Lepton Flavour Violation in String-Inspired Models

Lepton flavour violation has been proposed as a significant test of supersymmetric unification. Here we show that such signals are also a generic feature of supersymmetric string unified models in which there is no simple unified gauge group. In realistic models of this kind which involve third family Yukawa unification and large values of $\tan \beta$, there are generally heavy right-handed (singlet) neutrinos of intermediate mass $M_{\nu}$, whose couplings violate lepton flavour. To illustrate these effects we calculate the rates for $\mu \to e \gamma$ and $\tau \to \mu gamma$ in the minimal supersymmetric $SU(4)\times SU(2)\times SU(2)$ model. Including only the minimum irreducible contributions, we find that both rates are enhanced relative to similar models with low $\tan \beta$, with $\tau \to \mu gamma$ providing a decisive test of such models in the near future.