Small-x Dipole Evolution Beyond the Large-N_c Limit

TL;DR

This work addresses the challenge of small-$x$ QCD evolution by extending the Mueller dipole model beyond the large-$N_c$ limit with colour-suppressed interactions and saturation effects, implemented in a frame-independent Monte Carlo that enforces energy--momentum conservation. The core innovation is the dipole swing, a color-recoupling mechanism that generates pomeron loops and multi-dipole interactions, thereby achieving a more complete description of high-energy scattering. The method reproduces gamma*$p$ cross sections from HERA and total $pp$ cross sections from ISR to Tevatron, indicating improved frame independence and a practical path to studying exclusive final states. With only a few tunable parameters, this framework provides a scalable tool for exploring high-energy QCD evolution—including saturation and fluctuations—relevant for LHC phenomenology and beyond.

Abstract

We present a method to include colour-suppressed effects in the Mueller dipole picture. The model consistently includes saturation effects both in the evolution of dipoles and in the interactions of dipoles with a target in a frame-independent way. When implemented in a Monte Carlo simulation together with our previous model of energy--momentum conservation and a simple dipole description of initial state protons and virtual photons, the model is able to reproduce to a satisfactory degree both the gamma*-p cross sections as measured at HERA as well as the total p-p cross section all the way from ISR energies to the Tevatron and beyond.