Subleading-Nc corrections in non-linear small-x evolution

We explore the subleading-Nc corrections to the large-Nc Balitsky–Kovchegov (BK) evolution equation by comparing its solution to that of the all-Nc Jalilian-Marian–Iancu–McLerran–Weigert–Leonidov–Kovner (JIMWLK) equation. In earlier simulations it was observed that the difference between the solutions of JIMWLK and BK is unusually small for a quark dipole scattering amplitude, of the order of 0.1%, which is two orders of magnitude smaller than the naively expected . In this paper we argue that this smallness is not accidental. We provide analytical arguments showing that saturation effects and correlator coincidence limits fixed by group theory constraints conspire with the particular structure of the dipole kernel to suppress subleading-Nc corrections reducing the difference between the solutions of JIMWLK and BK to 0.1%. We solve the JIMWLK equation with improved numerical accuracy and verify that the remaining 1/Nc corrections, while small, still manage to slow down the rapidity-dependence of JIMWLK evolution compared to that of BK. We demonstrate that a truncation of JIMWLK evolution in the form of a minimal Gaussian generalization of the BK equation captures some of the remaining 1/Nc contributions leading to an even better agreement with JIMWLK evolution. As the 1/Nc corrections to BK include multi-reggeon exchanges one may conclude that the net effect of multi-reggeon exchanges on the dipole amplitude is rather small.