Search for flavor-changing neutral-current couplings between the top quark and the Z boson with LHC Run2 proton-proton collisions at $\sqrt{s} = 13\,\text{TeV}$ with the ATLAS detector

A search for flavor-changing neutral-current couplings between a top quark, an up or charm quark and a $Z$ boson is presented, using proton-proton collision data at $\sqrt{s} = 13\,\text{TeV}$ collected by the ATLAS detector at the Large Hadron Collider. The analyzed dataset corresponds to an integrated luminosity of 139 fb$^{-1}$. The search targets both events where a single top quark is produced as $gq\rightarrow tZ$ (with $q = u, c$) and top-quark pair events, with one top quark decaying through the $t \rightarrow qZ$ channel. The analysis selects events with three leptons (electrons or muons), a $b$-tagged jet, possible additional jets and missing transverse momentum. The data are consistent with the background-only hypothesis and 95% confidence-level limits on the branching ratios are set. These are $6.2 \times 10^{-5}$ ($13\times 10^{-5}$) for $t\rightarrow Zu$ ($t\rightarrow Zc$) for a left-handed coupling, and $6.6 \times 10^{-5}$ ($12\times 10^{-5}$) in the case of a right-handed coupling.