Physics Archives – Page 4 of 147

Two channels for the top–antitop excess

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^–1of 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.”

Final collisions for the RHIC

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.

The kaon stays on script

**Wired for rarity** Photomultiplier readout on the MUV3 detector at NA62, designed to study the K⁺ → π⁺νν̅ decay against an overwhelming background. Credit: P Traczyk, CERN

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, K_L → 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 K_L → π⁰νν in the 2030s.

On the hunt for cosmic clocks

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.

A sharper probe of a rare B_s decay

CMS figure 1 — **Fig. 1.** Differential branching fraction (left) and angular observable F_L (right) for the B_s → φμ⁺μ^– process as functions of q², compared with SM predictions. The branching fraction lies 4.2σ below Flavio expectations in the low-q² region. Credit: CMS Collab. CMS-PAS-BPH-23-003

The B_s → 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 B_s → 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 B_s → 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 B_s → φμ⁺μ^– decay as a function of q², using 138 fb^–1 of data collected at 13 TeV centre-of-mass energy. The B_s-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 B_s → J/ψφ, which shares the same final state, allowing many systematic uncertainties to cancel. The angular observables F_L and A₆ are extracted in each q² bin, from an unbinned maximum-likelihood fit to the three-dimensional distributions of the B_s candidates’ invariant mass and two angular variables.

While the angular observables F_L 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.

Jets boost nuclear coalescence

ALICE figure 1 — **Fig. 1.** Deuteron coalescence parameter B₂ in jets (full markers) and out of jets (open markers) in p–Pb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV (circle markers), compared with the measurements in pp collisions at a centre-of-mass energy of 13 TeV (square markers). Credit: ALICE Collab, 2026 arXiv:2602.22880

The production mechanism of light (anti)nuclei in hadronic collisions has been studied in several experiments over the past decades, but is still not fully understood. One candidate mechanism is baryon coalescence, in which nuclei can form from preexisting nucleons only if they are close in phase space. The ALICE collaboration has now reported the first measurement of deuteron production in and out of jets in p–Pb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV. The results are consistent with the enhancement expected from coalescence models.

Jets, the collimated emission of hadrons produced by the hadronisation of high-energy quarks, are a natural testing ground for coalescence, as the nucleons they contain are typically close in phase space. Comparing yields inside and outside jets can then test the mechanism directly. In the ALICE analysis, the coalescence probability is investigated by calculating the coalescence parameter B_A, defined as the ratio between the nucleus invariant yield and the proton invariant yield raised to the mass number A of the nucleus. This quantity is calculated both in (B^jet_A) and out of jets (B^UE_A, where UE represents the underlying event). In the latter, the density of produced particles is expected to be lower, and thus coalescence should be less likely. If proximity in phase-space affects the coalescence probability, B^jet_A should therefore exceed B^UE_A. Otherwise, the two should be similar.

Three regions of equal width are used to study the jet-correlated production: “toward”, “away” and “transverse” to the jet axis, with the direction approximated by the highest-transverse-momentum particle in the event. The in-jet contribution is obtained from the toward region by subtracting the underlying event, captured by the transverse region. B^jet₂ appears to be enhanced with respect to B^UE₂ (see figure 1), as expected from coalescence models.

Compared to previous studies in pp data, the system formed in p–Pb collisions is slightly larger and produces more particles, providing additional constraints on coalescence. The enhancement of B^jet₂ with respect to B^UE₂ is found to be larger in p-Pb than in the corresponding pp measurement at 13 TeV. This difference could be explained by the different source sizes and, possibly, by different particle-species compositions of the jets.

Further investigations of the coalescence parameter in and out of jets will be carried out with data from Run 3 of the LHC, which includes software-triggered pp data samples up to three orders of magnitude larger than those collected in Run 2. The full exploitation of this data will allow for the inclusion of A = 3 nuclei (³He, triton) and the extension of the transverse momentum coverage to higher values, providing additional information to constrain the processes behind the formation of light (anti)nuclei.

Two new CP tests for Higgs couplings

ATLAS figure 1 — **Fig. 1.** Distribution of the optimal observable (OO) in the VBF H → γγ analysis. Events are weighted by ln(1 + S/B), where S and B are the best-fit signal and background yields. The lower panel shows the background-subtracted data compared with the best-fit VBF result (c_HW̃ = 0.24) and two CP-violating scenarios, corresponding to two values of the c_HW̃ coefficient excluded at 95% CL by shape-only fits. Credit: arXiv:2603.20087

The origin of the observed asymmetry between matter and antimatter in the universe, of the order of one part in 10 billion, is one of the open questions in particle physics. In 1967, Andrei Sakharov showed that one of the necessary conditions to generate such an imbalance in the early universe is the violation of the combined charge-conjugation and parity (CP) symmetry. In the Standard Model (SM), CP violation arises from a complex phase in the quark mixing matrix, but this contribution is too small to account for the observed asymmetry. Additional sources of CP violation are therefore required, such as contributions from the neutrino sector or from other sources beyond the SM.

The discovery of the Higgs boson has opened a new sector in which to search for additional CP-violating interactions. The ATLAS experiment at the LHC recently reported the results of two new analyses that probe the CP properties of Higgs-boson interactions with electroweak gauge bosons, including the structure of its couplings to longitudinally and transversely polarised W and Z bosons.

The first comes from a measurement of Higgs production through the vector boson fusion mechanism (VBF), with the Higgs boson decaying into two photons. It is the first ATLAS measurement to use Run 3 data, collected between 2022 and 2024, to probe CP violation in the interaction between the Higgs boson and electroweak gauge bosons.

The Higgs decay into two photons provides a clean experimental signature with a large sample of Higgs events. The analysis exploits CP-odd observables, which change sign under a CP transformation, and whose average over the full phase space vanishes in the absence of CP violation. Any observed asymmetry would therefore signal a violation of CP symmetry. One such quantity, used in this analysis, is the optimal CP-odd observable. It is constructed from the per-event ratio of the CP-odd SM–BSM interference term to the SM matrix element squared, evaluated from the reconstructed four-momenta of the Higgs boson and the two jets.

ATLAS figure 2 — **Fig. 2.** Best-fit values and 95% CL intervals for three CP-violating Wilson coefficients, from a simultaneous fit combining all Run 2 CP analyses sensitive to the Higgs coupling to electroweak gauge bosons. The results are shown for both the linear and the linear-plus-quadratic scenarios, with the latter yielding tighter but less theoretically clean limits. Expected limits with statistical uncertainties only are shown for comparison. Credit: arXiv:2603.20117

No asymmetry was observed (see figure 1), and results were interpreted in the Standard Model effective field theory (SMEFT) framework. SMEFT provides a systematic, model-independent parameterisation of potential new-physics effects at energy scales beyond those directly accessible at the LHC, by extending the SM Lagrangian with higher-dimensional operators parameterised by Wilson coefficients. Constraints on these coefficients are broadly reinterpretable across a wide class of BSM scenarios. This new result improves the Run 2 limits by more than a factor of two.

The second result combines measurements from several Higgs-boson decay channels and production modes recorded during Run 2 to extract even stronger limits on possible CP-violating interactions. This combination uses the full Run 2 results from VBF H → ττ, H → WW^*, VBF H → γγ, H → ZZ^* and WH with H → bb̅. A strategy similar to that of the Run 3 VBF H → γγ analysis was employed in all the single channels by looking for asymmetries in CP-odd observables and interpreting the results in terms of SMEFT. In addition to constraining individual sources of CP violation, the combination disentangles the effects from multiple sources, since different channels probe different combinations of operators. This allows simultaneous limits to be set on three CP-violating Wilson coefficients, yielding the most stringent constraints to date (see figure 2).

These new results highlight the breadth of the ATLAS programme aimed at understanding the CP properties of the Higgs boson and its interactions with electroweak gauge bosons. They also demonstrate the growing potential of the Run 3 dataset to address open questions in the Higgs sector of the SM.

An upgraded take on CP violation

LHCb figure 1 — **Fig. 1.** Two-dimensional likelihood contours of the CP-violating observables x^DK and y^DK, measured separately for B⁺ (blue) and B^– (red). Shaded areas indicate the 68% and 95% confidence levels from profile-likelihood scans, while the dashed and solid lines are obtained from two-dimensional Gaussian approximations. The two vectors differ by a relative rotation of 2γ. Credit: LHCb-PAPER-2026-010

The LHCb collaboration has announced its first Run 3 measurement of the CKM angle γ, a key parameter describing CP violation in the quark sector. The measurement, consistent with previous results, yields γ = (68.1 ± 6.7)°, and will provide an important input to the world average.

The matter–antimatter asymmetry in the universe remains one of the central puzzles in physics, and violation of the combined charge–parity (CP) symmetry is a prerequisite for it. Within the Standard Model (SM), the only established source of CP violation is the complex phase in the Cabibbo–Kobayashi–Maskawa (CKM) matrix, which governs the weak transitions between quarks of different flavour. Precise experimental tests of the CKM framework thus play a central role in understanding CP violation and weak interactions.

Among the CKM parameters, the angle γ is central to testing the SM paradigm. Its direct determination through B → DK decays provides a clean theoretical environment and is largely insensitive to physics beyond the SM. Indirect determinations, obtained from global fits to other CKM observables under the assumption of CKM unitarity, currently achieve higher precision, but involve loop-level processes where new physics could enter, and are limited by challenging theoretical uncertainties. A precise direct determination of γ is therefore essential to compare the two approaches and provide a stringent test of the CKM picture.

LHCb is the best-positioned experiment to produce such a determination. Its highly efficient tracking system, excellent vertex resolution and powerful particle-identification capabilities allow it to reconstruct large samples of B → DK decays. As a result, LHCb measurements currently dominate the world average on γ, with a recent combination yielding γ = (62.8 ± 2.6)°. After a major upgrade of the detector from 2019 to 2022, the experiment now records data at five times the previous luminosity and operates with a highly-efficient, fully software-based trigger, significantly increasing its physics reach.

LHCb has now measured the CKM angle γ in the golden channel B^± → DK^±, with D → K⁰_Sh⁺h^–, using data collected by the upgraded LHCb detector between July and October 2024, corresponding to an integrated luminosity of 5.8 fb^–1. The decay B^± → Dπ^± is included to help control detector-induced effects and reduce the dependence on simulation, leading to a high-precision measurement. About 16,000 B^± → DK^± and 240,000 B^± → Dπ^±signal candidates were observed by inspecting their invariant-mass spectra.

Despite using only four months of Run 3 data, corresponding to a lower integrated luminosity than the combined Run 1 and Run 2 dataset, the signal yield is 17% larger. This illustrates the substantial gain in performance from the upgraded detector and reoptimised software trigger system. CP-violating observables are extracted through a simultaneous analysis of data across the decay phase space (see figure 1). A clear signature of direct CP violation is visible as the opening angle between the B⁺ and B^– vectors: the two differ by a relative rotation of 2γ, directly encoding the CP-violating phase. Interpreting these observables in terms of the underlying physics parameters yields γ = (68.1 ± 6.7)°.

Excellent precision has already been achieved with a small fraction of the LHCb Run 3 dataset. The full dataset, projected to increase the sample size by a factor of four, is expected to produce the most precise direct measurement of γ. The planned LHCb Upgrade II, with its much larger dataset and enhanced detector capabilities in the HL-LHC era, will further strengthen tests of the SM through increased sensitivity to CP violation.

Rencontres de Moriond turns 60

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

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

Branching out

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

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

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

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

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

Golden era

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

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

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

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

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

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

Drilling down on dark matter

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

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

Search and research

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

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

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

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

Thermal history

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

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

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

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

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

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

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Delving deep

Branching out

Golden era

Search and research

Thermal history