  It is now clear that the masses of the neutrino sector are much lighter than
those of the other three sectors.There are many attempts to explain the
neutrino masses radiatively by means of inert Higgses, which don't have vacuum
expectation values. Then one can discuss cold dark matter candidates, because
of no needing so heavy particles and having a $Z_2$ parity symmetry
corresponding to the R-parity symmetry of the MSSM. The most famous work would
be the Zee model. Recently a new type model along this line of thought was
proposed by E. Ma. We introduce a flavor symmetry based on a dihedral group
$D_6$ to constrain the Yukawa sector. For the neutrino sector, we find that the
maximal mixing of atmospheric neutrinos is realized, it can also be shown that
only an inverted mass spectrum, the value of $|V_{MNS_{13}}|$ is 0.0034 and so
on. For the fermionic CDM candidates, we find that the mass of the CDM and the
inert Higgs should be larger than about 230 and 300 GeV, respectively. If we
restrict ourselves to a perturbative regime, they should be lighter than about
750 GeV.

High Energy Physics - Phenomenology