ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2021). Search for heavy resonances decaying into a pair of Z bosons in the l+ l- l '+ l '- and l+ l-nu barnu final states using 139 fb-1 of proton-proton collisions at root s = 13 TeV with the ATLAS detector. Eur. Phys. J. C, 81(4), 332–39pp.
Abstract: A search for heavy resonances decaying into a pair of Z bosons leading to l+l-l ' +l '- and l+l-nu nu <overbar></mml:mover> final states, where l stands for either an electron or a muon, is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV collected from 2015 to 2018 that corresponds to the integrated luminosity of 139 fb<mml:mo>-1 recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. Different mass ranges spanning 200 GeV to 2000 GeV for the hypothetical resonances are considered, depending on the final state and model. In the absence of a significant observed excess, the results are interpreted as upper limits on the production cross section of a spin-0 or spin-2 resonance. The upper limits for the spin-0 resonance are translated to exclusion contours in the context of Type-I and Type-II two-Higgs-doublet models, and the limits for the spin-2 resonance are used to constrain the Randall-Sundrum model with an extra dimension giving rise to spin-2 graviton excitations.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2021). Search for new phenomena with top quark pairs in final states with one lepton, jets, and missing transverse momentum in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 174–66pp.
Abstract: A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of s = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb(-1). No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2021). Measurement of the CP-violating phase phi(s) in B-s(0) -> J/psi phi decays in ATLAS at 13 TeV. Eur. Phys. J. C, 81(4), 342–36pp.
Abstract: A measurement of the B-0(s) -> J/psi phi decay parameters using 80.5 fb(-1) of integrated luminosity collected with the ATLAS detector from 13 TeV proton-proton collisions at the LHC is presented. The measured parameters include the CP-violating phase phi(s), the width difference Delta Gamma(s) between the B-s(0) meson mass eigenstates and the average decay width Gamma(s). The values measured for the physical parameters are combined with those from 19.2 fb(-1) of 7 and 8 TeV data, leading to the following: phi(s) = -0.087 +/- 0.036 (stat.) +/- 0.021 (syst.) rad Delta Gamma(s) = 0.0657 +/- 0.0043 (stat.) +/- 0.0037 (syst.) ps(-1) Gamma(s) = 0.6703 +/- 0.0014 (stat.) +/- 0.0018 (syst.) ps(-1) Results for phi(s) and Delta Gamma(s) are also presented as 68% confidence level contours in the phi(s)-Delta Gamma(s) plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. phi(s) and Delta Gamma(s) measurements are in agreement with the Standard Model predictions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2018). Combination of searches for heavy resonances decaying into bosonic and leptonic final states using 36 fb(-1) of proton-proton collision data at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 98(5), 052008–32pp.
Abstract: Searches for new heavy resonances decaying into different pairings of W, Z, or Higgs bosons, as well as directly into leptons, are presented using a data sample corresponding to 36.1 fb(-1) of pp collisions at root s = 13 TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting bosonic decay modes in the qqqq, vvqq, evqq, eeqq, evev, eevv, evee, eeee, qqbb, vvbb, evbb, and eebb final states are combined, searching for a narrow-width resonance. Likewise, analyses selecting the leptonic ev and ee final states are also combined. These two sets of analyses are then further combined. No significant deviation from the Standard Model predictions is observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 Kaluza-Klein excitation of the graviton. Cross section limits are set at the 95% confidence level using an asymptotic approximation and are compared with predictions for the benchmark models. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The data exclude a heavy vector-boson triplet with mass below 5.5 TeV in a weakly coupled scenario and 4.5 TeV in a strongly coupled scenario, as well as a Kaluza-Klein graviton with mass below 2.3 TeV.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for long-lived particles produced in pp collisions at root s=13 TeV that decay into displaced hadronic jets in the ATLAS muon spectrometer. Phys. Rev. D, 99(5), 052005–36pp.
Abstract: A search for the decay of neutral, weakly interacting, long-lived particles using data collected by the ATLAS detector at the LHC is presented. The analysis in this paper uses 36.1 fb(-1) of proton-proton collision data at root s =13 TeV recorded in 2015-2016. The search employs techniques for reconstructing vertices of long-lived particles decaying into jets in the muon spectrometer exploiting a two-vertex strategy and a novel technique that requires only one vertex in association with additional activity in the detector that improves the sensitivity for longer lifetimes. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Properties of g -> b(b)over-bar at small opening angles in pp collisions with the ATLAS detector at root s=13 TeV. Phys. Rev. D, 99(5), 052004–26pp.
Abstract: The fragmentation of high-energy gluons at small opening angles is largely unconstrained by present measurements. Gluon splitting to b-quark pairs is a unique probe into the properties of gluon fragmentation because identified b-tagged jets provide a proxy for the quark daughters of the initial gluon. In this study, key differential distributions related to the g -> b (b) over bar process are measured using 33 fb(-1) of root s = 13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2016. Jets constructed from charged-particle tracks, clustered with the anti-k(t) jet algorithm with radius parameter R = 0.2, are used to probe angular scales below the R = 0.4 jet radius. The observables are unfolded to particle level in order to facilitate direct comparisons with predictions from present and future simulations. Multiple significant differences are observed between the data and parton shower Monte Carlo predictions, providing input to improve these predictions of the main source of background events in analyses involving boosted Higgs bosons decaying into b-quarks.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for heavy long-lived multicharged particles in proton-proton collisions at root s=13 TeV using the ATLAS detector. Phys. Rev. D, 99(5), 052003–26pp.
Abstract: A search for heavy long-lived multicharged particles is performed using the ATLAS detector at the LHC. Data with an integrated luminosity of 36.1 fb(-1) collected in 2015 and 2016 from proton-proton collisions at root s = 13 TeV are examined. Particles producing anomalously high ionization, consistent with long-lived massive particles with electric charges from vertical bar q vertical bar = 2e to vertical bar q vertical bar = 7e, are searched for. No events are observed, and 95% confidence level cross-section upper limits are interpreted as lower mass limits for a Drell-Yan production model. Multicharged particles with masses between 50 and 980-1220 GeV (depending on their electric charge) are excluded.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for four-top-quark production in the single-lepton and opposite-sign dilepton final states in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 99(5), 052009–36pp.
Abstract: A search for four-top-quark production, t (t) over bart (t) over bar, is presented. It is based on proton-proton collision data with a center-of-mass energy root s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during the years 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb(-1). Data are analyzed in both the single-lepton and opposite-sign dilepton channels, characterized by the presence of one or two isolated electrons or muons with high-transverse momentum and multiple jets. A data-driven method is used to estimate the dominant background from top-quark pair production in association with jets. No significant excess above the Standard Model expectation is observed. The result is combined with the previous same-sign dilepton and multilepton searches carried out by the ATLAS Collaboration and an observed (expected) upper limit of 5.3 (2.1) times the four-top-quark Standard Model cross section is obtained at 95% confidence level. Additionally, an upper limit on the anomalous four-top-quark production cross section is set in the context of an effective field theory model.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Measurements of inclusive and differential fiducial cross-sections of t(t)over-bar gamma production in leptonic final states at root s=13 TeV in ATLAS. Eur. Phys. J. C, 79(5), 382–41pp.
Abstract: Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1fb-1, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13TeV. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521 +/- 9(stat.)+/- 41(sys.)fb and 69 +/- 3(stat.)+/- 4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for large missing transverse momentum in association with one top-quark in proton-proton collisions at s=13 TeV with the ATLAS detector. J. High Energy Phys., 05(5), 041–50pp.
Abstract: This paper describes a search for events with one top-quark and large missing transverse momentum in the final state. Data collected during 2015 and 2016 by the ATLAS experiment from 13 TeV proton-proton collisions at the LHC corresponding to an integrated luminosity of 36.1 fb(-1) are used. Two channels are considered, depending on the leptonic or the hadronic decays of the W boson from the top quark. The obtained results are interpreted in the context of simplified models for dark-matter production and for the single production of a vector-like T quark. In the absence of significant deviations from the Standard Model background expectation, 95% confidence-level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the models considered.
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