Higher order QCD and quark mass corrections to the Z widths in the standard model

Abstract

We estimate, in the MS scheme, the strenghts of the higher order QCD ( α s 2, α s 3) and quark mass ( m f 2, m f 4, α s( m f 2, m f 4)) corrections to the Z widths in the standard model (SM). Then, we give improved SM predictions of these widths. The observables Γ H/ Γ e and σ H, which can be predicted and measured accurately are almost insensitive to the changes of the Higgs (40 GeV ⩽ M H ⩽ 1 TeV) and top quark ( M z⩽ M t⩽230 GeV) masses. Then, they should be useful in the high precision tests of the SM> Improved estimates of some quantities related to the b b width which are sensitive to the top quark mass are also given.