An experimental approach to optimise and test perturbative QCD to O(αs2)

Production rates of multijet hadronic final states observed in e+e- annihilation are compared with recent QCD calculations using renormalisation group improved perturbation theory to O(αs2). The scale parameter Λsub(anti M)sub(anti S) and the renormalisation scale μ2 are adjusted in order to reproduce the observed 2-, 3- and 4-jet event production rates. Small scales of μ2≅0.002.E2cm provide a significantly better description of the overall 4-jet production rates and of 2- and 3-jet rates at small jet pair masses than calculations using μ2=E2cm. The adjusted value of Λsub(anti M)sub(anti S) depends on the choice of μ2. It decreases by a factor of two when going from μ2=E2cm to μ2=0.002.E2cm and results in Λsub(anti M)sub(anti S)=95 MeV±30 MeV for 0.001≤μ2/E2cm≤0.020. The predictions of an Abelian vector theory complete to O(αA2) are not compatible with the observed dynamics of jet production. The experimental evidence for the energy dependence of αs, obtained from 3-jet event production rates observed in the center of mass energy range between 22 GeV and 56 GeV, does not depend on the detailed choice of μ2. (orig.)