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

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: MZ=91 195±9 MeV, ΓZ=2 494±10 MeV, Γhad=1748±10 MeV, Γl=83.49±0.46 MeV, 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 Nν=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 weak current to charged leptons to be: anti gVl=-0.0378-0.0042+0.0045, anti gAl=-0.4998±0.0014. Within the framework of the Standard Model, and including our measurements of the Z→b anti b forward-backward asymmetry and partial decay width, we derive an effective electroweak mixing angle of sin2 anti θW=0.2326±0.0012. We obtain an estimate for the strong coupling constant, αS=0.142±0.013, and for the top-quark mass, mt=1584032±19(Higgs) GeV, where the second error arises due to the uncertainty in the Higgs-boson mass. (orig.)