Flavour-Violating Charged Lepton Decays in Seesaw-Type Models

Abstract

Analytic expressions of lepton-flavour- and lepton-number-violating decays of charged leptons are derived in the context of general $SU(2{)}_L\otimes U(1{)}_Y$ 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 flavour-dependent graphs due to the presence of heavy neutral leptons of Dirac or Majorana nature is emphasized. Numerical estimates reveal that the decays ${\tau }^{-}\to e^{-}{e}^{-}{e}^{+}$ or ${\tau }^{-}\to e^{-}{\mu }^{-}{\mu }^{+}$ can be as large as $\sim 10^{-6}$, which may be observed in LEP experiments or other $\tau$ factories.

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