Reevaluation of the Hadronic Contribution to $α(M_Z^2)$ (revised)

Abstract

We reevaluate the hadronic part of the electromagnetic vacuum expectation value using the standard dispersion integral approach that utilizes the hadronic cross section measured in $\ee$ experiments as input. Previous analyses are based upon point-by-point trapezoidal integration which has the effect of weighting all inputs equally. We use a technique that weights the experimental inputs by their stated uncertainties, includes correlations, and incorporates some refinements. We find the hadronic contribution to the fractional change in the electromagnetic coupling constant at $q^2=M_Z^2$ to be $0.02666\pm0.00075$, which leads to a value of the electromagnetic coupling constant, $α^{-1}(M_Z^2) = 129.08\pm0.10$. This value significantly shifts the Standard Model predictions for the effective weak mixing angle measured at the $Z$ pole and moderately shifts the predicted $Z$ width.