The second joint ECFA (European Committee for Future Accelerators), NuPECC (Nuclear Physics European Collaboration Committee) and APPEC (AstroParticle Physics European Consortium) symposium, JENAS, was held from 3 to 6 May in Madrid, Spain. Senior and junior members of the astroparticle, nuclear and particle-physics communities presented their challenges and discussed common issues with the goal of achieving a more comprehensive assessment of overlapping research topics. For many of the more than 160 participants, it was their first in-person attendance at a conference after more than two years due to the COVID-19 pandemic.
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.
Cosmology, along with quantum mechanics, is probably among the most misunderstood physics topics for the layperson. Many misconceptions exist, for instance whether the universe had a beginning or not, what the cosmic expansion is, or even what exactly is meant by the term “Big Bang”. Will Kinney’s book An Infinity of Worlds: Cosmic Inflation and the Beginning of the Universe clarifies and corrects these misconceptions in the most accessible way.
Kinney’s main aim is to introduce cosmic inflation – a period of exponential expansion conjectured to have taken place in the very early universe – to a general audience. He starts by discussing the Standard Model of cosmology and how we know that it is correct. This is done most successfully and in a very succinct way. In only 24 pages, the book clarifies all the relevant concepts about what it means for the universe to expand, its thermal history and what a modern cosmologist means by the term Big Bang.
The book continues with an accessible discussion about the motivation for inflation. There are plenty of comments about the current evidence for the theory, its testability and future directions, along with discussions about the multiverse, quantum gravity, the anthropic principle and how all these combine together.
A clear understanding
There are two main points that the author manages to successfully induce the reader to reflect on. The first is the extreme success of the cosmic microwave background (CMB) as a tool to understand cosmology: its black-body spectrum established the Big Bang; its analysis demonstrated the flatness of the universe and its dark contents and motivated inflation; its fluctuations play a large part in our understanding of structure formation in the universe; and, along with the polarisation of the CMB, photons provide a window into the dynamics of inflation. Kinney notes that there are also plenty of features that have not been measured, which are especially important for inflation, such as the B-modes of the CMB and primordial gravitational waves, meaning that CMB-related observations have a long way to go.
The second main point is the importance of a clear understanding of what we know and what we do not know in cosmology. The Big Bang, which is essentially the statement that the universe started as a hot plasma of particles and cooled as it expanded, is a fact. The evidence, which goes well beyond the observation of cosmic expansion, is explained very well in Kinney’s book. Beyond that there are many unknowns. Despite the excellent motivation for and the significant observational successes of inflationary models, they are yet to be experimentally verified. It is probably safe to assume, along with the author, that we will know in the future whether inflation happened or not. Even if we establish that it did and understand its mechanism, it is not clear what we can learn beyond that. Most inflationary models make statements about elements, such as the inflationary multiverse, that in principle cannot be observed.
Steven Weinberg once commented that we did not have to wait to see the dark side of the moon to conclude that it exists. Whether this analogy can be extended successfully to include inflation or string theory is definitely debatable. What is certain, however, is that there will be no shortage of interesting topics and discussions in the years to come about cosmology and fundamental physics in general. Kinney’s book can serve as a useful introduction for the general public, but also for physics students and even physicists working in different fields. As such, this book is a valuable contribution to both science education and dissemination.
The use of deep learning in particle physics has exploded in recent years. Based on INSPIRE HEP’s database, the number of papers in high-energy physics and related fields referring to deep learning and similar topics has grown 10-fold over the last decade. A textbook introducing these concepts to physics students is therefore timely and valuable.
When teaching deep learning to physicists, it can be difficult to strike a balance between theory and practice, physics and programming, and foundations and state-of-the-art. Born out of a lecture series at RWTH Aachen and Hamburg universities, Deep Learning for Physics Research by Martin Erdmann, Jonas Glombitza, Gregor Kasieczka and Uwe Klemradt does an admiral job of striking this balance.
The book contains 21 chapters split across four parts: deep-learning basics, standard deep neural-networks, interpretability and uncertainty quantification, and advanced concepts.
In part one, the authors cover introductory topics including physics data, neural-network building blocks, training and model building. Part two surveys and applies different neural-network structures, including fully connected, convolutional, recurrent and graph neural-networks, while also reviewing multi-task learning. Part three covers introspection, interpretability, uncertainty quantification, and revisits different objective functions for a variety of learning tasks. Finally, part four touches on weakly supervised and unsupervised learning methods, generative models, domain adaptation and anomaly detection. Helping to lower the barrier to entry for physics students to use deep learning in their work, the authors contextualise these methods in real physics-research studies, which is an added benefit compared to similar textbooks.
Deep learning borrows many concepts from physics, which can provide a way of connecting similar ideas in the two fields. A nice example explained in the book is the cross-entropy loss function, which has its origins in the definition of entropy according to Gibbs and Boltzmann. Another example that crops up, although rather late in part three, is the connection between the mean-squared-error loss function and the log-likelihood function for a Gaussian probability distribution, which may be more familiar to physics students accustomed to performing maximum likelihood fits.
Hands-on
Accompanying the textbook is a breadth of free, online Jupyter notebooks (executable Python code in an interactive format), which are available at http://deeplearningphysics.org. These curated notebooks are paired with different chapters and immerse students in hands-on exercises. Both the problem and corresponding solution notebooks are available online,and are accessible to students even without expensive computing hardware as they can be launched on free cloud services such as Google Colab or Binder. In addition, students who have a CERN account can launch the notebooks on CERN’s service for web-based analysis (SWAN) platform.
Advanced exercises include the training and evaluation of a denoising autoencoder for speckle removal in X-ray images and a Wasserstein generative adversarial network for the generation of cosmic-ray-induced air-shower footprints. What is truly exciting about these exercises is their use of physics research examples, many taken from recent publications. Students can see how close their homework exercises and solutions are to cutting-edge research, which can be highly motivating.
In a book spanning less than 300 pages (excluding references), it is impossible to cover everything, especially as new deep-learning methods are developed almost daily. For a more theoretical understanding of the fundamentals of deep learning, readers are advised to consult the classic Deep Learning by Ian Goodfellow, Yoshua Bengio and Aaron Courville, while for more recent deep-learning developments in particle physics they are directed to the article “A Living Review of Machine Learning for Particle Physics” by Matthew Feickert and Benjamin Nachman.
With continued interest in deep learning, coverage of a variety of real physics-research examples and a breadth of accessible, online exercises, Deep Learning in Physics Research is poised to be a standard textbook on the bookshelf of physics students for years to come.
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.
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 δCP 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.
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 physicsis 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.
In the dark abysses of the Mediterranean Sea, what promises to be the world’s largest neutrino telescope, KM3NeT, is rapidly taking shape. Using transparent seawater as the detection medium, its large three-dimensional arrays of photosensors will instrument a volume of more than one cubic kilometre and detect the faint Cherenkov light induced by the passage of charged particles produced in nearby neutrino interactions. The main physics goals of KM3NeT are to detect high-energy cosmic neutrinos and identify their astrophysical origins, as well as to study the fundamental properties of the neutrino itself.
KM3NeT (the Cubic Kilometre Neutrino Telescope) is the successor to the ANTARES neutrino telescope, which operated continuously from 2008 and has recently been decommissioned (see “The ANTARES legacy” panel). KM3NeT comprises two detectors: ARCA (Astroparticle Research with Cosmics in the Abyss), located at a depth of 3500 m offshore from Sicily, and ORCA (Oscillation Research with Cosmics in the Abyss), located at a depth of 2450 m offshore from southern France. ARCA is a sparse detector of about 1 km3 that is optimised for the detection of TeV–PeV neutrinos, while ORCA is a 7 Mt-dense detector optimised for sub-TeV neutrinos. The KM3NeT collaboration comprises more than 250 scientists from 16 countries.
The key technology is the digital optical module (DOM) – a pressure-resistant glass sphere hosting 31 three-inch photomutiplier tubes, various calibration devices and the readout electronics (see “Modular” image). A total of 18 DOMs are hosted on a single detection line, and the lines are anchored to the seafloor and held taut by a submerged buoy. The ORCA detector will comprise around 100 lines and the ARCA detector will have twice as many. The bases of the lines are connected via cables on the seafloor to junction boxes, from which electro-opticalcables many tens of kilometres long bring the data to shore along optical fibres. Information on every single photon is transmitted to the shore stations, where trigger algorithms are applied to select interesting events for offline analysis.
From the light pattern recorded by the DOMs, the energy and the direction of a neutrino can be estimated. Furthermore, the neutrino flavour can also be distinguished; muon neutrino charged–current (CC) interactions produce an extended track-like signature (see “Subsea shower” image) whereas electron– and tau–neutrino CC interactions, as well as neutral-current interactions, produce more compact shower-like events. By selecting up-going neutrinos, i.e. those that have travelled from the other side of Earth, the large background from down-going atmospheric muons can be rejected and a clean sample of neutrinos obtained.
The first KM3NeT detection line was connected in 2016 and currently a total of 32 lines are operating at the two sites. The first science results with these partial detectors have already been obtained.
Fundamental neutrino properties
Sixty-six years after their discovery, neutrinos remain the most mysterious of the fermions. As they whiz through the universe, barely interacting with any other particles, they have the unique ability to oscillate between their three different types or flavours (electron, muon and tau). The observation of neutrino oscillations in the late 1990s implies that neutrinos have a non-zero mass, contrary to the Standard Model expectation. Understanding the origin and order of the neutrino masses could therefore unlock a path to new physics. Numerous neutrino experiments around the world are closing in on the neutrino’s properties, using both artificial (accelerator and reactor) and natural (atmospheric and extraterrestrial) neutrino sources.
The KM3NeT/ORCA array is optimised for the detection of atmospheric neutrinos, produced when cosmic rays strike atomic nuclei at an altitude of around 15 km. Such interactions produce a cascade of particles on Earth’s surface, mostly pions and kaons, which decay to neutrinos capable of traversing the entire planet. About two thirds of these are muon neutrinos and antineutrinos, and the remainder are electron neutrinos and antineutrinos.
Measuring the directions and energies of the detected atmospheric neutrinos allows the oscillatory behaviour of neutrinos to be studied, and thus elements of the leptonic “PMNS” mixing matrix to be determined. The measured direction is used as a proxy for the distance the atmospheric neutrino has travelled through Earth between its points of production and detection. First preliminary results with six ORCA lines and one year of data clearly show the expected disappearance of muon neutrinos with increasing baseline/energy. The corresponding constraints on θ23 (the mixing angle between the m2 and m3 states) and Δm232 (the mass difference of the squared masses) already start to be competitive with multi-year results from the current long-baseline accelerator experiments (see “Physics debut” figure).
Building a telescope anchored deep at the bottom of the sea requires skill, patience and expertise. KM3NeT would not be on its way without the invaluable experience gained from its older sibling, the ANTARES telescope. ANTARES operated continuously for more than 15 years, and pioneered solutions to construct and operate a neutrino detector in the challenging environment of the deep sea. Despite ANTARES containing only 12 detector lines compared to 86 in IceCube, its superior angular resolution (due to the intrinsic water properties) and its Northern Hemisphere location provided competitive results and valuable insights and constraints in various domains.
Following IceCube’s discovery of a diffuse flux of cosmic neutrinos, the ANTARES all-flavour neutrino data sample revealed a mild (1.8σ) excess of high-energy events consistent with the neutrino signal detected by IceCube. ANTARES also contributed strongly to the multi-messenger endeavour, participating in the search for a neutrino counterpart to major alerts from the LIGO/Virgo gravitational-wave interferometers, IceCube, ground-based imaging air Cherenkov telescopes, as well as X- and gamma-ray satellites. For instance, the TXS0506+056 blazar is the second most significant point source, with a local significance of 2.8σ, strengthening its case as the first high-energy neutrino source. ANTARES also distributed its own neutrino alerts with an unprecedented low latency for a neutrino telescope.
Its energy threshold of a few tens of GeV allowed the study of atmospheric muon neutrino disappearance due to neutrino oscillations and to constrain the “3+1” neutrino model. In this domain, results consistent with world best-fit values were obtained, as well as competitive limits on non-standard interactions. The data were also used to search for dark-matter particles that would have accumulated in astrophysical bodies like the Sun or the galactic centre before annihilating or decaying into neutrinos. Since no excesses were found, competitive limits were set that reduce the parameter space to be explored by direct, indirect (including KM3NeT) and collider dark-matter experiments.
Recently superseded in sensitivity by KM3NeT, ANTARES was finally decommissioned in February 2022.
A longer-term physics goal of KM3NeT is to determine the neutrino mass ordering, i.e. whether the third neutrino mass eigenstate is heavier or lighter than the first two. This is important to help constrain the plethora of theoretical models proposed to explain the neutrino masses. Due to the large distances travelled by atmospheric neutrinos as they pass through Earth’s mantle and core, subtle matter effects come into play and distort the expected oscillation pattern in the zenith angle/energy plane. By comparing the observed distortions to those expected for either “normal” or “inverted” mass ordering, and thanks to the large neutrino sample collected, the neutrino mass ordering can be determined.
A 115-line configuration of ORCA operating for three years is expected to provide a three-sigma sensitivity for most θ23 values. KM3NeT could therefore be the first detector to unambiguously determine the neutrino mass ordering, on a time scale in advance of the planned long-baseline accelerator experiments. New-physics scenarios (for example, non-standard interactions, neutrino decays and sterile neutrinos) that modify the oscillation patterns recorded in both ORCA and ARCA have already been explored. While no significant deviations from the Standard Model have been observed, the enhanced sensitivity as the detectors grow will push the existing limits and probe uncharted territories.
Neutrino astronomy
At the beginning of the 1960s, it was realised that the neutrino could play a special role in the study of the universe at large. Weakly interacting with matter and electrically neutral, it enables exploration at greater distances and higher energies than is possible with conventional electromagnetic probes. In addition, neutrinos are the unambiguous smoking gun of hadronic acceleration processes occurring at their source.
Since the observation of a significant flux of cosmic high-energy neutrinos in the TeV–PeV range by the IceCube Neutrino Observatory at the South Pole in 2013, the focus of neutrino astronomers has been to identify the astrophysical origins of these neutrinos. Amongst the diverse possible sources, a multi-messenger approach has identified the first: the flaring blazar TXS0506+056. While other source candidates have appeared, such as tidal disruption events and radio-bright blazars, the currently identified source population(s) cannot fully explain the detected flux. Having a neutrino telescope with a sensitivity similar to that of IceCube and with a complementary field of view allows the full neutrino sky to be continuously monitored. KM3NeT’s location in the Northern Hemisphere provides an optimal view of the galactic plane and makes it the ideal instrument to detect, characterise and resolve sources that may emit galactic neutrinos.
Soon, KM3NeT will start sending alerts to its multi- messenger partners – including conventional electromagnetic telescopes but also other neutrino telescopes such as IceCube and Baikal/GVD – when a neutrino candidate with a high probability of astrophysical origin is detected. This is right on time for the fourth observing run of the LIGO, Virgo and KAGRA gravitational-wave interferometers. While so far no neutrinos have been observed from binary compact systems detected through gravitational waves, a joint detection would reveal unique information on the high-energy processes in the environment of the mergers. Furthermore, the exceptional pointing resolution of KM3NeT would significantly reduce the region of interest where electromagnetic partners should search for a counterpart. The ARCA detector, for example, will benefit from the low optical scattering of deep seawater to reconstruct the direction of muon-neutrino events to less than 0.1 degrees at 100 TeV and around 1 degree for the electron/tau neutrino flavours.
Last but not least, KM3NeT is already waiting for the next close-by core-collapse supernova. Such astrophysical events are rare: the first and only one ever detected in neutrinos, SN1987a, occurred 35 years ago. The KM3NeT DOMs are continuously monitoring for a short-duration increase in counting rates on many DOMs simultaneously – the signature of a flash of MeV supernova neutrinos passing through the detectors – and the detector is networked with other neutrino telescopes via the SuperNova Early Warning System (SNEWS). If a galactic supernova would happen today, the number of neutrinos detected by SNEWS would be four orders of magnitude more than for SN1987a!
Whether the cosmic-neutrino sources are point-like, extended, transient or variable, the KM3NeT collaboration has developed reconstruction techniques, event selections and statistical frameworks to identify them and determine their characteristics. Disentangling the galactic from the extragalactic components, the steady from the transient and the electromagnetically bright from the obscure are on KM3NeT’s to-do list for the coming decade.
Marine science
KM3NeT is important not only for particle physics, but is also a powerful tool for marine sciences. The acquisition of long-term oceanographic data helps researchers understand and eventually mitigate the harmful effects of global processes, such as climate change and anthropogenic impact, as well as study episodic events such as earthquakes, tsunamis, biodiversity changes and pollution – all of which are difficult to study with short-term conventional marine expeditions. To this end, the seafloor infrastructures of first the ANTARES and now the KM3NeT sites are unique cabled marine observatories. They are open to all scientific communities, and as such are important nodes of the European Multidisciplinary Seafloor and water-column Observatory, EMSO.
Sixty-six years after their discovery, neutrinos remain the most mysterious of the fermions
Furthermore, the KM3NeT optical sensors and the acoustics sensors (used for the positioning of the DOMs) themselves provide unique information on deep-sea bioluminescence and bioacoustics. The ANTARES collaboration has several publications studying deep-sea bioluminescence and acoustic detection of cetaceans, and recently KM3NeT invited citizen scientists to analyse its optical and acoustic data via the Zooniverse platform in the context of the EU project REINFORCE.
The KM3NeT detectors will continue to grow in size and sensitivity as additional new lines are installed over the next five years. With three major neutrino telescope facilities now online – Baikal/GVD, IceCube and KM3NeT – neutrino astronomy is truly entering its golden era.
During the past several decades of intense experimental and theoretical research, particle physicists have come to rely on the Standard Model to describe phenomena at the smallest scales and highest energies. This highly predictive, relativistic spontaneously-broken gauge theory has pointed the way to a sequence of discoveries, including that of the W and Z bosons, the gluons, and the charm and top quarks. At each point, it gave us an approximate mass scale or energy range to explore, which told us what kind of facilities we needed to build to observe predicted phenomena. Finally, in 2012, its most remarkable prediction – the existence of a Higgs particle associated with an apparently fundamental scalar field responsible for electroweak symmetry breaking – was confirmed. There are, however, big questions in particle physics to which we don’t know the answers.
Every seven to 10 years since 1982, high-energy physicists in the US have undertaken a community planning exercise to identify the most important questions for the following two decades and the facilities, infrastructure and R&D needed to pursue them. For many years these efforts, which are sponsored by the Division of Particles and Fields (DPF) of the American Physical Society (APS) and include scientists from other countries and related fields, concluded with a summer workshop in Snowmass, Colorado. The planning exercise focuses on scientific issues, whereas establishing project priorities is the task of a Particle Physics Project Prioritization Panel “P5”, charged by the US Department of Energy (DOE) and the National Science Foundation (NSF).
The latest study, “Snowmass 2021” (CERN Courier January/February 2022 p43) was meant to conclude in July 2021, but had to be delayed due to the COVID-19 pandemic. Despite the challenges, our community accomplished an amazing amount of work. The final discussions and synthesis of all the white papers, seminars, workshops and other materials took place at the University of Washington in Seattle from 17–26 July 2022. At the end of themeeting, Hitoshi Murayama (UC Berkeley and the University of Tokyo) was named chairperson of the new P5 subpanel, which will take input from Snowmass 2021.
Snowmass in context
The last US community planning exercise was held in 2013. The subsequent P5 report synthesised the questions identified into five physics drivers: use the Higgs boson as a tool for discovery; pursue the physics associated with neutrino mass; identify the new physics of dark matter; understand cosmic acceleration; and explore the unknown. It also made 29 project-oriented recommendations. The two projects assigned the highest priority were participation in the High-Luminosity LHC and the ATLAS and CMS experiments; and the construction of the LBNF/DUNE long-baseline neutrino experiment, which will detect neutrinos produced at Fermilab interacting in massive underground detectors 1300 km away in the Homestake mine in South Dakota.
Nearly a decade since the last Snowmass/P5 exercise, some elements of the recommended experimental programme have taken data and have succeeded in pushing the boundaries of our knowledge. But despite some hints, they have not yet produced a result that points us in a specific direction. Snowmass 2021 reconfirmed the relevance of the physics drivers, and added a proposal for a sixth: flavour physics as a tool for discovery. Specifically, we don’t understand why three generations of matter particles exist nor the origin of the mass patterns that they exhibit. We do not know why the quark and the lepton mixing matrices are so different, or whether CP violation exists in the neutrino sector and how it relates to the observed matter–antimatter asymmetry of the universe. There are, currently, several tantalising hints of new particles and interactions that could explain various anomalies in the weak decays of B mesons and the anomalous magnetic moment of the muon. Depending on what the near-future brings, dedicated next-generation flavour experiments are likely to be required and could play a key role in the quest for physics beyond the Standard Model.
Snowmass 2021 was organised into 10 working groups or “frontiers”: accelerator, cosmic, community engagement, computational, energy, instrumentation, neutrinos, rare processes and precision measurements, theory, and underground facilities and infrastructure. Each frontier divided its work into several topical groups, taking into account input from the 2020 update of the European strategy for particle physics and other international studies. More than 500 new white papers were produced. An early-career organisation assisted young physicists in contributing to the Snowmass process and international participation was encouraged, with leaders of international institutes and laboratories including Fabiola Gianotti (CERN), Masanori Yamauchi (KEK) and Yifang Wang (IHEP) giving presentations during special plenary sessions at the Seattle workshop. In describing the US programme, Fermilab director Lia Merminga emphasised the importance of international collaboration, citing the close relationship between the US and CERN.
There was broad agreement that a successful future programme should include a healthy breadth and balance of physics topics, experiment sizes and timescales, supported via a dedicated, robust and ongoing funding process. Completion of existing experiments and execution of DUNE and the HL-LHC programmes are critical for addressing the science drivers in the near-term. Strong and continued support for formal theory, phenomenology and computational theory is needed, as are stronger, targeted efforts connecting theory to experiment. Both R&D directed to specific future projects and generic research needs to be supported in critical enabling technologies such as accelerators, instrumentation/detectors and computation, and in new ones such as quantum science and machine learning. Finally, a cohesive, strategic approach to promoting diversity, equity and inclusion, and to improving outreach and engagement, is required.
A panoply of ideas were discussed at Snowmass 2021. Here, in the context of the 10 frontiers, we list some of the larger projects and programmes that are proposed to be carriedout, or at least started, in the next two decades, and some important conclusions concerning enabling technologies and infrastructure, with the disclaimer that these may change as the final Snowmass frontier reports are written.
The cosmic frontier
The cosmic frontier is focused on understanding how fundamental physics shapes the behaviour of the universe, in particular concerning the nature of dark matter (DM) and dark energy. The space of DM models encompasses a dizzying array of possibilities representing many orders of magnitude in mass and couplings, making the DM programme one of the most interdisciplinary investigations in high-energy and particle physics. The cosmic frontier DM programme will “delve deep, search wide” by employing a range of direct searches for WIMPs interacting with targets on Earth or produced at accelerators, indirect searches for the products of DM annihilation and probes based on analyses of cosmic structure. A complementary thrust is building the next generation of cosmological probes. The next big project in this arena is CMB-S4, a system of telescopes to study the cosmic microwave background and address the mystery of cosmic inflation, which is expected to operate through to at least 2036 (CERN Courier March/April 2022 p34). Additional projects that would start after 2029 are Spec-S5 (the follow-on spectroscopic device to DESI), a project to carry out line intensity mapping (LIM), and planning efforts to increase the sensitivity of gravity wave detection by at least a factor of 10 (103 in sensitive volume)beyond what will be achieved by LIGO/Virgo.
The immediate goal for the energy frontier is to carry out the 2014 P5 recommendations to complete the HL-LHC upgrade and execute its physics programme. A new aspect of the proposed programme is the emergence of a variety of auxiliary experiments, examples of which are FASER (operational) and MATHULSA (proposed), that can use the existing LHC interaction regions to explore parts of discovery space in the far-forward regions. These are mid-scale detectors in cost and complexity, and provide room for additional innovation at the HL-LHC. The energy frontier supports the construction of a global e+e– Higgs factory as soon as possible. Either a linear collider or a circular collider can provide the necessary sensitivity, and a programme of directed detector and accelerator R&D for a Higgs factory is needed immediately to enable US participation. To ensure the long-term viability of the field, the energy frontier wants to begin accelerator and detector R&D towards a 10 TeV muon collider or a 100 TeV-scale hadron collider, in collaboration with partners worldwide. Finally, the US energy-frontier community has expressed renewed interest and ambition to develop options for an energy-frontier collider that could be sited in the US, specifically either an e+e– Higgs factory or a muon collider, while maintaining its international collaborative partnerships and obligations with, for example, CERN future-collider R&D projects.
The neutrino frontier
What are the neutrino masses? Are neutrinos their own antiparticles? How are the masses ordered? What is the origin of neutrino mass and flavour? Do neutrinos and antineutrinos oscillate differently? And are there new particles and interactions that can be discovered? These are among the fascinating questions elaborated by the neutrino frontier. The DUNE R&D programme, propelled by the development of large-scale liquid-argon detectors in the US and Europe, in particular through the CERN Neutrino Platform, has demonstrated the power and feasibility of this technique. Following the completion of DUNE Phase 1 by 2030, DUNE Phase 2 is the neutrino community’s highest priority project for 2030–2040. The Phase 2 project has three components: a replacement of the Fermilab 8 GeV Booster to deliver 2.4 MW to the DUNE target and possibly to provide beam for other experiments; the construction of an additional 20 kT (fiducial) of far-detectors at Homestake; and a fully capable near-detector complex at Fermilab to provide very precise control of the systematic uncertainties for the far-detector measurements, besides carrying out a rich physics programme of their own. DUNE will perform definitive studies of neutrino oscillations, test the three-flavour paradigm, search for new neutrino interactions, and will resolve the mass hierarchy question and hopefully observe CP violation. There are many other aspects of neutrino physics that merit study, including the absolute mass, the search for neutrinoless double beta decay (which bears on the issue of whether the neutrino is a Dirac or a Majorana fermion), the measurement of cross sections, and the search for sterile neutrinos. Several of these will be part of the US neutrino programme, either based in the US or through collaboration abroad.
Rare processes and precision measurements
The rare processes and precision measurements frontier is currently working on two mid-sized US projects at Fermilab endorsed by P5 in 2014: the Muon g−2 experiment, which has produced exciting results and will continue to take data for at least a few more years; and the Mu2e experiment, which is under construction. The programme also has important investments in flavour physics through support of the Belle II experiment in Japan and LHCb at CERN. Priorities for the next few years are to complete g−2, begin taking data with Mu2e, and continue collaboration at Belle II and LHCb, including participation in future upgrades. Looking ahead, the central themes are to understand quark and lepton flavour and its violation measurements, and the search for dark-matter production in the mass range from sub-MeV to a few GeV in fixed-target proton and electron experiments. There is a proposal to study muon science in an advanced muon facility at Fermilab that would greatly improve the search for lepton-flavour violation in µ → eγ, µN → eN and µ → 3e decays. This would require an intense proton beam with unique characteristics and accumulator rings to manage the production of muon beams with different energies and time profiles.
Theory frontier
Theoretical particle physics seeks to provide a predictive mathematical description of matter, energy, space and time that synthesises our knowledge of the universe, analyses and interprets existing experimental results and motivates future experimental investigation. Theory connects particle physics to other areas (e.g. gravity and cosmology) and extends the boundaries of our understanding (e.g. quantum information). Together, fundamental, phenomenological and computational theory form a vibrant ecosystem whose health is essential to all aspects of the US high-energy physics programme. The theory frontier recommends, among others, invigorated support for a broad programme of research as part of a balanced portfolio and an emphasis on targeted initiatives to connect theory to experiment.
Nearly a decade since the last Snowmass exercise, the recommended experimental programme has succeeded in pushing the boundaries of our knowledge
Accelerator frontier
The accelerator frontier, which has many crossovers with the energy frontier, aims to prepare for the next generations of major accelerator-based particle physics projects to explore the energy, neutrino and rare-process-and-precision frontiers. In the near term, a multi-MW beam-power upgrade of the Fermilab proton accelerator complex is required for DUNE phase 2. Studies are required to understand what other requirements the Fermilab accelerator complex needs to meet if the same upgrade is to be used for related rare-decay and precision experiments. In the energy frontier, a global consensus for an e+e– Higgs factory as the next collider has been reaffirmed. While some options (e.g. the International Linear Collider) have mature designs, other options (such as FCC-ee, C3, HELEN and CLIC) require further R&D to understand if they are viable. In order to further explore the energy frontier, a very high-energy circular hadron collider or a multi-TeV muon collider will be needed, both of which require substantial study to see if construction is feasible in the decade starting 2040 or beyond. It is proposed that the US establish a national integrated R&D programme on future colliders to carry out technology R&D and accelerator design studies for future collider concepts. Since machines of this magnitude will require international collaboration, the US R&D programme must be well-aligned and consistent with international efforts. Also under consideration are new acceleration techniques, such as wakefield acceleration, and ERLs, along with proposed R&D programmes that could indicate how they would contribute to the design of future colliders.
Computational frontier
Software and computing are essential to all high-energy physics experiments and many theoretical studies. However, computing has entered a new “post-Moore’s law” phase. Speed-ups in processing now come from the use of heterogeneous resources such as GPUs and FPGAs developed in the commercial sector, with significant implications for the way we develop and maintain software. We are also beginning to rely on community hardware resources such as high-performance computing centres and the cloud rather than dedicated experiment resources. Finally, new machine-learning approaches are changing the way we work. This new computing environment requires new approaches to address the long-term development, maintenance and user support of essential software packages and cross-cutting R&D efforts. Additionally, strong investment in career development for software and computing researchers is needed to ensure future success. The computational frontier therefore recommends the creation of a standing coordinating panel for software and computing under the auspices of the APS DPF, mirroring the Coordinating Panel for Advanced Detectors established in 2012.
Instrumentation frontier
Improved instrumentation is the key to progress in neutrino physics, collider physics and the physics of the cosmic and rare-processes frontiers. Many aspects now at the cutting-edge of detector development were hardly present 10 years ago, including quantum sensors, machine-learning and precision timing. Funding for instrumentation in the US, however, is actually declining. Key elements of a rejuvenated instrumentation effort include programmes to develop and maintain a sufficiently large and diverse workforce, including physicists, engineers and technicians at universities and national laboratories; double the US detector R&D budget over the next five years and modify funding models to enable R&D consortia; expand and sustain support for innovative detector R&D and establish a separate review process for such pathfinding endeavours; and develop and maintain critical facilities, centres and capabilities for sharing knowledge and tools.
Underground experiments address some of the most important areas of particle physics, including the search for dark matter, neutrino physics (including neutrinoless double beta decay and atmospheric neutrinos), cosmic-ray physics and searches for proton decay. The underground frontier concluded that future experiments and their enabling R&D require more space than is currently planned. They proposed a possible addition of the underground space at a depth of 4850 feet at SURF/Homestake and possible additional space at a depth of 7400 feet. These would open up space to develop new experiments and would provide the opportunity for SURF to host next-generation dark-matter or neutrinoless double beta decay experiments.
Community engagement
The community engagement frontier concentrated on seven areas: interaction with industry; career pipeline and development; diversity, equity and inclusion; physics education; public education and outreach; public policy and government engagement; and environmental and societal impacts. The inclusion of this broad array of issues as a “frontier” was a novel aspect of Snowmass 2021 and led to the formulation of many proposals for consideration and implementation by the community as a whole. These issues impact the ability of all frontiers to successfully complete their work, and some, such as the need to broaden representation, are highlighted by other frontiers too. While many recommendations apply directly to the DOE and NSF programmes and could be considered by P5, many others are directed to the HEP community as a whole. We in DPF are considering how best to pursue these issues with government agencies, APS and other groups.
The exciting road ahead
Almost three months since the Seattle workshop, the individual frontier reports are now nearly all complete and the process of synthesising the results has begun. One important theme is to stay the course on the programme approved by the last P5 in the hopes that the hints and anomalies that have shown up since then will provide some guidance for physics beyond the Standard Model. The second theme is that, in the absence of a specific target, we will have to plan a very diverse programme of experiments, theoretical studies and machine and detector R&D in which we broadly explore the large space of possibilities. In all cases, a global effort will be required, and much thought is being applied to ensuring that the US can play an appropriate role.
A global effort will be required, and much thought is being applied to ensuring that the US can play an appropriate role
We believe that members of the US high-energy physics community left the Seattle workshop with an appreciation of the great opportunities present in each frontier, the interconnections between the frontiers and the connections to programmes in the rest of the world. We hope that our report will help P5 produce recommendations that we can unite behind, as we did in 2014. That has proven to be an effective step in convincing the public and policy makers that we have conducted a rigorous process and achieved a consensus that is worthy of their support.
Studies of rare B-meson decays at the LHC provide a sensitive probe of physics beyond the Standard Model (SM) and allow us to explore energy scales much higher than those directly accessible. A key factor in the success of these studies is the availability of precise theoretical predictions that can be compared with experimentally accessible processes. The dimuon decays B0S→ μ+μ– and B0→ μ+μ– are a case in point. In particular, studies of these decays could help researchers to understand the nature of several anomalies seen in other rare B-meson decays.
The CMS collaboration recently reported a new measurement of the B0S→ μ+μ– branching fraction and effective lifetime, as well as the result of a search for the B0→ μ+μ– decay, using data recorded during LHC Run 2. This new study benefits not only from a large event sample but also from advanced machine-learning algorithms, which are used to uncover the rare signal events out of the overwhelming background. The B0S→ μ+μ– signal is very clearly seen (see figure 1), leading to more precise measurements than previously achieved. The B0S→ μ+μ– branching fraction is measured to be (3.8 ± 0.4) × 10–9, the relative uncertainty of 11% being a remarkable improvement with respect to that of the previous CMS result, 23%.
This measured value is consistent with the SM prediction of (3.7 ± 0.1) × 10–9, and reduces a previous tension between theory and experiment, which was based on the combination of the previous CMS result with the ATLAS and LHCb values. The variation in the central value of the CMS measurements is mostly driven by the use of a larger data sample and by the change of the B-hadron fragmentation fraction ratio (by about 8%). The measured effective lifetime of the B0S→ μ+μ–decay, 1.8 ± 0.2 ps, is also consistent with the SM prediction. The precision of this measurement is approaching the level necessary to probe the CP properties of B0S→ μ+μ–, which could differ from the SM prediction. Finally, the B0→ μ+μ– decay remains unseen.
CMS physicists are looking forward to continuing these rare-decay studies with the large data samples to be collected during LHC Run 3. Besides the improved precision expected for B0S→ μ+μ– measurements, seeing the first evidence of B0→ μ+μ– is high on their wish list.
The LHCb experiment is designed to study heavy-flavour particles containing beauty and charm quarks. Nevertheless, thanks to the large strangeness production cross-sections at the LHC as well as the excellent reconstruction performance of LHCb at low momenta, the experiment is also able to produce precise results in strange decays, complementary to those from dedicated experiments such as NA62 and KOTO. The collaboration has recently released a “trillio-scale” upper limit on the branching fraction of the decay K0S→ μ+μ–μ+μ–, being the first at this scale at the LHC. The same dataset was used to search for K0L→ μ+μ–μ+μ–, yielding the world best upper limit f and the first LHC result on a K0L decay.
According to the Standard Model (SM), K0S (K0L) mesons decay into four muons at a very small rate of a few 10–14 (10–13). The decay rates of these processes are very sensitive to possible contributions from new, yet-to-be discovered particles such as dark photons, which could significantly enhance or suppress the decay rate via quantum interference with the SM amplitude. Despite the unprecedented K0-meson production rate at the LHC, performing this search is challenging due to the low transverse momentum (typically a few hundred MeV) of the muons. LHCb exploits its unique capability to select, in real time, low transverse-momentum muons – a capability that has improved in recent years thanks to the versatility of its online trigger system. The analysis used machine learning to discriminate long-lived particles from combinatorial background, as well as a data-driven and detailed map of the detector material around the interaction point. The invariant mass of the four-muon system is used as a control variable to statistically separate the potential signal from the remaining combinatorial background.
No selected event consistent with the decay of K0S into four muons, which should appear in the region around the K0S mass of 498 MeV, was observed (see figure 1). In the absence of a signal, upper limits on the respective branching fractions are set to 5.1 × 10–12 for the K0S decay mode and 2.3 × 10–9 for the K0L mode at 90% CL. These results represent the world’s most precise searches for these decays, and the branching fraction for K0S→ μ+μ–μ+μ– is the most stringent upper limit on a K0S decay mode.
The upgraded LHCb detector, which started data-taking this year, offers excellent opportunities to further improve the search precision and eventually find evidence of this decay. In addition to the increased luminosity, the LHCb upgrade has a fully software trigger, which is expected to significantly improve the efficiency for K0 decays into four muons and other decays with very soft final-state particles.
