Physics Archives – Page 25 of 142

Neutrino scattering sizes up the proton

More than a century after its discovery, physicists are still working hard to understand how fundamental properties of the proton – such as its mass and spin – arise from its underlying structure. A particular puzzle concerns the proton’s size, which is an important input to understand nuclei, for example. Inelastic electron–proton scattering experiments in the late 1950s revealed the spatial distribution of charge inside the proton, allowing its radius to be deduced. A complementary way to determine this “charge radius”, and which relies on precise quantum-electrodynamics calculations, is to measure the shift it produces in the lowest energy levels of the hydrogen atom. Over the decades, numerous experiments have measured the proton’s size with increasing precision.

By 2006, based on results from scattering and spectroscopic measurements, the Committee on Data for Science and Technology (CODATA) had established the proton charge radius to be 0.8760(78) fm. Then, in 2010, came a surprise: the CREMA collaboration at the Paul Scherrer Institut (PSI) reported a value of 0.8418(7) fm based a novel, high-precision spectroscopic measurement of muonic hydrogen. Disagreeing with previous spectroscopic measurements, and lying more than 5σ below the CODATA world average, the result gave rise to the “proton radius puzzle”. While the most recent electron–proton scattering and hydrogen-spectroscopy measurements are in closer agreement with the latest muonic-hydrogen results, the discrepancies with earlier experiments are not yet fully understood.

Now, the MINERνA collaboration has brought a new tool to gauge the proton’s size: neutrino scattering. Whereas traditional scattering measurements probe the proton’s electric or magnetic charge distributions, which are encoded in vector form factors, scattering by neutrinos allows the analogous axial-vector form factor F_A, which characterises the proton’s weak charge distribution, to be measured. In addition to providing a complementary probe of proton structure, F_A is key to precise measurements of neutrino-oscillation parameters at experiments such as DUNE, Hyper-K, NOvA and T2K.

MINERνA is a segmented scintillator detector with hexagonal planes made from strips of triangular cross-section, which are assembled into planes perpendicular to the incoming beam. By studying how a beam of muon antineutrinos produced by Fermilab’s NuMI neutrino beamline interacts with a polystyrene target, which contains hydrogen closely bonded to carbon, the MINERνA researchers were able to make the first high-statistics measurement of the ν_μ p → μ⁺ n cross-section using the hydrogen atom in polystyrene. Extracting F_A from 5580 ± 180 signal events (observed over an estimated background of 12,500), they measured the nucleon axial charge radius to be 0.73(17) fm, in agreement with the electric charge radius measured with electron scattering.

“If we weren’t optimists, we’d say [this measurement] was impossible,” says lead author Tejin Cai, who proposed the idea of using a polystyrene target to access neutrino-hydrogen scattering while a PhD student at the University of Rochester. “The hydrogen and carbon are chemically bonded, so the detector sees interactions on both at once. But then, I realised that the very nuclear effects that made scattering on carbon complicated also allowed us to select hydrogen and would allow us to subtract off the carbon interactions.”

A new experiment called AMBER, at the M2 beamline of CERN’s Super Proton Synchrotron, is about to open another perspective on the proton charge radius. AMBER is the successor to COMPASS, which played a major role towards resolving the proton “spin crisis” (the finding, by the European Muon Collaboration in 1987, that quarks account for less than a third of the total proton spin) by studying the contribution to the proton spin from gluons. Instead of electrons, AMBER will use muon scattering at unprecedented energies (around 100 GeV) to access the small momentum-transfer needed to measure the proton radius. A future experiment at PSI called MUSE, meanwhile, aims to determine the proton radius through simultaneous measurements of muon– and electron–proton scattering.

AMBER is scheduled to start with a pilot run in September 2023 and to operate for up to three years, with the goal to find a value for the proton radius in the range 0.84–0.88 fm, as expected from previous experiments, and with an uncertainty of about 0.01 fm. “Some colleagues say that there is no proton-radius puzzle, only problematic measurements,” says AMBER spokesperson Jan Friedrich of TU Munich. “The discrepancy between theory and experiments, as well as between individual experiments, will have to shrink and align as much as possible. After all, there is only one true proton radius.”

TeV photons challenge standard explanations

**Afterglow** GRB 221009A captured by NASA’s Neil Gehrels Swift Observatory approximatively one hour after the record-breaking gamma-ray emission, showing bright rings formed by X rays scattering off dust layers within our galaxy. Credit: NASA/Swift/A Beardmore (University of Leicester)

Gamma-ray bursts (GRBs) are the result of the most violent explosions in the universe. They are named for their bright burst of high-energy emission, mostly in the keV to MeV region, which can last from milliseconds to hundreds of seconds, and are followed by an afterglow that covers the full electromagnetic spectrum. The extreme nature and important role in the universe of these extragalactic events – for example in the production of heavy elements, potential cosmic-ray acceleration or even mass-extinction events on Earth-like planets – makes them one of the most studied astrophysical phenomena.

Since their discovery in 1967, detailed studies of thousands of GRBs show that they are the result of cataclysmic events, such as neutron-star binary mergers. The observed gamma-ray emission is produced (through a yet-unidentified mechanism) within relativistic jets that decelerate when they strike interstellar matter, resulting in the observed afterglow.

But interest in GRBs goes beyond astrophysics. Due to the huge energies involved, they are also a unique lab to study the laws of physics at their extremes. This once again became clear on 9 October 2022, when a GRB was detected that was not only the brightest ever but also appeared to have produced an emission that is difficult to explain using standard physics.

Eye-catching emission

“GRB 221009A” immediately caught the eye of the multi-messenger community, its gamma-ray emission being so bright that it saturated many observatories. As a result, it was also observed by a wide range of detectors covering the electromagnetic spectrum, including at energies exceeding 10 TeV. Two separate ground-based experiments – the Large High Altitude Air Shower Observatory (LHAASO) in China and the Carpet-2 air-shower array in Russia – claimed detections of photons with an energy of 18 TeV and 251 TeV, respectively. This is significantly higher, by an order of magnitude, than the previous record for TeV emission from GRBs reported by the MAGIC and HESS telescopes in 2019 (CERN Courier January/February 2020 p10). Adding further intrigue, such high-energy emission from GRBs should not be able to reach Earth at all.

For photons with energies exceeding several TeV, electron–positron pair-production with optical photons starts to become possible. Although the cross section for this process only just exceeds its threshold at an energy of 2.6 TeV, it is compensated by the billions of light years of space filled with optical light that the TeV photons need to traverse before reaching us. Despite uncertainties in the density of this so-called extragalactic background light, a rough calculation using the distance of GRB 221009A (z = 0.151) suggests that the probability for an 18 TeV photon to reach Earth is around 10^–8.

Clearly we need to wait for the detailed analyses by LHAASO and Carpet-2 to confirm the measurements

The reported measurements have thus far only been provided through alerts shared among the multi-messenger community, while detailed data analyses are still ongoing. Their significance, however, led to tens of beyond-the-Standard Model (BSM) explanations being posted on the arXiv preprint server within days of the alert. While each differs in the specific mechanism hypothesised, the overall idea is similar: instead of being produced directly in the GRB, the photons are posited to be a secondary product of BSM particles produced during or close to the GRB. Examples range from light scalar particles or right-handed neutrinos produced in the GRB and decaying within our galaxy, to photons that converted into axions close to the GRB and turned back into photons in the galactic magnetic field before reaching Earth.

Clearly the community needs to wait for the detailed analyses by the LHAASO and Carpet-2 collaborations to confirm the measurements. The published energy resolution of LHAASO keeps open the possibility that their results can be explained with Standard Model physics, while the 251 TeV emission from Carpet-2 is more difficult to attribute to known systematic effects. This result could, however, be explained by secondary particles resulting from an ultra-high energy cosmic-ray (UHECR) produced in the GRB which, although would not represent new physics, would still confirm GRBs as a source of UHECRs for the first time. Analysis results from both collaborations are therefore highly anticipated.

STEREO rejects sterile neutrino

ILL high-flux reactor — **No news** ILL’s high-flux reactor, at which the STEREO collaboration conducted their measurement of the ν_e spectrum. Credit: J L Baudet/ILL

The STEREO experiment, located at the high-flux research reactor at the Institut Laue-Langevin (ILL), Grenoble, is the latest to cast doubt on the existence of an additional, sterile neutrino state. Based on the full dataset collated from October 2017 until the experiment shut down in November 2020, the results support the conclusions of a global analysis of all neutrino data, that a normalisation bias in the beta-decay spectrum of ²³⁵U is the most probable explanation for a deficit of electron neutrinos seen at reactor experiments during the past decade.

The confirmation of neutrino oscillations 25 years ago showed that the lepton content of a given neutrino evolves as it propagates, generating a change of flavour. Numerous experiments based on solar, atmospheric, accelerator, reactor and geological neutrino sources have determined the oscillation parameters in detail, reaffirming the three-neutrino picture obtained by precise measurements of the Z boson’s decay width at LEP. However, several anomalies have also shown up, one of the most prominent being the so-called reactor antineutrino anomaly. Following a re-evaluation of the expected ν_e flux from nuclear reactors by a team at CEA and Subatech in 2011, a deficit in the number of ν_e detected by reactor neutrino experiments appeared. Combined with a longstanding anomaly reported by short-baseline accelerator-neutrino experiments such as LSND and a deficit in ν_e seen in calibration data for the solar-neutrino detectors GALLEX and SAGE, excitement grew that an additional neutrino state – a sterile or right-handed neutrino with non-standard interactions that arises in many extensions of the Standard Model – might be at play.

We anticipate that this result will allow progress towards finer tests of the fundamental properties of neutrinos

Designed specifically to investigate the sterile-neutrino hypothesis, STEREO was positioned about 10 m from the ILL reactor core to measure the evolution of the antineutrino energy spectrum from ²³⁵U fission at short distances with high precision. Comprising six cells filled with gadolinium-doped liquid scintillator positioned at different distances from the reactor core, producing six spectra, the setup allows the hypothesis that ν_e undergo a fast oscillation into a sterile neutrino to be tested independently of the predicted shape of the emitted ν_e spectrum.

The measured antineutrino energy spectrum, based on 107,558 detected antineutrinos, suggests that the previously reported anomalies originate from biases in the nuclear experimental data used for the predictions, while rejecting the hypothesis of a light sterile neutrino with a mass of about 1 eV. “Our result supports the neutrino content of the Standard Model and establishes a new reference for the ²³⁵U antineutrino energy spectrum,” writes the team. “We anticipate that this result will allow progress towards finer tests of the fundamental properties of neutrinos but also to benchmark models and nuclear data of interest for reactor physics and for observations of astrophysical or geoneutrinos.”

Gallium remains

STEREO’s findings fit those reported recently by other neutrino-oscillation experiments. A 2021 analysis by the MicroBooNE collaboration at Fermilab, for example, favoured the Standard Model over an anomalous signal seen by its nearby experiment MiniBooNE, assuming the latter was due to the existence of a non-standard neutrino. Yet the story of the sterile neutrino is not over. In 2022, new results from the Baksan Experiment on Sterile Transitions (BEST) further confirmed the deficit in the ν_e flux emitted from radioactive sources as seen by the SAGE and GALLEX experiments – the so-called gallium anomaly – which, if interpreted in the context of neutrino oscillations, is consistent with ν_e → ν_s oscillations with a relatively large squared mass difference and mixing angle.

“Under the sterile neutrino hypothesis, a signal in MicroBooNE, MiniBooNE or LSND would require the sterile neutrino to mix with both ν_e and ν_μ, whereas for the gallium anomaly, mixing with ν_e alone is sufficient,” explains theorist Joachim Kopp of CERN. “Even though the reactor anomaly seems to be resolved, we’d still like to understand what’s behind the others.”

ALICE looks through the Milky Way

**Annihilation** Schematic showing a ³He annihilating in the gas of the ALICE time projection chamber (red) and a ³He that does not undergo an inelastic reaction and reaches the time-of-flight detector (blue), with dashed curves representing charged (anti)particles produced in the ³He annihilation. Source: *Nat. Phys.* 19 61

Antinuclei can travel vast distances through the Milky Way without being absorbed, concludes a novel study by the ALICE collaboration. The results, published in December, indicate that the search for ³He in space is a highly promising way to probe dark matter.

First observed in 1965 in the form of the antideuteron at CERN’s Proton Synchrotron and Brookhaven’s Alternating Gradient Synchrotron, antinuclei are exceedingly rare. Since they annihilate on contact with regular matter, no natural sources exist on Earth. However, light antinuclei have been produced and studied at accelerator facilities, including recent precision measurements of the mass difference between deuterons and antideuterons and between ³He and ³He by ALICE, and between the hypertriton and antihypertriton by the STAR collaboration at RHIC.

Antinuclei can in principle also be produced in space, for example in collisions between cosmic rays and the interstellar medium. However, the expected production rates are very small. A more intriguing possibility is that light antinuclei are produced by the annihilation of dark-matter particles. In such a scenario, the detection of antinuclei in cosmic rays could provide experimental evidence for the existence of dark-matter particles. Space-based experiments such as AMS-02 and PAMELA, along with the upcoming Antarctic balloon mission GAPS, are among a few experiments that are able to detect light antinuclei. But to be able to interpret future results, precise knowledge of the production and disappearance probabilities of antinuclei is vital.

The latter is where the new ALICE study comes in. The unprecedented energies of proton–proton and lead–lead collisions at the LHC produce, on average, as many nuclei as antinuclei. By studying the change in the rate of ³He as a function of the distance to the production point, the collaboration was able to determine the inelastic cross section, or disappearance probability, of ³He nuclei for the first time. These values were then used as input for astrophysics simulations.

Two models of the ³He flux expected near Earth after the nuclei’s journey from sources in the Milky Way were considered: one assumes that the sources are cosmic-
ray collisions with the interstellar medium, and the other annihilations of hypothetical weakly interacting massive particles (WIMPs). For each model, the Milky Way’s transparency to ³He – that is, its ability to let the nuclei through without being absorbed – was estimated. The WIMP dark-matter model led to a transparency of about 50%, whereas for the cosmic-ray model the transparency ranged from 25 to 90%, depending on the energy of the antinucleus. These values show that ³He originating from dark-matter or cosmic-ray collisions can travel distances of several kiloparsecs in the Milky Way without being absorbed, even from as far away as the galactic centre.

“This new result illustrates the close connection between accelerator-based experiments and observations of particles produced in the cosmos,” says ALICE spokesperson Marco van Leeuwen. “In the near future, these studies will be extended to ⁴He and to the lower-momentum region with much larger datasets.”

Preparing for post-LS3 scenarios

Proposed experimental programmes — **Future physics** New experimental programmes proposed at CERN’s North Area, including a high-intensity beam facility in the ECN3 cavern, are a focus of current PBC studies. Credit: CERN-HOMEWEB-PHO-2019-027-1

The Physics Beyond Colliders (PBC) study was launched in 2016 to explore the opportunities offered by CERN’s unique accelerator and experimental-area complex to address some of the outstanding questions in particle physics through experiments that are complementary to the high-energy frontier. Following the recommendations of the 2020 update of the European strategy for particle physics, the CERN directorate renewed the mandate of the PBC study, continuing it as a long-term activity.

The fourth PBC annual workshop took place at CERN from 7 to 9 November 2022. The aim was to review the status of the studies, with a focus on the programmes under consideration for the start of operations after Long Shutdown 3 (LS3), scheduled for 2026–2029.

The North Area (NA) at CERN, where experiments are driven by beams from the Super Proton Synchrotron (SPS), is at the heart of many present and proposed explorations for physics beyond the Standard Model. The NA includes an underground cavern (ECN3), which can host unique high-energy/high-intensity proton beams. Several proposals for experiments have been made, all of which require higher intensity proton beams than are currently available. It is therefore timely to identify the synergies and implications of a future ECN3 high-intensity programme on the otherwise ongoing NA technical consolidation programme.

The following proposals are being considered within the PBC study group:

• HIKE (High Intensity Kaon Experiment) is a proposed expansion of the current NA62 programme to study extremely rare decays of charged kaons and, in a second phase, those of neutral kaons. This would be complemented by searches for visible decays of feebly interacting particles (FIPs) that could emerge on-axis from the dump of an intense proton beam within a thick absorber that would contain all other known particles, except muons and neutrinos;

• SHADOWS (Search for Hidden And Dark Objects With the SPS) would search for visible FIP decays off-axis and could run in parallel to HIKE when operated in beam-dump mode. The proposed detector is compact and employs existing technologies to meet the challenges of reducing the muon background;

• SHiP (Search for Hidden Particles) would allow a full investigation of hidden sectors in the GeV mass range. Comprehensive design studies for SHiP and the Beam Dump Facility (BDF) in a dedicated experimental area were published in preparation for the European strategy update. During 2021, an analysis of alternative locations using existing infrastructure at CERN revealed ECN3 to be the most promising option;

• Finally, TauFV (Tau Flavour Violation) would conduct searches for lepton-flavour violating tau-lepton decays.

The HIKE, SHADOWS and BDF/SHiP collaborations have recently submitted letters of intent describing their proposals for experiments in ECN3. The technical feasibility of the experiments, their physics potential and implications for the NA consolidation are being evaluated in view of a possible decision by the beginning of 2023. A review of the experimental programme in the proposed high-intensity facility will take place during 2023, in parallel with a detailed comparison of the sensitivity to FIPs in a worldwide context.

A vibrant programme

The NA could also host a vibrant ion-physics programme after LS3, with NA60++ aiming to measure the caloric curve of the strong-force phase transition with lead–ion beams, and NA61++ proposing to explore the onset of the deconfined nuclear medium, extending the scan in the momentum/ion space with collisions of lighter ion beams. The conceptual implementation of such schemes in the accelerators and experimental area is being studied and the results, together with the analysis of the physics potential, are expected during 2023.

The search for long-lived particles with dedicated experiments and the exploration of fixed-target physics is also open at the LHC. The proposed forward-physics facility, located in a cavern that could be built at a distance of 600 m along the beam direction from LHC Interaction Point 1, would take advantage of the large flux of high-energy particles produced in the very forward direction in LHC collisions. It is proposed to host a comprehensive set of detectors (FASER2, FASERν2, AdvSND, FORMOSA, FLArE) to explore a broad range of new physics and to study the highest energy neutrinos produced by accelerators. A conceptual design report of the facility, including detector design, background analysis and mitigation measures, civil engineering and integration studies is in preparation. Small prototypes of the MATHUSLA, ANUBIS and CODEX-b detectors aiming at the search for long-lived particles at large angles from LHC collisions are also being built for installation during the current LHC run.

The North Area at CERN is at the heart of many present and proposed explorations for physics beyond the Standard Model

A gas-storage cell (SMOG2) was installed in front of the LHCb experiment during the last LHC long shutdown, opening the way to high-precision fixed-target measurements at the LHC. The storage cell enhances the density of the gas and therefore the rate of the collisions by up to two orders of magnitude as compared to the previous internal gas target. SMOG2 has been successfully commissioned with neon gas, demonstrating that it can be operated in parallel to LHCb. Future developments include the injection of different types of gases and a polarised gas target to explore nucleon spin-physics at the LHC.

Crystal clear

Fixed-target experiments are also being developed that would extract protons from LHC beams by channelling the beam halo with a bent crystal. The extracted protons would impinge on a target and be used for measurements of proton structure functions (“single crystal setup”) or estimation of the magnetic and electric dipole moments of short-lived heavy baryons (“double crystal setup”). In the latter case, the measurement would be based on the baryon spin precession in the strong electric field of a second bent crystal installed immediately downstream from the baryon-production proton target. A proof-of-principle experiment of the double-crystal setup is being designed for installation in the LHC to determine the channelling efficiency for long crystals at TeV energies, as well as to demonstrate the control and management of the secondary halo and validate the estimate of the achievable luminosity.

The technology know-how at CERN can also benefit non-accelerator experiments

The technology know-how and experience available at CERN can also benefit non-accelerator experiments such as the Atom Interferometer Observatory and Network (AION), proposed to be installed in one of the shafts at Point 4 of the LHC for mid-frequency gravitational-wave detection and ultra-light dark-matter searches, as well as the development of superconducting cavities for the Relic Axion Detector Experimental Setup (RADES) and for the heterodyne detection of axion-like particles.

During the workshop, progress on the possible applications of a gamma factory at CERN, as well as the status of the design of a Charged-Particle EDM Prototype Ring and of the R&D for novel monitored or tagged neutrino beamlines, were also presented.

Neutrinos reveal active galaxy’s inner depths

Highly energetic cosmic rays reach Earth from all directions and at all times, yet it has been challenging to conclusively identify their sources. Being charged, cosmic rays are easily deflected by interstellar magnetic fields during their propagation and thereby lose any information about where they originated from. On the other hand, highly energetic photons and neutrinos remain undeflected. Observations of high-energy photons and neutrinos are therefore crucial clues towards unravelling the mystery of cosmic-ray sources and accelerators.

Four years ago, the IceCube collaboration announced the identification of the blazar TXS 0506+056 as a source of high-energy cosmic neutrinos, the first of its kind (CERN Courier September 2018 p7). This was one of the early examples of multi-messenger astronomy wherein a high-energy neutrino event detected by IceCube, which was coincident in direction and time with a gamma-ray flare from the blazar, prompted an investigation into this object as a potential astrophysical neutrino source.

Point source

In the following years, IceCube made a full-sky scan for point-like neutrino sources, and in 2020, the collaboration found an excess coincident with the Seyfert II galaxy, NGC1068, that was inconsistent with a background-only hypothesis. However, with a statistical significance of only 2.9σ, it was insufficient to claim a detection. In November 2022, after a more detailed analysis with a longer live-time and improved methodologies, the collaboration confirmed NGC1068 to be a point source of high-energy neutrinos at a significance of 4.2σ.

IceCube’s measurements usher in a new era of neutrino astronomy

Messier 77, also known as the Squid Galaxy or NGC1068, is located 47 million light years away in the constellation Cetus and was discovered in 1780. Today, we know it to be an active galaxy: at its centre lies an active galactic nucleus (AGN), which is a luminous and compact region powered by a super massive black hole (SMBH), surrounded by an accretion disk. Specifically, it is a Seyfert II galaxy, which is an active galaxy that is viewed edge-on, with the line-of-sight passing through the accretion region that obscures the centre.

The latest search used data from the fully completed IceCube detector. Several calibrations and alignments were also made to the data-acquisition procedure and an advanced event reconstruction algorithm was deployed. The search was conducted in the Northern Hemisphere of the sky, i.e. by detecting neutrinos from “below”, so that Earth could screen background atmospheric muons.

Three different searches were carried out to locate possible point-like neutrino sources. The first involved scanning the sky for a statistically significant excess over background, while the other two used a catalogue of 110 sources that was developed in the 2020 study, the difference between the two being the statistical methods used. The results showed an excess of 79⁺²²_–20 muon–neutrino events, with the main contribution coming from neutrinos in the energy range of 1.5 to 15 TeV, while the all-flavour flux is expected to be a factor of three higher. All the events contributing to the excess were well-reconstructed within the detector, with no signs of anomalies, and the results were found not to be dominated by just one or a few individual events. The results were also in line with phenomenological models that predict the production of neutrinos and gamma rays in sources such as NGC1068.

IceCube’s measurements usher in a new era of neutrino astronomy and take researchers a step closer to understanding not only the origin of high-energy cosmic rays but also the immense power of massive black holes, such as the one residing inside NGC1068.

Hunting dark matter with invisible Higgs decays

CMS figure 1 — **Fig. 1.** The DM–nucleon coupling strength derived from the new CMS result, in the context of several models, as a function of the DM candidate mass. Limits from direct-detection experiments are also shown for comparison. Source: CMS-PAS-HIG-21-007

In the Standard Model (SM) of particle physics, the only way the Higgs boson can decay without leaving any traces in the LHC detectors is through the four-neutrino decay, H → ZZ → 4ν, which has an expected branching fraction of only 0.1%. This very small value can be seen as a difficulty but is also an exciting opportunity. Indeed, several theories of physics beyond the SM predict considerably enhanced values for the branching fraction of invisible Higgs-boson decays. In one of the most interesting scenarios, the Higgs boson acts as a portal to the dark sector by decaying to a pair of dark matter (DM) particles. Measurements of the “Higgs to invisible” branching fraction are clearly among the most important tools available to the LHC experiments in their searches for direct evidence of DM particles.

The CMS collaboration recently reported the combined results of different searches for invisible Higgs-boson decays, using data collected at 7, 8 and 13 TeV centre-of-mass energies. To find such a rare signal among the overwhelming background produced by SM processes, the study considers events in most Higgs-boson production modes: via vector boson (W or Z) fusion, via gluon fusion and in association with a top quark–antiquark pair or a vector boson. In particular, the analysis looked at hadronically decaying vector bosons or top quark–antiquark pairs. A typical signature for invisible Higgs-boson decays is a large missing energy in the detector, so that the missing transverse energy plays a crucial role in the analysis. No significant signal has been seen, so a new and stricter upper limit is set on the probability that the Higgs boson decays to invisible particles: 15% at 95% confidence level.

This result has been interpreted in the context of Higgs-portal models, which introduce a dark Higgs sector and consider several dark Higgs-boson masses. The extracted upper limits on the spin-independent DM-nucleon scattering cross section, shown in figure 1 for a range of DM mass points, have better sensitivities than those of direct searches over the 1–100 GeV range of DM masses. Once the Run 3 data will be added to the analysis, much stricter limits will be reached or, if we are lucky, evidence for DM production at the LHC will be seen.

Testing flavour symmetry with the Higgs boson

ATLAS figure 1 — **Fig. 1.** Best-fit value (red star) of the branching ratios B(H → eτ) and B(H → μτ), given in %, and likelihood contours at 68% (yellow) and 95% (grey) confidence levels, obtained from the simultaneous measurement of H → eτ and H → μτ signals based on the MC-template method, compared to the Standard Model expectation (black cross). Source: ATLAS-CONF-2022-060

Lepton number is a quantum number that represents the difference in the number of leptons and antileptons participating in a process, while lepton flavour is a corresponding quantity that accounts for each generation of lepton (e, μ or τ) separately. Lepton number is always conserved but lepton flavour violation (LFV) is known to exist in nature, as this phenomenon has been observed in neutrino oscillations – the transition of a neutral lepton of a given flavour to one with a different flavour. This observation motivates searches for additional manifestations of LFV that may be the result of beyond-the-Standard Model (SM) physics, key among which is the search for LFV decays of the Higgs boson.

The ATLAS collaboration has recently announced the results of searches for H → eτ and H → μτ decays based on the full Run 2 data set, which was collected at a centre-of-mass energy of 13 TeV. The unstable τ lepton decays to an electron or a muon and two neutrinos, or to one or more hadrons and one neutrino. Most of the background events in these searches arise from SM processes such as Z → ττ, the production of top–antitop and weak-boson pairs, as well as from events containing misidentified or non-prompt leptons (fake leptons). These fake leptons originate from secondary decays, for example of charged pions. Several multivariate analysis techniques were used for each final state to provide the maximum separation between signal and background events.

To ensure the robustness of the measurement, two background estimation methods were employed: a Monte Carlo (MC) template method in which the background shapes were extracted from MC and normalised to data, and a “symmetry method”, which used only the data and relied on an approximate symmetry between prompt electrons and prompt muons. Any difference between the branching fractions B(H → eτ_μ) and B(H → μτ_e), where the subscripts μ and e represent the decay modes of the τ lepton, would break this symmetry. In both cases, contributions from events containing fake leptons were estimated directly from the data.

The MC-template method enables the measurement of the branching ratios of the LFV decay modes. Searches based on the MC-template method for background estimation involve both leptonic and hadronic decays of τ leptons. A simultaneous measurement of the H → eτ and H → μτ decay modes was performed. For the H → μτ (H → eτ) search, a 2.5 (1.6) standard deviation upward fluctuation above the SM background prediction is observed. The observed (expected) upper limits on the branching fractions B(H → eτ) and B(H → μτ) at 95% confidence level are slightly below 0.2% (0.1%), which are the most stringent limits obtained by the ATLAS experiment on these quantities. The result of the simultaneous measurement of the H → eτ and H → μτ branching fractions is compatible with the SM prediction within 2.2 standard deviations (see figure 1).

The observed upper limits on the branching fractions are the most stringent limits obtained by the ATLAS experiment

The symmetry method is particularly sensitive to the difference in the two LFV decay branching ratios. For this measurement, only the fully leptonic final states were used. Special attention was paid to correctly account for asymmetries induced by the different detector response to electrons and muons, especially regarding the trigger and offline efficiency values for lepton reconstruction, identification and isolation, as well as regarding contributions from fake leptons. The measurement of the branching ratio difference indicates a small but not significant upward deviation for H → μτ compared to H → eτ. The best-fit value for the difference between B(H → μτ_e) and B(H → eτ_μ) is (0.25 ± 0.10)%.

The expected twice-larger LHC Run 3 dataset at the higher centre-of-mass energy of 13.6 TeV will shed further light on these results.

B to D decays reduce uncertainty on γ

LHCb figure 1 — **Fig. 1.** Distribution of B⁺ and B^– to D K^± candidates, where the contribution from correctly reconstructed decays can be seen in magenta, in one bin of the D-meson decay phase space bins. The large CP violation is clearly visible as a difference in height between the two peaks. Source: LHCb-PAPER-2022-017

The Cabibbo–Kobayashi–Maskawa (CKM) matrix describes the couplings between the quarks and the weak charged current, and contains within it a phase γ that changes sign under consideration of antiquarks rather than quarks. In the Standard Model (SM), this phase is the only known difference in the interactions of matter and anti-matter, a consequence of the breaking of charge-parity (CP) symmetry. While the differences within the SM are known to be far too small to explain the matter-dominated universe, it is still of paramount importance to precisely determine this phase to provide a benchmark against which any contribution from new physics can be compared.

A new measurement recently presented by the LHCb collaboration uses a novel method to determine γ using decays of the type B^± → D[K^∓π^±π^±π^∓]h^± (h = π, K). CP violation in such decays is a consequence of the interference between two tree-level processes with a weak phase that differs by γ, and thus provide a theoretically clean probe of the SM. The new aspect of this measurement compared to those performed previously lies in the partitioning of the five-dimensional phase space of the D-decay into a series of independent regions, or bins. In these bins, the asymmetries between B⁺ and B^– meson decay rates can receive large enhancements from the hadronic interactions in the D-meson decay. The enhancement for one of such bins can be seen in figure 1, which shows the invariant mass spectrum of the B⁺ and B^– meson candidates, where the correctly reconstructed decays peak at around 5.3 GeV. The observed asymmetry in this region is around 85%, which is the largest difference in the behaviour of matter and antimatter ever measured. Observables from the different bins are combined with information on the hadronic interactions in the D-meson decay from charm-threshold experiments to obtain γ = 55 ± 9°, which is compatible with previous determinations and is the second most precise single measurement.

The matter–antimatter asymmetry reaches 85% in a certain region, the largest ever observed

The LHCb average value of γ is then determined by combining this analysis with the measurements in many other B and D decays, where in all cases the SM contribution is expected to be dominant. Measurements of charm decays are also included to better constrain both the parameters of charm mixing, which also play an important role in the measurements of B-meson decays at the current level of precision and help to constrain the hadronic interactions in some of the D decays. In particular, included for the first time in this combination is a measurement of y_CP, which is proportional to the difference in lifetimes of the two neutral charm mesons, and was determined using two-body decays of the D meson using the entire LHCb data set collected so far.

The overall impact of these additional analyses reduces the uncertainty on γ by more than 10%, corresponding to adding around a year of data taking across all decay modes.

The improvements in the knowledge of y_CP is also dramatic, reducing the uncertainty by around 40%. While the value of γ is found to be compatible with determinations that would be more susceptible to new physics, the precision of the comparison is starting to approach the level of a few degrees, at which discrepancies may start to be observable.

Given that the current uncertainties on many of the key input analyses to the combination are predominately statistical in nature, measurements of these fundamental flavour-physics parameters with the upgraded LHCb detector, and beyond, are an intriguing prospect for new-physics searches.

Hidden charm in the quark–gluon plasma

ALICE figure 1 — **Fig. 1.** The J/ψ (blue) and ψ(2S) (red) nuclear modification factor (R_AA) as a function of centrality, quantified by the average number of nucleons participating in the collision (<N_part>). The line at R_AA = 1 corresponds to the absence of modification between lead–lead and proton–proton interactions. Comparisons with the TAMU and SHMc models are also shown. Source: arXiv:2210.08893

For almost 40 years, charmonium, a bound state of a heavy charm–anticharm pair (hence also called a hidden charm), has provided a unique probe to study the properties of the quark–gluon plasma (QGP), the state of matter composed by deconfined quarks and gluons present in the early instants of the universe and produced experimentally in ultrarelativistic heavy-ion collisions. Charmonia come in a rich variety of states. In a new analysis investigating how these different bound charmonium states are affected by the QGP, the ALICE collaboration has opened a novel way to study the strong interaction at extreme temperatures and densities.

In the QGP, the production of charmonium is suppressed due to “colour screening” by the large number of quarks and gluons present. The screening, and thus the suppression, increases with the temperature of the QGP and is expected to affect different charmonium states to different degrees. The production of the ψ(2S) state, for example, which is 10 times more weakly bound and two times larger in size than the most tightly bound state, the J/ψ, is expected to be more suppressed.

This hierarchical suppression is not the only fate of charmonia in the quark–gluon plasma. The large number of charm quarks and antiquarks in the plasma – up to about 100 in head-on lead–lead collisions – also gives rise to a mechanism, called recombination, that forms new charmonia and counters the suppression to a certain extent. This process is expected to depend on the type and momentum of the charmonia, with the more weakly bound charmonia being produced through recombination later in the evolution of the plasma and charmonia with the lowest (transverse) momentum having the highest recombination rate.

Previous studies, using data first from the Super Proton Synchrotron and then from the LHC, have shown that the production of the ψ(2S) state is indeed more suppressed than that of the J/ψ, and ALICE has also previously provided evidence of the recombination mechanism in J/ψ production. But so far, no studies of ψ(2S) production at low transverse particle momentum had been precise enough to provide conclusive results in this momentum regime, preventing a complete picture of ψ(2S) production from being obtained.

The ALICE collaboration has now reported the first measurements of ψ(2S) production down to zero transverse momentum, based on lead–lead collision data from the LHC collected in 2015 and 2018. The results indicate that the ψ(2S) yield is largely suppressed with respect to a proton–proton baseline, almost a factor of two more suppressed than the J/ψ. The suppression, shown as a function of the collision centrality (N_part) in the figure, is quantified through the nuclear modification factor (R_AA), which compares the particle production in lead–lead collisions with respect to the expectations based on proton–proton collisions.

Theoretical predictions based on a transport approach that includes suppression and recombination of charmonia in the QGP (TAMU) or on the Statistical Hadronisation Model (SHMc), which assumes charmonia to be formed only at hadronisation, describe the J/ψ data, while the ψ(2S) production is underestimated in central events by the SHMc. This observation represents one of the first indications that dynamical effects in the QGP, as taken into account in the transport models, are needed to reproduce the yields of the various charmonium states. It also shows that precision studies, including these and those of other charmonia, and foreseen for Run 3 of the LHC, may lead to a final understanding of the modification of the force binding these states in the extreme environment of the QGP.

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