Harmless R-parity violation from Z12−I compactification of E8×E8′ heterotic string

In a recent Z12−I orbifold model, an approximate Z2 symmetry which forbids the baryon number violating operators up to sufficiently high orders is found. The dimension-4 ΔB≠0 operators of the MSSM fields occur at dimension 10. The effective dimension-5 ΔB≠0 operators derived from these are harmless if some VEVs of neutral singlets are O(110) times the string scale. The main reason for forbidding these ΔB≠0 operators up to such a high order is the large order N=12 of ZN since the H-momentum rule is (−1,1,1) mod (12,3,12). For a lower order N<12, the ΔB≠0 operators would appear at lower dimensions.