Top-quark pair production in association with a $Z$ boson in the 4$\ell$ channel with the ATLAS experiment

The cross section of the $t\bar{t}Z$ and $t\bar{t}W$ processes are measured in a simultaneous fit using 36.1 $\text{fb}^{-1}$ of of proton--proton collisions at a centre-of-mass energy of $\sqrt{s}=13\ \text{TeV}$ recorded by the ATLAS experiment at the LHC. In addition, a fit is performed in the 4$\ell$ channel only, resulting in a cross section of $\sigma_{t\bar{t}Z} = 1.07 \pm 0.26 \,\text{pb}$. This result is consistent with the combined fit and agrees with the prediction by the Standard Model.