# Investigation into the limits of perturbation theory at low Q^2 using HERA deep inelastic scattering data

## TL;DR

This study analyzes HERA deep inelastic scattering data across a wide Q^2 range, testing the applicability of perturbative QCD and Regge models, and exploring the transition between these regimes.

## Contribution

It provides a comprehensive phenomenological analysis of HERA data, extending the understanding of QCD limits at low Q^2 and introducing a new fit with the ALLM parameterisation.

## Key findings

- Perturbative QCD describes data down to a few GeV^2.
- Regge formalism fits data up to Q^2 ≈ 0.65 GeV^2.
- A new fit with ALLM parameterisation successfully describes the entire dataset.

## Abstract

A phenomenological study of the final combined HERA data on inclusive deep inelastic scattering (DIS) has been performed. The data are presented and investigated for a kinematic range extending from values of the four-momentum transfer, $Q^2$, above 10$^4$ GeV$^2$ down to the lowest values observable at HERA of $Q^2$ = 0.045 GeV$^2$ and Bjorken $x$, $x_{\rm Bj}$ = 6 $\cdot$ 10$^{-7}$. The data are well described by fits based on perturbative quantum chromodynamics (QCD) using collinear factorisation and evolution of the parton densities encompassed in the DGLAP formalism from the highest $Q^2$ down to $Q^2$ of a few GeV$^2$. The Regge formalism can describe the data up to $Q^2 \approx$ 0.65 GeV$^2$. The complete data set can be described by a new fit using the ALLM parameterisation. The region between the Regge and the perturbative QCD regimes is of particular interest.