Publications

This Letter reports the observation of W⁺W⁻γ triboson production in 140 fb⁻¹ of data collected by the ATLAS detector from protonproton collisions at a centre-of-mass energy of s = 13 TeV at the LHC. Events with an opposite-charge eμ pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the W⁺W⁻γ→e^±μ^∓νν¯γ final state is 6.2 ± 0.8 (stat.) ± 0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1_−0.7^+1.0 fb. Constraints on the Wilson coefficients of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective field theory framework.

A measurement of the top-quark pole mass mtpole is presented in tt¯ events with an additional jet, tt¯ + 1-jet, produced in pp collisions at s=13 TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb⁻¹. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the tt¯ + 1-jet differential cross-section as a function of the inverse of the invariant mass of the tt¯ + 1-jet system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process pp→tt¯j2→3, where top quarks are taken as stable particles, and the process pp→bb¯l+νl−ν¯j2→7, which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a χ² fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the 2 → 3 measurement: mtpole=170.7±0.3stat.±1.4syst.±0.3scale±0.2PDF⊕αS GeV, which is in good agreement with other top-quark mass results.

A search for single production of a vector-like quark Q, which could be either a singlet T, with charge 23, or a Y from a (T, B, Y) triplet, with charge −43, is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb⁻¹ recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis targets Q → Wb decays where the W boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are set on the cross-section times branching ratio, and on the coupling of the Q to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet T, the 95% confidence level limit on the coupling strength κ ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the (T, B, Y) triplet, the limits on κ vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.

A search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying τ-lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a τ-slepton or a τ-sneutrino. Three channels are examined, requiring either exactly one hadronically decaying τ-lepton and no other leptons, exactly one hadronically decaying τ-lepton and at least one other lepton, or two or more hadronically decaying τ-leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach, are used. The search uses 140 fb-1 and 51.8 fb-1 of pp collision data recorded by the ATLAS detector at the Large Hadron Collider during 20152018 at s=13 TeV and 20222023 at s=13.6 TeV, respectively. Gluino masses below 2.25 TeV and squark masses up to 1.7 TeV

One correction is noted for the paper [1], which does not affect any of the other results reported. The right vertical axis (gaγγ) of figure 8 was not defined consistently with the description in the caption and was incorrectly lower by a factor of 10.

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions.This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of R=1.0 and its dependence on the jet substructure. The measurement uses 1.72 nb^-1 of Pb+Pb data and 255 pb^-1 of pp data, both at s_NN=5.02 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering R=0.2 calorimetric jets and are measured for transverse momentum above 200 GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ( R _AA). The jet R _AA is measured as a function of jet p _T, the charged k_t splitting scale (d₁₂), and the angular separation (? R ₁₂) of two leading sub-jets. The jet R _AA gradually decreases with increasing d₁₂, implying significantly stronger suppression of large-radius jets with larger k_t splitting scale. The jet R _AA gradually decreases for ? R ₁₂ in the range 0.01-0.2 and then remains consistent with a constant for ? R ₁₂ ? 0.2. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.

Measurements of tt¯ℓ+ℓ- production in the region of high dilepton invariant mass with effective field theory (EFT) interpretations are presented. They are performed using final states with three isolated leptons (electrons or muons) and are based on s=13 TeV protonproton collision data with an integrated luminosity of 140fb-1, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Measurements of the tt¯ℓ+ℓ- signal strength and cross-section upper-limits are performed inclusively in lepton flavour and separately for electrons and muons. The study also aims to probe anomalous four-fermion interactions including to test for possible lepton flavor universality violation. No significant deviations from the Standard Model predictions are observed and the measurements are interpreted through the EFT formalism to provide new constraints on the relevant operators.

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using 140 fb⁻¹ of proton-proton collision data at (Formula presented) ¼ 13 TeV recorded in 20152018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicated analysis channel is employed to target Z-boson associated long-lived particle production, including an axionlike particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photophobic axionlike particle model considered, this search produces the strongest limits to date for proper decay lengths greater than Oð10Þ cm.

A measurement of off-shell Higgs boson production is performed in the H * → WW channel. The measurement uses a protonproton collision dataset with an integrated luminosity of 140 fb⁻¹ collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Final states in which both W bosons decay leptonically are targeted, and events are categorised based on the flavour of the final-state leptons, the jet multiplicity, and the output of neural network-based classifiers. The data are found to be compatible with the Standard Model expectation. An observed (expected) upper bound on the 95 % symmetric confidence level interval is set on the rate of off-shell Higgs boson production at a value of 3.4 (4.4) times the Standard Model prediction. These results are combined with the results from the measurement of on-shell Higgs boson production in the same final states to obtain an observed (expected) upper bound at 95 % confidence level on the Higgs boson total width of 13.1 (17.3) MeV.

A calibration of the ATLAS flavour-tagging algorithms using a new calibration procedure based on optimal transportation maps is presented. Simultaneous, continuous corrections to the b-jet, c-jet, and light-flavour jet classification probabilities from jet-tagging algorithms in simulation are derived for b-jets using tt¯→eμννbb data. After application of the derived calibration maps, closure between simulation and observation is achieved for jet flavour observables used in ATLAS analyses of Large Hadron Collider (LHC) Run 2 proton-proton collision data. This continuous calibration opens up new possibilities for the future use of jet flavour information in LHC analyses and also serves as a guide for deriving high-dimensional corrections to simulation via transportation maps, an important development for a broad range of inference tasks.

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark (U1) as predicted by an ultraviolet-complete extension of the SM, referred to as the 4321 model. Given the assumed decay of U1 into third-generation SM fermions, the final state can contain multiple τ-leptons and b-quarks. This search is based on a dataset of pp collisions at s=13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of up to 140fb-1. No significant excess above the SM background prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an R-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).

Corrections to two figures are noted. In the presentation of exclusion limits for the axion-like particle models, the branching ratio for the decay of the associated W and Z bosons into electron or muon final states was not factored out. For the associated W production mode, this represents a numerical factor of 0.214, while for the Z production model it is a factor of 0.0673. The limits in the plot hence become weaker according to these numerical factors. In addition, the Y-axis labels have been updated to explicitly include the branching ratio of the axion-like particle to gluons. The modified figures are figure 9(a) and figure 10(a).

A search for the production of a Higgs boson in association with a single top quark, tH, is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb⁻¹ at a centre-of-mass energy of 13 TeV, collected by the ATLAS detector at the LHC. The search targets Higgs-boson decays into bb¯, WW^*, ZZ^*, and ττ, accompanied by an isolated lepton (electron or muon) from the top-quark decay. Multivariate techniques are employed to enhance the separation between signal and background processes. The observed signal strength, μ_tH, defined as the ratio between the measured cross-section and the predicted Standard Model value, is μ_tH = 8.1 ± 2.6 (stat.) ± 2.0 (syst.). The significance of the observed (expected) signal above the background-only expectation is 2.8 (0.4) standard deviations. The corresponding observed (expected) upper limit at the 95% confidence level on the tH cross-section is found to be 13.9 (6.1) times the value predicted by the Standard Model. An interpretation with an inverted sign of the top-quark Yukawa coupling is performed, and the signal strength and corresponding limit are reported.

A search for exotic decays of the Higgs boson H into new scalar or pseudoscalar particles that subsequently decay into b-quarks is presented. The search considers ZH production with several decay scenarios for the Higgs boson; first to a pair of identical scalars, H → 2a → 4b, second to a pair of scalars with different masses (m_a1a2), either directly, H → a₁a₂ → 4b, or via a longer decay chain, H → a₁a₂ → 3a₁ → 6b. The analysis uses proton-proton collision data at (Formula presented) ¼ 13 TeV collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb⁻¹. No significant excess above the Standard Model prediction is observed. The search sets upper limits at 95% confidence level on the ratio of the Higgs boson production cross section to the SM prediction times the branching ratio of Higgs bosons decaying into 4b or 6b, between 4% and 25% for σðZHÞ=σ_SMðZHÞ × BðH → 2a → 4bÞ, between 24% and 38% for σðZHÞ=σ_SMðZHÞ × BðH → a₁a₂ → 4bÞ, and between 10% and 20% for σðZHÞ=σ_SMðZHÞ × BðH → a₁a₂ → 3a₁ → 6bÞ, depending on the masses of the scalar particles.

Measurements of jet substructure are key to probing the energy frontier at colliders, and many of them use track-based observables which take advantage of the angular precision of tracking detectors. Theoretical calculations of track-based observables require track functions, which characterize the transverse momentum fraction r_q carried by charged hadrons from a fragmenting quark or gluon. This letter presents a direct measurement of r_q distributions in dijet events from the 140 fb⁻¹ of protonproton collisions at s=13 TeV recorded with the ATLAS detector. The data are corrected for detector effects using machine-learning methods. The scale evolution of the moments of the r_q distribution is sensitive to non-linear renormalization group evolution equations of QCD, and is compared with analytic predictions. When incorporated into future theoretical calculations, these results will enable a precision program of theory-data comparison for track-based jet substructure observables.

The jet energy calibration and its uncertainties are derived from measurements of the calorimeter response to single particles in both data and Monte Carlo simulation using protonproton collisions at s=13 TeV collected with the ATLAS detector during Run 2 at the Large Hadron Collider. The jet calibration uncertainty for anti-kT jets with a jet radius parameter of Rjet=0.4 and in the central jet rapidity region is about 2.5% for transverse momenta (pT) of 20 GeV, about 0.5% for pT=300 GeV and 0.7% for pT=4 TeV. Excellent agreement is found with earlier determinations obtained from pT-balance based in situ methods (Z/γ+jets). The combination of these two independent methods results in the most precise jet energy measurement achieved so far with the ATLAS detector with a relative uncertainty of 0.3% at pT=300 GeV and 0.6% at 4 TeV. The jet energy calibration is also derived with the single-particle calorimeter response measurements separately for quark- and gluon-induced jets and furthermore for jets with Rjet varying from 0.2 to 1.0 retaining the correlations between these measurements. Differences between inclusive jets and jets from boosted top-quark decays, with and without grooming the soft jet constituents, are also studied.

This Letter reports the first evidence of electroweak production of same-sign (Formula presented) boson pairs where at least one of the (Formula presented) bosons is longitudinally polarized and the most stringent constraint to date for the production of two longitudinally polarized same-sign (Formula presented) bosons. The dataset used corresponds to an integrated luminosity of (Formula presented) of proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the ATLAS detector during run 2 of the Large Hadron Collider. The study is performed in final states including two same-sign leptons (electrons or muons), missing transverse momentum, and at least two jets with a large invariant mass and a large rapidity difference. Two independent fits are performed targeting the production of same-sign (Formula presented) bosons with at least one, or two longitudinally polarized (Formula presented) bosons. The observed (expected) significance of the production with at least one longitudinally polarized (Formula presented) boson is 3.3 (4.0) standard deviations. An observed (expected) 95% confidence level upper limit of 0.45 (0.70) fb is reported on the fiducial production cross section of two longitudinally polarized same-sign (Formula presented) bosons.

This paper reports measurements of the transverse energy per unit pseudorapidity (dE_T/dη) produced in Au+Au collisions at ^√s_NN = 200 GeV, performed with the sPHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The results cover the pseudorapidity range |η| T/dη are presented for a range of centrality intervals and the average dE_T/dη as a function of the number of participating nucleons, N_part, is compared to a variety of Monte Carlo heavy-ion event generators. The results are in agreement with previous measurements at RHIC, and feature an improved granularity in η and improved precision in low-N_part events.

A search for a pseudoscalar a produced in association with a top-quark pair, or in association with a single top quark plus a W boson, with the pseudoscalar decaying into b-quarks (a→bb¯), is performed using the full Run 2 data sample using a dileptonic decay mode signature. The search covers pseudoscalar boson masses between 12 and 100 GeV and involves both the kinematic regime where the decay products of the pseudoscalar are reconstructed as two standard b-tagged small-radius jets, or merged into a large-radius jet due to its Lorentz boost. No significant excess relative to expectations is observed. Assuming a branching ratio BR(a→bb¯)=100%, the range of pseudoscalar masses between 50 and 80 GeV is excluded at 95% confidence level for a coupling of the pseudoscalar to the top quark of 0.5, while a coupling of 1.0 is excluded at 95% confidence level for the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.

Measurements of the total and differential Higgs boson production cross-sections, via WH and ZH associated production using H → WW^* → ℓνℓν and H → WW^* → ℓνjj decays, are presented. The analysis uses proton-proton events delivered by the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector between 2015 and 2018. The data correspond to an integrated luminosity of 140 fb⁻¹. The sum of the WH and ZH cross-sections times the H → WW^* branching fraction is measured to be 0.44−0.09+0.10stat.−0.05+0.06syst. pb, in agreement with the Standard Model prediction. Higgs boson production is further characterised through measurements of the differential cross-section as a function of the transverse momentum of the vector boson and in the framework of Simplified Template Cross-Sections.

The mass of the top quark is measured using top-quark-top-antiquark pair events with high transverse momentum top quarks. The dataset, collected with the ATLAS detector in protonproton collisions at s=13 TeV delivered by the Large Hadron Collider, corresponds to an integrated luminosity of 140 fb⁻¹. The analysis targets events in the lepton-plus-jets decay channel, with an electron or muon from a semi-leptonically decaying top quark and a hadronically decaying top quark that is sufficiently energetic to be reconstructed as a single large-radius jet. The mean of the invariant mass of the reconstructed large-radius jet provides the sensitivity to the top quark mass and is simultaneously fitted with two additional observables to reduce the impact of the systematic uncertainties. The top quark mass is measured to be m_t=172.95±0.53 GeV, which is the most precise ATLAS measurement from a single channel.

The production of D^± and Ds± charmed mesons is measured using the D^±/Ds±→ ϕ(μμ)π^± decay channel with 137 fb⁻¹ of s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider during the years 20162018. The charmed mesons are reconstructed in the range of transverse momentum 12 T

A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a W boson along with a muon or electron. Two channels are explored: a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with 140 fb⁻¹ of s = 13 TeV proton-proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as 10⁻⁷ are excluded at the 95% confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 fb⁻¹ of proton-proton collisions at s = 13 TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.

High precision single-differential W±-boson production cross-sections as a function of electron or muon transverse momentum pT or their pseudorapity η, as well as double-differential cross-sections as functions of these variables, are measured in protonproton collisions at centre-of-mass energies s=5.02 TeV and 13 TeV. The W-boson charge asymmetry as a function of lepton η is also measured. The data, collected in dedicated runs at reduced instantaneous luminosity with the ATLAS detector at the Large Hadron Collider, correspond to integrated luminosities of 255 pb-1 at 5.02 TeV and 338 pb-1 at 13 TeV. The measurements are in agreement with Standard-Model predictions calculated at next-to-next-to-leading-order in the strong coupling constant αs including transverse-momentum resummation at next-to-next-to-leading logarithmic accuracy using several parton distribution functions. The impact of the measured differential cross-sections as a function of lepton η on the determination of these functions is studied using a profiling technique.

A measurement of the B0 meson lifetime using B0→J/ψK∗0 decays in data from 13 TeV protonproton collisions with an integrated luminosity of 140fb-1 recorded by the ATLAS detector at the LHC is presented. The measured effective lifetime is (Formula presented.) The average decay width extracted from the effective lifetime, using parameters from external sources, is (Formula presented.) where the uncertainties are statistical, systematic and from external sources. The earlier ATLAS measurement of Γs in the Bs0→J/ψϕ decay was used to derive a value for the ratio of the average decay widths Γd and Γs for B0 and Bs0 mesons respectively, of (Formula presented.) The measured lifetime, average decay width and decay width ratio are in agreement with theoretical predictions and with measurements by other experiments. This measurement provides the most precise result of the effective lifetime of the B0 meson to date.

The jet cross section and jet-substructure observables in p +p collisions at root s =200 GeV were measured by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC). Jets are reconstructed from charged-particle tracks and electromagnetic-calorimeter clusters using the anti-k(t) algorithm with a jet radius of R =0.3 for jets with transverse momentum within 8.0

Neural simulation-based inference (NSBI) is a powerful class of machine-learning-based methods for statistical inference that naturally handles high-dimensional parameter estimation without the need to bin data into low-dimensional summary histograms. Such methods are promising for a range of measurements, including at the Large Hadron Collider, where no single observable may be optimal to scan over the entire theoretical phase space under consideration, or where binning data into histograms could result in a loss of sensitivity. This work develops a NSBI framework for statistical inference, using neural networks to estimate probability density ratios, which enables the application to a full-scale analysis. It incorporates a large number of systematic uncertainties, quantifies the uncertainty due to the finite number of events in training samples, develops a method to construct confidence intervals, and demonstrates a series of intermediate diagnostic checks that can be performed to validate the robustness of the method. As an example, the power and feasibility of the method are assessed on simulated data for a simplified version of an off-shell Higgs boson couplings measurement in the four-lepton final states. This approach represents an extension to the standard statistical methodology used by the experiments at the Large Hadron Collider, and can benefit many physics analyses.

Correction to one figure and the corresponding numbers in the text are noted for the paper. A wrong cross-section was used for the theory prediction of t¯tA(→ t¯t), which was estimated to differ by around 1% with respect to the one of t¯tH(→ t¯t). It was found later that the production cross-section for t¯tA(→ t¯t) can be up to 60% higher than the one of t¯tH(→ t¯t) for tan β ∼ 1. This affects the theoretical prediction shown in figure 7, and the limits in the tan β vs mH/A plane shown in figure 8(a). The changes in the text are noted for sections 8 and 9. In the scenario where the scalar H and pseudo-scalar A bosons have equal masses and both contribute to BSM t¯tt¯t production, values of tan β below 1.9 and 0.7 are excluded for mH/A between 400 GeV and 1000 GeV, respectively, at 95% CL instead of 1.6 and 0.6. In the scenario where only the pseudo-scalar A boson contributes to BSM t¯tt¯t production, values of tan β below 1.5 and 0.5 are excluded for mA between 400 GeV and 1000 GeV, respectively, at 95% CL instead of 1.2 and 0.5. The original plot for this scenario is not included in the paper as it was nearly identical to the one where only the scalar H boson contributes to BSM t¯tt¯t production, and was therefore omitted. After correction, the updated limits in the tan β vs mA plane are shown in figure 8(c).

A measurement of off-shell Higgs boson production in the H^∗ → ZZ → 4ℓ decay channel is presented. The measurement uses 140 fb⁻¹ of protonproton collisions at √s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider and supersedes the previous result in this decay channel using the same dataset. The data analysis is performed using a neural simulation-based inference method, which builds per-event likelihood ratios using neural networks. The observed (expected) off-shell Higgs boson production signal strength in the ZZ → 4ℓ decay channel at 68% CL is 0.87^+0.75_−0.54 (1.00^+1.04_−0.95). The evidence for off-shell Higgs boson production using the ZZ → 4ℓ decay channel has an observed (expected) significance of 2.5σ (1.3σ). The expected result represents a significant improvement relative to that of the previous analysis of the same dataset, which obtained an expected significance of 0.5σ. When combined with the most recent ATLAS measurement in the ZZ → 2ℓ2ν decay channel, the evidence for off-shell Higgs boson production has an observed (expected) significance of 3.7σ (2.4σ). The off-shell measurements are combined with the measurement of on-shell Higgs boson production to obtain constraints on the Higgs boson total width. The observed (expected) value of the Higgs boson width at 68% CL is 4.3^+2.7_−1.9 (4.1^+3.5_−3.4) MeV.

A search is presented for a heavy scalar (H) or pseudo-scalar (A) predicted by the two-Higgs-doublet models, where the H/A is produced in association with a top-quark pair (tt¯H/A), and with the H/A decaying into a tt¯ pair. The full LHC Run 2 protonproton collision data collected by the ATLAS experiment is used, corresponding to an integrated luminosity of 139fb-1. Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the tt¯+jets background in the regime with high jet and b-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An H/A-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the tt¯H/A→tt¯tt¯ production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an H/A with mass between 400 GeV and 1000 GeV, respectively. Assuming that both the H and A contribute to the tt¯tt¯ cross-section, tanβ values below 1.7 or 0.7 are excluded for a mass of 400 GeV or 1000 GeV, respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a tt¯ pair.

The EIC Comprehensive Chromodynamics Experiment (ECCE) detector has been designed to address the full scope of the proposed Electron Ion Collider (EIC) physics program as presented by the National Academy of Science and provide a deeper understanding of the quarkgluon structure of matter. To accomplish this, the ECCE detector offers nearly acceptance and energy coverage along with excellent tracking and particle identification. The ECCE detector was designed to be built within the budget envelope set out by the EIC project while simultaneously managing cost and schedule risks. This detector concept has been selected to be the basis for the EIC project detector.

The large amount of data recorded with the ATLAS detector at the Large Hadron Collider, corresponding to 140 fb⁻¹ of pp collisions at a centre-of-mass energy of s=13TeV, has brought our knowledge of the top quark to a higher level. The measurement of the topantitop quark pair-production cross-section has reached a precision of 1.8% and the cross-section was measured differentially up to several TeV in multiple observables including the top-quark transverse momentum and top-quark-pair invariant mass. Single-top-quark production was studied in all production modes. Rare production processes where the top quark is associated with a vector boson, and four-top-quark production, have become accessible and cross-section measurements for several of these processes have reached uncertainties of around 10% or smaller. Innovative measurements of the top-quark mass and properties have also emerged, including the observation of quantum entanglement in the top-quark sector and tests of lepton-flavour universality using top-quark decays. Searches for flavour-changing neutral currents in the top-quark sector have been significantly improved, reaching branching-ratio exclusion limits ranging from 10⁻³ to 10⁻⁵. Many of these analyses have been used to set limits on Wilson coefficients within the effective field theory framework.

The Lund jet plane (LJP) is measured for the first time in tt¯ events, using 140 fb-1 of s=13 TeV pp collision data collected with the ATLAS detector at the LHC. The LJP is a two-dimensional observable of the sub-structure of hadronic jets that acts as a proxy for the kinematics of parton showers and hadron formation. The observable is constructed from charged particles and is measured for R=1.0 anti-kt jets with transverse momentum above 350 GeV containing the full decay products of either a top quark or a daughter W boson. The other top quark in the event is identified from its decay into a b-quark, an electron or a muon and a neutrino. The measurement is corrected for detector effects and compared with a range of Monte Carlo predictions sensitive to different aspects of the hadronic decays of the heavy particles. In the W-boson-initiated jets, all the predictions are incompatible with the measurement. In the top quark initiated jets, disagreement with all predictions is observed in smaller subregions of the plane, and with a subset of the predictions across the fiducial plane. The measurement could be used to improve the tuning of Monte Carlo generators, for better modelling of hadronic decays of heavy quarks and bosons, or to improve the performance of jet taggers.

A summary of precision measurements sensitive to electroweak, QCD and quark-flavour effects performed by the ATLAS Collaboration at the Large Hadron Collider is reported. The measurements are predominantly performed on protonproton (pp) collision data recorded at a centre-of-mass energy of 13 TeV taken from 2015 to 2018, with an integrated luminosity of up to 140 fb⁻¹, with some results based on pp and Pb+Pb data recorded at lower nucleon centre-of-mass energies. The results cover a wide range of topics, from strong production of particles at low energies and the spectroscopy of hadrons to perturbative QCD with hadronic jets and electroweak and strong production of single and multiple vector bosons. They provide precise measurements of fundamental constants and stringent tests of the Standard Model with unprecedented precision and in energy ranges never explored before. They are also used to explore the proton structure and to perform model-independent searches for new physics.

Charged Higgs bosons produced either in top-quark decays or in association with a top quark, subsequently decaying via (Formula presented), are searched for in (Formula presented) of proton-proton collision data at (Formula presented) recorded with the ATLAS detector. Depending on whether the top quark is produced together with the (Formula presented) decays hadronically or semileptonically, the search targets (Formula presented) or (Formula presented) final states, in both cases with a (Formula presented)-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of (Formula presented), upper limits at 95% confidence level are set on the production cross section of the charged Higgs boson times the branching fraction (Formula presented) in the range 4.5 pb-0.4 fb. In the mass range 80-160 GeV, assuming the Standard Model cross section for (Formula presented) production, this corresponds to upper limits between 0.27% and 0.02% on (Formula presented).

A search is performed for dark matter particles produced in association with a resonantly produced pair of (Formula presented)-quarks with (Formula presented) using (Formula presented) of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. This signature is expected in extensions of the standard model predicting the production of dark matter particles, in particular those containing a dark Higgs boson (Formula presented) that decays into (Formula presented). The highly boosted (Formula presented) topology is reconstructed using jet reclustering and a new identification algorithm. This search places stringent constraints across regions of the dark Higgs model parameter space that satisfy the observed relic density, excluding dark Higgs bosons with masses between 30 and 150 GeV in benchmark scenarios with (Formula presented) mediator masses up to 4.8 TeV at 95% confidence level.

In ultrarelativistic heavy ion collisions at the LHC, each nucleus acts a sources of high-energy real photons that can scatter off the opposing nucleus in ultraperipheral photonuclear ((Formula presented)) collisions. Hard scattering processes initiated by the photons in such collisions provide a novel method for probing nuclear parton distributions in a kinematic region not easily accessible to other measurements. ATLAS has measured production of dijet and multijet final states in ultraperipheral (Formula presented) collisions at (Formula presented) using a dataset recorded in 2018 with an integrated luminosity of (Formula presented). Photonuclear final states are selected by requiring a rapidity gap in the photon direction; this selects events where one of the outgoing nuclei remains intact. Jets are reconstructed using the anti-(Formula presented) algorithm with radius parameter, (Formula presented). Triple-differential cross sections, unfolded for detector response, are measured and presented using two sets of kinematic variables. The first set consists of the total transverse momentum ((Formula presented)), rapidity, and mass of the jet system. The second set uses (Formula presented) and particle-level nuclear and photon parton momentum fractions, (Formula presented) and (Formula presented), respectively. The results are compared with leading-order perturbative QCD calculations of photonuclear jet production cross sections, where all leading order predictions using existing fits fall below the data in the shadowing region. More detailed theoretical comparisons will allow these results to strongly constrain nuclear parton distributions, and these data provide results from the LHC directly comparable to early physics results at the planned Electron-Ion Collider.

A search for a dark photon, a new light neutral particle, which decays promptly into collimated pairs of electrons or muons is presented. The search targets dark photons resulting from the exotic decay of the Standard Model Higgs boson, assuming its production via the dominant gluon-gluon fusion mode. The analysis is based on 140fb-1 of data collected with the ATLAS detector at the Large Hadron Collider from proton-proton collisions at a center-of-mass energy of 13 TeV. Events with collimated pairs of electrons or muons are analysed and background contributions are estimated using data-driven techniques. No significant excess in the data above the Standard Model background is observed. Upper limits are set at 95% confidence level on the branching ratio of the Higgs boson decay into dark photons between 0.001% and 5%, depending on the assumed dark photon mass and signal model.

In 2022 and 2023, the Large Hadron Collider produced approximately two billion hadronic interactions each second from bunches of protons that collide at a rate of 40 MHz. The ATLAS trigger system is used to reduce this rate to a few kHz for recording. Selections based on hadronic jets, their energy, and event topology reduce the rate to Ô(10) kHz while maintaining high efficiencies for important signatures resulting in b-quarks, but to reach the desired recording rate of hundreds of Hz, additional real-time selections based on the identification of jets containing b-hadrons (b-jets) are employed to achieve low thresholds on the jet transverse momentum at the High-Level Trigger. The configuration, commissioning, and performance of the real-time ATLAS b-jet identification algorithms for the early LHC Run 3 collision data are presented. These recent developments provide substantial gains in signal efficiency for critical signatures; for the Standard Model production of Higgs boson pairs, a 50% improvement in selection efficiency is observed in final states with four b-quarks or two b-quarks and two hadronically decaying τ-leptons.

A search for the production of top-quark pairs with the same electric charge (tt or tt¯) is presented. The analysis uses proton-proton collision data at s = 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb⁻¹. Events with two same-charge leptons and at least two b-tagged jets are selected. Neural networks are employed to define two selections sensitive to additional couplings beyond the Standard Model that would enhance the production rate of same-sign top-quark pairs. No significant signal is observed, leading to an upper limit on the total production cross-section of same-sign top-quark pairs of 1.6 fb at 95% confidence level. Corresponding limits on the three Wilson coefficients associated with the Otu1, OQu1, and OQu8 operators in the Standard Model Effective Field Theory framework are derived.

A search for T and Y vector-like quarks produced in proton-proton collisions at a centre-of-mass energy of 13 TeV and decaying into Wb in the fully hadronic final state is presented. The search uses 139 fb⁻¹ of data collected by the ATLAS detector at the LHC from 2015 to 2018. The final state is characterised by a hadronically decaying W boson with large Lorentz boost and a b-tagged jet, which are used to reconstruct the invariant mass of the vector-like quark candidate. The main background is QCD multijet production, which is estimated using a data-driven method. Upon finding no significant excess in data, mass limits at 95% confidence level are obtained as a function of the global coupling parameter, κ. The observed lower limits on the masses of Y quarks with κ = 0.5 and κ = 0.7 are 2.0 TeV and 2.4 TeV, respectively. For T quarks, the observed mass limits are 1.4 TeV for κ = 0.5 and 1.9 TeV for κ = 0.7.

A search for the production of a Higgs boson and one or more charm quarks, in which the Higgs boson decays into a photon pair, is presented. This search uses proton-proton collision data with a centre-of-mass energy of s = 13 TeV and an integrated luminosity of 140 fb⁻¹ recorded by the ATLAS detector at the Large Hadron Collider. The analysis relies on the identification of charm-quark-containing jets, and adopts an approach based on Gaussian process regression to model the non-resonant di-photon background. The observed (expected, assuming the Standard Model signal) upper limit at the 95% confidence level on the cross-section for producing a Higgs boson and at least one charm-quark-containing jet that passes a fiducial selection is found to be 10.6 pb (8.8 pb). The observed (expected) measured cross-section for this process is 5.3 ± 3.2 pb (2.9 ± 3.1 pb).

A search for a light charged Higgs boson produced in decays of the top quark, t→H±b with H±→cs, is presented. This search targets the production of top-quark pairs tt¯→WbH±b, with W→ℓν (ℓ=e,μ), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits b-quark and c-quark identification techniques as well as multivariate methods to suppress the dominant tt¯ background. The data analysed correspond to 140fb-1 of pp collisions at s=13TeV recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction B(t→H±b), assuming B(t→Wb)+B(t→H±(→cs)b)=1.0, are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60 and 168 GeV.

This Letter presents a search for highly ionizing magnetic monopoles in (Formula presented) of ultraperipheral (Formula presented) collision data at (Formula presented) collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20-150 GeV. Depending on the model, monopoles with a single Dirac magnetic charge and mass below 80-120 GeV are excluded.

PHENIX presents a simultaneous measurement of the production of direct γ and π0 in d+Au collisions at sNN=200 GeV over a pT range of 7.5 to 18 GeV/c for different event samples selected by event activity, i.e., charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor, RdAu,EXPπ0, is 0.925±0.023(stat)±0.15(scale), consistent with unity for minimum-bias (MB) d+Au collisions. For event classes with low and moderate event activity, RdAu,EXPπ0 is consistent with the MB value within 5% uncertainty. This result confirms that the previously observed enhancement of high-pT π0 production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5% of events with the highest event activity, RdAu,EXPπ0 is suppressed by 20% relative to the MB value with a significance of 4.5σ, which may be due to final-state effects. This suppression corresponds to a pT shift of δpT=0.213±0.055 Gev/c at 9 Gev/c.

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured low-mass vector-meson (ω + ρ and φ) production through the dimuon decay channel at forward rapidity (1.2 NN = 200 GeV. The low-mass vector-meson yield and nuclear-modification factor were measured as a function of the average number of participating nucleons, N_part\u203a, and the transverse momentum p_T. These results were compared with those obtained via the kaon decay channel in a similar p_T range at midrapidity. The nuclear-modification factors in both rapidity regions are consistent within the uncertainties. A comparison of the ω + ρ and J/ψ mesons reveals that the light and heavy flavors are consistently suppressed across both p_T and N_part\u203a. In contrast, the φ meson displays a nuclear-modification factor consistent with unity, suggesting strangeness enhancement in the medium formed.

A search is performed for light, spin-0 bosons decaying into two photons in the 66 to 110 GeV mass range, using 140 fb⁻¹ of proton-proton collisions at s = 13 TeV produced by the Large Hadron Collider and collected by the ATLAS detector. Multivariate analysis techniques are used to define event categories that improve the sensitivity to new resonances beyond the Standard Model. A model-independent search for a generic spin-0 particle and a model-dependent search for an additional low-mass Higgs boson are performed in the diphoton invariant mass spectrum. No significant excess is observed in either search. Mass-dependent upper limits at the 95% confidence level are set in the model-independent scenario on the fiducial cross-section times branching ratio into two photons in the range of 8 fb to 53 fb. Similarly, in the model-dependent scenario upper limits are set on the total cross-section times branching ratio into two photons as a function of the Higgs boson mass in the range of 19 fb to 102 fb.

A combination of searches for the single production of vectorlike top quarks ((Formula presented)) is presented. These analyses are based on proton-proton collisions at (Formula presented) recorded in 2015-2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of (Formula presented). The (Formula presented) decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a (Formula presented) boson ((Formula presented) and (Formula presented)). The individual searches used in the combination are differentiated by the number of leptons ((Formula presented), (Formula presented)) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, (Formula presented), of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.

Many extensions of the Standard Model, including those with dark matter particles, propose new mediator particles that decay into hadrons. This paper presents a search for such low mass narrow resonances decaying into hadrons using 140 fb⁻¹ of proton-proton collision data recorded with the ATLAS detector at a centre-of-mass energy of 13 TeV. The resonances are searched for in the invariant mass spectrum of large-radius jets with two-pronged substructure that are recoiling against an energetic photon from initial state radiation, which is used as a trigger to circumvent limitations on the maximum data recording rate. This technique enables the search for boosted hadronically decaying resonances in the mass range 20100 GeV hitherto unprobed by the ATLAS Collaboration. The observed data are found to agree with Standard Model predictions and 95% confidence level upper limits are set on the coupling of a hypothetical new spin-1 Z resonance with Standard Model quarks as a function of the assumed Z-boson mass in the range between 20 and 200 GeV.

This paper reports a search for a light CP-odd scalar resonance with a mass of 20 GeV to 90 GeV in 13 TeV proton-proton collision data with an integrated luminosity of 140 fb⁻¹ collected with the ATLAS detector at the Large Hadron Collider. The analysis assumes the resonance is produced via gluon-gluon fusion and decays into a τ⁺τ⁻ pair which subsequently decays into a fully leptonic μ+νμν¯τe−ν¯eντ or e+νeν¯τμ−ν¯μντ final state. No significant excess of events above the predicted Standard Model background is observed. The results are interpreted within a flavour-aligned two-Higgs-doublet model, and a model-independent cross-section interpretation is also given. Upper limits at 95% confidence level between 3.0 pb and 68 pb are set on the cross-section for producing a CP-odd Higgs boson that decays into a τ⁺τ⁻ pair.

This Letter presents a differential cross-section measurement of Lund subjet multiplicities, suitable for testing current and future parton shower Monte Carlo algorithms. This measurement is made in dijet events in 140 fb⁻¹ of s=13 TeV protonproton collision data collected with the ATLAS detector at CERN's Large Hadron Collider. The data are unfolded to account for acceptance and detector-related effects, and are then compared with several Monte Carlo models and to recent resummed analytical calculations. The experimental precision achieved in the measurement allows tests of higher-order effects in QCD predictions. Most predictions fail to accurately describe the measured data, particularly at large values of jet transverse momentum accessible at the Large Hadron Collider, indicating the measurement's utility as an input to future parton shower developments and other studies probing fundamental properties of QCD and the production of hadronic final states up to the TeV-scale.

Properties of the underlying-event in pp interactions are investigated primarily via the strange hadrons KS0, Λ and Λ¯, as reconstructed using the ATLAS detector at the LHC in minimum-bias pp collision data at s=13 TeV. The hadrons are reconstructed via the identification of the displaced two-particle vertices corresponding to the decay modes, Λ→π-p and Λ¯→π+p¯. These are used in the construction of underlying-event observables in azimuthal regions computed relative to the leading charged-particle jet in the event. None of the hadronisation and underlying-event physics models considered can describe the data over the full kinematic range considered. Events with a leading charged-particle jet in the range of 10

The need for large-scale production of highly accurate simulated event samples for the extensive physics programme of the ATLAS experiment at the Large Hadron Collider motivates the development of new simulation techniques. Building on the recent success of deep learning algorithms, variational autoencoders and generative adversarial networks are investigated for modelling the response of the central region of the ATLAS electromagnetic calorimeter to photons of various energies. The properties of synthesised showers are compared with showers from a full detector simulation using geant4. Both variational autoencoders and generative adversarial networks are capable of quickly simulating electromagnetic showers with correct total energies and stochasticity, though the modelling of some shower shape distributions requires more refinement. This feasibility study demonstrates the potential of using such algorithms for ATLAS fast calorimeter simulation in the future and shows a possible way to complement current simulation techniques.

During the 20152018 data-taking period, the Large Hadron Collider delivered proton-proton bunch crossings at a centre-of-mass energy of 13 TeV to the ATLAS experiment at a rate of roughly 30 MHz, where each bunch crossing contained an average of 34 independent inelastic proton-proton collisions. The ATLAS trigger system selected roughly 1 kHz of these bunch crossings to be recorded to disk. Offline algorithms then identify one of the recorded collisions as the collision of interest for subsequent data analysis, and the remaining collisions are referred to as pile-up. Pile-up collisions represent a trigger-unbiased dataset, which is evaluated to have an integrated luminosity of 1.33 pb⁻¹ in 20152018. This is small compared with the normal trigger-based ATLAS dataset, but when combined with vertex-by-vertex jet reconstruction it provides up to 50 times more dijet events than the conventional single-jet-trigger-based approach, and does so without adding any additional cost or requirements on the trigger system, readout, or storage. The pile-up dataset is validated through comparisons with a special trigger-unbiased dataset recorded by ATLAS, and its utility is demonstrated by means of a measurement of the jet energy resolution in dijet events, where the statistical uncertainty is significantly reduced for jet transverse momenta below 65 GeV.

Charged particle reconstruction in the presence of many simultaneous protonproton (pp) collisions in the LHC is a challenging task for the ATLAS experiments reconstruction software due to the combinatorial complexity. This paper describes the major changes made to adapt the software to reconstruct high-activity collisions with an average of 50 or more simultaneous pp interactions per bunch crossing (pile-up) promptly using the available computing resources. The performance of the key components of the track reconstruction chain and its dependence on pile-up are evaluated, and the improvement achieved compared to the previous software version is quantified. For events with an average of 60pp collisions per bunch crossing, the updated track reconstruction is twice as fast as the previous version, without significant reduction in reconstruction efficiency and while reducing the rate of combinatorial fake tracks by more than a factor two.

A search is presented for direct pair production of the stop, the supersymmetric partner of the top quark, in a decay through an Formula Presented-parity violating coupling to a charged lepton and a Formula Presented-quark. The dataset corresponds to an integrated luminosity of Formula Presented of proton-proton collisions at a center-of-mass energy of Formula Presented collected between 2015 and 2018 by the ATLAS detector at the LHC. The final state has two charged leptons (electrons or muons) and two Formula Presented-jets. The results of the search are interpreted in the context of a Minimal Supersymmetric Standard Model with an additional Formula Presented gauge symmetry that is spontaneously broken. No significant excess is observed over the Standard Model background, and exclusion limits on stop pair production are set at 95% confidence level. The corresponding lower limits on the stop mass for 100% branching ratios to a Formula Presented-quark and an electron, muon, or tau-lepton are 1.9 TeV, 1.8 TeV and 800 GeV, respectively, extending the reach of previous LHC searches.

Measurements of the Higgs boson production times decay rates and differential cross-sections have recently been performed by the ATLAS experiment in several decay channels using up to 139 fb⁻¹ of proton-proton collision data at s = 13 TeV recorded at the Large Hadron Collider. This paper presents multiple interpretations of these Higgs boson measurements. Measurements of production-mode cross-sections, simplified template cross-sections and fiducial differential cross-sections in different decay channels are reparameterised in terms of the impact of Standard Model effective field theory operators, and constraints are reported on the corresponding Wilson coefficients. Production and decay rate measurements are interpreted in UV-complete extensions of the Standard Model, namely the two-Higgs-doublet model (2HDM) near the alignment limit and the Minimal Supersymmetric Standard Model (MSSM) for various MSSM benchmark scenarios. The constraints on the 2HDM parameters (cos(β − α), tanβ) and the MSSM parameters (m_A, tanβ) are complementary to those obtained from direct searches for additional Higgs bosons.

A search is presented for new particles produced in proton-proton collisions at a centre-of-mass energy of 13 TeV that result in final states comprising a massive vector (W or Z) boson that decays hadronically and large missing transverse momentum. The data sample was collected with the ATLAS experiment at the Large Hadron Collider from 2015 to 2018 and corresponds to an integrated luminosity of 140 fb⁻¹. No significant excess over the Standard Model expectation is observed. Model-independent 95% confidence-level limits on the visible cross-section that range from 0.3 fb to 79.5 fb are obtained for non-Standard-Model processes. Exclusion limits are also presented for models with axion-like particles, for two-Higgs-doublet models with a pseudo-scalar mediator between the Standard Model and the dark sector, for the invisible decay of the Higgs boson and for pair-produced weakly interacting dark matter candidates.

This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb⁻¹ of p+Pb data collected at s_NN = 8.16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one b-tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be σ_tt¯=58.1±2.0stat._−4.4^+4.8syst. nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be R_pA=1.090±0.039stat._−0.087^+0.094syst. The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.

High-momentum two-particle correlations are a useful tool for studying jet-quenching effects in the quark-gluon plasma. Angular correlations between neutral-pion triggers and charged hadrons with transverse momenta in the range 4-12 GeV/c and 0.5-7 GeV/c, respectively, have been measured by the PHENIX experiment in 2014 for Au+Au collisions at sNN=200 GeV. Suppression is observed in the yield of high-momentum jet fragments opposite the trigger particle, which indicates jet suppression stemming from in-medium partonic energy loss, while enhancement is observed for low-momentum particles. The ratio and differences between the yield in Au+Au collisions and p+p collisions, IAA and ΔAA, as a function of the trigger-hadron azimuthal separation, Δφ, are measured for the first time at the BNL Relativistic Heavy Ion Collider. These results better quantify how the yield of low-pT associated hadrons is enhanced at wide angle, which is crucial for studying energy loss as well as medium-response effects.

The associated production of Higgs and W bosons via vector-boson fusion is highly sensitive to the relative sign of the Higgs boson couplings to W and Z bosons. In this Letter, two searches for this process are presented, using 140 fb⁻¹ of proton-proton collision data at √s = 13 TeV recorded by the ATLAS detector at the LHC. The first search targets scenarios with opposite-sign couplings of the W and Z bosons to the Higgs boson, while the second targets standard model-like scenarios with same-sign couplings. Both analyses consider Higgs boson decays into a pair of b quarks and W boson decays with an electron or muon. The data exclude the opposite-sign coupling hypothesis with a significance beyond 5σ, and the observed (expected) upper limit set on the cross section for vector-boson fusion WH production is 9.0 (8.7) times the standard model value at 95% confidence level.

A search for a new Z gauge boson predicted by L_µ-L_τ models, based on charged-current Drell-Yan production, pp→W±(∗)→Zμ±ν→μ±μ μ±ν, is presented. The data sample used corresponds to an integrated luminosity of 140 fb-1 of proton-proton collisions at s=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The search examines a final state of 3μ plus large missing transverse momentum. Upper limits are set on the Z production cross section times branching ratio in the mass range of 5-81 GeV. After combining with the previous Z search using the neutral-current Drell-Yan production with a 4μ final state, the most stringent exclusion limits to date are achieved in the parameter space of the Z coupling strength and mass.

This paper describes measurements of the transverse momentum spectra of W and Z bosons produced in protonproton collisions at centre-of-mass energies of s=5.02 TeV and s=13 TeV with the ATLAS experiment at the Large Hadron Collider. Measurements are performed in the electron and muon channels, W→ℓν and Z→ℓℓ (ℓ=e or μ), and for W events further separated by charge. The data were collected in 2017 and 2018, in dedicated runs with reduced instantaneous luminosity, and correspond to 255 and 338 pb^-1 at s=5.02 TeV and 13 TeV, respectively. These conditions optimise the reconstruction of the W-boson transverse momentum. The distributions observed in the electron and muon channels are unfolded, combined, and compared to QCD calculations based on parton shower Monte Carlo event generators and analytical resummation. The description of the transverse momentum distributions by Monte Carlo event generators is imperfect and shows significant differences largely common to W^-, W⁺ and Z production. The agreement is better at s=5.02 TeV, especially for predictions that were tuned to Z production data at s=7 TeV. Higher-order, resummed predictions based on DYTurbo generally match the data best across the spectra. Distribution ratios are also presented and test the understanding of differences between the production processes.

A search for the exclusive hadronic decays Formula Presented, Formula Presented, and Formula Presented is performed using up to Formula Presented of proton-proton collisions recorded with the ATLAS detector at a center-of-mass energy of Formula Presented. If observed, these rare processes would provide a unique test bench for the quantum chromodynamics factorization formalism used to calculate cross sections at colliders. Additionally, at future colliders, these decays could offer a new way to measure the Formula Presented boson mass through fully reconstructed decay products. The search results in the most stringent upper limits to date on the branching fractions Formula Presented, Formula Presented, Formula Presented at 95% confidence level.

Measurements of jet cross-section ratios between inclusive bins of jet multiplicity are performed in Formula Presented of proton-proton collisions with Formula Presented center-of-mass energy, recorded with the ATLAS detector at CERNs Large Hadron Collider. These ratios are constructed from double-differential cross-section measurements that are made in bins of jet multiplicity and other observables that are sensitive the energy scale and angular distribution of radiation due to the strong interaction in the final state. Additionally, the scalar sum of the two leading jets transverse momenta is measured triple differentially, in bins of the third jets transverse momentum and of jet multiplicity. These measurements are unfolded to account for acceptance and detector-related effects. The measured distributions are used to construct ratios of the inclusive jet-multiplicity bins, which have been shown to be sensitive to the strong coupling Formula Presented while being less sensitive than other observables to systematic uncertainties and parton distribution functions. The measured distributions are compared with state-of-the-art QCD calculations, including next-to-next-to-leading-order predictions for two- and three-jet events. These predictions are generally found to model the data well and perform best in bins with a modest requirement on the third jets transverse momentum. Significant differences between data and Monte Carlo predictions are observed in events with large rapidity gaps and invariant masses of the leading jet pair. Studies leading to reduced jet energy scale uncertainties significantly improve the precision of this work and are documented herein.

The ratio of branching ratios of the W boson to muons and electrons, R_W^μ/e=B(W→μν)/B(W→eν), has been measured using 140fb^-1 of pp collision data at s=13 TeV collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the tt¯ production cross-section in the ee, eμ and μμ dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio R_Z^μμ/ee for the Z boson measured in inclusive Z→ee and Z→μμ events. By using the precise value of R_Z^μμ/ee determined from e⁺e^- colliders, the ratio R_W^μ/e is determined to be (Formula presented.) The three uncertainties correspond to data statistics, experimental systematics and the external measurement of R_Z^μμ/ee, giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in W-boson decays at the 0.5% level.

A search for a hypothetical heavy scalar particle, X, decaying into a singlet scalar particle, S, and a Standard Model Higgs boson, H, using 140 fb⁻¹ of proton-proton collision data at the centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the LHC is presented. The explored mass range is 300 ≤ m_X ≤ 1000 GeV and 170 ≤ m_S ≤ 500 GeV. The signature of this search is one or two leptons (e or μ) from the decay of vector bosons originating from the S particle, S → W^±W^∓/ZZ, and two photons from the Higgs boson decay, H → γγ. No significant excess is observed above the expected Standard Model background. The observed (expected) upper limits at the 95% confidence level on the cross- section for gg → X → SH, assuming the same S → WW/ZZ branching ratios as for a SM-like heavy Higgs boson, are between 530 (800) fb and 120 (170) fb.

The energy and mass measurements of jets are crucial tasks for the Large Hadron Collider experiments. This paper presents a new calibration method to simultaneously calibrate these quantities for large-radius jets measured with the ATLAS detector using a deep neural network (DNN). To address the specificities of the calibration problem, special loss functions and training procedures are employed, and a complex network architecture, which includes feature annotation and residual connection layers, is used. The DNN-based calibration is compared to the standard numerical approach in an extensive series of tests. The DNN approach is found to perform significantly better in almost all of the tests and over most of the relevant kinematic phase space. In particular, it consistently improves the energy and mass resolutions, with a 30% better energy resolution obtained for transverse momenta pT > 500 GeV.

Entanglement is a key feature of quantum mechanics¹³, with applications in fields such as metrology, cryptography, quantum information and quantum computation⁴⁸. It has been observed in a wide variety of systems and length scales, ranging from the microscopic⁹¹³ to the macroscopic¹⁴¹⁶. However, entanglement remains largely unexplored at the highest accessible energy scales. Here we report the highest-energy observation of entanglement, in topantitop quark events produced at the Large Hadron Collider, using a protonproton collision dataset with a centre-of-mass energy of √s = 13 TeV and an integrated luminosity of 140 inverse femtobarns (fb)⁻¹ recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the topantitop quark production threshold, at which the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from the limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D = −0.537 ± 0.002 (stat.) ± 0.019 (syst.) for 340GeV

This Letter presents results from a combination of searches for Higgs boson pair production using Formula Presented of proton-proton collision data at Formula Presented recorded with the ATLAS detector. At 95% confidence level (CL), the upper limit on the production rate is 2.9 times the standard model (SM) prediction, with an expected limit of 2.4 assuming no Higgs boson pair production. Constraints on the Higgs boson self-coupling modifier Formula Presented, and the quartic Formula Presented coupling modifier Formula Presented, are derived individually, fixing the other parameter to its SM value. The observed 95% CL intervals are Formula Presented and Formula Presented, respectively, while the expected intervals are Formula Presented and Formula Presented in the SM case. Constraints obtained for several interaction parameters within Higgs effective field theory are the strongest to date, offering insights into potential deviations from SM predictions.

A search for dark mesons originating from strongly-coupled, SU(2) dark flavor symmetry conserving models and decaying gaugephobically to pure Standard Model final states containing top and bottom quarks is presented. The search targets fully hadronic final states and final states with exactly one electron or muon and multiple jets. The analyzed data sample corresponds to an integrated luminosity of 140 fb⁻¹ of proton-proton collisions collected at s = 13 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess over the Standard Model background expectation is observed and the results are used to set the first direct constraints on this type of model. The two-dimensional signal space of dark pion masses mπ_D and dark rho-meson masses mρ_D is scanned. For mπ_D/mρ_D = 0.45, dark pions with masses mπ_DD/mρ_D = 0.25 masses mπ_D

A search for heavy Majorana neutrinos in scattering of same-sign W boson pairs in protonproton collisions at s=13 TeV at the LHC is reported. The dataset used corresponds to an integrated luminosity of 140 fb⁻¹, collected with the ATLAS detector during 20152018. The search is performed in final states including a same-sign ee or eμ pair and at least two jets with large invariant mass and a large rapidity difference. No significant excess of events with respect to the Standard Model background predictions is observed. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. New constraints are set on the values of the |V_eN|² and |V_eNV_μN^\u204e| parameters for heavy Majorana neutrino masses between 50 GeV and 20 TeV, where V_ℓN is the matrix element describing the mixing of the heavy Majorana neutrino mass eigenstate with the Standard Model neutrino of flavour ℓ=e,μ. The sensitivity to the Weinberg operator is investigated and constraints on the effective ee and eμ Majorana neutrino masses are reported. The statistical combination of the ee and eμ channels with the previously published μμ channel is performed.

The identification of top quark decays where the top quark has a large momentum transverse to the beam axis, known as top tagging, is a crucial component in many measurements of Standard Model processes and searches for beyond the Standard Model physics at the Large Hadron Collider. Machine learning techniques have improved the performance of top tagging algorithms, but the size of the systematic uncertainties for all proposed algorithms has not been systematically studied. This paper presents the performance of several machine learning based top tagging algorithms on a dataset constructed from simulated proton-proton collision events measured with the ATLAS detector at √s = 13 TeV. The systematic uncertainties associated with these algorithms are estimated through an approximate procedure that is not meant to be used in a physics analysis, but is appropriate for the level of precision required for this study. The most performant algorithms are found to have the largest uncertainties, motivating the development of methods to reduce these uncertainties without compromising performance. To enable such efforts in the wider scientific community, the datasets used in this paper are made publicly available.

Measurements of inclusive, differential cross-sections for the production of events with missing transverse momentum in association with jets in proton-proton collisions at s = 13 TeV are presented. The measurements are made with the ATLAS detector using an integrated luminosity of 140 fb⁻¹ and include measurements of dijet distributions in a region in which vector-boson fusion processes are enhanced. They are unfolded to correct for detector resolution and efficiency within the fiducial acceptance, and are designed to allow robust comparisons with a wide range of theoretical predictions. A measurement of differential cross sections for the Z → νν process is made. The measurements are generally well-described by Standard Model predictions except for the dijet invariant mass distribution. Auxiliary measurements of the hadronic system recoiling against isolated leptons, and photons, are also made in the same phase space. Ratios between the measured distributions are then derived, to take advantage of cancellations in modelling effects and some of the major systematic uncertainties. These measurements are sensitive to new phenomena, and provide a mechanism to easily set constraints on phenomenological models. To illustrate the robustness of the approach, these ratios are compared with two common Dark Matter models, where the constraints derived from the measurement are comparable to those set by dedicated detector-level searches.

Differential cross-sections for top-quark pair production, inclusively and in association with jets, are measured in pp collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC using an integrated luminosity of 140 fb⁻¹. The events are selected with one charged lepton (electron or muon) and at least four jets. The differential cross-sections are presented at particle level as functions of several jet observables, including angular correlations, jet transverse momenta and invariant masses of the jets in the final state, which characterise the kinematics and dynamics of the top-antitop system and the hard QCD radiation in the system with associated jets. The typical precision is 5%15% for the absolute differential cross-sections and 2%4% for the normalised differential cross-sections. Next-to-leading-order and next-to-next-to-leading-order QCD predictions are found to provide an adequate description of the rate and shape of the jet-angular observables. The description of the transverse momentum and invariant mass observables is improved when next-to-next-to-leading-order QCD corrections are included.

A search for leptoquark pair production decaying into te^-t¯e⁺ or tμ^-t¯μ⁺ in final states with multiple leptons is presented. The search is based on a dataset of pp collisions at s=13TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb-1. Four signal regions, with the requirement of at least three light leptons (electron or muon) and at least two jets out of which at least one jet is identified as coming from a b-hadron, are considered based on the number of leptons of a given flavour. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No excess above the Standard Model background prediction is observed and 95% confidence level limits on the production cross section times branching ratio are derived as a function of the leptoquark mass. Under the assumption of exclusive decays into te^- (tμ^-), the corresponding lower limit on the scalar mixed-generation leptoquark mass mLQ_mixd is at 1.58 (1.59) TeV and on the vector leptoquark mass mU~₁ at 1.67 (1.67) TeV in the minimal coupling scenario and at 1.95 (1.95) TeV in the YangMills scenario.

Several processes studied by the ATLAS experiment at the Large Hadron Collider produce low-momentum Formula Presented-flavored hadrons in the final state. This paper describes the calibration of a dedicated tagging algorithm that identifies Formula Presented-flavored hadrons outside of hadronic jets by reconstructing the soft secondary vertices originating from their decays. The calibration is based on a proton-proton collision dataset at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of Formula Presented. Scale factors used to correct the algorithms performance in simulated events are extracted for the Formula Presented-tagging efficiency and the mistag rate of the algorithm using a data sample enriched in Formula Presented events. Several orthogonal measurement regions are defined, binned as a function of the multiplicities of soft secondary vertices and jets containing a Formula Presented-flavored hadron in the event. The mistag rate scale factors are estimated separately for events with low and high average numbers of interactions per bunch crossing. The results, which are derived from events with low missing transverse momentum, are successfully validated in a phase space characterized by high missing transverse momentum and therefore are applicable to new physics searches carried out in either phase space regime.

A search for the nonresonant production of Higgs boson pairs in the Formula Presented channel is performed using Formula Presented of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The analysis strategy is optimized to probe anomalous values of the Higgs boson self-coupling modifier Formula Presented and of the quartic Formula Presented (Formula Presented) coupling modifier Formula Presented. No significant excess above the expected background from Standard Model processes is observed. An observed (expected) upper limit Formula Presented is set at 95% confidence-level on the Higgs boson pair production cross section normalized to its Standard Model prediction. The coupling modifiers are constrained to an observed (expected) 95% confidence interval of Formula Presented (Formula Presented) and Formula Presented (Formula Presented), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The results are also interpreted in the context of effective field theories via constraints on anomalous Higgs boson couplings and Higgs boson pair production cross sections assuming different kinematic benchmark scenarios.

The observation of the electroweak production of single-top-quarks is made using 255 pb⁻¹ of proton-proton collision data recorded at s=5.02 TeV with the ATLAS detector at the Large Hadron Collider. An event selection is used to identify single-top-quark candidates arising from t-channel production with the top quark decaying semi-leptonically. Events passing the selection are then used to measure the inclusive cross-section for the combined production of single-top-quarks and antiquarks, σ(tq+t¯q), and the ratio R_t between these two. They are measured to be σ(tq+t¯q)=27.1_−4.1^+4.4(stat.)_−3.7^+4.4(syst.) pb and R_t=2.73_−0.82^+1.43(stat.)_−0.29^+1.01(syst.). The individual single-top-quark (tq) and single-top-antiquark (t¯q) production cross-sections are measured to be σ(tq)=19.8_−3.1^+3.9(stat.)_−2.2^+2.9(syst.) pb and σ(t¯q)=7.3_−2.1^+3.2(stat.)_−1.5^+2.8(syst.) pb. All measurements are in good agreement with the Standard Model predictions.

A measurement of the production of W bosons with opposite electric charges in association with two jets is presented based on 140 fb⁻¹ of data collected by the ATLAS detector in proton-proton collisions at s = 13 TeV. The analysis is sensitive to the scattering of W bosons, which is of particular interest in the ATLAS physics programme as it can be used to probe the electroweak symmetry breaking mechanism of the Standard Model. This signal is observed with a significance of 7.1 standard deviations above the background expectation, while 6.2 standard deviations were expected. The measured cross-section is determined in a signal-enriched fiducial volume and is found to be 2.7 ± 0.5 fb, which is consistent with the theoretical prediction of 2.20_−0.13^+0.14 fb.

A search for charged-lepton-flavor violating Formula Presented (Formula Presented, Formula Presented) interactions is presented, considering both top-quark production and decay. The data analyzed correspond to Formula Presented of proton-proton collisions at a center-of-mass energy of Formula Presented recorded with the ATLAS detector at the Large Hadron Collider. The analysis targets events containing two muons with the same electric charge, a hadronically decaying Formula Presented-lepton and at least one jet, with exactly one Formula Presented-tagged jet, produced by a Formula Presented interaction. Agreement with the Standard Model expectation within 1.6 standard deviations is observed, and limits are set at the 95% confidence level (CL) on the charged-lepton-flavor violation branching ratio of Formula Presented. An effective field theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavor of the associated light quark and the Lorentz structure of the coupling. These range from Formula Presented for Formula Presented to Formula Presented for Formula Presented. An additional interpretation is performed for scalar leptoquark production inducing charged-lepton-flavor violation, with fixed intergenerational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from Formula Presented to Formula Presented for leptoquark masses between 0.5 and 2.0 TeV.

A search is reported for long-lived dark photons with masses between 0.1 GeV and 15 GeV, from exotic decays of Higgs bosons produced via vector-boson-fusion. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are probed. This search uses the full LHC Run 2 (20152018) data sample collected in protonproton collisions at s=13 TeV, corresponding to an integrated luminosity of 139 fb^-1. Dominant backgrounds from Standard Model processes and non-collision sources are estimated using data-driven techniques. The observed event yields in the signal regions are consistent with the expected background. Upper limits on the Higgs boson to dark photon branching fraction are reported as a function of the dark photon mean proper decay length or of the dark photon mass and the coupling between the Standard Model and the potential dark sector. This search is combined with previous ATLAS searches obtained in the gluongluon fusion and WH production modes. A branching fraction above 10% is excluded at 95% CL for a 125 GeV Higgs boson decaying into two dark photons for dark photon mean proper decay lengths between 173 and 1296 mm and mass of 10 GeV.

Presented is the search for anomalous Higgs boson decays into two axion-like particles (ALPs) using the full Run 2 data set of 140fb^-1 of proton-proton collisions at a centre-of-mass energy of 13TeV recorded by the ATLAS experiment. The ALPs are assumed to decay into two photons, providing sensitivity to recently proposed models that could explain the (g-2)_μ discrepancy. This analysis covers an ALP mass range from 100 to 62GeV and ALP-photon couplings in the range 10^-7TeV^-1

A measurement of the invisible width of the Z boson using events with jets and missing transverse momentum is presented using 37 fb⁻¹ of 13 TeV protonproton data collected by the ATLAS detector in 2015 and 2016. The ratio of Z→inv to Z→ℓℓ events, where inv refers to non-detected particles and ℓ is either an electron or a muon, is measured and corrected for detector effects. Events with at least one energetic central jet with p_T≥110 GeV are selected for both the Z→inv and Z→ℓℓ final states to obtain a similar phase space in the ratio. The invisible width is measured to be 506±2(stat.)±12(syst.) MeV and is the single most precise recoil-based measurement. The result is in agreement with the most precise determination from LEP and the Standard Model prediction based on three neutrino generations.

A statistical combination of various searches for pair-produced leptoquarks is presented, using the full LHC Run 2 (20152018) data set of 139 fb⁻¹ collected with the ATLAS detector from protonproton collisions at a centre-of-mass energy of s=13 TeV. All possible decays of the leptoquarks into quarks of the third generation and charged or neutral leptons of any generation are investigated. Since no significant deviations from the Standard Model expectation are observed in any of the individual analyses, combined exclusion limits are set on the production cross-sections for scalar and vector leptoquarks. The resulting lower bounds on leptoquark masses exceed those from the individual analyses by up to 100 GeV, depending on the signal hypothesis.

A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to Formula Presented of Formula Presented collision data at Formula Presented recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: Formula Presented, Formula Presented, and Formula Presented. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the type-I two-Higgs-doublet model and the minimal supersymmetric standard model, and constrain parameter space not previously excluded by other searches.

Measurements of integrated and differential cross-sections for electroweak W^±Z production in association with two jets (W^±Zjj) in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of s = 13 TeV are used, corresponding to an integrated luminosity of 140 fb⁻¹. The W^±Zjj candidate events are reconstructed using leptonic decay modes of the gauge bosons. Events containing three identified leptons, either electrons or muons, and two jets are selected. Processes involving pure electroweak W^±Zjj production at Born level are separated from W^±Zjj production involving a strong coupling. The measured integrated fiducial cross-section of electroweak W^±Zjj production per lepton flavour is σ_{WZjj−EW→ℓνlljj} = 0.368 ± 0.037 (stat.) ± 0.059 (syst.) ± 0.003 (lumi.) fb, where ℓ and ℓ are either an electron or a muon. Respective cross-sections of electroweak and strong W^±Zjj production are measured separately for events with exactly two jets or with more than two jets, and in three bins of the invariant mass of the two jets. The inclusive W^±Zjj production cross-section, without separating electroweak and strong production, is also measured to be σ_{WZjj→ℓνlljj} = 1.462 ± 0.063 (stat.) ± 0.118 (syst.) ± 0.012 (lumi.) fb, per lepton flavour. The inclusive W^±Zjj production cross-section is measured differentially for several kinematic observables. Finally, the measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confidence level intervals on dimension-8 operators.

Inelastic beam-gas collisions at the Large Hadron Collider (LHC), within a few hundred metres of the ATLAS experiment, are known to give the dominant contribution to beam backgrounds. These are monitored by ATLAS with a dedicated Beam Conditions Monitor (BCM) and with the rate of fake jets in the calorimeters. These two methods are complementary since the BCM probes backgrounds just around the beam pipe while fake jets are observed at radii of up to several metres. In order to quantify the correlation between the residual gas density in the LHC beam vacuum and the experimental backgrounds recorded by ATLAS, several dedicated tests were performed during LHC Run 2. Local pressure bumps, with a gas density several orders of magnitude higher than during normal operation, were introduced at different locations. The changes of beam-related backgrounds, seen in ATLAS, are correlated with the local pressure variation. In addition the rates of beam-gas events are estimated from the pressure measurements and pressure bump profiles obtained from calculations. Using these rates, the efficiency of the ATLAS beam background monitors to detect beam-gas events is derived as a function of distance from the interaction point. These efficiencies and characteristic distributions of fake jets from the beam backgrounds are found to be in good agreement with results of beam-gas simulations performed with the Fluka Monte Carlo programme.

A search for the single production of a vectorlike top partner (Formula Presented) with mass greater than 1 TeV decaying into a Formula Presented boson and a top quark is presented, using the full Run 2 dataset corresponding to Formula Presented of Formula Presented collisions at Formula Presented, collected in 2015-2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a pair of electrons or muons with opposite-sign charges which form a Formula Presented-boson candidate, as well as by the presence of Formula Presented-tagged jets and forward jets. Events with exactly two or at least three leptons are categorized into two independently optimized analysis channels. No significant excess above the background expectation is observed and the results from the two channels are statistically combined to set exclusion limits at 95% confidence level on the masses and couplings of Formula Presented. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (Formula Presented) of the vectorlike quark. For singlet Formula Presented quarks, Formula Presented values between 0.22 and 0.64 are excluded for masses between 1000 and 1975 GeV. For Formula Presented quarks in the doublet scenario, where the production cross section is much lower, Formula Presented values between 0.54 and 0.88 are excluded for masses between 1000 and 1425 GeV.

The PHENIX experiment has performed a systematic study of identified charged-hadron (π±, K±, p, p¯) production at midrapidity in p+Al, He3+Au, and Cu+Au collisions at sNN=200GeV and U+U collisions at sNN=193GeV. Identified charged-hadron invariant transverse-momentum (pT) and transverse-mass (mT) spectra are presented and interpreted in terms of radially expanding thermalized systems. The particle ratios of K/π and p/π have been measured in different centrality ranges of large (Cu+Au and U+U) and small (p+Al and He3+Au) collision systems. The values of K/π ratios measured in all considered collision systems were found to be consistent with those measured in p+p collisions. However, the values of p/π ratios measured in large collision systems reach the values of ≈0.6, which is a factor of ≈2 larger than in p+p collisions. These results can be qualitatively understood in terms of the baryon enhancement expected from hadronization by recombination. Identified charged-hadron nuclear-modification factors (RAB) are also presented. Enhancement of proton RAB values over meson RAB values was observed in central He3+Au, Cu+Au, and U+U collisions. The proton RAB values measured in the p+Al collision system were found to be consistent with RAB values of φ, π±, K±, and π0 mesons, which may indicate that the size of the system produced in p+Al collisions is too small for recombination to cause a noticeable increase in proton production.

Three searches for the direct production of τ-sleptons or charginos and neutralinos in final states with at least two hadronically decaying τ-leptons are presented. For chargino and neutralino production, decays via intermediate τ-sleptons or W and h bosons are considered. The analysis uses a dataset of pp collisions corresponding to an integrated luminosity of 139 fb⁻¹, recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed and supersymmetric particle mass limits at 95% confidence level are obtained in simplified models. For direct production of χ~₁⁺χ~₁⁻, chargino masses are excluded up to 970 GeV, while χ~₁^± and χ~₂⁰ masses up to 1160 GeV (330 GeV) are excluded for χ~₁^±χ~₂⁰/χ~₁⁺χ~₁⁻ production with subsequent decays via τ-sleptons (W and h bosons). Masses of τ-sleptons up to 500 GeV are excluded for mass degenerate τ~_L,R scenarios and up to 425 GeV for τ~_L-only scenarios. Sensitivity to τ~_R-only scenarios from the ATLAS experiment is presented here for the first time, with τ~_R masses excluded up to 350 GeV.

Measurements of inclusive and differential production cross-sections of a top-quark-top-antiquark pair in association with a W boson (tt¯W) are presented. They are performed by targeting final states with two same-sign or three isolated leptons (electrons or muons) and are based on s = 13 TeV proton-proton collision data with an integrated luminosity of 140 fb⁻¹, recorded from 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. The inclusive tt¯W production cross-section is measured to be 880 ± 80 fb, compared to a reference theoretical prediction of 745 ± 50 (scale) ± 13 (2-loop approx.) ± 19 (PDF, α_s) fb. Differential cross-section measurements characterise this process in detail for the first time. Several particle-level observables are compared with a variety of theoretical predictions, which generally agree well with the normalised differential cross-section results. Additionally, the relative charge asymmetry of tt¯W⁺ and tt¯W⁻ is measured inclusively to be A_C^rel = 0.33 ± 0.05, in very good agreement with the theoretical prediction of 0.322 ± 0.003 (scale) ± 0.007 (PDF), as well as differentially.

Angular correlations between heavy quarks provide a unique probe of the quark-gluon plasma created in ultrarelativistic heavy-ion collisions. Results are presented of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy quarks produced in 5.02 TeV Formula Presented and Formula Presented collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying Formula Presented and Formula Presented, respectively. The distributions of azimuthal angle separation Formula Presented for muon pairs having pseudorapidity separation Formula Presented, are measured in different Formula Presented centrality intervals and compared to the same distribution measured in Formula Presented collisions at the same center-of-mass energy. Results are presented separately for muon pairs with opposite-sign charges, same-sign charges, and all pairs. A clear peak is observed in all Formula Presented distributions at Formula Presented, consistent with the parent heavy-quark pairs being produced via hard-scattering processes. The widths of that peak, characterized using Cauchy-Lorentz fits to the Formula Presented distributions, are found to not vary significantly as a function of Formula Presented collision centrality and are similar for Formula Presented and Formula Presented collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quark-gluon plasma.

Precision measurements of electron reconstruction, identification, and isolation efficiencies and photon identification efficiencies are presented. They use the full Run 2 data sample collected by the ATLAS experiment in pp collisions at a centre-of-mass energy of 13 TeV during the years 20152018, corresponding to an integrated luminosity of 139 fb⁻¹. The measured electron identification efficiencies have uncertainties that are around 30%50% smaller than the previous Run 2 results due to an improved methodology and the inclusion of more data. A better pile-up subtraction method leads to electron isolation efficiencies that are more independent of the amount of pile-up activity. Updated photon identification efficiencies are also presented, using the full Run 2 data. When compared to the previous measurement, a 30%40% smaller uncertainty is observed on the photon identification efficiencies, thanks to the increased amount of available data.

A search for R-parity-violating supersymmetry in final states with high jet multiplicity is presented. The search uses 140 fb⁻¹ of proton-proton collision data at s = 13 TeV collected by the ATLAS experiment during Run 2 of the Large Hadron Collider. The results are interpreted in the context of R-parity-violating supersymmetry models that feature prompt gluino-pair production decaying directly to three jets each or decaying to two jets and a neutralino which subsequently decays promptly to three jets. No significant excess over the Standard Model expectation is observed and exclusion limits at the 95% confidence level are extracted. Gluinos with masses up to 1800 GeV are excluded when decaying directly to three jets. In the cascade scenario, gluinos with masses up to 2340 GeV are excluded for a neutralino with mass up to 1250 GeV.

The ATLAS detector is installed in its experimental cavern at Point 1 of the CERN Large Hadron Collider. During Run 2 of the LHC, a luminosity of ℒ = 2 × 10³⁴ cm^-2 s^-1 was routinely achieved at the start of fills, twice the design luminosity. For Run 3, accelerator improvements, notably luminosity levelling, allow sustained running at an instantaneous luminosity of ℒ = 2 × 10³⁴ cm^-2 s^-1, with an average of up to 60 interactions per bunch crossing. The ATLAS detector has been upgraded to recover Run 1 single-lepton trigger thresholds while operating comfortably under Run 3 sustained pileup conditions. A fourth pixel layer 3.3 cm from the beam axis was added before Run 2 to improve vertex reconstruction and b-tagging performance. New Liquid Argon Calorimeter digital trigger electronics, with corresponding upgrades to the Trigger and Data Acquisition system, take advantage of a factor of 10 finer granularity to improve triggering on electrons, photons, taus, and hadronic signatures through increased pileup rejection. The inner muon endcap wheels were replaced by New Small Wheels with Micromegas and small-strip Thin Gap Chamber detectors, providing both precision tracking and Level-1 Muon trigger functionality. Trigger coverage of the inner barrel muon layer near one endcap region was augmented with modules integrating new thin-gap resistive plate chambers and smaller-diameter drift-tube chambers. Tile Calorimeter scintillation counters were added to improve electron energy resolution and background rejection. Upgrades to Minimum Bias Trigger Scintillators and Forward Detectors improve luminosity monitoring and enable total proton-proton cross section, diffractive physics, and heavy ion measurements. These upgrades are all compatible with operation in the much harsher environment anticipated after the High-Luminosity upgrade of the LHC and are the first steps towards preparing ATLAS for the High-Luminosity upgrade of the LHC. This paper describes the Run 3 configuration of the ATLAS detector.

The invariant yield of electrons from open-heavy-flavor decays for 1

This paper presents the measurement of fiducial and differential cross sections for both the inclusive and electroweak production of a same-sign W-boson pair in association with two jets (W^±W^±jj) using 139 fb⁻¹ of proton-proton collision data recorded at a centre-of-mass energy of s = 13 TeV by the ATLAS detector at the Large Hadron Collider. The analysis is performed by selecting two same-charge leptons, electron or muon, and at least two jets with large invariant mass and a large rapidity difference. The measured fiducial cross sections for electroweak and inclusive W^±W^±jj production are 2.92 ± 0.22 (stat.) ± 0.19 (syst.) fb and 3.38 ± 0.22 (stat.) ± 0.19 (syst.) fb, respectively, in agreement with Standard Model predictions. The measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confidence level intervals on dimension-8 operators. A search for doubly charged Higgs bosons H^±± that are produced in vector-boson fusion processes and decay into a same-sign W boson pair is performed. The largest deviation from the Standard Model occurs for an H^±± mass near 450 GeV, with a global significance of 2.5 standard deviations.

A search for the decay of the Higgs boson to a Z boson and a light, pseudoscalar particle, a, decaying respectively to two leptons and to two photons is reported. The search uses the full LHC Run 2 protonproton collision data at s=13 TeV, corresponding to 139 fb⁻¹ collected by the ATLAS detector. This is one of the first searches for this specific decay mode of the Higgs boson, and it probes unexplored parameter space in models with axion-like particles (ALPs) and extended scalar sectors. The mass of the a particle is assumed to be in the range 0.133 GeV. The data are analysed in two categories: a merged category where the photons from the a decay are reconstructed in the ATLAS calorimeter as a single cluster, and a resolved category in which two separate photons are detected. The main background processes are from Standard Model Z boson production in association with photons or jets. The data are in agreement with the background predictions, and upper limits on the branching ratio of the Higgs boson decay to Za times the branching ratio a→γγ are derived at the 95% confidence level and they range from 0.08% to 2% depending on the mass of the a particle. The results are also interpreted in the context of ALP models.

This Letter presents the first study of Higgs boson production in association with a vector boson (V ¼ W or Z) in the fully hadronic qqbb final state using data recorded by the ATLAS detector at the LHC in proton-proton collisions at √s = 13 TeV and corresponding to an integrated luminosity of 137 fb⁻¹. The vector bosons and Higgs bosons are each reconstructed as large-radius jets and tagged using jet substructure techniques. Dedicated tagging algorithms exploiting b-tagging properties are used to identify jets consistent with Higgs bosons decaying into bb̄. Dominant backgrounds from multijet production are determined directly from the data, and a likelihood fit to the jet mass distribution of Higgs boson candidates is used to extract the number of signal events. The VH production cross section is measured inclusively and differentially in several ranges of Higgs boson transverse momentum: 250450, 450650, and greater than 650 GeV. The inclusive signal yield relative to the standard model expectation is observed to be μ = 1.4^+1.0_−0.9 and the corresponding cross section is 3.1 ± 1.3(stat)^+1.8_−1.4(syst) pb.

This paper presents for the first time a precise measurement of the production properties of the Z boson in the full phase space of the decay leptons. This is in contrast to the many previous precise unfolded measurements performed in the fiducial phase space of the decay leptons. The measurement is obtained from protonproton collision data collected by the ATLAS experiment in 2012 at s=8 TeV at the LHC and corresponding to an integrated luminosity of 20.2 fb^-1. The results, based on a total of 15.3 million Z-boson decays to electron and muon pairs, extend and improve a previous measurement of the full set of angular coefficients describing Z-boson decay. The double-differential cross-section distributions in Z-boson transverse momentum p_T and rapidity y are measured in the pole region, defined as 80

This paper presents the electron and photon energy calibration obtained with the ATLAS detector using 140 fb⁻¹ of LHC proton-proton collision data recorded at √s = 13 TeV between 2015 and 2018. Methods for the measurement of electron and photon energies are outlined, along with the current knowledge of the passive material in front of the ATLAS electromagnetic calorimeter. The energy calibration steps are discussed in detail, with emphasis on the improvements introduced in this paper. The absolute energy scale is set using a large sample of Z-boson decays into electron-positron pairs, and its residual dependence on the electron energy is used for the first time to further constrain systematic uncertainties. The achieved calibration uncertainties are typically 0.05% for electrons from resonant Z-boson decays, 0.4% at E_T ∼ 10 GeV, and 0.3% at E_T ∼ 1 TeV; for photons at E_T ∼ 60 GeV, they are 0.2% on average. This is more than twice as precise as the previous calibration. The new energy calibration is validated using J/ → ee and radiative Z-boson decays.

A search for pair production of squarks or gluinos decaying via sleptons or weak bosons is reported. The search targets a final state with exactly two leptons with same-sign electric charge or at least three leptons without any charge requirement. The analysed data set corresponds to an integrated luminosity of 139 fb⁻¹ of proton-proton collisions collected at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Multiple signal regions are defined, targeting several SUSY simplified models yielding the desired final states. A single control region is used to constrain the normalisation of the WZ + jets background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of several supersymmetric models featuring R-parity conservation or R-parity violation, yielding exclusion limits surpassing those from previous searches. In models considering gluino (squark) pair production, gluino (squark) masses up to 2.2 (1.7) TeV are excluded at 95% confidence level.

Corrections to two figures, one table and the corresponding numbers in the text are noted for the paper. Systematic uncertainties arising from the comparison of the nominal tt¯tt¯ simulation with alternative samples generated with Sherpa and with MadGraph5_aMC@NLO+Herwig7 were not applied when deriving limits on the top-quark Yukawa coupling, Higgs oblique parameter and EFT operators. This affects Figs. 8 and 9, and Table 8. (Figure presented.) (Figure presented.) (Table presented.) Two-dimensional negative log-likelihood contours for |κ_tcos(α)| versus |κ_tsin(α)| at 68% and 95%, where κ_t is the top-Higgs Yukawa coupling strength parameter and α is the mixing angle between the CP-even and CP-odd components. The gradient-shaded area represents the observed likelihood value as a function of κ_t and α. Both the tt¯tt¯ signal and tt¯H background yields in each fitted bin are parameterised as a function of κ_t and α. The blue cross shows the SM expectation, while the black cross shows the best fit value The negative log-likelihood values as a function of the Higgs oblique parameter H^. The solid line represents the observed likelihood while the dashed line corresponds to the expected one. The dashed region shows the non-unitary regime Expected and observed 95% CL intervals on EFT coupling parameters assuming one EFT parameter variation in the fit Operators Expected C_i/Λ² [TeV ^-2] Observed C_i/Λ² [TeV ^-2] O_QQ¹ [-2.5,3.2] [-4.0,4.5] O_Qt¹ [-2.6,2.1] [-3.8,3.4] O_tt¹ [-1.2,1.4] [-1.9,2.1] O_Qt⁸ [-4.3,5.1] [-6.9,7.6] The changes in the text are noted for Sects. 9.1, 9.2 and 10. In Sect. 9.1, for the case when the tt¯tt¯ and tt¯H yields in each bin of the GNN distribution are parameterised as a function of κ_t and α and fixing the top-quark Yukawa coupling to be CP-even only, the observed limit is |κ_t|

The identification of jets originating from quarks and gluons, often referred to as quark/gluon tagging, plays an important role in various analyses performed at the Large Hadron Collider, as Standard Model measurements and searches for new particles decaying to quarks often rely on suppressing a large gluon-induced background. This paper describes the measurement of the efficiencies of quark/gluon taggers developed within the ATLAS Collaboration, using √s = 13 TeV protonproton collision data with an integrated luminosity of 140 fb⁻¹ collected by the ATLAS experiment. Two taggers with high performances in rejecting jets from gluon over jets from quarks are studied: one tagger is based on requirements on the number of inner-detector tracks associated with the jet, and the other combines several jet substructure observables using a boosted decision tree. A method is established to determine the quark/gluon fraction in data, by using quark/gluon-enriched subsamples defined by the jet pseudorapidity. Differences in tagging efficiency between data and simulation are provided for jets with transverse momentum between 500 GeV and 2 TeV and for multiple tagger working points.

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at s = 13 TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb⁻¹. The search for A → ZH is performed in the ℓ⁺ℓ⁻tt¯ and νν¯bb¯ final states and surpasses the reach of previous searches in different final states in the region with m_H > 350 GeV and m_A > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed m(tt¯) and m(bb¯) distributions in the ℓ⁺ℓ⁻tt¯ and νν¯bb¯ channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models.

This paper presents a search for a new Z resonance decaying into a pair of dark quarks which hadronise into dark hadrons before promptly decaying back as Standard Model particles. This analysis is based on proton-proton collision data recorded at s = 13 TeV with the ATLAS detector at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of 139 fb⁻¹. After selecting events containing large-radius jets with high track multiplicity, the invariant mass distribution of the two highest-transverse-momentum jets is scanned to look for an excess above a data-driven estimate of the Standard Model multijet background. No significant excess of events is observed and the results are thus used to set 95% confidence-level upper limits on the production cross-section times branching ratio of the Z to dark quarks as a function of the Z mass for various dark-quark scenarios.

Differential cross-sections are measured for the production of four charged leptons in association with two jets. These measurements are sensitive to final states in which the jets are produced via the strong interaction as well as to the purely-electroweak vector boson scattering process. The analysis is performed using proton-proton collision data collected by ATLAS at s = 13 TeV and with an integrated luminosity of 140 fb⁻¹. The data are corrected for the effects of detector inefficiency and resolution and are compared to state-of-the-art Monte Carlo event generator predictions. The differential cross-sections are used to search for anomalous weak-boson self-interactions that are induced by dimension-six and dimension-eight operators in Standard Model effective field theory.

A search for nonresonant Higgs boson pair production in the bb¯γγ final state is performed using 140 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. This analysis supersedes and expands upon the previous nonresonant ATLAS results in this final state based on the same data sample. The analysis strategy is optimised to probe anomalous values not only of the Higgs (H) boson self-coupling modifier κ_λ but also of the quartic HHVV (V = W, Z) coupling modifier κ_2V. No significant excess above the expected background from Standard Model processes is observed. An observed upper limit μ_HHλ 2V

Semi-visible jets, with a significant contribution to the event's missing transverse momentum, can arise in strongly interacting dark sectors. This results in an event topology where one of the jets can be aligned with the direction of the missing transverse momentum. The first search for semi-visible jets produced via a t-channel mediator exchange is presented. The analysis uses proton-proton collisions with an integrated luminosity of 139 fb⁻¹ and a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during the Run 2 of the LHC. No excess over Standard Model predictions is observed. Assuming a coupling strength of unity between the mediator, a Standard Model quark and a dark quark, mediator masses up to 2.7 TeV are excluded at the 95% confidence level. Upper limits on the coupling strength are also derived.

The inclusive top-quark-pair production cross section σ_tt¯ and its ratio to the Z-boson production cross section have been measured in protonproton collisions at s=13.6 TeV, using 29 fb⁻¹ of data collected in 2022 with the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge electron-muon pair and b-tagged jets, and assuming Standard Model decays, the top-quark-pair production cross section is measured to be σ_tt¯=850±3(stat.)±18(syst.)±20(lumi.) pb. The ratio of the tt¯ and the Z-boson production cross sections is also measured, where the Z-boson contribution is determined for inclusive e⁺e⁻ and μ⁺μ⁻ events in a fiducial phase space. The relative uncertainty on the ratio is reduced compared to the tt¯ cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, R_tt¯/Z=1.145±0.003(stat.)±0.021(syst.)±0.002(lumi.) is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.

A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 20152018 at s=13 TeV from pp collisions corresponding to an integrated luminosity of 139 fb⁻¹ are examined. Particles producing anomalously high ionization, consistent with long-lived spin-½ massive particles with electric charges from |q|=2e to |q|=7e are searched for. No statistically significant evidence of such particles is observed, and 95% confidence level cross-section upper limits are calculated and interpreted as the lower mass limits for a DrellYan plus photon-fusion production mode. The least stringent limit, 1060 GeV, is obtained for |q|=2e particles, and the most stringent one, 1600 GeV, is for |q|=6e particles.

A search for flavour-changing neutral interactions involving the top quark, the Higgs boson and an up-type quark q (q = c, u) is presented. The proton-proton collision data set used, with an integrated luminosity of 139 fb⁻¹, was collected at (Formula presented.) TeV by the ATLAS experiment at the Large Hadron Collider. Both the decay process t → qH in tt¯ production and the production process pp → tH, with the Higgs boson decaying into two photons, are investigated. No significant excess is observed and upper limits are set on the t → cH and the t → uH branching ratios of 4.3 × 10⁻⁴ and 3.8 × 10⁻⁴, respectively, at the 95% confidence level, while the expected limits in the absence of signal are 4.7 × 10⁻⁴ and 3.9 × 10⁻⁴. Combining this search with ATLAS searches in the H → τ⁺τ⁻ and H → bb¯ final states yields observed (expected) upper limits on the t → cH branching ratio of 5.8 × 10⁻⁴ (3.0 × 10⁻⁴) at the 95% confidence level. The corresponding observed (expected) upper limit on the t → uH branching ratio is 4.0 × 10⁻⁴ (2.4 × 10⁻⁴).

A search for new heavy scalars with flavour-violating decays in final states with multiple leptons and b-tagged jets is presented. The results are interpreted in terms of a general two-Higgs-doublet model involving an additional scalar with couplings to the top-quark and the three up-type quarks (ρ_tt, ρ_tc, and ρ_tu). The targeted signals lead to final states with either a same-sign top-quark pair, three top-quarks, or four top-quarks. The search is based on a data sample of proton-proton collisions at (Formula presented.) TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb⁻¹. Events are categorised depending on the multiplicity of light charged leptons (electrons or muons), total lepton charge, and a deep-neural-network output to enhance the purity of each of the signals. Masses of an additional scalar boson m_H between 200 − 630 GeV with couplings ρ_tt = 0.4, ρ_tc = 0.2, and ρ_tu = 0.2 are excluded at 95% confidence level. Additional interpretations are provided in models of R-parity violating supersymmetry, motivated by the recent flavour and (g − 2)_μ anomalies.

A search for a new massive charged gauge boson, W, is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of (Formula presented.) TeV, and corresponds to an integrated luminosity of 139 fb⁻¹. The reconstructed tb invariant mass is used to search for a W boson decaying into a top quark and a bottom quark. The result is interpreted in terms of a W boson with purely right-handed or left-handed chirality in a mass range of 0.56 TeV. Different values for the coupling of the W boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in the s-channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on the W → tb production cross-section times branching ratio as a function of the W-boson mass and in the plane of the coupling vs the W-boson mass.

Searches for electroweak production of wino-like chargino pairs, χ~₁⁺χ~₁⁻, and of wino-like chargino and next-to-lightest neutralino, χ~₁^±χ~₂⁰, are presented. The models explored assume that the charginos decay into a W boson and the lightest neutralino, χ~₁^±→W^±χ~₁⁰. The next-to-lightest neutralinos are degenerate in mass with the chargino and decay to χ~₁⁰ and either a Z or a Higgs boson, χ~₂⁰→Zχ~₁⁰ or hχ~₁⁰. The searches exploit the presence of a single isolated lepton and missing transverse momentum from the W boson decay products and the lightest neutralinos, and the presence of jets from hadronically decaying Z or W bosons or from the Higgs boson decaying into a pair of b-quarks. The searches use 139 fb⁻¹ of (Formula presented.) TeV proton-proton collisions data collected by the ATLAS detector at the Large Hadron Collider between 2015 and 2018. No deviations from the Standard Model expectations are found, and 95% confidence level exclusion limits are set. Chargino masses ranging from 260 to 520 GeV are excluded for a massless χ~₁⁰ in chargino pair production models. Degenerate chargino and next-to-lightest neutralino masses ranging from 260 to 420 GeV are excluded for a massless χ~₁⁰ for χ~₂⁰→Zχ~₁⁰. For decays through an on-shell Higgs boson and for mass-splitting between χ~₁^±/χ~₂⁰ and χ~₁⁰ as small as the Higgs boson mass, mass limits are improved by up to 40 GeV in the range of 200260 GeV and 280470 GeV compared to previous ATLAS constraints.

A search for heavy right-handed Majorana or Dirac neutrinos N_R and heavy right-handed gauge bosons W_R is performed in events with energetic electrons or muons, with the same or opposite electric charge, and energetic jets. The search is carried out separately for topologies of clearly separated final-state products (\u201cresolved\u201d channel) and topologies with boosted final states with hadronic and/or leptonic products partially overlapping and reconstructed as a large-radius jet (\u201cboosted\u201d channel). The events are selected from pp collision data at the LHC with an integrated luminosity of 139 fb ^{- 1} collected by the ATLAS detector at s=13 TeV. No significant deviations from the Standard Model predictions are observed. The results are interpreted within the theoretical framework of a left-right symmetric model, and lower limits are set on masses in the heavy right-handed W_R boson and N_R plane. The excluded region extends to about m(W_R) = 6.4 TeV for both Majorana and Dirac N_R neutrinos at m(N_R) R with masses of less than 3.5 (3.6) TeV are excluded in the electron (muon) channel at m(W_R) = 4.8 TeV for the Majorana neutrinos, and limits of m(N_R) up to 3.6 TeV for m(W_R) = 5.2 (5.0) TeV in the electron (muon) channel are set for the Dirac neutrinos. These constitute the most stringent exclusion limits to date for the model considered.

The evaluation of the measurement of double-spin asymmetries for charge-separated pions and kaons produced in deep-inelastic scattering from the proton using the ECCE detector design concept is presented, for the combinations of lepton and hadron beam energies of 5 × 41 GeV² and 18 × 275 GeV². The study uses unpolarised simulated data that are processed through a full GEANT simulation of the detector. These data are then reweighted at the parton level with DSSV helicity distributions and DSS fragmentation functions, in order to generate the relevant asymmetries, and subsequently analysed. The performed analysis shows that the ECCE detector concept provides the resolution and acceptance, with a broad coverage in kinematic phase space, needed for a robust extraction of asymmetries. This, in turn, allows for a precise extraction of sea-quark helicity distributions.

A search for supersymmetry targeting the direct production of winos and higgsinos is conducted in final states with either two leptons (e or μ) with the same electric charge, or three leptons. The analysis uses 139 fb⁻¹ of pp collision data at (Formula presented.) TeV collected with the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess over the Standard Model expectation is observed. Simplified and complete models with and without R-parity conservation are considered. In topologies with intermediate states including either Wh or WZ pairs, wino masses up to 525 GeV and 250 GeV are excluded, respectively, for a bino of vanishing mass. Higgsino masses smaller than 440 GeV are excluded in a natural R-parity-violating model with bilinear terms. Upper limits on the production cross section of generic events beyond the Standard Model as low as 40 ab are obtained in signal regions optimised for these models and also for an R-parity-violating scenario with baryon-number-violating higgsino decays into top quarks and jets. The analysis significantly improves sensitivity to supersymmetric models and other processes beyond the Standard Model that may contribute to the considered final states.

A search for new phenomena giving rise to pairs of opposite electrically charged muons with impact parameters in the millimeter range is presented, using 139 fb⁻¹ of s=13 TeV pp collision data from the ATLAS detector at the LHC. The search targets the gap in coverage between existing searches targeting final states with leptons with large displacement and prompt leptons. No significant excess over the background expectation is observed and exclusion limits are set on the mass of long-lived scalar supersymmetric muon-partners (smuons) with much lower lifetimes than previously targeted by displaced muon searches. Smuon lifetimes down to 1 ps are excluded for a smuon mass of 100 GeV, and smuon masses up to 520 GeV are excluded for a proper lifetime of 10 ps, at 95% confidence level. Finally, model-independent limits are set on the contribution from new phenomena to the signal-region yields.

Searches for long-lived particles (LLPs) are among the most promising avenues for discovering physics beyond the Standard Model at the Large Hadron Collider (LHC). However, displaced signatures are notoriously difficult to identify due to their ability to evade standard object reconstruction strategies. In particular, the ATLAS track reconstruction applies strict pointing requirements which limit sensitivity to charged particles originating far from the primary interaction point. To recover efficiency for LLPs decaying within the tracking detector volume, the ATLAS Collaboration employs a dedicated large-radius tracking (LRT) pass with loosened pointing requirements. During Run 2 of the LHC, the LRT implementation produced many incorrectly reconstructed tracks and was therefore only deployed in small subsets of events. In preparation for LHC Run 3, ATLAS has significantly improved both standard and large-radius track reconstruction performance, allowing for LRT to run in all events. This development greatly expands the potential phase-space of LLP searches and streamlines LLP analysis workflows. This paper will highlight the above achievement and report on the readiness of the ATLAS detector for track-based LLP searches in Run 3.

This Letter reports the observation of single top quarks produced together with a photon, which directly probes the electroweak coupling of the top quark. The analysis uses Formula Presented of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be Formula Presented, to be compared with the standard model prediction of Formula Presented at next-to-leading order in QCD.

This Letter presents the measurement of the fiducial and differential cross-sections of the electroweak production of a Zγ pair in association with two jets. The analysis uses 140 fb⁻¹ of LHC protonproton collision data taken at s=13 TeV recorded by the ATLAS detector during the years 20152018. Events with a Z boson candidate decaying into either an e⁺e⁻ or μ⁺μ⁻ pair, a photon and two jets are selected. The electroweak component is extracted by requiring a large dijet invariant mass and by using the information about the centrality of the system and is measured with an observed and expected significance well above five standard deviations. The fiducial pp→Zγjj cross-section for the electroweak production is measured to be 3.6 ± 0.5 fb. The total fiducial cross-section that also includes contributions where the jets arise from strong interactions is measured to be 16.8_−1.8^+2.0 fb. The results are consistent with the Standard Model predictions. Differential cross-sections are also measured using the same events and are compared with parton-shower Monte Carlo simulations. Good agreement is observed between data and predictions.

A search for pair-produced scalar or vector leptoquarks decaying into a b-quark and a τ -lepton is presented using the full LHC Run 2 (20152018) data sample of 139 fb ^{- 1} collected with the ATLAS detector in protonproton collisions at a centre-of-mass energy of s=13 TeV. Events in which at least one τ -lepton decays hadronically are considered, and multivariate discriminants are used to extract the signals. No significant deviations from the Standard Model expectation are observed and 95% confidence-level upper limits on the production cross-section are derived as a function of leptoquark mass and branching ratio B into a τ -lepton and b-quark. For scalar leptoquarks, masses below 1460 GeV are excluded assuming B= 100 %, while for vector leptoquarks the corresponding limit is 1650 GeV (1910 GeV) in the minimal-coupling (YangMills) scenario.

We performed feasibility studies for various measurements that are related to unpolarized TMD distribution and fragmentation functions for the ECCE detector proposal. The processes studied include semi-inclusive Deep inelastic scattering (SIDIS) where single hadrons (pions and kaons) were detected in addition to the scattered DIS lepton. The single hadron cross sections and multiplicities were extracted as a function of the DIS variables x and Q², as well as the semi-inclusive variables z, which corresponds to the momentum fraction the detected hadron carries relative to the struck parton and P_T, which corresponds to the transverse momentum of the detected hadron relative to the virtual photon. The expected statistical precision of such measurements is extrapolated to accumulated luminosities of 10 fb⁻¹ and potential systematic uncertainties are approximated given the deviations between true and reconstructed yields. The expected uncertainties are then used to obtain the expected impact on the related TMD distribution and fragmentation functions. We find that the ECCE detector proposal fulfills the physics requirements on these channels as detailed in the EIC Yellow Report.

The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with pp collisions at a centre-of-mass energy s=13 TeV. The absolute luminosity scale is determined using van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosity for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2 pp data sample corresponds to an integrated luminosity of 140.1 ± 1.2 fb ^{- 1} , i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 20172018 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of 338.1 ± 3.1 pb ^{- 1} .

A search is made for potential ccc̄ c̄ tetraquarks decaying into a pair of charmonium states in the four ffiffi muon final state using proton-proton collision data at ps ¼ 13 TeV, corresponding to an integrated luminosity of 140 fb⁻¹ recorded by the ATLAS experiment at LHC. Two decay channels, J=ψ þ J=ψ → 4μ and J=ψ þ ψ ð2SÞ → 4μ, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-J=ψ channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the J=ψ þ ψ ð2SÞ channel. The fitted masses and decay widths of the structures are reported.

This Letter reports the observation of Formula Presented-lepton-pair production in ultraperipheral lead-lead collisions Formula Presented and constraints on the Formula Presented-lepton anomalous magnetic moment Formula Presented. The dataset corresponds to an integrated luminosity of Formula Presented of LHC Formula Presented collisions at Formula Presented recorded by the ATLAS experiment in 2018. Selected events contain one muon from a Formula Presented-lepton decay, an electron or charged-particle track(s) from the other Formula Presented-lepton decay, little additional central-detector activity, and no forward neutrons. The Formula Presented process is observed in Formula Presented collisions with a significance exceeding 5 standard deviations and a signal strength of Formula Presented assuming the standard model value for Formula Presented. To measure Formula Presented, a template fit to the muon transverse-momentum distribution from Formula Presented-lepton candidates is performed, using a dimuon (Formula Presented) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for Formula Presented is Formula Presented.

A search for physics beyond the Standard Model inducing periodic signals in the dielectron and diphoton invariant mass spectra is presented using 139 fb⁻¹ of s = 13 TeV pp collision data collected by the ATLAS experiment at the LHC. Novel search techniques based on continuous wavelet transforms are used to infer the frequency of periodic signals from the invariant mass spectra and neural network classifiers are used to enhance the sensitivity to periodic resonances. In the absence of a signal, exclusion limits are placed at the 95% confidence level in the two-dimensional parameter space of the clockwork gravity model. Model-independent searches for deviations from the background-only hypothesis are also performed. [Figure not available: see fulltext.]

This letter presents a measurement of the nuclear modification factor of large-radius jets in Formula Presented Formula Presented collisions by the ATLAS experiment. The measurement is performed using Formula Presented and Formula Presented of Formula Presented and Formula Presented data, respectively. The large-radius jets are reconstructed with the anti-Formula Presented algorithm using a radius parameter of Formula Presented, by reclustering anti-Formula Presented Formula Presented jets, and are measured over the transverse momentum (Formula Presented) kinematic range of Formula Presented and absolute pseudorapidity Formula Presented. The large-radius jet constituents are further reclustered using the Formula Presented algorithm in order to obtain the splitting parameters, Formula Presented and Formula Presented, which characterize the transverse momentum scale and angular separation for the hardest splitting in the jet, respectively. The nuclear modification factor, Formula Presented, obtained by comparing the Formula Presented jet yields to those in Formula Presented collisions, is measured as a function of jet transverse momentum (Formula Presented) and Formula Presented or Formula Presented. A significant difference in the quenching of large-radius jets having single subjet and those with more complex substructure is observed. Systematic comparison of jet suppression in terms of Formula Presented for different jet definitions is also provided. Presented results support the hypothesis that jets with hard internal splittings lose more energy through quenching and provide a new perspective for understanding the role of jet structure in jet suppression.

A key open question in the study of multiparticle production in high-energy Formula Presented collisions is the relationship between the \u201cridge\u201di.e., the observed azimuthal correlations between particles in the underlying event that extend over all rapiditiesand hard or semihard scattering processes. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event. To address this question, two-particle correlations are measured in Formula Presented collisions at Formula Presented using data collected by the ATLAS experiment at the LHC, with an integrated luminosity of Formula Presented, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, Formula Presented, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge.

A search for dark matter produced in association with a Higgs boson in final states with two hadronically decaying τ-leptons and missing transverse momentum is presented. The analysis uses 139 fb⁻¹ of proton-proton collision data at s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. No evidence of physics beyond the Standard Model is found. The results are interpreted in terms of a 2HDM+a model featuring two scalar Higgs doublets and a pseudoscalar singlet field. Exclusion limits on the parameters of the model in selected benchmark scenarios are derived at 95% confidence level. Model-independent limits are also set on the visible cross-section for processes beyond the Standard Model producing missing transverse momentum in association with a Higgs boson decaying into τ-leptons.

This paper reports the first ATLAS measurement of the B_s⁰→ μμ effective lifetime. The measurement is based on the data collected in 20152016, amounting to 26.3 fb⁻¹ of 13 TeV LHC proton-proton collisions. The proper decay-time distribution of 58 ± 13 background-subtracted signal candidates is fit with simulated signal templates parameterised as a function of the B_s⁰ effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the B_s⁰→ μμ lifetime is 0.99_−0.07^+0.42 (stat.) ± 0.17 (syst.) ps and it is found to be consistent with the Standard Model.

A search is presented for a heavy resonance Formula Presented decaying into a Standard Model Higgs boson Formula Presented and a new particle Formula Presented in a fully hadronic final state. The full Large Hadron Collider run 2 dataset of proton-proton collisions at Formula Presented collected by the ATLAS detector from 2015 to 2018 is used and corresponds to an integrated luminosity of Formula Presented. The search targets the high Formula Presented-mass region, where the Formula Presented and Formula Presented have a significant Lorentz boost in the laboratory frame. A novel application of anomaly detection is used to define a general signal region, where events are selected solely because of their incompatibility with a learned background-only model. It is constructed using a jet-level tagger for signal-model-independent selection of the boosted Formula Presented particle, representing the first application of fully unsupervised machine learning to an ATLAS analysis. Two additional signal regions are implemented to target a benchmark Formula Presented decay into two quarks, covering topologies where the Formula Presented is reconstructed as either a single large-radius jet or two small-radius jets. The analysis selects Higgs boson decays into Formula Presented, and a dedicated neural-network-based tagger provides sensitivity to the boosted heavy-flavor topology. No significant excess of data over the expected background is observed, and the results are presented as upper limits on the production cross section Formula Presented) for signals with Formula Presented between 1.5 and 6 TeV and Formula Presented between 65 and 3000 GeV.

Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the μ+μ-γ final state with a protonproton collision data sample corresponding to an integrated luminosity of 139 fb ^{- 1} collected at s=13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/ψγ , ψ(2S)γ , and Υ(1S,2S,3S)γ are found to be 2.0 × 10 ^{- 4} , 10.5 × 10 ^{- 4} , and (2.5 , 4.2 , 3.4) × 10 ^{- 4} , respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 × 10 ^{- 6} , 2.4 × 10 ^{- 6} , and (1.1 , 1.3 , 2.4) × 10 ^{- 6} . An observed 95% CL interval of (- 133 , 175) is obtained for the κ_c/ κ_γ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (- 37 , 40) is obtained for κ_b/ κ_γ .

A search for flavor-changing neutral-current couplings between a top quark, an up or charm quark, and a Formula Presented boson is presented, using proton-proton collision data at Formula Presented collected by the ATLAS detector at the Large Hadron Collider. The analyzed data set corresponds to an integrated luminosity of Formula Presented. The search targets both single-top-quark events produced as Formula Presented (with Formula Presented, Formula Presented) and top-quark-pair events, with one top quark decaying through the Formula Presented channel. The analysis considers events with three leptons (electrons or muons), a Formula Presented-tagged jet, possible additional jets, and missing transverse momentum. The data are found to be consistent with the background-only hypothesis and 95% confidence-level limits on the Formula Presented branching ratios, assuming only tensor operators of the Standard Model effective field theory framework contribute to the Formula Presented vertices. These are Formula Presented (Formula Presented) for Formula Presented (Formula Presented) for a left-handed Formula Presented coupling, and Formula Presented (Formula Presented) in the case of a right-handed coupling. These results are interpreted as 95% CL upper limits on the strength of the corresponding couplings, yielding limits for Formula Presented and Formula Presented (Formula Presented and Formula Presented) of 0.15 (0.16), and limits for Formula Presented and Formula Presented (Formula Presented and Formula Presented) of 0.22 (0.21), assuming a new-physics energy scale Formula Presented of 1 TeV.

A measurement of the polarisation of W bosons produced in top-quark decays is presented, using protonproton collision data at a centre-of-mass energy of s=13 TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139 fb⁻¹. The measurement is performed selecting tt¯ events decaying into final states with two charged leptons (electrons or muons) and at least two b-tagged jets. The polarisation is extracted from the differential cross-section distribution of the cosθ^\u204e variable, where θ^\u204e is the angle between the momentum direction of the charged lepton from the W boson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the W boson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the cosθ^\u204e angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be f₀=0.684±0.005(stat.)±0.014(syst.), f_L=0.318±0.003(stat.)±0.008(syst.) and f_R=−0.002±0.002(stat.)±0.014(syst.), in agreement with the Standard Model prediction.

A measurement of the charge asymmetry in top-quark pair (tt¯) production in association with a photon is presented. The measurement is performed in the single-lepton tt¯ decay channel using protonproton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN at a centre-of-mass-energy of 13 TeV during the years 20152018, corresponding to an integrated luminosity of 139 fb⁻¹. The charge asymmetry is obtained from the distribution of the difference of the absolute rapidities of the top quark and antiquark using a profile likelihood unfolding approach. It is measured to be A_C=−0.003±0.029 in agreement with the Standard Model expectation.

Inclusive and differential measurements of the topantitop (Formula presented.) charge asymmetry (Formula presented.) and the leptonic asymmetry (Formula presented.) are presented in protonproton collisions at s = 13 TeV recorded by the ATLAS experiment at the CERN Large Hadron Collider. The measurement uses the complete Run 2 dataset, corresponding to an integrated luminosity of 139 fb⁻¹, combines data in the single-lepton and dilepton channels, and employs reconstruction techniques adapted to both the resolved and boosted topologies. A Bayesian unfolding procedure is performed to correct for detector resolution and acceptance effects. The combined inclusive (Formula presented.) charge asymmetry is measured to be (Formula presented.), which differs from zero by 4.7 standard deviations. Differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the (Formula presented.) system. Both the inclusive and differential measurements are found to be compatible with the Standard Model predictions, at next-to-next-to-leading order in quantum chromodynamics perturbation theory with next-to-leading-order electroweak corrections. The measurements are interpreted in the framework of the Standard Model effective field theory, placing competitive bounds on several Wilson coefficients. [Figure not available: see fulltext.].

This paper describes a search for the single production of an up-type vector-like quark (T) decaying as T → Ht or T → Zt. The search utilises a dataset of pp collisions at s = 13 TeV collected with the ATLAS detector during the 20152018 data-taking period of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb⁻¹. Data are analysed in final states containing a single lepton with multiple jets and b-jets. The presence of boosted heavy resonances in the event is exploited to discriminate the signal from the Standard Model background. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross section of T quarks in different decay channels. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (κ) of the vector-like quark. For singlet T quarks, κ values above 0.53 are excluded for all masses below 2.3 TeV. At a mass of 1.6 TeV, κ values as low as 0.35 are excluded. For T quarks in the doublet scenario, where the production cross section is much lower, κ values above 0.72 are excluded for all masses below 1.7 TeV, and this exclusion is extended to κ above 0.55 for low masses around 1.0 TeV.

A search for the charged-lepton-flavor-violating process Formula Presented is presented, using Formula Presented of Formula Presented Formula Presented collision data collected by the ATLAS experiment at the LHC. An excess in the Formula Presented invariant mass spectrum near the Formula Presented boson mass would be a striking signature of new physics. No excess is observed, and an upper limit Formula Presented is placed on the branching fraction at 95% confidence level, which is the most stringent limit to date.

A test of C P invariance in Higgs boson production via vector-boson fusion has been performed in the H → γ γ channel using 139 fb - 1 of proton-proton collision data at s = 13 TeV collected by the ATLAS detector at the LHC. The optimal observable method is used to probe the C P structure of interactions between the Higgs boson and electroweak gauge bosons, as described by an effective field theory. No sign of C P violation is observed in the data. Constraints are set on the parameters describing the strength of the C P -odd component in the coupling between the Higgs boson and the electroweak gauge bosons in two effective field theory bases: d ˜ in the HISZ basis and c H W ˜ in the Warsaw basis. The results presented are the most stringent constraints on C P violation in the coupling between Higgs and weak bosons. The 95% C.L. constraint on d ˜ is derived for the first time and the 95% C.L. constraint on c H W ˜ has been improved by a factor of 5 compared to the previous measurement.

A search for the pair-production of vector-like quarks optimized for decays into a Z boson and a third-generation Standard Model quark is presented, using the full Run 2 dataset corresponding to 139fb⁻¹ of pp collisions at s=13TeV, collected in 20152018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a Z boson with high transverse momentum, reconstructed from a pair of same-flavour leptons with opposite-sign charges, as well as by the presence of b-tagged jets and high-transverse-momentum large-radius jets reconstructed from calibrated smaller-radius jets. Events with exactly two or at least three leptons are used, which are further categorized by the presence of boosted W, Z, and Higgs bosons and top quarks. The categorization is performed using a neural-network-based boosted object tagger to enhance the sensitivity to signal relative to the background. No significant excess above the background expectation is observed and exclusion limits at 95% confidence level are set on the masses of the vector-like partners T and B of the top and bottom quarks, respectively. The limits depend on the branching ratio configurations and, in the case of 100% branching ratio for T→Zt and 100% branching ratio for B→Zb, this search sets the most stringent limits to date, allowing m_T>1.60TeV and m_B>1.42TeV, respectively.

The identification of b-jets, referred to as b-tagging, is an important part of many physics analyses in the ATLAS experiment at the Large Hadron Collider and an accurate calibration of its performance is essential for high-quality physics results. This publication describes the calibration of the light-flavour jet mistagging efficiency in a data sample of protonproton collision events at s=13 TeV corresponding to an integrated luminosity of 139 fb ^{- 1} . The calibration is performed in a sample of Z bosons produced in association with jets. Due to the low mistagging efficiency for light-flavour jets, a method which uses modified versions of the b-tagging algorithms referred to as flip taggers is used in this work. A fit to the jet-flavour-sensitive secondary-vertex mass is performed to extract a scale factor from data, to correct the light-flavour jet mistagging efficiency in Monte Carlo simulations, while simultaneously correcting the b-jet efficiency. With this procedure, uncertainties coming from the modeling of jets from heavy-flavour hadrons are considerably lower than in previous calibrations of the mistagging scale factors, where they were dominant. The scale factors obtained in this calibration are consistent with unity within uncertainties.

This article presents a collection of simulation studies using the ECCE detector concept in the context of the EIC's exclusive, diffractive, and tagging physics program, which aims to further explore the rich quarkgluon structure of nucleons and nuclei. To successfully execute the program, ECCE proposed to utilize the detector system close to the beamline to ensure exclusivity and tag ion beam/fragments for a particular reaction of interest. Preliminary studies confirm the proposed technology and design satisfy the requirements. The projected physics impact results are based on the projected detector performance from the simulation at 10 or 100 fb⁻¹ of integrated luminosity. Additionally, insights related to a potential second EIC detector are documented, which could serve as a guidepost for future development.

A search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) produced in association with a pair of top quarks, with the Higgs boson decaying into a pair of top quarks (H/A → tt¯) is reported. The search targets a final state with exactly two leptons with same-sign electric charges or at least three leptons. The analysed dataset corresponds to an integrated luminosity of 139 fb ⁻¹ of protonproton collisions collected at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Two multivariate classifiers are used to separate the signal from the background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of a type-II two-Higgs-doublet model. The observed (expected) upper limits at 95% confidence level on the tt¯ H/ A production cross-section times the branching ratio of H/A → tt¯ range between 14 (10) fb and 6 (5) fb for a heavy Higgs boson with mass between 400 GeV and 1000 GeV, respectively. Assuming that only one particle, either the scalar H or the pseudo-scalar A, contributes to the tt¯ tt¯ final state, values of tan β below 1.2 or 0.5 are excluded for a mass of 400 GeV or 1000 GeV, respectively. These exclusion ranges increase to tan β below 1.6 or 0.6 when both particles are considered. [Figure not available: see fulltext.]

Many extensions of the Standard Model predict the production of dark matter particles at the LHC. Sufficiently light dark matter particles may be produced in decays of the Higgs boson that would appear invisible to the detector. This Letter presents a statistical combination of searches for H→invisible decays where multiple production modes of the Standard Model Higgs boson are considered. These searches are performed with the ATLAS detector using 139 fb⁻¹ of protonproton collisions at a centreofmass energy of [Formula presented] at the LHC. In combination with the results at [Formula presented] and [Formula presented], an upper limit on the H→invisible branching ratio of 0.107 (0.077) at the 95% confidence level is observed (expected). These results are also interpreted in the context of models where the 125 GeV Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross-section of weakly interacting massive particles and nucleons.

The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of s=13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model tt¯ events; similarly, at a c-jet identification efficiency of 30%, a light-jet (b-jet) rejection factor of 70 (9) is obtained.

A search for resonances in events with at least one isolated lepton (e or μ) and two jets is performed using 139 fb ⁻¹ of s = 13 TeV protonproton collision data recorded by the ATLAS detector at the LHC. Deviations from a smoothly falling background hypothesis are tested in three- and four-body invariant mass distributions constructed from leptons and jets, including jets identified as originating from bottom quarks. Model-independent limits on generic resonances characterised by cascade decays of particles leading to multiple jets and leptons in the final state are presented. The limits are calculated using Gaussian shapes with different widths for the invariant masses. The multi-body invariant masses are also used to set 95% confidence level upper limits on the cross-section times branching ratios for the production and subsequent decay of resonances predicted by several new physics scenarios. [Figure not available: see fulltext.].

This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly suppressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in pp collisions at s=13 TeV during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb⁻¹. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant excess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left- and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t→uγ)

A search for Higgs boson pair production in events with two b-jets and two τ-leptons is presented, using a protonproton collision dataset with an integrated luminosity of 139 fb ⁻¹ collected at s = 13 TeV by the ATLAS experiment at the LHC. Higgs boson pairs produced non-resonantly or in the decay of a narrow scalar resonance in the mass range from 251 to 1600 GeV are targeted. Events in which at least one τ-lepton decays hadronically are considered, and multivariate discriminants are used to reject the backgrounds. No significant excess of events above the expected background is observed in the non-resonant search. The largest excess in the resonant search is observed at a resonance mass of 1 TeV, with a local (global) significance of 3.1σ (2.0σ). Observed (expected) 95% confidence-level upper limits are set on the non-resonant Higgs boson pair-production cross-section at 4.7 (3.9) times the Standard Model prediction, assuming Standard Model kinematics, and on the resonant Higgs boson pair-production cross-section at between 21 and 900 fb (12 and 840 fb), depending on the mass of the narrow scalar resonance. [Figure not available: see fulltext.]