REVISING A CLASS OF 3-GENERATION MODELS - MASS-SPECTRUM, RENORMALIZATION-GROUP AND PROTON-DECAY CONSTRAINTS

The mass spectrum of an entire class of three-generation superstring models is analysed. The requirement of a higgs light enough to allow for the standard electroweak breaking is found to be incompatible with the following constraints: (i) the perturbative evolution of gauge couplings up to the compactification scale, (ii) a correct prediction of the electroweak mixing angle, and (iii) the observed proton lifetime. This indicates that discrete symmetries and order-of-magnitude mass ratios are not enough to reproduce the standard model. Unless "accidental" cancellations occur in the mass matrix, this problem appears generic to many models.