Measurement of cross sections and leptonic forward-backward asymmetries at the Z pole and determination of electroweak parameters

Abstract

We report on the measurement of the leptonic and hadronic cross sections and leptonic forward-backward asymmetries at the Z peak with the L3 detector at LEP. The total luminosity of 40.8 pb^-1 collected in the years 1990, 1991 and 1992 corresponds to 1.09·10⁶ hadronic and 0.98·10⁵ leptonic Z decays observed. These data allow us to determine the electroweak parameters. From the cross sections we derive the properties of the Z boson: M_Z=91195±9MeVΓ_Z=2494±10MeV Γ_had = 1748±10MeVΓ_e=83.49±0.46MeV assuming lepton universality. We obtain an invisible width of Γinv= 496.5±7.9 MeV which, in the Standard Model, corresponds to a number of light neutrino species of N v=2.981±0.050. Using also the three leptonic forward-backward asymmetries and the average tau polarization, we determine the effective vector and axial-vector coupling constants of the neutral eak current to charged leptons to be: g^-ev=-0.0378^+0.0045-e_-0.0042gA=-0.4998±0.0014. Within the framework of the Standard Model, and including our measurements of the Z→ bb^- forward-backward asymmetry and partial decay width, we derive an effective electroweak mixing angle of sin²θ^-_W=0.2326±0.0012. We obtain an estimate for the strong coupling constant, αS=0.142 ± 0.013 and for the top-quark mass,m _t =158 -40 ⁺³² ±19(Higgs) GeV, where the second error arises due to the uncertainty in the Higgs-boson mass.