A study of jet production rates and a test of QCD on the Z0 resonance

Relative production rates of multijet hadronic final states of Z0 boson decays, observed in e+e− annihilation around 91 GeV centre of mass energy, are presented. The data can be well described by analytic O(αs2) QCD calculations and by QCD shower model calaculations with parameters as determined at lower energies. A first judgement of ΛMS and of the renormalization scale μ2 in O(αs2) QCD results in values similar to those obtained in the continuum of e+e− annihilations. Significant scaling violations are observed when the 3-jet fractions are compared to the corresponding results from smaller centre of mass energies. They can be interpreted as being entirely due tot the energy dependence of αs, as proposed by the nonabelian nature of QCD, The possibility of an energy independent coupling constant can be excluded with a significance of 5.7 standard deviations.