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.
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Submitted on : Friday, June 15, 2007 - 5:08:57 PM
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M. Davier. The hadronic contribution to (g-2) of the muon. Tau06 International Workshop, Sep 2006, Pisa, Italy. pp.288-296, ⟨10.1016/j.nuclphysbps.2007.03.023⟩. ⟨in2p3-00155193⟩

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