EFFECTS OF SUPERHEAVY NEUTRINOS IN LOW-ENERGY WEAK PROCESSES

We examine the validity of the decoupling theorem for a superheavy neutrino in the standard model augmented, in one case, by a sequential fourth generation of lepton doublet, and, in another, by a singlet neutrino field. We present a detailed R-xi gauge calculation of the mu --> e-gamma decay amplitude with its exact dependence on the neutrino mass m. Our calculation shows clearly that the resultant amplitude is nonvanishing In the large-m limit because of the contribution coming from the longitudinal gauge bosons. In such diagrams the usual decoupling theorem is invalidated as the Yukawa coupling grows with m. Other higher-order processes such as mu --> 3e, mu-e conversion in the nucleus, etc., as well as the rho-parameter, are also studied. The large-m behavior of various amplitudes are summarized in a table. If the source of the large mass does not involve a large coupling constant, as is the case in the seesaw model for neutrino masses, then the decoupling property is recovered when the mass dependence of the heavy-light mixing angle is taken into account. In several cases, notably the induced mu-eZ vertex and the rho-parameter, this recovery of the decoupling suppression involves some complicated cancellations.