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 excited tau-leptons and leptoquarks in the final state with tau-leptons and jets in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 199–46pp.
Abstract: A search is reported for excited tau-leptons and leptoquarks in events with two hadronically decaying tau-leptons and two or more jets. The search uses proton-proton (pp) collision data at root s = 13 TeV recorded by the ATLAS experiment during the Run 2 of the Large Hadron Collider in 2015-2018. The total integrated luminosity is 139 fb(-1). The excited tau-lepton is assumed to be produced and to decay via a four-fermion contact interaction into an ordinary tau-lepton and a quark-antiquark pair. The leptoquarks are assumed to be produced in pairs via the strong interaction, and each leptoquark is assumed to couple to a charm or lighter quark and a tau-lepton. No excess over the background prediction is observed. Excited tau-leptons with masses below 2.8 TeV are excluded at 95% CL in scenarios with the contact interaction scale Lambda set to 10 TeV. At the extreme limit of model validity where Lambda is set equal to the excited tau-lepton mass, excited tau-leptons with masses below 4.6 TeV are excluded. Leptoquarks with masses below 1.3 TeV are excluded at 95% CL if their branching ratio to a charm quark and a tau-lepton equals 1. The analysis does not exploit flavour-tagging in the signal region.
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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). Search for a new scalar resonance in flavour-changing neutral-current top-quark decays t → qX (q = u, c), with X → b(b)over-bar, in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 199–51pp.
Abstract: A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle X decaying into a bottom anti-bottom quark pair is presented. The search focuses on top-quark pair production where one top quark decays to qX, with X -> b (b) over bar, and the other top quark decays according to the Standard Model, with the W boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at least four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from b-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13TeV, recorded with the ATLAS detector at the LHC. The 95% confidence-level upper limits between 0.019% and 0.062% are derived for the branching fraction B(t -> uX) and between 0.018% and 0.078% for the branching fraction B(t -> cX), for masses of the scalar particle X between 20 and 160 GeV.
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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). Measurement of the B0s → μμ effective lifetime with the ATLAS detector. J. High Energy Phys., 09(9), 199–32pp.
Abstract: This paper reports the first ATLAS measurement of the B-s(0) -> μμeffective lifetime. The measurement is based on the data collected in 2015-2016, amounting to 26.3 fb(-1) of 13TeV LHC proton-proton collisions. The proper decay-time distribution of 58 +/- 13 background-subtracted signal candidates is fit with simulated signal templates parameterised as a function of the B-s(0) effective lifetime, with statistical uncertainties extracted through a Neyman construction. The resulting effective measurement of the B-s(0) -> μμlifetime is 0.99(-0.07)(+0.42) (stat.) +/- 0.17 (syst.) ps and it is found to be consistent with the Standard Model.
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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., Cabrera Urban, S., Cantero, J., et al. (2023). A search for heavy Higgs bosons decaying into vector bosons in same-sign two-lepton final states in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 200–51pp.
Abstract: A search for heavy Higgs bosons produced in association with a vector boson and decaying into a pair of vector bosons is performed in final states with two leptons (electrons or muons) of the same electric charge, missing transverse momentum and jets. A data sample of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018 is used. The data correspond to a total integrated luminosity of 139 fb(-1). The observed data are in agreement with Standard Model background expectations. The results are interpreted using higher-dimensional operators in an effective field theory. Upper limits on the production cross-section are calculated at 95% confidence level as a function of the heavy Higgs boson's mass and coupling strengths to vector bosons. Limits are set in the Higgs boson mass range from 300 to 1500 GeV, and depend on the assumed couplings. The highest excluded mass for a heavy Higgs boson with the coupling combinations explored is 900 GeV. Limits on coupling strengths are also provided.
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