Yukawa coupling unification with supersymmetric threshold corrections

Radiative corrections to the down-type fermion masses at the supersymmetric threshold are enhanced by the ratio of vacuum expectation values, tan β. This will have a profound impact on the unification of Yukawa couplings in supersymmetric grand unified theories (SUSY-GUT). We present an example of such a model with a horizontal gauge symmetry that naturally explains the fermion mass hierarchy and the small mixing angles of the Kobayashi-Maskawa (KM) matrix. The unification of the lepton and the down-quark Yukawa couplings is achieved without introducing large Higgs multiplets. (orig.)