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

A measurement of the $t\overline{t}W$ and $t\overline{t}Z$ production cross section in pp collisions at $\sqrt{s}=$ 8 TeV with the ATLAS detector using $pp$ collision data corresponding to an integrated luminosity of 20.3 fb. Final states with two, three or four leptons are considered. A simultaneous fit is applied to all regions and measures a $t\overline{t}W$ and $t\overline{t}Z$ cross section of $369^{+100}_{-91}$ fb and $176^{+58}_{-52}$ fb, respectively. The significance of the $t\overline{t}W$ signal is $5.0\sigma$, while the $t\overline{t}Z$ signal has a significance of $4.2\sigma$. The background-only hypothesis is rejected at $7.1\sigma$.