Updated estimate of the muon magnetic moment using revised results from e+e- annihilation

Abstract

A new evaluation of the hadronic vacuum polarization contribution to the muon magnetic moment is presented. We take into account the reanalysis of the low-energy \ee annihilation cross section into hadrons by the CMD-2 Collaboration. The agreement between \ee and $\tau$ spectral functions in the $\pi \pi$ channel is found to be much improved. Nevertheless, significant discrepancies remain in the center-of-mass energy range between 0.85 and $1.0\mathrm{GeV}/c^2$.The deviations from the measurement at BNL are found to be $(22.1\pm 7.2\pm 3.5\pm 8.0)10^{-10}$ (1.9 $\sigma$) and $(7.4\pm 5.8\pm 3.5\pm 8.0)10^{-10}$ (0.7 $\sigma$) for the \ee- and $\tau$-based estimates, respectively, where the second error is from the nonhadronic contributions and the third one from the BNL measurement. Taking into account the ${\rho }^{-}-{\rho }^0$ mass splitting determined from the measured spectral functions increases the $\tau$-based estimate and leads to a worse discrepancy between the two estimates.

Comments: 9 pages, 7 figures, proceedings of the SIGHAD03 workshop (Pisa, October 2003) on e+e- annihilation cross sections at low energy