Measurement of $t\bar{t}W$ and $t\bar{t}Z$ production cross sections in leptonic final states in 8 TeV $pp$ collisions at 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 $20.3 \,\textrm{fb}^{-1}$ 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 $\sigma_{ttZ} = 176^{+58}_{-52} \,\mathrm{fb}$ and $\sigma_{ttW} = 369^{+100}_{-91} \,\mathrm{fb}$, 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 $t\bar{t}W$.