Michel parameters and tau-neutrino helicity from decay correlations in Z --> tau+ tau-

The Michel parameters and the average τ-neutrino helicity are measured using correlations between the decays of the τ+ and τ− produced on the Z resonance and observed in the ALEPH detector at LEP. The Michel parameters, ϱl, ηl, ξl, (δξ)l, are determined from with l = (e, μ), and the average τ neutrino helicity, 〈h(ντ)〉, from τ → hν with h = (π, ϱ, a1). The results obtained with e−μ universality are: ϱl = 0.751±0.039±0.022, ηl = −0.04±0.15±0.11, ξl = 1.18±0.15±0.06, (δξ)l = 0.88±0.11±0.07, and the average τ neutrino helicity 〈h(ντ)〉 = −1.006±0.032±0.019. No significant deviation from the Standard Model V-A prediction is observed

Search for CP violation in the decay Z ---> tau+ tau-

Data collected by ALEPH in the years 1990, 1991 and 1992 have been used to update a previous search for CP violation in the decay of the Z into tau(+)tau(-). The measurement of the weak dipole form factor of the tau lepton has been performed by studying correlations between the tau leptons. No signal of CP violation was found. The weak dipole form factor is found to be over tilde d(tau) = (+0.15 +/- 0.58(stat) +/- 0.38(sys)) 10(-17)e x cm, obtained with 19628 identified tau(+)tau(-) events. This gives an upper limit on the weak dipole form factor of |over tilde d(tau)| < 1.5 . 10(-17)e x cm at the 95% confidence level

Michel parameters and tau-neutrino helicity from decay correlations in Z --> tau+ tau-

The Michel parameters and the average τ-neutrino helicity are measured using correlations between the decays of the τ+ and τ− produced on the Z resonance and observed in the ALEPH detector at LEP. The Michel parameters, ϱl, ηl, ξl, (δξ)l, are determined from with l = (e, μ), and the average τ neutrino helicity, 〈h(ντ)〉, from τ → hν with h = (π, ϱ, a1). The results obtained with e−μ universality are: ϱl = 0.751±0.039±0.022, ηl = −0.04±0.15±0.11, ξl = 1.18±0.15±0.06, (δξ)l = 0.88±0.11±0.07, and the average τ neutrino helicity 〈h(ντ)〉 = −1.006±0.032±0.019. No significant deviation from the Standard Model V-A prediction is observed

STUDY OF THE SUBJET STRUCTURE OF QUARK AND GLUON JETS

Measurements of the subjet structure of quark and gluon jets in hadronic Z decays are presented. The analysis is based on one million hadronic events recorded by the ALEPH detector. Roughly symmetric three-jet events are selected with a coarse jet-resolution cut-off, y(1). Gluon jets are identified with a purity of 94.6% in those events where evidence of long-lived heavy-flavour hadrons in the other two jets is found. The jets are then analyzed using a smaller cut-off y(0) (&lt; y(1)) so that subjets are resolved. The properties of the jets (subjet multiplicities [N-q], [N-g] and rates R(n)(g)((q)) for n = 1,2,3,4) are determined and are found to be in good agreement with the expectations of perturbative QCD as long as the subjet resolution parameter y(0) is sufficiently large to keep non-perturbative effects small. In particular, the ratio [N-g - 1]/[N-q - 1], which to leading order in QCD is given by the ratio of colour factors C-A/C-F = 9/4, is measured to be 1.96 +/- 0.15 for y(0) = 2 . 10(-3), but falls to 1.29 +/- 0.03 for y(0) = 1.6 . 10(-5)

An Upper limit for the tau-neutrino mass from tau --> 5 pi (pi0) tau-neutrino decays

From a sample of 152,000 τ decays collected by the ALEPH detector at LEP an upper limit of 24 MeV at 95% CL on the τ neutrino mass has been determined. The limit is obtained using a two dimensional likelihood fit of the visible energy and the invariant mass distribution of 25 τ → 5π(π0)ντ events