A constraint on Yukawa-coupling unification from lepton-flavor violating processes

Abstract

We present a new constraint on a lepton mixing matrix V from lepton-flavor violating (LFV) processes in supersymmetric standard models with massive neutrinos. Here we assume Yukawa-coupling unification f ν3 ≃ f top, in which τ-neutrino Yukawa coupling f ν3 is unified into top-quark Yukawa coupling f top at the unification scale M ∗≃3×10 16 GeV. We show that the present experimental bound on μ→ eγ decay already gives a stringent limit on the lepton mixing (typically V 13<0.02 for V 23=1/ 2 ). Therefore, many existing neutrino-mass models are strongly constrained. Future improvement of bounds on LFV processes will provide a more significant impact on the models with the Yukawa-coupling unification. We also stress that a precise measurement of a neutrino mixing ( V MNS) e3 in future neutrino experiments would be very important, since the observation of non-zero ( V MNS) e3 , together with negative experimental results for the LFV processes, have a robust potential to exclude a large class of SUSY standard models with the Yukawa-coupling unification.