Unification picture in the minimal supersymmetric SU(5) model with string remnants

The significant heavy threshold effect is found in the minimal supersymmetric SU(5) model with two adjoint scalars, one of which is interpreted as a massive string mode largely decoupled from the lower-energy particle spectra. This threshold related with the generic mass splitting of the basic adjoint moduli is shown to alter properly the running of gauge couplings, thus giving a natural solution to the string-scale grand unification. The further symmetry condition of the (top-bottom) Yukawa and gauge coupling superunification at a string scale results in the perfectly working predictions for the top and bottom quark masses in the absence of any large supersymmetric threshold corrections. [S0556-2821(98)06617-X].