SUSY SU(6) - GIFT FOR DOUBLET-TRIPLET SPLITTING AND FERMION MASSES

The supersymmetric SU(6) model equipped with the flavour-blind discrete gauge symmetry Z(3) is considered. It provides a simultaneous solution to the doublet-triplet splitting problem, the mu-problem and leads to a natural understanding of fermion flavour. The Higgs doublets arise as Goldstone modes of the spontaneously broken accidental global SU(6) x U(6) symmetry of the Higgs superpotential. Their couplings to fermions have peculiarities leading to a consistent picture of the quark and lepton masses and mixing, without invoking the horizontal symmetry or zero texture concepts. In particular, the only particle that has a direct O(1) Yukawa coupling with the Higgs doublet is the top quark. Other fermion masses arise from higher order operators, with a natural mass hierarchy described in terms of the small ratios epsilon(Sigma) = V-Sigma/V-H and epsilon(H) = V-H/M, where V-H and V-Sigma are the corresponding SU(6) and SU(5) symmetry breaking scales, and M is a large (Planck or string) scale, The model automatically implies almost precise b - tau Yukawa unification. Specific mass formulas are also obtained, relating the down quark and charged lepton masses. Neutrinos get small (similar to 10(-5) eV) masses which can be relevant for solving the solar neutrino problem via long wavelength vacuum oscillations.