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rpp2010-rev-light-neutrino-types - 1 THE NUMBER OF LIGHT...

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– 1– THE NUMBER OF LIGHT NEUTRINO TYPES FROM COLLIDER EXPERIMENTS Revised March 2008 by D. Karlen (University of Victoria and TRIUMF). 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].
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