Measurement of the top pair-production in association with a W or Z boson in pp collisions at 13 TeV

We present a measurement of the cross section of top quark pairs produced in association with a W or Z boson, in proton-proton collisions at the LHC at a center-of-mass energy of $13~\mathrm{TeV}$. The data sample used corresponds to an integrated luminosity of $12.9~\mathrm{fb}^{-1}$ collected in 2016. The measurement is performed in same-sign, three- and four-lepton final states where the jet and b-jet multiplicities are exploited to enhance the signal-to-background ratio. The $\mathrm{t\overline{t}W}$ and $\mathrm{t\overline{t}Z}$ production cross sections are measured to be $\sigma(\mathrm{t\overline{t}Z})=0.70^{+0.16}_{-0.15}(\text{stat.})^{+0.14}_{-0.12}(\text{sys.})$ pb and $\sigma(\mathrm{t\overline{t}W})=0.98~^{+0.23}_{-0.22}(\text{stat.})~^{+0.22}_{-0.18}(\text{sys.})$ pb with an expected (observed) significance of 2.6 (3.9) and 5.8 (4.6) standard deviations from the background-only hypothesis respectively. The measured cross sections are in agreement with the standard model prediction.