Incorporation of QCD effects in basic corrections of the electroweak theory

We study the incorporation of QCD effects in the basic electroweak corrections Δ^r, Δ^rW, and Δr. They include perturbative O(ααₛ) contributions and tt̄ threshold effects. The latter are studied in the resonance and Green-function approaches, in the framework of dispersion relations that automatically satisfy relevant Ward identities. Refinements in the treatment of the electroweak corrections, in both the modified minimal subtraction (-MS) and the on-shell schemes of renormalization, are introduced, including the decoupling of the top quark in certain amplitudes, its effect on ^e²(mZ) and sin²^θW(mZ), the incorporation of recent results on the leading irreducible O(α²) corrections, and simple expressions for the residual, i.e., "nonelectromagnetic," parts of Δ^r, Δ^rW, and Δr. The results are used to obtain accurate values for mW and sin²^θW(mZ), as functions of mₜ and mH. The higher-order effects induce shifts in these parameters comparable to the expected experimental accuracy, and they increase the prediction for mₜ derived from current measurements. The -MS and the on-shell calculations of Δr, in a recently proposed formulation, are compared and found to be in excellent agreement over the wide ranges 60 GeV≤mH≤1 TeV, mZ≤mₜ≤250 GeV.