Two-particle correlations in high-energy collisions and the gluon four-point function

We derive the rapidity evolution equation for the gluon four-point function in the dilute regime and at small x from the JIMWLK functional equation. We show that beyond leading order in Nc the mean field (Gaussian) approximation where the four-point function is factorized into a product of two-point functions is violated. We calculate these factorization breaking terms and show that they contribute at leading order in Nc to correlations of two produced gluons as a function of their relative rapidity and azimuthal angle, for generic (rather than back-to-back) angles. Such two-particle correlations have been studied experimentally at the BNL-RHIC collider and could be scrutinized also for $pp$ (and, in the future, also $AA$) collisions at the CERN-LHC accelerator.