The hadronic contribution to (g-2) of the muon

Abstract

The evaluation of the hadronic contribution to the muon magnetic anomaly $a_{\mu }$ is revisited, taking advantage of new experimental data on $e^{+}{e}^{-}$ annihilation into hadrons: SND and CMD-2 for the ${\pi }^{+}{\pi }^{-}$ channel, and \babar for multihadron final states. Discrepancies are observed between KLOE and CMD-2/SND data, preventing one from averaging all the $e^{+}{e}^{-}$ results. The long-standing disagreement between spectral functions obtained from $\tau$ decays and $e^{+}{e}^{-}$ annihilation is still present, and not accounted by isospin-breaking corrections, for which new estimates have been presented. The updated Standard Model value for $a_{\mu }$ based on $e^{+}{e}^{-}$ annihilation data is now reaching a precision better than experiment, and it disagrees with the direct measurement from BNL at the 3.3$\sigma$ level, while the $\tau$-based estimate is in much better agreement. The $\tau$/$e^{+}{e}^{-}$ discrepancy, best revealed when comparing the measured branching fraction for ${\tau }^{-}\to {\pi }^{-}{\pi }^0{\nu }_{\tau }$ to its prediction from the isospin-breaking-corrected $e^{+}{e}^{-}$ spectral function, remains a serious problem to be understood.

Comments: 9 pages, 7 figures, invited talk at the Tau06 International Workshop, Pisa, September 19-22 2006