Measurement of the t t ¯ W tt̄W and t t ¯ Z tt̄Z production cross sections in pp collisions at s = 8 √s=8 TeV with the ATLAS detector

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at s = 8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb -1 collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the t tW and t tZ processes simultaneously yields a significance of 5.0 (4.2) over the background-only hypothesis for t tW (t tZ) production. The measured cross sections are t tW = 369 +100 -91 fb and t tZ = 176 +58 -52 fb. The background-only hypothesis with neither t tW nor t tZ production is excluded at 7.1. All measurements are consistent with next-to-leading-order calculations for the t tW and t tZ processes.