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Hadronic decays of the $\tau$ lepton provide a clean environment to study hadron dynamics in an energy regime dominated by resonances, with the interesting information captured in the spectral functions. Recent results from ALEPH on exclusive channels are presented, with some emphasis on the $\pi \piz$ final state which plays a crucial role for the determination of the hadronic contribution to the muon anomalous magnetic moment. Inclusive spectral functions are the basis for QCD analyses, delivering an accurate determination of the strong coupling constant, quantitative information on nonperturbative contributions and a measurement of the mass of the strange quark.
The status of the ISR analysis with BaBar is presented. Preliminary results are given on the process $e^+e^- \to 2\pi^+ 2\pi^-$ and $J/\psi$ decays including a determination of its total width and the branching ratios into $2\pi^+ 2\pi^ -, K^+ K^- \pi^+ \pi ^-$, and $2K^+ 2K^-$ modes.
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 $e^+e^-$ annihilation cross section into hadrons by the CMD-2 Collaboration. The agreement between $e^+e^-$ 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 $1.0 {\rm GeV}/c^2$.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 $e^+e^-$ 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^- - ...
The evaluation of the hadronic contribution to the muon magnetic anomaly $a_\mu$ is reviewed, including a new estimate using precise results on the $\pi^+\pi^- $ spectral function from the KLOE Collaboration. It is found that the KLOE data confirm to some extent the previous $e^+e^-$ annihilation data in this channel, and accentuate the disagreement with the isospin-breaking-corrected spectral function from $\tau^-\to\pi^+\pi^0\nu_\tau$ decays. Correcting for the empirical difference in the mass of the charged and the neutral $\rho$ locally improves, but does not resolve this discrepancy. A preliminary reevaluation (including the KLOE data) of the $e^+e-^$-based Standard Model prediction of $a_\mu$ results in a deviation of 2.7 standard deviations from the BNL measurement.
A study of several 3- and 4-body hadronic final states ($\pi^+\pi^-\pi^0, \pi^+\pi^-\pi^+\pi^-, K^+K^-\pi^+\pi^- and K^+K^-K^+K^-$) accompanied by a hard photon is presented. These states are produced from $e^+e^-$ collisions at the c.m. energy near the $\UPSILON (4S)$ resonance using a $90 fb^-1$ data sample collected with the BABAR detector at the PEP-II collider. The invariant mass of the hadronic final state determines the virtual photon energy, so that the data can be compared with direct $e^+e^-$ cross sections. Cross sections have been obtained from threshold up to 4.5 GeV with systematic errors at the 5% level. The accuracy of the results is comparable with the best direct $e^+e^-$ results overall, but achieves amuch better precision in 1.4-2.5 GeV region where data are sparse. In addition to light meson spectroscopy these data...
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