The Number of Light Neutrino Types

The most precise measurements of the number of light neutrino types, N ν , come from studies of Z production in e + e − collisions. The invisible partial width, Γ inv , is determined by subtracting the measured visible partial widths, corresponding to Z decays into quarks and charged leptons, from the total Z width. The invisible width is assumed to be due to N ν light neutrino species each contributing the neutrino partial width Γ ν as given by the Standard Model. In order to reduce the model dependence, the Standard Model value for the ratio of the neutrino to charged leptonic partial widths, (Γ ν /Γ) SM = 1.991±0.001, is used instead of (Γ ν) SM to determine the number of light neutrino types: N ν = Γ inv Γ Γ Γ ν SM. (1) The combined result from the four LEP experiments is N ν = 2.984 ± 0.008 [1]. In the past, when only small samples of Z decays had been recorded by the LEP experiments and by the Mark II at SLC, the uncertainty in N ν was reduced by using Standard Model fits to the measured hadronic cross sections at several center-of-mass energies near the Z resonance. Since this method is much more dependent on the Standard Model, the approach described above is favored. Before the advent of the SLC and LEP, limits on the number of neutrino generations were placed by experiments at lower-energy e + e − colliders by measuring the cross section of the process e + e − → ννγ. The ASP, CELLO, MAC, MARK J, and VENUS experiments observed a total of 3.9 events above background [2], leading to a 95% CL limit of N ν < 4.8. This process has a much larger cross section at center-of-mass energies near the Z mass and has been measured at LEP by the ALEPH, DELPHI, L3, and OPAL experiments [3]. These experiments have observed several thousand such events, and CITATION: C. Amsler et al.