An experimental approach to optimise and test perturbative QCD toO(α ² _s )

Abstract

Production rates of multijet hadronic final states observed ine ⁺ e ⁻ annihilation are compared with recent QCD calculations using renormalisation group improved perturbation theory toO(α ² _s ). The scale parameter$\Lambda _{\overline {MS} }$ and the renormalisation scaleμ ² are adjusted in order to reproduce the observed 2-,3-, and 4-jet event production rates. Small scales ofμ ²≈0.002·E ²_cm 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 usingμ ²=E ²_cm . The adjusted value of$\Lambda _{\overline {MS} }$ depends on the choice ofμ ². It decreases by a factor of two when going fromμ ²=E ²_cm toμ ²=0.002·E ²_cm and results in$\Lambda _{\overline {MS} } = 95 MeV \pm 30 MeV$ MeV±30 MeV for 0.001≦μ ²/E ²_cm ≦0.020. The predictions of an Abelian vector theory complete toO(α ² _s ) are not compatible with the observed dynamics of jet production. The experimental evidence for the energy dependence of α _s , 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 ofμ ².