This Letter analyzes the sensitivity of event geometry estimators to the initial-state kinematics of hard scattering in proton–lead collisions. This analysis uses dijets as a proxy for the parton–parton scattering configuration, correlating it with event geometry estimators, namely the energy deposited in the Zero-Degree Calorimeter and the transverse energy recorded in the Forward Calorimeter in the Pb-going direction. The analysis uses data recorded by the ATLAS detector at the Large Hadron Collider with a nucleon–nucleon center-of-mass energy of 8.16 TeV, corresponding to an integrated luminosity of 56 nb⁻¹. The jets are measured within the pseudorapidity interval −2.8  <  η  <  4.5, where positive η values correspond to the direction of the proton beam. Results are presented as a function of the Bjorken-x of the parton originating from the proton, x_p. Both event geometry estimators are found to be dependent on x_p, with the energy deposited in the Zero-Degree Calorimeter about six times less sensitive to x_p compared with the transverse energy deposited in the Forward Calorimeter.

Events with t-channel single top quarks are used to probe effective field theory operators in s=13 TeV proton–proton collision data corresponding to 140 fb−1 recorded by the ATLAS detector at the Large Hadron Collider. To extract a maximum amount of information from these events, the joint fourfold probability distribution for four kinematic angles is simultaneously analysed. A method leveraging Fourier techniques to handle the challenges arising from this approach is employed. The relevant effective field theory operators are those sensitive to top-quark decay and t-channel production vertices. Their Wilson coefficients are tightly constrained, with results compatible with Standard Model predictions.

This paper presents differential cross-section measurements of events containing a charged lepton, missing transverse momentum, two b-jets, and two light jets, consistent with the W+W−bb¯ final state. The analysis is based on 140 fb−1 of proton-proton collision data at s=13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider (2015–2018). Differential cross-sections are unfolded at particle level for a range of kinematic observables and in three kinematic regions. Both regions and observables are optimised for sensitivity to the interference between W+W−bb¯ produced with singly and doubly resonant top quarks. Results are compared with various theoretical predictions and deviations are observed in regions sensitive to the interference between singly and doubly resonant top-quark production. The unfolded results provide new information about the modelling of interference and off-shell effects of the W+W−bb¯ final state, improving the understanding of top-quark production mechanisms and providing new inputs for future Monte Carlo generator development.

This paper reports the observation of electroweak diboson (WW/WZ/ZZ) production in association with a high-mass dijet system, in which final states with one boson decaying leptonically and the other boson decaying hadronically are studied. The hadronically decaying W/Z boson is reconstructed as either two small-radius jets or one large-radius jet with jet substructure requirements. The data analyzed correspond to an integrated luminosity of 140 fb−1 of proton–proton collisions at a center-of-mass energy of s√=13 TeV collected with the ATLAS detector during the 2015-2018 data taking at the Large Hadron Collider. The electroweak production of WW/WZ/ZZ in association with two jets is observed in a phase space dominated by vector-boson scattering with a significance of 7.4σ (expected 6.1σ) and the signal strength is determined to be 1.28+0.23−0.21. The corresponding production cross section in a fiducial phase space is measured in addition. The signal strengths of both electroweak and QCD associated diboson productions are furthermore measured in a two-dimensional fit, the result of which agrees with the Standard Model prediction. The data are interpreted in the context of a dimension-8 effective field theory to probe anomalous quartic gauge couplings resulting in the first set of exclusion limits on the Wilson coefficients in the semileptonic channel reported by the ATLAS Collaboration. The observed limits for the S02, T0 and M0 operators are (−3.96<fS02/Λ4<3.96) TeV−4, (−0.25<fT0/Λ4<0.22) TeV−4, (−1.26<fM0/Λ4<1.25) TeV−4.

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system’s radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (𝑝T) dependence of radial flow fluctuations (𝑣0⁡(𝑝T)) over 0.5 <𝑝T <10  GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at √𝑠NN=5.02  TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in 𝑝T, and centrality-independent shape in 𝑝T. The comparison with a hydrodynamic model demonstrates the sensitivity of 𝑣0⁡(𝑝T) to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.

The ATLAS experiment has performed a measurement of coherent exclusive J/ψ → μ⁺μ⁻ production in ultraperipheral Pb+Pb collisions at sNN−−−√=5.36 TeV. The data was recorded at the Large Hadron Collider (LHC) during 2023, and corresponds to an integrated luminosity of 79 μb⁻¹. Exclusive J/ψ candidates were selected with a dedicated track-sensitive trigger based on the ATLAS transition radiation tracker. The analysis involves reconstruction of the dimuon invariant mass based on muon tracks from the inner detector, as the muon transverse momentum range of interest precludes the use of the standard muon reconstruction and identification algorithms. Differential cross sections are measured as a function of J/ψ rapidity and are compared with theoretical predictions. After extrapolation to sNN−−−√=5.02 TeV, they are also compared with previous measurements performed by other experiments using data from LHC Run 2. While the results agree reasonably well with theoretical predictions, they are in tension with previous Run-2 results for the central rapidity region.

The top-quark mass is measured using top-quark decays producing an isolated lepton and J/ψ meson reconstructed in its μ⁺μ⁻ decay mode. The data sample was recorded with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of s√=13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb⁻¹. The measurement is based on the invariant mass m(ℓμ⁺μ⁻) of the system made of the isolated lepton ℓ from the W boson decay and the non-isolated μ⁺μ⁻ pair from a J/ψ decay of a b-hadron, exploiting its sensitivity to the top-quark mass. An unbinned maximum-likelihood fit to the m(ℓμ⁺μ⁻) distribution is performed to extract the top-quark mass. The top-quark mass is measured to be m_top = 172.17 ± 0.80 (stat) ± 0.81 (syst) ± 1.07 (recoil) GeV, with a total uncertainty of 1.56 GeV. The third uncertainty arises from changing the dipole parton shower gluon-recoil scheme used in top-quark decays.

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair () invariant mass in proton-proton collisions using 140 fb⁻¹ of data at √s = 13 TeV collected in 2015–2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The system is reconstructed in the single-lepton final state, requiring exactly one isolated electron or muon and at least four jets with at least two identified as originating from b-quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of Y_t < 2.1 relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.

A search is conducted in proton–proton collisions at the Large Hadron Collider for photon-induced production pp → pp + γγ, (γγ → VX) of a visible particle V decaying into a pair of same-flavour charged leptons (e+e− or μ+μ−) and an undetected invisible component X. Measurements of the outgoing proton energies by the ATLAS forward proton spectrometer allow the full photon–photon four-momentum to be reconstructed. By subtracting the visible four-momentum of the central system measured with the ATLAS detector, the “missing mass” of any event components not detected in the central region can be reconstructed, enabling the reconstruction of X without knowing its properties, thus allowing the search to be model-independent. A search for a narrow resonance is performed in the missing-mass spectrum between 100 GeV and 900 GeV. The analysis uses data collected in 2017 from proton–proton collisions at a centre-of-mass energy of s=13 TeV, corresponding to an integrated luminosity of 14.7 fb−1. No significant excess over the Standard Model expectation is observed and upper limits at 95% confidence level are set on the fiducial cross sections for three different signal models in the range between 128 and 2.5 fb. Additionally, model-independent limits are set on the visible cross section of BSM processes, for two sets of selection criteria. Both individual lepton flavour decay channels of the visible boson and a combination of the two channels are considered.

This paper presents the first observation of top-quark pair production in association with two photons (tt¯γγ). The measurement is performed in the single-lepton decay channel using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb−1 recorded during Run 2 at a centre-of-mass energy of 13 TeV. The tt¯γγ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be 2.42+0.58−0.53fb, corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of tt¯γγ to top-quark pair production in association with one photon is determined, yielding (3.30+0.70−0.65)×10−3.