Search for pair-produced scalar and vector leptoquarks decaying into third-generation quarks and first- or second-generation leptons in pp collisions with the ATLAS detector

International audienceA search for pair-produced scalar and vector leptoquarks decaying into quarks and leptons of different generations is presented. It uses the full LHC Run 2 (2015–2018) data set of 139 fb$^{−1}$ collected with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. Scalar leptoquarks with charge −(1/3)e as well as scalar and vector leptoquarks with charge +(2/3)e are considered. All possible decays of the pair-produced leptoquarks into quarks of the third generation (t, b) and charged or neutral leptons of the first or second generation (e, μ, ν) with exactly one electron or muon in the final state are investigated. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section are provided for eight models as a function of the leptoquark mass and the branching ratio of the leptoquark into the charged or neutral lepton. In addition, lower limits on the leptoquark masses are derived for all models across a range of branching ratios. Two of these models have the goal of providing an explanation for the recent B-anomalies. In both models, a vector leptoquark decays into charged and neutral leptons of the second generation with a similar branching fraction. Lower limits of 1980 GeV and 1710 GeV are set on the leptoquark mass for these two models.[graphic not available: see fulltext

Search for supersymmetry in final states with missing transverse momentum and three or more b-jets in 139 fb−1^{-1} of proton–proton collisions at s=13\sqrt{s} = 13 TeV with the ATLAS detector

International audienceA search for supersymmetry involving the pair production of gluinos decaying via off-shell third-generation squarks into the lightest neutralino $(\tilde{\chi }^0_1)$ is reported. It exploits LHC proton–proton collision data at a centre-of-mass energy $\sqrt{s} = 13$ TeV with an integrated luminosity of 139 fb$^{-1}$ collected with the ATLAS detector from 2015 to 2018. The search uses events containing large missing transverse momentum, up to one electron or muon, and several energetic jets, at least three of which must be identified as containing b-hadrons. Both a simple kinematic event selection and an event selection based upon a deep neural-network are used. No significant excess above the predicted background is found. In simplified models involving the pair production of gluinos that decay via off-shell top (bottom) squarks, gluino masses less than 2.44 TeV (2.35 TeV) are excluded at 95% CL for a massless $\tilde{\chi }^0_1.$ Limits are also set on the gluino mass in models with variable branching ratios for gluino decays to $b\bar{b}\tilde{\chi }^0_1,t\bar{t}\tilde{\chi }^0_1$ and $t\bar{b}\tilde{\chi }^-_1/\bar{t}b\tilde{\chi }^+_1.

Differential tt‾ t\overline{t} cross-section measurements using boosted top quarks in the all-hadronic final state with 139 fb−1^{−1} of ATLAS data

International audienceMeasurements of single-, double-, and triple-differential cross-sections are presented for boosted top-quark pair-production in 13 TeV proton–proton collisions recorded by the ATLAS detector at the LHC. The top quarks are observed through their hadronic decay and reconstructed as large-radius jets with the leading jet having transverse momentum (p$_{T}$) greater than 500 GeV. The observed data are unfolded to remove detector effects. The particle-level cross-section, multiplied by the $t\overline{t}\to WWb\overline{b}$ branching fraction and measured in a fiducial phase space defined by requiring the leading and second-leading jets to have p$_{T}$> 500 GeV and p$_{T}$> 350 GeV, respectively, is 331 ± 3(stat.) ± 39(syst.) fb. This is approximately 20% lower than the prediction of ${398}_{-49}^{+48}$ fb by Powheg+Pythia 8 with next-to-leading-order (NLO) accuracy but consistent within the theoretical uncertainties. Results are also presented at the parton level, where the effects of top-quark decay, parton showering, and hadronization are removed such that they can be compared with fixed-order next-to-next-to-leading-order (NNLO) calculations. The parton-level cross-section, measured in a fiducial phase space similar to that at particle level, is 1.94 ± 0.02(stat.) ± 0.25(syst.) pb. This agrees with the NNLO prediction of ${1.96}_{-0.17}^{+0.02}$ pb. Reasonable agreement with the differential cross-sections is found for most NLO models, while the NNLO calculations are generally in better agreement with the data. The differential cross-sections are interpreted using a Standard Model effective field-theory formalism and limits are set on Wilson coefficients of several four-fermion operators.[graphic not available: see fulltext

Exclusive dielectron production in ultraperipheral Pb+Pb collisions at sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV with ATLAS

International audienceExclusive production of dielectron pairs, γγ → e$^{+}$e$^{−}$, is studied using ${\mathcal{L}}_{\textrm{int}}$ = 1.72 nb$^{−1}$ of data from ultraperipheral collisions of lead nuclei at $\sqrt{s_{\textrm{NN}}}$ = 5.02 TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon–photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum ${p}_{\textrm{T}}^e$> 2.5 GeV, absolute electron pseudorapidity |η$^{e}$| 5 GeV, and dielectron transverse momentum ${p}_{\textrm{T}}^{ee}$< 2 GeV. Differential cross-sections are measured as a function of m$_{ee}$, average ${p}_{\textrm{T}}^e$, absolute dielectron rapidity |y$_{ee}$|, and scattering angle in the dielectron rest frame, |cos θ$^{*}$|, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be $215\pm 1{\left(\textrm{stat}.\right)}_{-20}^{+23}\left(\textrm{syst}.\right)\pm 4\left(\textrm{lumi}.\right)$μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs Starlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high |y$_{ee}$| and |cos θ$^{*}$| values.[graphic not available: see fulltext

Search for pair production of third-generation leptoquarks decaying into a bottom quark and a τ\tau -lepton with the ATLAS detector

International audienceA search for pair-produced scalar or vector leptoquarks decaying into a b-quark and a $\tau$-lepton is presented using the full LHC Run 2 (2015–2018) data sample of 139 fb$^{-1}$ collected with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of $\sqrt{s} =13$ TeV. Events in which at least one $\tau$-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 $\mathcal {B}$ into a $\tau$-lepton and b-quark. For scalar leptoquarks, masses below 1460 GeV are excluded assuming $\mathcal {B}=100$%, while for vector leptoquarks the corresponding limit is 1650 GeV (1910 GeV) in the minimal-coupling (Yang–Mills) scenario

Measurement of single top-quark production in the s-channel in proton–proton collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

International audienceA measurement of single top-quark production in the s-channel is performed in proton–proton collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb$^{−1}$. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two b-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is $\sigma ={8.2}_{-2.9}^{+3.5}$ pb, consistent with the Standard Model prediction of ${\sigma}^{\textrm{SM}}={10.32}_{-0.36}^{+0.40}$ pb.[graphic not available: see fulltext

Search for heavy, long-lived, charged particles with large ionisation energy loss in pppp collisions at s=13 TeV\sqrt{s} = 13~\text{TeV} using the ATLAS experiment and the full Run 2 dataset

International audienceThis paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb$^{−1}$ of proton–proton collisions at $\sqrt{s}$ = 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, dE/dx. Trajectories reconstructed solely by the inner tracking system and a dE/dx measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to $\mathcal{O}$(1) ns with a mass, measured using the Bethe–Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of R-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.[graphic not available: see fulltext

Search for pair production of third-generation leptoquarks decaying into a bottom quark and a τ\tau-lepton with the ATLAS detector

A search for pair-produced scalar or vector leptoquarks decaying into a $b$-quark and a $\tau$-lepton is presented using the full LHC Run 2 (2015-2018) data sample of 139 fb$^{-1}$ collected with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Events in which at least one $\tau$-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 into the $\tau$-lepton. For scalar leptoquarks, masses below 1490 GeV are excluded assuming a 100% branching ratio, while for vector leptoquarks the corresponding limit is 1690 GeV (1960 GeV) in the minimal-coupling (Yang-Mills) scenario.A search for pair-produced scalar or vector leptoquarks decaying into a $b$-quark and a $\tau$-lepton is presented using the full LHC Run 2 (2015-2018) data sample of 139 fb$^{-1}$ collected with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Events in which at least one $\tau$-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 into the $\tau$-lepton. For scalar leptoquarks, masses below 1490 GeV are excluded assuming a 100% branching ratio, while for vector leptoquarks the corresponding limit is 1690 GeV (1960 GeV) in the minimal-coupling (Yang-Mills) scenario

Search for long-lived, massive particles in events with displaced vertices and multiple jets in pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

International audienceA search for long-lived particles decaying into hadrons is presented. The analysis uses 139 fb$^{−1}$ of pp collision data collected at $\sqrt{s}$ = 13 TeV by the ATLAS detector at the LHC using events that contain multiple energetic jets and a displaced vertex. The search employs dedicated reconstruction techniques that significantly increase the sensitivity to long-lived particles decaying in the ATLAS inner detector. Background estimates for Standard Model processes and instrumental effects are extracted from data. The observed event yields are compatible with those expected from background processes. The results are used to set limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model, and on scenarios with pair-production of supersymmetric particles with long-lived electroweakinos that decay via a small R-parity-violating coupling. The pair-production of electroweakinos with masses below 1.5 TeV is excluded for mean proper lifetimes in the range from 0.03 ns to 1 ns. When produced in the decay of $m\left(\overset{\sim }{g}\right)$ = 2.4 TeV gluinos, electroweakinos with $m\left({\overset{\sim }{\chi}}_1^0\right)$ = 1.5 TeV are excluded with lifetimes in the range of 0.02 ns to 4 ns.[graphic not available: see fulltext

Measurement of the total cross section and ρ\rho -parameter from elastic scattering in pp collisions at s=13\sqrt{s}=13 TeV with the ATLAS detector

International audienceIn a special run of the LHC with $\beta ^{\star } = 2.5$ km, proton–proton elastic-scattering events were recorded at $\sqrt{s} = 13$ TeV with an integrated luminosity of 340~\upmu {\text {b}}^{-1} using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from $-t = 2.5 \cdot 10^{-4}$ GeV$^{2}$ to $-t = 0.46$ GeV$^{2}$ using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section $\sigma _{\text {tot}}$, parameters of the nuclear slope, and the $\rho$-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit $t \rightarrow 0$. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t-dependence. The results for $\sigma _{\text {tot}}$ and $\rho$ are $\begin{aligned} \sigma _{\text {tot}}(pp\rightarrow X) = 104.7 \pm 1.1 \; \text{ mb },\quad \rho = 0.098 \pm 0.011 . \end{aligned}$The uncertainty in $\sigma _{\text {tot}}$ is dominated by the luminosity measurement, and in $\rho$ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude