  We study multi-particle production with rapidity correlations in
proton-nucleus collisions at high energy in the Color Glass Condensate
framework. The high-energy evolution responsible for such correlations is
governed by a generalization of the JIMWLK equation describing the simultaneous
evolution of the strong nuclear color fields in the direct amplitude and the
complex conjugate amplitude. This functional equation can be used to derive
ordinary evolution equations for the cross-sections for particle production,
but the ensuing equations appear to be too complicated to be useful in
practice, including in the limit of a large number of colors Nc. We propose an
alternative formulation based on a Langevin process, which is valid for generic
Nc and is better suited for numerical implementations. For illustration, we
present the stochastic equations which govern two gluon production with
arbitrary rapidity separation.
