Abstract
It is shown that within the framework of the E8×E8 heterotic superstring if the Yukawa superpotential of the effective four-dimensional E6 gauge theory is required to lead in a natural way to (i) no rapid proton decay, (ii) Fritzsch-type quark and charged-lepton mass matrices, and (iii) naturally small neutrino masses generated through radiative corrections, then for three generations a minimal Z2×Z5 symmetry is required. A minimal model based on this discrete symmetry is suggested and discussed. The generalization for more than three generations is analyzed. © 1987 The American Physical Society.