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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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 lepton-flavour violation in high-mass dilepton final states using 139 fb-1 of pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 082–50pp.
Abstract: A search is performed for a heavy particle decaying into different-flavour, dilepton final states, using 139 fb(-1) of proton-proton collision data at root s = 13TeV collected in 2015-2018 by the ATLAS detector at the Large Hadron Collider. Final states with electrons, muons and hadronically decaying tau leptons are considered (e mu, e tau or μtau). No significant excess over the Standard Model predictions is observed. Upper limits on the production cross-section are set as a function of the mass of a Z' boson, a supersymmetric tau-sneutrino, and a quantum black-hole. The observed 95% CL lower mass limits obtained on a typical benchmark model Z' boson are 5.0TeV (e mu), 4.0TeV (e tau), and 3.9TeV (mu tau), respectively.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2021). Search for Lepton-Flavor Violation in Z-Boson Decays with tau Leptons with the ATLAS Detector. Phys. Rev. Lett., 127(27), 271801–20pp.
Abstract: A search for lepton-flavor-violating Z -> e tau and Z -> μtau decays with pp collision data recorded by the ATLAS detector at the LHC is presented. This analysis uses 139 fb(-1) of Run 2 pp collisions at root s = 13 TeV and is combined with the results of a similar ATLAS search in the final state in which the tau lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying tau leptons significantly improves the sensitivity reach for Z -> l tau decays. The Z -> l tau branching fractions are constrained in this analysis to B(Z -> e tau) < 7.0 x 10(-6) and B (Z -> μtau) < 7.2 x 10(-6) at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: B(Z -> e tau) < 5.0 x 10(-6) and B(Z -> μtau) < 6.5 x 10(-6) at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2022). Search for invisible Higgs-boson decays in events with vector-boson fusion signatures using 139 fb(-1) of proton-proton data recorded by the ATLAS experiment. J. High Energy Phys., 08(8), 104–66pp.
Abstract: A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb(-1) of pp collision data at a centre-of-mass energy of root s =13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2023). Search for high-mass Wγ and Zγ resonances using hadronic W/Z boson decays from 139 fb-1 of pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 125–47pp.
Abstract: A search for high-mass charged and neutral bosons decaying to W gamma and Z gamma final states is presented in this paper. The analysis uses a data sample of root s = 13TeV proton-proton collisions with an integrated luminosity of 139 fb(-1) collected by the ATLAS detector during LHC Run 2 operation. The sensitivity of the search is determined using models of the production and decay of spin-1 charged bosons and spin-0/2 neutral bosons. The range of resonance masses explored extends from 1.0TeV to 6.8TeV. At these high resonance masses, it is beneficial to target the hadronic decays of the W and Z bosons because of their large branching fractions. The decay products of the high-momentum W/Z bosons are strongly collimated and boosted-boson tagging techniques are employed to improve the sensitivity. No evidence of a signal above the Standard Model backgrounds is observed, and upper limits on the production cross-sections of these bosons times their branching fractions to W gamma and Z gamma are derived for various boson production models.
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