Abstract |
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[eng] 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. 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 eleven 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 $a_1$ 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 $\rho$ resonance
lineshape, and a QCD analysis of the nonstrange hadronic decays using spectral
moments, yielding the value $\alpha_s(m^2_\tau) = 0.340 \pm 0.005_{\rm exp} \pm
0.014_{\rm th}$. The evolution to the $Z$ mass scale yields $\alpha_s(M_Z^2) =
0.1209 \pm 0.0018$. 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.
Comment: 139 pages, 75 figures, submitted to Physics Reports |