Measurement of 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 pair production in association with a W or Z boson, in proton-proton collisions at a center-of-mass energy of $13~\mathrm{TeV}$ at the LHC. The data sample used corresponds to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$, collected in 2016 by the CMS experiment. The measurement is performed in same-charge dilepton, 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 $\sigma(\mathrm{t\overline{t}W})= 0.80~^{+0.12}_{-0.11}(\text{stat.})~^{+0.13}_{-0.12}(\text{sys.})$pb and $\sigma(\mathrm{t\overline{t}Z})= 1.00^{+0.09}_{-0.08}(\text{stat.})~^{+0.12}_{-0.10}(\text{sys.})$pb with an expected (observed) significance of 4.6 (5.5) and 9.5 (9.9) standard deviations from the background-only hypothesis respectively. The measured cross sections are in agreement with the standard model prediction. We use these measurements to constrain the Wilson coefficients for four dimension-six operators which would modify $\mathrm{t\overline{t}Z}$ and $\mathrm{t\overline{t}W}$ production. $$\textit{This document has been revised with respect to the version dated May 15, 2017.}$$