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

A measurement of the associated production of $t\bar{t}$ events with a vector boson ($W$, $Z$) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using 36.1 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, three or four leptons (electrons or muons) and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The $t\bar{t}Z$ and $t\bar{t}W$ production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values for the production cross sections are $\sigma_{t\bar{t}Z} = 0.95 \pm 0.08_{\text{stat.}} \pm 0.10_{\text{syst.}}\,\text{pb} = 0.95 \pm 0.13 \,\text{pb}$ and $\sigma_{t\bar{t}W} = 0.87 \pm 0.13_{\text{stat.}} \pm 0.14_{\text{syst.}}\,\text{pb} = 0.87 \pm 0.19 \,\text{pb}$ in agreement with the Standard Model prediction. The measurement of the $t\bar{t}Z$ cross section is used to set constraints on effective field theory operators which modify the $t\bar{t}Z$ vertex.