A precise determination of the number of families with light neutrinos and of the Z boson partial widths

Abstract

More extensive and precise results are reported on the parameters of Z decay. On the basis of 20 000 Z decays collected with the ALEPH detector at LEP we find M z=91.182±0.026 (exp.) ±0.030 (beam) GeV, Γ z=2.541±0.056 GeV and σ had 0=41.4±0.8 nb. The partial widths for the hadronic and leptonic channels are Γ had=1804±44 MeV, Γ e +e − =82.1±3.4 MeV, Γ μ + μ − =87.9±6.0 MeV and Γ τ + τ − =86.1±5.6 MeV, in good agreement with the standard model. On the basis of the average leptonic width Γ ℓ +ℓ − =83.9±2.2 MeV, the effective weak mixing angle is found to be sin 2 θ w( M z)=0.231±0.008. Usin g the partial widths calculated in the standard model, the number of light neutrino families is N ν =3.01±0.15 (exp.)±0.05 (theor.).