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Abstract:

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.

Abstract:

We report results from an updated search for neutral current (NC) resonant $\mathrm{\Delta }(1232)$ baryon production and subsequent $\mathrm{\Delta }$ radiative decay (NC $\mathrm{\Delta }\to N\gamma$ ). We consider events with and without final state protons; events with a proton can be compared with the kinematics of a $\mathrm{\Delta }(1232)$ baryon decay, while events without a visible proton represent a more generic phase space. In order to maximize sensitivity to each topology, we simultaneously make use of two different reconstruction paradigms, Pandora and Wire-Cell, which have complementary strengths, and select mostly orthogonal sets of events. Considering an overall scaling of the NC $\mathrm{\Delta }\to N\gamma$ rate as an explanation of the MiniBooNE anomaly, our data exclude this hypothesis at 94.4% CL. When we decouple the expected correlations between NC $\mathrm{\Delta }\to N\gamma$ events with and without final state protons, our data exclude an interpretation in which all excess events have associated protons at $2.0\sigma$ , and are consistent with an interpretation in which all excess events have no associated protons at  $0.63\sigma$ .

Abstract:

This article reports on a search for dijet resonances using $132{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded at $\sqrt{s}=13\mathrm{TeV}$ by the ATLAS detector at the Large Hadron Collider. The search is performed solely on jets reconstructed within the ATLAS trigger to overcome bandwidth limitations imposed on conventional single-jet triggers, which would otherwise reject data from decays of sub-TeV dijet resonances. Collision events with two jets satisfying transverse momentum thresholds of $p_{\mathrm{T}}\ge 85\mathrm{GeV}$ and jet rapidity separation of $|y^{*}|<0.6$ are analysed for dijet resonances with invariant masses from 375 to $1800\mathrm{GeV}$ . A data-driven background estimate is used to model the dijet mass distribution from multijet processes. No significant excess above the expected background is observed. Upper limits are set at 95% confidence level on coupling values for a benchmark leptophobic axial-vector $Z^{\prime }$ model and on the production cross section for a new resonance contributing a Gaussian-distributed line-shape to the dijet mass distribution.

Abstract:

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 $140{\mathrm{fb}}^{-1}$ of proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$ recorded in 2015–2018. 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 $\mathcal{O}(10)\mathrm{cm}$ .

Abstract:

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 p p 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 ( d 12 ), and the angular separation ( Δ R 12 ) of two leading sub-jets. The jet R AA gradually decreases with increasing d 12 , implying significantly stronger suppression of large-radius jets with larger k t splitting scale. The jet R AA gradually decreases for Δ R 12 in the range 0.01 − 0.2 and then remains consistent with a constant for Δ 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.