A search for flavour-changing neutral currents (FCNC) processes in proton-proton ($pp$) collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider (LHC) is presented. The analysed data collected during the years of 2015 and 2016 corresponds to an integrated luminosity of 36.1 fb$^{-1}$. A search considering top-quark pair-production events is performed, with one top-quark decaying through the dominant Standard Model (SM) mode $t$ $to$ $Wb$, and the other through the $t$ $to$ $qZ$ ($q$=$u$,$c$) FCNC channel. The data are consistent with the SM expectation and the observed and expected upper limits on the branching ratio of $t$ $to$ $uZ$ and $t$ $to$ $cZ$ are set at 95% confidence level representing an improvement of about a factor 3 compared with the Run-1 data results from the ATLAS Collaboration.
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