The hadronic contribution to $(g-2)_\mu$

Abstract

The evaluation of the hadronic contribution to the muon magnetic anomaly $a_\mu$  is reviewed, including a new estimate using precise results on the $\pi^+\pi^-$ spectral function from the KLOE Collaboration. It is found that the KLOE data confirm to some extent the previous $e^+e^-$ annihilation data in this channel, and accentuate the disagreement with the isospin-breaking-corrected spectral function from $\tau^-\to\pi^+\pi^0\nu_\tau$ decays. Correcting for the empirical difference in the mass of the charged and the neutral $\rho$ locally improves, but does not resolve this discrepancy. A preliminary reevaluation (including the KLOE data) of the e^+e-^-based Standard Model prediction of $a_\mu$  results in a deviation of 2.7 standard deviations from the BNL measurement