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

Abstract

The production cross section of a top-quark pair in association with a <em>Z</em> 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 <em>ZZ</em> background. A total of 48 events are observed in the signal regions and a <em>t<span style="text-decoration:overline">t</span>Z</em> cross section of σ_{<em>t<span style="text-decoration:overline">t</span>Z</em>} = 1.07^+0.28_-0.25 (stat.)^+0.10_-0.09 (syst.)pb is extracted, which is compatible with the Standard Model calculation. Combined with other channels, the measured cross section is σ_{<em>t<span style="text-decoration:overline">t</span>Z</em>} = 0.95 ± 0.08 , (stat.) ± 0.10 (syst.)pb. Data is unfolded in the 4<em>l</em> channel, to compute the differential cross section for <em>p</em>_T(Z(<em>ll</em>)) and Δ<em>Φ</em>_{<em>Z(ll)</em>}, variables sensitive to EFT operators, that modify the <em>t<span style="text-decoration:overline">t</span>Z</em> vertex.