Measurements are presented of the cross section ratios R ℓ = σ ℓ ( e + e − →ℓ + ℓ − ) σ h ( e + e − →hadrons) for ℓ=e, μ and τ using data taken from a scan around the Z 0 . The results are R e =(5.09± o .32±0.18)%, R μ =(0.46±0.35±0.17)% and R τ =(4.72±0.38±0.29)% where, for the ratio R e , the t -channel contribution has been subtracted. These results are consistent with the hypothesis of lepton universality and test this hypothesis at the energy scale s ∼8300 GeV 2 . The absolute cross sections σ ℓ (e + e − →ℓ + ℓ − ) have also been measured. From the cross sections the leptonic partial widths Γ e =(83.2±3.0±2.4) MeV, (Γ e Γ μ ) 1 2 =(84.6±3.0±2.4) MeV and (Γ e Γ τ ) 1 2 =(82.6±3.3±3.2) MeV have been extracted. Assuming lepton universality the ratio Γ ℓ Γ h =(4.89±0.20±0.12) × 10 −2 w was obtained, together with Γ ℓ =(83.6±1.8±2.2) MeV. The number of light neutrino species is determined to be N v =3.12±0.24±0.25. Al the data are consistent with the predictions of the standard model.
E+ E- final state is t-channel subtracted.
No t-channel subtraction. Statistical errors only.
Statistical errors only.
None
No description provided.
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No description provided.
The pure QED reaction e + e − → γγ has been studied at centre of mass energies around the mass of the Z 0 boson using data recorded by the OPAL detector at LEP. The results are in good agreement with the QED prediction. Lower limits on the cutoff parameters of the modified electron propagator are found to be Λ + >89 GeV and Λ. The lower limit on the mass of an excited electron is 82 GeV assuming the coupling constant λ =1. Upper limits on the branching ratios of Z 0 → γγ , Z 0 → π 0 γ and Z 0 → ηγ are set at 3.7×10 −4 , 3.9×10 −4 and 5.8×10 −4 respectively. Two events from the reaction e + e − → γγγ have been observed, consistent with the QED prediction. An upper limit on the branching ratio of Z 0 → γγγ is set at 2.8×10 −4 . All the limits are given at 95% confidence level.
No description provided.
Data read from graph.
We report on a measurement of the mass of the Z 0 boson, its total width, and its partial decay widths into hadrons and leptons. On the basis of 25 801 hadronic decays and 1999 decays into electrons, muons or taus, selected over eleven energy points between 88.28 GeV and 95.04 GeV, we obtain from a combined fit to hadrons and leptons a mass of M z =91.154±0.021 (exp)±0.030 (LEP) GeV, and a total width of Γ z =2.536±0.045 GeV. The errors on M z have been separated into the experimental error and the uncertainty due to the LEP beam energy. The measured leptonic partial widths are Γ ee =81.2±2.6 MeV, Γ μμ =82.6± 5.8 MeV, and Γ ττ =85.7±7.1 MeV, consistent with lepton universality. From a fit assuming lepton universality we obtain Γ ℓ + ℓ − = 81.9±2.0 MeV. The hadronic partial width is Γ had =1838±46 MeV. From the measured total and partial widths a model independent value for the invisible width is calculated to be Γ inv =453±44 MeV. The errors quoted include both the statistical and the systematic uncertainties.
Errors are statistical and point to point systematic luminosity error of 1 pct.
Measured values of e+ e- --> e+ e- cross section.
Corrected cross section. Corrections are for t-channel effects and loss of acollinear events near the boundary of the acceptance.
Hadronic decays of Z 0 bosons are studied in the Delphi detector. Global event variables and singel particles inclusive distributions are compared with QCD-based predictions. The mean charged multiplicity is found to be 20.6±1.0 (stat+syst). The mean values of the sphericity, aplanarity, thrust, minor value, p in T and p out T are compared with values found at lower energy e + e − colliders.
Corrected Sphericity distribution. Statistical errors only.
Corrected Aplanarity distribution. Statistical errors only.
Corrected Q3-Q2 distribution. Statistical errors only.
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.).
Penetrating charged particle track selection.
Calorimeter selection.
Average cross section.
We have made a precise measurement of the cross section for e + e − →Z 0 →hadrons with the L3 detector at LEP, covering the s range from 88.28 to 95.04 GeV. From a fit to the Z 0 mass, total width, and the hadronic cross section to be M Z 0 =91.160 ± 0.024 (experiment) ±0.030(LEP) GeV, Γ Z 0 =2.539±0.054 GeV, and σ h ( M Z 0 )=29.5±0.7 nb. We also used the fit to the Z 0 peak cross section and the width todetermine Γ invisible =0.548±0.029 GeV, which corresponds to 3.29±0.17 species of light neutrinos. The possibility of four or more neutrino flavors is thus ruled out at the 4σ confidence level.
No description provided.
Total hadronic cross section.
We have measured inclusive distributions for charged particles in hadronic decays of the Z boson. The variables chosen for study were charged-particle multiplicity, scaled momentum, and momenta transverse to the sphericity axes. The distributions have been corrected for detector effects and are compared with data from e+e− annihilation at lower energies and with the predictions of several QCD-based models. The data are in reasonable agreement with expectations.
Mean corrected charged particle multiplicity.
Corrected charged particle X distributions. Errors are statistical and systematic combined.
Corrected charged particle PTIN distributions. Errors are statistical and systematic combined.
We report on a measurement of the processes e + e − →e + e − , e + e − → μ + μ − , and e + e − → τ + τ − near the Z 0 pole. On the basis of 163 e + e − , 101 μ + μ − and 87 τ + τ − events we obtain Γ ee =89±4±4 MeV, Γ μμ =85±9±6 MeV and Γ ττ =87±10±8 MeV, compatible with the standard model. Combining these with our previous results on hadronic Z 0 decays, we find a hadronic width Γ had =1787±81±90 MeV and an invisible width Γ inv =552±85±71 MeV.
Statistical errors only.
Statistical errors only.
We have measured both the rates and the forward-backward asymmetry of ℓ + ℓ − from Z 0 →ℓ + ℓ − (where ℓ= μ , τ ) with the L3 detector. We obtained Γ ℓℓ =88±4±3 MeV and the vector neutral current coupling constant, g v =0.00±0.07 and the axial vector neutral current coupling constant, g A =−0.515±0.015.
No description provided.
No description provided.
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