Impact parameter dependent colour glass condensate dipole model

TL;DR

This paper enhances the colour glass condensate dipole model by including impact parameter dependence, fitting HERA data well with a free anomalous dimension, and compares its predictions to various structure functions and scattering processes.

Contribution

The paper introduces an impact parameter dependent version of the CGC dipole model with a free anomalous dimension, improving data fit and providing insights into saturation scales and process discriminators.

Findings

Optimal gamma_s = 0.46 close to BK equation solutions

Impact parameter saturation scale generally less than 0.5 GeV^2

Model less effective for observables sensitive to small dipoles

Abstract

We show that the colour glass condensate dipole model of Iancu, Itakura and Munier, improved to include the impact parameter dependence, gives a good fit to the total gamma* p cross section measured at HERA if the anomalous dimension at the saturation scale, gamma_s, is treated as a free parameter. We find that the optimum value of gamma_s = 0.46 is close to the value determined from numerical solution of the Balitsky-Kovchegov equation. The impact parameter dependent saturation scale is generally less than 0.5 GeV^2 in the HERA kinematic regime for the most relevant impact parameters b ~ 2-3 GeV^{-1}. We compare predictions of the model to data on the longitudinal and heavy flavour structure functions, exclusive diffractive vector meson production and deeply virtual Compton scattering at HERA. The model is found to be deficient for observables sensitive to moderately small dipole sizes, where an alternative model with explicit DGLAP evolution performs better. The energy dependence of exclusive diffractive processes is shown to provide an important discriminator between different dipole model cross sections.