Search for flavour-changing neutral current top-quark decays t → qZ in proton-proton collisions at √s=13 TeV with the ATLAS detector

Abstract

Inclusive J/ψ production is studied in p–Pb interactions at a centre-of-mass energy per nucleon-nucleon collision √ sNN = 8.16 TeV, using the ALICE detector at the CERN LHC. The J/ψ meson is reconstructed, via its decay to a muon pair, in the centreof-mass rapidity intervals 2.03 < ycms < 3.53 and −4.46 < ycms < −2.96, where positive and negative ycms refer to the p-going and Pb-going direction, respectively. The transverse momentum coverage is pT < 20 GeV/c. In this paper, ycms- and pT-differential cross sections for inclusive J/ψ production are presented, and the corresponding nuclear modification factors RpPb are shown. Forward results show a suppression of the J/ψ yield with respect to pp collisions, concentrated in the region pT . 5 GeV/c. At backward rapidity no significant suppression is observed. The results are compared to previous measurements by ALICE in p–Pb collisions at √ sNN = 5.02 TeV and to theoretical calculations. Finally, the ratios RFB between forward- and backward-ycms RpPb values are shown and discussed.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 √ s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb−1 , are analysed. The search is performed using topquark pair events, with one top quark decaying through the t → qZ (q = u, c) flavourchanging 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 × 10−4 (2.4 × 10−4 ) on the t → uZ branching ratio and 2.4 × 10−4 (3.2 × 10−4 ) on the t → cZ branching ratio, constituting the most stringent limits to date.