Implications of the electroweak precision data: a 1996 update

The most reliable prediction for the Higgs-boson mass, MH, is obtained in a fit to the leptonic Z-resonance observables Γ1 and SW−2 combined with the W-boson mass, MW, and top-quark mass, mt, measurements. The corresponding bounds on MH are independent of potential uncertainties related to Rb, Rc, and αs(MZ2), and they are not significantly further improved by including also the experimental information on the inclusive (hadronic and total) Z-boson decays. At the 1σ level, we obtain MH ≲ 360 GeV using Sw−2(LEP + SLD) and MH ≲ 540 GeV using SW−2(LEP). Our analysis in terms of effective parameters confirms previous conclusions with increased accuracy. In the mass parameter, Δx, and the mixing parameter, ε, pure fermion loops are sufficient, while for the coupling parameter, Δy, (MH-insensitive) bosonic contributions are essential for consistency with experiment, thus providing indirect empirical evidence for the non-Abelian structure of the theory.