A study of the reaction e+e− → μ+μ− around the Z0 pole

Abstract

Measurements of the cross section and forward-backward asymmetry for the reaction e+e− → μ+μ− using the DELPHI detector at LEP are presented. The data come from a scan around the Z0 peak at seven centre of mass energies, giving a sample of 3858 events in the polar angle region 22° < θ < 158°. From a fit to the cross section for 43° < θ < 137°, a polar angle region for which the absolute efficiency has been determined, the square root of the product of the Z0 → e+e− and Z0 → μ+μ− partial widths is determined to be (ΓeΓμ)12 = 85.0 ± 0.9(stat.) ± 0.8(syst.) MeV. From this measurement of the partial width, the value of the effective weak mixing angle is determined to be sin2(θw) = 0.2267 ± 0.0037. The ratio of the hadronic to muon pair partial widths is found to be Γh/Γμ = 19.89 ± 0.40(stat.) ± 0.19(syst.). The forward-backward asymmetry at the resonance peak energy ECMS = 91.22 GeV is found to be AFB = 0.028 ± 0.020(stat.) ± 0.005(syst.). From a combined fit to the cross section and forward-backward asymmetry data, the products of the electron and muon vector and axial-vector coupling constants are determined to be VeVμ = 0.0024 ± 0.0015(stat.) ± 0.0004(syst.) and AeAμ = 0.253 ± 0.003(stat.) ± 0.003 (syst.). The results are in good agreement with the expectations of the minimal standard model.