Whither grand unification: experimental tests of B-L gauge groups

The failure of rank 4 SU(5) grand unification to account for the absence of proton decay makes it even more important to verify the predictions of the rank 5 unification groups that embed B-L local symmetry. These are SO(10) and SU(2)sub(L)xSU(2)sub(R)SU(4)sub(C). Tests of the B-L unification groups which can be carried out in the near future are suggested. These depend on a relatively low mass scale associated with the breaking of SU(4)sub(C) to U(1)sub(B-L)xSU(3)sub(C) and if n-n-bar oscillations and/or Ksub(L) → μe-bar decay are detected, it would further the understanding of the basic quark - lepton symmetry that is expressed through the gauging of the B-L symmetry. (UK)