From 18 to 22 May 2026, the 28th annual PLANCK conference – held in conjunction with the 6th EuCAPT Symposium – brought more than 250 theoretical physicists to CERN to confront a set of dilemmas that are, at root, about scale. Where should the hunt for dark matter focus, when its mass might span 90 orders of magnitude? Why is the Higgs mass so tiny compared to the Planck scale, with no signs of a mechanism to stabilise the gap? Can one learn about particle production in the early universe, when evidence is spread thinly over the farthest sky?
Microscopic to galactic
Faced with these puzzles, theorists are seeking answers from systems ranging from the microscopic to the galactic. Jordy de Vries (NIKHEF) discussed novel methods to look for subtle differences in how the strong force affects matter and antimatter using tiny molecular dipole moments. At the opposite end, Kai Schmitz (Münster University) reported on work using the rhythms of pulsars spread across the Milky Way to detect gravitational waves produced in the first moments after the Big Bang.
The framework of effective field theory (EFT) threaded through the conference. By capturing the physics at a given scale and folding higher-scale effects into a handful of parameters, it proves ubiquitously powerful for advancing complex calculations. Giulia Isabella (UCLA) presented new results applying scattering methods – originally developed for particle physics – to calculate gravitational-wave signals from black hole mergers. Mikael Chala (University of Granada) discussed the use of EFTs in calculations of thermal phase transitions, such as those that occurred in the early universe. Anders Eller Thomsen (University of Bern) presented precision calculations of the Standard Model EFT used to predict subtle effects of undiscovered particles with masses beyond the energy range of the LHC.
The electroweak scale, at which electroweak symmetry breaks and particles acquire mass through the Brout–Englert–Higgs mechanism, continues to play a unique role across a variety of fundamental puzzles. The possibility that a strong electroweak phase transition led to a universe filled with matter was discussed in talks by Jorinde van de Vis (CERN), Dave Sutherland (University of Glasgow) and Maria Cristina Fiore (University of Granada), anticipating future tests of this scenario with gravitational waves, Higgs measurements and resonance searches.
A highly varied field, united around big questions
The idea of a “WIMP miracle”, in which the observed abundance of dark matter is due to a weakly interacting particle with mass around the electroweak scale, is one of the most compelling and long-standing dark-matter explanations. Weishuang Linda Xu (Stanford University) presented work interpreting gamma-ray observations of the galactic centre, arguing that this dark-matter framework can be fully tested once the next generation of telescopes comes online.
Many talks presented novel model-building ideas, including some related to the recent burst of activity around models explaining the observed hierarchies in fermion masses, many of which answer other theoretical puzzles. Marta Zamoro (Autonomous University of Madrid) showed how both the mass hierarchies and the strong CP problem could be explained through a model with extra copies of the Standard Model QCD group. Javier Lizana (University of Castilla–La Mancha) and Simone Marciano (University of Valencia) presented different models in which a dark-matter candidate arises within theories predicting hierarchical fermion masses. Each of these predicts novel phenomena, particularly in flavour-changing processes that could be measured at LHCb.
PLANCK 2026 presented a picture of a highly varied field, united around big questions and emerging techniques. Speakers and participants looked towards the new opportunities offered by upcoming measurements, colliders and telescopes, building a theoretical toolkit to interpret their insights across all scales.
The 16th Workshop on Critical Point and Onset of Deconfinement (CPOD26), held at CERN from 13 to 17 April 2026, brought together more than 100 researchers focused on the phase structure of strongly-interacting matter. The workshop continued the long-running CPOD series, which centres on understanding how and when ordinary hadronic matter transforms into quark–gluon plasma (QGP), and whether a critical point exists in the QCD phase diagram.
The series adopts a plenary-only format, to encourage unified discussion across theory and experiment and bridge gaps between different approaches to the same underlying questions. In this same spirit, the programme emphasised clarity and synthesis rather than specialised talks for a narrow audience. Many of the review talks were devoted to emerging subfields, making it easier to connect developments across traditionally separate areas.
Critical point
The central scientific theme was the search for the hypothetical QCD critical point, where the smooth crossover between hadronic matter and the QGP gives way to a genuine first-order transition, deconfining and chiral-restoring at once. Identifying it remains one of the most important open problems in high-energy nuclear physics.
Speakers examined how matter behaves under extreme temperature and density – conditions recreated in relativistic heavy-ion collisions. While lattice QCD provides invaluable results at vanishing net-baryon density, extending these calculations to high net-baryon density remains difficult due to the infamous “sign problem”. Still, new exclusion limits were presented, ruling out parts of the phase diagram as possible locations for the critical point.
On the experimental side, several contributions highlighted results from ongoing heavy-ion programmes, probing a wide range of observables with beam-energy and system-size scans. These efforts are designed to uncover non-trivial structures in the excitation functions of observables that might signal critical behaviour – or the onset of deconfinement. Particular attention was given to event-by-event fluctuations and correlations, as well as to methodology, since signals can be distorted by the dynamical evolution of the collision system.
Central were, of course, the properties of the QGP. Presentations addressed transport coefficients, collective flow, dileptons and the role of fluctuations in the QGP, aiming to understand how such properties vary across different regions of the phase diagram. The connection between early-time dynamics and final-state observables was a recurring theme, with hydrodynamic and transport models playing an important role.
A key takeaway from CPOD26 was the importance of integrating multiple approaches
Understanding hadronisation – the transition from QGP back to hadrons – is essential for linking theoretical descriptions of deconfined matter to experimental measurements. Various models of particle production and freeze-out were compared, with emphasis on how they affect fluctuation observables and potential signatures of critical phenomena. Subtler effects, such as possible indications of isospin-symmetry breaking, were also noted.
The workshop’s scope extended to dense QCD matter in astrophysical environments, particularly in neutron stars, with observations from modern astrophysics providing complementary constraints on the QCD equation of state at high density. This interdisciplinary perspective highlighted how insights from astrophysics and heavy-ion collisions can inform each other, contributing to a more complete picture of the phase diagram.
Looking to the future, several talks focused on new experimental facilities and detector upgrades. Speakers discussed developments at the FAIR complex at GSI – home to the CBM and HADES experiments – and at CERN, including the proposed new fixed-target experiment NA60+/DICE, the upgrade of NA61/SHINE and the next-generation ALICE 3 detector for the HL-LHC. The possibility of a fixed-target experiment at the Electron–Ion Collider (EIC) was also discussed. These efforts aim to improve sensitivity to rare signals and extend coverage into previously unexplored regions of the phase diagram.
Intriguing hints
A key takeaway from CPOD26 was the importance of integrating multiple approaches, especially given the uncertainty about the existence and location of the critical point. Experimental hints are intriguing but not yet conclusive, and theoretical predictions still face challenges in achieving quantitative agreement with data. The need for new, precise measurements at collision energies between 3 and 10 GeV was stressed throughout the workshop.
By bringing together diverse perspectives in a focused and collaborative setting, CPOD26 highlighted both how far the community has progressed in refining its tools and how much remains to be understood about the fundamental behaviour of strongly interacting matter. The next workshop is planned in Lanzhou (China), on 16–20 August 2027.
From 27 to 29 May 2026, LoopFest returned to Brookhaven National Laboratory, where the conference series started in 2002. Since then, LoopFest has been held each year across universities and national laboratories in the US and, more recently, Canada. With around 60 participants, this year’s edition provided a forum for discussing the latest results in precision quantum field theory (QFT), along with their applications to interpreting current experimental results and anticipating the precision reach of future experiments, primarily in collider physics. Its cousin event, the Loops and Legs symposium, was hosted last April in Bayreuth, Germany.
Talks highlighted impressive developments in multi-loop and multi-leg calculations, in matching fixed-order higher-order calculations with multi-purpose parton-shower event generators, and in the evaluation of parton distribution functions for precision measurements. Areas central to the precision-physics programme of the LHC, such as differential measurements of Higgs-boson and top-quark processes, received particular attention, while dedicated talks turned to future e+e– machines. There, the main challenge will be for theory to equal the extreme experimental accuracy expected for electroweak precision observables.
Exquisite control
Matching the precision of the High-Luminosity LHC and future e+e– colliders is vital for future discoveries. This endeavour will require exquisite control of theoretical predictions, at the (sub)percent and permille level, respectively, and already poses issues that are both technical and conceptual. Most importantly, aiming for extreme precision could not only enable the indirect discovery of new physics, but also lead to new insights into the fundamental structures of quantum field theory. Several talks stressed this point, some reviewing cutting-edge studies of amplitudes in N = 4 super Yang–Mills theories, others the impact of recent advances in quantum amplitudes for gravitational waves from black-hole scattering.
If anomalies emerge from precision studies, their interpretation will be equally challenging. Multiple contributions traced the state of the art of Standard Model effective field theory (SMEFT) calculations. SMEFT extends the Standard Model with a series of higher-dimensional operators built from its own fields, so that the imprint of unknown heavy particles is captured by small shifts in couplings. It is a common way to explore physics beyond the Standard Model under very general assumptions.
A dedicated effort to provide the dimension-6 SMEFT Lagrangian at the first non-trivial order of quantum corrections is reaching completion and will become a fundamental building block for future studies. Results from a global fit of the SMEFT Lagrangian – including, for the first time, a broad spectrum of electroweak, Higgs-boson, top-quark, Drell-Yan, di-boson and flavour observables – were presented as a proof of concept of how the SMEFT could help explore models of new physics.
New ways to apply artificial intelligence/machine learning and quantum information science have opened up
Finally, the focus shifted to how the groundbreaking ideas that have transformed the theoretical calculation of scattering amplitudes have also opened new ways to apply artificial intelligence/machine learning (AI/ML) and quantum information (QI) science. This is creating new opportunities to rethink how loop calculations are performed, potentially overcoming longstanding computational bottlenecks and enabling qualitatively new approaches and conceptual frameworks. For instance, recent work has explored the application of reinforcement learning, supervised regression and self-supervised learning to some of these problems.
At the same time, generative models – including normalising flows and diffusion-inspired methods – have already demonstrated impressive gains in Monte Carlo integration and importance sampling, the random-sampling methods used to evaluate the high-dimensional integrals underlying cross-section predictions. Since precision QFT calculations are quantum-mechanical at heart, a genuinely quantum computation of precision scattering opens the door to synergies with QI studies of collider processes. Some talks asked how far a future quantum computer could help study QFT processes from a QI perspective, to best exploit the precision data produced by colliders like the LHC.
Extended reach
Overall, LoopFest conveyed the impressive theoretical progress achieved in recent years, enabling the exploration of new physics through precision measurements at current and future experiments. Ground-breaking methods developed during the last two decades have progressed rapidly and are now entering a mature phase, providing a robust framework within which future technical and conceptual challenges can be identified and addressed. Combined with new emergent technologies such as AI/ML and QI, this could extend the reach of current precision-physics programmes far beyond expectations.
Cosmological filaments form the backbone of the cosmic web, the vast, interconnected network that defines the universe on the largest scales. Stretching across tens to hundreds of millions of light-years, they link galaxies and galaxy clusters along the pathways where matter assembles under gravity. They may also hold the key to one of the deepest questions in modern physics: the nature of dark matter.
For astrophysicists, filaments first drew attention as a potential reservoir of missing baryons. Big Bang nucleosynthesis and precision measurements of the cosmic microwave background agree on how much ordinary matter the universe should hold, but the census of stars, galaxies and hot gas comes up short. The leading explanation is a warm, diffuse gas permeating cosmic filaments, too faint to detect in any single observation but increasingly accessible through statistical techniques at X-ray and radio wavelengths.
More recently, dark-matter hunters have begun to recognise the potential of filaments as probes of new physics. Filaments are not only vast but overwhelmingly dark-matter-dominated, with lower astrophysical backgrounds than traditional search targets such as the galactic centre. New simulations are pinning down their dark-matter density profiles with enough precision to make quantitative predictions, and recent theoretical work has opened detection channels that could turn these structures into laboratories for physics beyond the Standard Model.
Dark matter and the cosmic web
Our scientific understanding of stars and the structures they inhabit has grown remarkably over the past century. We now know that galaxies are vast collections of stars, and clusters are collections of galaxies. These immense systems do not float randomly; they are woven into an intricate “cosmic web” resembling that of a spider. Gravity shapes this web and governs the motion of the celestial bodies within it. Yet many observations defy expectations. Galaxies rotate too quickly, clusters bend light too strongly, and the cosmic web holds together with more gravitational pull than visible matter would allow. Something unseen must be at work. A new, invisible “dark matter” component must dominate the mass of the universe.
Dark-matter candidates in the spotlight
Dark matter accounts for roughly 85% of the matter in the cosmos, and about 27% of its content once dark energy is included, yet its nature remains unknown. Several well-motivated candidates have emerged, each predicting distinct signatures that indirect searches, including those targeting cosmic filaments, could probe. Weakly interacting massive particles, sterile neutrinos, primordial black holes and axions are among the most prominent.
Weakly interacting massive particles These hypothetical particles naturally arise in several extensions of the Standard Model and possess two defining features: they are massive, and they interact only through gravity and the weak force.
Sterile neutrinos Unlike the three known active neutrino species, they do not interact through the weak force. Their existence is motivated by extensions of the Standard Model that aim to explain both neutrino masses and the matter–antimatter asymmetry of the universe.
Primordial black holes Unlike stellar black holes, which form from collapsing stars, primordial black holes are hypothetical relics of the early universe, born from the collapse of exceptionally dense regions of matter moments after the Big Bang.
Axions Originally proposed to solve the strong CP problem, axions are hypothetical particles whose production mechanism can account for the observed dark-matter abundance, elegantly linking two of modern physics’ greatest mysteries.
Dark matter accounts for roughly 85% of the matter in the cosmos and dictates how cosmic structures form and evolve. Yet, despite decades of international effort and extraordinary experimental ingenuity, its nature remains a puzzle. The Standard Model of particle physics, describing all known fundamental particles, can’t account for the observational effects of dark matter. In response, theorists have proposed a wide range of models that include dark-matter candidates (see “Dark-matter candidates in the spotlight” panel). A well-motivated dark-matter theory, one that truly excites theorists, typically meets three criteria. First, it accounts for the observed cosmic abundance of dark matter. Second, it yields clear, testable predictions. And third, it resolves multiple open questions in fundamental physics.
Rich landscape
While the theoretical landscape is rich, testing it requires identifying cosmic environments where dark matter’s signatures might be detectable. One of the most powerful strategies is indirect detection – the search for faint cosmic messengers produced when dark matter annihilates, decays or interacts with ordinary matter. These signatures may appear as electromagnetic waves, neutrinos or charged cosmic rays. Observing these messengers requires high sensitivity and careful modelling of both the dark-matter signal and the astrophysical backgrounds. Progress, therefore, depends on close collaboration between particle physicists, astrophysicists and cosmologists, integrating theoretical predictions with multi-messenger observations.
Choosing optimal targets is crucial for indirect dark-matter searches. Traditional efforts have focused on the galactic centre and on dwarf satellite galaxies of the Milky Way. The galactic centre is expected to host the highest dark-matter density, but it also contains intense and complex astrophysical backgrounds, which is why the origin of a long-debated gamma-ray excess observed by Fermi-LAT remains uncertain (see “Gamma-ray excess” figure). Dwarf galaxies, by contrast, are dark-matter-dominated and relatively free of astrophysical emission. However, their stellar populations are orders of magnitude smaller than that of the Milky Way. This limits the available kinematic tracers – observables whose spatial distribution correlates with the underlying matter density field – and leads to sizable uncertainties in the predicted signals.
Unconventional environments
Recently, unconventional but promising probes have gained attention, such as cosmological filaments. Filaments are a natural outcome of anisotropic gravitational collapse in an expanding universe. Matter can collapse under gravity in some directions while still expanding in others, producing elongated structures that are bound across their width but continue to grow along their length. Not all cosmic filaments are alike. Some lie within galaxy clusters, linking individual galaxies over relatively short distances. Others extend far beyond cluster boundaries, forming vast inter-cluster bridges that connect galaxy clusters and even superclusters across tens and hundreds of megaparsecs. The longer the filament, the thinner and more diffuse it tends to be. This reflects the way gravity draws matter out of underdense regions and funnels it into elongated bridges between massive nodes.
Together, galaxy clusters and the diffuse filaments that connect them form the cosmic web and make up most of the baryonic matter. Yet the very properties that make filaments so fundamental to cosmic structure also make them extraordinarily difficult to observe. Their emission is faint, diffuse and easily overwhelmed by brighter astrophysical sources, posing a major challenge for direct detection across the electromagnetic spectrum.
To overcome this limitation, astronomers have turned to a statistical technique known as “image stacking”. In stacking analyses, many observations of similar systems are superimposed. Any emission associated with filaments then adds coherently, while random noise and unrelated astrophysical signals average away. The result is a significant enhancement in sensitivity, allowing extremely weak, extended emission to emerge that otherwise would remain invisible.
A potent technique
The power of this approach relies on numbers: the larger the sample that can be stacked, the stronger and more reliable the resulting signal. Image stacking is therefore a potent but data-hungry technique, one that becomes increasingly effective as modern surveys deliver ever-larger datasets. This requirement poses a particular challenge for filaments, whose precise locations are generally unknown. Since cosmological filaments connect massive structures, a natural strategy is to use galaxy clusters as signposts: by stacking observations of regions between pairs of clusters, the faint emission from the filamentary bridges that link them can be statistically enhanced.
Cluster catalogues have expanded dramatically over the past decade. Today, surveys based on optical imaging, weak gravitational lensing and the Sunyaev–Zel’dovich effect, in which scattering with high-energy electrons distorts the cosmic microwave background, collectively identify tens of thousands of clusters across the sky. While progress is remarkable, it may still fall short of what is needed to robustly detect the extremely faint emission expected from typical filaments. This limitation motivates the search for alternative tracers. A reliable proxy for galaxy clusters available in far greater numbers, potentially in the millions, would enormously increase the statistical power of stacking analyses.
Particularly effective proxies for galaxy clusters are luminous red galaxies (LRGs). These massive, early-type galaxies have been observed and catalogued for decades and are known to be excellent tracers of the large-scale structure of the universe. LRGs typically reside in, or near, the centres of galaxy clusters, making them reliable signposts of the densest regions of the cosmic web. Pairs of LRGs that are close to one another in the sky and in physical distance can therefore be used as proxies for nearby cluster pairs. Statistically, such pairs are likely to be connected by inter-cluster bridges or filaments, even if the filaments themselves cannot be directly identified in individual observations.
By applying this stacking technique to pairs of LRGs drawn from the Sloan Digital Sky Survey, whose catalogues contain millions of such galaxies, together with radio maps from the GLEAM and OVRO-LWA surveys, researchers have identified an intriguing anomaly. The radio emission associated with stacked filaments (see “Stacked maps” figure) exceeds theoretical predictions for diffuse filamentary gas by more than an order of magnitude.
One possible interpretation is that this excess arises from secondary radiation produced by dark matter (see “Simulations, observations and theory” figure). In this scenario, weakly interacting massive particles with masses of a few GeV decay into electrons, which then spiral through filament magnetic fields and emit synchrotron radiation at radio wavelengths. For the magnetic field strengths inferred in the stacking analysis, the amplitude of the observed signal is consistent with that expected from a dark-matter flux of this kind.
As with other anomalies, this interpretation remains debated. A more conventional explanation attributes the emission to astrophysical particle acceleration in strong accretion shocks, generated as matter falls into filaments and galaxy clusters. While shocks can in principle produce radio synchrotron emission, reproducing the observed excess appears to require acceleration efficiencies higher than those typically assumed in simulations. Significant uncertainties persist in filament properties, such as their magnetic field strengths and shock characteristics, which complicate the modelling of expected signals and remain an active area of research.
Cosmic filaments may also open a window onto more exotic dark-matter scenarios. Recent work has shown that if heavy dark matter decays into gravitons – the hypothetical quantum carriers of the gravitational interaction – these can convert into photons via the Gertsenshtein effect (see “Graviton-to-photon” figure), closely analogous to the Primakoff conversion of axions, as they propagate through the large-scale magnetic fields threading filaments. This process generates an irreducible extragalactic gamma-ray background, allowing such scenarios to be constrained with Fermi-LAT data and offering promising sensitivity for future gamma-ray observatories.
A bright future for the dark universe
For millennia, humanity has been inspired by the starry sky. Philosophers, poets and scientists alike have gazed upward, their minds filled with questions, joy and awe. Dante, one of Italy’s greatest poets, expressed this enduring fascination in the closing line of Inferno in The Divine Comedy:
Centuries after Dante, the sky continues to guard many of its secrets. However, we are now entering a golden era for indirect dark-matter searches. Future facilities, most notably the Square Kilometre Array (SKA), currently under construction in South Africa and Australia, will deliver unprecedented sensitivity to the diffuse structures of the cosmic web, and may soon be capable of directly imaging large filaments, characterising their properties and turning these vast structures into powerful probes of physics beyond the Standard Model.
These observational advances are being matched by progress on the theoretical front. Cosmological simulations are reaching new levels of realism, while the growing use of machine-learning and artificial-intelligence techniques is beginning to transform how filamentary structures are identified, modelled and interpreted. These developments promise a far more precise characterisation of filament properties, sharpening their role as laboratories for fundamental physics. The cosmic web may not keep its secrets much longer.
Born of one kind, a neutrino can die another. Its three flavours, electron, muon and tau, do not correspond to states of definite mass, but to quantum superpositions of three distinct masses. As neutrinos propagate, the mixture reshuffles and the flavour at arrival can differ from the one at production. None of this is predicted by the Standard Model, making the observation of neutrino oscillations one of the clearest signals of physics beyond it.
Neutrino oscillations provide a unique probe of new physics, acting as an interferometer that is sensitive to neutrino mass differences down to the sub-eV level. Precise measurements at the next-generation accelerator-based oscillation experiments, Hyper-Kamiokande in Japan and DUNE in the US, are poised to answer several critical questions. What is the ordering of the three neutrino masses, given their two measured mass-squared differences? Do neutrinos and antineutrinos oscillate differently? Are there additional, as yet undetected, neutrino states? These long-baseline neutrino facilities, in which a beam of neutrinos is sent to a detector hundreds of kilometres away, will produce much larger datasets than current-generation experiments. However, they suffer from a fundamental limitation: we do not know the precise energy or intensity of the neutrino beams when they set out. Reaching ultimate precision on neutrino-oscillation parameters is therefore no longer a matter of statistics, but one of messy nuclear-physics questions related to the details of weak-interaction cross sections and proton-induced hadron production.
Modern neutrino beams use the famous “magnetic horn” design, developed by Simon van der Meer at CERN in 1961 (see “In focus” image). Protons strike a target to produce pions, which the horn focuses into a volume for them to decay to neutrinos and leptons. The trouble is, the resulting neutrino beam covers a wide range of energies (about 0.5 GeV and 2.5 GeV for Hyper-K and DUNE, respectively), with a shape and intensity that depend on the details of tough-to-model proton–nucleus collisions. To make matters worse, the broadband neutrino flux forces the neutrino energy to be estimated from the products of neutrino–nucleus interactions, which are notoriously difficult to model accurately. Together, these challenges form a barrier to ultimate precision in neutrino-oscillation measurements.
There is, in principle, a way around both problems: neutrino tagging. Proposed by Bruno Pontecorvo in 1979, this technique associates a measurement of the four momenta of the pion and muon in a π+→ μ+νμ decay with a measurement of a neutrino in a downstream detector. Four-momentum conservation then fixes the neutrino kinematics event-by-event. As a result, the neutrino energy is known for each interaction and the flux is perfectly constrained, nullifying the key challenges for neutrino-oscillation experiments and producing well-controlled muon, pion and kaon beams as byproducts. The idea was first attempted in the 1990s at the dedicated Tagged Neutrino Facility (TNF) at the Serpukhov accelerator in Protvino (see “Dream, deferred” image). TNF recorded two candidate events in its brief pilot run, before the dissolution of the Soviet Union brought the work to a halt.
The downside with neutrino tagging is one of scale: for every neutrino seen in a massive, 100-tonne detector close to the beam, there are about 1013 pion decays. To collect a reasonable 105 neutrino interactions per year, one would therefore need to identify at least 1011 individual muons per second and successfully identify the minute fraction of them that are associated with observed neutrinos. Such a measurement demands beamline detectors with timing resolutions of 10 to 100 ps and a neutrino detector with sub-ns timing resolution, with the beamline detectors operating in a high-radiation environment.
Promising performance
These challenges proved too much for the 1990s, and the idea lay dormant for three decades after the closure of TNF. However, a revolution in detector and electronics technology has since changed what is possible (see “The fast-timing revolution” panel) – beginning with the NA62 experiment at CERN.
The fast-timing revolution
R&D for the high-luminosity phase of the LHC are expected to push fast-timing sensors beyond the performance of NA62’s GigaTracker. With bunches crossing every 25 ns and up to 200 proton collisions in each, the ATLAS and CMS upgrades require timing resolutions below 50 ps to disentangle overlapping vertices. In a silicon detector, incoming particles release small electric charges, with internal electric fields then steering them toward electrodes to be collected and measured. Faster timing demands more charge, a shorter drift for the signal to form quickly and smaller collection electrodes for sharper and stronger pulses. In conventional planar sensors, these requirements are often in conflict with one another. Since the collected charge is proportional to the sensor thickness, which also sets the drift distance, thinner sensors give faster signals but fewer carriers. Potential solutions span a wide range of architectures. Low-gain avalanche detectors (LGADs), for instance, combine thin, 50 μm sensors with a gain layer to amplify charge, compensating for the lower number of carriers produced in a thin substrate while preserving a short drift. They achieve resolutions of 20 to 50 ps and will equip the new ATLAS and CMS timing layers.
Many roads to fast timing
For the upgrade of the LHCb vertex locator, which must resist radiation levels of 1016 to 1017 1 MeV neutron equivalents per cm2 (neq/cm2), a gain layer would erode too quickly. Three-dimensional sensors sidestep the problem geometrically. Their electrodes run along the sides of each pixel rather than on the top and bottom surfaces, so charges move sideways over distances below 50 μm while the sensor remains thick enough to generate large signals. Initially developed for radiation-hard pixel detectors for ATLAS and CMS, these sensors were later redesigned for timing and have reached resolutions as good as 10 ps. The remaining challenge lies in the electronics. Readout chips must match the sensors’ speed and radiation tolerance, and recent prototypes in 28 nm CMOS have achieved 30 ps resolution over areas of a few mm2. Large-area designs are currently underway.
Depleted monolithic active pixel sensors integrate the sensor and readout electronics on the same chip. With resolutions of 10 to 200 ps and pixels smaller than 100 × 100 μm2, they are much cheaper than three-dimensional sensors and LGADs, and therefore better suited to instrumenting large surfaces and achieving a lower material budget. Some R&D initiatives are also exploring the use of Cherenkov radiation to detect charged particles, a process much faster than ionisation in silicon. In these detectors, the prompt Cherenkov light is converted into photoelectrons and amplified in a thin gaseous detector, producing signals with time resolutions of a few tens of picoseconds. While highly promising, extending this approach to finely segmented detectors operating at very high rates remains an open problem. All these new tracking technologies offer promising perspectives beyond high-energy physics, for example in real-time monitoring of the proton and ion beams used in cancer therapy.
In order to study the very rare kaon decay K+→ π+νν, the NA62 collaboration faced a similar timing challenge in the late 2010s. At the time, pixel detectors, mainly developed for experiments at the LHC, were only recording the position of particles, their time being given by the proton bunch crossing (50–25 ns). New R&D was started to face the challenge of integrating timing capabilities into every pixel (of which there are more than one thousand per cm2). Within a few years, the “TDCPix” chip was designed (see “Pixel timekeeper” image), achieving a hit time resolution of 130 ps and starting a new field of 4D tracking (measuring particle trajectories in space and time). The price to pay for this performance was a significant increase in the power density absorbed by the pixel, exceeding 2W/cm2. Absorbing this power required developing an innovative cooling technology: a 200 μm-thin silicon plate integrating a dense microfluidic cooling circuit. These two innovations led to the GigaTracker beam spectrometer (see “Fourth dimension” figure) – the first 4D tracking detector in high-energy physics, which has been in operation since 2015.
With the GigaTracker in hand, the NA62 collaboration achieved the main goal of measuring the K → πνν decay, and was able to put neutrino tagging to the test. The facility’s high-intensity kaon beam also serves as a neutrino source, since the kaons predominantly decay as K+→ μ+ν. Due to the intensity of the neutrino beam and its mean energy of 40 GeV, a non-negligible number of neutrinos interact in the experiment’s electromagnetic calorimeter, a 20-tonne volume of liquid krypton. The analysis of data collected in 2022 revealed one neutrino interaction candidate that could be matched to a detected parent decay. The neutrino’s energy was estimated to be 52 GeV, with a record relative precision of 0.3%. For reference, with a few exceptions (such as pion and kaon decays at rest), neutrino energies from conventional neutrino beams are known with an uncertainty of at least 10%, and not event-by-event.
NA62’s proof-of-concept for neutrino tagging, combined with the broader advance of fast-timing detectors, have together made it possible to revisit the original TNF idea. Developed in parallel with the first tagged-neutrino analysis by NA62, the NuTag collaboration investigated the conditions under which a tagged beam would enable measurements inaccessible with conventional neutrino beams. These efforts have led to the proposed nuSCOPE facility at CERN, which emerged within CERN’s Physics Beyond Colliders study group by combining the tagged-beam concept from NuTag with the slow-extraction-driven monitored neutrino beam pioneered by the ENUBET collaboration. Rather than using a pulsed magnetic horn, the ENUBET setup relies on slow extraction from the SPS and lines the decay tunnel with particle detectors that identify the charged leptons produced with neutrinos, constraining the flux at the percent level.
Legacy measurements
The idea of nuSCOPE echoes that of TNF (see “Beam to neutrino” figure). The first step is to direct a slow-extracted proton beam from the SPS onto a target to produce secondary pions and kaons that are then momentum-selected, using a series of dipoles and quadrupoles, to form an 8.5 GeV meson beam with a narrow momentum range. The mesons then traverse a set of ultra-fast detectors before decaying to predominantly muons and neutrinos. The muons reach a second set of fast detectors, whilst a few neutrinos interact in a dedicated detector 25 metres downstream. With sufficient timing and spatial resolution, each neutrino interaction can be associated with a measured individual meson decay. For the first time, the energy of the incoming neutrino would be known at the sub-percent level on an event-by-event basis. Measurements of neutrino cross sections, currently the dominant source of systematic uncertainty projected for DUNE and Hyper-Kamiokande, may then reach an accuracy of about 1%. Such datasets could serve as reference, “legacy” measurements for neutrino physics for decades to come.
The implications extend well beyond standard oscillation physics. Short-baseline oscillations induced by sterile neutrinos, for instance, could produce rapid patterns that would get washed out by energy smearing in conventional beams. The facility would also deliver intense, well-characterised muon and pion beams, opening additional avenues for rare process searches and precision measurements. Looking further ahead, one can even imagine how such techniques might reshape future long-baseline experiments. Depending on what DUNE, Hyper-Kamiokande and the reactor-based JUNO experiment in China will discover, the next leap in precision may not come just from higher intensities, but from beams whose properties are known with exquisite accuracy.
The top quark was never meant to bind. And yet a year ago, CMS reported an excess of top-quark–antiquark pairs in dilepton events near the production threshold, consistent with the fleeting formation of a top-quark–antiquark quasi-bound state: toponium. ATLAS confirmed the effect just a few months later, rejecting a pure perturbative QCD interpretation at 7.7σ (CERN Courier September/October 2025 p9). CMS has now extended the case to an independent decay process.
The analysis, presented at this year’s Rencontres de Moriond, looks at events in which one top decays into a charged lepton, a neutrino and a bottom quark, and the other into jets. In 138 fb–1 of Run 2 data at 13 TeV, the enhancement exceeds the pure-QCD prediction by more than five standard deviations, with an excess cross section of 5.1 ± 0.9 pb.
“Establishing a signal in both channels was very important,” says Regina Demina, who leads the University of Rochester CMS group. “The lepton + jets channel has higher statistics, thanks to the larger hadronic branching ratio of the W boson, and a single neutrino makes the kinematics easier to reconstruct. The systematic uncertainties differ from those in the dilepton channel.”
The charm and bottom quarks live long enough to bind tightly with their antiparticles, and the resulting mesons appear as sharp, narrow peaks in the cross section. The top quark, by contrast, decays too quickly, with a width comparable to the binding energy that would hold a top-quark–antiquark system together. Any such state would manifest as a broad threshold enhancement, smeared over the smooth QCD continuum.
“The formation of bound states of charm or bottom quarks is a well-established effect, which allowed theorists to refine our understanding of the QCD binding potential,” says Yu-Heng Yu, a graduate student at the University of Rochester who worked on the analysis. “Yet it came as a surprise that, given the very short lifetime of top quarks, such a quasi-bound state still manages to form in a small fraction of events.”
The lepton + jets channel demanded two methodological adjustments. The first replaces the invariant mass of the top-quark–antiquark pair, whose resolution is limited near threshold, with their relative velocity as the discriminating observable. “If they form a bound state, the relative velocity should be much smaller than when they are produced independently,” says Otto Hindrichs, also at Rochester. The second concerns the parity-sensitive observables that distinguish a pseudoscalar from a scalar interpretation of the bump. “These variables require a reconstruction method that identifies the down-type jet from the hadronic W decay,” explains Hindrichs. “To achieve this, we developed a machine-learning technique that improves the correct identification of the top-quark decay products.”
Some puzzles remain. The 5.1 pb cross section sits below the 8.8 pb measured in dilepton events, and the non-relativistic QCD reference of about 6.4 pb. “We do observe somewhat different signal strengths in the lepton + jets and dilepton channels, and we are actively investigating this difference,” says Yu.
“With the current sensitivity, interpretations beyond the Standard Model cannot be excluded,” Hindrichs adds. “A pseudoscalar heavy Higgs decaying into top-quark pairs would interfere strongly with the continuum, creating a characteristic peak-dip structure in the invariant tt mass. With enough statistics, this feature could be used to differentiate it from a quasi-bound state.”
The top-quark–antiquark threshold enhancement in e+e– collisions was analysed by Fadin and Khoze in 1987, and extended to hadron colliders by Fadin, Khoze and Sjöstrand in 1990, before the 1995 discovery of the top quark at Fermilab. The standard assumption was that any signal would have to wait for a next-generation e+e– collider reaching the threshold, which would provide the cleanest measurement of the top-quark mass. “Even with Run 3 data, we will not be able to resolve the structure of the threshold region itself, because the resolution on both the invariant mass and the relative velocity is too limited,” says Demina. “But the increased statistics should allow us to probe the spin-parity content of the bump, testing whether it carries scalar or vector contributions alongside the pseudoscalar one.”
On 6 February 2026, beams of oxygen ions circulated through the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory for the last time. A leading facility in the study of hadronic matter and the strong force since 2000, RHIC now hands its tunnel and many of its components to its successor, the Electron–Ion Collider (EIC).
“Experiencing the challenges of first trying to get beams to circulate during commissioning in the fall of 1999, one could not have dreamed how far the performance of this machine would come,” said Wolfram Fischer, chair of Brookhaven’s Collider-Accelerator Department. “We’ve pushed well beyond the original design in terms of the number of collisions we can produce, the energy range of those collisions, the variety of ions we’ve collided, and our ability to align the spins of protons and maintain a high degree of this alignment or polarisation.”
RHIC was conceived above all to study the quark–gluon plasma (QGP). In QGP, quarks and gluons, normally confined inside protons and neutrons, roam free under extreme temperature and density. The early universe is thought to have existed in this state for a fraction of a second after the Big Bang, before cooling into the ordinary matter around us.
Theorists had expected this primordial soup to behave as a weakly coupled gas of quarks and gluons. Gold–ion collision data from RHIC’s four original detectors, BRAHMS, PHENIX, PHOBOS and STAR, found instead a strongly coupled liquid. By 2005 the collaborations had concluded that they were producing one of the lowest-viscosity substances ever observed, a nearly “perfect” liquid. Later runs traced how this extreme state of matter swirls, flows and cools, and revealed that even small collision systems can briefly form tiny droplets, overturning earlier ideas about how QGP forms.
RHIC transformed nuclear physics by demonstrating the remarkable consequences of ‘boiling the vacuum’
“RHIC transformed nuclear physics by demonstrating the remarkable consequences of ‘boiling the vacuum,’” said theorist Raju Venugopalan, paraphrasing T D Lee’s description of matter governed by quantum chromodynamics.
Beyond QGP, STAR and PHENIX measurements in polarised proton–proton collisions established that gluons carry a significant share of the proton’s spin. In the final run, sPHENIX, the faster successor to PHENIX, became the first detector to record a continuous streaming dataset from RHIC’s spin-polarised proton collisions – thus eliminating the need for triggers.
The final run also gave a sense of the scale of modern physics data: sPHENIX alone recorded more than 200 petabytes of raw data, more than every previous RHIC dataset combined, including 40 billion gold–ion collision events. Analysis of RHIC data will continue for at least another decade. Much of RHIC’s infrastructure will then live on in the EIC, including its ion sources, pre-accelerator chain and one of its superconducting storage rings. A new electron ring will share the tunnel, crossing the ion beam at points where polarised electrons and ions will collide. The EIC will enable precision measurements that reveal how quarks and gluons are organised within protons or atomic nuclei, helping physicists to understand how mass, spin, and nuclear structure emerge from the strong force.
Less than one in 10 billion positively-charged kaons decay into a pion and a neutrino–antineutrino pair. The NA62 experiment has now measured the rate of this rare process with an uncertainty 40% smaller than its previous result and a central value closer to the Standard Model (SM) prediction (CERN Courier November/December 2024 p11).
“The K+→π+νν decay is a golden mode of flavour physics,” says NA62 spokesperson Giuseppe Ruggiero. “It is highly suppressed in the SM, but its branching ratio can be predicted to better than 10% precision. The decay is also highly sensitive to new physics, with many models predicting dramatic changes to the branching ratio. Such modifications may come from indirect effects of new physics at or above the 100 TeV scale.”
The scarcity of the decay called for a kaon factory. At NA62, a high-intensity proton beam from the Super Proton Synchrotron strikes a beryllium target, producing around 500 million secondary particles per second. About 6% are positively charged kaons. From that flux, the experiment must isolate the signal against backgrounds many orders of magnitude larger. The first 5σ observation, on data collected through 2022, was reported in 2024. The branching ratio came out at (13.0+3.3–3.0) × 10–11, consistent within 1.7σ with the SM prediction of around 8 × 10–11, despite a central value about 50% higher. Two years of additional data have now doubled the signal sample, and the central value has come down to (9.6+1.9–1.8) × 10–11, reaching a sub-20% precision.
Two new machine-learning techniques drove the increase in precision. “Reconstructing beam particles in the harsh environment of up to a gigahertz of incoming particles is challenging,” says Joel Swallow of CERN, lead data analyst of the study. “To tackle this, we deployed a transformer encoder to pick out a kaon as it enters the experiment. Meanwhile, a combined convolutional and feed-forward neural network was developed for pion identification, which effectively uses images of the energy deposits in the calorimeters to more efficiently and accurately identify pions.”
Two new machine-learning techniques drove the increase in precision
“Had the central value stayed where it was, the precision of the new measurement would have been sensitive to a 3σ excess,” says Ruggiero. “If there had been an excess that large, this measurement was perfectly positioned to find it. Evidently, nature is a bit more subtle.”
The new result tightens constraints on beyond-SM scenarios that would have predicted larger branching ratios, including those involving leptoquarks or heavy Z′ bosons. Still, the dominant uncertainty remains statistical, and additional data from 2025 and 2026 will improve the precision further.
The neutral counterpart, KL→ π0νν, has yet to be observed. The current upper limit on its branching ratio, set by Japan’s KOTO experiment at J-PARC, sits two orders of magnitude above the SM prediction. “Measuring both the charged and neutral modes is important,” says Ruggiero. “Together, they enable a fully independent reconstruction of the unitarity triangle from kaon decays alone. Even if, in the end, the charged mode is consistent with the SM, it does not rule out significant enhancements from new physics to the neutral mode.” The proposed KOTO-II, at J-PARC, is targeting a measurement of KL→ π0νν in the 2030s.
The galactic centre (GC) is one of the most extreme places we know – a dense stellar cluster filled with turbulent plasma, orbiting the four-million-solar-mass black hole Sagittarius A* (Sgr A*). For decades, astronomers have expected this region to host a rich population of pulsars. Yet only a handful have been detected, and none within a parsec of Sgr A*. A deep survey with the Green Bank Telescope, part of the Breakthrough Listen (BL) programme, has now delivered both a stringent non-detection of the expected population and an intriguing millisecond pulsar candidate near Sgr A*.
Pulsars are rapidly rotating, highly magnetised neutron stars, whose periodic radio emission sweeps across Earth like a cosmic lighthouse. Their stable periods make them among the most precise clocks in nature. Ever since Jocelyn Bell Burnell’s 1967 detection of the B1919+21 pulsar, more than three thousand have been catalogued in our galaxy.
Many should populate the GC. The region hosts a dense concentration of massive stars that evolve and die in supernovae, leaving behind neutron stars. Population-synthesis models estimate the number of pulsars within the central parsec at hundreds, perhaps thousands. Moreover, the 2013 discovery of a magnetar (J1745-2900) just arcseconds from Sgr A* confirmed that neutron stars can survive, and be detected, in this environment.
Delving deep
Why, then, are they so elusive? Radio pulses are scattered by clumps of ionised gas along the line of sight, blurring them in time. The effect is severe everywhere, but worse near the dense GC, where it can stretch millisecond pulses to seconds at standard observing frequencies. Higher frequencies are scattered far less, and so pass through more cleanly. The BL GC survey took advantage of this, focusing on high radio frequencies of 8–12 GHz, well above the band typically used for pulsar searches. The observations total more than 20 hours between 2021 and 2023, with 11 hours on the innermost 1.4 arcminutes around Sgr A*. The result is one of the deepest pulsar searches ever performed in this region.
At the achieved sensitivity, the survey should have detected roughly 10% of the millisecond pulsars, rotating hundreds of times per second, and up to half of the slower, canonical pulsars expected if the GC population resembled that of the wider galaxy. It came up empty – almost.
In a one-hour scan, the survey identified a candidate consistent with an 8.19 millisecond pulsar, dubbed the Breakthrough Listen Pulsar (BLPSR). The signal was coherent across both time and frequency throughout the observation, with statistical tests on randomised data giving a chance occurrence rate of roughly one in a thousand (about 3σ) from its statistical properties alone, and closer to one in a million (approaching 5σ) when its coherent signal power is included.
These figures make a chance-origin unlikely on a single trial, though they are not, on their own, sufficient to establish a pulsar. The candidate did not reappear in subsequent observations, and a much stronger case is required before asserting an astrophysical origin. If confirmed, BLPSR would be the first millisecond pulsar found in the immediate GC environment, and an encouraging sign that more may yet lurk in the central parsec, just below current detection thresholds.
Still, the shortage of detections raises real questions. GC pulsars could be intrinsically fainter, older, or differently distributed than expected. Strong scattering may persist at higher frequencies through complex, localised structures in the interstellar medium. Selection effects, including long periods and unfavourable beaming geometry, could also play a larger role than usually assumed.
Millisecond pulsars are extraordinarily stable rotators, and serve as precision clocks for measuring gravitational effects. A confirmed millisecond pulsar in close orbit around Sgr A* may open a new window on strong-field gravity, allowing precision tests of general relativity in the immediate vicinity of a supermassive black hole.
The connection to fundamental physics extends further. Wide-band, high-resolution radio data of the kind used here have also been turned to the search for axion dark matter, where axion-to-photon conversion in stellar magnetic fields would imprint narrow spectral features. Modern radio surveys are increasingly designed for this kind of breadth, with the same observations used to search for pulsars, signatures of dark matter, and potential signs of extraterrestrial technology.
The path forward needs deeper, more sensitive searches, supported by advances in instrumentation and analysis. The Square Kilometre Array and the next-generation Very Large Array promise to overcome the current sensitivity and frequency limitations. Open data are equally important. By releasing GC observations publicly, BL enables the broader community to pursue independent analyses and complementary science cases.
If confirmed, a millisecond pulsar near Sgr A* would be a step forward in our understanding of the GC, and a potential new probe of physics in its most extreme regimes.
The Bs→ φμ+μ– process, in which a bottom quark decays into a strange quark and a pair of oppositely charged muons, is a powerful probe of physics beyond the Standard Model (SM). For the first time, the CMS collaboration has measured its branching fraction as a function of q2, the squared invariant mass of the dimuon pair. In the low-q2 region, from 1.1 to 6 GeV2, the result lies 4.2σ below SM predictions obtained from a range of form-factor calculations.
In the SM, the weak nuclear force is mediated by the heavy gauge bosons W+, W– and Z0. Transitions mediated by the Z0 boson in which fundamental particles, such as quarks, change their flavour without altering their electric charge are known as flavour-changing neutral current (FCNC) processes. These transitions are absent at tree level in the SM and can only happen via complex, higher-order “penguin” or “box” loop diagrams. Moreover, the Glashow–Iliopoulos–Maiani mechanism ensures that contributions from the up-type quarks in the loop largely cancel, heavily suppressing FCNCs. As a result, these rare processes provide a sensitive probe for physics beyond the SM.
The Bs→ φμ+μ– decay is an FCNC transition where a bottom quark decays to a strange one, with the intermediate loop dominated by a top quark. Recent studies of similar processes have revealed tensions between experimental measurements and theoretical predictions for both the branching fraction and angular observables. Specifically, using 9 fb–1 of data collected at 7, 8 and 13 TeV centre-of-mass energies, the LHCb collaboration observed that the Bs→ φμ+μ– branching fraction lies 3.6σ below the SM prediction (CERN Courier September/October 2021 p15).
In this new result, the CMS collaboration reports its first differential measurement of the branching fraction of the Bs→ φμ+μ– decay as a function of q2, using 138 fb–1 of data collected at 13 TeV centre-of-mass energy. The Bs-meson candidate is reconstructed in the K+K–μ+μ– final state by requiring soft-muon identification and high-purity hadronic tracks. The two hadron tracks, assigned the kaon mass hypothesis, are paired to form the φ-meson candidate. The narrow natural width of the φ resonance enables a clean selection with low background.
Signal events are extracted from extended, unbinned maximum-likelihood fits to the K+K–μ+μ– invariant mass distribution over various q2 intervals. The branching fraction is then measured relative to the normalisation channel Bs→ J/ψφ, which shares the same final state, allowing many systematic uncertainties to cancel. The angular observables FL and A6 are extracted in each q2 bin, from an unbinned maximum-likelihood fit to the three-dimensional distributions of the Bs candidates’ invariant mass and two angular variables.
While the angular observables FL and A6 are consistent with expectations, the analysis reveals an up to 4.2σ tension between the measured branching fraction and SM predictions (see figure 1). Still, the current sensitivity is limited by statistical constraints. The inclusion of Run 3 data will significantly reduce these uncertainties, yielding the improved precision required to address the persistent anomalies in the beauty quark sector.
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