Radiative Lepton Masses with Heavy Lepton Seed

Abstract

We construct a $Z_8$ model for leptons where all Yukawa couplings are of order unity, but known lepton masses are generated radiatively, {\it order by order}. The seed is provided by fourth generation leptons $E$ and $N$, which receive (Dirac) mass in usual way. Two additional Higgs doublets with nontrivial $Z_8$ charge are introduced to give nearest neighbor Yukawa couplings. Hence, nonstandard Higgs bosons are flavor changing in an unusual way. Loop masses are generated when $Z_8$ is {\it softly} broken down to $Z_2$. However, $e$ and $μ$ mass generation require new Higgs bosons to be at weak scale. Neutral scalar mixing underlies $m_μ/m_τ\gg m_e/m_μ$, $m_τ/m_E$. The $Z_2$ symmetry forbids $μ\to eγ$ and $τ\to μγ$. The most stringent bound comes from $τ\to μμ^\pm e^\mp$. The model has interesting implications for $τ\to eγ$, $μ\bar e \to \barμe$ conversion, $μ\to eν_e\barν_μ$, and FCNC decays of $E$ and $N$ (such as $E\to τe^\pmμ^\mp$).