Di-Higgs production as a probe of flavor changing neutral Yukawa couplings

TL;DR

This paper explores how di-Higgs production can indirectly constrain flavor-changing neutral Yukawa couplings involving top partners, providing bounds that surpass current unitarity limits and are relevant for future collider experiments.

Contribution

It introduces a method to constrain flavor-changing neutral Yukawa couplings via di-Higgs production, considering simplified models with top partners and analyzing their experimental bounds.

Findings

Current di-Higgs constraints surpass unitarity bounds.

HL-LHC can bound couplings within (-0.4, 0.4) for certain parameters.

Top quark EDM provides stronger bounds in some scenarios.

Abstract

Top partners are well motivated in many new physics models. Usually, vector like quarks $T_{L,R}$ are introduced to avoid the quantum anomaly. It is crucial to probe their interactions with the standard model particles. However, flavor changing neutral couplings are always difficult to detect directly in the current and future experiments. In this paper, we will show how to constrain the flavor changing neutral Yukawa coupling $Tth$ through the di-Higgs production indirectly. We consider the simplified model including a pair of gauge singlet $T_{L,R}$. Under the perturbative unitarity and experimental constraints, we choose $m_T=400~\mathrm{GeV},s_L=0.2$ and $m_T=800~\mathrm{GeV},s_L=0.1$ as benchmark points. After the analysis of amplitude and evaluation of the numerical cross sections, we find that the present constraints from di-Higgs production have already surpassed the unitarity bound because of the $(y_{L,R}^{tT})^4$ behavior. For the case of $m_T=400~\mathrm{GeV}$ and $s_L=0.2$, $\mathrm{Re}y_{L,R}^{tT}$ and $\mathrm{Im}y_{L,R}^{tT}$ can be bounded optimally in the range $(-0.4, 0.4)$ at HL-LHC with $2\sigma$ CL. For the case of $m_T=800~\mathrm{GeV}$ and $s_L=0.1$, $\mathrm{Re}y_{L,R}^{tT}$ and $\mathrm{Im}y_{L,R}^{tT}$ can be bounded optimally in the range $(-0.5, 0.5)$ at HL-LHC with $2\sigma$ CL. The anomalous triple Higgs coupling $\delta_{hhh}$ can also affect the constraints on $y_{L,R}^{tT}$. Finally, we find that the top quark electric dipole moment can give stronger bounds of $y_{L,R}^{tT}$ in the off-axis regions for some scenarios.