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

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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(nu3) similar or equal to f(top), in which tau -neutrino Yukawa coupling f(nu3) is unified into top-quark Yukawa coupling f(top) at the unification scale M-* similar or equal to 3 x 10(16) GeV. We show that the present experimental bound on mu --> e gamma decay already gives a stringent limit on the lepton mixing (typically V-13 < 0.02 for V-23 = 1/<root>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 he 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. (C) 2001 Published by Elsevier Science B.V.