Charged-lepton decays from soft flavour violation

TL;DR

This paper explores a two-Higgs-doublet model with soft lepton flavor violation, showing it can produce observable charged-lepton decay rates while suppressing radiative decays, and potentially explain the muon g-2 anomaly.

Contribution

It demonstrates a parameter space where charged-lepton decays are near experimental bounds, radiative decays are suppressed, and the muon g-2 discrepancy can be resolved within this model.

Findings

Charged-lepton decay branching ratios can approach experimental limits.

Radiative decays are suppressed by the seesaw scale, making them experimentally invisible.

Extra scalars can account for the muon g-2 anomaly.

Abstract

We consider a two-Higgs-doublet extension of the Standard Model, with three right-handed neutrino singlets and the seesaw mechanism, wherein all the Yukawa-coupling matrices are lepton flavour-diagonal and lepton flavour violation is soft, originating solely in the non-flavour-diagonal Majorana mass matrix of the right-handed neutrinos. We consider the limit $m_R \to \infty$ of this model, where $m_R$ is the seesaw scale. We demonstrate that there is a region in parameter space where the branching ratios of all five charged-lepton decays $\ell_1^- \to \ell_2^- \ell_3^+ \ell_3^-$ are close to their experimental upper bounds, while the radiative decays $\ell_1^- \to \ell_2^- \gamma$ are invisible because their branching ratios are suppressed by $m_R^{-4}$. We also consider the anomalous magnetic moment of the muon and show that in our model the contributions from the extra scalars, both charged and neutral, can remove the discrepancy between its experimental and theoretical values.