Measurement of the $t\overline{t}Z$ and $t\overline{t}W$ cross sections in proton-proton collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS detector

A measurement of the associated production of a top-quark pair ($t\overline{t}$) with a vector boson ($W$, $Z$) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using $36.1\text{}\text{}{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same- or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The $t\overline{t}Z$ and $t\overline{t}W$ production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are ${\sigma }_{t\overline{t}Z}=0.95±0.0{8}_{\mathrm{stat}}±0.1{0}_{\mathrm{syst}}\text{}\text{}\mathrm{pb}$ and ${\sigma }_{t\overline{t}W}=0.87±0.1{3}_{\mathrm{stat}}±0.1{4}_{\mathrm{syst}}\text{}\text{}\mathrm{pb}$ in agreement with the Standard Model predictions. The measurement of the $t\overline{t}Z$ cross section is used to set constraints on effective field theory operators which modify the $t\overline{t}Z$ vertex.