Measurement of top quark–antiquark pair production in association with a W or Z boson in pp collisions at √s=8 TeV
A measurement of the cross section for the production of top quark–antiquark pairs ( tt¯ ) in association with a vector boson V (W or Z) in proton-proton collisions at √s=8 TeV is presented. The results are based on a dataset corresponding to an integrated luminosity of 19.5 fb −1 recorded with the CMS detector at the LHC. The measurement is performed in three leptonic (e and μ ) channels: a same-sign dilepton analysis targeting ttW events, and trilepton and four-lepton analyses designed for ttZ events. In the same-sign dilepton channel, the ttW cross section is measured as σttW=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 ttZ cross section, σttZ=200+80−70(stat)+40−30(syst)fb−1 , is obtained with a significance of 3.1 standard deviations. The measured cross sections are compatible with standard model predictions within their experimental uncertainties. The inclusive ttV process is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels.