Measurement of the cross section of top quark pair production in association with a 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 Z boson, using proton-proton collisions at a center-of-mass energy of $13~\mathrm{TeV}$. The data sample used corresponds to an integrated luminosity of $2.7~\mathrm{fb}^{-1}$. The measurement is performed in three- and four-lepton final states where the jet and b-jet multiplicities were exploited to enhance the signal over background ratio. The $\mathrm{t\overline{t}Z}$ production cross section is measured to be $1065^{+352}_{-313}(\mathrm{stat.})^{+168}_{-142}(\mathrm{sys.})~\mathrm{fb}$ with a significance of $3.6$ standard deviations from the background-only hypothesis.