Naturally suppressed proton decay in a class of three generation superstring models

We identify a class of three generation superstring models in which the most dangerous proton decay processes occurring through dimension five operators are naturally suppressed. The mechanism relies on the combined effect of a Z3 matter parity with a Z3 R-symmetry and is more effective in the case of a low U(1)B-L breaking scale. The importance of the mechanism stems from the fact that the discrete symmetries used are among the ones arising upon spontaneous compactification of the heterotic superstring on the only known Calabi-Yau space leading to three quark-lepton families. (orig.)