ATLAS collaboration (2858 authors)Aguilar-Saavedra, Juan AntonioAmor Dos Santos, Susana PatriciaAnjos, NunoAraque, Juan PedroCantrill, RobertCarvalho, JoãoCastro, Nuno FilipeConde Muiño, PatriciaDa Cunha Sargedas De Sousa, Mario JoseFiolhais, MiguelGalhardo, BrunoGomes, AgostinhoGonçalo, RicardoJorge, PedroLopes, LourencoMachado Miguens, JoanaMaio, AméliaManeira, JoséOnofre, AntónioPalma, AlbertoPedro, RutePina, João AntonioSantos, HelenaSaraiva, JoãoSilva, JoséTavares Delgado, AdemarVeloso, FilipeWolters, Helmut2019-02-042019-02-042015-11-24http://dx.doi.org/10.1007/JHEP11(2015)172http://hdl.handle.net/10400.26/26935The 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 $ processes.engjournal article2019-02-04