Measurement of the t¯tZ and t¯tW cross sections in proton-proton collisions at √s=13 TeV with the ATLAS detector

Abstract

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 ¯ t Z and t ¯ 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 σ t ¯ t Z = 0.95 ± 0.0 8 stat ± 0.1 0 syst pb and σ t ¯ t W = 0.87 ± 0.1 3 stat ± 0.1 4 syst pb in agreement with the Standard Model predictions. The measurement of the t ¯ t Z cross section is used to set constraints on effective field theory operators which modify the t ¯ t Z vertex.