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

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 p s = 13TeV, corresponding to an integrated luminosity of 36.1 fb1, 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 _ 104 (3:2 _ 104) on the t > cZ branching ratio, constituting the most stringent limits to date.