Charged lepton flavor violation in light of Muon $g-2$

TL;DR

This paper explores how the muon g-2 anomaly within a two Higgs doublet model predicts enhanced charged lepton flavor violation processes, which upcoming experiments could detect, providing insights into new physics beyond the Standard Model.

Contribution

It demonstrates that explaining muon g-2 via exotic scalars in a two Higgs doublet model leads to testable predictions for charged lepton flavor violation processes in upcoming experiments.

Findings

Enhanced rates of muon and tau flavor violating decays predicted.

Muon to electron gamma decay within MEG II sensitivity.

Tau to mu gamma decay probes tau Yukawa couplings.

Abstract

The recent confirmation of the muon $g-2$ anomaly by the Fermilab g-2 experiment may harbinger a new era in $\mu$ and $\tau$ physics. In the context of general two Higgs doublet model, the discrepancy can be explained via one-loop exchange of sub-TeV exotic scalar and pseudoscalars, namely $H$ and $A$, that have flavor changing neutral couplings $\rho_{\tau\mu}$ and $\rho_{\mu\tau}$ at $\sim 20$ times the usual tau Yukawa coupling, $\lambda_\tau$. Taking $\rho_{\ell\ell^\prime}\sim \lambda_{ \rm min(\ell, \ell^\prime)}$, we show that the above solution to muon $g-2$ then predicts enhanced rates of various charged lepton flavor violating processes, which should be accessible at upcoming experiments. We cover muon related processes such as $\mu \to e \gamma$, $\mu \to eee$ and $\mu N \to e N$, and $\tau$ decays $\tau \to \mu \gamma$ and $\tau \to \mu\mu\mu$. A similar one-loop diagram with $\rho_{e\tau}= \rho_{\tau e} = {\cal O}(\lambda_e)$ induces $\mu \to e\gamma$, bringing the rate right into the sensitivity of the MEG~II experiment. The $\mu e\gamma$ dipole can be probed further by $\mu \to 3e$ and $\mu N \to e N$. With its promised sensitivity range and ability to use different nuclei, the $\mu N \to eN$ conversion experiments can not only make discovery, but access the extra diagonal quark Yukawa couplings $\rho_{qq}$. For the $\tau$ lepton, we find that $\tau \to \mu\gamma$ would probe $\rho_{\tau\tau}$ down to $\lambda_\tau$ or lower, while $\tau \to 3\mu$ would probe $\rho_{\mu\mu}$ to ${\cal O}(\lambda_{\mu})$.