NON-DECOUPLING OF HEAVY NEUTRINOS AND LEPTON FLAVOR VIOLATION

We consider a class of models predicting new heavy neutral fermionic s tates, whose mixing with the light neutrinos can be naturally signific ant and produce observable effects below the threshold for their produ ction. We update the indirect limits on the flavour non-diagonal mixin g parameters that can be derived from unitarity, and show that signifi cant rates are in general expected for one-loop-induced rare processes due to the exchange of virtual heavy neutrinos, involving the violati on of the muon and electron lepton numbers. In particular, the amplitu des for mu-e conversion in nuclei and for mu --> ee(+)e(-) show a non- decoupling quadratic dependence on the heavy neutrino mass M, while mu --> e gamma is almost independent of the heavy scale above the electr oweak scale. These three processes are then used to set stringent cons traints on the flavour-violating mixing angles. In all the cases consi dered, we point out explicitly that the non-decoupling behaviour is st rictly related to the spontaneous breaking of the SU(2) symmetry.