Radiative Lepton Masses with Heavy Lepton Seed

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 $\mu$ mass generation require new Higgs bosons to be at weak scale. Neutral scalar mixing underlies $m_{\mu }\mathrm{/}{m}_{\tau }\gg m_e\mathrm{/}{m}_{\mu }$, $m_{\tau }\mathrm{/}{m}_E$. The $Z_2$ symmetry forbids $\mu \to e\gamma$ and $\tau\to \mu\gamma$\mathrm{.}Themoststringentboundcomesfrom$\tau\to \mu\mu^\pm e^\mp$. The model has interesting implications for $\tau \to e\gamma$, $\mu \bar e \to \bar\mu e$conversion,$\mu\to e\nu_e\bar\nu_\mu$,andFCNCdecaysof$E$and$N$(suchas$E\to \tau e^\pm\mu^\mp$).