Physicists Find No Evidence of Fundamental Symmetry Breaking in Particle Collisions
Scientists at CERN searched for violations of a fundamental physics law using five years of collision data and found none—confirming the Standard Model's predictions. The result narrows the search space for physics beyond current theory, guiding where particle physics research should focus next.
Originaltitel: Search for 𝐶𝑃 violation in events with top quarks and Z bosons at √𝑠 =13 and 13.6 TeV
<p>A search for the violation of the charge-parity (CP) symmetry in the production of top quarks in association with Z bosons is presented, using events with at least three charged leptons and additional jets. The search is performed in a sample of proton-proton collision data collected by the CMS experiment at the CERN LHC in 2016–2018 at a center-of-mass energy of 13 TeV and in 2022 at 13.6 TeV, corresponding to a total integrated luminosity of 173 f b<sup>−1</sup>. 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 symmetric around zero, CP-violating modifications of the SM would introduce asymmetries. Two CP-odd operators and in the SM effective field theory are considered that may modify the interactions between top quarks and electroweak 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 < 𝑐ItW < 2.5 and −0.2 < 𝑐ItZ < 2.0 are set at 95 % confidence level. The largest discrepancy is observed in where data is consistent with positive values, with an observed local significance with respect to the SM hypothesis of 2.5 standard deviations, when only linear terms are considered.</p>