Measurement of the ttZ and ttW 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 (t 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 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 tZ and t tW production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are σt tZ 1⁄4 0.95 � 0.08stat � 0.10syst pb and σt tW 1⁄4 0.87 � 0.13stat � 0.14syst pb in agreement with the Standard Model predictions. The measurement of the t tZ cross section is used to set constraints on effective field theory operators which modify the t tZ vertex.