FERMION MASSES AND MIXING IN SO(10)

We attempt to incorporate the observed structure of quark and lepton masses and mixing within the framework of SO(10) grand unification supplemented by a Peccei-Quinn U(1) symmetry that also distinguishes the three chiral families. All dimensionless couplings are assumed to be of order unity. The hierarchical structure among the charged fermion masses, including the t-b mass splitting, becomes related to the "small" (approximately 10(-2) ratio M(c)/M(x) (or its powers), where M(c) [M(x)] denote the SU(4)c [SO(10)] breaking scales. An upper bound on the top quark mass is obtained which for m(s)(1 GeV) = 175 MeV and 0.04 less-than-or-equal-to \V(cb)\ less-than-or-equal-to 0.055 yields 120 GeV less-than-or-similar-to m(t)phys less-than-or-similar-to 150 GeV. In the neutral sector the tau-neutrino may be cosmologically significant, whereas nu-c and nu-mu can play a role in the MSW effect. The mixing angles in the lepton sector are related via the SU(4)c symmetry to the quark mixing angles. One finds that asymptotically, theta-mu-e congruent-to (m(s)/m-mu)theta-12, theta-tau-mu congruent-to theta-23 and theta-tau-e congruent-to-theta-13.