Measurement of Z^0 decays to hadrons, and a precise determination of the number of neutrino species

We have made a precise measurement of the cross section for e^+e^-->Z^0->hadrons with the L3 detector at LEP, covering the s range from 88.28 to 95.04 GeV. From a fit to the Z^0 mass, total width, and the hadronic cross section to be M"Z^0=91.160 +/- 0.024 (experiment) +/-0.030(LEP) GeV,...
Ausführliche Beschreibung

We have made a precise measurement of the cross section for e^+e^-->Z^0->hadrons with the L3 detector at LEP, covering the s range from 88.28 to 95.04 GeV. From a fit to the Z^0 mass, total width, and the hadronic cross section to be M"Z^0=91.160 +/- 0.024 (experiment) +/-0.030(LEP) GeV, Γ"Z^0=2.539+/-0.054 GeV, and σ"h(M"Z^0)=29.5+/-0.7 nb. We also used the fit to the Z^0 peak cross section and the width todetermine Γ"i"n"v"i"s"i"b"l"e=0.548+/-0.029 GeV, which corresponds to 3.29+/-0.17 species of light neutrinos. The possibility of four or more neutrino flavors is thus ruled out at the 4σ confidence level. Ausführliche Beschreibung