Improved Determination of $\alpha(M_{\rm Z}^2$) and the Anomalous Magnetic Moment of the Muon

Abstract

We reevaluate the hadronic contribution to the running of the QED fine structure constant \aqed at $s=M_{\rm Z}^2$. We use data from \ee\ annihilation and $\tau$ decays at low energy and at the $q\bar{q}$ thresholds, where resonances occur. Using so-called spectral moments and the Operator Product Expansion (OPE), it is shown that a reliable theoretical prediction of the hadronic production rate $R(s)$ is available at relatively low energies. Its application improves significantly the precision on the hadronic vacuum polarization contribution. We obtain \daqedhZ $=(277.8\pm2.6)\times10^{-4}$ yielding $\alpha^{-1}(M_{\rm Z}^2)=128.923\pm0.036$. Inserting this value in a global electroweak fit using current experimental input, we constrain the mass of the Standard Model Higgs boson to be $M_{\rm Higgs}=129^{+103}_{-62}~{\rm GeV}$. Analogously, we improve the precision of the hadronic contribution to the anomalous magnetic moment of the muon for which we obtain $a_\mu^{\rm had}=(695.1\pm7.5)\times 10^{-10}$.