High Energy Evolution of Dipole Gluon Distribution Beyond Eikonal Approximation

Abstract

At high energy, the dipole gluon distribution is described at eikonal order by the Wilson-line dipole correlator, whose high-energy evolution is governed by the Balitsky-Kovchegov equation. Going beyond the eikonal approximation, we identify the subeikonal operator representing the dipole gluon distribution, consisting of a Wilson line with a single insertion of the light-cone chromoelectric field, and derive its high-energy evolution equation in the large-$N_c$ limit under the single-logarithmic approximation. The resulting nonlinear evolution is coupled to the Wilson-line dipole correlator and incorporates gluon saturation effects. In the dilute limit, the subeikonal distribution grows as a power-law of the energy with exponent $α_s N_c/2π$, less than one fifth of the eikonal value, while in the saturation regime it obeys the same Levin--Tuchin law as the eikonal correlator. These results constitute the first closed nonlinear evolution equation for the dipole gluon distribution at subeikonal order, a step toward precision small-$x$ phenomenology and toward connecting small-$x$ physics with the moderate-$x$ dynamics.