Top-quark pair production in association with a Z boson in the final state with four charged leptons

The production cross section of a top-quark pair in association with a Z boson is measured in the channel with four reconstructed leptons, using 36.1 fb$^{−1}$of proton–proton collisions, collected with the ATLAS detector at the Large Hadron Collider at CERN. A profile likelihood fit is performed, using four signal regions and a control region for the $ZZ$ background. A total of 48 events are observed in the signal regions and a $t\bar{t}Z$ cross section of $\sigma_{t\bar{t}Z} = 1.07\,^{+0.28}_{-0.25} \mathrm{(stat.\!)\,}^{+0.10}_{-0.09}\mathrm{(syst.\!)}$ pb is extracted, which is compatible with the Standard Model calculation. Combined with other channels, the measured cross section is $\sigma_{t\bar{t}Z} = 0.95 \pm 0.08\, \mathrm{(stat.\!)}\pm 0.10\, \mathrm{(syst.\!)}$ pb. Data is unfolded in the $4\ell$ channel, to compute the differential cross section for $p_{\mathrm{T}}(Z(\ell\ell))$ and $\Delta\phi(Z(\ell\ell))$, variables sensitive to EFT operators, that modify the $t\bar{t}Z$ vertex.