Hadronic \tau decays and QCD

Hadronic decays of the $\tau$ lepton provide a clean source to study hadron dynamics in an energy regime dominated by resonances, with the interesting information captured in the spectral functions. Recent results on exclusive channels are reviewed. Inclusive spectral functions are the basis for QCD analyses, delivering an accurate determination of the strong coupling constant and quantitative information on nonperturbative contributions. Strange decays yield a determination of the strange quark mass.Comment: 13 page

e+e- results from BABAR and implications for g-2

The BABAR collaboration has nearly completed a program of precise measurements of the cross sections for the dominant channels of e+e- --> hadrons from threshold to an energy of 3-5 GeV using the initial-state radiation (ISR) method, i.e. the measurement of the cross sections e+e- --> gamma hadrons with the energetic photon detected at large angle to the beams. These data are used as input to vacuum polarization dispersion integrals, in particular the hadronic contribution to the muon g-2 anomaly. In addition to the recently measured pi+pi- cross section, giving the dominant contibution, many multihadronic channels have been investigated, with some recent examples presented here. We give preliminary results for the process e+e- --> K+K-(gamma) using 232 fb-1 of data collected with the BABAR detector at e+e- center-of-mass energies near 10.6 GeV. The lowest-order contribution to the hadronic vacuum polarization term in the muon magnetic anomaly is obtained for this channel: amu-KK-LO=(22.95 +-0.14(stat) +-0.22(syst)) 10^-10, which is about a factor of three more precise than the previous world average value.Comment: 4 pages, 5 figures, talk given at the Tau Workshop, September 2012, Nagoy

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

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 $1.0 {\rm GeV}/c^2$.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 \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.Comment: 9 pages, 7 figures, proceedings of the SIGHAD03 workshop (Pisa, October 2003) on e+e- annihilation cross sections at low energ

Progress on R measurement through ISR with BaBar

The status of the ISR analysis with BaBar is presented. Preliminary results are given on the process e+e- --> 2pi+ 2pi- and J/psi decays including a determination of its total width and the branching ratios into 2pi+ 2pi-, K+ K- pi+ pi-, and 2K+ 2K- modes.Comment: 6 pages, 6 figures, proceedings of the SIGHAD03 workshop (Pisa, October 2003) on e+e- annihilation cross sections at low energ

Comments on "An Update of the HLS Estimate of the Muon g-2"by M.Benayoun {\it et al.}, arXiv:1210.7184v3

In a recent paper \cite{benayoun} M.Benayoun {\it et al.} use a specific model to compare results on the existing data for the cross section of the process $e^+e^-\rightarrow \pi^+\pi^-$ and state conclusions about the inconsistency of the BABAR results with those from the other experiments. We show that a direct model-independent comparison of the data at hand contradicts this claim. Clear discrepancies with the results of Ref. \cite{benayoun} are pointed out. As a consequence we do not believe that the lower value and the smaller uncertainty obtained for the prediction of the muon magnetic anomaly are reliable results.Comment: 7 pages,2 figures, comment

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

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.Comment: 9 pages, 7 figures, invited talk at the Tau06 International Workshop, Pisa, September 19-22 200

Spectral functions from Hadronic Tau Decays and QCD

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.Comment: 14 pages, 18 figures, invited talk at the Tau02 Workshop, Santa Cruz, Sept. 200

Measurement of Branching Fractions in Tau Decays

Full LEP-I data collected by the ALEPH detector during 1991-1995 running are analyzed in order to measure the $\tau$ decay branching fractions. The analysis follows the global method used in the published study based on 1991-1993 data, with several improvements, especially concerning the treatment of photons and $\pi^0$'s. Extensive systematic studies are performed, in order to match the large statistics of the data sample corresponding to 327148 measured and identified $\tau$ decays. Preliminary values for the branching fractions are obtained for the 2 leptonic channels and 11 hadronic channels defined by their respective numbers of charged particles and $\pi^0$'s. Using previously published ALEPH results on final states with charged and neutral kaons, corrections are applied so that branching ratios for exclusive final states without kaons are derived. Some physics implications of the results are given, in particular concerning universality in the leptonic charged weak current, isospin invariance in $a_1$ decays, and the separation of vector and axial-vector components of the total hadronic rate.Comment: Invited Talk at Tau02 International Workshop, Sept. 10-13 2002, Santa Cruz, California, 19 pages, 13 figure