants (325 on site) from 31 countries, 159 of whom were students, and demonstrated the high level of research conducted in the Balkan states.

Roadmaps to the future

The first roundtable “HEP – roadmaps to the future” showed the strong collaboration between CERN and the Balkan states. Four out of 23 CERN Member States come from the region (Bulgaria, Greece, Serbia and Romania); two out of three Associate Member States in the pre-stage to membership are Cyprus and Slovenia; and two out of seven Associate Member States are Croatia and Turkey. A further four countries have cooperation agreements with CERN, and more than 400 CERN users come from the Balkans. 

Kicking off the HEP roundtable discussions, CERN director for research and computing Joachim Mnich presented the recently launched accelerator and detector R&D roadmaps in Europe. Paris Sphicas (CERN and the University of Athens) reported on the future of particle-physics research, during which he underlined the current challenges and opportunities. These included: dark matter (for example the search for WIMPs in the thermal parameter region, the need to check simplified models such as axial-vector and di-lepton resonances, and indirect searches); supersymmetry (the search for “holes” in the low-mass region that will exist even after the LHC); neutrinos (whether neutrinos are Majorana or Dirac particles, their mass measurement and exploration of a possible “sterile” sector); as well as a comprehensive review of the Higgs sector. 

CERN’s Emmanuel Tsesmelis, who was awarded the Balkan Physical Union charter and honorary membership in recognition of his contributions to cooperation between the Balkan states and CERN, reflected on the proposed Future Circular Collider (FCC). Describing the status of the FCC feasibility study, due to be completed by the end of 2025, he stressed that the success of the project relies on strong global participation. His presentation initiated a substantial discussion about the role of the Balkan countries, which will be continued in May 2023 at the 11th LHCP conference in Belgrade.

The roundtable devoted to quantum technologies (QTs), chaired by Enrique Sanchez of the European Physical Society (EPS), was another highlight with strong relevance to HEP. Various perspectives on the different QT sectors – computing and simulation, communication, metrology and sensing – were discussed, touching upon the impact they could have on society at large. Europe plays a leading role in quantum research, concluded the panel. However, despite increased interest in QTs, including at CERN, issues such as how to obtain appropriate funding to enhance European technological leadership, remain. Discussions highlighted the opportunities for new generations of physicists from the Balkans to help build this “second quantum revolution”. 

In addition to the roundtables, four high-level scientific satellite events took place, attracting a further 150 on-site participants: the COST Workshop on Theoretical Aspects of Quantum Gravity; the SEENET–MTP Assessment Meeting and Workshop; the COST School on Quantum Gravity Phenomenology in the Multi-Messenger Approach; and the CERN–SEENET–MTP–ICTP PhD School on Gravitation, Cosmology and Astroparticle Physics. The latter is part of a unique regional programme in HEP initiated by SEENET–MTP (Southeastern European Network in Mathematical and Theoretical Physics) and CERN in 2015, and joined by the ICTP in 2018, which has contributed to the training of more than 200 students in 12 SEENET countries. 

The BPU11 Congress, the largest event of its type in the region since the beginning of the COVID-19 pandemic, contributed to closer cooperation between the Balkan countries and CERN, ICTP, SISSA, the Central European Initiative and others. It was possible thanks to the support of the EPS, ICTP and CEI-Trieste, CERN, EPJ, as well as the Serbian ministry of science and institutions active in physics and mathematics in Serbia. In addition to the BPU11 PoS Proceedings, several articles based on invited lectures will be published in a focus issue of EPJ Plus “On Physics in the Balkans: Perspectives and Challenges”, as well as in a special issue of IJMPA.

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The ep/eA programme finds itself at a crossroad between nuclear and particle physics, with synergies with astroparticle physics. It has the potential to empower the High-Luminosity LHC (HL-LHC) physics programme in a unique way, and allows for a deeper exploration of the electroweak and strong sectors of the Standard Model beyond what can be achieved with proton–proton collisions alone. In many cases, adding LHeC to HL-LHC data can significantly improve the precision of Higgs-boson measurements – similar to the improvements expected when moving from the LHC to HL-LHC.

The innovative spirit of the ep/eA community was demonstrated during the workshop “Electrons for the LHC – LHeC/FCCeh and PERLE” held at IJCLab from 26 to 28 October. As the ep/eA community moves from the former HERA facility and from the Electron-Ion Collider, currently under construction at Brookhaven, to higher energies at LHeC and FCC-eh, the threshold will be reached to study electroweak, top and Higgs physics in deep-inelastic scattering (DIS) processes for the first time. In addition, these programmes enable the exploration of the low Bjorken-x frontier orders of magnitude beyond current DIS results. At this stage, it is unclear what physics will be unlocked if hadronic matter is broken into even smaller pieces. In recent years, particle physicists have learned that the ultimate precision for Higgs-boson physics lies in the complementarity of e⁺e^–, pp and ep collisions, as embedded in the FCC programme, for example. Exploiting this complementarity is key to exploring new territories via the Higgs and dark sectors, as well as zooming in on potential anomalies in our data.

The October workshop underlined the advantage of a joint ep/pp/eA/AA/pA interaction experiment, and the need to further document its added scientific value. For example, a precision of 1 MeV on the W-boson mass could be within reach. In short, the ep data allows constraints to be placed on the most important systematic uncertainty when measuring the W-boson mass with pp data.

Reduced power 

Participants also addressed how to reduce the power consumption of LHeC and FCC-eh. PERLE, an expanding international collaboration revolving around a multi-turn demonstrator facility being pursued at IJCLab in Orsay for energy recovery linacs (ERLs) at high beam currents, is ready to become Europe’s leading centre for developing and testing sustainable accelerating systems.  

At this stage, it is unclear what physics will be unlocked if hadronic matter is broken into even smaller pieces

As demonstrated at the workshop, with additional R&D on ERLs and ep colliders we might be able to further reduce the power consumption of the LHeC (and FCC-eh) to as low as 50 MW. These values are to be compared with the GW power consumption if there was no energy recovery and therefore provide a power-economic avenue to extend the Higgs precision frontier beyond the HL-LHC. ERLs are not uniquely applicable to eA colliders, but have been discussed for future linear and circular e⁺e^– colliders too. With PERLE and other sustainable accelerating systems, the ep/eA programme has the ambition to deliver a demonstration of ERL technology at high beam current, potentially towards options for an ERL-based Higgs factory.

Workshop participants are engaged to further develop an ep/eA programme with the ability to significantly enrich this overall strategy with a view to finding cracks in the Standard Model and/or finding new phenomena that further our understanding of nature at the smallest and largest scales.

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With the imminent commissioning of the Cherenkov Telescope Array (CTA), which will comprise more than 100 telescopes located in the northern and southern hemispheres, gamma-ray astronomy is about to enter a new era. This was taken as an opportunity to discuss the statistical methods used to analyze data from Cherenkov telescopes at a dedicated PHYSTAT workshop hosted by the university of Berlin. More than 300 participants, including several statisticians, registered for PHYSTAT-Gamma from 28 to 30 September to discuss concrete statistical problems, find synergies between fields, and set the employed methods in a broader context.

Three main topics were addressed at the meeting across 13 talks and multiple discussion sessions: statistical analysis of data from gamma-ray observatories in a multi-wavelength context, connecting statisticians and gamma-ray astronomers, and astrophysical sources across different wavelengths. Many concrete physical questions in gamma-ray astronomy must be answered in an astrophysical context, which becomes visible only by observing the electromagnetic spectrum. A mutual understanding of the statistical methods and systematic errors is therefore needed. Josh Speagle (University of Toronto) proclaimed a potential ‘datapocalypse’ in the heterogeneity and amount of soon-to-be-expected astronomical data. Similarities between analyses in X- and gamma-ray astronomy gave hope for reducing the data heterogeneity. Further cause for optimism arose from new approaches for combining data from different observatories.

The second day of PHYSTAT-Gamma focused on building connections between statisticians and gamma-ray astronomers. Eric Feigelson (Penn State) gave an overview of astrostatistics, followed by deeper discussions of Bayesian methods in astronomy by Tom Loredo (Cornell) and techniques for fitting astrophysical models to data with bootstrap methods by Jogesh Babu (Penn State). The session concluded with an overview of statistical methods for the analysis of astronomical time series by Jeff Scargle (NASA).

The final day centered on the problem of how to match astrophysical sources across different wavelengths. CTA is expected to detect gamma rays from more than 1000 sources. Identifying the correct counterparts at other wavelengths will be essential to study the astrophysical context of the gamma-ray emission. Applying Bayesian methods, Tamas Budavari (Johns Hopkins) discussed the current state of the problem from a statistical point of view, followed by in-depth talks and discussions among experts from X-ray, gamma-ray, and radio astronomy.

Topics across all sessions were the treatment of systematic errors and the formats for exchanging data between experiments. Technical considerations appear to dominate the definition of data formats in astronomy currently. However, for example, as Fisher famously showed with the introduction of sufficiency, statistical aspects can help to find useful representations of data and might also be considered in the definition of future data formats.

PHYSTAT-gamma was only the first attempt to discuss statistical aspects of gamma-ray astronomy. For example, the LHCf experiment at CERN will help to improve the prediction of the gamma-ray flux, which is expected from astrophysical hadron colliders and measured by gamma-ray observatories like CTA. However, modeling uncertainties from particle physics must be treated appropriately to improve the constraints on astrophysical processes. The discussion of this and many further topics is planned for follow-up meetings.

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The triennial workshop “Physics of fundamental Symmetries and Interactions – PSI2022” took place for the sixth time at the Paul Scherrer Institut (PSI) in Switzerland from 17 to 22 October, bringing the worldwide fundamental symmetries community together. More than 190 participants including some 70 young scientists welcomed the close communication of an in-person meeting built around 35 invited and 25 contributed talks.

A central goal of the meeting series is to deepen relations between disciplines and scientists. This year, exceptionally, participants connected with the FIPs workshop at CERN on the second day of the conference, due to the common topics discussed.

With PSI’s leading high-intensity muon and pion beams, many topics in muon physics and lepton-flavour violation were highlighted. These covered rare muon decays (μ → e + γ, μ → 3e) and muon conversion (μ → e), muonic atoms and proton structure, and muon capture. Presentations covered complementary experimental efforts at J-PARC, Fermilab and PSI. The status of the muon g-2 measurement was reviewed from an experimental and theoretical perspective, where lattice-QCD calculations from 2021 and 2022 have intensified discussions around the tension with Standard Model expectations.

Fundamental physics using cold and ultracold neutrons was a second cornerstone of the programme. Searches for a neutron electric dipole moment (EDM) were discussed in contributions by collaborations from TRIUMF, LANL, SNS, ILL and PSI, complemented by presentations on searches for EDMs in atomic and molecular systems. Along with new results from neutron-beta-decay measurements, the puzzle of the neutron lifetime keeps the community busy, with improving “bottle” and “beam” measurements presently differing by more than 5 standard deviations. Several talks highlighted possible explanations via neutron oscillations into sterile or mirror states.

The current status of direct neutrino-mass measurements and future outlook down into the meV range was covered together with updates on searches for neutrinoless double-beta decay. An overview of the hunt for the unknown at the dark-matter frontier was presented together with new limits and plans from various searches. Ultraprecise atomic clocks were discussed allowing checks of general relativity and the Standard Model, and for searches beyond established theories. The final session covered the latest results from antiproton and antihydrogen experiments at CERN, demonstrating the outstanding precision achieved in CPT tests with these probes. The workshop was a great success and participants look forward to reconvening at PSI2025.

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The 12^th Higgs Hunting workshop, which took place in Paris and Orsay from 12 to14 September, presented an overview of recent and new results in Higgs-boson physics. The results painted an increasingly detailed picture of Higgs-boson properties, thanks to the many analyses now reporting results based on the full LHC Run 2 dataset, with an integrated luminosity of about 140 fb^-1. Searches for phenomena beyond the Standard Model (BSM) were also presented.

Highlights included new results from CMS on decays of Higgs bosons to b quarks and to invisible final states, and a new limit from ATLAS on lepton-flavour violating decays of the Higgs boson. Events with two Higgs bosons in the final state were used to set limits on interactions involving three Higgs bosons and between two Higgs bosons and two weak vector bosons. All the results remain compatible with Standard Model expectations, except for a small number of intriguing tensions in some BSM searches, such as small excesses in a search for heavier partners of the Higgs boson decaying to W-boson pairs and in a search for resonances produced alongside a Z boson and decaying to a pair of Higgs bosons. These deviations from theory will be followed up by ATLAS and CMS in further analyses using Run 2 and Run 3 data.

This year’s workshop was special as the event marked the tenth anniversary of the Higgs-boson discovery in 2012. Two historical talks given by the former ATLAS and CMS spokespersons Peter Jenni (University of Freiburg & CERN) and Jim Virdee (Imperial College) highlighted the long-term efforts that laid the foundation for the Higgs-boson discovery in 2012.

The workshop also hosted an in-depth discussion on future accelerators and related detector R&D. It focused on future efforts in Europe, the US and Latin America, and featured presentations by Karl Jakobs (University of Freiburg and chair of the European Committee for Future Accelerators), Meenashi Narain (Brwon University and convener of the energy frontier group of the Snowmass process), Maria-Teresa Tova (National University of La Plata) and representative for the Latin American strategy effort) and Emmanuel Perez (CERN), who discussed recent improvements in physics analyses at future colliders.

Recent theory developments were also extensively covered, in particular recent developments in higher-order computations by Michael Spira (PSI), which highlighted the agreement between experimental results and predictions. A review of recent theory progress towards future colliders was also presented by Gauthier Durieux (CERN), while Carlos Wagner (Enrico Fermi Institute, & Kavli Institute for Cosmological Physics) discussed the new-physics that can be explored via precise measurements of Higgs-boson couplings. Finally, a “vision” presentation by Marcela Carena (Fermilab) highlighted new opportunities for the study of electroweak baryogenesis in relation to Higgs-boson measurements.

Many experimental sessions were held regarding recent results on a wide variety of topics, some which will be relevant in upcoming Run 3 measurements. This includes measurements related to potential CP-violating effects in the Higgs sector, as well as effective field theories (EFTs). This latter topic allows a general description of deviations from Standard Model  predictions in Higgs-boson measurements and beyond, and much improved measurements in this direction are expected in Run 3. The search for  Higgs-boson pair production was also an important focus at the Paris meeting. The latest Run 2 analyses showed greatly improved sensitivity compared to earlier rounds, and further improvements are expected in Run 3. While sensitivity to the Standard Model signal is not expected until the High-Luminosity LHC, these searches should set strong constraints on BSM effects in the Higgs sector.

Concluding talks were given by Fabio Maltoni (Louvain) and Giacinto Piacquadio (Stony Brook), and the next Higgs Hunting workshop will be held in Orsay and Paris from 11 to 13 September 2023.

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Initially, the vast number of solutions led to the impression that any quantum field theory could be obtained as an EFT of string theory, hindering the predictive power of the theory. In fact, recent developments have shown that quite the opposite is true: many respectable-looking field theories become inconsistent when coupled to quantum gravity and can never be obtained as EFTs of string theory. This set is known as the “swampland” of quantum field theories. The task of the swampland programme is to determine the structure and boundaries of the swampland, and from there extract the predictive power of string theory. Over the past few years, deep connections between the swampland and a fundamental understanding of open questions in high-energy physics ranging from the hierarchy of fundamental scales to the origin and fate of the universe, have emerged.

The workshop Back to the Swamp, held at Instituto de Física Teórica UAM/CSIC in Madrid from 26 to 28 September, gathered leading experts in the field to discuss recent progress in our understanding of the swampland, as well as its implications for particle physics and cosmology. In the spirit of the two previous conferences Vistas over the Swampland and Navigating the Swampland, also hosted at IFT, the meeting featured 22 scientific talks and attracted about 100 participants.

The swampland programme has led to a series of conjectures that have sparked debate about how to connect string theory with the observed universe, especially with models of early-universe cosmology. This was reflected with several talks on the subject, ranging from new scrutiny of current proposals to obtain de Sitter vacua, which might not be consistently constructed in quantum gravity, new candidates for quintessence models that introduce a scalar field to explain the observed accelerated expansion  of the universe, and scenarios where dark matter is composed of primordial black holes. Several talks covered the implications of the programme for particle physics and quantum field theories in general. Topics included axion-based proposals to solve the strong-CP problem from the viewpoint of quantum gravity, as well as how axion physics and approximate symmetries can link swampland ideas with experiment and how the mathematical concept of “tameness” could describe those quantum field theories that are compatible with quantum gravity. Progress on the proposal to characterize large field distances and field-dependent weak couplings as emergent concepts, general bounds on supersymmetric quantum field theories from consistency of axionic string worldsheet theories, and several proposals on how dispersive bound and the boostrap programme are also relevant for swampland ideas. Finally, several talks covered more formal topics, such as a sharpened formulation of the distance conjecture, new tests of the tower weak gravity conjecture, the discovery of new corners in the string theory landscape, and arguments in favour of and against Euclidean wormholes.

The new results demonstrated the intense activity in the field and highlighted several current aspects of the swampland programme. It is clear that the different proposals and conjectures driving the programme have sharpened and become more interconnected. Each year, the programme attracts more scientists working in different specialities of string theory, and proposals to connect the swampland with experiment take a larger fraction of the efforts.

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So far, the experimental effort has been driven by theoretical arguments that favoured the existence of new particles with relatively large couplings to the Standard Model (SM) and masses commensurate the mass of the Higgs boson. Searching for these particles has been one of the main goals of the physics programme of the LHC. However, several beyond-the-SM theories predict the existence of light (sub-GeV) particles, which interact very weakly with the SM fields. Such feebly interacting particles (FIPs) can provide elegant explanations to several unresolved problems in modern physics. Furthermore, searching for them requires specific and distinct techniques, creating new experimental challenges along with innovative theoretical efforts.

FIPs are currently one of the most debated and discussed topics in fundamental physics and were recommended by the 2020 update of the European strategy for particle physics as a compelling field to explore in the next decade. The FIPs 2022 workshop held at CERN from 17 to 21 October was the second in a series dedicated to the physics of FIPs, attracted 320 experts from collider, beam-dump and fixed-target experiments, as well as from the astroparticle, cosmology, axion and dark-matter communities gathered to discuss the progress in experimental searches and new developments in underlying theoretical models.

The main goal of the workshop was to create a base for a multi-disciplinary and interconnected approach. The breadth of open questions in particle physics and their deep interconnection requires a diversified research programme with different experimental approaches and techniques, together with a strong and focused theoretical involvement. In particular, FIPs 2022, which is strongly linked with the Physics Beyond Colliders initiative at CERN, aimed at shaping the FIPs programme in Europe. Topics under discussion include the impact that FIPs might have in stellar evolution, Λ_CDM cosmological-model parameters, indirect dark-matter detection, neutrino physics, gravitational-wave physics and AMO (atoms-molecular-optical) physics. This is in addition to searches currently performed at colliders and extracted beam lines worldwide.

The main sessions were organised around three main themes: light dark matter in particle and astroparticle physics and cosmology; ultra-light FIPs and their connection with cosmology and astrophysics; and heavy neutral leptons and their connection with neutrino physics. In addition, young researchers in the field presented and discussed their work in the “new ideas” sessions.

FIPs 2022 aimed not only to explore new answers to the unresolved questions in fundamental physics, but to analyse the technical challenges and necessary infrastructure and collaborative networks required to answer them. Indeed, no single experiment or laboratory would be able by itself to cover the large parameter space in terms of masses and couplings that FIPs models suggest. Synergy and complementarity among a great variety of experimental facilities are therefore paramount, calling for a deep collaboration across many laboratories and cross-fertilisation among different communities and experimental techniques. We believe that a network of interconnected laboratories can become a sustainable, flexible and efficient way of addressing the particle physics questions in the next millennium.

The next appointment for the community is the retreat/school “FIPs in the ALPs” to be held in Les Houches from 15 to 19 May 2023, to be followed by the next edition of the FIPs workshop at CERN in autumn 2024.

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The Physics Beyond Colliders (PBC) study was launched in 2016 to explore the opportunities offered by CERN’s unique accelerator and experimental-area complex to address some of the outstanding questions in particle physics through experiments that are complementary to the high-energy frontier. Following the recommendations of the 2020 update of the European strategy for particle physics, the CERN directorate renewed the mandate of the PBC study, continuing it as a long-term activity.

The fourth PBC annual workshop took place at CERN from 7 to 9 November 2022. The aim was to review the status of the studies, with a focus on the programmes under consideration for the start of operations after Long Shutdown 3 (LS3), scheduled for 2026–2029.

The North Area (NA) at CERN, where experiments are driven by beams from the Super Proton Synchrotron (SPS), is at the heart of many present and proposed explorations for physics beyond the Standard Model. The NA includes an underground cavern (ECN3), which can host unique high-energy/high-intensity proton beams. Several proposals for experiments have been made, all of which require higher intensity proton beams than are currently available. It is therefore timely to identify the synergies and implications of a future ECN3 high-intensity programme on the otherwise ongoing NA technical consolidation programme. 

The following proposals are being considered within the PBC study group:

• HIKE (High Intensity Kaon Experiment) is a proposed expansion of the current NA62 programme to study extremely rare decays of charged kaons and, in a second phase, those of neutral kaons. This would be complemented by searches for visible decays of feebly interacting particles (FIPs) that could emerge on-axis from the dump of an intense proton beam within a thick absorber that would contain all other known particles, except muons and neutrinos;

• SHADOWS (Search for Hidden And Dark Objects With the SPS) would search for visible FIP decays off-axis and could run in parallel to HIKE when operated in beam-dump mode. The proposed detector is compact and employs existing technologies to meet the challenges of reducing the muon background;

• SHiP (Search for Hidden Particles) would allow a full investigation of hidden sectors in the GeV mass range. Comprehensive design studies for SHiP and the Beam Dump Facility (BDF) in a dedicated experimental area were published in preparation for the European strategy update. During 2021, an analysis of alternative locations using existing infrastructure at CERN revealed ECN3 to be the most promising option;

• Finally, TauFV (Tau Flavour Violation) would conduct searches for lepton-flavour violating tau-lepton decays.

The HIKE, SHADOWS and BDF/SHiP collaborations have recently submitted letters of intent describing their proposals for experiments in ECN3. The technical feasibility of the experiments, their physics potential and implications for the NA consolidation are being evaluated in view of a possible decision by the beginning of 2023. A review of the experimental programme in the proposed high-intensity  facility will take place during 2023, in parallel with a detailed comparison of the sensitivity to FIPs in a worldwide context.

A vibrant programme

The NA could also host a vibrant ion-physics programme after LS3, with NA60++ aiming to measure the caloric curve of the strong-force phase transition with lead–ion beams, and NA61++ proposing to explore the onset of the deconfined nuclear medium, extending the scan in the momentum/ion space with collisions of lighter ion beams. The conceptual implementation of such schemes in the accelerators and experimental area is being studied and the results, together with the analysis of the physics potential, are expected during 2023.

The search for long-lived particles with dedicated experiments and the exploration of fixed-target physics is also open at the LHC. The proposed forward-physics facility, located in a cavern that could be built at a distance of 600 m along the beam direction from LHC Interaction Point 1, would take advantage of the large flux of high-energy particles produced in the very forward direction in LHC collisions. It is proposed to host a comprehensive set of detectors (FASER2, FASERν2, AdvSND, FORMOSA, FLArE) to explore a broad range of new physics and to study the highest energy neutrinos produced by accelerators. A conceptual design report of the facility, including detector design, background analysis and mitigation measures, civil engineering and integration studies is in preparation. Small prototypes of the MATHUSLA, ANUBIS and CODEX-b detectors aiming at the search for long-lived particles at large angles from LHC collisions are also being built for installation during the current LHC run.

The North Area at CERN is at the heart of many present and proposed explorations for physics beyond the Standard Model

A gas-storage cell (SMOG2) was installed in front of the LHCb experiment during the last LHC long shutdown, opening the way to high-precision fixed-target measurements at the LHC. The storage cell enhances the density of the gas and therefore the rate of the collisions by up to two orders of magnitude as compared to the previous internal gas target. SMOG2 has been successfully commissioned with neon gas, demonstrating that it can be operated in parallel to LHCb. Future developments include the injection of different types of gases and a polarised gas target to explore nucleon spin-physics at the LHC.

Crystal clear

Fixed-target experiments are also being developed that would extract protons from LHC beams by channelling the beam halo with a bent crystal.  The extracted protons would impinge on a target and be used for measurements of proton structure functions (“single crystal setup”) or estimation of the magnetic and electric dipole moments of short-lived heavy baryons (“double crystal setup”). In the latter case, the measurement would be based on the baryon spin precession in the strong electric field of a second bent crystal installed immediately downstream from the baryon-production proton target. A proof-of-principle experiment of the double-crystal setup is being designed for installation in the LHC to determine the channelling efficiency for long crystals at TeV energies, as well as to demonstrate the control and management of the secondary halo and validate the estimate of the achievable luminosity.

The technology know-how at CERN can also benefit non-accelerator experiments

The technology know-how and experience available at CERN can also benefit non-accelerator experiments such as the Atom Interferometer Observatory and Network (AION), proposed to be installed in one of the shafts at Point 4 of the LHC for mid-frequency gravitational-wave detection and ultra-light dark-matter searches, as well as the development of superconducting cavities for the Relic Axion Detector Experimental Setup (RADES) and for the heterodyne detection of axion-like particles.

During the workshop, progress on the possible applications of a gamma factory at CERN, as well as the status of the design of a Charged-Particle EDM Prototype Ring and of the R&D for novel monitored or tagged neutrino beamlines, were also presented.

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Opening the event, Joachim Mnich, CERN director for research and computing, noted that CERN is widely recognised, including by its member states, as an important platform for promoting applications of quantum technologies for both particle physics and beyond. “The journey has just begun, and the road is still long,” he said, “but it is certain that deep collaboration between physicists and computing experts will be key in capitalising on the full potential of quantum technologies.”

The conference was organised by the CERN Quantum Technology Initiative (CERN QTI), which was established in 2020, and followed a successful workshop on quantum computing in 2018 that marked the beginning of a range of new investigations into quantum technologies at CERN. CERN QTI covers four main research areas: quantum theory and simulation; quantum sensing, metrology and materials; quantum computing and algorithms; and quantum communication and networks. The first day’s sessions focused on the first two: quantum theory and simulation, as well as quantum sensing, metrology and materials. Topics covered included the quantum simulation of neutrino oscillations, scaling up atomic interferometers for the detection of dark matter, and the application of quantum traps and clocks to new-physics searches.

Building partnerships

Participants showed an interest in broadening collaborations related to particle physics. Members of the quantum theory and quantum sensing communities discussed ways to identify and promote areas of promise relevant to CERN’s scientific programme. It is clear that many detectors in particle physics can be enhanced – or even made possible – through targeted R&D in quantum technologies. This fits well with ongoing efforts to implement a chapter on quantum technologies in the European Committee for Future Accelerators’ R&D roadmap for detectors, noted Michael Doser, who coordinates the branch of CERN QTI focused on sensing, metrology and materials.

For the theory and simulation branch of CERN QTI, the speakers provided a useful overview of quantum machine learning, quantum simulations of high-energy collider events and neutrino processes, as well as making quantum-information studies of wormholes testable on a quantum processor. Elina Fuchs, who coordinates this branch of CERN QTI, explained how quantum advantages have been found for toy models of increased physical relevance. Furthermore, she said, developing a dictionary that relates interactions at high energies to lower energies will enhance knowledge about new-physics models learned from quantum-sensing experiments.

The conference demonstrated the clear potential of different quantum technologies to impact upon particle-physics research

The second day’s sessions focused on the remaining two areas, with talks on quantum-machine learning, noise gates for quantum computing, the journey towards a quantum internet, and much more. These talks clearly demonstrated the importance of working in interdisciplinary, heterogeneous teams when approaching particle-physics research with quantum-computing techniques. The technical talks also showed how studies on the algorithms are becoming more robust, with a focus on trying to address problems that are as realistic as possible.

A keynote talk from Yasser Omar, president of the Portuguese Quantum Institute, presented the “fleet” of programmes on quantum technologies that has been launched since the EU Quantum Flagship was announced in 2018. In particular, he highlighted QuantERA, a network of 39 funding organisations from 31 countries; QuIC, the European Quantum Industry Consortium; EuroQCI, the European Quantum Communication Infrastructure; EuroQCS, the European Quantum Computing and Simulation Infrastructure; and the many large national quantum initiatives being launched across Europe. The goal, he said, is to make Europe autonomous in quantum technologies, while remaining open to international collaboration. He also highlighted the role of World Quantum Day – founded in 2021 and celebrated each year on 14 April – in raising awareness around the world of quantum science.

Jay Gambetta, vice president of IBM Quantum, gave a fascinating talk on the path to quantum computers that exceed the capabilities of classical computers. “Particle physics is a promising area for looking for near-term quantum advantage,” he said. “Achieving this is going to take both partnership with experts in quantum information science and particle physics, as well as access to tools that will make this possible.”

Industry and impact

The third day’s sessions – organised in collaboration with CERN’s knowledge transfer group – were primarily dedicated to industrial co-development. Many of the extreme requirements faced by quantum technologies are shared with particle physics, such as superconducting materials, ultra-high vacuum, precise timing, and much more. For this reason, CERN has built up a wealth of expertise and specific technologies that can directly address challenges in the quantum industry. CERN strives to maximise the impact of all of its technologies and know-how on society in many ways to ease the transfer of CERN’s knowledge to industry and society. One focus is to see which technologies might help to build robust quantum-computing devices. Already, CERN’s White Rabbit technology, which provides sub-nanosecond accuracy and picosecond precision of synchronisation for the LHC accelerator chain, has found its way to the quantum community, noted Han Dols, business development and entrepreneurship section leader.

Several of the day’s talks focused on challenges around trapped ions and control systems. Other topics covered included the potential of quantum computing for drug development, measuring brain function using quantum sensors, and developing specialised instrumentation for quantum computers. Representatives of several start-up companies, as well as from established technology leaders, including Intel, Atos and Roche, spoke during the day. The end of the third day was dedicated to crucial education, training and outreach initiatives. Google provided financial support for 11 students to attend the conference, and many students and researchers presented posters.

Marieke Hood, executive director for corporate affairs at the Geneva Science and Diplomacy Anticipator (GESDA) foundation, also gave a timely presentation about the recently announced Open Quantum Institute (OQI). CERN is part of a coalition of science and industry partners proposing the creation of this institute, which will work to ensure that emerging quantum technologies tackle key societal challenges. It was launched at the 2022 GESDA Summit in October, during which CERN Director-General Fabiola Gianotti highlighted the potential of quantum technologies to help achieve key UN Sustainable Development Goals. “The OQI acts at the interface of science and diplomacy,” said Hood. “We’re proud to count CERN as a key partner for OQI, its experience of multinational collaboration will be most useful to help us achieve these ambitions.”

The final day of the conference was dedicated to hands-on workshops with three different quantum-computing providers. In parallel, a two-day meeting of the “Quantum Computing 4HEP” working group, organised by CERN, DESY and the IBM Quantum Network, took place.

Qubit by qubit

Overall, the QT4HEP conference demonstrated the clear potential of different quantum technologies to impact upon particle-physics research. Some of these technologies are here today, while others are still a long way off. Targeted collaboration across disciplines and the academia–industry interface will help ensure that CERN’s research community is ready to maximise on the potential of these technologies.

“Widespread quantum computing may not be here yet, but events like this one provide a vital platform for assessing the opportunities this breakthrough technology could deliver for science,” said Enrica Porcari, head of the CERN IT department. “Through this event and the CERN QTI, we are building on CERN’s tradition of bringing communities together for open discussion, exploration, co-design and co-development of new technologies.”

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The conference programme began with a warm welcome from Eberhard Widmann (Austrian Academy of Sciences) who was delighted to resume the SSP series, the last one held in Aachen in spring 2018. As proposed by the International Advisory Committee, the scientific programme this year focused more on fundamental symmetries and interactions in theory and laboratory experiments compared to previous editions and included topics such as dark matter and cosmology.  51 invited and contributed talks, as well as 17 posters were presented, highlighting scientific achievements worldwide.

These included topics on searches for lepton-flavour violation and symmetries in heavy quark decays at BELLE in Japan, BESIII in Beijing, muon-decay experiments at the Paul Scherrer Institute, and the first direct test of T and CPT symmetries in Φ decays at DAΦNE in Frascati. Prospects to discover physics beyond the Standard Model, such as the g-2 measurement at Fermilab, or at high-energy colliders were also presented, as well as searches for the electric dipole moments (EDM) of the neutron, deuteron, muon and in atoms and molecules. Double β-decay experiments, sterile-neutrino searches and flavour oscillations were also discussed. Results and upper limits on CPT tests with antihydrogen, muonium and positronium were reported.

The meeting ended with presentations on advanced instrumentation and on upcoming future facilities at PSI, DESY, Mainz university and J-PARC. Many participants from regions such as China attended the conference online. Discussions on various subjects followed during the poster session, where master and PhD students presented their work and results. Stefan Paul (TU Munich) gave a public lecture in the picturesque Festsaal of the Austrian Academy of Sciences about the shortest length scales that humankind has explored so far and how laboratory experiments test theoretical models describing the beginning of the universe.

SSP2022 was a successful and enjoyable conference, which created many fruitful and at times lively discussions in the field of symmetries in subatomic physics. The many contributions together with the social events around the conference programme provided an inspiring environment for animated discussions. SSP2022 benefited from being a relatively small-scale conference and the natural lightness it brings when meeting new colleagues and carrying out in-depth conversations on physics topics that we are passionate about.

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The conference opened with an overview of the LHCb experiment, where the first milestones of the Upgrade I commissioning were presented. The new fully software trigger scheme of LHCb, with the highest data processing scheme of any LHC experiment, has been successfully implemented for the full LHCb detector.

The hot-off-the-press result on the simultaneous determination of the ratios R(D*)= BR(B→D*τ^–ν_τ)/BR(B→D*μ^–ν_μ) and R(D)= BR(B^–→D⁰τ^–ν_τ)/BR(B^–→D⁰μ^–ν_μ) was shown. This result, which superseded the previous LHCb measurement, is 1.9 σ away from the Standard Model (SM) expectation. Another highlight was the first observation of the decay Λ⁰_b→Λ⁺_cτ^–ν_τ, and its use to test lepton flavour universality using the ratio of the tauonic to muonic decay, R(Λ⁺_c), which is yet in agreement with the SM. The newest precision extractions of the moduli of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements |V_cb| and |V_ub| were discussed, showing that the long-standing puzzle of inclusive versus exclusive measurements keeps being a hot topic with many upcoming developments in the near future.

Within the mixing and CP violation (CPV) stream, a major highlight was the measurement of the time-integrated CP asymmetry in D^o→K⁺K^– decays, leading to the first determination of the direct CP asymmetries in both D^o→K⁺K^– and D^o→π⁺π^– in the latter case constituting the first evidence for CPV in a single charm decay. These results led to exciting discussions about the size of U-spin breaking and possible underlying mechanisms. A new theoretical methodology for the derivation of amplitude U-spin sum rules was presented, making sum rules feasible for any system at any order in the expansion in the symmetry-breaking terms.

Further major results were the determination of the charm mixing parameters
y_CP-y^Kπ_CP, very large local CP asymmetries seen in B⁺→h⁺h’^– h’⁺ (with h,h’=π,K), as well as a new simultaneous determination of the weak phase γ together with charm mixing and decay parameters. On the theory side, it was also presented the completion of the next-to-next-to-leading order (NNLO) QCD calculation of the width difference of B^o_s mesons, allowing for an improved comparison with the corresponding experimental results.

The versatility of LHCb was showcased by covering rare beauty, kaon and charm decays, along with new tests on lepton-flavor universality violation

The rare decays session again showcased the versatility of LHCb by covering rare beauty, kaon and charm decays, including the most recent results on lepton-flavor universality violation. Recent progress on handling QCD corrections of b→sℓ⁺ℓ^–, which are important for the interpretation of the B anomalies, were presented, and it was shown a new method for the extraction of CKM matrix elements using time-dependent kaon decays. Lots of future opportunities lie in the measurements of rare charmed baryon decays which are very little probed so far.

New exciting results were shown in the spectroscopy stream, where one new pentaquark and three new tetraquark states were presented, showing the leading contribution of LHCb to the discovery of exotics and yet-not-understood states. Progress on QCD predictions along with new data-driven and machine-learning based methods were discussed.

The BSM session gave a great overview of a diverse range of beyond-the-SM models including leptoquarks, Z’ models, axion-like particles (ALPs) as well as models with extra dimensions. Importantly, these models induce correlations between the B anomalies and other anomalies like g-2 or the Cabibbo angle anomaly. Complementary and partially competitive constraints on the viable model space come from direct searches and high-p_T observables.

Interesting discussions took place in the electroweak precision-measurements session, where the LHCb W-boson mass measurement was presented, which is in line with the world average and in tension with the recent precise CDF measurement at the 4σ level. This measurement will soon be complemented with the full Run 2 dataset.

The workshop closed with a grand overview given in the keynote talk by Alexander Lenz. The next instance of the Implications Workshop will take place at CERN in October 2023.

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The workshop aimed to bring together the physics community, detector magnet designers and industry to exchange ideas and concepts, foster collaboration, and to discuss the needs and R&D development goals for future superconducting detector magnets. A key goal was to address the issue of the commercial availability of aluminium-stabilised Nb-Ti/Cu conductor technology.

Fifteen physics-experiment projects, which had either been approved or are in the design phase, presented their needs and plans for superconducting detector magnets. These experiments covered a wide range of physics programmes for existing and future colliders, non-colliders and a space-based experiment. The presented projects showed a strong demand for aluminium-stabilised Nb-Ti/Cu conductor technology. Other conductor technologies that were featured during the workshop included cable-in-conduit technology (CICC) and aluminium-stabilised high-temperature-superconducting (HTS) technology.

Presentations by leading industrial partners showed that the industrial capability to produce superconducting detector magnets does exist, as long as a suitable conductor is available. It was also shown that aluminium-stabilised Nb-Ti/Cu conductors are currently not commercially available, although an R&D effort is currently on-going with IHEP in China. In particular, the co-extrusion process needed to clad the Nb-Ti/Cu Rutherford cable with aluminium is a key missing ingredient in industry. At the same time, the presentations showed that other ingredients, such as Nb-Ti/Cu wire production, the cabling of strands into a Rutherford cable, the high-purity aluminium stabiliser itself and the technique for welding-on of aluminium-alloy reinforcements for high-strength conductors, are still available.

The main conclusion of the workshop was that, given the need for aluminium-stabilised Nb-Ti/Cu conductors for future superconducting detector magnet projects, it is important that the commercial availability of this conductor is re-established, which would require a leading effort from international institutes through collaboration and cooperation with industry. This world-leading effort will advance technologies to be transferred openly to industry and other laboratories. Of particular importance is the co-extrusion technology needed to bond the aluminium stabiliser to the Rutherford cable. Hybrid-structure technology through electron- beam welding or other approaches to maximise the performance of an Al-stabilised superconductor combined with high-strength Al-alloy is needed for high-stress detector magnets. Back-up solutions such as copper-coated and soldered aluminium stabilisers, copper-based stabilisers and CICC should also be considered. In the long term, aluminium-stabilised HTS technology will be important for specific detector-magnet applications.

The workshop was received with strong interest and enthusiasm, and it is expected that another will be organised in one to two years, depending on the progress being made.

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The FCC-ee, a proposed 91 km future circular collider at CERN foreseen to begin operations in the 2040s, would deliver enormous samples of collision data at a wide range of energies, allowing for ultra-precise studies of the Higgs, W and Z bosons, and the top quark. For example, when running at the Z resonance the FCC-ee will produce – in little more than one minute – a data set the same size as that the LEP collider accumulated in the 1990s during its entire period of operation. For this reason, unlocking the full potential of FCC-ee data will require exquisite systematic control at a level far beyond that achieved at previous colliders.

A beautiful and unique attribute of circular e⁺e^– colliders is that the beams can naturally acquire transverse polarisation, and the precession frequency of the polarisation vector divided by the circulation frequency around the ring is directly proportional to the beam energy. This property allows the energy to be determined with very high precision through applying an oscillating magnetic field which, when in phase with the precession, depolarises the beams. This technique of resonant depolarisation underpins the precise knowledge of the mass and other properties of many particles that now serve as “standard candles”.

A key example is the measurement of the mass and width of the Z boson, and associated electroweak observables, which was the major achievement of the LEP programme. FCC-ee offers the possibility of improving the precision of these measurements by a factor of around 500 – a gigantic advance in precision that will allow for ultra-sensitive tests of the self-consistency of the Standard Model, and provide excellent sensitivity to new heavy particles that may affect the measurements through quantum corrections or mixing. Achieving the best possible knowledge of the collision energy is essential to accomplish this programme, and was the focus of the second FCC Energy Calibration, Polarization and Mono-chromatisation (EPOL) workshop held at CERN from 19 to 30 September, which was a follow-up to the first workshop that took place in 2017.

The two-week workshop was attended by more than 100 accelerator physicists, particle physicists and engineers from around the world; some remote and others participating in person. Presentations focused not only on the challenges at the FCC-ee, but also encompassed activities and initiatives at other facilities. The first week highlighted the plans for polarimetry measurements at the future Electron Ion Collider in the US. Complementary projects were presented from SuperKEKb in Japan, where the accelerator is stress-testing many aspects of the FCC-ee design, CEPC in China and other machines around the world.

Earth tides

The collision-energy calibration is a central consideration in the design and proposed operation strategy of the FCC-ee, in contrast to LEP where it was essentially an afterthought. At LEP, resonant depolarisation measurements were performed in dedicated calibration periods a few times per year. At FCC-ee these measurements will take place continually. This is essential, as a hard-learned lesson from LEP is that the beam energy is not constant, but varies throughout a fill, and also evolves over longer timescales. The gravitational pull of the moon distorts the tunnel in “Earth tides”, and modifies the relative trajectory of the beam through the quadrupole magnets, leading to energy changes that at LEP were around 10 MeV over a few hours during Z running, but will be 20 times larger at FCC-ee. Seasonal changes in the water level of Lac Leman lead to similar effects. At FCC-ee these distortions will be combatted by continuous adjustment of the radio frequency (RF) cavities, as is now routinely done in the LHC.

Additional challenges that were discussed in the workshop included the requirements on the laser polarimeters that will monitor the polarisation levels of the e⁺ and e^– beams, the shifts in collision energy that will occur at each interaction point through the combined effect of synchrotron radiation and the boost provided by the RF system, as well as spurious dispersions folded with collision offsets. Here the project will benefit from the considerable progress achieved since LEP in both the reliability and precision of beam position and dispersion measurements. A particular highlight of the discussions was an agreement that it will be feasible to perform resonant depolarisation measurements at higher energies for use in the determination of the mass of the W boson, which was not possible at LEP, allowing this important parameter of nature to be measured around a factor 20–40 times better than at present.

The workshop concluded with a list of future tasks to be tackled and open questions. These questions will be addressed as part of the ongoing FCC Feasibility Study, with updates planned for the mid-term review, scheduled for the middle of 2023, and the final report in 2025.

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The LHC collaborations that study the top quark presented a wealth of recent results based on Run 2 data, many of which were shown for the first time, and even included a measurement with the very first data collected in Run 3. CMS and ATLAS presented new top-quark mass results, new measurements of top-quark production asymmetries, new cross-section measurements as well as searches for new production and decay modes, both within and beyond the Standard Model (SM). These included ttW and four top-quark production, and processes involving flavour-changing-neutral-current interactions that could produce sizable rates beyond the SM prediction.

Earlier this year, CMS released a preliminary mass measurement that profiles all uncertainties, including a finely split set of signal-modelling uncertainties based on variations of Monte Carlo generators. To account for the limited statistical power for some of these variations, this precision analysis implements a fully consistent treatment of the resulting fluctuations leading to a 380 MeV uncertainty. ATLAS presented a top-quark mass measurement of 172.63 ± 0.20 (stat) ± 0.67 (syst) ± 0.37 (recoil) GeV. The last uncertainty represents the ambiguity in assigning the recoil of gluon emissions in the top-quark decay chain that was neither considered in Run 1 analyses nor in the CMS measurement and requires further studies. The large difference in the modelling uncertainties assigned by both collaborations underlines the importance to overcome the limitations of Monte Carlo generators for these precision measurements.

Run 2 of the LHC opens up new production processes that could not be probed at the Tevatron or in Run 1. Recently, ATLAS announced the observation of the rare production process of a single top quark and a photon, thus completing the list of associated top-quark production processes with SM gauge bosons. CMS followed with a brand-new analysis of the four top-quark production process, the rarest process accessible by the LHC to date. Together with combined ATLAS analyses, there is now very strong evidence that this elusive process exists. While most results in the classical top-quark pair and single-top production modes agree very well with the SM predictions, slight excesses are seen in several rare production modes, such as ttW and four-top production. None of these excesses are statistically significant, but they form an interesting pattern that requires experimental results and theory predictions to be considered extra carefully, while keeping an eye open for more exotic explanations.

Theory ahead

Theory contributions at TOP 2022 revolved around two major themes: precision calculations and beyond-SM models. For the former, several groups presented new calculations that enable a more precise comparison of measurements with SM predictions. These calculations provide an integrated treatment of the top-quark and boson decays, including off-shell effects, which are small in the total cross section, but which can be significantly enhanced locally in some corners of phase space. Including these effects is therefore relevant for the highest-precision differential measurements at the LHC. For the second theme, the most popular approach is to expand around the SM with minimal model dependence using effective field theory. This is complemented by more focussed efforts in concrete new-physics scenarios, including composite Higgs (and top) models as well as leptoquarks. A dedicated theory mini-workshop discussed the interplay of top-quark measurements with results in flavour physics.

Perhaps the most exciting result, the first at Run 3, was presented by CMS. On 5 July, just two months before the conference, the LHC switched back on after a three-year shutdown and started to produce the first proton-proton collisions at a record centre-of-mass energy of 13.6 TeV. Stretching over the next few years, Run 3 will increase the size of available datasets involving top quarks by a factor of three to four. Both ATLAS and CMS made a tremendous effort to prepare the detectors, to collect and check the quality of the data, and to provide preliminary calibrations for leptons and jets. In a race against the clock, CMS isolated the top-quark pair production process in the data collected in July and August in time for the conference. Even at this very early stage, the data are understood well enough that a cross-section measurement with a total uncertainty below 8% was possible by making use of the top-quark events themselves to calibrate most of the relevant experimental uncertainties in situ.

With these first results showing that the LHC and the experiments are smoothly operating, TOP22 kicks off the Run 3 top-quark physics programme. We can look back on a very exciting edition of the TOP conference and look forward to meeting again in Michigan in 2023.

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Safety is a priority for CERN. It spans all areas of occupational health and safety, including the protection of the environment and the safe operation of facilities. Continuous exchanges with similar research infrastructures on best practices and techniques ensures that CERN maintains the highest standards. From 25 to 28 October, more than 100 people from CERN and research institutes worldwide gathered in the Globe of Science and Innovation at CERN for the International Technical Safety Forum (ITSF). This key conference in matters of health and safety is a forum for exchanging new ideas, processes, procedures and technologies in personnel, environmental and equipment safety among a variety of high-energy physics, synchrotron and other research infrastructures.

It is a pleasure to share new ways of thinking and acting in matters of occupational health & safety and environmental protection

Yves Loertscher

“In its 25-year existence, the Forum has evolved with the times, all the while increasing its attractiveness for experts to share their knowledge, experience and challenges,” says Ralf Trant of the CERN technology department. “The scope has broadened from high-energy physics to a wider range of disciplines and participating institutes, in Europe and beyond with Asian labs joining in addition to American institutes, who have been involved since the beginning.”

Opening the event, Benoît Delille, head of the CERN Health, Safety & Environment (HSE) unit, noted: “For colleagues from different institutes who visit CERN for the first time, it is an occasion for us to share the values on which this Organization is built, that we are proud of, and also how we make them come to life through the prism of Safety.” A first session on environmental protection and sustainability saw CERN share its approach to minimise its environmental footprint in key domains, alongside a presentation from the European Spallation Source (ESS) on environmental management during its post-construction phase. Sessions including continuous improvements in health & safety, fire safety, equipment certification, incidents and lessons learned, risk assessment and technical risks unfolded during the week, ending with new projects and challenges, safety culture and behaviour and safety training.

“Listening to your colleagues from other research institutes informing about occurred events, lessons learned and recent developments in safety assessment is the pure essence of ITSF,” said Peter Jakobsson, head of environment, safety, health & quality at ESS and member of the ITSF organising committee, who chaired the “Incidents and lessons learned” session. “We openly share information in different subject safety areas such as fire hazards, handling of chemicals and inspection of pressurised equipment. In doing so, we all learn from each other to create a safe work environment for our staff and scientific users: a true sign of the safety culture that we all strive for.”

In addition to a rich programme of presentations, the event featured an interactive fire workshop in which participants shared ongoing projects and challenges related to fire safety in accelerator facilities. CERN also shared its experiences of the fire-induced radiological integrated assessment (FIRIA) project whose objective is to develop a general methodology for assessing the fire-related risks present in CERN’s facilities and provide a forum to keep experts connected and updated. Participants also enjoyed visits of the installations, complemented with a tour of the CERN safety training centre in Prévessin on the final day.

“This event gave us the possibility to share our knowledge through presentations but also through networking breaks, visits and social events,” said Yves Loertscher, head of the CERN HSE occupational health & safety group and organiser of this year’s ITSF event. “After a break of almost three years owing to the pandemic, it is a pleasure to interact directly with peers again and share new ways of thinking and acting in matters of occupational health & safety and environmental protection”.

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On 7 September colleagues and friends of Gabriele Veneziano gathered at CERN for an informal celebration of the renowned theorist’s 80th birthday. While a visitor in the CERN theory division (TH) in 1968, Veneziano wrote a paper “Construction of a crossing-simmetric, Regge-behaved amplitude for linearly rising trajectories”. It was an attempt to explain the strong interaction, but ended up marking the beginning of string theory. During the special TH colloquium, talks by Paolo Di Vecchia (NBI&Nordita), Thibault Damour (IHES) and others explored this and numerous other aspects of Veneziano’s work, much of which was undertaken during his 30 year-long career at CERN. Concluding the day’s proceedings, Veneziano thanked his mentors, CERN TH and chance – “the chance of having lived through one of most interesting periods in the history of physics… during which, through a wonderful cooperation between theory and experiment, enormous progress has been made in our understanding of nature at its deepest level.”

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Focal point

The symposium began with the research highlights and strategies of the three research fields. A major part of this concerned the progress and plans of the six joint projects that have emerged since the first JENAS event in 2019: dark matter (iDMEu initiative); gravitational waves for fundamental physics; machine-learning optimised design of experiments; nuclear physics at the LHC; storage rings to search for charged-particle electric dipole moments; and synergies between the LHC and future electron–ion collider experiments. The discussions on the joint projects were complemented by a poster session where young scientists presented the details of many of these activities.

The goal was achieving a more comprehensive assessment of overlapping research topics

Detector R&D, software and computing, as well as the application of artificial intelligence, are important examples where large synergies between the three fields can be exploited. On detector R&D there is interest in collaborating on important research topics such as those identified in the 2021 ECFA roadmap on detector R&D. In this roadmap, colleagues from the astro­particle and nuclear-physics communities were involved. Likewise, the challenges of processing and handling large datasets, distributed computing, as well as developing modern analysis methods for complex data analyses involving machine learning, can be addressed together.

Overview talks and round-table discussions related to education, outreach, open science and knowledge transfer allowed participants to emphasise and exchange best practices. In addition, the first results of surveys on diversity and the recognition of individual achievements in large collaborations were presented and discussed. For the latter, a joint APPEC–ECFA–NuPECC working group has presented an aggregation of best practices already in place. A major finding is that many collaborations have already addressed this topic thoroughly. However, they are encouraged to further monitor progress and consider introducing more of the best practices that were identified.  

Synergy

One day was dedicated to presentations and closed-session discussions with representatives from both European funding agencies and the European Commission. The aim was to evaluate whether appropriate funding schemes and organisational structures can be established to better exploit the synergies between astroparticle, nuclear and particle physics, and thus enable a more efficient use of resources. The positive and constructive feedback will be taken into account when carrying out the common projects and towards the preparation of the third JENAS event, which is planned to take place in about three years’ time.

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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.

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

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

Talking points

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

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

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

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

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

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

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

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

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The International Union of Pure and Applied Physics (IUPAP) is an offspring of the International Research Council, a temporary body created after the First World War to rebuild and promote research across the sciences. IUPAP was established in 1922 with 13 member countries and held its first general assembly in Paris the following year. Originally, neither the International Research Council nor IUPAP included any of the countries of the Central Powers (Germany, Austria–Hungary, Bulgaria and the Ottoman Empire). Many lessons in science diplomacy had to be learned before IUPAP and the other scientific unions became truly international and physicists from all countries could apply to join. Today, with 60 member countries, the union strongly advocates that no scientist shall be excluded from the scientific community as long as their work is based on ethics and the principles of science in its highest ideals – an aspect that certainly will be further elaborated by the working group on ethics established by IUPAP in October last year. 

Information exchange

Among IUPAP’s commissions covering all the different disciplines of physics  is the Commission on Symbols, Units, Nomenclature, Atomic Masses and Fundamental Constants (C2), formed in 1931. The task of this commission is to promote the exchange of information and views among the members of the international scientific community in the general field of fundamental constants. As an example, the international system of units (SI) was originally recommended by IUPAP in 1960, and C2 has maintained its role in recommending further improvements, including resolutions supporting the choice of constants to define the new SI as well as the decision to proceed with the redefinition of four of the seven units made in May 2019. 

From 11 to 13 July, around 250 physicists from some 70 countries gathered to celebrate the 100th birthday of IUPAP at a symposium held at the Abdus Salam International Centre for Theoretical Physics (ICTP) in Trieste, Italy. The symposium was one of the official events of the International Year of Basic Sciences for Sustainable Development, which was officially inaugurated only a few days earlier at the UNESCO headquarters in Paris. About 40% of the participants were physically present, while the rest connected online. Various panels composed of international experts discussed important issues in alignment with the IUPAP’s core aims, including: how to support and encourage early-career physicists, how to improve diversity in physics, how to strengthen the ties to physicists working in industry, how to improve the quality of physics education, and how to promote physics in less developed countries.

IUPAP continues to promote physics as an essential tool for development and sustainability in the next century

A number of influential scientists, including Giorgio Parisi (La Sapienza) and Laura Greene (Florida State University), described their roles in providing evidence-based advice to their respective governments on science and shared best practices that could be useful across borders. Other prominent speakers included William Phillips (Maryland), who covered the quantum reform of modern metric systems; Donna Strickland (Waterloo), who discussed the physics of high-intensity lasers; and Takaaki Kajita (Tokyo), who presented 100 years of neutrino physics via an online connection with the International Conference on High Energy Physics (ICHEP) in Bologna. Climate scientist Tim Palmer (Oxford) argued that a supercomputing facility modelled on the organisation of CERN would enable a step-change in quantifying climate change, while Stewart Prager (Princeton) outlined a new project sponsored by the American Physical Society to engage physicists in reducing nuclear threat. Dedicated panels discussed the development of physics in Africa and the Middle East, Asia and the Pacific, and Latin America. It is clear that in these regions IUPAP has a large potential to foster further international collaboration.

IUPAP enhances the vital role of young physicists, among others, through the award of early-career scientist prizes. In Trieste, several recent recipients of the prize were invited to present their research. The talks were all striking and left the audience with high hopes for the future of physics. Furthermore, the logistics in the auditorium and the handling of all the questions that came in from online participants were smoothly taken care of by members of the International Association of Physics Students.

The centennial symposium was an opportunity to reflect on IUPAP’s role in promoting international cooperation and to welcome Ukraine as a new member. The decision to admit Ukraine was expedited to send a strong signal of support for the war-torn country – a war that has not spared its scientific institutions and the people who work there, as expressed by the president of the Ukrainian Academy of Sciences Anatoly Zagorodny in a powerful online presentation. IUPAP has issued a statement strongly condemning the Russian aggression in Ukraine, while also expressing the principle that no scientist should be excluded from union-sponsored conferences, as long as he or she carries out work not contributing to weapons development. To overcome difficulties related to conferences, IUPAP has put in place that excluded scientists can participate using the Union as their affiliation – similar to the model applied for the Olympic Games.

IUPAP has served the physics community for 100 years and has strong ambitions to continue to assist in the worldwide development of physics and to promote physics as an essential tool for development and sustainability in the next century.

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European Laboratories for Accelerator Based Sciences (EURO-LABS) aims to provide unified transnational access to leading research infrastructures across Europe. Taking over from previously running independent programmes, it brings together the nuclear physics, the high-energy accelerator, and the high-energy detector R&D communities. With 33 partners from European countries, EURO-LABS forms a large network of laboratories and institutes ranging from modest sized test infrastructures to large-scale ESFRI facilities such as SPIRAL2.  Its goal is to enable research at the technological frontiers in accelerator and detector development and to open wider avenues in both basic and applied research in diverse topics, from optimal running of reactors to mimicking reactions in the stars. Within this large network, EURO-LABS will ensure diversity and actively support researchers from different nationalities, gender, age, grade, and variety of professional expertise.

Sharing information to support users at test facilities is pivotal. Targeted improvements such as new isotope-enriched targets for high-quality standard medical radioisotope production, improved beam- profile monitors, or magnetic-field measurement instruments in cryogenic conditions will further enhance the capabilities of  facilities to address the challenges of the coming decades. Through an active and open data management plan following the FAIR principle, EURO-LABS will act as a gateway for information to facilitate research across disciplines and provide training for young researchers.

Funded by the European Commission, EURO-LABS started on 1 September and will run until August 2026. At the kick off meeting, held in Bologna from 3 to 5 October, presentations offered a detailed overview of the research infrastructures and facilities providing particle and ion beams at energies from meV to GeV. Exchanges during the meeting gave participants a view of the strengths and synergies on offer, planting the seeds for fruitful collaborations.

Prospects for testing and developing techniques for present and future accelerators were among the highlights of the meeting. In the high-energy accelerator sector, this requires state of the art test benches for cryogenic equipment such as magnets, superconducting cavities and associated novel materials, electron and plasma beams, as well as specialised test-beam facilities. Facilities at CERN, DESY and PSI, for example, allow the study of performances and radiation effects on detectors for the HL-LHC and beyond while also enabling nuclei to be explored under extreme conditions. Benefiting from past experiences, a streamlined procedure for handling transnational-access applications to all research infrastructures across the different fields of EURO-LABS was defined.

On the last day of the meeting, the consortium’s governing board, chaired by Edda Gschwendtner (CERN), met for the first time. The governing board further appointed Navin Alahari (GANIL, France) as EURO-LABS scienfitic coordinator, Paolo Giacomelli (INFN-BO, Italy) as project corodinator, Maria Colonna (INFN-LNS, Italy), Ilias Efhymiopoulos (CERN) and Marko Mikuz (Univ.Lubljana, Slovenia) as deputy scientific coordinator and work-package and Maria J G Borge (CSIC, Spain) and Adam Maj (IFJ, Poland) as work-package leaders.

With all facilities declaring their readiness to receive the first transnational users, the next annual meeting will be hosted by IFJ-PAN in Krakow, Poland.

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The 13th International Particle Accelerator Conference (IPAC’22), which took place in Bangkok from 12 to 17 June, marked the return of an in-person event after two years due to the COVID pandemic. Hosted by the Synchrotron Light Research Institute, it was the first time that Thailand has hosted an IPAC conference, with around 800 scientists, engineers, technicians, students and industrial partners from 37 countries in attendance. The atmosphere was understandably electric. Energy and enthusiasm filled the rooms, as delegates had the chance to meet with colleagues and friends from around the world.

The conference began with a blessing from princess Maha Chakri Sirindhorn, who attended the two opening plenary sessions. The scientific programme included excellent invited and contributed talks, as well as outstanding posters, highlighting scientific achievements worldwide. Among them were the precise measurement of the muon’s anomalous magnetic dipole moment (g-2) at Fermi­lab, and the analysis at synchrotron light sources of soil samples obtained from near-Earth asteroid 162173 Ryugu by the Hayabusa2 space mission, which gave a glimpse into the origin of the Solar System.

In total, 88 invited and contributing talks on a wide array of particle accelerator-related topics were presented. These covered updates of new collider projects such as the Electron Ion Collider (EIC), proposed colliders (FCC, ILC and CEPC), as well as upgrade plans for existing facilities such as BEPCII and SuperKEKB, and new photo-source projects such as NanoTerasu and Siam Photon Source II. A talk about the power efficiency of accelerators drew a lot of attention given increasing global concern about sustainability. Accelerator-based radiotherapy continued to be the main topic in the accelerator application category, with a special focus on designing an affordable and low-maintenance linac for deployment in low- and middle-income countries and other challenging environments (CERN Courier January/February 2022 p30). 

Raffaella Geometrante (KYMA) hosted a popular industry session on accelerator technology. Completely revamped from past editions, its aim was to substantially improve the dynamics between laboratories and industry, while also addressing other topics on accelerator innovations and disruptive technologies. 

An engaging outreach talk “Looking into the past with photons” highlighted how synchrotron radiation has become an indispensable tool in archaeological and paleontological research, enabling investigations of the relationship between past civilisations in different corners of the world. A reception held during an evening boat cruise along the Chao Phraya River took participants past majestic palaces and historic temples against a backdrop of traditional Thai music and performances.

IPAC’22 was a successful and memorable conference, seen as a symbol of our return to normal scientific activities and face-to-face interaction. It was also one of the most difficult IPAC conferences to organise – prohibiting or impeding participation from several regions, particularly China and Taiwan, as the world begins to recover from the most prevalent health-related crisis in a century. It was mentioned in the opening session that many breakthroughs in combating the coronavirus pandemic were achieved with the use of particle accelerators: the molecular structure of the virus, which is essential information for subsequent rational drug design, was solved at synchrotron light sources.

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More than 500 participants from over 30 countries attended the annual meeting of the Future Circular Collider (FCC) collaboration, which is pursuing a feasibility study for a visionary post-LHC research infrastructure at CERN. Organised as a hybrid event at Sorbonne University in Paris from 30 May to 3 June, the event demonstrated the significant recent progress en route to the completion of the feasibility study in 2025, and the technological and scientific opportunities on offer.

In their welcome talks, Ursula Bassler (CNRS) and Philippe Chomaz (CEA), chair of the FCC collaboration board, stressed France’s long-standing participation in CERN and reaffirmed the support of French physicists and laboratories in the different areas of the FCC project. CERN Director-General Fabiola Gianotti noted that the electron–positron stage, FCC-ee, could begin operations within a few years of the end of the HL-LHC – a crucial step in keeping the community engaged across different generations – while the full FCC programme would offer 100 years of trailblazing physics at both the energy and intensity frontiers. Beyond its outstanding scientific case, FCC requires coordinated R&D in many domains, such as instrumentation and engineering, raising opportunities for young generations to contribute with fresh ideas. These messages echoed those in other opening talks, in particular by Jean-Eric Paquet, director for research and innovation at the European Commission, who highlighted FCC’s role as a world-scale research infrastructure that will allow Europe to maintain its leadership in fundamental research.

A new era

Ten years after the discovery of the Higgs boson, the ATLAS and CMS collaborations continue to establish its properties and interactions with other particles. The discovery of the Higgs boson completes the Standard Model but leaves many questions unanswered; a new era of exploration has opened that requires a blend of large leaps in precision, sensitivity and eventually energy. Theorist Christophe Grojean (DESY) described how the diverse FCC research programme (CERN Courier May/June 2022 p23) offers an extensive set of measurements at the electroweak scale, the widest exploratory potential for new physics, and the potential to address outstanding questions such as the nature of dark matter and the origin of the cosmic matter–antimatter asymmetry.

In recent months, teams from CERN have worked closely with external consultants and CERN’s host states to develop a new FCC layout and placement scenario (CERN Courier May/June 2022 p27). Key elements include the effective use of the European electricity grid, the launch of heat-recovery projects, cooling, agriculture and industrial use – as well as cutting-edge data connections to rural areas. Parallel sessions at FCC Week focused on the design of FCC-ee, which offers a high-luminosity Higgs and electroweak factory. Tor Raubenheimer (SLAC) showed it to be the most efficient lepton collider for energies up to the top-quark mass threshold and highlighted its complementarity to a future FCC-hh. Profiting from the FCC-ee’s high technological readiness, ongoing R&D efforts aim to maximise the efficiency and performance while optimising its environmental impact and operational costs. Many sessions were dedicated to detector development, where the breadth of new results showed that the FCC-ee is much more than a scaled-up version of LEP. It would offer unprecedented precision on Higgs couplings, electroweak and flavour variables, the top-quark mass, and the strong coupling constant, with ample discovery potential for feebly interacting particles. Participants also heard about the FCC-ee’s unique ability in ultra-precise centre-of-mass energy measurements, and the need for new beam-stabilisation and feedback systems.

The FCC programme builds on the large, stable global community that has existed for more than 30 years at CERN and in other laboratories worldwide

High-temperature superconductor (HTS) magnets are among key FCC-ee technologies under consideration for improved energy efficiency, also offering significant potential societal impact. They could be deployed in the FCC-ee final-focus sections, around the positron-production target, and even in the collider arcs. Another major focus is ensuring that the 92 km-circumference machine’s arc cells are effective, reliable and easy to maintain, with a complete arc half-cell mockup planned to be constructed by 2025. The exploration of existing and alternative technologies for FCC-ee is supported by two recently approved projects: the Swiss accelerator R&D programme CHART, and the EU-funded FCCIS design study. The online software requirements for FCC-ee are dominated by an expected physics event rate of ~200 kHz when running at the Z pole. Trigger and data acquisition systems sustaining comparable data rates are already being developed for the HL-LHC, serving as powerful starting points for FCC-ee.

Looking to the future

Finally, participants reviewed ongoing activities toward FCC-hh, an energy- frontier 100 TeV proton–proton collider to follow FCC-ee by exploiting the same infrastructure. FCC-hh studies complement those for FCC-ee, including the organisation of CERN’s high-field magnet R&D programme and the work of the FCC global conductor-development programme. In addition, alternative HTS technologies that could reach higher magnetic fields and higher energies while reducing energy consumption are being explored for FCC’s energy-frontier stage. The challenges of building and operating this new infrastructure and the benefits that can be expected for society and European industry were also discussed during a public event under the auspices of the French Physical Society. 

The FCC programme builds on the large, stable global community that has existed for more than 30 years at CERN and in other laboratories worldwide. The results presented during FCC Week 2022 and ongoing R&D activities will inspire generations of students to learn and grow. Participants from diverse fields and the high number of junior researchers who joined the meeting underline the attractiveness of the project. Robust international participation and long-term commitment to deliver ambitious projects are key for the next steps in the FCC feasibility study.

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Marking 10 years since the discovery of the Higgs boson, a two-day workshop held at the University of Birmingham on 30 June and 1 July brought together ATLAS and CMS physicists who were involved in the discovery and subsequent characterisation of the Higgs boson. Around 75 physicists, in addition to members of the public who attended a colloquium, celebrated this momentous discovery together with PhD students, early-career researchers and members of IOP’s history of physics group. In an informal atmosphere, participants recalled and gave insights on what had taken place, spicing it with personal stories that placed the human dimension of science under the spotlight.

The story of the Higgs-boson search was traced from the times of LEP and the Tevatron. Participants were reminded of the uncertainty and excitement during the final days of LEP: the hints of an excess of events at around 115 GeV and the ensuing controversy surrounding the decision to either stop the machine or extend its data-taking further. For the Tevatron, the focus was more on the relentless race against time until the LHC could provide an overwhelming dataset. It was considered plausible that the Tevatron could observe the Higgs boson first, leading CERN to delay a scheduled break in LHC data-taking following its 2011 run.

The timeline of the design, construction and commissioning of the LHC experi­ments was presented, with a particular focus on the excellent performance achieved by ATLAS and CMS since the beginning of Run 1. The parallel role of theory and the collaboration among theorists and experimentalists was also discussed. Speakers from the experiments involved in the Higgs-discovery analyses provided personal perspectives on the events leading up to the 4 July 2012 announcement.

With his unique perspective, former CERN Director-General Chris Llewellyn-Smith described the early discussions and approval of the LHC project during a well-attended public symposium. He recalled his discussions with former UK prime minister Margaret Thatcher, the role of the ill-fated US Superconducting Super Collider and the “byzantine politics” that led to the LHC’s approval in 1994. Most importantly, he emphasised that the LHC was not inevitable: scientists had to fight to secure funding and bring it to reality. Former ATLAS spokesperson David Charlton reflected on the preparation of the experiments, the LHC startup in 2008 and subsequent magnet problems that delayed the physics runs until 2010, noting the excellent performance of the machine and detectors that enabled the discovery to be made much earlier than expected.

The workshop would not have been complete without a discussion on what happened after the discovery. Precision measurements of the Higgs-boson couplings, observation of new decay and production modes, as well as the search for Higgs-boson pair-production were described, always with a focus on the challenges that needed to be overcome. The workshop closed with a look to the future, both in terms of experimental prospects of the High-Luminosity LHC and theory.

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Peter Higgs winner of the 2013 Nobel Prize in Physics.

“It was an overwhelming time for us. It took time to understand what had happened. I especially remember the excitement among the young researchers.” 

Rolf Heuer former CERN Director-General. 

“It took 14 years to build the LHC. At one point we had 1000 dipoles, each costing a million Swiss francs, stored on the surface, throughout rain and snow.” 

Lyn Evans former LHC project director.

“The first two years of measuring Standard Model physics were essential to give us confidence in the readiness of the two experiments to search for new physics.” 

Peter Jenni founding ATLAS spokesperson.

“A key question for CMS was: can tracking be done in a congested environment with just a few points, albeit precise ones? It was a huge achievement requiring more than 200 m² of active silicon.” 

Michel Della Negra founding CMS spokesperson.

“I remember on 4 July 2012 a magnificent presentation of a historical discovery. I would also like to celebrate the life of Robert Brout, a great physicist and important man.” 

François Englert winner of the 2013 Nobel Prize in Physics. 

“The gist of the theory behind the Higgs boson would easily compete with the most far-fetched conspiracy theory, yet it seems nature chose it.” 

Eliezer Rabinovici president of the CERN Council.

“The structure of the vacuum is intimately connected to how the Higgs boson interacts with itself. To probe this phenomenon at the LHC we can study the production of Higgs-boson pairs.” 

André David CMS experimentalist (CERN).

“Collaboration between experiment and theory is even more necessary now to find any hints for BSM physics.” 

Reisaburo Tanaka ATLAS experimentalist (Université Paris-Saclay).

“Precision Higgs physics is a telescope to high-scale physics, so I’m looking forward to the next 10 years of discovery.” 

Sally Dawson theorist (BNL). 

“Theory accuracy will be even more important to make the best of the HL-LHC data, especially in the case in which no evidence of new physics will show up… This is also crucial for the Monte Carlo tools used in the analyses.”

Massimiliano Grazzini theorist (University of Zurich).

“After 10 years we’ve measured the five main production and five major decay mechanisms of the Higgs boson.” 

Kerstin Tackmann ATLAS experimentalist (DESY).

“What we know so far – Mass: known to 0.11%. Width: closing in on SM value of 3.2^+2.5_–1.7   MeV (plus evidence of off-shell Higgs production). Spin 0: spin 1 & 2 excluded at 99.9% CL. CP structure: in accordance with SM CP-even hypotheses.”

Marco Delmastro ATLAS experimentalist (CNRS/IN2P3 LAPP).

“We have learned much about the 125 GeV Higgs boson since its discovery. The LHC Run 3 starts tomorrow: ready for the next decade of Higgs-boson exploration!”

Adinda de Wit CMS experimentalist (University of Zurich).

“The Higgs boson is linked to profound structural problems in the Standard Model. It is therefore an extraordinary discovery tool that calls for a broad experimental programme at the LHC and beyond.” 

Fabiola Gianotti CERN Director-General.

“Elusive non-resonant pairs of Higgs bosons are the prime experimental signature of the Higgs-boson self-coupling. We are all eager to analyse Run 3 data to further probe HH events!”

Arnaud Ferrari ATLAS experimentalist (Uppsala University).

“New physics can affect differently the different fermion generations. We have to precisely measure the couplings if we want to understand the Higgs boson’s nature.”

Andrea Marini CMS experimentalist (CERN).

“From its potential invisible, forbidden, and exotic decays to the possible existence of scalar siblings, the Higgs boson plays a fundamental role in searches for physics beyond the Standard Model.”

Roberto Salerno CMS experimentalist (CNRS/IN2P3 – LLR & École polytechnique).

“An incredible collaborative effort has brought us this far. But there is much more to come, especially during Long Shutdown 3, with HL-LHC paving the way from Run 3 to ultimate performance. Interesting times ahead to say the least!”

Mike Lamont CERN director for accelerators and technology.

“The hard work and creativity in reconstruction and analysis techniques are already evident since the last round of projections. Imagine what we can do in the next 20 years!”

Elizabeth Brost ATLAS experimentalist (BNL).

“The Higgs is the first really new elementary particle we’ve seen. We need to study it to death!”  

Nima Arkani-Hamed theorist (IAS).

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Involving around 1500 participants, 17 parallel sessions, 900 talks and 250 posters, ICHEP2022 (which took place in Bologna from 6 to 13 July) was a remarkable week of physics, technology and praxis. The energy and enthusiasm among the more than 1200 delegates who were able to attend in person was palpable. As the largest gathering of the community since the beginning of the pandemic – buoyed by the start of LHC Run 3 and the 10th anniversary of the Higgs-boson discovery – ICHEP2022 served as a powerful reminder of the importance of non-digital interactions.

Roberto Tenchini’s (INFN Pisa) heroic conference summary began with a reminder: it is 10 years since ICHEP included a session titled “Standard Model”, the theory being so successful that it now permeates most sessions. As an example, he highlighted cross-section predictions tested over 14 orders of magnitude at the LHC. Building on the Higgs@10 symposium at CERN on 4 July, the immense progress in understanding the properties and interactions of the Higgs boson (including legacy results with full Run 2 statistics in two papers by ATLAS and CMS published in Nature on 4 July) was centre stage. CERN Director-General Fabiola Gianotti gave a sweeping tour of the path to discovery and emphasised the connections between the Higgs boson and profound structural problems in the SM. Many speakers highlighted the concomitant role of the Higgs boson in exploring new physics, dashing notions that future precision measurements are “business as usual”. Chiara Mariotti (INFN Torino) pointed out that only 3% of the total Higgs data expected at the LHC has been analysed so far.

Hot topics

Another hot electroweak topic was CDF’s recent measurement of the mass of the W boson, as physicists try to understand what could cause it to lie so far from its prediction and from previous measurements. Andrea Rizzi (Pisa) confirmed that CMS is working hard on a W-mass analysis that will bring crucial information, on a time-scale to be decided. Patience is king with such a complex analysis, he said: “we are really trying to do the measurement the way we want to do it.”

CMS presented a total of 85 parallel talks and 28 posters, including new searches related to b-anomalies with taus, and the most precise measurement of B_s → μ⁺μ^–. Among new results presented by ATLAS in 71 parallel talks and 59 posters were the observation of a four charm–quark state consistent with one seen by LHCb, joint-polarisation measurements of the W and Z bosons, and measurements of the total proton–proton cross section and the ratio of the real vs imaginary parts of the elastic-scattering amplitude. ATLAS and CMS also updated participants on many searches for new particles, in particular leptoquarks. Among highlights were searches by ATLAS for events with displaced vertices, which could be caused by long-lived particles, and by CMS for resonances decaying to Higgs bosons and pairs of either photons or b quarks, which show interesting excesses. “Se son rose fioriranno!” said Tenchini. 

The sigmas are rather higher for exotic hadrons. LHCb presented the discovery of a new strange pentaquark (with a minimum quark content ccuds) and two tetraquarks (one corresponding to the first doubly charged open-charm tetraquark with csud), taking the number of hadrons discovered at the LHC so far to well over 60, and introducing a new exotic-hadron naming scheme for “particle zoo 2.0” (Exotic hadrons brought into order by LHCb). LHCb also reported the first evidence for direct CP violation in the charm system (LHCb digs deeper in CP-violating charm decays) and a new precise measurement of the CKM angle γ. Vladimir Gligorov (LPNHE) described how, in addition to the flavour factories LHCb and Belle II, experiments including ATLAS, CMS, BESIII, NA62 and KOTO will be crucial to enable the next level of understanding in quark mixing. Despite no significant new results having been presented, the status of tests of lepton flavour universality (LFU) in B decays by LHCb generated lively discussions, while Toshinori Mori (Tokyo) described exciting prospects for LFU tests in charged-lepton flavour experiments, in particular MEG-II, which has just started operations at PSI, and the upcoming Mu2e and MUonE experiments.

ICHEP2022 served as a powerful reminder of the importance of non-digital interactions

Moving to leptons that are known to mix, neutrinos continue to play very important roles in understanding the smallest and largest scales, said Takaaki Kajita (Tokyo) via a link from the IUPAP Centennial Symposium taking place in parallel at ICTP Trieste. Status reports on DUNE, Hyper-K, JUNO, KM3NeT and SNB showed how these detectors will help constrain the still poorly-known PNMS matrix that describes leptonic mixing, while new results from NOvA and STEREO further reveal anomalous behaviour. Among the major open questions in neutrino physics summed-up by theorist Joachim Kopp (Mainz and CERN) were: how do neutrinos interact? What explains the oscillation anomalies? And how do supernova neutrinos oscillate?

Several plenary presentations showcased the increasing complementarity with astroparticle physics and cosmology, with the release of the first-science images from the James Webb Space Telescope on 12 July adding spice (Webb opens new era in observational astrophysics). Multiband gravitational-wave astronomy across 12 or more orders of magnitude in frequency will bloom in the next decade, predicted Giovanni Andrea Prodi (Trento), while larger datasets and synchronisation of experiments offer a bright future in all messengers, said Gwenhael De Wasseige (Louvain): “We are just at the beginning of the story.” The first results from the Lux–Zeplin experiment were presented, setting the tightest limits on spin-independent WIMP–nucleon cross-sections for WIMP masses above 9 GeV (CERN Courier September/October 2022 p13), while the increasingly crowded plot showing limits from direct searches for axions illustrate the vibrancy and shifting focus of dark-matter research. Indeed, among several sessions devoted to the exploration of high-energy QCD in heavy-ion, proton–lead and proton–proton collisions, Andrea Dainese (INFN Padova) described how the LHC is not only a collider of nuclei but an (anti-)nuclei factory relevant for dark-matter searches.

The unique ability of theorists to put numerous results and experiments in perspective was on full display. We should all renew the enthusiasm that built the LHC, and be a lot more outspoken about the profound ideas we explore, urged Veronica Sanz (Sussex); after all, she said, “we are searching for something that we know should be somewhere.” A timely talk by Gavin Salam (Oxford) summarised the latest understanding of QCD effects relevant to the muon g-2 and W-mass anomalies and also to future Higgs-boson measurements, concluding that, as we approach high precision, we should expect to be confronted by conceptual problems that we could, so far, ignore.

The unique ability of theorists to put numerous results and experiments in perspective was on full display

Accelerators (including a fast-paced summary of the HL-LHC niobium-tin magnet programme from Lucio Rossi), detectors (68 talks and posters revealing an increasingly holistic approach to detector design), computing (highlighting a period of rapid evolution thanks to optimisation, modernisation, machine-learning algorithms and increasing hardware diversity), industry, diversity and outreach were addressed in detail. A highly acclaimed outreach event in Bologna’s Piazza Maggiore on the evening of 12 July saw thousands of people listen to Fabiola Gianotti, Guido Tonelli, Gian Giudice and Antonio Zoccoli discuss the implications of the Higgs-boson discovery.

Only the narrowest snapshot of proceedings is possible in such a short report. What was abundantly clear from ICHEP2022 is that, following the discovery of the Higgs boson and as-yet no new particles beyond the SM, the field is in a fascinating and challenging period where confusion is more than matched by confidence that new physics must exist. The strategic direction of the field was addressed in two wide-ranging round-table discussions where laboratory directors and senior physicists answered questions submitted by participants. Much discussion concerned future colliders, and addressed a perceived worry in some quarters that the field is entering a period of decline. For anyone following the presentations at ICHEP2022, nothing could be further from the truth.

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The Synchrotron-light for Experimental Science and Applications in the Middle East (SESAME) is a 2.5 GeV third-generation synchrotron radiation (SR) source developed under the auspices of UNESCO and modelled after CERN. Located in Allan, Jordan, it aims to foster scientific and technological excellence as well as international cooperation amongst its members, which are currently Cyprus, Egypt, Iran, Israel, Jordan, Pakistan, Palestine and Turkey. As a user facility, SESAME hosts visiting scientists from a wide range of disciplines, allowing them to access advanced SR techniques that link the functions and properties of samples and materials to their micro, nano and atomic structure.

The location of SESAME is known for its richness in archaeological and cultural heritage. Many important museums, collections, research institutions and universities host departments dedicated to the study of materials and tools that are inextricably linked to prehistory and human history, demanding interdisciplinary research agendas and teams. As materials science and condensed-matter physics play an increasing role in understanding and reconstructing the properties of artefacts, SESAME offers a highly versatile tool for the researchers, conservators and cultural-heritage specialists in the region.

The high photon flux, small source size and low divergence available at SR sources allow for advanced spectroscopy and imaging techniques that are well suited for studying ancient and historical materials, and which often present very complex and heterogeneous structures. SR techniques are non-destructive, and the existence of several beamlines at SR facilities means that samples can easily be transferred and reanalysed using complementary techniques.

SESAME offers a versatile tool for researchers, conservators and cultural-heritage specialists in the region

At SESAME, an infrared microspectroscopy beamline, an X-ray fluorescence and absorption spectroscopy beamline, and a powder diffraction beamline are available, while a soft X-ray beamline called “HESEB” has been designed and constructed by five Helmholtz research centres and is now being commissioned. Next year, the BEAmline for Tomography at SESAME (BEATS) will also be completed, with the construction and commissioning of a beamline for hard X-ray full-field tomography. BEATS involves the INFN, The Cyprus Institute and the European SR facilities ALBA-CELLS (Spain), DESY (Germany), ESRF (France), Elettra (Italy), PSI (Switzerland) and SOLARIS (Poland).

To explore the potential of these beamlines, the First SESAME Cultural Heritage Day took place online on 16 February with more than 240 registrants in 39 countries. After a welcome by SESAME director Khaled Toukan and president of council Rolf Heuer, Mohamed ElMorsi (Conservation Centre, National Museum of Egyptian Civilization), Marine Cotte (ESRF) and Andrea Lausi (SESAME) presented overviews of ancient Egyptian cultural heritage, heritage studies at the ESRF, and the experimental capabilities of SESAME, respectively. This was followed by several research insights obtained by studies at SESAME and other SR facilities: Maram Na’es (TU Berlin) showed the reconstruction of colour in Petra paintings; Heinz-Eberhard Mahnke and Verena Lepper (Egyptian Museum and Papyrus Collection, FU/HU Berlin and HZB) explained how to analyse ancient Elephantine papyri using X-rays and tomography; Amir Rozatian (University of Isfahan) and Fatma Marii (University of Jordan) determined the material of pottery, glass, metal and textiles from Iran and ancient glass from the Petra church; and Gonca Dardeniz Arıkan (Istanbul University) provided an overview of current research into the metallurgy of Iran and Anatolia, the origins of glassmaking, and the future of cultural heritage studies in Turkey. Palaeontology with computed tomography and bioarchaeological samples were highlighted in talks by Kudakwashe Jakata (ESRF) and Kirsi Lorentz (The Cyprus Institute).

During the following discussions, it was clear that institutions devoted to the research, preservation and restoration of materials would benefit from developing research programmes in close cooperation with SESAME. Because of the multiple applications in archaeology, palaeontology, palaeo-environmental science and cultural heritage, it will be necessary to establish a multi-disciplinary working group, which should also share its expertise on practical issues such as handling, packaging, customs paperwork, shipping and insurance. 

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Tens of thousands of accelerators around the world help create radiopharmaceuticals, treat cancer, preserve food, monitor the environment, strengthen materials, understand fundamental physics, study the past, and even disclose crimes.

A first of its kind international conference, Accelerators for Research and Sustainable Development: From Good Practices Towards Socioeconomic Impact was organised by the International Atomic Energy Agency (IAEA) at its headquarters in Vienna from 23 to 27 May. It was held as a hybrid event attended by around 500 scientists from 72 IAEA member states. While focusing mainly on applications of accelerator science and technology, the conference was geared towards accelerator technologists, operators, users, entrepreneurs, and other stakeholders involved in applications of accelerator technologies as well as policy makers and regulators.

The far-reaching capabilities of accelerator technology help countries progress towards sustainable development

Rafael Mariano Grossi

“The far-reaching capabilities of accelerator technology help countries progress towards sustainable development,” said IAEA director general Rafael Mariano Grossi in his opening address. “IAEA’s work with accelerators helps to fulfil a core part of its ‘Atoms for Peace and Development’ mandate.” He also highlighted how accelerator technology plays a critical role in two IAEA initiatives launched over the past year: Rays of Hope, aimed at improving access to radiotherapy and cancer care in low- and middle-income countries, and NUTEC plastics, supporting countries in addressing plastic waste issues in the ocean and on land. Finally, he described IAEA plans to establish an accelerator of its own: a state-of-the-art ion-beam facility in Seibersdorf, Austria that will support research and help educate and train scientists.

The conference included sessions dedicated to case studies demonstrating socioeconomic impact as well as best practices in effective management, safe operation, and the sustainability of present and future accelerator facilities. It showcased the rich diversity in types of accelerators – from large-scale synchrotrons and spallation neutron sources, or medical cyclotrons and e-beam irradiators used for industrial applications, to smallscale electrostatic accelerators and compact-accelerator based neutron sources – and included updates in emerging accelerator technologies, such as laser-driven neutron and X-ray sources and their future applications. Six plenary sessions featuring 16 keynote talks captured the state of the art in various application domains, accompanied by 16 parallel and two poster sessions by young researchers.

During the summary and highlights session, important developments and future trends were presented:

Accelerator technologies evolve very fast, presenting a challenge for regulatory bodies to authorise and inspect accelerator facilities and activities. This conference demonstrated that thanks to recent technological breakthroughs in accelerator technology and associated instrumentation, accelerators are becoming an equally attractive alternative to other sources of ionising radiation such as gamma irradiators or research reactors, among other conventional techniques. Based on the success of this conference, it is expected that the IAEA will start a new series of accelerator community gatherings periodically from now on every two to three years.

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Science and technology are key instruments for a society’s economic growth and development. Yet Africa’s science, innovation and education have been chronically under-funded. Transferring knowledge, building research capacity and developing competencies through training and education are major priorities for Africa in the 21st century. Physics combines these priorities by extending the frontiers of knowledge and inspiring young people. It is therefore essential to make basic knowledge of emerging technologies available and accessible to all African citizens to build a steady supply of trained and competent researchers. 

In this spirit, the African School of Fundamental Physics and Applications was initiated in 2010 as a three-week biennial event. To increase networking opportunities among participants, the African Conference on Fundamental and Applied Physics (ACP) was included as a one-week extension of the school. The first edition was held in Namibia in 2018 and the second, co-organised jointly by Mohammed V University and Cadi Ayyad University in Morocco, was rebranded ACP2021, originally scheduled to take place in December but postponed due to COVID-19. The virtual event held from 7 to 11 March attracted more than 600 registrants, an order of magnitude higher than its first edition. 

The ACP2021 scientific programme covered the three major physics areas of interest in Africa defined by the African Physical Society: particles and related applications; light sources and their applications; and cross-cutting fields covering accelerator physics, computing, instrumentation and detectors. The programme also included topics in quantum computing and quantum information, as well as machine learning and artificial intelligence. Furthermore, ACP2021 focused on topics related to physics education, community engagement, women in physics and early-career physicists. The agenda was stretched to accommodate different time zones and 15 parallel sessions took place.

Welcome speeches by Hassan Hbid (Cadi Ayyad University) and by Mohammed Rhachi (Mohammed V University) were followed by a plenary talk by former CERN Director-General Rolf Heuer, “Science bridging Cultures and Nations” and an overview of the African Strategy for Fundamental and Applied Physics (ASFAP). Launched in 2021, the ASFAP aims to increase African education and research capabilities, build the foundations and frameworks to attract the participation of African physicists, and establish a culture of awareness of grassroots physics activities contrary to the top-down strategies initiated by governments. Shamila Nair-Bedouelle (UNESCO) conveyed a deep appreciation of and support for the ASFAP initiative, which is aligned with the agenda of the United Nations Sustainable Development Goals. A rich panel discussion followed, raising different views on physics education and research roadmaps in Africa.

A central element of the ACP2021 physics programme is the ASFAP community planning meeting, where physics and community-engagement groups discussed progress in soliciting the community input that is critical for the ASFAP report. The report will outline the direction for the next decade to encourage and strengthen higher education, capacity building and scientific research in Africa.

The motivation and enthusiasm of the ACP2021 participants was notable, and the efforts in support of research and education across Africa were encouraged. The next ACP in 2023 will be hosted by South Africa. 

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The first Italian workshop on the Future Circular Collider (FCC) took place in Rome from 21 to 22 March and was attended by around 120 researchers.

The FCC study is exploring the technical and financial feasibility of a 91 km-circumference collider situated under French and Swiss territory near CERN, thus exploiting existing infrastructures. In a first phase (FCC-ee) the tunnel would host an electron–positron collider at energies from 90 to 365 GeV, which would be replaced by a proton–proton collider (FCC-hh) with a centre-of-mass energy of at least 100 TeV, almost an order of magnitude higher than that of the LHC. The proposed roadmap foresees the R&D for the 16 T superconducting dipole magnets needed to keep the FCC-hh proton beams on track to take place in parallel with FCC-ee construction and operation. 

“The FCC is a large infrastructure that would allow Europe to maintain its worldwide leadership in high-energy physics research. This project is therefore of strategic importance in the international science scenario of the coming years,” remarked INFN president Antonio Zoccoli in his introduction. “INFN has great potential and could make a significant contribution to its implementation. In this perspective, it is important to clearly identify the main activities in which to invest, assemble the necessary human resources and identify possible industrial partners.”

The workshop was opened by FCC study leader Michael Benedikt, who gave an overview of the FCC feasibility study, while deputy study leader Frank Zimmermann covered the technological challenges, design features and machine studies for FCC-ee. Opportunities for technological development related to the FCC-ee were then presented, along with machine studies, in which INFN are already involved. Scientific and technological R&D areas where collaborations could be strengthened or initiated were also identified, prompting an interesting discussion with CERN colleagues. 

INFN is already well integrated both in the FCC coordination structure and several ongoing studies, having participated in the project since its beginning, and provides important contributions on all aspects of the FCC study. These range from accelerator and detector R&D, such as the development of superconducting magnets, to experimental and theoretical physics studies. This is made evident by the strong Italian involvement in FCC-related European programmes, such as EuroCirCol for FCC-hh and FCC-IS for FCC-ee, and AIDAinnova on innovative detector technologies for future accelerators. INFN is committed to the development of superconducting magnets for FCC-hh, for which substantial additional funding could come from a project in the context of the next-generation funding programme Horizon Europe.

The second day of the workshop focused on the work that experimental and theoretical physicists have been carrying out to deeply understand the scientific potential of the visionary FCC project, the specific requests for the detectors and the associated R&D activities.

This workshop was the first in a series organised by INFN to promote and support the FCC project and pursue the key technological R&D needed to demonstrate its feasibility by the next update of the European strategy for particle physics.

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David Cox, a giant in the world of statistics, passed away earlier this year at the age of 97. As he had been a contributor to PHYSTAT workshops and was a supporter of its activities, a seminar held on 23 March was dedicated to his memory. Brad Efron (Stanford) referred to Cox as the world’s most famous statistician – an assessment confirmed by Cox being the first recipient of the International Prize in Statistics, roughly the equivalent of a Nobel Prize. The citation mentioned a lifelong series of contributions to statistics spanning many subjects. In particular, it emphasised his work on what is now called Cox’s proportional hazards model, which provides a very useful way to implement regression analysis of survival times (the times to an event of interest such as the death of a person or failure of a machine). His contribution is ranked 16th in Nature’s list of most-cited papers in any subject.

Heather Battey (Imperial College), who collaborated closely with Cox for the past five years, described how he was still very active until his very last days, and highlighted his helpful and charming personality. 

Long-time collaborator Nancy Reid (Toronto) concurred, admiring his ability to see through extraneous detail and concentrate on the essence of the problem. She remembers going with him to watch Verdi’s Ernani, sung in Italian, in Budapest when they were both attending a statistics meeting there. So that Reid wouldn’t be completely lost, Cox kindly summarised the lengthy and convoluted plot by telling her “The tenor is in love with the soprano, and the baritone is trying to keep them apart.” 

It was a special pleasure to have Cox available at our meetings, and he was always prepared to explain statistical issues in informal discussions with particle physicists. Bob Cousins (UCLA) recalled the talks Cox had given at PHYSTAT meetings in 2005, 2007 and 2011. He compared and contrasted frequentist statistics and the “five faces” of Bayesian statistics, repeatedly warning of the dangers of “treacherous” uniform prior probability densities used in attempts to represent ignorance. He alluded to a general key problem in frequentist statistics, that of ensuring that the long run used to calibrate coverage is relevant to the specific data sample being analysed. He also discussed in more technical detail issues of testing multiple hypotheses, including graphical methods. Cox and Reid further offered published thoughts on problems presented to them by LHC physicists. Cousins concluded that we would do well to read Cox’s contributions again.

PHYSTAT is pleased and honoured to have had the opportunity of paying its respect to a very eminent statistician and a wonderful person. His memory will long be with us.

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Despite several COVID waves, the organisers of the 6th edition of the International Summer School on Intelligent Signal Processing for Frontier Research and Industry (INFIERI) made this school an in-person event. The INFIERI school was successfully held at UAM from August 23 to September 4 thanks to the unprecedented speed of the vaccine roll out, the responsible behaviour of the school participants and the proper applied logistics.

Against a backdrop of topics ranging from cosmology to the human body and particle physics, the programme covered advanced technologies such as semiconductors, deep sub-micron 3D technologies, data transmission, artificial intelligence and quantum computing.

Topics were presented in lectures and keynote speeches, and the teaching was reinforced via hands-on laboratory sessions, allowing students to practise applications in realistic conditions across a range of areas, such as: theoretical physics, accelerators, quantum communication, Si Photonics and nanotechnology. The latter included medical applications to new mRNA vaccines, which have long been under investigation for cancer treatment, besides their use against COVID-19. For instance, they could analyse combined real PET/MRI images using machine-learning techniques to find biomarkers of illness in a hospital setting, or study the irradiation of a biomaterial using a proton beam. Worldwide experts from academia, industry and laboratories such as CERN either gave lectures or ran lab sessions, most of them attending in person, often for the entire duration of the school.

During the last day, the students presented posters on their own research projects – the high number and quality of presentations reflecting the cross-disciplinary facets and the excellence of the participants. Many were then selected to be part of the in-preparation proceedings of the Journal of Instrumentation.

The next INFIERI school will only offer in-person attendance, which is considered essential to the series, but if the pandemic continues it will exploit some of the learning gained from the 6th edition.

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Keeping with the tradition of Moriond, several new experimental results were presented by major experimental collaborations, with participants enjoying ample opportunities to debate cases where measurements and theoretical predictions do not agree. Held 10 years after the Higgs discovery, the conference started with a review of how the Higgs boson came of age – from early exploration to a precision era. An exciting mix of new precision results and interesting observations in Higgs physics were presented, including the first measurement of the Higgs-charm coupling as well as studies of off-shell Higgs production and di-Higgs production by the ATLAS and CMS collaborations.

The first observation of tqγ production by ATLAS as well as many measurements in top-quark physics, including a mass measurement based on single top quarks by CMS, were discussed. Many recent studies of Z and W bosons and their interactions were reported, including a new CMS result that resolved an earlier mild LEP tension in the decay rates of W bosons to leptons, and the observation of triple-W production at the LHC by ATLAS. The LHCb collaboration presented its first measurement of the W mass, while CMS discussed the first observation of WW and triple-J/ψ production in double-parton scattering.

Several sessions were devoted to flavour measurements and anomalies, including possible lepton-flavour universality violations in B-meson decays. LHCb presented the most precise value of the CKM matrix angle γ measured in a single experiment, as well as the most precise measurement of the charm-mixing parameter y_CP. New results on lepton-flavour universality attracted a lot of attention. Among them are LHCb’s measurement of the ratio of Br(B⁺ → K⁺μ⁺μ^–) to Br(B⁺ → K⁺e⁺e^–), which is 3.1σ away from the SM, new LHCb limits on rare B⁰ decays, and the CMS measurement of the Drell–Yan forward–backward asymmetry difference between di-muons and di-electrons. The status of selected Standard Model (SM) calculations was described with the conclusion that the predictions are robust and therefore possible deficiencies of the SM a very unlikely source of the flavour anomalies. A number of talks demonstrated that there are many ways to accommodate the flavour anomalies into a consistent physics picture, which predicts subtle signals at the LHC that could have easily evaded detection so far.

Several speakers emphasised the importance of new creative analysis concepts

Continuing the topic of searches for new physics, several speakers emphasised the importance of new creative analysis concepts, including searching for anomalous energy losses, non-pointing tracks, delayed photons, displaced jets, displaced collimated leptons and tagging missing mass with forward detectors. Among the results of many interesting searches presented at Moriond, a 3σ excess in the number of highly ionising particles reported by the ATLAS collaboration caused some excitement and discussion, indicating that further studies (and statistics!) are very much needed.

Several talks presented theoretical predictions at high orders of perturbative QCD for basic SM processes at the LHC and future lepton colliders, such as the Drell–Yan and jet-production processes. These tour de force computations, representing cutting-edge applications of quantum field theory to collider physics, force us to think about how such advances in the theory of hard hadron collisions can be used to search for physics beyond the SM. Several talks addressed this issue by considering specific physics examples pointing towards new, exciting opportunities during LHC Run 3.

Emphasising the need for a refined knowledge of the fundamental input parameters used to describe hadron collisions, four new extractions of the strong coupling constant were reported, based on HERA, CDF, LEP and CMS data. The role of precision deep-inelastic scattering (HERA) and W/Z (ATLAS/CMS) data in constraining parton distribution functions was clearly elucidated.

An element of nonperturbative QCD that keeps theorists on their toes is hadronic spectroscopy

Turning towards the non-perturbative sector of QCD, a measurement of Λ_c production down to zero transverse momentum allowed the ALICE collaboration to extract the total charm cross-section in pp collisions. Interestingly, the fraction of Λ_c is significantly above the e⁺e^– baseline. Jet substructure measurements presented by ALICE and CMS allow a detailed comparison to Monte Carlo event generators. Furthermore, the first direct observation of the dead-cone effect, a suppression of forward gluon radiation in case of a massive emitter, was presented by the ALICE collaboration using charm-tagged jets.

An element of non-perturbative QCD that keeps theorists on their toes is hadronic spectroscopy. This trend continued at Moriond where the discoveries of several new states were presented, including the same-sign doubly charmed T⁺_cc (c–c–u–d) (LHCb) and the Z^–_cs (c–c–s–u) (BES III). The exploration of the χ_c1, earlier known as X(3872), with the hope of revealing its molecular or tetraquark nature, continues in pp as well as in PbPb collisions.

The best constraint of the charm diffusion coefficient in the quark–gluon plasma (ALICE), jet quenching studies with Z-hadron correlations (CMS) and surprising results on ridge structures in γp and γPb collisions (ATLAS) were presented during a dedicated heavy-ion session. Interestingly, by studying the abundant nuclei produced in heavy-ion collisions, the ALICE collaboration ruled out simple coalescence models for antideuteron production in PbPb collisions.

Finally, the current status of the muon anomalous magnetic moment was reviewed. The experimental value presented last year by the Fermilab g-2 collaboration shows a 1.5–4.2σ discrepancy with the SM prediction, depending on the theoretical baseline. An interesting comparison between continuum and lattice computations of the hadronic vacuum polarisation contributions was presented, and a new lattice result on hadronic light-by-light scattering was described, indicating that this “troublemaking” contribution is being brought under theoretical control.

Exciting experimental results and developments in the theory of QCD and high-energy interactions that, perhaps, remained somewhat hidden during the pandemic years, were on full display at Moriond, making the 56th edition of this conference a resounding success.

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The ever maturing technology of silicon photomultipliers (SiPMs) has a range of advantages over traditional photomultiplier tubes (PMTs). As such, SiPMs are quickly replacing PMTs in a range of physics experiments. The technology is already included in the LHCb SciFi tracker and is foreseen to be used in CMS’ HGCAL, as well as in detectors at proposed future colliders. For these applications the important advantages of SiPMs over PMTs are their higher photo-detection efficiencies (by roughly a factor of two), their lower operating voltage (30-70 V compared to kV’s) and their small size, which allows them to be integrated in compact calorimeters. For space-based instruments — such as the POLAR-2 gamma-ray mission, which aims to use 6400 SiPM channels (see image) — a further advantage is the lack of a glass window, which gives SiPMs the mechanical robustness required during launch. There is, however, a disadvantage with SiPMs: dark current, which flows when the device is not illuminated and is greatly aggravated after exposure to radiation.

In order to strengthen the community and make progress on this technological issue, a dedicated workshop was held at CERN in a hybrid format from 25 to 29 April. Organized by the University of Geneva and funded by the Swiss National Science Foundation, the event attracted around 100 experts from academia and industry. The participants included experts in silicon radiation damage from the University of Hamburg who showed both the complexity of the problem and the need for further studies. Whereas the non-ionizing energy loss concept used to predict radiation damage in silicon is linearly correlated to the degradation of semiconductor devices in a radiation field, this appears to be violated for SiPMs. Instead, dedicated measurements for different types of SiPMs in a variety of radiation fields are required to understand the types of damage and their consequences on the SiPMs’ performance. Several such measurements, performed using both proton and neutron beams, were presented at the April workshop, while plans were made to coordinate such efforts in the future, for example by performing tests of one type of SiPMs at different facilities followed by identical analysis of the irradiated samples. In addition, an online platform to discuss upcoming results was established.

The lack of a glass window gives SiPMs the mechanical robustness required during launch

The damage sustained by radiation manifests itself mainly in the form of an increased dark current. As presented at the workshop, this increase can cause a vicious cycle because the increased current can cause self-heating, which further increases the highly temperature-dependent dark current. These issues are of great importance for future space missions as they influence the power budget, causing the scientific performance to degrade over time. Data from the first SiPM based in-orbit detectors, such as the SIRI mission by the US Naval Research Lab, the Chinese-led GECAM and GRID detectors and the Japanese-Czech GRBAlpha payload, were presented. It is clear that although SiPMs have advantages over PMTs, the radiation, which is highly dependent on the satellite’s orbit, can cause a significant degradation in performance that limits low-earth orbit missions to several years in space. Based on these results, a future Moon mission has decided against the use of SiPMs and reverted to PMTs.

Solutions to radiation damage in SiPMs were also discussed at length. These are mainly in the form of speeding up the annealing of the damage by exposing SiPMs to hotter environments for short periods. Additionally, cooling of the SiPM during data taking will not only decrease the dark current directly, but could also reduce the radiation damage itself, although further research on this topic is required.

Overall, the workshop indicated significant further studies are required to predict the impact of radiation damage on future experiments.

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Around 140 physicists convened for one of the first in-person international particle-physics conferences in the COVID-19 era. The Moriond conference on electroweak interactions and unified theories, which took place from 12 to 19 March on the Alpine slopes of La Thuile in Italy, was a wonderful chance to meet friends and colleagues, to have spontaneous exchanges, to listen to talks and to prolong discussions over dinner.

The LHC experiments presented a suite of impressive results based on increasingly creative and sophisticated analyses, including first observations of rare Standard Model (SM) processes and the most recent insights in the search for new physics. ATLAS reported the first observation of the production of a single top quark in association with a photon, a rare process that is sensitive to the existence of new particles. CMS observed for the first time the electroweak production of a pair of opposite-sign W bosons, which is crucial to investigate the mechanism of electroweak symmetry breaking. The millions of Higgs bosons produced so far at the LHC have enabled detailed measurements and open a new window on rare phenomena, such as the rate of Higgs-boson decays to a charm quark–antiquark pair. CMS presented the world’s most stringent constraint on the coupling between the Higgs boson and the charm quark, improving their previous measurement by more than a factor of five, while ATLAS measurements demonstrated that it is weaker than the coupling between the Higgs boson and the bottom quark. On the theory side, various new signatures for extended Higgs sectors were proposed.

The LHC experiments presented a suite of impressive results based on increasingly creative and sophisticated analyses

Of special interest is the search for heavy resonances decaying to high-mass dijets. CMS reported the observation of a spectacular event with four high transverse-momentum jets, forming an invariant mass of 8 TeV. CMS now has two such events, exceeding the SM prediction with a local significance of 3.9σ, or 1.6σ when taking into account the full range of parameter space searched. Moderate excesses with a global significance of 2–2.5σ were observed in other channels, for example in a search by ATLAS for long-lived, heavy charged particles and in a search by CMS for new resonances that decay into two tau pairs. Data from Run 3 and future High-Luminosity LHC runs will show whether these excesses are statistical fluctuations of the SM expectation or signals of new physics.

Flavour anomalies

The persistent set of tensions between predictions and measurements in semi-leptonic b → s ℓ⁺ℓ^– decays (ℓ = e, μ) were much discussed. LHCb has used various decay modes mediated by strongly suppressed flavour-changing neutral currents to search for deviations from lepton flavour universality (LFU). Other measurements of these transitions, including angular distributions and decay rates (for which the predictions are affected by troublesome hadronic corrections) as well as analyses of charged-current b→ cτ^– ν decays from BaBar, Belle and LHCb also show a consistent pattern of deviations from LFU. While none are individually significant enough to constitute clear evidence of new physics, they represent an intriguing pattern that can be explained by the same new-physics models. Theoretical talks on this subject proposed additional observables (based on baryon decays or leptons at high transverse momenta) to get more information on operators beyond the SM that would contribute to the anomalies. Updates from LHCb on several b → s ℓ⁺ℓ^–-related measurements with the full Run 1 and Run 2 datasets are eagerly awaited, while Belle II also has the potential to provide essential independent checks. The integrated SuperKEKB luminosity has now reached a third of the full Belle dataset, with Belle II presenting several impressive new results. These include measurements of the b → s ℓ⁺ℓ^– decay branching fractions with a precision limited by the sample size and precise measurements of charmed particle lifetimes, including the individual world-best D and Λ⁺_c  lifetimes, proving the excellent tracking and vertexing capabilities of the detector.

The other remarkable deviation from the SM prediction is the anomalous magnetic moment of the muon (g–2)_μ, for which the SM prediction and the recent Fermilab measurement stand 4.2σ apart – or less, depending on whether the hadronic vacuum polarisation contribution to (g–2)_μ is calculated using traditional “dispersive” methods or a recent lattice QCD calculation. The jury is still out on the theory side, but the ongoing analysis of Run 2 and Run 3 data at Fermilab will soon reduce the statistical uncertainty by more than a factor of two. The hottest issues in neutrinos – in particular their masses and mixing – were reviewed. The current leading long-baseline experiments – NOvA in the US and T2K in Japan – have helped to refine our understanding of oscillations, but the neutrino mass hierarchy and CP-violating phase remain to be determined. A great experimental effort is also being devoted to the search for neutrinoless double-beta decay (NDBD) which, if found, would prove that neutrinos are Majorana particles and have far-reaching implications in cosmology and particle physics. The GERDA experiment at Gran Sasso presented its final result, placing a lower limit on the NDBD half-life of 1.8 × 10²⁶ years.

While tensions between solar-neutrino bounds and the reactor antineutrino anomaly are mostly resolved, the gallium anomaly remains

Another very important question is the possible existence of “sterile” neutrinos that do not participate in weak interactions, for which theoretical motivations were presented together with the robust experimental programme. The search for sterile neutrinos is motivated by a series of tensions in short-baseline experiments using neutrinos from accelerators (LSND, Mini-BooNE), nuclear reactors (the “reactor antineutrino anomaly”) and radioactive sources (the “gallium anomaly”), which cannot be accounted for by the standard three-neutrino framework. In particular, MicroBooNE has neither confirmed nor excluded the electron-like low-energy excess observed by MiniBooNE. While tensions between solar-neutrino bounds and the reactor antineutrino anomaly are mostly resolved, the gallium anomaly remains.

Dark matter and cosmology

The status of dark-matter searches both at the LHC and via direct astrophysical searches was comprehensively reviewed. The ongoing run of the 5.9 tonne XENONnT experiment, for example, should elucidate the 3.3σ excess observed by XENON1T in low-energy electron recoil events. The search for axions, which provide a dark-matter candidate as well as a solution to the strong-CP problem, cover different mass ranges depending on the axion coupling strength. The parameter space is wide, and Moriond participants heard how a discovery could happen at any moment thanks to experiments such as ADMX. The status of the Hubble tension was also reviewed.

The many theory talks described various beyond-the-SM proposals – including extra scalars and/or fermions and/or gauge symmetries – aimed at explaining LFU violation, (g–2)_μ, the hierarchy among Yukawa couplings, neutrino masses and dark matter. Overall, the broad spectrum of informative presentations brilliantly covered the present status of open questions in phenomenological high-energy physics and shine a light on the many rich paths that demand further exploration.

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Between February 23-25, the Kavli Institute of Theoretical Physics (KITP) in Santa Barbara, California, hosted the Theory Frontier conference of the US Particle Physics Community Planning Exercise, “Snowmass 2021“. Organised by the Division of Particles and Fields of the American Physical Society (APS DPF), Snowmass aims to identify and document a scientific vision for the future of particle physics in the U.S. and abroad. The event brought together theorists from the entire spectrum of high-energy physics, fostering dialogue and revealing common threads, to sketch a decadal vision for high-energy theory in advance of the main Snowmass Community Summer Study in Seattle on 17-26 July.

It was also one of the first large in-person meetings for the US particle physics community since the start of the COVID-19 pandemic.

The conference began in earnest with Juan Maldacena’s (IAS) vision for formal theory in the coming decade. Highlighting promising directions in quantum field theory and quantum gravity, he surveyed recent developments in “bootstrap” techniques for conformal field theories, amplitudes and cosmology; implications of quantum information for understanding quantum field theories; new dualities in supersymmetric and non-supersymmetric field theories; progress on the black-hole information problem; and constraints on effective field theories from consistent coupling to quantum gravity. Following talks by Eva Silverstein (U. Stanford) on quantum gravity and cosmology and Xi Dong (UC Santa Barbara) on geometry and entanglement, David Gross (KITP) brought the morning to a close by recalling the role of string theory in the quest for unification and emphasising its renewed promise in understanding QCD.

Clay Cordova (Chicago), David Simmons-Duffin (Caltech), Shu Heng Shao (IAS) and Ibrahima Bah (Johns Hopkins) followed with a comprehensive overview of recent progress in quantum field theory. Cordova’s summary of supersymmetric field theory touched on the classification of superconformal field theories, improved understanding of maximally supersymmetric theories in diverse dimensions, and connections between supersymmetric and non-supersymmetric dynamics. Simmons-Duffin made a heroic attempt to convey the essentials of the conformal bootstrap in a 15-minute talk, while Shao surveyed generalised global symmetries and Bah detailed geometric techniques guiding the classification of superconformal field theories.

The first afternoon began with Raman Sundrum’s (Maryland) vision for particle phenomenology, in which he surveyed the pressing questions motivating physics beyond the Standard Model, some promising theoretical mechanisms for answering them, and the experimental opportunities that follow. Tim Tait (UC Irvine) followed with an overview of dark- matter models and motivation, drawing a contrast between the more top-down perspective on dark matter prevalent during the previous Snowmass process in 2013 (also hosted by KITP) and the much broader bottom-up perspective governing today’s thinking. Devin Walker (Dartmouth) and Gilly Elor (Mainz) brought the first day’s physics talks to a close with bosonic dark matter and new ideas in baryogenesis.

The final session of the first day was devoted to issues of equity and inclusion in the high-energy theory community, with  DPF early-career member Julia Gonski (Columbia) making a persuasive case giving a voice to early-career physicists in the years between Snowmass processes.  Connecting from Cambridge, Howard Georgi (Harvard) delivered a compelling speech on the essential value of diversity in physics, recalling Ann Nelson’s legacy and reminding the packed auditorium that “progress will not happen at all unless the good people who think that there is nothing they can do actually wake up and start doing.” This was followed by a panel discussion moderated by Devin Walker (Dartmouth) and featuring Georgi, Bah, Masha Baryakhtar (Washington), and Tien-Tien Yu (Oregon) in dialogue about their experiences.

Developments across all facets of the high-energy theory community are shaping new ways of exploring the universe from the shortest length scales to the very longest

The second and third days of the conference spanned the entire spectrum of activity within high-energy theory, consolidated around quantum information science with talks by Tom Hartman (Cornell), Raphael Bousso (Berkeley), Hank Lamm (Fermilab) and Yoni Kahn (Illinois). Marius Wiesemann (MPI), Felix Kling (DESY) and Ian Moult (Yale) discussed simulations for collider physics, and Michael Wagman (Fermilab), Huey-Wen Lin (Michigan State) and Thomas Blum (Connecticut) emphasised recent progress in lattice gauge theory. Recent developments in precision theory were covered by Bernhard Mistlberger (CTP), Emanuele Mereghetti (LANL) and Dave Soper (Oregon) and the status of scattering-amplitudes applications by Nima Arkani-Hamed (IAS), Mikhail Solon (Caltech) and Henriette Elvang (Michigan). Masha Baryakhtar (Washington), Nicholas Rodd (CERN) and Daniel Green (UC San Diego) reviewed astroparticle and cosmology theory, followed by an overview of effective field theory approaches in cosmology and gravity by Mehrdad Mirbabayi (ICTP) and Walter Goldberger (Yale); Isabel Garcia Garcia (KITP) discussed alternative approaches to effective field theories in gravitation. Recent findings in neutrino theory were covered by Alex Friedland (SLAC), Mu Chun Chen (UC Irvine) and Zahra Tabrizi (Northwestern). Bridging these themes with talks on amplitudes and collider physics, machine learning for particle theory and cosmological implications of dark sector models were talks by Lance Dixon (SLAC), Jesse Thaler (MIT) and Neal Weiner (New York). Connections with the many other “frontiers” in the Snowmass process were underlined by Laura Reina (Florida State), Lian-Tao Wang (Chicago), Pedro Machado (Fermilab), Flip Tanedo (UC Riverside), Steve Gottlieb (Indiana), and Alexey Petrov (Wayne State).

The rich and broad programme of the Snowmass Theory Conference demonstrates the vibrancy of high-energy theory at this interesting juncture for the field, following the discovery of the final missing piece of the Standard Model, the Higgs boson, in 2012. Subsequent developments across all facets of the high-energy theory community are shaping new ways of exploring the universe from the shortest length scales to the very longest. The many thematic threads and opportunities covered in the conference bode well for the final Snowmass discussions with the whole community in Seattle this summer.

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GW observations are a new way to explore the universe’s deepest mysteries. They allow researchers to test gravity in extreme conditions, to get important clues on the mathematical structure and possible extension of general relativity, and to understand the origin of matter and the evolution of the universe. As more GW observations with increased detector sensitivities spur astrophysical and theoretical investigations, the analysis and interpretation of GW data faces new challenges which require close collaboration with all GW researchers. The Oaxaca workshop focused on a topic that is currently receiving a lot of attention: the development of efficient machine-learning (ML) methods and numerical-analysis algorithms for the detection and analysis of GWs. The programme gave participants an overview of new-physics phenomena that could be probed by current or next-generation GW detectors, as well as data-analysis tools that are being developed to search for astrophysical signals in noisy data.

Since their first detections in 2015, the LIGO and Virgo detectors have reached an unprecedented GW sensitivity. They have observed signals from binary black-hole mergers and a handful of signals from binary neutron star and mixed black hole-neutron star systems. In discussing the role that numerical relativity plays in unveiling the GW sky, Pablo Laguna and Deirdre Shoemaker (U. Texas) showed how it can help in understanding the physical signatures of GW events, for example by distinguishing black hole-neutron star binaries from binary black-hole mergers. On the observational side, several talks focused on possible signatures of new physics in future detections. Adam Coogan (U. de Montréal and Mila) and Gianfranco Bertone (U. of Amsterdam, and chair of EuCAPT) discussed dark-matter halos around black holes. Distinctive GW signals  could help to determine whether dark matter is made of a cold, collisionless particle via signatures of intermediate mass-ratio inspirals embedded in dark-matter halos. In addition, primordial black holes could be dark-matter candidates.

Bernard Mueller (U. Monash) and Pablo Cerdá-Durán (U. de Valencia) described GW emission from core-collapse supernovae. The range of current detectors is limited to the Milky Way, where the rate of supernovae is about one per century. However, if and when a galactic supernova happens, its GW signature will be within reach of existing detectors. Lorena Magaña Zertuche (U. of Mississippi) talked about the physics of black-hole ringdown – the process whereby gravitational waves are emitted in the aftermath of a binary black-hole merger – which is crucial for understanding astrophysical black holes and testing general relativity. Finally, Leïla Haegel (U. de Paris) described how the detection of GW dispersion would indicate the breaking of Lorentz symmetry: if a GW propagates according to a modified dispersion relation, its frequency modes will propagate at different speeds, changing  the phase evolution of the signals with respect to general relativity.

Machine learning
Applications of different flavours of ML algorithms to GW astronomy, ranging from the detection of GWs to their characterisation in detector simulations, were the focus of the rest of the workshop.

ML has seen a huge development in recent years and has been increasingly used in many fields of science. In GW astronomy, a variety of supervised, unsupervised, and reinforcement ML algorithms, such as deep learning, neural networks, genetic programming and support vector machines, have been developed. They have been used to successfully deal with noise in the detector, signal processing, data analysis for signal detections and for reducing the non-astrophysical background of GW searches. These algorithms must be able to deal with large data sets and demand  a high accuracy to model  theoretical waveforms and to perform  searches at the limit of instrument sensitivities. The next step for a successful use of ML in GW science will be the integration of ML techniques with more traditional numerical-analysis methods that have been developed for the modelling, real-time detection, and analysis of signals.

The BIRS workshop provided a broad overview of the latest advances in this field, as well as open questions that need to be solved to apply robust ML techniques to a wide range of problems. These include reliable background estimation, modelling gravitational waveforms in regions of the parameter space not covered by full numerical relativity simulations, and determining populations of GW sources and their properties. Although ML for GW astronomy is in its infancy, there is no doubt that it will play an increasingly important role in the detection and characterization of GWs leading to new discoveries.

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The overall FCC programme – comprising an electron-positron Higgs and electroweak factory (FCC-ee) as a first stage followed by a high-energy proton-proton collider (FCC-hh) – combines the two key strategies of high-energy physics. FCC-ee offers a unique set of precision measurements to be confronted with testable predictions and opens the possibility for exploration at the intensity frontier, while FCC-hh would enable further precision and the continuation of open exploration at the energy frontier. The February workshop saw advances in our understanding of the physics potential of FCC-ee, and discussions of the possibilities provided at FCC-hh and at a possible FCC-eh facility.

The overall FCC programme combines the two key strategies of high-energy physics: precision measurements at the intensity frontier and the open exploration at the energy frontier

The proposed R&D efforts for the FCC align with the requests of the 2020 update of the European strategy for particle physics and the recently published accelerator and detector R&D roadmaps established by the Laboratory Directors Group and ECFA. Key activities of the FCC feasibility study, including the development of a regional implementation scenario in collaboration with the CERN host states, were presented.

Over the past several months, a new baseline scenario for a 91 km-circumference layout has been established, balancing the optimisation of the machine performance, physics output and territorial constraints. In addition, work is ongoing to develop a sustainable operational model for FCC taking into account human and financial resources and striving to minimise its environmental impact. Ongoing testing and prototyping work on key FCC-ee technologies will demonstrate the technical feasibility of this machine, while parallel R&D developments on high-field magnets pave the way to FCC-hh.

Physics programme
A central element of the overall FCC physics programme is the precise study of the Higgs sector. FCC-ee would provide model-independent measurements of the Higgs width and its coupling to Standard Model particles, in many cases with sub-percent precision and qualitatively different to the measurements possible at the LHC and HL-LHC. The FCC-hh stage has unique capabilities for measuring the Higgs-boson self-interactions, profiting from previous measurements at FCC-ee. The full FCC programme thus allows the reconstruction of the Higgs potential, which could give unique insights into some of the most fundamental puzzles in modern cosmology, including the breaking of electroweak symmetry and the evolution of the universe in the first picoseconds after the Big Bang.

Presentations and discussions throughout the week showed the impressive breadth of the FCC programme, extending far beyond the Higgs factory alone. The large integrated luminosity to be accumulated by FCC-ee at the Z-pole enables high-precision electroweak measurements and an ambitious flavour-physics programme. While the latter is still in the early phase of development, it is clear that the number of B mesons and tau-lepton pairs produced at FCC-ee significantly surpasses those at Belle II, making FCC-ee the flavour factory of the 2040s. Ongoing studies are also revealing its potential for studying interactions and decays of heavy-flavour hadrons and tau leptons, which may provide access to new phenomena including lepton-flavour universality-violating processes. Similarly, the capabilities of FCC-ee to study beyond-the-Standard Model signatures such as heavy neutral leptons have come into further focus. Interleaved presentations on FCC-ee, FCC-hh and FCC-eh physics also further intensified the connections between the lepton- and hadron-collider communities.

The impressive potential of the full FCC programme is also inspiring theoretical work. This ranges from overarching studies on our understanding of naturalness, to concrete strategies to improve the precision of calculations to match the precision of the experimental programme.

The physics thrusts of the FCC-ee programme inform an evaluation of the run plan, which will be influenced by technical considerations on the accelerator side as well as by physics needs and the overall attractiveness and timeliness of the different energy stages (ranging from the Z pole at 91 GeV to the tt threshold at 365 GeV). In particular, the possibility for a direct measurement of the electron Yukawa coupling by extensive operation at the Higgs pole (125 GeV) raises unrivaled challenges, which will be further explored within the FCC feasibility study. The main challenge here is to reduce the spread in the centre-of-mass energy by a factor of around ten while maintaining the high luminosity, requiring a monochromatisation scheme long theorised but never applied in practice.

Detectors status and plan
Designing detectors to meet the physics requirements of FCC-ee physics calls for a strong R&D programme. Concrete detector concepts for FCC-ee were discussed, helping to establish a coherent set of requirements to fully benefit from the statistics and the broad variety of physics channels available.

The primary experimental challenge at FCC-ee is how to deal with the extremely high instantaneous luminosities. Conditions are the most demanding at the Z pole, with the luminosity surpassing 10³⁶ cm^-2s^-1 and the rate of physics events exceeding 100 kHz. Since collisions are continuous, it is not possible to employ “power pulsing” of the front-end electronics as has been developed for detector concepts at linear colliders. Instead, there is a focus on the development of fast, low-power detector components and electronics, and on efficient and lightweight solutions for powering and cooling. With the enormous data samples expected at FCC-ee, statistical uncertainties will in general be tiny (about a factor of 500 smaller than at LEP). The experimental challenge will be to minimise systematic effects towards the same level.

The mind-boggling integrated luminosities delivered by FCC-ee would allow Standard Model particles – in particular the W, Z and Higgs bosons and the top quark, but also the b and c quarks and the tau lepton – to be studied with unprecedented precision. The expected number of Z bosons produced (5×10¹²) is more than five orders of magnitude larger than the number collected at LEP, and more than three orders of magnitude larger than that envisioned at a linear collider. The high-precision measurements and the observation of rare processes made possible by these large data samples will open opportunities for new-physics discoveries, including the direct observation of very weakly-coupled particles such as heavy-neutral leptons, which are promising candidates to explain the baryon asymmetry of the universe.

With overlapping requirements, designs for FCC-ee can follow the example of detectors proposed for linear colliders.

The detectors that will be located at two (possibly four) FCC-ee interaction points must be designed to fully profit from the extraordinary statistics. Detector concepts under study feature: a 2 T solenoidal magnetic field (limited in strength to avoid blow-up of the low-emittance beams crossing at 30 mrad); a small-pitch, thin-layers vertex detector providing an excellent impact-parameter resolution for lifetime measurements; a highly transparent tracking system providing a superior momentum resolution; a finely segmented calorimeter system with excellent energy resolution for electrons and photons, isolated hadrons and jets; and a muon system. To fully exploit the heavy-flavour possibilities, at least one of the detector systems will need efficient particle-identification capabilities allowing π/K separation over a wide momentum range, for which there are ongoing R&D efforts on compact, light RICH detectors.

With overlapping requirements, designs for FCC-ee can follow the example of detectors proposed for linear colliders. The CLIC-inspired CLD concept – featuring a silicon-pixel vertex detector and a silicon tracker followed by a 3D-imaging, highly granular calorimeter system (a silicon-tungsten ECAL and a scintillator-steel HCAL) surrounded by a superconducting solenoid and muon chambers interleaved with a steel return yoke – is being adapted to the FCC-ee experimental environment. Further engineering effort is needed to make it compatible with the continuous-beam operation at FCC-ee. Detector optimisation studies are being facilitated by the robust existing software framework which has been recently integrated into the FCC study.

The IDEA (International Detector for Electron-positron Accelerator) concept, specifically developed for a circular electron-positron collider, brings in alternative technological solutions. It includes a five-layer vertex detector surrounded by a drift chamber, enclosed in a single-layer silicon “wrapper”. The distinctive element of the He-based drift chamber is its high transparency. Indeed, the material budget of the full tracking system, including the vertex detector and the wrapper, amounts to only about 5% (10%) of a radiation length in the barrel (forward) direction. The drift chamber promises superior particle-identification capabilities via the use of a cluster-counting technique that is currently under test-beam study. In the baseline design, a thin low-mass solenoid is placed inside a monolithic, 2 m-deep, dual-readout fibre calorimeter. An alternative (more expensive) design also features a finely segmented crystal ECAL placed immediately inside the solenoid, providing an excellent energy resolution for electrons and photons.

Recently, work has started on a third FCC-ee detector concept comprising: a silicon vertex detector; a light tracker (drift chamber or full-silicon device); a thin, low-mass solenoid; a highly-granular noble liquid-based ECAL; a scintillator-iron HCAL; and a muon system. The current baseline ECAL design is based on lead/steel absorbers and active liquid-argon, but a more compact option based on tungsten and liquid-krypton is an interesting option. The concept design is currently being implemented inside the FCC software framework.

All detector concepts are under evolution and there is ample room for further innovative concepts and ideas.

Closing remarks
Circular colliders reach higher luminosities than linear machines because the same particle bunches are used over many turns, while detectors can be installed at several interaction points. The FCC-ee programme greatly benefits from the possibility of having four interaction points to allow the collection of more data, systematic robustness and better physics coverage — especially for very rare processes that could offer hints as to where new physics could lie. In addition, the same tunnel can be used for an energy-frontier hadron collider at a later stage.

The FCC feasibility study will be submitted by 2025, informing the next update of the European strategy for particle physics. Such a machine could start operation at CERN within a few years after the full exploitation of the HL-LHC in around 2040. CERN, together with its international partners, therefore has the opportunity to lead the way for a post-LHC research infrastructure that will provide a multi-decade research programme exploring some of the most fundamental questions in physics. The geographical distribution of participants in the 5^th FCC physics workshop testifies to the global attractiveness of the project. In addition, the ongoing physics and engineering efforts, the cooperation with the host states, the support from the European physics community and the global cooperation to tackle the open challenges of this endeavour, are reassuring for the next steps of the FCC feasibility study.

The post Spotlight on FCC physics appeared first on CERN Courier.

Bruno Touschek was born in Vienna on 3 February 1921. His mother came from a well-to-do Jewish family and his father was a major in the Austrian Army. Bruno witnessed the tragic consequences of racial discrimination that prevented him from both completing his high school and university studies in Austria. But he also experienced the hopes of the post-war era and played a role in the post-war reconstruction.  With the help of his friends, he continued his studies in Hamburg, where he worked on the 15 MeV German betatron proposed by Rolf Widerøe and learnt about electron accelerators. After the war he obtained his PhD at the University of Glasgow in 1949 , where he was involved in theoretical studies and in the building of a 300 MeV electron synchrotron. Touschek emerged from the early-post war years as one of the first physicists in Europe endowed with a unique expertise in the theory and functioning of accelerators. His genius was nurtured by close exchanges with Arnold Sommerfeld, Werner Heisenberg, Max Born and Wolfgang Pauli, among others, and flourished in Italy, where he arrived in 1953 called by Edoardo Amaldi, his first biographer and first Secretary-General of CERN.

In 1960 he proposed and built the first electron-positron storage ring, Anello di Accumulazione (AdA), which started operating in Frascati in February 1961. The following year, in order to improve the injection efficiency, a Franco-Italian collaboration was born that brought AdA to Orsay. It was here that the “Touschek effect“, describing the loss and scattering of charged particles in storage rings, was discovered and the proof of collisions in an electron-positron ring was obtained.

AdA paved the way to the electron-positron colliders ADONE in Italy, ACO in France, VEPP-2 in the USSR and SPEAR in the US. Bruno spent the last year of his life at CERN, from where – already quite ill – he was brought to Innsbruck, Austria, where he passed away on 25 May 1978 aged just 57.

Bruno Touschek’s  life and scientific contributions were celebrated at a memorial symposium from 2 to 4 December, held in the three institutions where Touschek has left a lasting legacy: Sapienza University of Rome, INFN Frascati National Laboratories and Accademia Nazionale dei Lincei. Contributions also came from the Irène Joliot-Curie Laboratoire, and sponsorship from the Austrian Embassy in Italy.

In addition to Touschek’s impact on the physics of particle colliders, the three-day symposium addressed the present-day landscape. Carlo Rubbia and Ugo Amaldi gave a comprehensive overview of the past and future of particle colliders, followed by talks about physics at ADONE and LEP, and future machines, such as a muon collider, the proposed Future Circular Collider at CERN and the Circular Electron Positron Collider in China, as well as new developments in accelerator techniques. ADONE’s construction challenges were remembered. Developments in particle physics since the 1960s – including the quark model, dual models and string theory, spontaneous symmetry breaking and statistical physics – were described in testimonies from the  universities of Rome, Frascati, Nordita and Collège de France.

Touschek’s direct influence was captured in talks by his former students, from Rome and the Frascati theory group, which he founded in the mid 1960s. His famous lectures on statistical mechanics, given from 1959 to 1960, were remembered by many speakers. Giorgio Parisi, who graduated with Nicola Cabibbo, recollected the years in Frascati after the observation of a large hadron multiplicity in e⁺ e^– annihilations made by ADONE, and the ideas leading to QCD.

The final day of the symposium, which took place at the Accademia dei Lincei where Touschek had been a foreign member since 1972, turned to future strategies in high-energy physics, including neutrinos and other messengers from the universe. Also prominent were the many benefits brought to society by particle accelerators, reaffirming the intrinsic broader value of fundamental research.

Touschek’s life and scientific accomplishments have been graphically illustrated in the three locations of the symposium, including displays of his famous drawings on academic life in Roma and Frascati. LNF’s visitor center was dedicated to Touschek, in the presence of his son Francis Touschek.

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The Neutron Time Of Flight (n_TOF) facility at CERN, a project proposed by former Director General Carlo Rubbia in the late 1990s, started operations in 2001. Its many achievements during the past two decades, and future plans in neutron science worldwide, were the subject of a one-day hybrid event NSTAPP – Neutrons in Science, Technology and Applications organised by the n_TOF collaboration at CERN on 22 November.

At n_TOF, a 20 GeV/c proton beam from the Proton Synchrotron (PS) strikes an actively cooled pure-lead  neutron spallation target. The generated neutrons are water-moderated to produce a spectrum that covers 11 orders of magnitude in energy from GeV down to meV. At the beginning, n_TOF was equipped with a single experimental station, located 185 m downstream from the spallation target. In 2014, a major upgrade saw the construction and operation of a new experimental test area located 20 m above the production target to allow measurements of very low-mass samples. Last year, during Long Shutdown 2, a new third-generation, nitrogen-cooled spallation target was installed and successfully commissioned to prolong the experiment’s lifetime by ten years. At the same time, a new close-to-target irradiation and experimental station called NEAR was added to perform activation measurements relevant nuclear astrophysics and measurements in collaboration with the R2E (Radiation to Electronics) project that are difficult at other facilities.

Advancing technology

During 20 years of activities, the n_TOF collaboration has carried out more than 100 experiments with considerable impact on nuclear astrophysics, advanced nuclear technologies and applied nuclear sciences, including novel medical applications. Understanding the origin of the chemical elements through the slow-neutron-capture process has been a particular highlight. The high instantaneous neutron flux, which is only available at n_TOF thanks to the short proton pulse delivered by the PS, provided key reaction rates relevant to big-bang nucleosynthesis and stellar evolution (the former attempting to explain the discrepancy between the predicted and existing amount of lithium by investigating ⁷Be creation and destruction, and the latter determining the chemical history of our galaxy).

Basic nuclear data are also essential for the development of nuclear-energy technology. It was this consideration that motivated Rubbia to propose a spallation neutron source at CERN in the first place, prompting a series of accurate neutron cross-section measurements on minor actinides and fission products. Neutron reaction processes on thorium, neptunium, americium, curium, in addition to minor isotopes of uranium and plutonium, have been all measured at n_TOF. These measurements provide the nuclear data necessary for the development of advanced nuclear systems, such as the increase of safety margins in existing nuclear plants as well as to enable generation-IV reactors and accelerator-driven systems, or even enabling new fuel cycles which reduce the amount of long-lived nuclear species.

Basic nuclear data are also essential for the development of nuclear-energy technology

Contributions from external laboratories, such as J-PARC (Japan), the Chinese Spallation Neutron Source (China), SARAF (Israel), GELINA (Belgium), GANIL (France) and Los Alamos (US), highlighted synergies in the measurement of neutron-induced capture, fission and light-charged-particle reactions for nuclear astrophysics, advanced nuclear technologies, and medical applications.  Moreover, technologies developed at CERN have also influenced the creation of two startups, Transmutex and Newcleo. The former focuses on accelerator-driven systems for energy production, for which the first physics validation was executed at the FEAT and TARC experiments at the CERN PS in 1999, while the latter plans to develop critical reactors based on liquid lead.

With the recent technical upgrades and the exciting physics programme in different fields, such as experiments focusing on the breaking of isospin symmetry in neutron-neutron scattering and pursuing its core experimental activities, the n_TOF facility has a bright future ahead.

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The 30^th International Symposium on Lepton Photon Interactions at High Energies, hosted online by the University of Manchester from 10 to 14 January, saw more than 500 physicists from around the world engaged in a broad science programme. The Lepton Photon series dates to the 1960s and takes place every two years. This was the first time the conference was meant to return to the UK in over 50 years, with its original August time slot moved to January due to Covid-19 restrictions. The agenda was stretched to improve accessibility in different time zones. Posters were presented via pre-recorded videos and three prizes were awarded following a public vote.

With 2022 marking the ten-year anniversary of the Higgs-boson discovery, it was appropriate that the conference kicked-off with an experimental Higgs-summary talk. Both the ATLAS and CMS collaborations showcased their latest high-precision measurements of Higgs-boson properties and searches for physics beyond the Standard Model using the Higgs boson as a portal. ATLAS presented a new combination of the Higgs total and differential cross-section measurements in the two-photon and four-lepton channels, while CMS shared the first full Run-2 search for resonant di-Higgs production in several multi-lepton final states.

The LHC experiments continue to demonstrate the power of hadron colliders to test the electroweak sector. Notable new results included the first observation of opposite-charge WWjj production at CMS, the first tri-boson (WWW) observation at ATLAS, and LHCb entering the game of W-boson mass measurements. A highlight of the talks covering QCD topics was a combined fit of the parton distribution function of the proton to differential cross-section measurements from ATLAS and HERA data. A wide range of new-physics searches were presented, including a dark-photon search from ATLAS with the full Run-2 data, and a CMS search for new scalars decaying into final states with Higgs-bosons.

In flavour physics, the pattern of anomalies in rare leptonic and semi-leptonic processes continues to intrigue. Highlights in this area included new tests of lepton universality from LHCb in Λ_b⁰ → Λ_c⁺ℓ^–? decays (ℓ=e, μ, τ) , where the decay involving a τ lepton was observed for the first time, and from Belle in Ω_?⁰→Ω⁻ℓ⁺? decays, where the ratio of the e-μ final-state branching ratios was found to be in agreement with the expectation of unity and where the μ decay had been measured for the first time. Similar studies of rare leptonic decays are now also taking place in the charm sector. The BESIII collaboration tested in one study the e-μ universality in a second decay mode and confirmed its agreement with the Standard Model. Participants also heard about the latest searches for the ultra-rare decay K→π?? from KOTO, searching for the neutral kaon decay mode, and from NA62, which now has a 3.4σ evidence for the charged kaon decay mode.

With the 2021 update on muon g-2 from Fermilab, and with the MEG-II, DeeMe and Mu3e experiments getting ready to search for muon-to-electron transitions, there is much excitement about charged-lepton physics. CP violation in beauty and charm remains a hot topic, with updates from LHCb, Belle and BES-III on D⁰ and B_s oscillations and the CKM angle γ. In all these areas, the theoretical community continues to push the boundaries to make improved predictions. Among other things, theorists presented the latest global fits of Wilson coefficients, and several welcome developments in lattice QCD. 

The highlights from the neutrino sector included the low-energy excess search by MicroBooNE and the observation of the CNO cycle of solar neutrinos by Borexino. The latest results from the long-baseline experiments – T2K and recently NovA– are starting to hint at large CP violating effects in neutrino oscillations.

A series of talks on dark-matter searches spanned collider experiments, direct detection and astrophysical signatures. Some interesting anomalies persist, such as the DAMA annual modulation and the XENON1T low-energy excess. These will be challenged by a suite of next-generation detectors, such as PANDAX-4T, XENONnT, LZ and DarkSide-20k.

The conference also included a rich programme of talks covering astrophysics with an emphasis on gravitational waves and multi-messenger astronomy. Hot-off-the-press was a combined search for spatial correlations between neutrinos and ultra-high energy cosmic rays, using data from ANTARES, IceCube, Auger and TA collaborations, with no sign yet of a connection.

As well as many new results from experiments in operation, the conference included sessions devoted to R&D in accelerators, detectors, software and computing, covering both collider and non-collider experiments. With many new facilities proposed in the medium and long terms, technological challenges, which include power consumption, data rates and radiation tolerance, are immense and demand significant efforts in harnessing promising avenues such as high-temperature superconductors, quantum sensors or specialised computer accelerators. Common to all areas is the need to train and retain highly skilled people to lead these efforts in future.

A firm part of the Lepton Photon plenary programme are discussions around diversity, inclusion and outreach. A lively panel discussion covered many aspects of the former two topics and ended with a key message to the whole community: be an ally and take an active stance in support of minorities. The conference ended with traditional reports from the IUPAP commission on particles and fields and from ICFA, followed by strategy updates from Snowmass and the African Strategy for Fundamental and Applied Physics. While Snowmass is an established process for regular updates of the US strategy for the field based on wide-spread community input both from the US and internationally, the African strategy is the first of its kind and is testament to the continent’s ambition and growing importance in physics research. The next conference will take place in Melbourne in July 2023.

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The first CERN South Asian High Energy Physics Instrumentation (SAHEPI) workshop, held in Kathmandu, Nepal, in 2017, came into place shortly after Pakistan (July 2015) and India (January 2017) became CERN Associate Member States and follows similar regional approaches in Latin America and South-east Asia. Also, within the South Asia region, CERN has signed bilateral international cooperation agreements with Bangladesh (2014), Nepal (2017) and Sri Lanka (2017). The second workshop took place in Colombo, Sri Lanka, in 2019. SAHEPI’s third edition took place virtually on 21 October 2021, hosted by the University of Mauritius in collaboration with CERN. Its aim was to consolidate the dialogue from the first two workshops while strengthening the scientific cooperation between CERN and the South Asia region.

“SAHEPI has been very successful in strengthening the scientific cooperation between CERN and the South Asia region and reinforcing intra-regional links,” said Emmanuel Tsesmelis, head of relations with Associate Members and non-Member States at CERN. “SAHEPI provides the opportunity for countries to enhance their existing contacts and to establish new connections within the region, with the objective of initiating new intra-regional collaborations in particle physics and related technologies, including the promotion of exchange of researchers and students within the region and also with CERN.”

Rising participation

Despite its virtual mode, SAHEPI-3 witnessed the largest participation yet, with 210 registrants. Representatives from Afghanistan, Bangladesh, Bhutan, India, Maldives, Mauritius, Nepal, Pakistan, and Sri Lanka attended, with at least one senior scientist and one student from each country. The workshop brought together physicists and policy makers from the South Asia region and neighbouring countries, together with representatives from CERN. Societal applications of technologies developed for particle physics were key highlights of SAHEPI-3, explained Archana Sharma, senior advisor for relations with international organisations at CERN:

“In this decade, disruptive innovation underpinning the importance of science and technology is making a huge impact towards the United Nations Sustainable Development Goals. CERN plays its role at the forefront, whether it is advances in science and technology or dissemination of that knowledge with an emphasis on inclusive engagement. We see the percolation of this initiative with increasing engagement from the region in CERN programmes.”

Participants reviewed the status and operation of present facilities in particle physics, and the scientific experimental programme, including the LHC and its high-luminosity upgrade at CERN, while John Ellis captivated participants with his talk “Answering the Big Question: From the Higgs boson to the dark side of the Universe”.  Sanjaye Ramgoolam topped off the workshop with a public lecture on “the simple and the complex” in elementary particle physics.

SAHEPI has been very successful in strengthening the scientific cooperation between CERN and the South Asia region and reinforcing intra-regional links

Emmanuel Tsesmelis

Country representatives presented several highlights of the ongoing experimental programmes in collaboration with CERN and other international projects. India’s contributions across the ALICE experiment (such as the development of the photon multiplicity detector), its plans to join the IPPOG outreach group, its activities for the Worldwide LHC computing grid, industrial involvement and contributions to CMS – where it is the seventh-largest country in terms of the number of members – were presented. For Afghanistan, representatives described the participation of the country’s first student in the CERN Summer Student School (2019) and the completion of master’s degrees by two faculty members based on measurements at ATLAS. The country hopes to team up with particle physicists outside Afghanistan to teach online courses at the physics faculty at Kabul University, provide postgraduate scholarships to students and involve more female faculty members at ICTP – the International Centre for Theoretical Physics.

Pakistan shared its contributions to the LHC experiments as well as accelerator projects such as CLIC/CTF3 and Linac4 and its role in the tracker alignment of CMS and Resistive Plate Chambers. Nepal representatives described the development of supercomputers at Kathmandu University (KU) and acknowledged the donation agreement between KU and CERN receiving servers and related hardware to set up a high-performance computing facility. In Sri Lanka, delegates highlighted a rising popularity of the CERN Summer Student Programme among university physics students following honours degrees. The country also mentioned its initiative of an island-wide online teacher training programme to promote particle physics. The representative from Bangladesh reported on the country’s long tradition in theoretical particle physics and plans for developing the experimental particle physics community in partnership with CERN. Maldives and Bhutan continue to be growing members from South Asia at CERN, with Bhutan preparing to host the second South Asia science education programme in a hybrid-mode this year.

Strengthening relations
Chief guest Leela Devi Dookun-Luchoomun, the Vice-Prime Minister and Minister of Education, Tertiary Education, Science and Technology of Mauritius, informed the audience about the formation of a research and development unit in her ministry and gave her strong support to a partnership between CERN and Mauritius. The Vice-Chancellor of the University of Mauritius, Dhanjay Jhurry, expressed his deep appreciation of SAHEPI and indicated his support for future initiatives via a partnership between CERN and the University of Mauritius.

The workshop and the initiative to reinforce particle-physics capacity in the region also form part of broader efforts for CERN to emphasise the role of fundamental research in development, notably to advance the United Nations Sustainable Development Goals agenda. In this regard, discussions took place for a follow-up on the first-of-its-kind professional development programme for high-school teachers of STEM subjects from South Asia, held in New Delhi in 2019, with Bhutan volunteering to host the next event in 2023 pandemic permitting. A poster competition engaged students from South Asia, and three prizes were announced to encourage further participation in big-science projects and towards capacity building in the local regions.

The motivation and enthusiasm of SAHEPI-participants was notable, and the efforts in support of research and education across the region were clear. Proceedings of the workshop will be presented to representatives of the governments from the participating countries to raise awareness at the highest political level of the growth of the community in the region and its value for broader societal development.

Discussions will follow in 2023 at SAHEPI-4, helping CERN continue to engage further with particle physics research and education across South Asia for the benefit of the field as a whole.

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Seven years after the direct detection of gravitational waves (GWs), particle physicists around the world are preparing for the next milestone in GW astronomy: the search for a cosmological stochastic GW background. Current and planned GW observatories roughly cover 12 orders of magnitude from the nanohertz to kilohertz regimes, in which astrophysical models predict sizable GW signals from the merging of compact objects such as black-hole and neutron-star mergers, as observed by the LIGO/Virgo collaborations. It is also expected that the universe contains a randomly distributed GW background, which is yet to be detected. This could be the result of various known and unknown astrophysical signals, which are too weak to be resolved individually, or could be due to hypothetical processes in the very early Universe, such as phase transitions at high temperatures. The most promising region to search for the latter is arguably the ultra-high frequency (UHF) regime encompassing megahertz and gigahertz GWs, which is beyond the reach of current detectors. The detection of such a stochastic GW background could therefore offer a powerful probe of the early universe and of physics beyond the Standard Model.

On 12-15 October a virtual workshop hosted by CERN explored theoretical models and detector concepts targeting the UHF GW regime. Following an initial meeting at ICTP Trieste in 2019 and the publication of a Living Review on UHF GWs, the goal of the workshop was to bring together theorists and experimentalists to discuss feasibility studies and prototypes of existing detector concepts as well as to review more recent proposals.  

The wide range of detector concepts discussed demonstrates the rapid evolution of this field and shows the difficulty in choosing the optimal strategy. Tailoring “light shining through wall” experiments for GWs is one promising approach. In the presence of a static magnetic field, general relativity in conjunction with electrodynamics allows GWs to generate electromagnetic radiation at the same frequency, similar to the conversion of the hypothetical axion into photons. In this case, the bounds placed on “axion to photon” couplings, for example as determined by the CAST and OSQAR experiments at CERN or the ALPS experiments at DESY, can be recast as GW bounds. 

The sheer variety of systems offers a new playground for creative ideas and underlines the cross-disciplinary nature of this field 

Another approach, echoing that of the very first GW searches in the late 1960s, is to detect the mechanical deformation induced by GWs at the base of resonant-bar detectors, which can be implemented in the UHF regime using centimetre-sized bulk acoustic wave devices common in radio-frequency engineering. Resonant microwave cavities are another approach to detect interactions between GWs and electromagnetism, and have been explored in the past, such as by the MAGO collaboration at CERN (2004-2007) or proposed as a modified version of the ADMX experiment at the University of Washington. Further proposals include the precise measurement of optically levitated nanoparticles, transitions in Bose Einstein condensates, mesoscopic quantum systems, cosmological detectors and magnon systems. The sheer variety of systems, the majority of which are much smaller and less costly than long-baseline interferometric detectors, offers a new playground for creative ideas and underlines the cross-disciplinary nature of this field. Working groups set up during the workshop will investigate some of the most promising ideas in more detail within the next months.

Complementing the discussion about detector concepts, theorists presented BSM models that predict violent processes in the early universe, which could source strong GW signals. These arise e.g. in some models of cosmic inflation, at the transition phase between cosmic inflation and the radiation dominated universe, or from spontaneous symmetry breaking processes. Since these processes occur isotropically everywhere in the Universe, the expected signal is a diffuse gravitational wave background. Moreover, some relics of these processes, such as topological defects and primordial black holes, may have survived until the late universe and may still be actively emitting gravitational waves. 

The current sensitivity of all proposed and existing detector concepts is several orders of magnitude away from the expected cosmological GW signals. Given that the first laser-interferometer GW detectors built in the 1970s were eight orders of magnitude below the sensitivity of the currently operating LIGO/Virgo/KAGRA observatories, however, there is every reason to think that the search for UHF GWs is the beginning and not the end of a story.  

The post Exploring the early universe with gravitational waves appeared first on CERN Courier.

The ALICE detector has undergone significant overhauls during Long Shutdown 2 to prepare for the higher luminosities expected during Run 3 and 4 of the LHC, starting this year. Further upgrades of the inner tracking system and the addition of a new forward calorimeter are being planned for the next long shutdown, ahead of Run 4. A series of physics questions will still remain  inaccessible with Run 3 and 4, and major improvements in the detector performance and an ability to collect an even greater integrated luminosity are needed to address them in Run 5 and beyond. The ideas for a heavy-ion programme for Run 5 and 6 are part of the European strategy for particle physics. At the beginning of 2020, the ALICE collaboration formed dedicated working groups to work out the physics case, the physics performance, and a detector concept for a next-generation heavy-ion experiment called “ALICE 3”.

To advance the project further, the ALICE collaboration organised a hybrid workshop on October 18 and 19, attracting more than 300 participants. Invited speakers on theory and experimental topics reviewed relevant physics questions for the 2030s, and members of the ALICE collaboration presented detector plans and physics performance studies for ALICE 3. Two key areas are the understanding how thermal equilibrium is approached in the quark-gluon plasma (QGP) and the precise measurement of its temperature evolution.

Restoring chiral symmetry

Heavy charm and beauty quarks are ideal probes to understand how thermal equilibrium is approached in the QGP, since they are produced early in the collision and are traceable throughout the evolution of the system. Measurements of azimuthal distributions of charm and beauty hadrons, as well as charm-hadron pairs, are particularly sensitive to the interactions between heavy quarks and the QGP.  In heavy-ion collisions, heavy charm quarks are abundantly produced  and can hadronise into rare multi-charm baryons. The production yield of such particles is expected to be strongly enhanced compared to proton-proton collisions because the free propagation of charm quarks in the deconfined plasma allows the combination of quarks from different initial scatterings.

Electromagnetic radiation is a powerful probe of the temperature evolution of the QGP. Since real and virtual photons emitted throughout the evolution of the system are not affected by the strong interaction, differential measurements of dielectron pairs produced from virtual photons allow physicists to determine the temperature evolution in the plasma phase. Given the high temperature and density of the quark-gluon plasma, chiral symmetry is expected to be restored. ALICE 3 will allow us to study the mechanisms of chiral symmetry restoration from the imprint on the dielectron spectrum.

New specialised detectors are being considered to further extend the physics reach

To achieve the performance required for these measurements and the broader proposed ALICE 3 physics programme, a novel detector concept has been envisioned. At its core is a tracker based on silicon pixel sensors, covering a large  pseudo-rapidity range and installed within a new superconducting magnet system. To achieve the ultimate pointing resolution, a retractable high-resolution vertex detector is to be placed in the beampipe. The tracking is complemented by particle identification over the full acceptance, realised with different technologies, including silicon-based time-of-flight sensors. Further specialised detectors are being considered to further extend the physics reach.

ALICE 3 will exploit completely new detector components to significantly extend the detector capabilities and to fully exploit the physics potential of the LHC. The October workshop marked the start of the discussion of ALICE 3 with the community at large and of the review process with the LHC experiments committee.

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The inaugural Sparks! Serendipity Forum attracted 49 leading computer scientists, policymakers and related experts to CERN from 17 to 18 September for a multidisciplinary science-innovation forum. In this first edition, participants discussed a range of ethical and technical issues related to artificial intelligence (AI), which has deep and developing importance for high-energy physics and its societal applications. The structure of the discussions was designed to stimulate interactions between AI specialists, scientists, philosophers, ethicists and other professionals with an interest in the subject, leading to new insights, dialogue and collaboration between participants.

World-leading cognitive psychologist Daniel Kahneman opened the public part of the event by discussing errors in human decision making, and their impact on AI. He explained that human decision making will always have bias, and therefore be “noisy” in his definition, and asked whether AI could be the solution, pointing out that AI algorithms might not be able to cope with the complexity of decisions that humans have to make. Others speculated as to whether AI could ever achieve the reproducibility of human cognition – and if the focus should shift from searching for a “missing link” to considering how AI research is actually conducted by making the process more regulated and transparent.

Introspective AI

Participants discussed both the advantages and challenges associated with designing introspective AI, which is capable of examining its own processes and could be beneficial in making predictions about the future. Participants also questioned, however, whether we should be trying to make AI more self-aware and human-like. Neuroscientist Ed Boyden explored introspection through the lens of neural pathways, and asked whether we can design introspective AI before we understand introspection in brains. Following the introspection theme, philosopher Luisa Damiano addressed the reality versus fiction of “social-embodied” AI – the idea of robots interacting with us in our physical world – arguing that such a possibility would require careful ethical considerations. 

AI is already a powerful, and growing, tool for particle physics

Many participants advocated developing so-called “strong” AI technology that can solve problems it has not come across before, in line with specific and targeted goals. Computer scientist Max Welling explored the potential for AI to exceed human intelligence, and suggested  that AI can potentially be as creative as humans, although further research is required. 

On the subject of ethics, Anja Kaspersen (former director of the UN Office for Disarmament Affairs) asked: who makes the rules? Linking to military, humanitarian and technological affairs, she considered how our experience in dealing with nuclear weapons could help us deal with the development of AI. She said that AI is prone to ethics washing: the process of creating an illusory sense that ethical issues are being appropriately addressed when they are not. Participants agreed that we should seek to avoid polarising the community when considering risks associated with current and future AI, and suggested a more open approach to deal with the challenges faced by AI today and tomorrow. Skype co-founder Jann Tallin identified AI as one of the most worrying existential risks facing society today; the fact that machines do not consider whether their decisions are unethical demands that we consider the constraints of the AI design space within the realm of decision making. 

Fruits of labour

The initial outcomes of the Sparks! Serendipity Forum are being written up as a CERN Yellow Report, and at least one paper will be submitted to the journal Machine Learning Science and Technology. Time will tell what other fruits of the serendipitous interactions at Sparks! will bring. One thing is certain, however, AI is already a powerful, and growing, tool for particle physics. Without it, the LHC experiments’ analyses would have been much more tortuous, as discussed by Jennifer Ngadiuba and Maurizio Pierini (CERN Courier September/October 2021 p31)

Future editions of the Sparks! Seren­dipity Forum will tackle different themes in science and innovation that are relevant to CERN’s research. The 2022 event will be built around future health technologies, including the many accelerator, detector and simulation technologies that are offshoots of high-energy-physics research. 

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The leading role of CERN in fundamental research is complemented by its contribution to applications for the benefit of society. A strong example is the Heavy Ion Therapy Masterclass (HITM) school, which took place from 17 to 21 May 2021. Attracting more than 1000 participants from around the world, many of whom were young students and early-stage researchers, the school demonstrated the enormous potential to train the next generation of experts in this vital application. It was the first event of the European Union project HITRIplus (Heavy Ion Therapy Research Integration), in which CERN is a strategic partner along with other research infrastructures, universities, industry partners, the four European heavy-ion therapy centres and the South East European International Institute for Sustainable Technologies (SEEIIST). As part of a broader “hands-on training” project supported by the CERN and Society Foundation with emphasis on capacity building in Southeast Europe, the event was originally planned to be hosted in Sarajevo but was held online due to the pandemic. 

The school’s scientific programme highlighted the importance of developments in fundamental research for cancer diagnostics and treatment. Focusing on treatment planning, it covered everything needed to deliver a beam to a tumour target, including the biological response of cancerous and healthy tissues. The Next Ion Medical Machine Study (NIMMS) group delivered many presentations from experts and young researchers, starting from basic concepts to discussions of open points and plans for upgrades. Expert-guided practical sessions were based on the matRad open-source professional toolkit, developed by the German cancer research centre DKFZ for training and research. Several elements of the course were inspired by the International Particle Therapy Masterclasses.  

Virtual visits to European heavy-ion therapy centres and research infrastructures were ranked by participants among the most exciting components of the course. There were also plenty of opportunities for participants to interact with experts in dedicated sessions, including a popular session on entrepreneurship by the CERN Knowledge Transfer group. This interactive approach had a big impact on participants, several of which were motivated to pursue careers in related fields and to get actively involved at their home institutes. This future expert workforce will become the backbone for building and operating future heavy-ion therapy and research facilities that are needed to fight cancer worldwide (see Linacs to narrow radiotherapy gap).

Further support is planned at upcoming HITRIplus schools on clinical and medical aspects, as well as HITRIplus internships, to optimally access existing European heavy-ion therapy centres and contribute to relevant research projects. 

The post Training future experts in the fight against cancer appeared first on CERN Courier.

Systematic uncertainties, on the other hand, arise from many sources and may not cause a spread in results when experiments are repeated, but merely shift them away from the true value. Accumulating more data usually does not reduce the magnitude of a systematic effect. As a result, estimating systematic uncertainties typically requires much more effort than for statistical ones, and more personal judgement and skill is involved. Furthermore, statistical uncertainties between different analyses usually are independent; this often is not so for systematics.

The November event saw the largest number of statisticians at any PHYSTAT meeting

In particle-physics analyses, many systematics are related to detector and analysis effects. Examples include trigger efficiency; jet energy scale and resolution; identification of different particle types; and the strength of backgrounds and their distributions. There are also theoretical uncertainties which, as well as affecting predicted values for comparison with measured ones, can also influence the experimental variables extracted from the data. Another systematic comes from the intensity of accelerator beams (the integrated luminosity at the LHC for example), which is likely to be correlated for the various measurements made using the same beams.

At the LHC, it is in analyses with large amounts of data where systematics are likely to be most relevant. For example, a measurement of the mass of the W boson published by the ATLAS collaboration in 2018, based on a sample of 14 million W-boson decays, had a statistical uncertainty of 7 MeV but a systematic uncertainty of 18 MeV.

PHYSTAT-Systematics

Two big issues for systematics are how the magnitudes of the different sources are estimated, and how they are then incorporated in the analysis. The PHYSTAT-Systematics meeting concentrated on the latter, as it was thought that this was more likely to benefit from the presence of statisticians – a powerful feature of the PHYSTAT series, which started at CERN in 2000.

The 20 talks fell into three categories. The first were those devoted to analyses in different particle-physics areas: the LHC experiments; neutrino-oscillation experiments; dark-matter searches; and flavour physics. A large amount of relevant information was discussed, with interesting differences in the separate sub-fields of particle physics. For example, in dark-matter searches, upper limits sometimes are set using Yellin’s Maximum Gap method when the expected background is low, or by using Power Constrained Limits, whereas these tend not to be used in other contexts.

The second group followed themes: theoretical systematics; unfolding; mis-modelling; an appeal for experiments to publish their likelihood functions; and some of the many aspects that arise in using machine learning (where the machine-learning process itself can result in a systematic, and the increased precision of a result should not be at the expense of accuracy).

Finally, there was a series of talks and responses by statisticians. The November event saw the largest number of statisticians at any PHYSTAT meeting, and the efforts that they made to understand our intricate analyses and the statistical procedures that we use were much appreciated. It was valuable to have insights from a different viewpoint on the largely experimental talks. David van Dyk, for instance, emphasised the conceptual and practical differences between simply using the result of a subsidiary experiment’s estimate of a systematic to assess its effect on a result, and using the combined likelihood function for the main and the subsidiary measurements. Also, in response to talks about flavour physics and neutrino-oscillation experiments, attention was drawn to the growing impact in cosmology of non-parametric, likelihood-free (simulation-based likelihoods) and Bayesian methods. Likelihood-free methods came up again in response to a modelling talk based on LHC-experiment analyses, and the role of risk estimation was emphasised by statisticians. Such suggestions for alternative statistical strategies open the door to further discussions about the merits of new ideas in particular contexts.

A novel feature of this remote meeting was that the summary talks were held a week later, to give speakers Nick Wardle and Sara Algeri more time. In her presentation, Algeri, a statistician, called for improved interaction between physicists and statisticians in dealing with these interesting issues.

Overall, the meeting was a good step on the path towards having a systematic approach to systematics. Systematics is an immense topic, and it was clear that one meeting spread over four afternoons was not going to solve all the issues. Ongoing PHYSTAT activities are therefore planned, and the organisers welcome further suggestions.

The post A systematic approach to systematics appeared first on CERN Courier.

The status of theory improvements and phenomenological interpretations, such as those from effective field theory, were also presented. Highlights included the Higgs pair-production process, which is particularly challenging at the LHC due to its low rate. ATLAS and CMS showed greatly improved sensitivity in various final states, thanks to improvements in analysis techniques. Also shown were results on the scattering of weak vector bosons, a process that is strongly related to the Higgs sector, highlighting large improvements from both the larger datasets and the higher collision energy available in Run 2.

Several searches for phenomena beyond the Standard Model – in particular for additional Higgs bosons – were presented. No significant excesses have yet been found.

The historical talk “The LHC timeline: a personal recollection (1980-2012)” was given by Luciano Maiani, former CERN Director-General, and concluding talks were given by Laura Reina (Florida) and Paolo Meridiani (Rome). A further highlight was the theory talk from Nathaniel Craig, who discussed the progress being made in addressing six open questions. Does the Higgs boson have a size? Does it interact with itself? Does it mediate a Yukawa force? Does it fulfill the naturalness strategy? Does it preserve causality? And does it realise electroweak symmetry?

The next Higgs Hunting workshop will be held in Orsay and Paris from 12 to 14 September 2022.

The post Scrutinising the Higgs sector appeared first on CERN Courier.

Cold atoms offer exciting prospects for high-precision measurements based on emerging quantum technologies. Terrestrial cold-atom experiments are already widespread, exploring both fundamental phenomena such as quantum phase transitions and applications such as ultra-precise timekeeping. The final quantum frontier is to deploy such systems in space, where the lack of environmental disturbances enables high levels of precision.

This was the subject of a workshop supported by the CERN Quantum Technology Initiative, which attracted more than 300 participants online from 23 to 24 September. Following a 2019 workshop triggered by the European Space Agency (ESA)’s Voyage 2050 call for ideas for future experiments in space, the main goal of this workshop was to begin drafting a roadmap for cold atoms in space.

The workshop opened with a presentation by Mike Cruise (University of Birmingham) on ESA’s vision for cold atom R&D for space: considerable efforts will be required to achieve the technical readiness level needed for space missions, but they hold great promise for both fundamental science and practical applications. Several of the cold-atom teams that contributed white papers to the Voyage 2050 call also presented their proposals.

Atomic clocks

Next came a session on atomic clocks, including descriptions of their potential for refining the definitions of SI units, such as the second, and distributing this new time-standard worldwide, and potential applications of atomic clocks to geodesy. Next-generation spacebased atomic-clock projects for these and other applications are ongoing in China, the US (Deep Space Atomic Clock) and Europe.

This was followed by a session on Earth observation, featuring the prospects for improved gravimetry using atom interferometry and talks on the programmes of ESA and the European Union. Quantum space gravimetry could contribute to studies of climate change, for example, by measuring the densities of water and ice very accurately and with improved geographical precision.

Cold-atom experiments in space offer great opportunities to probe the foundations of physics

For fundamental physics, prospects for space-borne cold-atom experiments include studies of wavefunction collapse and Bell correlations in quantum mechanics, probes of the equivalence principle by experiments like STEQUEST, and searches for dark matter.

The proposed AEDGE atom interferometer will search for ultralight dark matter and gravitational waves in the deci-Hertz range, where LIGO/Virgo/KAGRA and the future LISA space observatory are relatively insensitive, and will probe models of dark energy. AEDGE gravitational- wave measurements could be sensitive to first-order phase transitions in the early universe, as occur in many extensions of the Standard Model, as well as to cosmic strings, which could be relics of symmetries broken at higher energies than those accessible to colliders.

These examples show that cold-atom experiments in space offer great opportunities to probe the foundations of physics as well as make frontier measurements in astrophysics and cosmology.

Several pathfinder experiments are underway. These include projects for terrestrial atom interferometers on scales from 10 m to 1 km, such as the MAGIS project at Fermilab and the AION project in the UK, which both use strontium, and the MIGA project in France and proposed European infrastructure ELGAR, which both use rubidium. Meanwhile, a future stage of AION could be situated in an access shaft at CERN – a possibility that is currently under study, and which could help pave the way towards AEDGE. Pioneering experiments using Bose-Einstein condensates on research rockets and the International Space Station were also presented.

A strong feature of the workshop was a series of breakout sessions to enable discussions among members of the various participating communities (atomic clocks, Earth observation and fundamental science), as well as a group considering general perspectives, which were summarised in a final session. Reports from the breakout sessions will be integrated into a draft roadmap for the development and deployment of cold atoms in space. This will be set out in a white paper to appear by the end of the year and presented to ESA and other European space and funding agencies.

Space readiness

Achieving space readiness for cold-atom experiments will require significant research and development. Nevertheless, the scale of participation in the workshop and the high level of engagement testifies to the enthusiasm in the cold-atom community and prospective user communities for deploying cold atoms in space. The readiness of the different communities to collaborate in drafting a joint roadmap for the pursuit of common technological and scientific goals was striking.

The post The quantum frontier: cold atoms in space appeared first on CERN Courier.

Former CERN Director-General (DG) Chris Llewellyn Smith swiftly did away with notions that the ISR was built without a physics goal. Viki Weisskopf (DG at the time) was well aware of the quark model, he said, and urged that the ISR be built to discover quarks. “The basic structure of high-energy collisions was discovered at the ISR, but you don’t get credit for it because it is so obvious now,” said Llewellyn Smith. Summarising the ISR physics programme, Ugo Amaldi, former DELPHI spokesperson and a pioneer of accelerators for hadron therapy, listed the observation of charmed-hadron production in hadronic interactions, studies of the Drell–Yan process, and measurements of the proton structure function as ISR highlights. He also recalled the frustration at CERN in late 1974 when the J/ψ meson was discovered at Brookhaven and SLAC, remarking that history would have changed dramatically had the ISR detectors also enabled coverage at high transverse momentum.

A beautiful machine

Amaldi sketched the ISR’s story in three chapters: a brilliant start followed by a somewhat difficult time, then a very active and interesting programme. Former CERN director for accelerators and technology Steve Myers offered a first-hand account, packed with original hand-drawn plots, of the battles faced and the huge amount learned in getting the first hadron collider up and running. “The ISR was a beautiful machine for accelerator physics, but sadly is forgotten in particle physics,” he said. “One of the reasons is that we didn’t have beam diagnostics, on account of the beam being a coasting beam rather than a bunched beam, which made it really hard to control things during physics operation.” Stochastic cooling, a “huge surprise”, was the ISR’s most important legacy, he said, paving the way for the SppS and beyond.

Former LHC project director Lyn Evans took the baton, describing how the confluence of electroweak theory, the SPS as collider and stochastic cooling led to rapid progress. It started with the Initial Cooling Experiment in 1977–1978, then the Antiproton Accumulator. It would take about 20 hours to produce a bunch dense enough for injection into the SppS , recalled Evans, and several other tricks to battle past the “26 GeV transition, where lots of horrible things” happened. At 04:15 on 10 July 1981, with just him and Carlo Rubbia in the control room, first collisions at 270 GeV at the SppS were declared.

Poignantly, Evans ended his presentation “The SPS and LHC machines” there. “The LHC speaks for itself really,” he said. “It is a fantastic machine. The road to it has been a long and very bumpy one. It took 18 years before the approval of the LHC and the discovery of the Higgs. But we got there in the end.”

Discovery machines

The parallel world of hadron-collider experiments was brought to life by Felicitas Pauss, former CERN head of international relations, who recounted her time as a member of the UA1 collaboration at the SppS during the thrilling period of the W and Z discoveries. Jumping to the present day, early-career researchers from the ALICE, ATLAS, CMS and LHCb collaborations brought participants up to date with the progress at the LHC in testing the Standard Model and the rich physics prospects at Run 3 and the HL-LHC.

Few presentations at the symposium did not mention Carlo Rubbia, who instigated the conversion of the SPS into a hadron collider and was the prime mover of the LHC, particularly, noted Evans, during the period when the US Superconducting Super Collider was under construction. His opening talk presented a commanding overview of colliders, their many associated Nobel prizes and their applications in wider society.

During a brief Q&A at the end of his talk, Rubbia reiterated his support for a muon collider operating as a Higgs factory in the LHC tunnel: “The amount of construction is small, the resources are reasonable, and in my view it is the next thing we should do, as quickly as possible, in order to make sure that the Higgs is really what we think it is.”

It seems in hindsight that the LHC was inevitable, but it was anything but

Christopher Llewellyn Smith

In a lively and candid presentation about how the LHC got approved, Llewellyn Smith also addressed the question of the next collider, noting it will require the unanimous support of the global particle-physics community, a “reasonable” budget envelope and public support. “It seems in hindsight that the LHC was inevitable, but it was anything but,” he said. “I think going to the highest energy is the right way forward for CERN, but no government is going to fund a mega project to reduce error bars – we need to define the physics case.”

Following a whirlwind “view from the US”, in which Young-Kee Kim of the University of Chicago described the Tevatron and RHIC programmes and collated congratulatory messages from the US Department of Energy and others, CERN DG Fabiola Gianotti rounded off proceedings with a look at the future of the LHC and beyond. She updated participants on the significant upgrade work taking place for the HL-LHC and on the status of the Future Circular Collider feasibility study, a high-priority recommendation of the 2020 update of the European strategy for particle physics which is due to be completed in 2025. “The extraordinary success of the LHC is the result of the vision, creativity and perseverance of the worldwide high-energy physics community and more than 30 years of hard work,” the DG stated. “Such a success demonstrates the strength of the community and it’s a necessary milestone for future, even more ambitious, projects.”

Videos from the one-off symposium, capturing the rich interactions between the people who made hadron colliders a reality, are available here.

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The inaugural CERN Flavour Anomalies Workshop took place on 20 October as part of this year’s Implications of LHCb Measurements and Future Prospects meeting. More than 500 experimentalists and theorists met in a hybrid format via Zoom and in person. Discussion centered on the longstanding tensions in B-physics measurements, and new project ideas. The workshop was dedicated to the memory of long-time LHCb collaborator Sheldon Stone (Syracuse), who made a plentiful contribution to CERN’s flavour programme.

The central topic of the workshop was the b anomalies: a persistent set of tensions between predictions and measurements in a number of semileptonic b-decays which are not as clear as unexpected peaks in invariant mass distributions. Instead, they manifest themselves as modifications to the branching fractions and angular distributions of certain flavour-changing neutral-current (FCNC) b-decays which have become more significant over the past decade. The latest LHCb measurement of the ratio (R_K) of B⁺ decays to a kaon and a muon or electron pair differs from the Standard Model (SM) by more than 3σ, and the ratio (R_K*) of B⁰ decays to an excited kaon and a muon or electron pair differs by more than 2σ. LHCb has also seen several departures from theory in measurements of angular distributions at the level of roughly 3σ significance. Finally, and coherent with these FCNC effects, BaBar, Belle and LHCb analyses of charged-current b→cτ^–ν̄  decays support lepton-flavour-universality (LFU) violation at a combined significance of roughly 3σ. Though no single measurement is statistically significant, the collective pattern is intriguing. 

Four of the major fitting groups showed a stunning agreement in fits to effective-field-theory parameters

But how robust are the SM predictions for these observables? Efforts include both theory-only and data-driven approaches for distinguishing genuine signs of beyond-the-SM (BSM) effects from hard-to-understand hadronic effects. A further aim is to understand what type of BSM models could produce the observed effects. Of particular interest was the question of how to incorporate information from high-pT searches at the LHC experiments. ATLAS and CMS are ramping up their efforts, and their ongoing B-physics programmes will hopefully soon confirm and complement LHCb’s results. Both experiments reported on work to address the main bottlenecks: the reconstruction of low-momentum leptons, and trigger challenges foreseen as a result of increased luminosities in Run 3. The complementarity of B-physics and direct searches was clear from results such as ATLAS and CMS searches for leptoquarks compatible with the flavour anomalies.

Theory consensus

The workshop saw, for the first time, a joint theory presentation by four of the major b→sℓ⁺ℓ^– fitting groups. They showed a stunning agreement in fits to effective-field-theory parameters which register as nonzero in the presence of BSM physics (see figure). The fits use observables that either probe LFU or help to constrain troublesome hadronic uncertainties. The observables include the now famous R_K, R_K* and R_pK (which studies Λ_b⁰ baryon decays to a proton, a charged kaon and a pair of muons or electrons), whose measurements are dominated by LHCb results; and results on the branching fraction for B_s→μ⁺μ^– from ATLAS, CMS and LHCb. Though the level of agreement diminishes when other observables and measurements are included, dominantly due to the different theoretical assumptions made by the four groups, all agree that substantial tensions with the SM are unavoidable.

New results from LHCb included first measurements of the LFU-sensitive ratios R_K*+ (which concerns B⁺→K*⁺ℓ⁺ℓ^– decays) and R_Ks (which concerns B⁰→K_S⁰ℓ⁺ℓ^– decays), and new measurements of branching fractions and angular observables for the decay B_s→ϕμ⁺μ^–, which is at present hampered by significant theory uncertainties. By contrast, many theoretical predictions for b→cτ^–ν̄ processes are now more precise than measurements, with the promise of further improvements thanks to dedicated lattice-QCD studies. Larger and more diverse datasets will be needed to reduce the experimental uncertainties.

As the end of the year approaches, it may not be too early to collect wishes for 2022. The most prevalent wishes involve new analysis results from ATLAS, CMS and LHCb on these burning topics, and a 2022 workshop to happen in person!

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In this year’s unusual Olympic summer, high-energy physicists pushed back the frontiers of knowledge and broke many records. The first one is surely the number of registrants to the EPS-HEP conference, hosted online from 26 to 30 July by the University of Hamburg and DESY: nearly 2000 participants scrutinised more than 600 talks and 280 posters. After 18 months of the COVID pandemic, the community showed a strong desire to meet and discuss physics with international colleagues. 

200 trillion b-quarks, 40 billion electroweak bosons, 300 million top quarks and 10 million Higgs bosons

The conference offered the opportunity to hear about analyses using the full LHC Run-2 data set, which is the richest hadron-collision data sample ever recorded. The results are breathtaking. As my CERN colleague Michelangelo Mangano explained recently to summer students, “The LHC works and is more powerful than expected, the experiments work and are more precise than expected, and the Standard Model works beautifully and is more reliable than expected.” About 3000 papers have been published by the LHC collaborations in the past decade. They have established the LHC as a truly multi-messenger endeavour, not so much because of the multitude of elementary particles produced – 200 trillion b-quarks, 40 billion electroweak bosons, 300 million top quarks and 10 million Higgs bosons – but because of the diversity of scientifically independent experiments that historically would have required different detectors and facilities, built and operated by different communities. “Data first” should always remain the leitmotif of the natural sciences. 

Paula Alvarez Cartelle (Cambridge) reminded us that the LHC has revealed new states of matter, with LHCb confirming that four or even five quarks can assemble themselves into new long-lived bound states, stabilised by the presence of two charm quarks. For theorists, these new quark-molecules provide valuable input data to tune their lattice simulations and to refine their understanding of the non-perturbative dynamics of strong interactions.

Theoretical tours de force

While Run 1 was a time for inclusive measurements, a multitude of differential measurements were performed during Run 2. Paolo Azzurri (INFN Pisa) reviewed the transverse momentum distribution of the jets produced in association with electroweak gauge bosons. These offer a way to test quantum chromodynamics and electroweak predictions at the highest achievable precision through higher-order computations, resummation and matching to parton showers. The work is fuelled by remarkable theoretical tours de force reported by Jonas Lindert (Sussex) and Lorenzo Tancredi (Oxford), which build on advanced mathematical techniques, including inspiring new mathematical developments in algebraic geometry and finite-field arithmetic. We experienced a historic moment: the LHC definitively became a precision machine, achieving measurements reaching and even surpassing LEP’s precision. This new situation also induced a shift more towards precision measurements, model-independent interpretations and Standard Model (SM) compatibility checks, and away from model-dependent searches for new physics. Effective-field-theory analyses are therefore gaining popularity, explained Veronica Sanz (Valencia and Sussex).

We know for certain that the SM is not the ultimate theory of nature. How and when the first cracks will be revealed is the big question that motivates future collider design studies. The enduring and compelling “B anomalies” reported by LHCb could well be the revolutionary surprise that challenges our current understanding of the structure of matter. The ratios of the decay widths of B mesons, either through charged or neutral currents, b→cℓν and b→sℓ⁺ℓ^–, could finally reveal that the electron, muon and tau lepton differ by more than just their masses.

The statistical significance of the lepton flavour anomalies is growing, reported Franz Muheim (Edinburgh and CERN), creating “cautious” excitement and stimulating the creativity of theorists like Ana Teixeira (Clermont-Ferrand), who builds new physics models with leptoquarks and heavy vectors with different couplings to the three families of leptons, to accommodate the apparent lepton-flavour-universality violations. Belle II should soon bring new additional input to the debate, said Carsten Niebuhr (DESY).

Long-awaited results

The other excitement of the year came from the long-awaited results from the muon g-2 experiment at Fermilab, presented by Alex Keshavarzi (Manchester). The spin precession frequency of a sample of 10 billion muons was measured with a precision of a few hundred parts per million, confirming the deviation from the SM prediction observed nearly 20 years ago by the E821 experiment at Brookhaven. With the current statistics, the deviation now amounts to 4.2σ. With an increase by a factor 20 of the dataset foreseen in the next run, the measurement will soon become systematics limited. Gilberto Colangelo (Bern) also discussed new and improved lattice computations of the hadronic vacuum polarisation, significantly reducing the discrepancy between the theoretical prediction and the experimental measurement. The jury is still out – and the final word might come from the g-2/EDM experiment at J-PARC.

Accelerator-based experiments might not be the place to prove the SM wrong. Astrophysical and cosmological observations have already taught us that SM matter only constitutes around 5% of the stuff that the universe is made of. The traditional idea that the gap in the energy budget of the universe is filled by new TeV-scale particles that stabilise the electroweak scale under radiative corrections is fading away. And a huge range of possible dark-matter scales opens up a rich and reinvigorated experimental programme that can profit from original techniques exploiting electron and nuclear recoils caused by the scattering of dark-matter particles. A front-runner in the new dark-matter landscape is the QCD axion originally introduced to explain why strong interactions do not distinguish matter from antimatter. Babette Döbrich (CERN) discussed the challenges inherent in capturing an axion, and described the many new experiments around the globe designed to overcome them.

Progress could also come directly from theory

Progress could also come directly from theory. Juan Maldacena (IAS Princeton) recalled the remarkable breakthroughs on the black-hole information problem. The Higgs discovery in 2012 established the non-trivial vacuum structure of space–time. We are now on our way to understanding the quantum mechanics of this space–time.

Like at the Olympics, where breaking records requires a lot of work and effort by the athletes, their teams and society, the quest to understand nature relies on the enthusiasm and the determination of physicists and their funding agencies. What we have learnt so far has allowed us to formulate precise and profound questions. We now need to create opportunities to answer them and to move ahead.

One cannot underestimate how quickly the landscape of physics can change, whether the B-anomalies will be confirmed or whether a dark-matter particle will be discovered. Let’s see what will be awaiting us at the next EPS-HEP conference in 2023 in Hamburg – in person this time!

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Africa’s science, innovation, education and research infrastructures have over the years been undervalued and under-resourced. This is particularly true in physics. The African Strategy for Fundamental and Applied Physics (ASFAP) initiative aims to define the education and physics priorities that can be most impactful for Africa. The first ASFAP community town hall was held from 12 to 15 July. The event was virtual, with 147 people participating, including international speakers and members of the ASFAP community. The purpose of the meeting was to initiate a broad and community-driven discussion and action programme, leading to a final strategy document in two to three years’ time.

The first day began with an overview of the ASFAP by Simon Connell (University of Johannesburg) on behalf of the steering committee and addresses by Shamila Nair-Bedouelle (UNESCO assistant director-general for natural sciences), Sarah Mbi Enow Anyang Agbor (African Union commissioner for human resources, science and technology) and Raissa Malu (member of the Democratic Republic of Congo’s Presidential Panel to the African Union). These honoured guests encouraged delegates to establish a culture of gender balance in African physics. Later, in a dedicated forum for women in physics, Iroka Chidinma Joy (chief engineer at the National Space Research and Development Agency) noted that women are drastically underrepresented in scientific fields across the continent, and pointed out a number of cultural, religious and social barriers that prevent women from pursuing higher education. Barriers can come as early as primary education: in most cases, girls are not encouraged to take leading roles in conducting science experiments in classrooms. Improved strategies should include outreach, mentorship, dedicated funding for women, the removal of age limits for women wishing to conduct scientific research or further their education, and awards and recognition for women who excel in scientific fields. 

Community-driven

Representatives of scientific organisations such as the African Physical Society, the Network of African Science Academies and the African Academy of Science all presented messages of support for ASFAP, and delegates from other regions, including Japan, China, India, Europe, the US and Latin America, all presented their regional strategies. The consensus is that strategic planning should be a bottom-up and community-driven process, even if this means it may take two to three years to produce a final report. 

The meeting was updated on the progress of a diverse and well-established range of working groups (WGs) on accelerators, astrophysics and cosmology; computing and the fourth industrial revolution (4IR); energy needs for Africa; instrumentation and detectors; light sources; materials physics; medical physics; nuclear physics; particle physics; and community engagement (CE), which comprises physics education (PE), knowledge transfer, entrepreneurship and stakeholder and governmental-agency engagement. The WGs must also maintain dynamic communications with each other as key topics often impact multiple working groups.

Marie Clémentine Nibamureke (University of Johannesburg) highlighted the importance of the CE WG’s vision “to improve science education and research in African countries in order to position Africa as a co-leader in science research globally”. Convener Jamal Mimouni (Mentouri University) stressed that for ASFAP to establish a successful CE programme, it is crucial to reflect on challenges in teaching and learning physics in Africa – and on why students may be reluctant to choose physics as their study field. Nibamureke explained that the CE WG is seeking to appoint liaison officers between all the ASFAP working groups. Sam Ramaila (University of Johannesburg), representing the PE WG, indicated four main points the group has identified as crucial for the transformation and empowering of physics practices in Africa: strengthening teacher training; developing 21st-century skills and competences; introducing the 4IR in physics teaching and learning; and attracting and retaining students in physics programmes. Ramaila identified problem-based learning, self-directed learning and technology-enhanced learning as new educational strategies that could make a difference in Africa if applied more widely. 

On the subject of youth engagement, Mounia Laassiri (Mohammed V University) led a young-person’s forum to discuss the major issues young African physicists face in their career progression: outreach, professional development and networking will be a central focus for this new forum going forwards, she explained, and the forum aims to encourage young physics researchers to take up leadership roles. So far, there are about 40 members of the young-people’s forum. Laassiri explained that the long-term vision, which goes beyond ASFAP, is to develop into an association of young physicists affiliated to the African Physical Society.

We are now soliciting inputs for the development of the African Strategy for Fundamental and Applied Physics

The ability to generate scientific innovation and technological knowledge, and translate this into new products, is vital for a society’s economic growth and development. The ASFAP is a key step towards unlocking Africa’s potential. We are now soliciting inputs for the development of the African Strategy for Fundamental and Applied Physics. Letters of interest may be submitted by individuals, research groups, professional societies, policymakers, education officials and research institutes on anything they think is an issue, needs to be improved, or is important for fundamental or applied physics education and research in Africa.

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

g-2 anomaly

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

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

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

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

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The conference surveyed topics relating to multi-loop and multi-leg calculations in quantum chromodynamics (QCD) and electroweak processes. In scattering processes, loops are closed particle lines and legs represent external particles. Both present computational challenges. Recent progress on many inclusive processes has been reported at three- or four-loop order, including for deep-inelastic scattering, jets at colliders, the Drell–Yan process, top-quark and Higgs-boson production, and aspects of bottom-quark physics. Much improved descriptions of scaling violations of parton densities, heavy-quark effects at colliders, power corrections, mixed QCD and electroweak corrections, and high-order QED corrections for e⁺e^– colliders have also recently been obtained. These will be important for many processes at the LHC, and pave the way to physics at facilities such as the proposed Future Circular Collider (FCC).

Quantum field theory provides a very elegant way to solve Einsteinian gravity

Weighty considerations

Although merging black holes can have millions of solar masses, the physics describing them remains classical, and quantum gravity happened, if at all, shortly after the Big Bang. Nevertheless, quantum field theory provides an elegant way to solve Einsteinian gravity. At this year’s Loop Summit, perturbative approaches to gravity were discussed that use field-theoretic methods at the level of the 5th and 6th post-Newtonian approximations, where the nth post-Newtonian order corresponds to a classical n-loop calculation between black-hole world lines. These calculations allow predictions of the binding energy and periastron advance of spiralling-in pairs of black holes, and relate them to gravitational-wave effects. In these calculations, the classical loops all link to world lines in classical graviton networks within the framework of an effective-field-theory representation of Einsteinian gravity.

Other talks discussed important progress on advanced analytic computation technologies and new mathematical methods such as computational improvements in massive Dirac-algebra, new ways to calculate loop integrals analytically, new ways to deal consistently with polarised processes, the efficient reduction of highly connected systems of integrals, the solution of gigantic systems of differential equations, and numerical methods based on loop-tree duality. All these methods will decrease the theory errors for many processes due to be measured in the high-luminosity phase of the LHC, and beyond.

Half of the meeting was devoted to developing new ideas in subgroups. In-person discussions are invaluable for highly technical discussions such as these — there is still no substitute for gathering around the blackboard informally and jotting down equations and diagrams. The next Loop Summit in this triennial series will take place in summer 2024.

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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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COVID-19 put the community on a steep learning curve regarding new forms of online communication and collaboration. Before the pandemic, a typical high-energy physics (HEP) researcher was expected to cross the world several times a year for conferences, collaboration meetings and detector shifts, at the cost of thousands of dollars and a sizeable carbon footprint. The online workshop Sustainable HEP — a new initiative this year — attracted more than 300 participants from 45 countries from 28 to 30 June to discuss how the lessons learned in the past two years might help HEP transition to a more sustainable future.

The first day of the workshop focused on how new forms of online interaction could change our professional travel culture. Shaun Hotchkiss (University of Auckland) stressed in a session dedicated to best-practice examples that the purpose of online meetings should not simply be to emulate traditional 20th-century in-person conferences and collaboration meetings. Instead, the community needs to rethink what virtual scientific exchange could look like in the 21st century. This might, for instance, include replacing traditional live presentations by pre-recorded talks that are pre-watched by the audience at their own convenience, leaving more precious conference time for in-depth discussions and interactions among the participants.

Social justice

The second day highlighted social-justice issues, and the potential for greater inclusivity using online formats. Alice Gathoni (British Institute in Eastern Africa) powerfully described the true meaning of online meetings to her: everyone wants to belong. It was only during the first online meetings during the pandemic that she truly felt a real sense of belonging to the global scientific community.

The third day was dedicated to existing sustainability initiatives and new technologies. Mike Seidel (PSI) presented studies on energy-recovery linacs and discussed energy-management concepts for future colliders, including daily “standby modes”. Other options include beam dynamics explicitly designed to maximise the ratio of luminosity to power, more efficient radio-frequency cavities, the use of permanent magnets, and high-temperature superconductor cables and cavities. He concluded his talk by asking thought-provoking questions such as whether the HEP community should engage with its international networks to help establish sustainable energy-supply solutions.

The workshop ended by drafting a closing statement that calls upon the HEP community to align its activities with the Paris Climate Agreement and the goal of limiting global warming to 1.5 degrees. This statement can be signed by members of the HEP community until 20 August.

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The 25th International Conference on Computing in High-Energy and Nuclear Physics (CHEP) gathered more than 1000 participants online from 17 to 21 May. Dubbed “vCHEP”, the event took place virtually after this year’s in-person event in Norfolk, Virginia, had to be cancelled due to the COVID-19 pandemic. Participants tuned in across 20 time zones, from Brisbane to Honolulu, to live talks, recorded sessions, excellent discussions on chat apps (to replace the traditional coffee-break interactions) and special sessions that linked job seekers with recruiters.

Given vCHEP’s virtual nature this year, there was a different focus on the content. Plenary speakers are usually invited, but this time the organisers invited papers of up to 10 pages to be submitted, and chose a plenary programme from the most interesting and innovative. Just 30 had to be selected from more than 200 submissions — twice as many as expected — but the outcome was a diverse programme tackling the huge issues of data rate and event complexity in future experiments in nuclear and high-energy physics (HEP).

Artificial intelligence

So what were the hot topics at vCHEP? One outstanding one was artificial intelligence and machine learning. There were more papers submitted on this theme than any other, showing that the field is continuing to innovate in this domain. 

Interest in using graph neural networks for the problem of charged-particle tracking was very high, with three plenary talks. Using a graph to represent the hits in a tracker as nodes and possible connections between hits as edges is a very natural way to represent the data that we get from experiments. The network can be effectively trained to pick out the edges representing the true tracks and reject those that are just spurious connections. The time needed to get to a good solution has improved dramatically in just a few years, and the scaling of the solution to dense environments, such as at the High-Luminosity LHC (HL-LHC), is very promising for this relatively new technique. 

ATLAS showed off their new fast-simulation framework

On the simulation side, work was presented showcasing new neural-network architectures that use a “bounded information-bottleneck autoencoder” to improve training stability, providing a solution that replicates important features such as how real minimum-ionising particles interact with calorimeters. ATLAS also showed off their new fast-simulation framework, which combines traditional parametric simulation with generative adversarial networks, to provide better agreement with Geant4 than ever before.

New architectures

Machine learning is very well suited to new computing architectures, such as graphics processing units (GPUs), but many other experimental-physics codes are also being rewritten to take advantage of these new architectures. IceCube are simulating photon transport in the Antarctic ice on GPUs, and presented detailed work on their performance analysis that led to recent significant speed-ups. Meanwhile, LHCb will introduce GPUs to their trigger farm for Run 3, and showed how much this will improve the energy consumption per event of the high-level trigger. This will help to meet the physical constraints of power and cooling close to the detector, and is a first step towards bringing HEP’s overall computing energy consumption to the table as an important parameter. 

LHCb will introduce GPUs to their trigger farm for Run 3

Encouraging work on porting event generation to GPUs was also presented — particularly appropriately, given the spiralling costs of higher order generators for HL-LHC physics. Looking at the long-term future of these new code bases, there were investigations of porting calorimeter simulation and liquid-argon time-projection chamber software to different toolkits for heterogeneous programming, a topic that will become even more important as computing centres diversify their offerings.

Keeping up with benchmarking and valuing these heterogeneous resources is an important topic for the Worldwide LHC Computing Grid, and a report from the HEPiX Benchmarking group pointed to the future for evaluating modern CPUs and GPUs for a variety of real-world HEP applications. Staying on the facilities topic, R&D was presented on how to optimise delivering reliable and affordable storage for HEP, based on CephFS and the CERN-developed EOS storage system. This will be critical to providing the massive storage needed in the future. The network between facilities will likely become dynamically configurable in the future, and how best to take advantage of machine learning for traffic prediction is being investigated.

Quantum computing

vCHEP was also the first edition of CHEP with a dedicated parallel session on quantum computing. Meshing very well with CERN’s Quantum Initiative, this showed how seriously investigations of how to use this technology in the future are being taken. Interesting results on using quantum support-vector machines to train networks for signal/background classification for B-meson decays were highlighted.

On a meta note, presentations also explored how to adapt outreach events to a virtual setup, to keep up public engagement during lockdown, and how best to use online software training to equip the future generation of physicists with the advanced software skills they will need.

Was vCHEP a success? So far, the feedback is overwhelmingly positive. It was a showcase for the excellent work going on in the field, and 11 of the best papers will be published in a special edition of Computing and Software for Big Science — another first for CHEP in 2021.

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New results on the production of strangeness in heavy-ion collisions were presented for a variety of collision energies and systems. In an experimental highlight, the ALICE collaboration reported that the number of strange baryons depends more on the final-state multiplicity than the initial-state energy. On the theory side, it was shown that several models can explain the suppression of strange particles at low multiplicities. ALICE also presented new measurements of the charm cross section and fragmentation functions in proton–proton (pp) collisions. When compared to e⁺e^– collisions, these results suggest that the universality of parton-to-hadron fragmentation may be broken. 

Moving on to heavy flavours, the ATLAS collaboration presented results for the suppression of heavy-flavour production compared to pp collisions and the angular anisotropy of heavy mesons in heavy-ion collisions. These measurements are crucial for constraining models of in-medium energy loss. Interestingly, while charm seems to follow the flow of the quark–gluon plasma, beauty does not seem to flow. Better statistics are needed to constrain theoretical models. On the theory side, extremely interesting new calculations using open quantum systems coupled with potential non-relativistic QCD calculations were used to compute both the suppression and anisotropic flow of bottomonium states.

Hints of extrema

Another important goal of the field is to determine experimentally whether a critical point exists in the phase diagram of strongly interacting matter, and, if so, where it is located. The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) presented results on higher order cumulants of net-proton fluctuations over a range of collision energies. Extrema as a function of beam energy are expected to indicate critical behaviour. New data from the Beam Energy Scan II programme at RHIC is expected to provide much-needed statistics to confirm hints of extrema in the data. On the theory side, new lattice QCD calculations of second-order net-baryon cumulants were presented, as well as new expansion schemes to extend the lattice-QCD equation of state to larger net baryon chemical potentials that are not computable directly, because of the fermion-sign problem. Another study included the lattice-QCD equation of state and susceptibilities in a hydrodynamic calculation to allow for a more direct comparison to experimental measurements of net-proton fluctuations. Significant differences between net-proton and net-baryon fluctuations were quantified. 

The study of the quark–gluon plasma’s vorticity via the measurement of the polarisation of hyperons was also a major topic. Theoretical calculations obtain the opposite sign to the data for the angular differential measurement. Attempts to solve this discrepancy presented at SQM 2021 featured shear-dependent terms and a stronger “memory” of the strange-quark spin.

Various new applications of machine learning and artificial intelligence were also discussed, for example, for determining the order of the phase transition and constraining the neutron-star equation of state. 

Overall, there were 41 plenary and 96 parallel talks at SQM 2021, poignantly including presentations in memory of Jean Cleymans, Jean Letessier, Dick Majka and Jack Sandweiss, who all made exceptional impacts on the field.

The next SQM conference will be held from 13 to 18 June 2022 in Busan, South Korea.

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In order to allow worldwide virtual participation, live plenary presentations were limited to two hours daily. Highlights included Harry Westfahl, Jr. (LNLS) on the scientific capabilities of fourth-generation storage-ring light sources; Thomas Glasmacher (FRIB) on the newly commissioned Facility for Rare Isotope Beams at Michigan State University; Norbert Holtkamp (SLAC) on the future of high-power free-electron lasers; Houjun Qian (DESY) on radio-frequency photocathode guns; and Young-Kee Kim (University of Chicago) on future directions in US particle physics. The closing plenary talk was a sobering presentation on climate change and the Brazilian Amazonia region by Paulo Artaxo (University of São Paulo).

The remainder of the talks were pre-recorded with live Q&A sessions, and 400 teleconferencing rooms per day were set up to allow virtual poster sessions. Highlights in topical sessions included “Women in Science: The Inconvenient Truth” by Márcia Barbosa (Universidade Federal do Rio Grande do Sul) and an industrial forum hosted by Raffaella Geometrante (KYMA) on the intersection between government accelerator projects and industry.

IPAC22 is currently planned as an in-person conference in Bangkok, Thailand, from 17 to 22 June next year.

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The LHCP format traditionally has plenary sessions in the mornings and late afternoons, with parallel sessions in the middle of the day. This “shape” was kept for the online meeting, with a shorter day to improve the practicality of joining from distant time zones. This resulted in a dense format with seven-fold parallel sessions, allowing all parts of the LHC programme, both experimental and theoretical, to be explored in detail. The overall vitality of the programme is illustrated by the raw statistics: a grand total of 238 talks and 122 posters were presented.

Last year saw a strong focus on the couplings to the second generation

Nine years on from the discovery of the 125 GeV Higgs boson, measurements have progressed to a new level of precision with the full Run-2 data. Both ATLAS and CMS presented new results on Higgs production, helping constrain the dynamics of the production mechanisms via differential and “simplified template” cross-section measurements. While the couplings of the Higgs to third-generation fermions are now established, last year saw a strong focus on the couplings to the second generation. After first evidence for Higgs decays to muons was reported from CMS and ATLAS results earlier in the year, ATLAS presented a new search with the full Run-2 data for Higgs decays to charm quarks using powerful new charm-tagging techniques. Both CMS and ATLAS showed updated searches for Higgs-pair production, with ATLAS being able to exclude a production rate more than 4.1 times the Standard Model (SM) prediction at 95% confidence. This is a process that should be observable with High-Luminosity LHC statistics, if it is as predicted in the SM. A host of searches were also reported, some using the Higgs as a tool to probe for new physics.

Puzzling hints

The most puzzling hints from the LHC Run 1 seem to strengthen in Run 2. LHCb presented analyses relating to the “flavour anomalies” found most notably in b→sµ⁺µ⁻ decays, updated to the full data statistics, in multiple channels. While no result yet passes a 5σ difference from SM expectations, the significances continue to creep upwards. Searches by ATLAS and CMS for potential new particles or effects at high masses that could indicate an associated new-physics mechanism continue to draw a blank, however. This remains a dilemma to be studied with more precision and data in Run 3. Other results in the flavour sector from LHCb included a new measurement of the lifetime of the Ω_c, four times longer than previous measurements (CERN Courier July/August 2021 p17) and the first observation of a mass difference between the mixed D⁰–D⁰ meson mass eigenstates (CERN Courier July/August 2021 p8).

A wealth of results was presented from heavy-ion collisions. Measurements with heavy quarks were prominent here as well. ALICE reported various studies of the differences in heavy-flavour hadron production in proton–proton and heavy-ion collisions, for example using D mesons. CMS reported the first observation of B_c meson production in heavy-ion collisions, and also first evidence for top-quark pair production in lead–lead collisions. ATLAS used heavy-flavour decays to muons to compare suppression of b- and c-hadron production in lead–lead and proton–proton collisions. Beyond the ions, ALICE also showed intriguing new results demonstrating that the relative rates of different types of c-hadron production differ in proton–proton collisions compared to earlier experiments using e⁺e⁻ and ep collisions at LEP and HERA.

Looking forward, the experiments reported on their preparations for the coming LHC Run 3, including substantial upgrades. While some work has been slowed by the pandemic, recommissioning of the detectors has begun in preparation for physics data taking in spring 2022, with the brighter beams expected from the upgraded CERN accelerator chain. One constant to rely on, however, is that LHCP will continue to showcase the fantastic panoply of physics at the LHC.

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The European Consortium for Astroparticle theory (EuCAPT) held its first annual symposium from 5 to 7 May. Hundreds of theoretical physicists from Europe and beyond met online to discuss the present and future of astroparticle physics and cosmology, in a dense and exciting meeting that featured 29 invited presentations, 42 lightning talks by young researchers, and two community-wide brainstorming sessions.  

Participants discussed a wide array of topics at the interface between particle physics, astrophysics and cosmology, with particular emphasis on the challenges and opportunities for these fields in the next decade. Rather than focusing on experimental activities and the discoveries they might enable, the sessions were structured around thematic areas and explored the interdisciplinary multi-messenger aspects of each. 

Two sessions were dedicated to cosmology, exploring the early and late universe. As stressed by Geraldine Servant (Hamburg), several unresolved puzzles of particle physics – such as the origin of dark matter, the baryon asymmetry, and inflation – are directly linked to the early universe, and new observational probes may soon shed new light on them.

Julien Lesgourgues (Aachen) showed how the very same puzzles are also linked to the late universe, and cautiously elaborated on a series of possible inconsistencies between physical quantities inferred from early- and late-universe probes, for example the Hubble constant. Those inconsistencies represent both a challenge and an extraordinary opportunity for cosmology, as they might “break” the standard Lambda–cold-dark-matter model of cosmology, and allow us to gain insights into the physics of dark matter, dark energy and gravity.

We are witnessing a proliferation of theoretically well-motivated models

New strategies to go beyond the standard models of particle physics and cosmology were also discussed by Marco Cirelli (LPTHE) and Manfred Lindner (Heidelberg), in the framework of dark-matter searches and neutrino physics, respectively. Progress in both fields is currently not limited by a lack of ideas – we are actually witnessing a proliferation of theoretically well-motivated models – but by the difficulty of identifying experimental strategies to conclusively validate or rule them out. Much of the discussion here concerned prospects for detecting new physics with dedicated experiments and multi-messenger observations. 

Gravitational waves have added a new observational probe in astroparticle physics and cosmology. Alessandra Buonanno (Max Planck Institute for Gravitational Physics) illustrated the exciting prospects for this new field of research, whose potential for discovering new physics is attracting enormous interest from particle and astroparticle theorists. The connection between cosmic rays, gamma rays and high-energy neutrinos was explored in the final outlook by Elena Amato (Arcetri Astrophysical Observatory), who highlighted how progress in theory and observations is leading the community to reconsider some long-held beliefs – such as the idea that supernova remnants are the acceleration sites of cosmic rays up to the so-called “knee” – and stimulating new ideas.

In line with EuCAPT’s mission, the local organisers and the consortium’s steering committee organised a series of community-building activities. Participants stressed the importance of supporting diversity and inclusivity, a continuing high priority for EuCAPT, while a second brainstorming session was devoted to the discussion of the EuCAPT white paper currently being written, which should be published by September. Last but not least, Hannah Banks (Cambridge), Francesca Capel (TU Munich) and Charles Dalang (University of Geneva) received prizes for the best lightning talks, and Niko Sarcevic (Newcastle) was awarded an “outstanding contributor” prize for the help and support she provides for the analysis of the EuCAPT census (pictured).

The next symposium will take place in 2022, hopefully in person, at CERN. 

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Although the FCC is a long-term project with a horizon up to the 22nd century, its timescales are rather tight. A post-LHC collider should be operational around the 2040s, ensuring a smooth continuation from the High-Luminosity LHC, so construction would need to begin in the early 2030s. Placement studies to balance geological and territorial constraints with machine requirements and physics performance suggest that the most suitable scenarios are based on a 92 km-circumference tunnel with eight surface sites.

The next steps are subsurface investigations of high-risk areas, surface-site initial-state analysis and verification of in-principle feasibility with local authorities. A “Mining the Future” competition has been launched to solicit ideas for how to best use the nine million cubic metres of molasse that would be excavated from the tunnel.

The present situation in particle physics is reminiscent of the early days of superconductivity

A highlight of the week was the exploration of the physics case of a post-LHC collider. Matthew Reece (Harvard University) identified dark matter, the baryon asymmetry and the origin of primordial density perturbations as key experimental motivations, and the electroweak hierarchy problem, the strong CP problem and the mystery of flavour mixing patterns as key theoretical motivations. The present situation in particle physics is reminiscent of the early days of superconductivity, he noted, when we had a phenomenological description of symmetry breaking in superconductivity, but no microscopic picture. Constraining the shape of the Higgs potential could allow a similar breakthrough for electroweak symmetry breaking. Regarding recent anomalous measurements, such as those of the muon’s magnetic moment, Reece noted that while these measurements could give us the coefficients of one higher dimension operator in an effective-field-theory description of new physics, only colliders can systematically produce and characterise the nature of any new physics. FCC-ee and FCC-hh both have exciting and complementary roles to play.

A key technology for FCC-ee is the development of efficient superconducting radio-frequency (SRF) cavities to compensate for the 100 MW synchrotron radiation power loss in all modes of operation from the Z pole up to the top threshold at 365 GeV. A staged RF system is foreseen as the baseline scenario, with low-impedance single-cell 400 MHz Nb/Cu cavities for Z running replaced by four-cell Nb/Cu cavities for W and Higgs operation, and later augmented by five-cell 800 MHz bulk Nb cavities at the top threshold.

As well as investigations into the use of HIPIMS coating and the fabrication of copper substrates, an innovative slotted waveguide elliptical (SWELL) cavity design was presented that would operate at 600 or 650 MHz. SWELL cavities optimise the surface area, simplify the coating process and avoid the need for welding in critical areas, which could reduce the performance of the cavity. The design profits from previous work on CLIC, and may offer a simplified installation schedule while also finding applications outside of high-energy physics. A prototype will be tested later this year.

Several talks also pointed out synergies with the RF systems needed for the proposed electron–ion collider at Brookhaven and the powerful energy-recovery linac for experiments (PERLE) project at Orsay, and called for stronger collaboration between the projects.

Machine design

Another key aspect of the study regards the machine design. Since the conceptual design report last year, the pre-injector layout for FCC-ee has been simplified, and key FCC-ee concepts have been demonstrated at Japan’s SuperKEKB collider, including a new world-record luminosity of 3.12 × 10³⁴ cm^–2 s^–1 in June with a betatron function of β_γ* = 1 mm. Separate tests squeezed the beam to just β_γ* = 0.8 mm in both rings.

Other studies reported during FCC Week 2021 demonstrated that hosting four experiments is compatible with a new four-fold symmetric ring. This redundancy is thought to be essential for high-precision measurements, and different detector solutions will be invaluable in uncovering hidden systematic biases. The meeting also followed up on the proposal for energy-recovery linacs (ERLs) at FCC-ee, potentially extending the energy reach to 600 GeV if deemed necessary during the previous physics runs. First studies for the use of the FCC-ee booster as a photon source were also presented, potentially leading to applications in medicine and industry, precision QED studies and fundamental-symmetry tests.

Participants also tackled concepts for power reduction and power recycling, to ensure that FCC is sustainable and environmentally friendly. Ideas relating to FCC-ee include making the magnets superconducting rather than normal conducting, improving the klystron efficiency, using ERLs and other energy-storage devices, designing “twin” dipole and quadrupole magnets with a factor-two power saving, and coating SRF cavities with a high-temperature superconductor.

All in all, FCC Week 2021 saw tremendous progress across different areas of the study. The successful completion of the FCC Feasibility Study (2021–2025) will be a crucial milestone for the future of CERN and the field.

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Large gas molecules absorb energy in vibrational and rotational modes of excitation

Freon-based gases are essential to many detectors currently used at CERN, especially for tracking and triggering. Examples run from muon systems, ring-imaging Cherenkov (RICH) detectors and time-projection chambers (TPCs) to wire chambers, resistive-plate chambers (RPCs) and micro-pattern gas detectors (MPGDs). While the primary gas in the mixture is typically a noble gas, adding a “quencher” gas helps achieve a stable gas gain, well separated from the noise of the electronics. Large gas molecules such as freons absorb energy in relevant vibrational and rotational modes of excitation, thereby preventing secondary effects such as photon feedback and field emission. Extensive R&D is needed to reach the stringent performance required of each gas mixture.

CERN has developed several strategies to reduce greenhouse gas (GHG) emissions from particle detectors. As demonstrated by the ALICE experiment’s TPC, upgrading gas-recirculation systems can reduce GHGs by almost 100%. When it is not possible to recirculate all of the gas mixture, gas recuperation is an option – for example, the recuperation of CF₄ by the CMS experiment’s cathode-stripchamber (CSC) muon detector and the LHCb experiment’s RICH-2 detector. A complex gas-recuperation system for the C₂H₂F₄ (R134a) in RPC detectors is also under study, and physicists are exploring the use of commonplace gases. In the future, new silicon photomultipliers could reduce chromatic error and increase photon yield, potentially allowing CF₄ to be replaced with CO₂. Meanwhile, in LHCb’s RICH-1 detector, C₄F₁₀ could possibly be replaced with hydrocarbons like C₄H₁₀ if the flammability risk is addressed.

Eco-gases

Finally, alternative “eco-gases” are the subject of intense R&D. Eco-gases have a low global-warming potential because of their very limited stability in the atmosphere as they react with water or decompose in ultraviolet light. Unfortunately, these conditions are also present in gaseous detectors, potentially leading to detector aging. In addition to their stability, there is also the challenge of adapting current LHC detectors, given that access is difficult and many components cannot be replaced.

Roberto Guida (CERN), Davide Piccolo (Frascati), Rob Veenhof (Uludağ University) and Piet Verwilligen (Bari) convened workshop sessions at the April event. Groups from Turin, Frascati, Rome, CERN and GSI presented results based on the new hydro-fluoro-olefin (HFO) mixture with the addition of neutral gases such as helium and CO₂ as a way of lowering the high working-point voltage. Despite challenges related to the larger signal charge and streamer probability, encouraging results have been obtained in test beams in the presence of LHClike background gamma rays. CMS’s CSC detector is an interesting example where HFO could replace CF₄. In this case, its decomposition could even be a positive factor, however further studies are needed.

We now need to create a compendium of simulations and measurements for “green” gases in a similar way to the concerted effort in the 1990s and 2000s that proved indispensable to the design of the LHC detectors. To this end, the INRS-hosted LXCAT database enables the sharing and evaluation of data to model non-equilibrium low-temperature plasmas. Users can upload data on electron- and ion-scattering cross sections and compare “swarm” parameters. The ETH (Zürich), Aachen and HZDR (Dresden) groups illustrated measurements of transport parameters, opening possibilities of collaboration, while the Bari group sought feedback and collaboration on a proposal to precisely measure transport parameters for green gases in MPGDs using electron and laser beams.

Obtaining funding for this work can be difficult due to a lack of expected technological breakthroughs in low-energy plasma physics

Future challenges will be significant. The volumes of detector systems for the High-Luminosity LHC and the proposed Future Circular Collider, for example, range from 10 to 100 m³, posing a significant environmental threat in the case of leaks. Furthermore, since 2014 an EU “F-gas” regulation has come into force, with the aim of reducing sales to one-fifth by 2030. Given the environmental impact and the uncertain availability and price of freon-based gases, preparing a mitigation plan for future experiments is of fundamental importance to the high-energy-physics community, and the next generation of detectors must be completely designed around eco-mixtures. Although obtaining funding for this work can be difficult, for example due to a lack of expected technological breakthroughs in low-energy plasma physics, the workshop showed that a vibrant cadre of physicists is committed to taking the field forward. The next workshop will take place in 2022.

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Proposals in this direction have appeared intermittently over the past 50 years, including during and after the construction of LEP and the LHC. Now that the existence of GWs has been established by the LIGO and VIRGO detectors, and as new, even larger storage rings are being proposed in Europe and China, this question has renewed relevance. We are on the cusp of the era of GW astronomy — a young and dynamic domain of research with much to discover, in which particle accelerators could conceivably play a major role.

From 2 February to 31 March this year, a topical virtual workshop titled “Storage Rings and Gravitational Waves” (SRGW2021) shone light on this tantalising possibility. Organised within the European Union’s Horizon 2020 ARIES project, the meeting brought together more than 100 accelerator experts, particle physicists and members of the gravitational-physics community to explore several intriguing proposals.

Theoretically subtle

GWs are extremely feebly interacting. The cooling and expanding universe should have become “transparent” to them early in its history, long before the timescales probed through other known phenomena. Detecting cosmological backgrounds of GWs would, therefore, provide us with a picture of the universe at earlier times that we can currently access, prior to photon decoupling and Big-Bang nucleosynthesis. It could also shed light on high-energy phenomena, such as high-temperature phase transitions, inflation and new heavy particles that cannot be directly produced in the laboratory.

In the opening session of the workshop, Jorge Cervantes (ININ Mexico) presented a vivid account of the history of GWs, revealing how subtle they are theoretically. It took about 40 years and a number of conflicting papers to definitively establish their existence. Bangalore S. Sathyaprakash (Penn State and Cardiff) reviewed the main expected sources of GWs: the gravitational collapse of binaries of compact objects such as black holes, neutron stars and white dwarfs; supernovae and other transient phenomena; spinning neutron stars; and stochastic backgrounds with either astrophysical or cosmological origins. The GW frequency range of interest extends from 0.1 nHz to 1 MHz (see figure “Sources and sensitivities”).

The frequency range of interest extends from 0.1 nHz to 1 MHz

Raffaele Flaminio (LAPP Annecy) reviewed the mindboggling precision of VIRGO and LIGO, which can measure motion 10,000 times smaller than the width of an atomic nucleus. Jörg Wenninger (CERN) reported the similarly impressive sensitivity of LEP and the LHC to small effects, such as tides and earthquakes on the other side of the planet. Famously, LEP’s beam-energy resolution was so precise that it detected a diurnal distortion of the 27 km ring at an amplitude of a single millimetre, and the LHC beam-position-monitor system can achieve measurement resolutions on the average circumference approaching the micrometre scale over time intervals of one hour. While impressive, given that these machines are designed with completely different goals in mind, it is still far from the precision achieved by LIGO and VIRGO. However, one can strongly enhance the sensitivity to GWs by exploiting resonant effects and the long distances travelled by the particles over their storage times. In one hour, protons at the LHC travel through the ring about 40 million times. In principle, the precision of modern accelerator optics could allow storage rings and accelerator technologies to cover a portion of the enormous GW frequency range of interest.

Resonant Responses

Since the invention of the synchrotron, storage rings have been afflicted by difficult-to-control resonance effects which degrade beam quality. When a new ring is commissioned, accelerator physicists work diligently to “tune” the machine’s parameters to avoid such effects. But could accelerator physicists turn the tables and seek to enhance these effects and observe resonances caused by the passage of GWs?

In accelerators and storage rings, charged particles are steered and focused in the two directions transverse to their motion by dipole, quadrupole and higher-order magnets — the “betatron motion” of the beam. The beam is also kept bunched in the longitudinal plane as a result of an energy-dependent path length and oscillating electric fields in radio-frequency (RF) cavities — the “synchrotron motion” of the beam. A gravitational wave can resonantly interact with either the transverse betatron motion of a stored beam, at a frequency of several kHz, or with the longitudinal synchrotron motion at a frequency of tens of hertz.

Katsunobu Oide (KEK and CERN) discussed the transverse betatron resonances that a gravitational wave can excite for a beam circulating in a storage ring. Typical betatron frequencies for the LHC are a few kHz, offering potentially sensitivity to GWs with frequencies of a similar order of magnitude. Starting from a standard 30 km ring, Oide-san proposed special beam-optical insertions with a large beta function, which would serve as “GW antennas” to enhance the resonance strength, resulting in 37.5 km-long optics (see figure “Antenna optics”). Among several parameters, the sensitivity to GWs should depend on the size of the ring. Oide derived a special resonance condition of k_GWR±2=Q_x, with R the ring radius, k_GW the GW wave number and Q_x the horizontal betatron tune. 

Suvrat Rao (Hamburg University) presented an analysis of the longitudinal beam response of the LHC. An impinging GW affects the revolution period, in a similar way to the static gravitational gradient effect due to the presence of the Mont Blanc (which alters the revolution time at the level of 10^-16 s) and the diurnal effect of the changing locations of sun and moon (10^-18 s) — the latter effect being about six orders of magnitude smaller than the tidal effect on the ring circumference.

The longitudinal beam response to a GW should be enhanced for perturbations close to the synchrotron frequency, which, for the LHC, would be in the range 10 to 60 Hz. Raffaele D’Agnolo (IPhT) estimated the sensitivity to the gravitational strain, h, at the synchrotron frequency, without any backgrounds, as h~10^-13, and listed three possible paths to further improve the sensitivity by several orders of magnitude. Rao also highlighted that storage-ring GW detection potentially allows for an earth-based GW observatory sensitive to millihertz GWs, which could complement space-based laser interferometers such as LISA, which is planned to be launched in 2034. This would improve the sky-localisation GW-source, which is useful for electromagnetic follow-up studies with astronomical telescopes.

Out of the ordinary

More exotic accelerators were also mooted. A “coasting-beam” experiment might have zero restoring voltage and no synchrotron oscillations. Cold “crystalline” beams of stable ordered 1D, 2D or 3D structures of ions could open up a whole new frequency spectrum, as the phonon spectrum which could be excited by a GW could easily extend up to the MHz range. Witek Krasny (LPNHE) suggested storing beams consisting of “in the LHC: decay times and transition rates could be modified by an incident GW. The stored particles could, for example, include the excited partially stripped heavy ions that are the basis of a “gamma factory”.

Finally on the storage-ring front, Andrey Ivanov (TU Vienna) and co-workers discussed the possibly shrinking circumference of a storage ring, such as the 1.4 km light source SPring-8 in Japan, under the influence of the relic GW background.

Delegates at SRGW2021 also proposed completely different ways of using accelerator technology to detect GWs. Sebastian Ellis (IPhT) explained how an SRF cavity might act as a resonant bar or serve as a Gertsenshtein converter, in both cases converting a graviton into a photon in the presence of a strong background magnetic field and yielding a direct electromagnetic signal — similar to axion searches. Related attempts at GW detection using cavities were pioneered in the 1970s by teams in the Soviet Union and Italy, but RF technology has made big strides in quality factors, cooling and insulation since then, and a new series of experiments appears to be well justified.

Another promising approach for GW detection is atomic-beam interferometry. Instead of light interference, as in LIGO and VIRGO, an incident GW would cause interference between carefully prepared beams of cold atoms. This approach is being pursued by the recently approved AION experiment using ultra-cold-strontium atomic clocks over increasingly large path lengths, including the possible use of an LHC access shaft to house a 100-metre device targeting the 0.01 to 1 Hz range. Meanwhile, a space-based version, AEDGE, could be realised with a pair of satellites in medium earth orbit separated by 4.4×10⁷ m.

Storage rings as sources

Extraordinarily, storage rings could act not only as GW detectors, but also as observable sources of GWs. Pisin Chen (NTU Taiwan) discussed how relativistic charged particles executing circular orbital motion can emit gravitational waves in two channels: “gravitational synchrotron radiation” (GSR) emitted directly by the massive particle, and  “resonant conversion” in which, via the Gertsenshtein effect, electromagnetic synchrotron radiation (EMSR) is converted into GWs.

Gravitons could be emitted via “gravitational beamstrahlung”

John Jowett (GSI, retired from CERN) and Fritz Caspers (also retired from CERN) recalled that GSR from beams at the SPS and other colliders had been discussed at CERN as early  as the 1980s. It was realised that these beams would be among the most powerful terrestrial sources of gravitational radiation although the total radiated power would still be many orders of magnitude lower than from regular synchrotron radiation. The dominant frequency of direct GSR is the revolution frequency, 10 kHz, while the dominant frequency of resonant EMSR-GSR conversion is a factor γ³ higher, around 10 THz at the LHC, conceivably allowing the observation of gravitons. If all particles and bunches of a beam excited the GW coherently, the space-time metric perturbation has been estimated to be as large as h_GSR~10^-18. Gravitons could also be emitted via “gravitational beamstrahlung” during the collision with an opposing beam, perhaps producing the most prominent GW signal at future proposed lepton colliders. At the LHC, argued Caspers, such signals could be detected by a torsion-balance experiment with a very sensitive, resonant mechanical pickup installed close to the beam in one of the arcs. In a phase-lock mode of operation, an effective resolution bandwidth of millihertz or below could be possible, opening the exciting prospect of detecting synthetic sources of GWs.

Towards an accelerator roadmap

The concluding workshop discussion, moderated by John Ellis (King’s College London), focused on the GW-detection proposals considered closest to implementations: resonant betatron oscillations near 10 kHz; changes in the revolution period using “low-energy” coasting ion-beams without a longitudinally focusing RF system; “heterodyne” detection using SRF cavities up to 10 MHz; beam-generated GWs at the LHC; and atomic interferometry. These potential components of a future R&D plan cover significant regions of the enormous GW frequency space.

Apart from an informal meeting at CERN in the 1990s, SRGW2021 was the first workshop to link accelerators and GWs and bring together the implicated scientific communities. Lively discussions in this emerging field attest to the promise of employing accelerators in a completely different way to either detect or generate GWs. The subtleties of the particle dynamics when embedded in an oscillating fabric of space and time, and the inherent sensitivity problems in detecting GWs, pose exceptional challenges. The great interest prompted by SRGW2021, and the tantalising preliminary findings from this workshop, call for more thorough investigations into harnessing future storage rings and accelerator technologies for GW physics.

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

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

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

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

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

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

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

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

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