Charged-lepton flavor violation

Processes that change individual charged-lepton family numbers are absent perturbatively in the minimal massless-neutrino Standard Model. A flavour-misaligned Higgs coupling can permit decays such as $h\to \mu \tau$. Neutrino masses and mixing permit extremely suppressed loop effects, while additional flavour structures can give larger rates.

An operator $({\bar{L}}_i\varphi e_{Rj})({\varphi }^{†}\varphi )/{\Lambda }^2$ contains three factors of $v+h$. Its contribution to the mass matrix is proportional to $v^3$, while its single-Higgs coupling is three times the corresponding mass-over-v contribution. With the normalization $-\sqrt{2}{\lambda }^{\prime }/{\Lambda }^2$, the mass-basis coupling is $Y_h=M_{\mathrm{diag}}/v+v^2{\hat{\lambda }}^{\prime }/{\Lambda }^2$. Nonzero off-diagonal coefficients permit charged-lepton flavour-violating Higgs decays.