arXane › Physics
Wednesday, 4 November 2026: 6 new papers, 8 cross-lists and 16 replacements in High Energy Physics - Experiment (hep-ex), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
The ATOMKI anomalies motivate the possibility of a new particle, the X17, with a mass of 17 MeV and weak couplings to nuclei and electrons. Such a particle may be produced at the LHC, travel 480 m in the far-forward direction, and decay to $e^+e^-$ in FASER. In this paper, previous FASER searches for dark photons and axion-like particles (ALPs) are combined and interpreted as searches for a candidate X17 particle, the protophobic vector boson. The searches use data collected from $pp$ collisions at $\sqrt{s} = 13.6$ TeV in LHC Run 3 from 2022 for the dark photon search and from 2022-2023 for the ALP search. No excess events are found above the expected background, and the combined null results exclude a previously-unconstrained "low-coupling window" with electron coupling $6 \times 10^{-5} < \epsilon_e < 1.2 \times 10^{-4}$. A "high-coupling window" remains.
The superposition principle at the heart of optics states that electromagnetic fields do not interact in vacuum. This principle is not universal, instead electromagnetic fields couple weakly via quantum vacuum fluctuations. Striking consequences are light-by-light scattering and vacuum birefringence. Experimental results from colliding X-ray free-electron and optical high-intensity lasers combined with dark-field detection and polarimetry for background suppression are reported. This approach yields the most stringent bound for light-by-light scattering in the low-energy regime achieved to date -- improving upon previous experimental limits by ten orders of magnitude -- and paves the way for precision measurements.
We measure the $e^+e^-\to\pi^+\pi^-(\gamma)$ cross section from threshold to 1.4 GeV using 460 fb$^{-1}$ of BaBar initial-state radiation data. The result is obtained from the $\pi^+\pi^-/\mu^+\mu^-$ ratio, reducing systematic uncertainties that are in common. Unlike the previous 2009 BaBar analysis, the pion and muon contributions are separated statistically through their angular distributions. The compatibility with the previous BaBar result is quantified including statistical and systematic correlations. The new measurement is also compared with other two-pion results as a function of energy and in terms of its hadronic vacuum polarization (HVP) contribution to the muon magnetic anomaly. The leading-order HVP contribution of the two-pion channel in the energy range of 0.3$-$1.4 GeV is $a_{\mu}^{\pi\pi}=525.9\pm 3.5_\mathrm{stat}\pm3.8_\mathrm{syst}$ and $517.8\pm 2.4_\mathrm{stat}\pm3.3_\mathrm{syst}$ in units of $10^{-10}$ for the new 2026 and combined 2009 and 2026 BaBar measurements, respectively.
We report precise results for the $e^+e^-\to \pi^+\pi^-(\gamma)$ cross section in the energy range between threshold and 1.4 GeV, relevant for the hadronic vacuum polarization contribution to the muon $g-2$ prediction. Using the full 460 fb$^{-1}$ of on- and off-$\Upsilon(4S)$ data, the cross section is derived from the ratio of $\pi^+\pi^-$ to $\mu^+\mu^-$ initial-state radiation (ISR) events, in which many systematic uncertainties cancel. The pion/muon separation does not rely on particle identification as in the 2009 BaBar analysis, but rather makes use of the well-known, different angular distributions in the two-particle center-of-mass system. The compatibility and correlation between the 2026 and 2009 measurements are discussed. The new measurement is also compared with other two-pion results. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the new 2026 and combined 2009 and 2026 BaBar measurements in the energy range of 0.3$-$1.4 GeV are $525.9\pm 3.5_\mathrm{stat}\pm3.8_\mathrm{syst}$ and $517.8\pm 2.4_\mathrm{stat}\pm3.3_\mathrm{syst}$ in units of $10^{-10}$, respectively.
A search for a Majorana-type heavy neutral lepton (HNL) produced in top quark decays is reported. The search is based on proton-proton collision data at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$, collected during 2016$-$2018 with the CMS detector at the CERN LHC. In the type-I seesaw model, a Majorana HNL can mediate lepton-number-violating top quark decays. The search targets top quark pair production events in which one top quark decays into a bottom quark and a W boson, which subsequently decays into a charged lepton (electron or muon) and an HNL. The Majorana HNL then decays into a W boson and a charged lepton with the same sign and flavor as the one produced with the HNL. The experimental signature features a same-sign, same-flavor lepton pair and jets. The HNL masses between 20 and 100 GeV are probed. The data agree with the standard model background prediction. Observed (expected) upper limits at 95% confidence level are set on the branching fraction of the top quark decay into an HNL, a charged lepton, and a bottom quark, varying between [1.1 (0.71)] $\times$10$^{-5}$ and [4.8 (3.0)] $\times$10$^{-5}$ in the dielectron channel, and between [3.7 (3.4)] $\times$10$^{-6}$ and [2.4 (1.3)] $\times$10$^{-5}$ in the dimuon channel.
Axion-like particles (ALPs) are a common benchmark for experiments at the lifetime frontier, as their suppressed couplings to Standard Model particles can render them long-lived on detector scales. This article presents the projected sensitivity of the ANUBIS detector at the High-Luminosity LHC (HL-LHC) to two scenarios: a photophobic ALP model and a modified version of the flavour-diagonal, fermion-coupled ALP benchmark of the Physics Beyond Colliders initiative that introduces an additional $h\to Za$ decay. For the photophobic case, the ANUBIS detector will reach a peak sensitivity of $\sigma_{Wa}\cdot Br(a\rightarrow gg)=6.2\times10^{-4}$~pb at $c\tau=1.95$ m for an ALP mass of $10$ GeV. This will improve on the current exclusion limits from the ATLAS experiment by two orders of magnitude and extend the $c\tau$ coverage by three orders of magnitude. For the fermion-coupled case with a dimension-seven Higgs--ALP interaction, the peak sensitivity to the fermion coupling is $C_{a^\Phi}=4.4\times10^{-8}$, reached for ALP masses between $16$ and $23$ GeV. Overall, the ANUBIS detector is shown to provide unique coverage of the parameter space of both models, in regions that can be probed neither by the main LHC experiments nor by forward detectors at the HL-LHC.
Context: Astronomical imaging aims to maximize signal capture while minimizing noise. Enhancing the signal-to-noise ratio directly on detectors is difficult and expensive, leading to extensive research in advanced post-processing techniques. Aims: Removing background noise from images is a valuable pre-processing step catalog-building tasks. We introduce BGRem, a machine learning (ML) based tool to remove background noise from astronomical images. Methods: BGRem uses a diffusion-based model with an attention U-Net as backbone, trained on simulated images for optical and gamma ({\gamma})-ray data from the MeerLICHT and Fermi-LAT telescopes. In a supervised manner, BGRem learns to denoise astronomical images over several diffusion steps. Results: BGRem performance was compared with a widely used tool for cataloging astronomical sources, SourceExtractor (SExtractor). It was shown that the amount of true positive sources using SExtractor increased by about 7% for MeerLICHT data when BGRem was used as a pre-processing step. We also show the generalizability of BGRem by testing it with optical images from different telescopes and also on simulated {\gamma}-ray data representative of the Fermi-LAT telescope. We show that in both cases, BGRem improves the source detection efficiency. Conclusions: BGRem can improve the accuracy in source detection of traditional pixel-based methods by removing complex background noise. Using zero-shot approach, BGRem can generalize well to a wide range of optical images. The successful application of BGRem to simulated {\gamma}-ray images, alongside optical data, demonstrates its adaptability to distinct noise characteristics and observational domains. This cross-wavelength performance highlights its potential as a general-purpose background removal framework for multi-wavelength astronomical surveys.
Aims: We develop a deep-learning-based pipeline to construct a list of {\gamma}-ray sources and to quantitatively assess the impact of background uncertainties on source detection and characterization. This work extends and enhances the existing AutoSource-ID algorithm, previously adopted on optical observations, for applications on {\gamma}-ray data. Methods: Source detection was performed through image segmentation using a U-Net architecture, while dedicated deep neural networks (DNNs) were employed for source localization, flux estimation, and the classification of detected sources as either genuine or spurious. The training data were generated from Monte Carlo simulations based on 10 years of Fermi-LAT observations. Results: We present the first Fermi-LAT {\gamma}-ray all-sky source list derived using a deep-learning-based method (1FDL), containing more than 3800 sources with estimated positions, fluxes, and probability scores that quantify the likelihood of each detection being genuine or spurious. We further evaluated the robustness of the pipeline against variations in the interstellar emission model (IEM) and compared the resulting source list with the 4FGL-DR2 catalog. Conclusion: We find that the our deep-learning-based approach achieves performance comparable to, or better than, the likelihood-based methods employed in the standard Fermi-LAT catalog pipeline when tested against changes in the IEM. The 1FDL source list is broadly consistent with the Fermi-LAT 4FGL-DR2 catalog obtained over the same 10-year observational period, including a few new sources confirmed in the recent Fermi-LAT 16-year source list (FL16Y). These results demonstrate the potential of DNN methods for the detection and characterization of {\gamma}-ray sources and provide a foundation for their application in future high-energy astrophysical analyses.
Coherent elastic neutrino--nucleus scattering (CE$\nu$NS) offers a low-energy probe of the weak mixing angle, $\sin^2\theta_W$, and possible new physics in the neutrino sector. Its reliable interpretation, however, requires independent constraints on the neutron distributions of the target nuclei. We exploit correlations across nuclei within an extended Skyrme energy-density-functional framework to propagate constraints from the CREX and PREX-II parity-violating electron-scattering (PVES) measurements to CE$\nu$NS targets. This yields neutron form factors and radii, with relative radius uncertainties below $1\%$. Using these inferred neutron form factors, we jointly analyze the COHERENT CsI, Ge, and Ar data and obtain $\sin^2\theta_W=0.231^{+0.016}_{-0.015}$, consistent with the Standard Model prediction. These results provide the first electroweak-experimentally constrained benchmarks for nuclear-structure studies, neutrino-sector electroweak tests, and CE$\nu$NS signal predictions in dark-matter detectors.
We present the first next-to-next-to-leading-order (NNLO) predictions for inclusive color-singlet toponium hadroproduction within NRQCD in the zero-width limit for the top quark. The central new result is the NNLO QCD correction to the short-distance coefficient for pseudoscalar toponium production. The predictions include electroweak and relativistic corrections, additional production channels, and perturbatively evaluated bound-state matrix elements. At the Large Hadron Collider proton-proton center-of-mass energy of $13\,\mathrm{TeV}$, we obtain a cross section of $4.87^{+0.63}_{-0.65}\,\mathrm{pb}$, with scale and parametric uncertainties combined. The NNLO corrections reduce the renormalization- and factorization-scale uncertainties by approximately a factor of four relative to NLO, to the few-percent level. The dominant remaining uncertainty arises from the Coulomb-scale dependence of the bound-state matrix elements.
We argue that the squark simplified model commonly used in collider analyses fails to represent realistic low-energy supersymmetric models and significantly underestimates squark production rates at hadron colliders. This has important implications for extracting squark mass limits at the LHC and assessing the potential reach of HL-LHC or the FCC-hh. In particular, we find that the current LHC squark mass limit is realistically in the range of 2.6-3.0 TeV, rather than the 1.7-1.8 TeV range that is typically quoted based on the simplified model.
Fragmentation of relativistic neon nuclei was studied with the Abrasion-Ablation Monte Carlo for Colliders model (AAMCC-MST), which incorporates pre-equilibrium clustering of spectator matter via a Minimum Spanning Tree (MST) algorithm, subsequent decays of the excited clusters, and Coulomb repulsion between final fragments. After validating the model against data on $^{20}$Ne and $^{22}$Ne fragmentation on various targets, the production of spectator nuclear fragments, neutrons, and protons in $^{20}$Ne-$^{20}$Ne collisions at the LHC was modeled at $\sqrt{s_{\rm NN}}=5.36$ TeV. The production cross sections, transverse and longitudinal momentum distributions for $^2$H, $^4$He, $^6$Li, $^{10}$B, $^{12}$C, $^{14}$N, and $^{16}$O fragments were calculated as these fragments may contaminate $^{20}$Ne beams circulating in the LHC. A simple parametrization describing relative width of the longitudinal momentum distributions calculated for these fragment species was constructed. To study the sensitivity of results to $\alpha$-clustering and nuclear density distributions in neon nuclei, two density profiles for $^{20}$Ne and $^{22}$Ne were used: a deformed Woods-Saxon, and a five-$\alpha$ bi-pyramidal configuration. In addition, a spherical profile was considered for $^{20}$Ne. Comparing results obtained with these profiles and with or without MST clustering shows that the calculated yields of spectator nucleons and fragments, their momentum distributions are sensitive to the details of nuclear structure of $^{20,22}$Ne and fragmentation modeling. The results provide the input for simulations of the transport of secondary forward nuclei in the LHC to estimate their interaction rates with circulating $^{20}$Ne beam nuclei, and also for calculations of responses of neutron and proton Zero Degree Calorimeters to $^{20}$Ne-$^{20}$Ne events.
ALICE experiment at the CERN's Large Hadron Collider, carried out a cosmic data-taking campaign in the period 2015-2018 (LHC Run 2), for a total of 62.5 days of real observation time. In this contribution, the analysis of these data is limited to multimuon events, i.e. events with more than four detected muons. In particular, the muon multiplicity distribution (MMD) is studied in the low-intermediate multiplicity range ($4 < N_{\mu} < 50$), while for multiplicities $N_{\mu} > 100$, dominated by large statistical fluctuations of the data, the rate of the high muon multiplicity events (HMM) is measured. The results are compared with Monte Carlo simulations using three of the main hadronic interaction models describing the air shower development in the atmosphere: QGSJET-II-04, EPOS-LHC, and SIBYLL 2.3d, and assuming two extreme compositions of primary cosmic rays, pure proton and pure iron, representing respectively the lightest and the heaviest composition. Although the models have difficulty precisely describing the trend of primary p and Fe as a function of cosmic ray primary energy, QGSJET-II-04 is the only model that reproduces reasonably well the MMD and the rate of HMM events assuming a heavy composition for the entire energy range studied.
The zero-energy antiproton absorption in light nuclei is considered. The ratios $R_{n/p}$ of antiproton-neutron versus antiproton-proton capture rates in the lightest nuclei $^2$H, $^3$He and $^4$He are analyzed, both from the experimental results and from some selected theoretical models. These models are used as key input in determining the level shifts and widths of the lowest states. We establish the capture orbit in $^3$He to be states of angular momentum $L=2$. Several N-\={N} optical potentials are used to compute the $R_{n/p}$ values for several heavy nuclei and the results are found to be consistent with the data. However, the old chamber experiments studying pionization in Helium, are not consistent with models. Only the Paris potential is likely to succeed provided baryonia states predicted by this model are better understood.