Search for flavor-changing neutral-current couplings between the top quark and the $Z$ boson with proton-proton collisions at $\sqrt{s} = 13$ 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-protoncollision data at $\sqrt{s} = 13$ TeV collected by the ATLAS detector at theLarge Hadron Collider. The analyzed dataset corresponds to an integratedluminosity of 139 fb$^{-1}$. The search targets both single-top-quark eventsproduced as $gq\rightarrow tZ$ (with $q = u, c$) and top-quark-pair events,with one top quark decaying through the $t \rightarrow Zq$ channel. Theanalysis considers events with three leptons (electrons or muons), a $b$-taggedjet, possible additional jets, and missing transverse momentum. The data arefound to be consistent with the background-only hypothesis and 95%confidence-level limits on the $t \rightarrow Zq$ branching ratios are set,assuming only tensor operators of the Standard Model effective field theoryframework contribute to the $tZq$ vertices. These are $6.2 \times 10^{-5}$($13\times 10^{-5}$) for $t\rightarrow Zu$ ($t\rightarrow Zc$) for aleft-handed $tZq$ coupling, and $6.6 \times 10^{-5}$ ($12\times 10^{-5}$) inthe case of a right-handed coupling. These results are interpreted as 95% CLupper limits on the strength of corresponding couplings, yielding limits for$|C_{uW}^{(13)*}|$ and $|C_{uB}^{(13)*}|$ ($|C_{uW}^{(31)}|$ and$|C_{uB}^{(31)}|$) of 0.15 (0.16), and limits for $|C_{uW}^{(23)*}|$ and$|C_{uB}^{(23)*}|$ ($|C_{uW}^{(32)}|$ and $|C_{uB}^{(32)}|$) of 0.22 (0.21),assuming a new-physics energy scale $\Lambda_\text{NP}$ of 1 TeV.