The Hadronic Contribution to (g−2)μ

The evaluation of the hadronic contribution to the muon magnetic anomaly aμ 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 aμ 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 3.3σ 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−→π−π0ντ to its prediction from the isospin-breaking-corrected e+e− spectral function, remains a serious problem to be understood.