Dynamical NNLO parton distributions

TL;DR

This paper determines NNLO parton distributions of the nucleon using recent DIS and hadronic data, comparing dynamical and standard approaches, and finds that current data cannot yet distinguish the small NNLO effects, with the dynamical approach yielding a slightly lower .

Contribution

It introduces a dynamical NNLO approach to determine parton distributions from low-scale inputs, providing more precise predictions and a different  value compared to standard methods.

Findings

Dynamical NNLO distributions are stable at Q^2=2-3 GeV^2.

Current DIS data cannot distinguish NNLO effects from NLO.

Dynamical approach yields (M_Z^2)=0.1124 b 0.0020.

Abstract

Utilizing recent DIS measurements (\sigma_r, F_{2,3,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 (Q_0^2 < 1 GeV^2). These are compared with `standard' NNLO distributions generated from positive input distributions at some fixed and higher resolution scale (Q_0^2 > 1 GeV^2). Although the NNLO corrections imply in both approaches an improved value of \chi^2, typically \chi^2_{NNLO} \simeq 0.9 \chi^2_{NLO}, 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 F_L(x,Q^2) become perturbatively stable already at Q^2 = 2-3 GeV^2 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 1\sigma uncertainty bands. Within the dynamical approach we obtain \alpha_s(M_Z^2)=0.1124 \pm 0.0020, whereas the somewhat less constrained `standard' fit gives \alpha_s(M_Z^2)=0.1158 \pm 0.0035.