QCD predictions for hadronic final states in e +e − annihilation

Abstract

We defend the applicability of asymptotically free QCD perturbation theory to the description of multi-jet hadronic final states and investigate its predictions for the process e +e − → hadrons both in the hadron “continuum” and in heavy Q Q resonances. We describe the distribution of the hadron energy flow in terms of variables (thrust, spherocity, acoplanarity) chosen to eliminate the sensitivity of the data and of the QCD predictions both to non-perturbative effects related to confinement, and to infra-red divergences which arise in perturbation theory. We present arguments that such analyses should be considered as genuine tests of perturbative QCD. We propose definitions of two- and three-jet final states which can be used to exploit maximally the perturbative QCD predictions. We show that particularly strong tests of the gluonic degrees of freedom are provided by jet distributions in the hadronic decays of heavy Q Q resonances and by the jet-photon angular correlations in sequential decays through even charge conjugation states. Predictions for the hadron energy distributions near new flavour thresholds are also discussed.