$ttW$ and $ttZ$ production cross section in leptonic final states at 8 TeV with ATLAS

A measurement of the production cross section of a pair of top quarks in association with a $W$ or $Z$ boson is presented, using $20.3 \,\mathrm{fb}^{-1}$ of $pp$ collision data collected by the ATLAS detector at $\sqrt{s} = 8 \,\mathrm{TeV}$. The measurement combines four separate analyses which consider final states with two opposite sign, two same sign, three and four leptons. A simultaneous fit to data of the $t\bar{t}W$ and $t\bar{t}Z$ signals measures their cross sections to be $\sigma_{t\bar{t}Z} = 176^{+58}_{-52} \;\mathrm{fb}$ and $\sigma_{t\bar{t}W} = 369^{+100}_{-91}\;\mathrm{fb}$, consistent with next-to-leading order theoretical calculations. Tested against the background-only hypothesis, these correspond to an observed (expected) significance of $4.2\sigma$ ($4.5\sigma$) for $t\bar{t}Z$ and $5.0\sigma$ ($3.2\sigma$) for $t\bar{t}W$.