Search for flavor-changing neutral-current couplings between the top quark and the Z boson with proton-proton collisions at √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–proton collision data at √𝑠 = 13  TeV collected by the ATLAS detector at the Large Hadron Collider. The analyzed data set corresponds to an integrated luminosity of 139  fb⁻¹. The search targets both single-top-quark events produced as 𝑔⁢𝑞→𝑡⁢𝑍 (with 𝑞=𝑢, 𝑐) and top-quark-pair events, with one top quark decaying through the 𝑡→𝑍⁢𝑞 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→𝑍⁢𝑞 branching ratios, assuming only tensor operators of the Standard Model effective field theory framework contribute to the 𝑡⁢𝑍⁢𝑞 vertices. These are 6.2 × 10⁻⁵ (13 × 10⁻⁵) for 𝑡→𝑍⁢𝑢 (𝑡→𝑍⁢𝑐) for a left-handed 𝑡⁢𝑍⁢𝑞 coupling, and 6.6 × 10⁻⁵ (12 × 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^(13)*_𝑢⁢𝑊| and |𝐶^(13)*_𝑢⁢𝐵| (|𝐶⁽³¹⁾_𝑢⁢𝑊| and |𝐶⁽³¹⁾𝑢⁢𝐵|) of 0.15 (0.16), and limits for |𝐶^(23)*_𝑢⁢𝑊| and |𝐶^(23)*_𝑢⁢𝐵| (|𝐶⁽³²⁾_𝑢⁢𝑊| and |𝐶⁽³²⁾_𝑢⁢𝐵|) of 0.22 (0.21), assuming a new-physics energy scale Λ_NP of 1 TeV.