Inclusive and differential cross section measurements of $t\bar{t}Z$ production in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector, including EFT and spin correlations interpretations

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. Final states with two, three and four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 140 fb$^{-1}$. The inclusive cross section is measured to be $\sigma_{t\bar{t}Z}=0.86\pm 0.04(\text{stat.})\pm 0.04(\text{syst.})$ pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the $t\bar{t}Z$ system. Both absolute and normalised differential cross section measurements are performed at particle- and parton-level for specific fiducial volumes, and are compared with theoretical predictions at NLO+NNLL. The results are interpreted in the framework of the Standard Model Effective Field Theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in $t\bar{t}Z$ is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations.

Baptiste Ravina

Senior Research Fellow

I’m an experimental particle physicist looking for new physics at the ATLAS experiment with machine learning.

Steffen Korn

Postdoc (quantumTANGO)

Elizaveta Shabalina

Senior Researcher