Search for tZ Flavour Changing Neutral Currents in top-quark decays with the ATLAS detector

A search for flavour-changing neutral current (FCNC) processes in top-quark decays is presented. Data collected from proton-proton collisions at the LHC 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 for top-quark pair-production events, with one top-quark decaying through the $t\rightarrow qZ$ ($q=u,c$) FCNC channel, and 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. No evidence for a signal is found and the observed (expected) upper limits on the $t\rightarrow uZ$ and $t\rightarrow cZ$ branching ratios of $1.7\times10^{-4}$ ($2.4\times10^{-4}$) and $2.3\times10^{-4}$ ($3.2\times10^{-4}$) are set at 95% confidence level, respectively.