Measurement of associated production of vector bosons and top quark-antiquark pairs at sqrt(s) = 7 TeV

The first measurement of vector-boson production associated with a top quark-antiquark pair in proton-proton collisions at sqrt(s) = 7 TeV is presented. The results are based on a data set corresponding to an integrated luminosity of 5.0 inverse femtobarms, recorded by the CMS detector at the LHC in 2011. The measurement is performed in two independent channels through a trilepton analysis of t t-bar Z events and a same-sign dilepton analysis of t t-bar V (V = W or Z) events. In the trilepton channel a direct measurement of the t t-bar Z cross section sigma(t t-bar Z) = 0.28 +0.14/-0.11 (stat) +0.06/-0.03 (syst) pb is obtained. In the dilepton channel a measurement of the t t-bar V cross section yields sigma(t t-bar V) = 0.43 +0.17/-0.15 (stat) +0.09/-0.07 (syst) pb. These measurements have a significance, respectively, of 3.3 and 3.0 standard deviations from the background hypotheses and are compatible, within uncertainties, with the corresponding next-to-leading order predictions of 0.137 +0.012/-0.016 and 0.306 +0.031/-0.053 pb.