Choosing the renormalization/factorization scale (QCD)

The basic issue is that all perturbative calculations involve a truncated expansion. For calculations to have predictive power, one must have some idea of the errors due to omission of uncalculated terms. In typical QCD calculations, the problem is compounded by the presence of an arbitrary scale parameter, mu . Without some knowledge of an appropriate range of values for the scale parameter, perturbative QCD calculations are without predictive power. Finally, a very prevalent subtraction scheme is minimal subtraction, the MS scheme, where the scale parameter has no manifest physical interpretation.