HEAVY-NEUTRINO EFFECTS ON TAU-LEPTON DECAYS

Minimal extensions of the standard model that are motivated by grand u nified theories or superstring models with an E6 symmetry can naturall y predict heavy neutrinos of Dirac or Majorana nature. Such heavy neut ral leptons violate the decoupling theorem at the one-loop electroweak order and hence offer a unique chance for possible lepton-flavor deca ys of the tau-lepton, e.g. tau --> eee or tau --> mumumu, to be seen i n LEP experiments. We analyze such decays in models with three and fou r generations.