Measurement of top quark–antiquark pair production in association with a W or Z boson in pp collisions at $$\sqrt{s} = 8$$$$\,\text {TeV}$$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 $$\upmu $$ ) 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.