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

Abstract

We have studied the reactions e+e-→hadrons, e+e-, μ+ μ- and τ+ τ-, in the energy range 88.2 {Mathematical expression} 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: {Mathematical expression} If we do not assume lepton universality, we obtain for the partial decay widths of the Z0 into e+e- μ+ μ- and τ+ τ-: {Mathematical expression} 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 Nv=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 {Mathematical expression} and {Mathematical expression}. In the framework of the Standard Model, we estimate the top quark mass to be mt=193-69+52±16 (Higgs) GeV, and we derive a value for the weak mixing angle of sin2θW=1-(MW/MZ)2=0.222 ± 0.008, corresponding to an effective weak mixing angle of {Mathematical expression}. © 1991 Springer-Verlag.