Predictive SUSY SO(10) model with very low tan β

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 m e,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 ε u, d, e which are related by the SO(10) symmetry properties. Their values are determined by the ratio of the GUT scale M x ⋍ 10 16 GeV to a higher (possibly string) scale M ⋍ 10 17−10 18 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 m s m d 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 M t < 165 GeV, while the lower bound M t > 160 GeV implies m s m d > 22 . tanβ can vary from 1.4 to 1.7. The proton decaying d = 5 operators qqql are naturally suppressed.