ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Search for flavour-changing neutral current top-quark decays t -> qZ in proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 176–41pp.
Abstract: A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of root s = 13TeV, corresponding to an integrated luminosity of 36.1 fb(-1), are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t -> qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t -> bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 x 10(-4) (2.4 x 10(-4)) on the t -> uZ branching ratio and 2.4 x 10(-4) (3.2 x 10(-4)) on the t -> cZ branching ratio, constituting the most stringent limits to date.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Angular analysis of B-d(0) -> K* mu(+)mu(-) decays in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 047–47pp.
Abstract: An angular analysis of the decay B-d(0) -> K*mu(+)mu(-) is presented, based on proton-proton collision data recorded by the ATLAS experiment at the LHC. The study is using 20.3 fb(-1) of integrated luminosity collected during 2012 at centre-of-mass energy of root s = 8TeV. Measurements of the K* longitudinal polarisation fraction and a set of angular parameters obtained for this decay are presented. The results are compatible with the Standard Model predictions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Measurements of differential cross sections of top quark pair production in association with jets in pp collisions at root s=13 TeV using the ATLAS detector. J. High Energy Phys., 10(10), 159–58pp.
Abstract: Measurements of di ff erential cross sections of top quark pair production in association with jets by the ATLAS experiment at the LHC are presented. The measurements are performed as functions of the top quark transverse momentum, the transverse momentum of the top quark-antitop quark system and the out-of-plane transverse momentum using data from pp collisions at p s = 13TeV collected by the ATLAS detector at the LHC in 2015 and corresponding to an integrated luminosity of 3.2 fb. The top quark pair events are selected in the lepton (electron or muon) + jets channel. The measured cross sections, which are compared to several predictions, allow a detailed study of top quark production.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 08(8), 006–46pp.
Abstract: A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a Z boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (h) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from b-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at root s = 13 TeV collected with the ATLAS detector at the LHC in 20152016, corresponding to an integrated luminosity of 36.1 fb(-1). No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks ((t) over tilde (1)) with long decay chains involving Z or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs ((t) over tilde (2)) decaying via (t) over tilde (2) -> Z (t) over tilde (1) or (t) over tilde (2) -> h (t) over tilde (1). The results exclude at 95% confidence level (t) over tilde (2) and (t) over tilde (1) masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for pair production of vector-like top quarks in events with one lepton, jets, and missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 08(8), 052–40pp.
Abstract: The results of a search for vector-like top quarks using events with exactly one lepton, at least four jets, and large missing transverse momentum are reported. The search is optimised for pair production of vector-like top quarks in the Z(->nu nu) t + X decay channel. LHC pp collision data at a centre-of-mass energy of root S = 13 TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb(-1). No significant excess over the Standard Model expectation is seen and upper limits on the production cross-section of a vector-like T quark pair as a function of the T quark mass are derived. The observed (expected) 95% CL lower limits on the T mass are 870 GeV (890 GeV) for the weak-isospin singlet model, 1.05 TeV (1.06 TeV) for the weak-isospin doublet model and 1.16 TeV (1.17 TeV) for the pure Zt decay mode. Limits are also set on the mass as a function of the decay branching ratios, excluding large parts of the parameter space for masses below 1 TeV.
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