Simple discrete symmetries in phenomenologically viable E6 superstring models

We analyze the simplest discrete symmetries of the superpotential of superstring-inspired E6 models necessary to simultaneously suppress (i) proton decay, (ii) neutrino masses, and (iii) tree-level flavor-changing neutral currents in E6 superstring models without any intermediate scale. We show that there are two and only two different Z2 symmetries that incorporate (i)--(iii). These symmetries put restrictions on the decays of exotic particles; in particular, the exotic quarks always decay semileptonically. A Z3-symmetric model satisfying (i)--(iii) and with μ→eγ forbidden is also constructed