Physics Archives – Page 5 of 147

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.

String pilgrimage to Santiago

**Modern methods** Researchers are converging on a common set of tools for string theory, holography and quantum gravity. Credit: IGFAE

One hundred researchers gathered in Santiago de Compostela from 21 to 23 January for Iberian Strings, the annual meeting of the vibrant Spanish and Portuguese string theory community. From the idea that black holes may test quantum gravity to the new, string-inspired ways of organising quantum field theories using symmetries and defects, the programme offered a broad overview of where string theory and holography currently sit. What stood out was the extent to which very different problems are now being tackled with a shared set of theoretical tools.

Black holes remain a clean laboratory for probing ideas about quantum gravity. Decades of work have shown they behave much like ordinary thermodynamic systems, with quantities such as temperature and entropy. A central question is how this simple large-scale behaviour arises from an underlying quantum description. Vijay Balasubramanian (University of Pennsylvania) emphasised that the challenge is not only reproducing the familiar area law – which links entropy to the area of the event horizon – but also understanding what different semiclassical calculations are really describing.

Calculations under control

One way to address this problem is to count the quantum states that give rise to a black hole’s entropy. To make progress, researchers often focus on settings where calculations are under better control. Gabriel Cardoso (IST Lisbon) discussed BPS black holes, highly symmetric solutions that allow precise calculations using holography. Stefano Trezzi (University of Barcelona) showed that near-extremal black holes, systems close to a zero-temperature limit, exhibit a universal near-horizon behaviour that provides a clean setting to study how quantum effects modify the semiclassical picture.

So much for static black holes; what about their evolution in time? Marija Tomašević (CERN) suggested that quantum effects can form a horizon where classical gravity would predict a naked singularity. Pablo A Cano (University of Murcia) and Marina David (KU Leuven) explored instead how black holes react when they are perturbed, emitting gravitational waves as they settle back to equilibrium through a process known as ringdown. Across these contributions, the focus was on separating what can be understood within controlled semiclassical calculations from what requires genuinely microscopic, quantum-gravitational input.

Some particle theories may have been gravity all along. And vice versa. These seemingly disparate worlds, with particle beams and colour confinement in one (particle physics) and curved spacetime in the other (gravity), may simply be two languages for the same physics. To translate between them, the particle side must live in one fewer dimension. Just as a hologram stores a 3D image on a 2D plate, a gravitational theory in D dimensions may be exactly equivalent to a non-gravitational quantum field theory in D–1 dimensions. This holographic correspondence is central to modern approaches to quantum gravity. The focus at the workshop was on its more applied uses, as a controlled way to learn about dynamics at strong coupling.

Elias Kiritsis (University of Crete) provided a concrete example. Using familiar spacetime physics, he studied how strongly interacting quantum systems respond to gentle deformations at low temperature, a standard probe of transport. In this setting, quantum effects can modify quantities such as the ratio of viscosity to entropy density beyond the semiclassical value.

To round the picture, Francesco Nitti (APC Paris), explored holographic models in which varying the curvature of spacetime can affect confinement, while Shota Komatsu (CERN) presented an overview of matrix-model methods in holography, emphasising how they can provide tractable descriptions of strong-coupling dynamics in specific regimes, such as large-N limits. Following ’t Hooft, theorists often treat the number of colours in an SU(N) gauge theory as a tunable parameter, providing a controlled simplification of strongly coupled dynamics.

Black holes remain a clean laboratory for probing ideas about quantum gravity

Working in simplified settings can be an effective way to make progress. In holography, a quantum field theory in two dimensions can map to a three-dimensional spacetime with a negative cosmological constant. Symmetries then constrain the gravity side, allowing us to pose – and sometimes answer – questions that would be far harder to tackle in higher dimensions or less symmetric settings. In this spirit, Stéphane Detournay (Université Libre de Bruxelles) showed how near-extremal black holes themselves can behave like two-dimensional systems, where effects due to thermodynamics, symmetry and quantum corrections can often be disentangled cleanly.

Rapid progress in understanding generalised symmetries and defects was a hot topic. Guillermo Arias-Tamargo (Imperial College London) described how recent work on non-invertible symmetries in non-linear sigma models pushes beyond the traditional picture of symmetries as simple group actions on local fields. In this modern framework, symmetries are realised through extended objects, such as defects or interfaces. Tracking how observables transform across these structures provides concrete constraints on the dynamics and phases of the theory.

A particularly sharp application came from José Calderón Infante (Caltech), who used defect-based arguments to rule out global shift symmetries in quantum gravity. Interfaces also featured prominently as physically meaningful probes, naturally connecting abstract symmetry ideas to concrete quantities such as boundary degrees of freedom and entropy-like measures – as discussed by Carlos Hoyos (Universidad de Oviedo).

The meeting covered a wide range of active topics, but controlled semiclassical arguments, low-dimensional holographic models and defect-based symmetry arguments resurfaced throughout the programme. In that sense, Iberian Strings provided an overview not only of open questions but also of modern methods.

The most elusive higgsinos

ATLAS figure 1 — **Fig. 1.** Observed and expected limits at 95% CL for 1ℓ1T (purple) and displaced track (red) searches are shown in the chargino-neutralino mass difference versus chargino mass plane, along with previous limits from the LEP2 (grey) and ATLAS (blue, light green and orange) experiments. Credit: ATLAS Collab., adapted from arXiv:2511.20042

Supersymmetry has so far eluded discovery at the LHC, yet it retains strong theoretical appeal as an extension of the Standard Model (SM), and potential hiding places remain. In two recent analyses, the ATLAS collaboration sets new bounds on compressed higgsino models, where the proposed particles lie very close in mass. The collaboration used machine-learning techniques to target some of the most elusive signatures at the LHC: low-momentum decay products.

Without extreme fine tuning, quantum corrections would drive the Higgs-boson mass far above the electroweak scale. Supersymmetry prevents this by introducing fermion partners for the SM bosons (and vice versa) so that their quantum contributions naturally cancel. The result is a partner for every SM particle – including higgsinos, the fermionic counterparts of the Higgs field. Higgsinos mix with the partners of the electroweak gauge bosons to form electrically neutral and charged states known as neutralinos (χ̃⁰) and charginos (χ̃^±). The lightest neutralino (χ̃⁰₁) is stable in a wide class of models and may naturally account for the observed dark-matter abundance.

In compressed scenarios, the tiny mass-splitting between these new particles poses a distinct experimental challenge. When a heavier state decays to χ̃⁰₁, the small mass difference leaves little energy for the accompanying SM particles. The visible decay products therefore carry very low momentum and may fall below reconstruction and identification thresholds. The new analyses focus precisely on this regime using the full Run 2 dataset collected at √s = 13 TeV, with two complementary strategies optimised for different values of the mass splitting.

Firstly, a “displaced track” search targets scenarios with a mass difference between the lightest chargino χ̃^±₁ and χ̃⁰₁ of 0.3 to 1 GeV, in which the χ̃^±₁ has a non-negligible lifetime and can travel a few millimetres before decaying into an invisible χ̃⁰₁ and a low-momentum charged pion. The resulting event signature is a pion track with a large transverse impact parameter and high missing transverse momentum from the neutralinos. Significant improvement in signal sensitivity is achieved by the use of two dedicated neural networks (NNs), where one exploits the global event kinematics and the other focuses on the displaced track characteristics.

A “one-lepton-one-track (1ℓ1T)” search instead targets scenarios with a larger mass splitting of 1 to 3 GeV, in which the heavier neutralino χ̃⁰₂ promptly decays into the χ̃⁰₁ and two low-momentum leptons. Since these could elude the existing ATLAS identification techniques, dedicated low-momentum electron and muon identification algorithms have been developed using NNs that exploit track and calorimeter information. The new algorithms are applied to leptons with momentum as low as 0.5 GeV for electrons and 1 GeV for muons, below the standard reconstruction thresholds, resulting in a signature consisting of one lepton and one lepton-like track. An additional NN enhances sensitivity for event classification, exploiting kinematic features that depend strongly on the mass splitting.

The observed data are consistent with the SM predictions, with no signs of new physics emerging in the targeted phase-space. Based on this result, lower limits on the higgsino masses are set at 95% confidence level (CL) (see figure 1). The 1ℓ1T search excludes a mass-splitting region between 0.8 and 2.0 GeV, extending previous limits from the LEP experiments up to a maximum χ̃^±₁ mass of 132 GeV for a 1.8 GeV mass splitting. The displaced track search extends the exclusion limits previously set by the ATLAS experiment by about 30 GeV, reaching a χ̃^±₁ mass of 199 GeV for a 0.6 GeV mass splitting. Together, the two searches exclude χ̃^±₁ masses below 126 GeV at 95% CL over the targeted mass splitting range. Limits set by the ATLAS collaboration now supersede those from the LEP experiments in all mass-splitting ranges.

With this result, ATLAS is now able to set limits over the full range of higgsino mass splittings that are interesting for naturalness, marking a significant milestone in the search for supersymmetry. The new Run 3 dataset, along with advanced analysis techniques, will push these searches even further – perhaps towards the discovery of physics beyond the SM.

Topics

Reset your password

Registration complete