RENORMALIZATION SCHEME DEPENDENCE AND THE PROBLEM OF THEORETICAL UNCERTAINTIES IN NEXT-NEXT-TO-LEADING ORDER QCD PREDICTIONS

Renormalization scheme uncertainties in the next-next-to-leading order QCD predictions are discussed. To obtain an estimate of these uncerta inties it is proposed to compare predictions in all schemes that do no t have unnaturally large expansion coefficients. A concrete prescripti on for eliminating the unnatural schemes is given, based on the requir ement that large cancellations in the expression for the characteristi c renormalization scheme invariant should be avoided. As an example th e QCD corrections to the Bjorken sum rule are considered. The importan ce of the next-next-to-leading order corrections for a proper evaluati on of perturbative QCD predictions is emphasized.