When JIMWLK evolution really matters: the example of incoherent diffraction

TL;DR

This paper demonstrates that the Gaussian Approximation fails for certain multi-gluon correlators in high energy scattering, especially in incoherent diffraction, where the full JIMWLK evolution predicts larger cross sections.

Contribution

It provides analytical and numerical evidence that the Gaussian Approximation is inadequate for four-gluon exchange correlators in specific high energy scattering processes.

Findings

Gaussian Approximation underestimates cross sections for four-gluon correlators.

Discrepancies between JIMWLK and GA are significant in weak scattering regimes.

JIMWLK predicts larger cross sections than GA across various kinematic regimes.

Abstract

We consider high energy scattering in the effective theory of the Color Glass Condensate. The most convenient degrees of freedom are Wilson lines encoding multiple gluon exchanges, whose evolution with energy follows the JIMWLK equation. Instead of using the latter, very often one resorts to a Gaussian Approximation (GA), which is known to be remarkably accurate in describing a wide class of multi-gluon correlators whose expansion in the weak scattering limit starts with an exchange of only two gluons. Here we demonstrate, both analytically and numerically, that such an approximation is not valid for correlators which start with an exchange of four gluons. As a main example, we focus on incoherent diffraction in photon-nucleus collisions and we show that the discrepancy between the JIMWLK and the GA results is driven by weak scattering and further persists in the regime where unitarity corrections begin to become important. The JIMWLK calculation leads to cross sections which are systematically larger in all kinematic regimes of interest.