Baryon Non-Conservation in Unified Theories, in the Light of Supersymmetry and Superstrings

The first part of this talk presents the general complexion of baryon and lepton number non-conservation that may arise in the context of quark-lepton unification. The second part presents the status of grand unification with and without supersymmetry and spells out the characteristic proton decay modes, which if seen, will clearly show supersymmetry. The main theme of this talk, that follows next, pertains to two issues: (i) the need to remove the mismatch between MSSM and string-unifications; and especially (ii) the need to resolve naturally the problem of rapid proton decay, that generically arises in SUSY unification. Seeking for a natural solution to this second problem, it is noted that SUSY GUTS, including SUSY SO(10) and E_6, can at best accommodate proton-stability by a suitable choice of the Higgs-multiplets and discrete symmetries, but not really explain it, because they do not possess the desired symmetries to suppress both d=4 and d=5 proton-decay operators. By contrast, following a recent work, I argue that a class of string-solutions, possessing three families, does possess the desired symmetries, which naturally safeguard proton-stability from all potential dangers. They also permit neutrinos to have desired light masses. This shows that, believing in supersymmetry, superstring is needed just to understand why the proton is so stable.