Abstract
Measurements of the tau lepton polarization and forward-backward polarization asymmetry near the Z resonance using the OPAL detector are described. The measurements are based on analyses of tau -> e nu_e nu_tau, tau -> mu nu_mu nu_tau, tau -> pi nu_tau, tau -> rho nu_tau and tau -> a1 nu_tau decays from a sample of 144810 e+e- -> tau+ tau- candidates corresponding to an integrated luminosity of 151 pb-1. Assuming that the tau lepton decays according to V-A theory, we measure the average tau polarization near Ecm = MZ to be <Ptau> = (-14.10 +/- 0.73 +/- 0.55)% and the tau polarization forward-backward asymmetry to be Afb = (-10.55 +/- 0.76 +/- 0.25)%, where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: Atau = 0.1456 +/- 0.0076 +/- 0.0057, Ae = 0.1454 +/- 0.0108 +/- 0.0036. These measurements are consistent with the hypothesis of lepton universality and combine to give Al = 0.1455 +/- 0.0073. Within the context of the Standard Model this combined result corresponds to sin^2(theta)(lept,effective) = 0.23172 +/- 0.00092. Combing these results with those from the other OPAL neutral current measurements yields a value of sin^2(theta)(lept,effective) = 0.23211 +/- 0.00068.
Highlights
Parity violation in the weak neutral current results in a polarization of final-state fermion−antifermion pairs produced in Z0 decay with the τ lepton being the only fundamental fermion whose polarization is experimentally accessible using the detectors at the LEP e+e− collider
In the Standard Model the effective electroweak mixing angle is determined from these couplings, the polarization measurements yield a value of sin2 θeleffpt
This paper describes a measurement of Pτ and AFpoBl using the full data sample collected with the OPAL detector at LEP during the period 1990-1995 which corresponds to an integrated luminosity of 151 pb−1
Summary
Parity violation in the weak neutral current results in a polarization of final-state fermion−antifermion pairs produced in Z0 decay with the τ lepton being the only fundamental fermion whose polarization is experimentally accessible using the detectors at the LEP e+e− collider. Once the small effects of photon exchange, photon-Z0 interference and photonic radiative corrections are taken into account, the measurement of Pτ is directly related to the ratio of the vector to axial vector coupling constants for τ leptons and that of AFpoBl to the ratio for electrons [2] These measurements test the hypothesis of lepton universality in the neutral current. 0e.n9e0rigsyu(s√eds)inatthtehaenpaleyaskis.ofMtho√setZo0f the selected events resonance and the (88%) were remainder, referred to as ‘off-peak data’, were recorded at several distinct s values within 3 GeV above and below the peak These new results supersede the measurements reported in Reference [6], which were based on an analysis of a 1990-1994 OPAL data sample that was restricted to the central region of the detector: | cos θτ−| < 0.68. The details of the specific variables used for each channel are described
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