Search for CP Violation in Events with Top Quarks and Z Bosons at √s = 13 and 13.6 TeV

Abstract

This article has 2436 authors. A search for the violation of the charge-parity (CP) symmetry in the production of top quarks in association with Zbosons is presented, using events with at least three charged leptons and additional jets. The search is performedin a sample of proton-proton collision data collected by the CMS experiment at the CERN LHC in 2016–2018 ata center-of-mass energy of 13 TeV and in 2022 at 13.6 TeV, corresponding to a total integrated luminosity of 173fb−1. For the first time in this final state, observables that are odd under the CP transformation are employed.Also for the first time, physics-informed machine-learning techniques are used to construct these observables.While for standard model (SM) processes the distributions of these observables are predicted to be symmetricaround zero, CP-violating modifications of the SM would introduce asymmetries. Two CP-odd operators I tW andI tZ in the SM effective field theory are considered that may modify the interactions between top quarks andelectroweak bosons. The obtained results are consistent with the SM prediction within two standard deviations,and exclusion limits on the associated Wilson coefficients of −2.7 < 𝑐I tW < 2.5 and −0.2 < 𝑐I tZ < 2.0 are set at 95 %confidence level. The largest discrepancy is observed in 𝑐I tZ where data is consistent with positive values, with anobserved local significance with respect to the SM hypothesis of 2.5 standard deviations, when only linear termsare considered.