Measurements of the inclusive and differential production cross sections of a top-quark–antiquark pair in association with a Z boson at $$\sqrt{s} = 13$$ TeV with the ATLAS detector

Abstract

Measurements of both the inclusive and differential production cross sections of a top-quark–antiquark pair in association with a Z boson ($$t{\bar{t}}Z$$ t t ¯ Z ) are presented. The measurements are performed by targeting final states with three or four isolated leptons (electrons or muons) and are based on $$\sqrt{s} = 13$$ s = 13 TeV proton–proton collision data with an integrated luminosity of 139 $$\hbox {fb}^{-1}$$ fb - 1 , recorded from 2015 to 2018 with the ATLAS detector at the CERN Large Hadron Collider. The inclusive cross section is measured to be $$\sigma _{t{\bar{t}}Z} = 0.99 \pm 0.05$$ σ t t ¯ Z = 0.99 ± 0.05 (stat.) $$\pm \, 0.08$$ ± 0.08 (syst.) pb, in agreement with the most precise theoretical predictions. The differential measurements are presented as a function of a number of kinematic variables which probe the kinematics of the $$t{\bar{t}}Z$$ t t ¯ Z system. Both absolute and normalised differential cross-section measurements are performed at particle and parton levels for specific fiducial volumes and are compared with theoretical predictions at different levels of precision, based on a $$\chi ^{2}/$$ χ 2 / ndf and p value computation. Overall, good agreement is observed between the unfolded data and the predictions.

Observing $$t{\bar{t}}Z$$ spin correlations at the LHC

“…The most recent results of tt Z cross section measurements by the ATLAS collaboration at 139.1 fb −1 [14] and the CMS collaboration at 77.5 fb −1 [15] show a good experimental 0123456789(). : V,-vol agreement of the inclusive rate with the NLO QCD and electroweak corrections calculation of Ref.…”

“…For most coefficients, except c rr and c kk , the total scale uncertainty is largely subdominant with respect to the statistical uncertainty on the generated Monte Carlo sample. Uncertainties arising from the PDF are known to have a negligible impact [14,15], compared to the above scale uncertainties, and are not considered here. In order to ease the comparison between our predictions for the tt Z spin density matrix and the previous results for the tt case, we also include in Table 1 the mixed NLO QCD and electroweak ("NLOW") calculations from Tables 7 and 8 of Ref.…”

“…Recent measurements of the inclusive and differential tt Z cross section by the ATLAS and CMS collaborations [14,15] are performed exclusively in the trilepton (3 ) and tetralepton (4 ) channels, where the tt system decays semi-and di-leptonically, respectively. The di-leptonically decaying tt in the tetralepton final state may be used for measurements of spin correlation and polarisaton, whereas the semileptonically decaying tt can only be used for polarisation measurements.…”

“…The ATLAS analysis [14] uses the full LHC Run 2 dataset, corresponding to an integrated luminosity of 139.1 fb −1 , and reports about 430 tt Z events in their 3 selection and about 90 events in their 4 selection. The CMS analysis [15], on the other hand, only uses the dataset collected during the 2016-2017 period and amounting to an integrated luminosity of 77.5 fb −1 .…”

“…We therefore derive our expected statistical uncertainties based on the assumption that 100 events can be used to determine the spin correlation and cross-correlation coefficients, and 600 events for the individual top polarisation coefficients. 3 From the ATLAS measurement [14], and in particular the normalised differential cross section distribution for | φ( + t , − t )| and p T (tt) in the 4 parton-level fiducial volume (Figures 14b, 14d, 15b and 15d of Ref. [14]), we estimate that an 8% total systematic uncertainty can be assigned to each of 4 bins (equally dividing the range [−1, 1]) for all the angular distributions we consider in this study.…”