[Paper Review] Search for CP violation using $ extrm{t}\overline{ extrm{t}} $ events in the lepton+jets channel in pp collisions at $ \sqrt{s} $ = 13 TeV

CMS Collaboration|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies3 citations

TL;DR

This study searches for CP violation in top quark pair production using 138 fb⁻¹ of proton-proton collision data at √s = 13 TeV collected by the CMS experiment. By analyzing four T-odd triple-product observables in the lepton+jets final state, the analysis measures the dimensionless chromoelectric dipole moment (CEDM) of the top quark as 0.04 ± 0.10 (stat) ± 0.07 (syst), finding no significant evidence for CP violation beyond the Standard Model prediction.

ABSTRACT

Results are presented on a search for CP violation in the production and decay of top quark-antiquark pairs in the lepton+jets channel. The search is based on data from proton-proton collisions at 13 TeV, collected with the CMS detector, corresponding to an integrated luminosity of 138 fb$^{-1}$. Possible CP violation effects are evaluated by measuring uncorrected asymmetries in observables constructed from linearly independent four-momentum vectors of the final-state particles. The dimensionless chromoelectric dipole moment of the top quark obtained from the observed asymmetries is measured to be 0.04 $\pm$ 0.10 (stat) $\pm$ 0.07 (syst), and the asymmetries exhibit no evidence for CP-violating effects, consistent with expectations from the standard model.

Motivation & Objective

• To search for CP violation in top quark pair production and decay using the lepton+jets final state in proton-proton collisions at √s = 13 TeV.
• To measure the top quark's dimensionless chromoelectric dipole moment (CEDM) as a probe for new physics beyond the Standard Model.
• To test the hypothesis that CP-violating effects in the top quark sector could signal physics beyond the Standard Model.
• To improve sensitivity to new physics by using T-odd observables constructed from final-state four-momentum vectors.

Proposed method

• The analysis uses four T-odd, CP-odd observables based on triple products of momentum and spin vectors in the top quark decay frame.
• Observables are constructed from the momenta of the lepton, b jets, and highest transverse momentum jet, with corrections for detector effects and reconstruction biases.
• The analysis employs a simultaneous fit to signal and background contributions in the lepton+jets final state, using control regions to constrain the background.
• The CEDM is extracted from measured asymmetries in the observables, with systematic uncertainties evaluated using data-driven methods and simulation validation.
• The CMS detector's particle-flow reconstruction and jet energy scale corrections are applied to improve momentum resolution and reduce systematic biases.
• The analysis uses Monte Carlo simulations matched with parton shower models (POWHEG+PYTHIA8) and includes corrections for underlying event and detector effects.

Experimental results

Research questions

• RQ1 Is there evidence for CP violation in top quark pair production and decay in the lepton+jets final state at 13 TeV?
• RQ2 What is the measured upper limit on the dimensionless chromoelectric dipole moment (CEDM) of the top quark?
• RQ3 Do the observed asymmetries in T-odd observables deviate from Standard Model predictions?
• RQ4 Can the CEDM be constrained using only reconstructable final-state particles in the lepton+jets channel?
• RQ5 What are the dominant sources of systematic uncertainty in the measurement of CP-violating asymmetries?

Key findings

• The measured dimensionless chromoelectric dipole moment (CEDM) of the top quark is 0.04 ± 0.10 (statistical) ± 0.07 (systematic), consistent with zero.
• No significant CP-violating asymmetries are observed in any of the four T-odd observables, indicating consistency with the Standard Model.
• The observed asymmetries are compatible with the Standard Model prediction, with no evidence for new physics contributions.
• The analysis achieves a sensitivity to the CEDM at the level of 0.10 in the statistical uncertainty, improving on previous results from 8 TeV data.
• Systematic uncertainties are dominated by jet energy scale and modeling of top quark decay kinematics, with minor contributions from lepton and b-jet identification.
• The results are published in the Journal of High Energy Physics and archived in the HEPData repository for public access.