Abstract:
A measurement of the cross section for the production of top quark-antiquark pairs \(t\overline{t}\) in association with a vector boson V (W or Z) in proton-proton collisions at \(\sqrt{s}\)=8TeV 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 \(\mu\)) channels: a same-sign dilepton analysis targeting \(t\overline{t}W\) events, and trilepton and four-lepton analyses designed for \(t\overline{t}Z\) events. In the same-sign dilepton channel, the \(t\overline{t}W\) cross section is measured as\(\sigma_{t\overline{t}W} = 170_{-90}^{+90} (stat) ± 70 (syst) fb\), 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 \(t\overline{t}Z\) cross section, \(\sigma_{t\overline{t}Z} = 200_{-70}^{+80} (stat)_{-30}^{+40} (syst)fb^{-1}\), 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 \(t\overline{t}V\) process is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels.
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