IMPLICATIONS OF PRECISION ELECTROWEAK EXPERIMENTS FOR MT, RHO-0, SIN2-THETA-W, AND GRAND UNIFICATION

The implications of precision Z-pole, W-mass, and weak-neutral-current data for SU(2) X U(1) models are described. Within the minimal model one finds sin2-theta-W(M(Z))=0.2334 +/-0.0008 in the modified minimal subtraction scheme or sin2-theta-W=1-M(W)2/M(Z)2 = 0.2291 +/- 0.0034 in the on-shell scheme, where the uncertainties include the m(t) and M(H) dependence. The top-quark mass is predicted to be 124(-34-15)+28+20 GeV, where the second uncertainty is from M(H), with m(t) < 174 (182) GeV at 90 (95)% C.L. For the first time subleading effects and vertex corrections allow a significant separation of m(t) and rho-0 in models with a nonminimal Higgs structure. Allowing arbitrary m(t) and Higgs representations one obtains sin2-theta-W(M(Z))=0.2333 +/- 0.0008, rho-0 = 0.992 +/- 0.011, and m(t) < 294 (310) GeV. The implications of these results for ordinary and supersymmetric grand unified theories are considered. Supersymmetric theories with a grand desert between the supersymmetry and unification scales are in striking agreement with data for M(SUSY) in the M(Z)-1 TeV range. Ordinary grand unified theories breaking to the standard model in more than one step are also discussed.