Study of Muon Pair Production at Lep.

The cross sections and forward-backward charge asymmetries of the process e^+e^ - to Z^0(gamma) to mu^+ mu^-(gamma) have been measured based on 3371 events collected by the L3 detector at LEP. The data are found to be consistent with lepton universality. Together with the cross section measurements of Z^0 to hadrons, the partial decay width of Z ^0 to mu^+mu^-(gamma) is found to be 83.9 +/- 0.9(stat) +/- 0.6(syst) MeV while the vector and axial vector coupling constants are: g_ V = -0.043_sp{ -0.015}{+0.026} and g_ A = -0.5006 +/- 0.0035. In the framework of the minimal Standard Model, a value of 0.2311_sp{-0.0032} {+0.0034} is obtained for the effective weak mixing angle sin^2{|theta _{rm w}}. From the combined fits to the data of Z ^0 decays to hadrons e^+e ^-(gamma), mu^+ mu^-(gamma), and tau ^+tau^-(gamma), the following precise values of Z^0 parameters are determined: M_ Z = 91.176 +/- 0.010 +/- 0.020(LEP) GeV, Gamma_ Z = 2.502 +/- 0.017 GeV, Gamma_ {had} = 1.726 +/- 0.022 GeV, Gamma_{ll } = 84.0 +/- 0.9 MeV, g_ V = -0.050 _sp{-0.012}{+0.016}, g_ A = -0.5002 +/- 0.0029, sin^2{ |theta_{rm w}} = 0.2305_sp{-0.0027}{+0.0028 }, in good agreement with the Standard Model. The number of light neutrino species is N_nu = 3.07 +/- 0.08. All these measurements are in favor of top mass: M_ T = 121_sp{ -37}{+32} +/- 20 GeV. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).