Soft and hard pomerons

TL;DR

This paper discusses the application of Regge theory and perturbative QCD to small-x structure-function data, highlighting the challenges in reconciling these approaches due to the need for resummation techniques at low x.

Contribution

It emphasizes the necessity of resummation in perturbative QCD at small x and the current lack of a proper method to perform this resummation.

Findings

Regge theory describes small-x data well from Q^2=0 upwards

Perturbative QCD requires resummation at small x

Current methods do not adequately handle resummation at low x

Abstract

Regge theory provides an excellent description of small-$x$ structure-function data from $Q^2=0$ up to the highest available values. The large-$Q^2$ data should also be described by perturbative QCD: the two descriptions must agree in the region where they overlap. However, at present there is a serious lack in our understanding of how to apply perturbative QCD at small $x$. The usual lowest-order or next-to-lowest order expansion is not valid, at least not until $Q^2$ becomes much larger than is usually assumed; a resummation is necessary, but as yet we do not know how to do this resummation.