Measurement of the 𝑡⁢¯𝑡⁢𝑍 and 𝑡⁢¯𝑡⁢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 (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 𝜎𝑡⁢ ¯ 𝑡 ⁢𝑍=0.95±0.0⁢8stat±0.1⁢0syst  pb and 𝜎𝑡⁢ ¯ 𝑡 ⁢𝑊=0.87±0.1⁢3stat±0.1⁢4syst  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.