FLAVOR-VIOLATING CHARGED LEPTON DECAYS IN SEESAW-TYPE MODELS

Analytic expressions of lepton-flavour- and lepton-number-violating de cays of charged leptons are derived in the context of general SU(2)(L) x U(1)(g)amma seesaw scenarios that are motivated by grand unified th eories (GUT's) or superstring models, in which left-handed and/or righ t-handed neutral singlets are present. Possible constraints imposed by cosmology and low-energy data are briefly discussed. The violation of the decoupling theorem in flavour-dependent graphs due to the presenc e of heavy neutral leptons of Dirac or Majorana nature is emphasized. Numerical estimates reveal that the decays tau(-) --> e(-)e(-)e(+) or tau --> e(-)mu(-)mu(+) can be as large as similar to 10(-6), which may be observed in LEP experiments or other tau factories.