2024 Articles

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

Aad, G.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Acharya, B. S.; Adam Bourdarios, C.; Adamczyk, L.; Addepalli, S. V.; Addison, M. J.; Adelman, J.; Adiguzel, A.; Adye, T.; Affolder, A. A.; Afik, Y.; Agaras, M. N.; Agarwala, J.; Aggarwal, A.; Agheorghiesei, C.; Ahmad, A.; Ahmadov, F.; Ahmed, W. S.; Ahuja, S.; Ai, X.; Aielli, G.; Aikot, A.; Ait Tamlihat, M.; Aitbenchikh, B.; Aizenberg, I.; Akbiyik, M.; Åkesson, T. P. A.; Akimov, A. V.; Akiyama, D.; Akolkar, N. N.; Aktas, S.; Al Khoury, Konie; Angerami, Aaron; Busch, Elena Laura; Brooijmans, Gustaaf H.; Cole, Brian A.

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a 𝑍 boson (𝑡𝑡̄𝑍) 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 𝜎_𝑡𝑡̄𝑍 = 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 𝑡𝑡̄𝑍 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 𝑡𝑡̄𝑍 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.