PREDICTIVE FERMION MASS MATRIX ANSATZE IN NONSUPERSYMMETRIC SO(10) GRAND UNIFICATION

We investigate the status of predictive fermion mass Ansatze in nonsup ersymmetric SO(10) grand unification which make use of the grand unifi cation scale conditions m(e) = m(d)/3, m(mu) = 3m(3), and \V-cb\ = <sq uare-root>m(c)/m(t) in nonsupersymmetric SO(10) grand unification. The gauge symmetry below an intermediate symmetry-breaking scale M(I) is assumed to be that of the standard model with either one Higgs doublet or two Higgs doublets. We find in both cases that a maximum of five s tandard model parameters may be predicted within 1 sigma experimental ranges. We find that the standard model scenario predicts the low ener gy /V-cb/ to be in a range which includes its experimental midvalue 0. 044 and which for a large top mass can extend to lower values than the range resulting in the supersymmetric case. In the two Higgs standard model case, we identify the regions of parameter space for which unif ication of the bottom quark and tau lepton Yukawa couplings is possibl e at grand unification scale. In fact, we find that unification of the top, bottom, and tau Yukawa couplings is possible with the running b- quark mass within the 1 sigma preferred range m(b) = 4.25 +/- 0.1 GeV provided alpha(3c)(M(z)) is near the low end of its allowed range. In this case, one may make six predictions which include \V-cb\ within it s 90% confidence limits. However unless the funning mass m(b) > 4.4 Ge V, third generation Yukawa coupling unification requires the top mass to be greater than 180 GeV. We compare these nonsupersymmetric cases t o the case of the minimal supersymmetric standard model embedded in th e SO(10) grand unified group. We also give an example of a possible me chanism, based on induced vacuum expectation values and a broken U(1)( 3) symmetry for generating the observed hierarchy of masses and a mass matrix texture.