Measurement of the Z resonance parameters at LEP

Abstract

The properties of the Z resonance are measured from the analysis of 4.5 million Z decays into fermion pairs collected with the ALEPH detector at LEP. The data arc consistent with lepton universality. The resonance parameters are measured to be MZ = (91.1885 ± 0.0031) GeV/c2, ΓZ = (2.4951 ± 0.0043) GeV. σ0had = (41.559 ± 0.058) nb and, combining the three lepton flavours, Rℓ = 20.725 ± 0.039. The corresponding number of light neutrino species is Nv = 2.983± 0.013 and the strong coupling constant is αs (MZ) = 0.114±0.004±0.002QCD + 0.005 log10 [MH/150 GeV/c2]. The lepton pair forward-backward asymmetry is measured to be A0,ℓFB = 0.0173 ± 0.0016 from which the effective weak mixing angle is derived: sin2 θlepteff = 0.23089 ± 0.00089. The measurement of the leptonic width Γℓℓ = 84.02 ± 0.15 MeV leads to a determination of the effective ρ parameter ρlepteff = 1.0064 ± 0.0018. The data support the Standard Model and favour a light Higgs.