Determination of Z0 resonance parameters and couplings from its hadronic and leptonic decays

From measurements of the cross sections for e+e−→ hadrons and the cross sections and forward-backward charge-asymmetries for ee−→e+e−, μ+μ− and π+π− at several centre-of-mass energies around the Z0 pole with the DELPHI apparatus, using approximately 150 000 hadronic and leptonic events from 1989 and 1990, one determines the following Z0 parameters: the mass and total width MZ = 91.177 ± 0.022 GeV, ΓZ = 2.465 ± 0.020 GeV, the hadronic and leptonic partial widths Γh = 1.726 ± 0.019 GeV, Γl = 83.4 ± 0.8 MeV, the invisible width Γinv = 488 ± 17 MeV, the ratio of hadronic over leptonic partial widths RZ = 20.70 ± 0.29 and the Born level hadronic peak cross section σ0 = 41.84±0.45 nb. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above (Γl = 83.7 ± 0.8rmMeV). From these results the number of light neutrino species is determined to be Nv = 2.94 ±0.10. The individual leptonic widths obtained are: Γe = 82.4±_1.2 MeV, Γu = 86.9±2.1 MeV and Γτ = 82.7 ± 2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are: Vl2 = 0.0003±0.0010 andAl2 = 0.2508±0.0027. These values correspond to the electroweak parameters: ϱeff = 1.003 ± 0.011 and sin2θWeff = 0.241 ± 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin2θWMS = 0.2338 ± 0.0027. All these values are in good agreement with the predictions of the MSM. Fits yield 43

Keywords:

• Quantum electrodynamics
• Lepton
• Weinberg angle
• Electroweak interaction
• Hadron
• Electron–positron annihilation
• Resonance
• Neutrino
• Particle physics
• Physics
• Standard Model
• Nuclear physics

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