Search for flavour-changing neutral current top quark decays $t\to qZ$ in proton-proton collisions at $\sqrt{s}=13$ 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 Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 36 fb$^{-1}$, are analysed. A search is performed using top-quark$-$top-antiquark events, with one top quark decaying through the $t\rightarrow qZ$ $(q = u, c)$ flavour-changing neutral-current channel, and the other through the dominant Standard Model mode $t\rightarrow bW$. Only decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are considered as signal. Consequently, the final-state topology is characterised by the presence of three isolated charged leptons, and at least two jets, one of them originating from a $b$-quark. Data are consistent with Standard Model background contributions and thus observed (expected) upper limits at 95% confidence level are set on the $t\to uZ$ branching ratio of $1.7\times10^{-4}$ ($2.4\times10^{-4}$) and on the $t\to cZ$ branching ratio of $2.3\times10^{-4}$ ($3.2\times10^{-4}$), constituting the most stringent limits to date.