HERA data and DGLAP evolution: theory and phenomenology

TL;DR

This paper critically analyzes HERA data to assess deviations from NLO DGLAP evolution, finding that while geometric scaling aligns with DGLAP, deviations at low x and Q^2 suggest the need for small-x resummation.

Contribution

It provides a detailed comparison of HERA data with DGLAP predictions, highlighting regions where small-x resummation improves the theoretical description.

Findings

Geometric scaling of HERA data is consistent with DGLAP.

Deviations at low x and Q^2 are not due to NNLO terms.

Deviations are compatible with small-x resummation.

Abstract

We examine critically the evidence for deviations from next-to-leading order perturbative DGLAP evolution in HERA data. We briefly review the status of perturbative small-x resummation and of global determinations of parton distributions. We show that the geometric scaling properties of HERA data are consistent with DGLAP evolution, which is also strongly supported by the double asymptotic scaling properties of the data. However, backward--evolution of parton distributions into the low x, low Q^2 region does show evidence of deviations between the observed behaviour and the next-to-leading order predictions. These deviations cannot be explained by missing next-to-next-to-leading order perturbative terms, and are consistent with perturbative small-x resummation.