The number of neutrinos and the Z line shape

Thanks to a precise calibration and monitoring of the LEP beam energy the Z resonance line shape has been precisely measured, hence permitting accurate determinations of the Z mass and width. A large effort was spent by the experiments for the measurement of the LEP luminosity, enabling the number of neutrino types to be precisely determined. The result, Nν=2.9841±0.0083, demonstrates the existence of only 3 neutrino species. To cite this article: A. Blondel, C. R. Physique 3 (2002) 1155–1164.