Determination of the number of light neutrino species from single photon production at LEP

A determination of the number of light neutrino families performed by measuring the cross section of single photon production in e+e− collision near the Z resonance is reported. From an integrated luminosity of 100pb−1, collected during the years 1991–94, we have observed 2091 single photon candidates with an energy above 1 GeV in the polar angular region 45°<θγ<135°. From a maximum likelihood fit to the single photon cross section, the Z decay width into invisible particles is measured to be Γinv=498±12(stat)±12(sys)MeV. Using the Standard Model couplings of neutrinos to the Z, the number of light neutrino species is determined to be Nν=2.98±0.07(stat)±0.07(sys).