Electroweak parameters of the z0 resonance and the standard model

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The reaction e+e- → γγ(γ) at Z0 energies

The total and differential cross-sections for the reaction e+e- → γγ(γ) are measured at centre of mass energies around 91 GeV using an integrated luminosity of 4.7 pb-1. The aggreement with QED prediction is good. Consequently there is no evidence for non-standard channels which would have the same experimental signature. The lower limits on the QED cuttoff parameters are Λ+ > 113 GeV and Λ- > 95 GeV. An upper limit on the effective coupling between a possible excited electron and the gamma is derived. At 95% confidence level the branching ratios for Z0 decay into π0γ, ηψ and γγγ are below 1.5 × 10-4, 2.8 × 10-4 and 1.4 × 10-4 respectively. © 1991

DETERMINATION OF ALPHA-S IN 2ND-ORDER QCD FROM HADRONIC-Z DECAYS

Distributions of event shape variables obtained from 120600 hadronic Z decays measured with the DELPHI detector are compared to the predictions of QCD based event generators. Values of the strong coupling constant alpha(s) are derived as a function of the renormalization scale from a quantitative analysis of eight hadronic distributions. The final result, alpha(s) (M(Z)) = 0.113 +/- 0.007, is based on second order perturbation theory and uses two hadronization corrections, one computed with a parton shower model and the other with a QCD matrix element model