Measurement of Top Quark-Antiquark Pair Production in Association with a W or Z Boson in pp Collisions at $\sqrt{s} = 8$ $\,\text {TeV}$

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