Neutron-Anti-Neutron Oscillation as a Test of Grand Unification1

We discuss the predictions for the neutron-anti-neutron (N − N) process in various supersymmetric and non-supersymmetric grand unified theories. In particular it is pointed out that in a class of superstring inspired grand unified theories (of E6 type) that satisfy the constraints of gauge coupling unification, breakdown of the B − L symmetry occurs at an intermediate scale leading in turn to ∆B = 1 type R-parity violating interactions naturally suppressed to the level of 10 to 10. This in turn implies an N −N transition time of order 10 to 10 sec. which may be observable in the next generation of proposed experiments. These models also satisfy the conditions needed for generating the cosmological baryon asymmetry of the right order of magnitude for a restricted range of the parameter space. Invited talk presented at the International workshop on proton decay and neutron oscillation at Oak Ridge National Laboratory from March 28-30, 1996. 2 Work supported by the National Science Foundation Grant #PHY-9119745 and a Distinguished Faculty Research Award by the University of Maryland for the year 1995-96.