Measurement of the tt¯Z and tt¯W cross sections in proton-proton collisions at √s =13 TeV with the ATLAS detector

A measurement of the associated production of a top-quark pair (tt¯) 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 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 tt^-Z and tt^-W production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are σ_tt¯Z=0.95±0.08_stat±0.10_syst  pb and σ_tt¯W=0.87±0.13_stat±0.14_syst  pb in agreement with the Standard Model predictions. The measurement of the 
tt¯Z cross section is used to set constraints on effective field theory operators which modify the tt¯Z vertex.