Lepton-Flavour Nonconservation and the Type of Neutrino Masses

The lepton-flavour nonconservation is studied in the left-right symmetric gauge theory with both Majorana and Dirac mass terms of neutrinos. The tree graphs with physical Higgs scalar exchange and the one-loop graphs with light (left-handed) and heavy (right-handed) gauge boson exchange contributing to µ → eγ and µ → eee are evaluated. The one-loop graphs with intermediate heavy Majorana neutrinos are shown to give more important effects than those with light Majorana neutrinos. The latter contributions to the rare decay rates are negligibly small, independently of the value of η≡M2WL/M2WR, because of the strong suppression by powers of light neutrino masses mνL. The former contributions are not directly related to mνL but depend on η. They can lead to relatively large values of the rare decay rates, unless η is inhibitingly small. The predictions for the rates of lepton-flavour changing processes µ → eγ, µ → eee, Z → µe, µN → eN, KL → µe and K+ → π+µe- in the present gauge model are compared with similar predictions in other types of gauge models with different types of neutrino masses.