Branching ratios and spectral functions of τ decays: Final ALEPH measurements and physics implications

Abstract

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 ? 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 ?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 ? decays. Branching fractions are obtained for the two leptonic channels and 11 hadronic channels defined by their respective numbers of charged particles and ?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 ? 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 ? resonance lineshape, and a QCD analysis of the non-strange hadronic decays using spectral moments, yielding the value ?s(m? 2) 0.340 ± 0.005exp ± 0.014th. The evolution to the Z mass scale yields ?s(MZ 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. © 2005 by Elsevier B.V. All rights reserved

An update of the HLS estimate of the muon g−2

“…Up to now, the broken HLS model (BHLS) [13]basically an empty shell-has been fed with all existing data sets 4 for what concerns the annihilation channels π 0 γ , ηγ , π + π − π 0 , K + K − , K 0 K 0 , with the spectra from ALEPH [21], CLEO [22] and BELLE [23] for the τ dipion decay 5 and with the V P γ /P γ γ partial width information extracted from the Review of Particle Properties (RPP) [24]. This already represents more than 40 data sets collected by different groups with different detectors; one may thus consider that the systematics affecting these data sets wash out to a large extent within a global fit framework.…”

Amplitude analyses of D 0 → π + π − π + π − and D 0 → K + K − π + π − decays

“…[39], the three-pion decay of the τ lepton was studied using a similar model for the a 1 (1260) propagator as used in the analysis presented here. From a simultaneous fit to ALEPH [63], ARGUS [64], OPAL [65] and CLEO [62] The a 1 (1640) resonance, the first radial excitation of the a 1 (1260) meson, was observed in ref. [66] decaying to σπ and f 2 (1270)π, and in ref.…”

Tau Decays and αs

“…[4]). The non-strange decay modes can be further decomposed into vector and axialvector components, corresponding to channels with an even and odd number of neutral or charged pions, respectively.…”

“…The approach of the OPE was also tested experimentally studying the evolution of the τ hadronic widths to masses smaller than the τ mass. The update in 2005 [4] used the full LEP-1 data set with the final branching ratio measurements based on extensive systematic studies.…”

“…Measurements performed by ALEPH [2][3][4][5], CLEO [6] and OPAL [7] are briefly reviewed in the following together with other determinations, focusing on the latest ALEPH results. Over the years, the two main improvements in the measurement of α s (m 2 τ ) were * Email address: zhangzq@lal.in2p3.fr (Zhiqing Zhang) from the increased precision of the branching ratio and spectral function measurements and the higher-order perturbative calculation.…”