Measurement of electroweak parameters from hadronic and leptonic decays of the Z⁰

Abstract

We have studied the reactions e⁺ e⁻ GeV. A total luminosity of 5.5 pb^{− 1}, corresponding to approximately 115000 hadronic and 10000 leptonic Z₀ decays, has been recorded with the L3 detector. From a simultaneous fit to all of our measured cross section data, we obtain assuming lepton universality: M_z = 91.181 ± 0.010 ± 0.02 (LEP) GeV, G_z = 2501 ± 17 MeV, G_had = 1742 ± 19 MeV, G_t = 83.6 ± 0.8 MeV. If we do not assume lepton universality, we obtain for the partial decay widths of the Z₀ into e⁺ e⁻ Ge = 83.3 ± 1.1 MeV, G_m = 84.5 ± 2.0 MeV, G_t = 84.0 ± 2.7 MeV. From the measured ratio of the invisible and the leptonic decay widths of the Z₀, we determine the number of light neutrino species to be N_v =3.05±0.10. We include our measurements of the forward-backward asymmetry for the leptonic channels in a fit to determine the vector and axial-vector neutral current coupling constants of charged leptons to the Z₀. We obtain [(g̅)]_V = − 0.046 ^+0.015_−0.012 and [(g̅)]A = − 0.500 ± 0.003. In the framework of the standard model, we estimate the top quark mass to bem t =193−69⁺⁵² ±16 (Higgs) GeV, and we derive a value for the weak mixing angle of sin²sin² θ_W = 0.2315 ± 0.0025.