Implications of precision electroweak experiments for m_t, ρ₀, sin²θ_W, 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 sin²θ̂_W(M_Z)=0.2334±0.0008 in the modified minimal subtraction scheme or sin²θ_W≡1-M_W²/M_Z²=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 ρ₀ in models with a nonminimal Higgs structure. Allowing arbitrary m_t and Higgs representations one obtains sin²θ̂_W(M_Z)=0.2333±0.0008, ρ₀=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.