Physics

Baryon Acoustic Oscillations from the C iv Forest with DESI DR2

(2026)

We present a measurement of baryon acoustic oscillations (BAO) in the cross correlation of triply ionized carbon (C iv) absorption with the positions of quasars (QSO) and emission-line galaxies (ELGs). We use quasars and ELGs from the second data release (DR2) of the Dark Energy Spectroscopic Instrument survey. Our data sample consists of 2.5 million quasars, 3.1 million ELGs, and the C iv absorption is measured along the line of sight of 1.5 million high-redshift quasars with z > 1.3. We measure the isotropic BAO signal at 4.2σ for the C IV×QSO cross correlation. This translates into a 3.0% precision measurement of the ratio of the isotropic distance scale, DV, and the sound horizon at the drag epoch, rd, with DV/rd(zeff = 1.92) = 30.3 ± 0.9. We make the first detection of the BAO feature in the C IV×ELG cross-correlation at a significance of 2.5σ and find DV/rd(zeff = 1.47) = 24.6 ± 1.0.

Observation of a Bc*+ Meson with the ATLAS Detector

(2026)

The first observation of a new meson state decaying into and a photon is presented using a dataset of collisions at a center-of-mass energy collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of . The mesons are reconstructed from a three-muon final state from decays, along with photons reconstructed via conversions to electron-positron pairs in the detector material. The new state is observed with a significance that exceeds 8 standard deviations. The mass difference between the new meson state and the ground-state meson is measured to be . This corresponds to a mass for the observed state of , where the last uncertainty is due to the precision of the best knowledge of the meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the system, the meson.

Measurement of coincident photon-initiated processes in ultra-peripheral Pb+Pb collisions with the ATLAS detector

(2026)

The Lorentz-contracted electromagnetic fields of the ions in ultra-relativistic heavy-ion collisions generate intense quasi-real photon fluxes. These lead to photon-induced interactions that are observed in ultra-peripheral collisions (UPCs), such as vector meson and lepton-pair production. The high photon flux also enables the occurrence of multiple photon-induced processes in a single collision. Presented is the first measurement of the coincident production of γ γ → μ + μ − and γ + A → ρ 0 + A in UPC Pb+Pb collisions at centre-of-mass energies of 5.02 TeV and 5.36 TeV with the ATLAS detector at the Large Hadron Collider. The coincident ρ 0 is identified via its decay into a π + π − pair, with the selected π + π − sample also containing the non-resonant continuum contribution. The rate of the coincident process relative to the exclusive γ γ → μ + μ − process is measured differentially in intervals of forward event activity, quantified by the Zero Degree Calorimeters. The relative rate of coincident ρ 0 production, including non-resonant π + π − contributions, is measured to be ( 9.3 ± 0.4 ( stat . ) ± 0.2 ( syst . ) ) × 10 − 3 , summed over forward event activity. Correlations between the dimuon kinematic properties, such as its mass, and the coincident ρ 0 meson production rate, are also presented. These measurements confirm the presence of multi photon-induced processes in UPC collisions, and can provide new insight into the impact parameter dependence of photon-induced vector meson production.

Combination of ATLAS and CMS Searches for Higgs Boson Pair Production at s=13 TeV

(2026)

This Letter presents a combination of searches for Higgs boson pair ( ) production performed by the ATLAS and CMS Collaborations using proton-proton collision datasets recorded at during the Large Hadron Collider Run 2, corresponding to integrated luminosities ranging between 126 and . The upper limit at the 95% confidence level on the total signal strength, defined as the ratio of the measured cross section to the standard model (SM) prediction, corresponds to 2.5, with an expected value of 1.7 (2.8) assuming the absence (presence) of the SM signal. The strength of the signal is measured to be relative to the SM prediction. The observed significance is found to be 1.1 standard deviations whereas 1.3 are expected for the SM signal. Constraints are set on the Higgs boson trilinear self-coupling and on the couplings of two Higgs bosons to two vector bosons, both normalized to the SM predictions and denoted as and , respectively. The observed individual constraints at the 95% confidence level are and , while the expected constraints assuming the presence of the SM signal are and .

Search for anomalies in vector-boson fusion production of the Higgs boson in H(→ γγ)jj events using 164 fb−1 of pp collision data collected at s=13.6 TeV with the ATLAS detector

(2026)

This article details two studies of Higgs boson properties using the vector-boson fusion production mode and the γγjj final state. Both efforts are based on a data sample corresponding to 164 fb−1 of s=13.6$$ \sqrt{s}=13.6 $$ TeV proton-proton collisions recorded by the ATLAS experiment at the Large Hadron Collider. The first study employs matrix-element-based optimal observables to constrain CP-odd couplings beyond the Standard Model within the Standard Model Effective Field Theory framework, expressed in the Warsaw basis. The second study exploits angular distributions to probe the Higgs boson’s couplings to longitudinally and transversely polarised W and Z bosons in the production of the Higgs boson. To maximise the sensitivity, the constraints of the CP-odd couplings are combined with those from a previous analysis performed in γγjj events in a data sample of proton-proton collisions at s=13$$ \sqrt{s}=13 $$ TeV, corresponding to an integrated luminosity of 140 fb−1. A significant improvement with respect to the previous analysis is achieved through the implementation of a new neural-network-based classification algorithm. All measurements are in agreement with the Standard Model prediction of a CP-even Higgs boson with the expected relative coupling strengths to longitudinally and transversely polarised vector bosons.

Measurement of Charged-Particle Production in sNN=9.62 TeV Proton-Oxygen Collisions as a Probe of Cosmic-Ray Air Showers with the ATLAS Detector

(2026)

This Letter presents a measurement of prompt charged-particle production in proton-oxygen interactions at center-of-mass energy with the ATLAS detector, corresponding to of integrated luminosity. A total of selected events have at least one track with transverse momentum and pseudorapidity . The measured fiducial proton-oxygen cross section is and the extrapolated inelastic proton-air cross section is . Measurements of charged-particle multiplicity, , and distributions are an order-of-magnitude more precise than differences between hadronic-interaction models. These results enable improved modeling of cosmic-ray air showers, which is important for astroparticle physics.

Measurements of Z-Boson Pair Entanglement in Decays of Higgs Bosons at the ATLAS Experiment

(2026)

Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of bosons are reported, using proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 and 13.6 TeV, recorded with the ATLAS detector. Measurements of angular observables sensitive to spin-density-matrix elements in the process yield coefficients and , consistent with their Standard Model predictions. A complementary hypothesis test using the full angular distribution, and relying on several Standard Model assumptions in the decays, provides substantially higher sensitivity to quantum correlations and disfavors the separable-state hypothesis at a significance of 4.7 standard deviations (expected ) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.

Search for New Physics via Low-Energy Electron Recoils with a 4.2 Tonne-Year Exposure from the LZ Experiment

(2026)

We report results from searches for new physics models through electron recoils using data collected by the LUX-ZEPLIN experiment during its first two science runs, with a total exposure of 4.2 tonne-years. The observed data are consistent with a background-only hypothesis. Constraints are derived for electromagnetic interactions of solar neutrinos, solar axionlike particles (ALPs), mirror dark matter, and the absorption of bosonic dark matter candidates. The inverse Primakoff process for ^{57}Fe deexcitation solar ALPs is considered for the first time. These results represent the most stringent constraints to date on keV-scale Primakoff and ^{57}Fe solar ALPs, bosonic dark matter, mirror dark matter, and neutrino millicharge, while remaining competitive for the other signal models investigated.

Search for charginos and neutralinos with B-L R-parity violating decays in s=13 TeV and 13.6 TeV pp collisions with the ATLAS detector

(2026)

A search is performed for the electroweak pair production of charginos and associated production of a chargino and neutralino, each of which decays through an -parity-violating coupling into a lepton and a , , or Higgs boson. This search targets the Higgs boson decay channel of the charginos and neutralinos, using events with three or more -tagged jets and one or two electrons or muons. The analyzed data corresponds to an integrated luminosity of and of proton-proton collision data produced by the Large Hadron Collider at center-of-mass energies of and , respectively, collected by the ATLAS experiment between 2015 and 2023. The data are found to be consistent with predictions from the Standard Model. The results are interpreted as limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model. Limits are also set on the production of charginos and neutralinos for a minimal supersymmetric Standard Model with an additional gauge symmetry that is spontaneously broken. Charginos and neutralinos with masses between 150 GeV and 1100 GeV are excluded at 95% confidence level for a scenario in which they decay via Higgs bosons, assuming equal decay branching fractions to each lepton flavor. Additional limits on chargino and neutralino production are derived for flavor-specific decay scenarios where the charginos decay into a single lepton flavor.

Search for quantum black holes in lepton+jet final states using proton–proton collisions at s = 13.6 TeV with the ATLAS detector

(2026)

A search for quantum black holes in electron+jet or muon+jet final states with high invariant mass is performed. The analysis uses data from s = 13.6 TeV pp collisions recorded by the ATLAS detector between 2022 and 2024 during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of 164 fb − 1 . This search is strongly motivated by a dramatic increase of the production cross-section by up to an order of magnitude for the highest masses considered, thanks to the small increase of 0.6 TeV in centre-of-mass energy between Run 2 and Run 3. No significant excess above the Standard Model background is observed, and 95% CL upper limits are set on the production cross-section times branching ratio in several benchmark models, reaching a mass scale of 9.4 TeV. These represent the strongest exclusion limits to date on quantum black hole production.

Physical Sciences

Physics