A precision measurement of the number of neutrino species

Abstract

We have measured the cross section for e⁺e⁻→hadrons over the center of mass energy range of the Z⁰ peak, from 88.22 to 95.03 GeV. We determine the Z⁰ mass M_z=91.164±0.013 (experiment) ±0.030 (LEP) GeV. Within the framework of the standard model we determine the invisible width, γ_invisible=0.502±0.018 GeV, and the number of light neutrino species, N_ν=3.01±0.11. We exclude the existence of a supersymmetric scalar neutrino having a mass less than 31.4 GeV, at the 95% confidence level. We performed a model independent combined fit to the e⁺e⁻→hadrons and e⁺e⁻→μ⁺μ⁻ data to determine total width, leptonic width and hadronic width of the Z⁰.