Evidence for the associated production of a vector boson (W, Z) and top quark pair in the dilepton and trilepton channels in pp collision data at $\sqrt{s}=8$ TeV collected by the ATLAS detector at the LHC

Measurements of the production cross sections of top quark pairs in association with a $W$ or $Z$ boson using a data set corresponding to 20.3 ${\rm fb}^{-1}$ of integrated luminosity collected by the ATLAS detector in 2012 are presented. Final states with two or three leptons are considered. Assuming $t\bar{t}W$ production at the Standard Model rate, predicted by next-to-leading-order QCD calculations, the ratio of the measured $t\bar{t}Z$ signal strength to the Standard Model expectation is found to be $\mu_{t\bar{t}Z} = 0.73^{+0.29}_{-0.26}$, corresponding to a 3.2 $\sigma$ excess over the background-only hypothesis. Assuming $t\bar{t}Z$ production at the Standard Model rate, the $t\bar{t}W$ signal strength is found to be $\mu_{t\bar{t}W} = 1.25^{+0.57}_{-0.48}$, corresponding to a 3.1 $\sigma$ excess over the background-only hypothesis. The combined $t\bar{t}Z$ and $t\bar{t}W$ signal strength, assuming the Standard Model cross section ratio for \ttz\ and \ttw\ predicted by next-to-leading-order QCD calculations, is found to be $\mu_{t\bar{t}V} = 0.89^{+0.23}_{-0.22}$, corresponding to a 4.9 $\sigma$ excess over the background-only hypothesis. A simultaneous measurement of the $t\bar{t}Z$ and $t\bar{t}W$ signal strengths yields $\mu_{t\bar{t}Z} = 0.71^{+0.28}_{-0.26}$ and $\mu_{t\bar{t}W} = 1.30^{+0.59}_{-0.48}$, corresponding to a 4.4 $\sigma$ excess over the background-only hypothesis. Observed signal significance from the simultaneous measurement yields $3.1 \sigma $ for both $t\bar{t}Z$ and $t\bar{t}W$ processes. All measurements are consistent with next-to-leading-order theoretical calculations for $t\bar{t}Z$ and $t\bar{t}W$ processes.