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

We estimate, in the anti(MS) scheme, the strengths of the higher order QCD (αs2, αs3) and quark mass (mf2, mf4, αs (mf2, mf4)) corrections to the Z widths in the standard model (SM). Then, we give improved SM predictions of these widths. The observables ΓH/Γe and σH, which can be predicted and measured accurately are almost insensitive to the changes of the Higgs (40 GeV ≤ MZ ≤ 1 TeV) and top quark (Mh ≤ Mt ≤ 230 GeV) masses. Then, they should be useful in the high precision tests of the SM. Improved estimates of some quantities related to the banti b width which are sensitive to the top quark mass are also given. (orig.)