Measurement of the Associated Production of Vector Bosons with Top-Antitop Pairs at 7 TeV

A measurement of vector boson production associated with top-antitop pairs in proton-proton collisions at $\sqrt{s} = 7$~TeV is presented. The results are based on data corresponding to an integrated luminosity of 4.98~fb$^{-1}$, recorded in 2011 by the CMS detector at the CERN LHC. The measurement is performed in two independent channels: a trilepton analysis, which searches exclusively for $t\bar{t}Z$ events, and a same-sign dilepton analysis, which searches inclusively for $t\bar{t}V\,\,(V=W,Z)$ events. Combining the individual significances from all seven decay channels, a $t\bar{t}V$ signal is established at a combined significance of 4.67 standard deviations from the background hypothesis. The combined $t\bar{t}V$ cross section is measured to be $\sigma_{ttV} = 0.51 \,\,\, ^{+0.15}_{-0.13} \,\,\,\mathrm{(stat)} \,\,\, ^{+0.05}_{-0.04}\,\,\, \mathrm{(syst)} \,\,\, \mathrm{pb} $, which is found to be somewhat larger but compatible with the next-to-leading order calculation. In the trilepton channel, a direct measurement of the $t\bar{t}Z$ cross section $\sigma_{ttZ} = 0.30 \,\,\,^{+0.14}_{-0.11} \,\,\,\mathrm{(stat)} \,\,\, ^{+0.04}_{-0.02}\,\,\, \mathrm{(syst)} \,\,\, \mathrm{pb}$ is obtained with a significance of 3.66 standard deviations from the background hypothesis.