Measurement of the $t\bar{t}W$ and $t\bar{t}Z$ production cross sections in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector.

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using $pp$ collision data corresponding to an integrated luminosity of 20.3/fb, collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. The fit to the data considering both signal processes simultaneously yields significances of $5.0\sigma$ and $4.2\sigma$ over the background-only hypothesis for the $t\bar{t}W$ and $t\bar{t}Z$ processes, respectively. The measured cross-sections are $\sigma_{t\bar{t}W} = 369^{+100}_{-91}$ fb and $\sigma_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1\sigma$. All measurements are consistent with next-to-leading-order theoretical calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.