Implications of precision electroweak experiments for mt, 0, sin2W, and grand unification

The implications of precision Z-pole, W-mass, and weak-neutral-current data for SU(2)×U(1) models are described. Within the minimal model one finds sin2W(MZ)=0.23340.0008 in the modified minimal subtraction scheme or sin2W1-MW2MZ2=0.22910.0034 in the on-shell scheme, where the uncertainties include the mt and MH dependence. The top-quark mass is predicted to be 124-34-15+28+20 GeV, where the second uncertainty is from MH, with mt<174 (182) GeV at 90 (95)% C.L. For the first time subleading effects and vertex corrections allow a significant separation of mt and 0 in models with a nonminimal Higgs structure. Allowing arbitrary mt and Higgs representations one obtains sin2W(MZ)=0.23330.0008, 0=0.9920.011, and mt<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 MSUSY in the MZ-1 TeV range. Ordinary grand unified theories breaking to the standard model in more than one step are also discussed.