Golden Probe of the Top Yukawa

TL;DR

This paper investigates how the Higgs decay to four leptons can be used to probe the CP properties of the top quark Yukawa coupling, showing sensitivity to interference effects at high luminosity and energy colliders.

Contribution

It demonstrates that the $h o 4 ext{leptons}$ channel can provide unique insights into the CP nature of the top Yukawa coupling, complementing existing methods.

Findings

Kinematic distributions are sensitive to interference effects.

Meaningful constraints can be obtained with ~300 fb$^{-1}$ at 14 TeV.

Higher luminosity or energy improves sensitivity.

Abstract

We perform a preliminary study of the ability of the Higgs decay to four leptons to shed light on the top quark Yukawa couplings. In particular we examine whether the $h\to 4\ell$ `golden channel' is sensitive to the $CP$ properties of the top quark couplings to the Higgs boson. We show that kinematic distributions are sensitive to interference of the next-to-leading order electroweak corrections with the tree level $ZZ$ contribution. This translates into a sensitivity to the top quark Yukawa couplings such that meaningful constraints on their $CP$ properties can begin to be obtained once $\sim 300$ fb$^{-1}$ of data has been collected at $\sim 14$ TeV, with significant improvements at higher luminosity or with a higher energy hadron collider. This makes the $h\to4\ell$ channel a useful probe of the top quark Yukawa couplings that is qualitatively different from already established searches in $h\to V\gamma$ two body decays, $tth$, and $gg\to h$. We also briefly discuss other potential possibilities for probing the top Yukawa $CP$ properties in $h\to2\ell\gamma$ and $\ell^+\ell^-\to h Z, h\gamma$.