Search for heavy neutral leptons in decays of W bosons using leptonic and semi-leptonic displaced vertices in √s = 13 TeV pp collisions with the ATLAS detector

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 coeffcients as small as 10−7 are excluded at the 95% confidence level. The results are interpreted under different assumptions on the favour of the leptons from the HNL decays

A search for dark matter produced in association with a dark Higgs boson decaying into a Higgs boson pair in 3b or 4b final states using pp collisions at √s = 13 TeV with the ATLAS detector

Abstract A search is performed for dark matter particles produced in association with a resonant pair of Higgs bosons using 140 fb −1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This signature is expected in some extensions of the Standard Model predicting the production of dark matter particles, and is interpreted in terms of a dark Higgs model containing a Z′ mediator in which the dark Higgs boson s decays into a pair of Higgs bosons. The dark Higgs boson is reconstructed through final states with at least three b-tagged jets, produced by the pair of Higgs boson decays, in events with significant missing transverse momentum consistent with the presence of dark matter. The observed data are found to be in good agreement with Standard Model predictions, constraining scenarios with dark Higgs boson masses within the range of 250 to 400 GeV and Z′ mediators up to 2.3 TeV.</jats:p

Azimuthal anisotropies of charged particles with high transverse momentum in Pb+Pb collisions at √sNN=5.02 TeV with the ATLAS detector

A measurement is presented of elliptic (v2) and triangular (v3) azimuthal anisotropy coefficients for charged particles produced in Pb+Pb collisions at √sNN = 5.02 TeV using a dataset corresponding to an integrated luminosity of 0.44 nb−1 collected with the ATLAS detector at the LHC in 2018. The values of v2 and v3 are measured for charged particles over a wide range of transverse momentum (pT), 1–400 GeV, and Pb+Pb collision centrality, 0–60%, using the scalar-product and multiparticle cumulant methods. These methods are sensitive to event-by-event fluctuations and nonflow effects in the measurements of azimuthal anisotropies. Positive values of v2 are observed up to a pT of approximately 100 GeV from both methods across all centrality intervals. Positive values of v3 are observed up to approximately 25 GeV using both methods, though the application of the three-subevent technique to the multiparticle cumulant method leads to significant changes at the highest pT. At high pT (pT 10 GeV), charged particles are dominantly from jet fragmentation. These jets, and hence the measurements presented here, are sensitive to the path-length dependence of parton energy loss in the quark-gluon plasma produced in Pb+Pb collisions

Measurements and interpretations of W±Z production cross-sections in pp collisions at √s =13 TeV with the ATLAS detector

Measurements of integrated and differential cross-sections for W±Z production 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±Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons or muons. The integrated cross-section per lepton flavour for the production of W±Z is measured in the detector fiducial region with a relative precision of 4%. The measured value is compared with the Standard Model prediction at a precision of up to next-to-next-to-leading-order in QCD and next-to-leading-order in electroweak. Cross-sections for W+Z and W−Z production and their ratio are presented. The W±Z production is also measured differentially as functions of various kinematic variables, including new observables sensitive to CP-violation effects. All measurements are compared with state-of-the-art Standard Model predictions from fixed-order calculations or Monte Carlo generators based on next-to-leading-order matrix elements interfaced with parton showers. An effective field theory interpretation of the measurements is performed, considering both CP-conserving and CP-violating dimension-6 operators modifying the W±Z production. In the absence of observed deviations from the Standard Model, limits on CP-conserving Wilson coefficients are extracted using the transverse mass of the W±Z system. For CP-violating coefficients a machine learning approach is used to construct an observable with enhanced sensitivity to CP-violation effects

Search for the production of a Higgs boson in association with a single top quark in pp collisions at √s = 13 TeV with the ATLAS detector

International audienceA 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 $140\mathrm{fb}^{-1}$ at a centre-of-mass energy of $13$ TeV, collected by the ATLAS detector at the LHC. The search targets Higgs-boson decays into $b\bar{b}$, $WW^{*}$, $ZZ^{*}$, 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 $μ_{tH}~=~8.1~\pm~2.6~\mathrm{(stat.)}~\pm~2.0~\mathrm{(syst.)}$. 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

Search for a new pseudoscalar decaying into a pair of bottom and antibottom quarks in top-associated production in √s = 13 TeV proton–proton collisions with the ATLAS detector

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 largeradius 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

Weakly supervised anomaly detection for resonant new physics in the dijet final state using proton-proton collisions at √s =13 TeV with the ATLAS detector

An anomaly detection search for narrow-width resonances beyond the Standard Model that decay into a pair of jets is presented. The search is based on 139  fb−1 of proton-proton collisions at √s =13  TeV recorded during 2015–2018 with the ATLAS detector at the Large Hadron Collider. The analysis is optimized without a particular signal model and aims to be sensitive to a broad range of new physics. It uses two different machine learning strategies to estimate the background in different signal regions. In each region, a weakly supervised classifier is trained to distinguish this background model from data. The analysis focuses on events with high transverse momentum jets reconstructed as large-radius jets. The mass and substructure of these jets are used as inputs to the classifiers. After a classifier-based selection, the distribution of the invariant mass of the two jets is used to search for potential local excesses. The model-independent results of both the anomaly detection methods show no signs of significant local excesses. In addition to model-independent results, a representative set of signal models is injected into the data, and the sensitivity of the methods to these scenarios is reported