Dynamical NNLO parton distributions

Abstract

Utilizing recent DIS measurements (σr, F2,3,L) and data on hadronic dilepton production we determine at NNLO (3-loop) of QCD the dynamical parton distributions of the nucleon generated radiatively from valencelike positive input distributions at an optimally chosen low resolution scale (Q02 < 1 GeV2). These are compared with `standard' NNLO distributions generated from positive input distributions at some fixed and higher resolution scale (Q02 > 1 GeV2). Although the NNLO corrections imply in both approaches an improved value of 2, typically 2NNLO 0.9 2NLO, present DIS data are still not sufficiently accurate to distinguish between NLO results and the minute NNLO effects of a few percent, despite of the fact that the dynamical NNLO uncertainties are somewhat smaller than the NLO ones and both are, as expected, smaller than those of their `standard' counterparts. The dynamical predictions for FL(x,Q2) become perturbatively stable already at Q2 = 2-3 GeV2 where precision measurements could even delineate NNLO effects in the very small-x region. This is in contrast to the common `standard' approach but NNLO/NLO differences are here less distinguishable due to the much larger 1σ uncertainty bands. Within the dynamical approach we obtain αs(MZ2)=0.1124 0.0020, whereas the somewhat less constrained `standard' fit gives αs(MZ2)=0.1158 0.0035.