Probing the CP structure of the top quark Yukawa coupling: Loop sensitivity vs. on-shell sensitivity

TL;DR

This paper investigates the CP structure of the top quark Yukawa coupling by comparing loop-induced effects in top pair production with direct on-shell Higgs production, using next-to-leading-order calculations within the Standard Model Effective Field Theory.

Contribution

It provides a comprehensive analysis contrasting loop sensitivity and on-shell sensitivity to CP violation in top Yukawa interactions at the LHC.

Findings

Loop sensitivity and on-shell sensitivity are complementary in probing CP violation.

Electroweak corrections with arbitrary CP mixtures are calculated at NLO in SMEFT.

The study highlights the importance of both methods for a wide parameter space coverage.

Abstract

The question whether the Higgs boson is connected to additional CP violation is one of the driving forces behind precision studies at the Large Hadron Collider. In this work, we investigate the CP structure of the top quark Yukawa interaction-one of the most prominent places for searching for New Physics-through Higgs boson loops in top quark pair production. We calculate the electroweak corrections including arbitrary CP mixtures at next-to-leading-order in the Standard Model Effective Field Theory. This approach of probing Higgs boson degrees of freedom relies on the large $t\bar{t}$ cross section and the excellent perturbative control. In addition, we consider all direct probes with on-shell Higgs boson production in association with a single top quark or top quark pair. This allows us to contrast loop sensitivity versus on-shell sensitivity in these fundamentally different process dynamics. We find that loop sensitivity in $t\bar{t}$ production and on-shell sensitivity in $t\bar{t}H$ and $tH$ provide complementary handles over a wide range of parameter space.