From minimal to realistic supersymmetric SU(5) grand unification

We construct and discuss a "realistic" example of SUSY SU(5) GUT model, withan additional U(1) flavour symmetry, that is not plagued by the need of largefine tunings, like those associated with doublet-triplet splitting in theminimal model, and that leads to an acceptable phenomenology. This includescoupling unification with a value of alpha_s(m_Z) in much better agreement withthe data than in the minimal version, an acceptable hierarchical pattern forfermion masses and mixing angles, also including neutrino masses and mixings,and a proton decay rate compatible with present limits (but the discovery ofproton decay should be within reach of the next generation of experiments). Inthe neutrino sector the preferred solution is one with nearly maximal mixingboth for atmospheric and solar neutrinos.Comment: 20 pages, LATEX, 2 figures; uses psfig.st