Harmless R-parity violation from Z12-I compactification of E8xE′8 heterotic string

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 Delta B not equal 0 operators of the MSSM fields occur at dimension 10. The effective dimension-5 Delta B not equal 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 Delta B not equal 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 Delta B not equal 0 operators would appear at lower dimensions. (c) 2007 Elsevier B.V. All rights reserved.