Evaluation of α(M^2_Z) and (g-2)_μ

Abstract

This talk summarizes the recent development in the evaluation of the leading order hadronic contributions to the running of the QED fine structure constant α(s), at $s=M_{\rm Z}^2$, and to the anomalous magnetic moments of the muon $(g-2)_μ$. The accuracy of the theoretical prediction of these observables is limited by the uncertainties on the hadronic contributions. Significant improvement has been achieved in a series of new analyses which is presented historically in three steps: (I), use of $τ$ spectral functions in addition to e^+e^- cross sections, (II), extended use of perturbative QCD and (III), application of QCD sum rule techniques. The most precise values obtained are: $Δα_{had} (M_2^Z) =(276.3\pm1.6)\times10^{-4}$, yielding $α^{-1}(M_{\rm Z}^2)=128.933\pm0.021$, and $a_μ^{\rm had}=(692.4\pm6.2)\times 10^{-10}$ with which one finds for the complete Standard Model prediction $a_μ^{\rm SM}=(11 659 159.6\pm6.7)\times10^{-10}$. For the electron $(g-2)_e$, the hadronic contribution is $a_e^{\rm had}=(187.5\pm1.8)\times 10^{-14}$.

9 pages, Talk given at the ICHEP'98 Conference, Vancouver, Canada, July 23-29, 1998