Inclusive and differential cross-section measurements of t¯tZ production in pp collisions at √s = 13 TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a
top-quark-top-antiquark pair in association with a Z boson (t¯tZ) are presented. Final states
with two, three or four isolated leptons (electrons or muons) are targeted. The measurements
use the data recorded by the ATLAS detector in pp collisions at √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 σt¯tZ = 0.86±0.04 (stat.)±0.04 (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¯tZ system. Both the absolute and normalised differential cross-section
measurements are performed at particle level and parton level for specific fiducial volumes,
and are compared with NLO+NNLL theoretical predictions. The results are interpreted in
the framework of 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¯tZ events 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.