A generic problem for a class of three-generation models

A promising example from a class of three-generation superstring models is analysed. Under broad assumptions, the existence of a Higgs doublet light enough to allow for the standard electroweak breaking is found to be incompatible with (i) the perturbative evolution of gauge couplings up to compactification scale, (ii) gauge unification, and (iii) a correct prediction of the electroweak mixing angle. This seems to indicate that discrete symmetries and order-of-magnitude mass ratios are not enough to reproduce the standard model. (orig.)