Measurement of top quark pair production in association with a W or Z boson using event reconstruction techniques

A measurement of top quark pair production in association with a W or Z boson is presented, using 8 TeV proton-proton 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 ($W \rightarrow \ell\nu$) or the Z boson decays to two charged leptons ($Z \rightarrow \ell\ell$), where $\ell$ refers to an electron or muon. The $\mathrm{t}\bar{\mathrm{t}}\mathrm{W}$ cross section is measured to be $\sigma(\mathrm{t}\bar{\mathrm{t}}\mathrm{W}) = 382^{+117}_{-102}$ fb with a significance of 4.8 standard deviations from the background-only hypothesis. The $\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}$ cross section is measured to be $\sigma(\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}) = 242^{+65}_{-55}$ fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to find bounds for five anomalous dimension-six operators which would affect the $\mathrm{t}\bar{\mathrm{t}}\mathrm{W}$ and $\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}$ cross sections.