Quasi-Degenerate Neutrino Mass Spectrum, \mu -> e + \gamma Decay and Leptogenesis

Abstract

In a large class of SUSY GUT models with see-saw mechanism of neutrino mass generation, lepton flavor violating (LFV) decays $\mu \to e+\gamma$, $\tau \to \mu +\gamma$, etc., are predicted with rates that are within the reach of present and planned experiments. A crucial element in these predictions is the matrix of neutrino Yukawa couplings $\text{ynu}$ which can be expressed in terms of the light and RH heavy neutrino masses, the neutrino mixing PMNS matrix $U$, and an orthogonal matrix $\mathbf{R}$. Leptogenesis can take place only if $\mathbf{R}$ is complex. Considering the case of quasi-degenerate neutrinos and assuming that $\mathbf{R}$ is complex, we derive simple analytical expressions for the $\mu \to e+\gamma$, $\tau \to \mu +\gamma$ and $\tau \to e+\gamma$ decay rates. Taking into account the leptogenesis constraints on the relevant parameters we show that the predicted rates of the LFV decays $\mu \to e+\gamma$, and $\tau \to e+\gamma$ are generically enhanced by a factor of $\sim 10^3$ to $\sim 10^6$ with respect to the rates calculated for real $\mathbf{R}$, while the $\tau \to \mu +\gamma$ decay rate is enhanced approximately by two orders of magnitude.

Comments: 17 pages, 1 figure; results and conclusions unchanged, typos corrected, references added; version submitted for publication