Quasi-Degenerate Neutrinos and Lepton Flavour Violation in Supersymmetric Models

In supersymmetric (SUSY) models the misalignment between fermion and sfermion families introduces unsuppressed flavour-changing processes. Even if the mass parameters are chosen to give no flavour violation, family dependent radiative corrections make this adjustment not stable. We analyze the rate of l --> l' gamma in SUSY-GUT models with three quasi-degenerate neutrinos and universal scalar masses at the Planck scale. We pay special attention to a recently proposed scenario where the low-energy neutrino mixings are generated from identical quark and lepton mixings at large scales. We show that: (i) To take universal slepton masses at the GUT scale is a very poor approximation, even in no-scale models. (ii) For large neutrino Yukawa couplings the decay mu --> e gamma would be observable in the planned experiment at PSI. (iii) For large values of tanbeta the tau coupling gives important corrections, pushing mu --> e gamma and tau --> mu gamma to accessible rates. In particular, the non-observation of these processes in the near future would exclude the scenario with unification of quark and lepton mixing angles. (iv) The absence of lepton flavour violating decays in upcoming experiments would imply a small tau coupling (i.e., a low value of tanbeta), small neutrino couplings, and large (>~ 250 GeV) SUSY-breaking masses