Universality of multi-particle production in QCD at high energies
  By studying the color structure of multi-particle production processes in p+A-type (dilute-dense) collisions, we find that higher-point functions beyond typical dipoles and quadrupoles, e.g., sextupoles, octupoles, etc., naturally appear in the cross sections, but are explicitly suppressed in the large-N_c limit. We evaluate the sextupole in the McLerran-Venugopalan model and find that, in general, its analytical form cannot be written as combination of dipoles and quadrupoles. Within the Color Glass Condensate framework, we present a proof that in the large-N_c limit, all multi-particle production processes in the collision of a dilute system off a dense can, up to all orders in \alpha_s, be described in terms of only dipoles and quadrupoles. 
Fabio Dominguez, Cyrille Marquet, Anna M. Stasto, Bo-Wen Xiao
hep-ph nucl-th