Hierarchical top-bottom mass relation in a superstring derived standard-like model

I propose a mechanism in a class of superstring standard-like models which explains the mass hierarchy between the top and bottom quarks. At the trilinear level of the superpotential only the top quark gets a nonvanishing mass term while the bottom quarks and tau lepton mass terms are obtained from nonrenormalizable terms. I construct a model which realized this mechanism. In this model the bottom quark and tau lepton Yukawa couplings are obtained from quartic order terms. I show that λb=λτ∝1/8 λt at the unification scale. A naive estimate yields mt∝175-180 GeV. (orig.)