Abstract :
The first fermion family might play a key role in understanding the structure of flavour: a role of the mass unification point. The GUT scale running masses me,u,d are rather close, which may indicate an approximate symmetry limit. Following this observation, we present a new predictive approach based on the SUSY SO(10) theory with tan β ∼ 1. The inter-family hierarchy is first generated in a sector of hypothetical superheavy fermions and then transferred inversely to ordinary quarks and leptons by means of the universal seesaw mechanism. The Yukawa matrices are simply parametrized by the small complex coefficients εu,d,e which are related by the SO(10) symmetry properties. Their values are determined by the ratio of the GUT scale Mx ⋍ 1016 GeV to a higher (possibly string) scale M ⋍ 1017−1018 GeV. The suggested ansatz correctly reproduces the fermion mass and mixing pattern. By taking as input the masses of leptons and c and b quarks, the ratio msmd and the value of the Cabibbo angle, the u, d, s quark masses, top mass and tan β are computed. The top quark is naturally in the 100 GeV range, but obeys an upper limit Mt < 165 GeV, while the lower bound Mt > 160 GeV implies msmd > 22. tanβ can vary from 1.4 to 1.7. The proton decaying d = 5 operators qqql are naturally suppressed.
