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

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 Mz=91.182±0.026 (exp.) ±0.030 (beam) GeV, Γz=2.541±0.056 GeV and σhad0=41.4±0.8 nb. The partial widths for the hadronic and leptonic channels are Γhad=1804±44 MeV, Γe+e−=82.1±3.4 MeV, Γμ+μ−=87.9±6.0 MeV and Γτ+τ−=86.1±5.6 MeV, in good agreement with the standard model. On the basis of the average leptonic width Γℓ+ℓ−=83.9±2.2 MeV, the effective weak mixing angle is found to be sin2θw(Mz)=0.231±0.008. Usin g the partial widths calculated in the standard model, the number of light neutrino families is Nν=3.01±0.15 (exp.)±0.05 (theor.)

Search for excited leptons in Z0 decay

We have looked for evidence of excited lepton production in Z0 decay observed in the ALEPH detector at LEP. We find no ℓℓγγ events and set new limits at 95% CL on ℓ∗ masses at 44.6, 44.6 and 41.2 GeV/c2 for respectively. Observed events in the ℓℓγ channels are consistent with radiative effects and we set limits for the first time on the coupling for ℓ∗ masses in the range up to 86 GeV/c2