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| Paper IPM / P / 12538 |
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| Abstract: | |||||||
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The large hierarchy between the neutrino mass scale and that of
the other fermions seems to be unnatural from a theoretical point
of view. Various strategies have been devised in order to generate
naturally small values of neutrino masses. One of these
techniques is neutrino mass generation at the loop level which
requires a mechanism, e.g., a symmetry, to forbid the lower
order contributions. Here, we study in detail the conditions on
this type of symmetries. We put special emphasis on the discrete
Zn symmetries as a simple example but our results can be also
extended to more general groups. We find that regardless of the
details of the symmetry, in certain cases the existence of a lower
order contribution to neutrino masses can be determined by the
topology of the diagrams with a given number of loops. Finally, we
discuss the lepton flavor violating rare decays in this class of
models, which generically appear
at the one loop level.
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