[NT/RBRC seminar] Eikonal Expansion for Classical Gluon Fields

by Mr Alexander DeBrizzi (University of Connecticut)

Friday 13 Nov 2026, 11:00 → 13:00 US/Eastern

CFNS library (https://bnl.zoomgov.com/j/1614715193?pwd=WkwxODVWdzZzb29zQnZRVGp3VTBDQT09)

The eikonal approximation is both a ubiquitous and natural approximation in high-energy physics. This approximation is applied, for example, in the McLerran-Venugopalan (MV) model and Color-Glass-Condensate (CGC) framework, which underlie many low-$x$ physics developments. The validity of this approximation requires that subeikonal terms are higher-order corrections to the leading eikonal behavior, and this has been assumed wherever the approximation has been applied. However, in this talk, we show that this is not correct and that a naive, complete expansion in eikonality is not a valid perturbative treatment for classical gauge fields. Specifically, we demonstrate that subeikonal fields make a leading contribution to the equations of motion for fields at a lower order in eikonality due to divergences associated with the fields at high momentum. Nevertheless, we find that the validity of the eikonal approximation can be restored if we separate the classical gauge field into low- and high-momentum modes, whereupon the color current in the equations of motion for the eikonal moments of the field are renormalized by effects of the interaction between the two modes. Additionally, we see that even for strong fields, in the framework of low-$x$ physics, the equation for the leading eikonal behavior is linear. We conclude that we can preserve the eikonal approximation, so as long as we understand correctly what in fact the color current in this approximation represents. preprint: arXiv:2608.22536