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 $\sqrt{s}=13\mathrm{TeV}$ collected by the ATLAS detector at the Large Hadron Collider. The analyzed data set corresponds to an integrated luminosity of $139{\mathrm{fb}}^{-1}$. The search targets both single-top-quark events produced as $gq\to tZ$ (with $q=u$, $c$) and top-quark-pair events, with one top quark decaying through the $t\to Zq$ channel. The analysis considers events with three leptons (electrons or muons), a $b$-tagged jet, possible additional jets, and missing transverse momentum. The data are found to be consistent with the background-only hypothesis and 95% confidence-level limits on the $t\to Zq$ branching ratios, assuming only tensor operators of the Standard Model effective field theory framework contribute to the $tZq$ vertices. These are $6.2\times {10}^{-5}$ ($13\times {10}^{-5}$) for $t\to Zu$ ($t\to Zc$) for a left-handed $tZq$ coupling, and $6.6\times {10}^{-5}$ ($12\times {10}^{-5}$) in the case of a right-handed coupling. These results are interpreted as 95% CL upper limits on the strength of the 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 ${\mathrm{\Lambda }}_{\mathrm{NP}}$ of 1 TeV.

• Received 30 January 2023
• Accepted 22 June 2023

DOI:https://doi.org/10.1103/PhysRevD.108.032019

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