Flavour violating charged lepton decays in a GUT and superstring inspired standard model

Abstract

Analytic expressions of lepton-flavour- and lepton-number-violating decays of charged leptons are derived in the context of general SU(2)L ⊗ U(1)Y seesaw scenarios that are motivated by grand unified theories (GUT’s) or superstring models, in which left-handed and/or right-handed neutral singlets are present. Possible constraints imposed by cosmology and low-energy data are briefly discussed. The violation of the decoupling theorem in flavourdependent graphs due to the presence of heavy neutral leptons of Dirac or Majorana nature is emphasized. Numerical estimates reveal that the decays τ − → e − e − e + or τ − → e − µ − µ + can be as large as ∼ 10 −6, which may be observed in LEP experiments or other τ factories.