A PERTURBATION-THEORY DRIVEN ANALYSIS OF THE EFFECTIVE QED COUPLING AT M-Z

An evaluation of the effective QED coupling at the scale M-z is presen ted. It employs the predictions of perturbative QCD far the cross sect ion of electron positron annihilation into hadrons up to order alpha(s )(2), including the full quark mass dependence, and of order alpha(s)( 3) in the high energy region. This allows to predict the input for the dispersion relations over a large part of the integration region. The perturbative piece is combined with data for the lower energies. The normalization of data from the heavy quark thresholds is deduced from a comparison between data and pQCD below and above the threshold regio n. The result for the hadronic contribution to the running of the coup ling Delta alpha(had)((5))(M-Z(2)) = (277.5 +/- 1.7) x 10(-4) leads to (alpha(M-Z(2)))(-1) = 128.927 +/- 0.023. Compared to previous analyse s which were based on data only the uncertainty is significantly reduc ed. This is due to our assumption of perturbative QCD to be valid down to 1.8 GeV with the exception of the charm threshold and to our model for the normalization of the data in the region between 3.7 and 5.0 G eV. The error estimate is based on the variation of the renormalizatio n scale mu(2) and the parametric uncertainty of alpha(s) and the quark masses. The absence of large nonperturbative contributions is assumed . (C) 1998 Elsevier Science B.V. All rights reserved.