Low energy hadronic contribution to the QED vacuum polarization

Recent improvements in the low energy $e^{+}{e}^{-}$ annihilation data and their influence on the determination of the hadronic contribution to the running of the QED fine structure constant at $m_Z$ are discussed. Using CMD-2 and KLOE measurements in the $\rho$ region we obtain $\Delta {\alpha }_{\mathrm{had}}^{(5)}(s)=0.02758\pm 0.00035$ at $s=m_Z^2$.

Figure 1

$R_{\mathrm{had}}$ including resonances. Measurements are shown with statistical errors. The relative uncertainty assigned to our parametrization is shown as band and given with numbers at the bottom.