Probing the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math> structure of the top quark Yukawa coupling: Loop sensitivity versus on-shell sensitivity

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 $t\overline{t}$ 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 $t\overline{t}$ production and on-shell sensitivity in $t\overline{t}H$ and $tH$ provide complementary handles over a wide range of parameter space.

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