Running $U_{e3}$ and BR($\mu \to e+\gamma$) in SUSY-GUTs

Abstract

In supersymmetric seesaw models based on SUSY-GUTs, it could happen that the neutrino PMNS mixing angles are related to the lepton flavour violating decay rates. In particular SO(10) frameworks, the smallest mixing angle would get directly correlated with the $\mu \to e+\gamma$ decay amplitude. Here we study this correlation in detail considering $U_{e3}$ as a free parameter between 0 and $U_{e3}$(CHOOZ). Large radiative corrections to $U_{e3}$ present in these models, typically of the order ~$\Delta U_{e3}\sim 10^{-3}$ (peculiar to hierarchial neutrinos), can play a major role in enhancing the Br($\mu \to e+\gamma$), especially when $U_{e3} \ler 10^{-3}$. For large tan$\beta$, even such small enhancements are sufficient to bring the associated Br($\mu \to e+\gamma$) into realm of MEG experiment as long as SUSY spectrum lies within the range probed by LHC. On the other hand, for some (negative) values of $U_{e3}$, suppressions can occur in the branching ratio, due to cancellations among different contributions. From a top-down perspective such low values of $U_{e3}$ at the weak scale might require some partial/full cancellations between the high scale parameters of the model and the radiative corrections unless $U_{e3}$ is purely of radiative origin at the high scale. We further emphasize that in Grand Unified theories there exist additional LFV effects related to the running above the GUT scale, that are also independent on the low energy value of $U_{e3}$. These new contributions can become competitive and even dominant in some regions of the parameter space.

16 pages, 9 figures; references added, improved discussions in text, to appear in JHEP

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