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

Abstract In a recent Z 12 − I orbifold model, an approximate Z 2 symmetry which forbids the baryon number violating operators up to sufficiently high orders is found. The dimension-4 Δ B ≠ 0 operators of the MSSM fields occur at dimension 10. The effective dimension-5 Δ B ≠ 0 operators derived from these are harmless if some VEVs of neutral singlets are O ( 1 10 ) times the string scale. The main reason for forbidding these Δ B ≠ 0 operators up to such a high order is the large order N = 12 of Z N since the H -momentum rule is ( − 1 , 1 , 1 ) mod ( 12 , 3 , 12 ). For a lower order N 12 , the Δ B ≠ 0 operators would appear at lower dimensions.