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

Abstract

The evaluation of the hadronic contribution to the muon magnetic anomaly $a_μ$ is revisited, taking advantage of new experimental data on $e^+e^-$ annihilation into hadrons: SND and CMD-2 for the $π^+π^-$ 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 $τ$ 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 $a_μ$ 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$σ$ level, while the $τ$-based estimate is in much better agreement. The $τ$/$e^+e^-$ discrepancy, best revealed when comparing the measured branching fraction for $τ^- \to π^- π^0 ν_τ$ to its prediction from the isospin-breaking-corrected $e^+e^-$ spectral function, remains a serious problem to be understood.