A Constraint on Yukawa-Coupling Unification from Lepton-Flavor Violating Processes

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 fν3 ≃ ftop, in which τ -neutrino Yukawa coupling fν3 is unified into top-quark Yukawa coupling ftop at the unification scale M∗ ≃ 3 × 1016 GeV. We show that the present experimental bound on μ → eγ decay already gives a stringent limit on the lepton mixing (typically V13 < 0.02 for V23 = 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 (VMNS)e3 in future neutrino experiments would be very important, since the observation of non-zero (VMNS)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.