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 ne utrinos. 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 t op-quark Yukawa coupling f(top) at the unification scale M-* similar or equ al 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 (typica lly V-13 < 0.02 for V-23 = 1/<root>2) Therefore, many existing neutrino-mas s models are strongly constrained. Future improvement of. bounds on LFV pro cesses will provide a more significant impact on the models with the Yukawa -coupling unification. We also stress that a precise measurement of a neutr ino mixing (V-MNS)(e3) in future neutrino experiments would he very importa nt, since the observation of non-zero (V-MNS)(e3) together with negative ex perimental results for the LFV processes, have a robust potential to exclud e a large class of SUSY standard models with the Yukawa-coupling unificatio n. (C) 2001 Published by Elsevier Science B.V.