Muon Flavor Violation in Two Higgs Doublet Model with Extra Yukawa Couplings

TL;DR

This paper investigates lepton flavor violation in a two Higgs doublet model with extra Yukawa couplings, showing how upcoming experiments can probe these effects and analyzing their compatibility with current B physics constraints.

Contribution

It demonstrates that LFV processes in this model are dominated by two-loop dipole transitions and assesses their experimental detectability considering existing flavor constraints.

Findings

LFV processes are mainly driven by two-loop dipole mechanisms.

Current B physics data restrict the size of extra Yukawa couplings.

Certain B decay modes remain consistent with the Standard Model, while others are beyond experimental reach.

Abstract

The new round of experiments, MEG II, COMET/Mu2e, and Mu3e, would soon start to push the $\mu \to e\gamma$, $\mu N \to eN$ conversion, and $\mu \to 3e$ frontier, while Belle II would probe $\tau \to \mu\gamma$ and $\tau \to 3\mu$. In the general two Higgs doublet model with extra Yukawa couplings, we show that all these processes probe the lepton flavor violating (LFV) dipole transition that arises from the two loop mechanism, with scalar-induced contact terms subdominant. This is because existing data suggest the extra Yukawa couplings $\rho_{\mu e},\, \rho_{ee} \lesssim \lambda_e$, while $\rho_{\tau\mu},\, \rho_{\tau\tau} \lesssim \lambda_\tau$ and $\rho_{tt} \lesssim \lambda_t$, with $\lambda_i$ the usual Yukawa coupling of the Standard Model (SM), where $\rho_{\mu e}\rho_{tt}$ and $\rho_{\tau\mu}\rho_{tt}$ enter the $\mu e\gamma$ and $\tau\mu\gamma$ two loop amplitudes, respectively. With the $B_s \to \mu\mu$ decay rate basically consistent with SM expectation, together with the $B_s$ mixing constraint, we show that $B_s \to \tau\tau$ would also be consistent with SM, while $B_s \to \tau\mu$ and $B \to K\tau\mu$ decays would be out of reach of projected sensitivities, in strong contrast with some models motivated by the B anomalies.