Infrared instability from nonlinear QCD evolution

Using the Balitsky–Kovchegov (BK) equation as an explicit example, we show that nonlinear QCD evolution leads to an instability in the propagation toward the infrared of the gluon transverse momentum distribution, if one starts with a state with an infrared cut-off. This effect takes the mathematical form of rapidly moving traveling wave solutions of the BK equation, which we investigate by numerical simulations. These traveling wave solutions are different from those governing the transition to saturation, which propagate towards the ultraviolet. The infrared wave speed, formally infinite for the leading order QCD kernel, is determined by higher order corrections. This mechanism could play a rôle in the rapid decrease of the mean free path in the Color Glass Condensate scenario for heavy ion collisions.