Abstract :
The accuracy of the conventional treatment of quark mass thresholds in the QCD running coupling constant αs, based on the step approximation to the β-function, is investigated. The errors of extrapolating αs(μ) from low energies to μ = Mz, implied by this approximation, are shown to be of the same magnitude as the typical next-to-next-to-leading order contributions to αs and tend to increase the resulting αs. The importance of the proper choice of matching points is emphasized.
