A measurement of τ polarization at LEP

Physics Letters B, 1990

using x-,~v decays to determine the x polarization. We describe how the accuracy can be significantly improved by (i) observing the decays x~pv and x-,alv with the subsequent 9-,nn and a~-,r~nn decays acting as polarization analysers, and (ii) using polarization correlations measurable in the double (x-x +) decay distribution. One of the main physics goals of the experiments at LEP, the large e-e + collider, is a precision measurement of sin20w. On the Z resonance the best determinations are provided by asymmetries, which fall into three categories [ 1 ]. First, we have the forwardbackward asymmetry Afn in the process e-e+-~ff where fis either a quark of a lepton,

A sample of Z0-->tau+tau- events observed in the DELPHI detector at LEP in 1991 and 1992 is analysed to measure the tau polarisation in the exclusive decay channelsevbar v,mu vbar v, pinu, rhonu and a1nu. The tau polarisation is also measured with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision than the use of the exclusive decay modes. The results have been combined with those published on the 1990 data. A measurement of the tau polarisation as a function of production angle yields the values for the mean tau polarisation tau=-0.148±0.022 and for the Z0 polarisation P Z=-0.136±0.027. These results are used to determine the ratio of vector to axial-vector effective couplings for tausbar v_tau /bar a_tau = 0.074 ± 0.011, and for electronsbar v_e /bar a_e = 0.068 ± 0.014, compatible with e-tau universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptonsbar v_l /...

The European Physical Journal C, 2001

The polarisation of τ 's produced in Z decay is measured using 160 pb −1 of data accumulated at LEP by the ALEPH detector between 1990 and 1995. The variation of the polarisation with polar angle yields the two parameters Ae = 0.1504 ± 0.0068 and Aτ = 0.1451 ± 0.0059 which are consistent with the hypothesis of e-τ universality. Assuming universality, the value Ae-τ = 0.1474 ± 0.0045 is obtained from which the effective weak mixing angle sin 2 θ eff W = 0.23147 ± 0.00057 is derived.

European Physical Journal C, 1992

From the analysis of a data sample corresponding to an integrated luminosity of 4.63 pb−1 taken during the 1990 run of LEP at centre of mass energies between 88.2 GeV an 94.2 GeV, the tau decays \(\tau ^ - \to e^ - \bar v_e v_\tau ,\tau ^ - \to \mu ^ - \bar v_\mu v_\tau ,\tau ^ - \to \pi ^ - (K^ - )v_\tau ,\tau ^ - \to \rho {}^ - v_\tau \) and their charge conjugates have been studied. The following branching ratios have been measured; \(BR(\tau ^ - \to e^ - \bar v_e v_\tau ) = 18.6 \pm 0.8(stat.) \pm 0.6(sys.)\% ,\) , \(BR\left( {\tau ^ - \to \mu ^ - \bar v_\mu v_\tau } \right) = 17.4 \pm 0.7 \pm 0.6\% ,\) , Br(τ− → π− (K−)vτ)=11.9±0.7±0.7%, BR (τ− → ρ− vτ)= 22.4±0.8±1.3%, in good agreement with world averages. The measured electronic and muonic branching ratios lead to a measurement of the strong coupling constant, αs (mτ) = 0.26 −0.12+0.09. Extrapolating the αs value fromm τ tom Z yields αs (mZ) = 0.109 −0.028+0.012. The average polarizationP τ of taus produced in Z → τs τs decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes isP τ=−0.24±0.07. This value ofP τ gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of υτ/aτ = 0.12 ± 0.04, and hence a value of the effective electroweak mixing parameter sin2 θW(m Z2).

Physics Letters B, 1997

in e + e − annihilation at √ s=58 GeV H. Hanai et al.

International Journal of Modern Physics A, 2001

Preliminary results for the measurement of the 7 lepton polarization and forward-backward asymmetry near the Z" resonance using the OPAL detector are presented. The measurement is based on analysis of 7-_)ei&evr, ~+&,z+, ~+r(K)v,, 7+p1+ and T+alv, decays from a sample of 144,810 e+e-+T+T-candidates corresponding to an integrated luminosity of 151 pb-'. Assuming that the r lepton decays according to V-A theory, we measure the average 7 polarization near &=Mz to be Pr) = (-14.11+ 0.73 f 0.56)% and the 7 polarization forward-backward asymmetry to be A:: = (-10.52 f 0.76 f 0.25)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality and, within the context of the Standard Model, combine to produce sin' OFgt = 0.23170 f 0.00089. 0920-5632/01/$ see front matter 0 2001 Elsevier Science B.V. All rights reserved PII SO920-5632(Ol)Ol202-6