Physics Archives – Page 2 of 144

Rencontres de Moriond turns 60

For 60 years, Rencontres de Moriond has brought theorists and experimentalists in high-energy physics into close, sustained contact. The 2026 Electroweak session, held in the Alpine town of La Thuile, Italy, from 15 to 22 March, gathered around 140 participants for a week covering flavour, neutrinos, dark matter, Higgs and beyond-the-Standard Model physics.

Several updates came from the flavour sector. The LHCb collaboration presented the first measurement of the CP-violating angle γ using a small fraction of the Run 3 dataset (see “An upgraded take on CP violation“). This result, compatible with previous determinations, demonstrated the improved sensitivity of the upgraded trigger. LHCb also reported the observation of a new doubly charmed hadron, the Ξ⁺_cc. The BES III collaboration showed many new measurements in charm and tau physics, while Belle II presented an updated measurement of R(D^*) – a test of lepton-flavour universality comparing τ leptons with electrons and muons in the B → D^*ℓν decay. This new result, with its improved precision, is consistent with both the Standard Model (SM) at 1.5σ and the world average at 1.3σ. The growing LHCb Run 3 dataset and the record peak luminosity reached by SuperKEKB will enable several interesting results in B-physics.

Branching out

NA62 presented a new result based on 2023–2024 data for the very rare decay K⁺ → π⁺νν (see “The kaon stays on script“), whose SM branching ratio of order 10^–10 makes it highly sensitive to new physics. Combined with previous NA62 data, the new result determines the branching ratio with a precision of about 20%, in agreement with the SM prediction.

In neutrino physics, new results addressed the sterile neutrino anomalies (Δm² ~1 eV²). The MicroBooNE experiment, using a combination of data from the Booster Neutrino Beam and the NuMI beam at Fermilab, excluded essentially all the parameter space favoured by the MiniBooNE and LSND anomalies at 95% confidence level. Similarly, the KATRIN experiment showed new results excluding almost all the parameter space favoured by the Gallium anomaly. The 3 + 1 sterile-neutrino explanation of the anomalies now seems to be excluded (see CERN Courier January/February 2026 p8), although new-physics alternatives are still under scrutiny. The JUNO experiment, which measures antineutrinos from nuclear reactors, released its first results based on 59 days of data-taking. With this small fraction of the target exposure, it has already achieved world-leading accuracy on the θ₁₂ and Δm²₂₁ mixing parameters. The main goal of JUNO is to establish the ordering of the three neutrino masses, and it should achieve 3–4σ significance in about six to seven years of data-taking, with detector performance already close to the design.

The limits from the LHC experiments provide strong guidance for theorists when building new models

New results from direct searches for dark matter in the 1 GeV–10 TeV range were presented by the LUX-ZEPLIN, XENONnT and DarkSide-50 experiments. The exclusion limits for the WIMP spin-independent cross-section are now approaching 10^–48 cm² for masses in the ~30–70 GeV region. In addition to the “standard” analyses, optimised for WIMPs above 10 GeV, dedicated techniques have been developed to cover the lower WIMP mass region (1–10 GeV). XENONnT and LUX-ZEPLIN are now entering the “neutrino fog”, an irreducible background from coherent elastic neutrino–nucleus scattering, and both report first signals from ⁸B solar neutrinos.

Many searches for phenomena beyond the SM at the LHC were presented by the ATLAS and CMS collaborations. In addition to the “classical” high-energy signatures, the experiments are now investing large resources to investigate the more challenging phase-space regions characteristic of models with feebly interacting particles and compressed-SUSY scenarios. New trigger strategies, such as scouting and trigger-level analyses, parked data and delayed streams, and end-of-fill triggers, have been developed to address these complicated topologies, while dedicated reconstruction, calibration and machine-learning techniques help identify non-conventional signatures. Despite great efforts, no significant signals have been found – but the limits from the LHC experiments provide strong guidance for theorists when building new models.

Golden era

The ATLAS and CMS collaborations presented several new results on Higgs-boson physics. New total and differential cross-section measurements in the “golden” H → ZZ^* → 4ℓ channel were shown, using about half of the LHC Run 3 data set (2022–2024). With this subset of the total collected data, the two experiments have already reached a precision on the total cross-section of less than 10%.

A new ATLAS measurement reached, for the first time, 3σ evidence for the inclusive H → bb production with transverse momentum above 450 GeV. When the detectors were designed, this process was considered unobservable due to the large QCD dijet background. The achievement comes from developing and calibrating in situ a new algorithm based on graph neural networks, optimised to identify boosted objects decaying into heavy-flavour jets.

The research programmes at existing and planned facilities point to strong progress in the coming years

Concerning di-Higgs (HH) production, which provides critical information about the Higgs boson self-couplings and constraints on its potential, ATLAS and CMS presented their legacy Run 2 combination. The measured signal strength is σ/σ_SM = 0.8^+0.9_–0.7, showing sensitivity to the SM signal at better than 1σ. The performance gains already seen with Run 3 data point to possible evidence for the process by combining the two experiments’ Run 2 and Run 3 datasets, assuming the SM production rate.

Thanks to the large collected datasets, ATLAS and CMS are now able to explore very rare processes in top-quark and multi-boson production. The latter are very powerful at constraining new-physics interactions in the framework of effective field theory, and several new results were presented, including top + boson, top + diboson and three-top production. It is also worth highlighting the first measurement of the |V_cb| CKM element using on-shell W bosons from top-quark decays presented by ATLAS.

In summary, Moriond Electroweak 2026 demonstrated steady experimental improvements in addressing the fundamental open questions in particle physics. Although the experimental challenges are arduous, the research programmes at existing and planned facilities point to strong progress in the coming years.

Drilling down on dark matter

The High Energy, Cosmology and Astroparticle Physics (HECAP) Abu Dhabi Workshop, held at New York University Abu Dhabi on Saadiyat Island from 13 to 15 January, brought together more than 30 researchers to discuss some of the deepest mysteries in fundamental physics.

A central theme was the effort to unravel the nature of dark matter (DM) and understand its production mechanisms, both in the early universe and in laboratory experiments. The traditional freeze-out paradigm, in which DM was once in thermal equilibrium with ordinary matter and froze out as the universe expanded and cooled, was contrasted with freeze-in scenarios, where DM is never in equilibrium and is produced through extremely weak interactions. In particular, Andrzej Hryczuk (NCBJ) analysed the production of multi-component dark sectors, including non-equilibrium effects such as conversions and cannibalisation processes, and Hyun Min Lee (Chung-Ang University) explored gravity-mediated DM scenarios. Complementary perspectives on DM phenomenology were presented by Nuria Rius (IFIC/UV), who discussed scenarios with warped extra dimensions where DM interacts gravitationally with Standard Model (SM) particles.

Search and research

The possible signatures of dark sectors at colliders were discussed by Giovanna Cottin (UC Chile), who emphasised that dark-sector particles may be long-lived and produce displaced vertices or other unconventional signatures at the LHC. The importance of dedicated searches and of reinterpreting existing LHC data was highlighted, together with the prospects offered by future facilities such as the Future Circular Collider.

A recurring theme was the connection between the origin of matter and the nature of dark matter. Cosmological observations directly establish a quark–antiquark asymmetry, conventionally identified with a baryon asymmetry under the assumption that the dark sector carries no compensating baryon number. Whether this assumption holds, and what its breakdown would imply, was a recurring question. Pilar Hernández (UV) discussed the link of this puzzle to neutrino masses, while a complementary scenario was presented in which the observed quark–antiquark asymmetry predicts the existence of DM, stabilised by the same symmetry that prevents proton decay.

Several presentations focused on the early universe as a probe of new physics. Javier Rubio (Universidad Complutense de Madrid) discussed Hubble-induced phase transitions triggered by the evolution of the scalar component of spacetime curvature after inflation, which can amplify field fluctuations, generate transient topological defects, and potentially lead to observable gravitational-wave signals. The role of early-universe dynamics in uncovering new physics was also highlighted by Basabendu Barman (SRM University), who discussed how cosmological observations may provide unique information about physics beyond the SM.

Closely related to these questions is the study of vacuum decay in scalar field theories. José Ramón Espinosa (IFT) showed that the standard semiclassical “bounce” picture can be extended to include pseudo-bounce and antibounce configurations, revealing a richer structure of vacuum decay channels than previously considered.

Thermal history

The reheating epoch following inflation was also discussed as a crucial stage in the thermal history of the universe, when it evolved from a nearly empty state to a hot radiation-dominated plasma. The details of this transition can significantly affect DM production and enlarge the viable parameter space for DM candidates, as discussed by Yann Mambrini (Université Paris-Saclay) and Kuldeep Deka (NYU Abu Dhabi).

Gravitational waves provide an important observational probe of these early-universe processes. Antonio Junior Iovino (NYU Abu Dhabi) discussed gravitational-wave signatures from primordial black holes across a wide range of frequencies and the prospects for detecting them with experiments ranging from pulsar timing arrays to the LIGO–Virgo–KAGRA network. Related aspects of gravitational-wave production in the early universe were addressed by Xunjie Xu (IHEP), who discussed the thermal generation of a cosmic gravitational-wave background.

A recurring theme was the connection between the origin of matter and the nature of dark matter

The workshop also covered precision tests of the SM. Yosef Nir (WIS, Rehovot) presented recent developments in flavour physics, including the puzzling measurement of the branching fraction of B_s → K⁰K⁰, which appears to be in tension with SM expectations based on flavour-symmetry relations. The decay proceeds dominantly through loop diagrams and is therefore sensitive to virtual contributions from new heavy particles. In addition, measurements of CP asymmetries in B → J/ψ π decays were discussed, as they help refine the determination of the CKM parameter sin(2β).

Alberto Casas (IFT) discussed how high-energy experiments can test fundamental aspects of quantum mechanics, including quantum entanglement and Bell non-locality, at unprecedented energy scales. In addition, Juan José Gómez Cadenas (DIPC) reviewed the status of searches for lepton-number violation and the progress of the NEXT experiment in probing neutrinoless double-beta decay.

Overall, the workshop provided an excellent forum to discuss recent developments at the interface of particle physics, cosmology and astroparticle physics, in particular in the search for physics beyond the SM. Many of the discussions illustrated how progress in understanding DM, the early universe and fundamental interactions increasingly relies on the interplay between theoretical work, laboratory experiments and astrophysical observations.

Garching gathers for the FCC

From 26 to 30 January 2026, the Max Planck Institute for Physics in Garching, near Munich, hosted the 9th FCC Physics Workshop, a major gathering of theorists and experimentalists advancing the physics case, addressing the experimental challenges and developing detector-concept candidates for the proposed Future Circular Collider (FCC). The event brought together hundreds of scientists from Europe and beyond to discuss the FCC and the broader strategy for the field after the LHC and its high-luminosity phase.

European physicists have recently recommended the electron–positron FCC (FCC-ee) as the preferred next flagship project in the ongoing update to the European Strategy for Particle Physics, with a final decision on construction anticipated around 2028 (CERN Courier January/February 2026 p7). This endorsement followed years of extensive design and feasibility studies, and provided an important backdrop to the Munich workshop, where participants worked to align physics goals with technical feasibility and long-term sustainability considerations.

The meeting marked a visible shift in tone and substance for the FCC programme, with conceptual exploration giving way to operational, benchmark-driven studies. Five days of discussion converged on a common message: if the FCC is to deliver as the next flagship collider at CERN, precision must be engineered at every level, from beam energy calibration and theoretical predictions to detector granularity, reconstruction algorithms, analysis software and governance structures.

The FCC’s first stage, FCC-ee, is conceived as a high-luminosity electron–positron collider operating at multiple centre-of-mass energies, including the Z pole, WW threshold, Higgsstrahlung maximum around 240–250 GeV and the top-quark pair threshold. The physics case has long emphasised unprecedented precision in electroweak observables, Higgs couplings and top-quark properties, alongside sensitivity to rare and exotic processes. What was striking in Munich was the degree to which this ambition is now translated into quantitative requirements and structured work plans.

The meeting marked a visible shift in tone and substance for the FCC programme

The Physics Studies work package presented a coordinated strategy across electroweak, Higgs, flavour, QCD, top and beyond-the-Standard Model (BSM) groups. The workshop highlighted the need for consistent benchmark processes, shared uncertainty frameworks and global fit strategies capable of combining hundreds of measurements into coherent constraints on new physics.

At FCC-ee luminosities, statistical uncertainties on many observables would improve by up to three orders of magnitude over previous electron–positron colliders. This shifts the limiting factor toward systematic effects: beam energy calibration, luminosity normalisation, detector alignment, flavour-tagging biases, uncertainties in higher-order calculations and parton-shower modelling. Matching this statistical power with equally ambitious control of systematics is a prerequisite for turning per-mil measurements into probes of new physics well beyond the direct kinematic reach.

The workshop made clear that the FCC physics case cannot be decoupled from detector performance. Precision Higgs and electroweak measurements demand excellent tracking momentum resolution and minimal material budgets to control multiple scattering and secondary interactions. Heavy-flavour and tau-physics programmes hinge on vertexing and impact-parameter resolution, with b- and c-tagging joined by emerging capabilities such as strange-quark tagging. Multi-jet final states from W, Z and Higgs decays bring jet-energy resolution to the fore. Meeting these goals favours highly granular calorimetry and particle-flow reconstruction, which combines information from all subsystems to identify and measure each particle in the event.

Beyond precision

At the same time, the programme also extends beyond canonical precision channels. Sensitivity to long-lived particles and feebly interacting states motivates continuous tracking and hermetic calorimetric coverage. Ultra-precise luminosity measurements at the 10^–5 level are integral to the detector architecture, and particle-identification capable of K/π separation over wide momentum ranges supports flavour and QCD studies.

Four detector concepts – ALLEGRO, CLD, IDEA and ILD – are under active development, exploring complementary technologies toward shared performance goals. A fifth, ALFA, has recently emerged, and the workshop encouraged further proposals. The timeline outlined in Munich foresees optimisation studies through 2027, system demonstrators by around 2030, scalable prototypes in the early 2030s, and conceptual design reports in 2033. While the final political decision is still pending, detector R&D is advancing in lockstep with physics requirements.

The Physics Software and Computing (PSC) work package presented its vision for supporting physics and detector studies. At its core is Key4hep, a community-driven framework for HEP experiments, prototyped by FCC together with other future collider projects and increasingly adopted by related R&D efforts. Key4hep provides modularity, interoperability and long-term sustainability, integrating past work from linear-collider facilities and current CEPC and EIC work. In Munich, updates were presented on full simulation geometries for several detector subsystems, integrated digitisation and reconstruction chains, and improved user workflows.

Large-scale production campaigns, data-management strategies and distributed-analysis frameworks are being aligned with CERN IT services and GRID tools, with machine-learning methods increasingly embedded in reconstruction and analysis workflows. Realistic simulation studies, incorporating beam-induced backgrounds and detailed geometries, are gaining importance, alongside the development of robust analysis algorithms that can be validated across simulation levels.

Core contributors

The PSC session also addressed human infrastructure. Recognising computing experts as core scientific contributors – with appropriate career paths and visibility – was considered essential to the long-term success of a data-intensive programme like the FCC.

One of the most distinctive aspects of the Munich workshop was the visible role of early-career professionals. The FCC Early Career Forum presented a draft document synthesising discussions from FCC Week 2025 and subsequent exchanges. Its focus on sustainability, communication, careers and governance resonated across sessions.

One of the most distinctive aspects of the workshop was the visible role of early-career professionals

Sustainability emerged as a central design consideration: environmental, economic and social aspects must be integrated from the earliest design phases through operation and decommissioning. Participants stressed the importance of engaging local and regional communities, and of clearly articulating how the FCC could contribute to broader societal goals.

The Munich workshop made clear that the FCC programme is entering a new phase of maturity and consolidation. With coordinated efforts across physics, detectors, computing and accelerator physics, the community is laying the groundwork for a project that promises to extend our understanding of fundamental interactions and prepare particle physics for its next frontier.

The Standard Model: a practical step-by-step guide

This new textbook offers an intermediate-level presentation of the Standard Model (SM). It assumes that students have a knowledge of relativistic quantum mechanics and are comfortable with the Dirac equation, the properties of Dirac spinors, and covariant notation, including how to write Lagrangians in it. It also assumes familiarity with Feynman diagrams in quantum electrodynamics and with the basic application of the Feynman rules. Even so, the book opens with a substantial revision chapter taking up about a quarter of the main text, covering discrete symmetries, the S-matrix and aspects of QED, with a complete calculation of Compton scattering between a photon and an electron. Although the author does not treat phase-space calculations – the integrals over the kinematics of final-state particles that turn a matrix element into a cross section or decay rate – as an independent topic, the complicated example of muon decay is worked out in detail.

A distinctive feature is the inclusion of fully worked-out examples, in which the algebra is carried out at much greater length than in most other textbooks. The effect is that of a blackboard lecture, rather than one of those slide presentations in which all the so-called “trivial” steps – that students rightly find anything but trivial – are omitted. Several of the examples broaden the physical understanding in ways rarely seen at this level: the hydrogen atom solved with the Dirac equation, coupled oscillators, mechanics problems involving torque, forces and inertia, all pressed into service to illuminate the underlying particle physics.

The Standard Model: A Practical Step-by-Step-Guide — Credit: Cambridge University Press

The heart of the volume is divided into two parts. The first sets out the basic elements of the SM, starting with the electroweak interactions mediated by photons, W and Z bosons, through gauge symmetry, spontaneous symmetry breaking and the Higgs boson, and ending with QCD. The second turns to applications. It includes an accessible treatment of higher-order corrections, flavour physics, flavour-changing currents and the CKM matrix that encodes the mixing between quark generations. It also features a chapter on QCD applications, covering the parton model and deep-inelastic scattering. A more elementary treatment is reserved for hadronisation, the non-perturbative process by which quarks and gluons turn into observable hadrons.

A clean section on the lowest-order calculation of gluon–gluon fusion Higgs-boson production at hadron colliders sits alongside the more standard material. Kaon oscillations, CP violation and neutrino oscillations close the book, alongside a 20-page experimental chapter of the kind one might expect in an introductory course. The book is presumably aimed at students with a stronger grounding in quantum field theory than in particle physics, who are now building from that base toward an understanding of the SM.

Overall, the book is faithful to its title. It sticks to the SM and avoids new-physics scenarios, save perhaps for neutrino oscillations, which some already classify as beyond it. Occasional bridges are built nonetheless. For instance, Majorana neutrinos appear in one of the exercises, while an accessible treatment of the θ-QCD term comes a breath away from discussing axions. The author does not shy away from using modern computational tools, with examples drawing on Mathematica, FeynCalc, the event generators MadGraph and Pythia, which simulate hard scattering and the subsequent parton showers and hadronisation, and the detector-simulation package Delphes. Each chapter ends with a small set of problems.

The result is a clear and engaging treatment, carefully tailored to its readership. Its fresh perspective, its unconventional examples and the painstaking attention to algebraic detail, make it a useful resource not only for students but also for instructors teaching introductory particle physics.

Die urknallmaschine

How to explain CERN to someone who’s never been there? That’s indeed not always easy, but this book can surely help.

The German-language Die Urknallmaschine (The Big Bang Machine) by Barbara Warmbein offers an authentic glimpse into the research at CERN and the unique, sometimes extraordinary, environment in which it happens.

Warmbein has a real talent for combining fundamental ideas in physics and engineering at CERN with illustrative analogies. She creates mental pictures that make complex ideas easier to grasp, and much more likely to stick. The level is accessible to anyone with a general interest in physics and remains engaging without becoming overly technical or intimidating. This makes the book a good choice not only for readers without a CERN background, but also for anyone looking for better ways to explain the laboratory to friends, family or visitors.

Warmbein starts with the big questions, the great mysteries of our universe, and gradually builds a bridge to CERN’s research. Along the way, she explores both what we already understand and what remains unknown, often linking these ideas to everyday experiences. She weaves in historical context, reminding us, for example, that around the year 1900 many physicists believed that their work was almost complete, just before quantum mechanics and special relativity changed everything once again.

After roughly 50 pages dedicated to well-pitched basics, Warmbein moves on to accelerators and detectors, before widening the perspective to CERN as an organisation. She traces its development over the past 70 years, highlighting both what happens on site and the global network of institutes and collaborations that make CERN possible.

A nice touch is the inclusion of small but interesting pieces of side information, details that even people working at CERN might not know, in a compact form. This adds an extra layer of discovery, even for long-time insiders. The applications of accelerators outside fundamental research are one such example. Warmbein presents her material in a warm, approachable way, capturing both the science and the human side of CERN in one stroke.

Mark Alastair Rayner 1983–2026

It was with profound shock and sadness that we learned of the passing of Mark Rayner, editor of CERN Courier, on 23 March due to sudden illness. His love of physics, talent for communication and editorial rigour raised the bar for this magazine.

Mark was born in Hounslow, England, on 7 October 1983 and studied physics at Worcester College, University of Oxford, from 2002 to 2006. In 2005 he spent three months at CERN as a Summer Student working on tests of the ATLAS transition-radiation-tracker end caps. He continued at Oxford with a PhD, participating in the Muon Ionisation Cooling Experiment (MICE) based at the Rutherford Appleton Laboratory. His thesis described the development of a novel technique for characterising the MICE muon beam and demonstrating its suitability for a muon cooling measurement, an essential step on the path towards a possible neutrino factory and muon collider.

In 2011, he moved from accelerator physics to neutrino physics, joining the University of Geneva both as a lecturer and as a researcher working on the T2K, Hyper-Kamiokande and BabyMIND experiments. Over the years, Mark supervised several students. In the process he deconstructed the weaknesses in the T2K detector system, realising that an upgrade of the detector setup at the source was necessary for the long-term programme. An upgrade was proposed, with a much simplified and better geometry, largely using detector techniques developed in MICE. It was approved in 2019 and is now successfully operational.

As a physicist, Mark stood out for his care and originality. He liked simplicity and elegance, and to understand the relative causality of correlated observations. He made many important contributions and was happy to do so, without seeking recognition.

A natural educator and communicator, Mark trained as an apprentice physics teacher at Ecole Internationale de Genève in 2018. The following year, he joined CERN as a senior fellow working on the Courier, where he played a major role in the launch of the magazine’s website and rose quickly to become deputy editor. When his fellowship ended, Mark took his exceptional skills to the World Economic Forum, where he managed the production of a portfolio of publications and tools relating to education, skills and learning, and served as lead author for the Future of Jobs report 2023.

Mark returned to CERN as a staff member in 2024, as editor of the Courier. Over a short period, his eye for design, his mastery with words, and his ability to interpret and display complex information in novel ways sharpened the impact of CERN’s flagship publication. He also paid particular attention to improving the visibility of gender diversity in these pages and to developing the magazine’s online presence, enabling him to connect particle physics with new audiences. He took great pride in his work and in engaging with authors to shape their stories. He had huge respect for those who devoted their lives to fundamental research in physics and was widely recognised for his dignity and professionalism among members of the international particle-physics community.

Above all, Mark cared deeply about everything he did, and especially about the well-being of others. His pursuit of excellence and his remarkable attention to detail set a standard that inspired those around him, and this is reflected in the deeply motivated team that he built and nurtured. He was highly cultured, played the flute, and sang in the Geneva Gospel Choir.

Mark was a man of great intellect, warmth and spirit, whose presence brought light to those fortunate enough to know him. He will be remembered with great respect and will be profoundly missed.

Jan Żylicz 1932–2026

On 16 February 2026, the Polish physics community suffered a painful loss – professor Jan Żylicz, an outstanding nuclear physicist, passed away in Warsaw.

Jan Lubart Żylicz was born on 7 January 1932 in Góra, in the Kashubian region, and completed his studies in physics at the University of Warsaw in 1955, under the supervision of Andrzej Sołtan. His work on beta spectroscopy of strongly deformed nuclei, conducted at the Institute of Nuclear Research, contributed to his 1961 PhD at the University of Warsaw. He continued his research on beta decay of rare-earth nuclei during a stay at the Niels Bohr Institute in Copenhagen from 1963 to 1965. One of his important contributions was the identification of the Coriolis effect’s role in rotating atomic nuclei, which served as the basis for his habilitation at the University of Warsaw in 1967.

His research stay at the CERN–ISOLDE facility, from 1970 to 1971, was devoted to the study of nuclides far from beta stability. This topic significantly influenced his subsequent scientific work, and he spent two further research stays in the mass separator group at GSI Darmstadt, from 1978 to 1979 and from 1986 to 1987. The close and long-lasting collaboration with GSI, which Jan initiated, played a crucial role in the scientific development of young scientists in the Warsaw group, many of whom completed postdoctoral fellowships there.

He led several large research projects, including studies of octupole correlations in actinide nuclei. This work was to some extent pioneering and contributed to growing interest in this topic among theorists and experimentalists. He devoted particular attention to exotic nuclides far from the beta-stability line, notably through the extensive research programme on Gamow–Teller transitions in the region of the doubly magic tin isotope ¹⁰⁰Sn, carried out mainly at GSI Darmstadt, but also at the ILL in Grenoble, the University of Jyväskylä and CERN–ISOLDE.

Jan had a talent for initiating valuable research programmes that could be carried out in Poland under the modest experimental conditions available in Warsaw at the end of communism. For example, he developed a new method for measuring the K-shell ionisation by charged particles, which was used for many years at the Warsaw Van de Graaff accelerator and yielded several results of practical importance. He was also interested in phenomena at the interface between nuclear and atomic physics, and proposed a programme to study radiative electron capture in forbidden transitions. Among Jan’s most original achievements are his works on the isomeric state of ²²⁹Th, and the idea of spin-mixing oscillations in the states of the hydrogen-like ion ²²⁹Th⁸⁹⁺. This work was ahead of its time – attempts to confirm the phenomena he predicted are currently underway at the ESR storage ring at GSI Darmstadt.

Associated with the University of Warsaw from 1972 to the end of his career, Jan established a new Nuclear Spectroscopy Group, which he headed until 1994, and served as director of the Institute of Experimental Physics from 1994 to 2002. In 2005, the Polish Physical Society awarded him its highest distinction – the Marian Smoluchowski Medal – and the European Physical Society honoured him with the title of EPS Fellow. He was also awarded the Knight’s Cross of the Order of Polonia Restituta.

Jan was an outstanding educator, whose lectures were valued for their clarity and for the passion with which he explained the essence of a problem. He attached particular importance to mentoring young academic staff, supporting and patiently motivating them. He sent them to international conferences and helped to organise research stays at leading Western institutions, which was especially important at a time when this was not as easy as it is today. He supervised 17 master’s theses and 12 doctoral dissertations, with six of his students later becoming professors of physics. Their successes brought him joy and pride, and he considered creating the conditions for the scientific development of his younger colleagues his principal achievement.

Jan Żylicz was a warm and kind man with an extraordinary sense of humour. Working with him gave us a sense of purpose, satisfaction and joy. He will forever be remembered as a model scholar and teacher.

Roger Barlow 1951–2026

Roger Barlow passed away suddenly on 1 February 2026 at his home in Wales. Roger had an illustrious career in particle physics and, latterly, also in accelerator physics. He was well known internationally for his work in statistics, in particular for his widely used textbook, Statistics: A Guide to the Use of Statistical Methods in the Physical Sciences, published in 1989.

Roger was born on 14 April 1951 in Canterbury. After attending Edinburgh Academy, he obtained a place to study for his first degree at Oxford. He then went to Cambridge, where he completed his PhD in 1977 on proton–deuteron interactions at CERN’s 2 metre bubble chamber. Roger then took up a research post at Oxford, working on the TASSO experiment, and contributed to the discovery of the gluon in 1979.

In 1980, Roger was appointed to a lectureship at the University of Manchester and joined the JADE Collaboration, where his work on event reconstruction and Monte Carlo simulation led to one of the early measurements of the B-meson lifetime. During his early years at Manchester, Roger moved on to the OPAL experiment, becoming leader of the Manchester team in 1991. On OPAL, he helped design, build, commission and operate the muon chambers, which were crucial for many Standard Model physics studies, including precision measurements of the Z boson.

Roger became the overall leader of the Manchester particle-physics group in 2005, after the retirement of Robin Marshall. The group was then also involved in ATLAS, D0, several neutrino experiments and BaBar, which Roger had joined. Under his leadership, the particle-physics group grew to more than 100 members. As a collaborator on BaBar, he helped design the electromagnetic endcap, and he supervised the construction of half of the detector in Manchester. His data analyses included setting new limits on the existence of second-class weak currents in tau–lepton decays. As BaBar wound down, he took his group into LHCb.

In the early 2000s, Roger began researching accelerator science, forming an accelerators group in Manchester and becoming a founding member of the Cockcroft Institute of Accelerator Science and Technology. He was principal investigator for the CONFORM project that led to the successful operation of EMMA, the world’s first non-scaling FFAG accelerator. This provided a proof of principle for a new type of accelerator with many potential applications. In 2011, he left Manchester for a post at the University of Huddersfield, where he formed another accelerator-science group.

In addition to his textbook, Roger produced several influential works on statistics, including a description of extended maximum likelihood, a highly cited paper on fitting using finite Monte Carlo samples, and a detailed paper on the treatment of systematic uncertainties.

Roger was a dedicated and skilled teacher, who cared deeply about educating the next generations. Among his many contributions to the public understanding of science, he introduced the Particle Physics Masterclasses for high-school students, which quickly expanded across the UK, before becoming truly international. In recognition of this, he was awarded the Institute of Physics’ Lise Meitner Medal and Prize in 2022.

Roger retired from Huddersfield in 2017, but continued to work on BaBar and LHCb, and to publish papers and lecture on statistics, right up until his passing.

Outside of physics, Roger was active in UK national politics as a member of the Liberal Democrats. He was selected three times to stand as a candidate for the UK parliament. He will be greatly missed by his wife, Ann, his children Edward and Eleanor, his extended family, and his many friends and colleagues across the world.

The mystery of the little red dots

Every new instrument needs its mysteries, and no discovery of the James Webb Space Telescope (JWST) has been more surprising than the “little red dots” it discovered in the early universe. Four years after their discovery, their nature is still an open question, with new papers purporting to solve the mystery on an almost daily basis.

These unexpected objects came into view in JWST’s first data release in 2022 thanks to its sharp images and sensitivity in the near infrared. By summer of 2023, a number of discovery papers had been written about them, identifying three traits in common: they were compact in size, had unusual “V-shaped” spectra and they showed emission from high-velocity hydrogen gas. Due to their compact size and red colour in the rest frame, they were dubbed little red dots. A few appeared in every pointing of the JWST imaging camera NIRCam, accounting for a few percent of all known galaxies in the first billion years of cosmic time. The race was on to determine their nature.

Two options initially appeared possible, but both were extraordinary and required a very precise tuning of parameters to fit the observations: too-dense galaxies or too-massive supermassive black holes. In either case, the objects had to be enshrouded in a cocoon of dust.

Galaxies or black holes?

The first paper assumed they were very massive galaxies, with their stars all assembled less than a billion years after the Big Bang. In favour of the galactic hypothesis were the V-shaped spectra, which are difficult to model without invoking massive stars. The vertex of the V-shape resembles a “Balmer break”, which is produced by the absorption of hydrogen atoms in the n = 2 level. Longward of the break, the optical continuum rises steeply toward the red, which this model attributed to the reddening of these stars by dust, with the UV being produced by starlight that was scattered out of the dust screen. However, the very high masses and early-universe star formation rates required for these models were difficult to reconcile with our understanding of the rate at which galaxies and their dark-matter halos assemble.

The first paper assumed they were very massive galaxies, with their stars all assembled less than a billion years after the Big Bang

The black-hole hypothesis was supported by evidence for very dense gas clouds moving at thousands of kilometres per second in the potential of a massive black hole. In this picture, surrounding dust would preferentially absorb ultraviolet light and re-emit it at longer wavelengths, producing the observed red colour. Though this explanation promised to alleviate the tension arising from the implied galaxy masses, it quickly became clear that these objects were not typical growing black holes. They were not detected in X-rays, nor did they show the characteristic 1000 K dust signature that is ubiquitous in actively accreting black holes. However, the most concerning piece of the black-hole interpretation was the implied black-hole masses. Applying local calibrations to the observed motion of gas in the little red dots implied black-hole masses of ten million to a billion suns, compared with galaxy masses of the same order – a stark contrast with local black holes, which have masses roughly a thousandth of their host galaxies. These overly massive black holes are hard to grow so far in advance of the galaxies, and also overproduce the total amount of black-hole mass created at such an early time.

Explaining their redness

Two major breakthroughs occurred in 2024 that clarified the nature of the little red dots. All the aforementioned models invoked heavy amounts of dust to suppress ultraviolet emission and produce the observed red colours. The conservation of energy implies that all the absorbed radiation should be re-emitted by the dust. However, multiple studies of populations and of luminous individual sources turned up non-detections of dust emission. These stringent limits on the far-infrared energy output were enough to conclusively rule out these entire classes of models, invoking reddening by dust to explain the observed red colours.

At the same time, campaigns to observe the broad population of little red dots discovered a remarkable class of sources with very little ultraviolet emission and extreme Balmer breaks. These breaks could not be produced by anything resembling a stellar population we have observed before, and served as conclusive evidence that normal stars cannot be responsible for producing the optical emission in little red dots; the photoabsorption by hydrogen in the n = 2 energy state must nevertheless be a crucial physical aspect of the little red dots, even if it wasn’t happening in the atmospheres of massive stars.

Plausible scenarios

The challenge is therefore to explain the characteristic red colour of the little red dots without dust obscuration. Any successful model would also need a substantial reservoir of hydrogen around to cause the hydrogen absorption that looked like starlight, but wasn’t. One plausible scenario that could satisfy these requirements is very dense gas arranged quasi-spherically around the black hole. In this scenario, the black holes powering the little red dots could be significantly less massive than we had originally thought, when we had assumed that dust was obscuring most of the light from the growing black hole.

The task is to explain the characteristic red colour of the little red dots without dust obscuration

In this new picture, the little red dots are powered by black holes that are accreting at much higher rates than are typically seen at later times. A higher accretion rate implies greater luminosity for a given black-hole mass, and therefore we infer much lower black-hole masses, perhaps closer to a million suns, and much more aligned with the measured galaxy masses. As a side benefit, lower black-hole masses are much more natural for objects that are so prevalent, because the number of low-mass dark-matter halos and low-mass galaxies is much higher than the number of high-mass systems.

Astronomers are still arguing about how this dense gas is configured and accretes onto the black hole, and everyone has their favourite model. We do not know if the geometry of the system is completely spherical, or if we are seeing a mixed-phase medium where the viewing angle is an important parameter. These details matter, because if we can pin down the characteristic size and density of these gas envelopes, we may be able to infer more robust black-hole masses for the population. There has been some recent speculation that the little red dots may be marking the end stages of black-hole seed growth, in which case they could be a critical missing link in our understanding of the formation of the first black holes. However, without more concrete constraints on black-hole mass, we cannot know for sure. At the same time, we need a much better theoretical understanding of what makes little red dots so distinct from the more typical growing black holes we have studied for decades, and why that mode of growth becomes so much less common as the universe ages.

One thing we do know for sure: the more we learn about the little red dots, the more complex and unexpected they become. We are excited to see what new wrinkles arise as we enter our fifth year of JWST operations.

All that antimatters in the universe

**Intersections** The interdisciplinary antimatter workshop explored laboratory, astrophysical and cosmological probes of antimatter. Credit: CERN

Applying the Standard Model (SM) to early cosmological times leads to an uninhabitable universe, with tiny and equal amounts of matter and antimatter. Yet the universe is habitable and the local universe strongly matter-dominated. Observations of the diffuse gamma-ray background and cosmic microwave background show no evidence for the presence of antimatter on large scales and rule out a matter–antimatter symmetric universe.

From 19 to 22 January, 80 particle physicists, astronomers and cosmologists gathered at CERN for the first “All that Antimatters in the Universe” workshop to explore the frontier between the laboratory and astrophysical perspectives on the matter–antimatter asymmetry of the universe.

Broad panorama

Julia Harz (Mainz University) reviewed a broad panorama of baryogenesis models in which physics beyond the SM produces a homogeneous matter excess within the first seconds after the Big Bang, before light elements are synthesised. She highlighted their features and potential tests and constraints, including searches at colliders like the LHC and indirectly with experiments such as those looking for neutrinoless double-beta decays.

Questioning our assumptions about antimatter was a central thread of the workshop, with several presentations highlighting non-standard baryogenesis models that allow domains of antimatter to survive the Big Bang, as well as others in which antimatter is hidden in compact nuggets that could also constitute dark matter. A lively discussion explored how to hunt for these scenarios using astrophysical and cosmological observables. For example, spectral distortions of the cosmic microwave background could indicate energy injections from matter–antimatter annihilation in the early universe. Observations at 21 cm-wavelengths offer another probe: these signals trace neutral hydrogen during the cosmic-dawn epoch, when the first stars and galaxies formed, and could reveal anomalous heating or ionisation patterns characteristic of antimatter annihilation.

Questioning assumptions about antimatter was a central thread of the workshop

The discrete symmetries of charge conjugation (C), parity (P) and time reversal (T) have been central to particle physics since the discovery that nature violates them individually, yet their combined action (CPT) appears to be preserved in all standard interactions. In a particularly sharp presentation, Gabriela Barenboim (University of Valencia) stressed that while much attention is devoted to the search for differences in the interactions between particles and antiparticles through CP-symmetry violation, the more fundamental possibility of CPT violation remains largely unexplored. Unlike CP violation, which can occur within the Standard Model, any breakdown of CPT symmetry would signal new physics and could manifest as differences in the intrinsic properties of particles and antiparticles, including their masses and lifetimes.

Leading stress-tests of CPT symmetry are now carried out at CERN’s Antimatter Factory (AF), whose experiments presented an array of impressive results at the workshop. Eric Hunter (CERN) highlighted the potential of boosting the yield of antihydrogen formation at the AF experiments, showing how this could improve our knowledge of antimatter physics enormously. Improved yields of antimatter replicas of naturally occurring matter-based atoms would enable higher precision tests of key electromagnetic transitions and gravitational interactions of antimatter.

Much attention went to antimatter in cosmic rays. Primary cosmic rays are particles accelerated at astrophysical sources such as supernova remnants and injected into the galaxy, whereas secondary cosmic rays are produced when those primaries collide with gas and dust in the interstellar medium. In standard galactic cosmic-ray models, antimatter is purely a secondary product of the interactions of primary cosmic rays with the interstellar medium. However, the AMS-02 experiment operating on the International Space Station has firmly established a positron excess requiring a primary source, possibly pulsars. AMS-02 antiproton data also show some anomalies, but uncertainties in the propagation models and interaction cross-sections remain large.

Mind the GAPS

Complementary searches for cosmic-ray antimatter are also carried out by balloon-borne experiments. Principal investigator Chuck Hailey (Columbia University) described how the GAPS balloon experiment, uniquely suited to probe low-energy antiprotons, antideuterons and antihelium, reported its first data from a 25-day flight completed in early 2026. The specificity of GAPS is the exploitation of the characteristic X-ray emission produced by short-lived bound states between antimatter nuclei and ordinary atoms, which results in excellent particle-identification and background-rejection capabilities.

The atmosphere at the workshop was excellent, with participants curious to learn from other communities and expand their horizons everywhere that antimatter matters in the universe, from the cosmos to the lab, via astrophysical systems. While antimatter still holds many mysteries, All that Antimatters in the Universe brought us one step closer to answering them.

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