Measurement of the inclusive and differential cross section of a top quark pair in association with a $Z$ boson at $13\,\text{TeV}$ with the ATLAS detector

Abstract: The inclusive as well as differential cross section of the associated production of top-antitop quark pairs and a $Z$ boson ($t\bar{t}Z$) is measured in final states with exactly three or four isolated leptons (electrons or muons). For this purpose, the full LHC Run 2 dataset of proton-proton collisions recorded by the ATLAS detector from $2015$ to $2018$, which corresponds to an integrated luminosity of $139\,\text{fb}^{-1}$, is used. The inclusive production cross section is measured to be $\sigma_{t\bar{t}Z}=1.05\pm0.05\,(\text{stat.})\,\pm0.09\,(\text{syst.})\,\text{pb}$, which is in agreement with the most precise Standard Model theoretical prediction. Absolute and normalised differential cross section measurements are performed as a function of various kinematic variables in order to probe the kinematics of the $t\bar{t}Z$ system within both parton- and particle-level phase spaces.