REALISTIC MODEL-BUILDING OF HORIZONTAL GAUGE INTERACTIONS AND QUARK-LEPTON MASS MATRICES

We attempt to explain the gross structure of mass matrices of quarks and leptons by the vector-like horizontal gauge interactions in association with critically enhanced radiative corrections due to vertical gauge interactions. The horizontal gauge symmetry G(H) is assumed to break down in TeV region into the product of two nearly decoupled global symmetries G approximately H(low) X G approximately H(high), each operating on three low-lying and k higher generations, respectively. By setting G approximately H(low) congruent-to SO(3), we realize nearly massless neutrinos in the low-lying three generations as the result of the dynamical seesaw mechanism. In k higher generations, the broken gauge interactions with G approximately H(high) generate Dirac mass matrices both in charged and neutral sectors. The mass difference between up and down sectors in the higher generations can be kept sufficiently small to be compatible with experimental-rho value, by assuming that masses of gauge bosons associated with G approximately H(high) are considerably small, compared with those associated with G approximately H(low) which are severely constrained by FCNC condition. Highly massive sideway gauge bosons associated with G(H)/G approximately H(low) X G approximately H(high) cause the bare mass-matrix transfer from higher to low-lying generations. We finally discuss a (3+3)-generation model with G(H) = SU(6).