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) 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 TeV proton–proton collision data with an integrated luminosity of 139 hbox {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 (stat.) pm , 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 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}/ndf and p value computation. Overall, good agreement is observed between the unfolded data and the predictions.

Highlights

  • Background estimationSeveral processes can lead to background contaminations in the signal regions

  • This paper presents measurements of the inclusive and differential ttZ production cross section in final states with three or four isolated leptons with the pAeTrLfoArmS eddetwecitthor√[2s1=]

  • Modelling uncertainties of W Z /Z Z + jets related to the μr and μf scales and the parton distribution functions (PDFs) choice are obtained with the same prescription as for the signal

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Summary

Introduction

Background estimationSeveral processes can lead to background contaminations in the signal regions. The W Z /Z Z + jets components with b- or c-jets are constrained to their MC predictions and normalisation uncertainties of 50% (W Z /Z Z + b) and 30% (W Z /Z Z +c) are assigned to them These uncertainties are evaluated from data/MC comparisons in Z + b/c events [82], and take into account differences in the heavyflavour jet fractions between Z + jets and W Z /Z Z + jets events. Uncertainties related to the μr and μf scales and the PDF of the t W Z background are evaluated in the same way as for the ttZ and W Z /Z Z + jets samples. It is evaluated by switching to an alternative diagram removal scheme (DR1 vs DR2) [57] and obtaining an uncertainty from the differences observed in the signal regions

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