Abstract |
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The full LEP-1 data set collected with the ALEPH detector at the Z pole during 1991-1995 is analysed 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, but several improvements are introduced, 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 over 300 000 measured and identified tau decays. Branching fractions are obtained for the two leptonic channels and I I 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 to the hadronic channels to derive branching ratios for exclusive final states without kaons. Thus the analyses of the full LEP-1 ALEPH data are combined to yield a complete description of tau decays, encompassing 22 non-strange and 11 strange hadronic modes. Some physics implications of the results are given, in particular related to universality in the leptonic charged weak current, isospin invariance in alpha(1) decays, and the separation of vector and axial-vector components of the total hadronic rate. Finally, spectral functions are determined for the dominant hadronic modes and updates are given for several analyses. These include: tests of isospin invariance between the weak charged and electromagnetic hadronic currents, fits of the p resonance lineshape, and a QCD analysis of the non-strange hadronic decays using spectral moments, yielding the value alpha(s) (m(tau)(2)) = 0.340 +/- 0.005(exp) +/- 0.014(th). The evolution to the Z mass scale yields alpha(s) (M-Z(2)) = 0.1209 +/- 0.0018. This value agrees well with the direct determination from the Z width and provides the most accurate test to date of asymptotic freedom in the QCD gauge theory. (c) 2005 Elsevier B.V. All rights reserved. |