2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/190484It is well known, although sometimes ignored, that not only the d=5 but also d=6 proton decay depends on fermion mixings. In general we study carefully the dependence of d=6 decay on fermion mixings using the effective operator approach. We find that without specifying a theory it is impossible to make clear predictions. Even in a given model, it is often not possible to determine all the physical parameters. We point out that it is possible to make a clear test of any grand unified theory with symmetric Yukawa couplings. We discuss in some detail realistic theories based on SU(5) and SO(10) gauge symmetry.12 pages, two new references, a new equation, and few correctionsHigh Energy Physics - PhenomenologyHigh Energy Physics - Experimenttext