Gauge unification, Yukawa hierarchy and the μ problem

The hierarchy in the Yukawa couplings may be the result of a gauged horizontal U(1) h symmetry. If the mixed anomalies of the Standard Model gauge group with U(1) h are cancelled by a Green-Schwarz mechanism, a relation between the gauge couplings, the Yukawa couplings and the μ-term arises. Assuming that at a high energy scale g32 = g22 = 53g12 and (memμmτ)(mdmsmb) ∼ λ (where λ is of the order of the cabibbo angle), the U(1) h symmetry solves the μ-problem with μ ∼ λm32.