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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). Searches for new phenomena in events with two leptons, jets, and missing transverse momentum in 139 fb-1 of √s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 83(6), 515–52pp.
Abstract: Searches for new phenomena inspired by super-symmetry in final states containing an e(+) e(-) or mu(+) mu(-) pair, jets, and missing transverse momentum are presented. These searches make use of proton-proton collision data with an integrated luminosity of 139 fb(-1), collected during 2015-2018 at a centre-of-mass energy root s = 13 TeVby the ATLAS detector at the Large Hadron Collider. Two searches target the pair production of charginos and neutralinos. One uses the recursive-jigsaw reconstruction technique to follow up on excesses observed in 36.1 fb(-1) of data, and the other uses conventional event variables. The third search targets pair production of coloured supersymmetric particles (squarks or gluinos) decaying through the next-to-lightest neutralino ((chi) over tilde (0)(2)) via a slepton ((l) over tilde) or Z boson into l(+) l(-) (chi) over tilde (0)(1), resulting in a kinematic endpoint or peak in the dilepton invariant mass spectrum. The data are found to be consistent with the Standard Model expectations. Results are interpreted using simplified models and exclude masses up to 900 GeV for electroweakinos, 1550 GeV for squarks, and 2250 GeV for gluinos.
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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). Searches for lepton-flavour-violating decays of the Higgs boson into eτ and μτ in √s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 07(7), 166–74pp.
Abstract: This paper presents direct searches for lepton flavour violation in Higgs boson decays, H -> e tau and H -> μtau, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy root s = 13 TeV, corresponding to an integrated luminosity of 138 fb(-1). Leptonic (tau -> l nu(l)nu(tau)) and hadronic (tau -> hadrons nu(tau)) decays of the tau-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, B(H -> e tau) < 0.20% (0.12%) and B(H -> μtau ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H -> e tau and H -> μtau signals. The best-fit branching ratio difference, B(H -> μtau) -> B(H -> e tau), measured with the Symmetry method in the channel where the tau-lepton decays to leptons, is (0.25 0.10)%, compatible with a value of zero within 2.5 sigma.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2020). Searches for lepton-flavour-violating decays of the Higgs boson in root s=13 TeV pp collisions with the ATLAS detector. Phys. Lett. B, 800, 135069–23pp.
Abstract: This Letter presents direct searches for lepton flavour violation in Higgs boson decays, H -> e tau and H -> μtau, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy root s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb(-1). No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47%(0.34(-0.10)(+0.13)%) and 0.28%(0.37(-0.10)(+0.14)%) for H -> e tau and H -> μtau, respectively.
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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). Searches for exclusive Higgs and Z boson decays into J/psi gamma, psi (2S) gamma, and Upsilon(nS) gamma at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 786, 134–155.
Abstract: Searches for the exclusive decays of the Higgs and Z bosons into a J/psi, psi(2S), or Upsilon(nS) (n = 1, 2, 3) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of 36.1 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence- level upper limits on the branching fractions of the Higgs boson decays to J/psi gamma, psi(2S) gamma and Upsilon(nS) gamma of 3.5 x 10(-4), 2.0 x 10(-3), and (4.9, 5.9, 5.7) x 10(-4), respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are 2.3 x 10(-6), 4.5 x 10(-6) and (2.8, 1.7, 4.8) 10(-6), respectively.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb-1 of ATLAS √s=13 TeV proton-proton collision data. Eur. Phys. J. C, 83(9), 781–33pp.
Abstract: Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the mu(+)mu(-) gamma final state with a proton-proton collision data sample corresponding to an integrated luminosity of 139 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/psi gamma, psi(2S)gamma, and Upsilon(1S, 2S, 3S)gamma are found to be 2.0 x 10(-4), 10.5x10(-4), and (2.5, 4.2, 3.4) x10(-4), respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 x 10(-6), 2.4 x 10(-6), and (1.1, 1.3, 2.4) x10(-6). An observed 95% CL interval of (-133, 175) is obtained for the kappa(c)/kappa(gamma) ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37, 40) is obtained for kappa(b)/kappa(gamma).
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