BARYON DECAY - FLIPPED SU (5) SURMOUNTS ANOTHER CHALLENGE

Effective four-dimensional field theories derived from string theories may contain superpotential terms that violate baryon number. Terms that are cubic in the standard model particles are well known to lead to excessively rapid proton decay, and the same is true for terms that are quartic in these light fields. This holds even if the terms contain many powers of fields with large intermediate vacuum expectation values, corresponding to asymmetric manifolds of compactification. We show that no such beta-violating superpotential interaction arises in flipped SU (5), thanks to an extra symmetry of the effective cubic d = 4 and quartic d = 5 superpotentials terms, induced at short distances. We conclude that in this model baryon decay proceeds predominantly through conventional d = 6 gauge boson exchange, and we recalculate its rate using the latest LEP data to estimate the heavy gauge boson masses.