Observation of top quark pairs produced in association with a vector boson in pp collisions at $ \sqrt{s}=8 $ TeV

Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb$^{−1}$, collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from $\mathrm{t}\overline{\mathrm{t}}\mathrm{W}$ or $\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}$ decays. The $\mathrm{t}\overline{\mathrm{t}}\mathrm{W}$ cross section is measured to be 382$_{− 102}^{+ 117}$ fb with a significance of 4.8 standard deviations from the background-only hypothesis. The $\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}$ cross section is measured to be 242$_{− 55}^{+ 65}$ fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the $\mathrm{t}\overline{\mathrm{t}}\mathrm{W}$ and $\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}$ cross sections.