Measurement of top quark-antiquark pair production in association with a W or Z boson in pp collisions at sqrt(s) = 8 TeV

A measurement of the cross section for the production of top quark-antiquark pairs (ttbar) in association with a vector boson V (W or Z) in proton-proton collisions at sqrt(s) = 8 TeV is presented. The results are based on a dataset corresponding to an integrated luminosity of 19.5 inverse femtobarns recorded with the CMS detector at the LHC. The measurement is performed in three leptonic (e and mu) channels: a same-sign dilepton analysis targeting ttbar W events, and trilepton and four-lepton analyses designed for ttbar Z events. In the same-sign dilepton channel, the ttbar W cross section is measured as sigma[ttbar W] = 170 +90/-80 (stat) +/- 70 (syst) fb, corresponding to a significance of 1.6 standard deviations over the background-only hypothesis. Combining the trilepton and four-lepton channels, a direct measurement of the ttbar Z cross section, sigma[ttbar Z] = 200 +80/-70 (stat) +40/-30 (syst) fb, is obtained with a significance of 3.2 standard deviations. The measured cross sections are compatible with standard model predictions within their experimental uncertainties. The inclusive ttbar V process is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels.