Search for flavour-changing neutral current t->qZ top quark decays 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 from proton-proton collisions at the ATLAS experiment at a centre-of-mass energy of √s=13 TeV, corresponding to an integrated luminosity of 36 fb-1, are analysed. A 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 decays of the Z boson to charged leptons and leptonic W boson decays are considered as signal. No evidence of t->qZ signal is found and thus observed (expected) upper limits at 95% confidencelevel are set on the t->uZ branching ratio of 1.7 × 10-4 (2.4 × 10-4) and on the t->cZ branching ratio of 2.4 × 10-4 (3.2×10-4).

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