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 $ TeV. The dataset corresponds to an integrated luminosity of 20.3 fb$^{−1}$ 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\overline{t}W $ and $ t\overline{t}Z $ processes simultaneously yields a significance of 5.0σ (4.2σ) over the background-only hypothesis for $ t\overline{t}W $ ( $ t\overline{t}Z $ ) production. The measured cross sections are $ {\sigma}_{t\overline{t}W}={369}_{-91}^{+100} $ fb and $ {\sigma}_{t\overline{t}Z}={176}_{-52}^{+58} $ fb. The background-only hypothesis with neither $ t\overline{t}W $ nor $ t\overline{t}Z $ production is excluded at 7.1σ. All measurements are consistent with next-to-leading-order calculations for the $ t\overline{t}W $ and $ t\overline{t}Z $
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