ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Search for Majorana neutrinos in same-sign WW scattering events from pp collisions at √s=13 TeV. Eur. Phys. J. C, 83(9), 824–26pp.
Abstract: A search for Majorana neutrinos in same-sign WW scattering events is presented. The analysis uses root s=13 TeV proton-proton collision data with an integrated luminosity of 140 fb(-1) recorded during 2015-2018 by the ATLAS detector at the Large Hadron Collider. The analysis targets final states including exactly two same-sign muons and at least two hadronic jets well separated in rapidity. The modelling of the main backgrounds, from Standard Model same-sign WW scattering and WZ production, is constrained with data in dedicated signal-depleted control regions. The distribution of the transverse momentum of the second-hardest muon is used to search for signals originating from a heavy Majorana neutrino with a mass between 50 GeV and 20 TeV. No significant excess is observed over the background expectation. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. In addition, the sensitivity to the Weinberg operator is investigated. Upper limits at the 95% confidence level are placed on the squared muon-neutrino-heavy-neutrino mass-mixing matrix element |V-mu N|(2) as a function of the heavy Majorana neutrino's mass m(N), and on the effective μμMajorana neutrino mass |m(mu mu)|.
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ATLAS Collaboration(Aad, G. et al), Akiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Search for magnetic monopoles and stable particles with high electric charges in √s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 11(11), 112–45pp.
Abstract: We present a search for magnetic monopoles and high-electric-charge objects using LHC Run 2 root s = 13TeV proton-proton collisions recorded by the ATLAS detector. A total integrated luminosity of 138 fb(-1) was collected by a specialized trigger. No highly ionizing particle candidate was observed. Considering the Drell-Yan and photon-fusion pair production mechanisms as benchmark models, cross-section upper limits are presented for spin-0 and spin-1/2 magnetic monopoles of magnetic charge 1g(D) and 2g(D) and for high-electric-charge objects of electric charge 20 <= vertical bar z vertical bar <= 100, for masses between 200 GeV and 4000 GeV. The search improves by approximately a factor of three the previous cross-section limits on the Drell-Yan production of magnetic monopoles and high-electric charge objects. Also, the first ATLAS limits on the photon-fusion pair production mechanism of magnetic monopoles and high-electric-charge objects are obtained.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for long-lived, massive particles in events with displaced vertices and multiple jets in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 200–49pp.
Abstract: A search for long-lived particles decaying into hadrons is presented. The analysis uses 139 fb(-1) of pp collision data collected at root s = 13TeV 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 longlived 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 1.5TeV is excluded for mean proper lifetimes in the range from 0.03 ns to 1 ns. When produced in the decay of m((g) over tilde) – 2.4TeV gluinos, electroweakinos with m((X) over tilde (0)(1)) = 1.5TeV are excluded with lifetimes in the range of 0.02 ns to 4 ns.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for leptoquarks decaying into the bt final state in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 001–55pp.
Abstract: A search for leptoquarks decaying into the b tau final state is performed using Run 2 proton-proton collision data from the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb(-1) at root s = 13TeV recorded by the ATLAS detector. The benchmark models considered in this search are vector leptoquarks with electric charge of 2/3e and scalar leptoquarks with an electric charge of 4/3e. No significant excess above the Standard Model prediction is observed, and 95% confidence level upper limits are set on the cross-section times branching fraction of leptoquarks decaying into b tau. For the vector leptoquark production two models are considered: the Yang-Mills and Minimal coupling models. In the Yang-Mills (Minimal coupling) scenario, vector leptoquarks with a mass below 1.58 (1.35) TeV are excluded for a gauge coupling of 1.0 and below 2.05 (1.99) TeV for a gauge coupling of 2.5. In the case of scalar leptoquarks, masses below 1.28 (1.53) TeV are excluded for a Yukawa coupling of 1.0 (2.5). Finally, an interpretation of the results with minimal model dependence is performed for each of the signal region categories, and limits on the visible cross-section for beyond the Standard Model processes are provided.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for leptonic charge asymmetry in t tbar W production in final states with three leptons at root s=13 TeV. J. High Energy Phys., 07(7), 033–54pp.
Abstract: A search for the leptonic charge asymmetry (A(c)(l)) of top-quark-antiquark pair production in association with a W boson (t (t) over barW) is presented. The search is performed using final states with exactly three charged light leptons (electrons or muons) and is based on root s = 13TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN during the years 2015-2018, corresponding to an integrated luminosity of 139 fb(-1). A profile-likelihood fit to the event yields in multiple regions corresponding to positive and negative differences between the pseudorapidities of the charged leptons from top-quark and top-antiquark decays is used to extract the charge asymmetry. At reconstruction level, the asymmetry is found to be -0.12 +/- 0.14 (stat.) +/- 0.05 (syst.). An unfolding procedure is applied to convert the result at reconstruction level into a charge-asymmetry value in a fiducial volume at particle level with the result of -0.11 +/- 0.17 (stat.) +/- 0.05 (syst.). The Standard Model expectations for these two observables are calculated using Monte Carlo simulations with next-to-leading-order plus parton shower precision in quantum chromodynamics and including next-to-leading-order electroweak corrections. They are -0.084(-0.003)(+0.005) (scale) +/- 0.006 (MC stat.) and -0.063(-0.004)(+0.007) (scale) +/- 0.004 (MC stat.) respectively, and in agreement with the measurements.
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