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

Abstract

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 $\sqrt{s}=13$ 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 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 are set, 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 corresponding couplings, yielding limits for $|C_{uW}^{(13)\ast }|$ and $|C_{uB}^{(13)\ast }|$ ($|C_{uW}^{(31)}|$ and $|C_{uB}^{(31)}|$) of 0.15 (0.16), and limits for $|C_{uW}^{(23)\ast }|$ and $|C_{uB}^{(23)\ast }|$ ($|C_{uW}^{(32)}|$ and $|C_{uB}^{(32)}|$) of 0.22 (0.21), assuming a new-physics energy scale ${\Lambda }_{\text{NP}}$ of 1 TeV.

arXiv comment50 pages in total, author list starting page 33, 4 figures, 8 tables, published in Phys. Rev. D. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2019-06/

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