Measurement of the $t\bar{t}W$ and $t\bar{t}Z$ production cross sections in $pp$ collisions at $\sqrt{s}=8\mathrm{T}\mathrm{e}\mathrm{V}$ 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 $\sqrt{s} = 8 $\mathrm{T}\mathrm{e}\mathrm{V}$$. The dataset corresponds to an integrated luminosity of $20.3{\mathrm{f}\mathrm{b}}^{-1}$ 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 $t\bar{t}W$ and $t\bar{t}Z$ processes simultaneously yields a significance of $5.0\sigma$ ($4.2\sigma$) over the background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The measured cross sections are ${\sigma }_{t\bar{t}W}=369_{-91}^{+100}$ fb and ${\sigma }_{t\bar{t}Z}=176_{-52}^{+58}$ 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 calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.