The casuality and/or energy-momentum conservation constraints on QCD   amplitudes in small x regime
  The causality and/or the energy-momentum constraints on the amplitudes of high energy processes are generalized to QCD. The constraints imply that the energetic parton may experience at most one inelastic collision only and that the number of the constituents in the light cone wave function of the projectile is increasing with the collision energy and the atomic number. 
B. Blok and L. Frankfurt