The classical field created in early stages of high energy nucleus–nucleus collisions

We show that a special choice of light-cone gauge can greatly simplify the calculation of the classical color field created in the initial stages of nucleus–nucleus collisions. Within this gauge, we can in particular construct explicitly the conserved color current and calculate exactly the gauge field immediately after the collision. This field is used as a boundary condition in an iterative solution of the Yang–Mills equations in the forward light-cone. In leading order, which corresponds to a linearization of the Yang–Mills equation in the forward light-cone, we obtain a simple formula for the spectrum of gluons produced in nucleus–nucleus collisions. This formula reproduces exactly the known formula for proton–nucleus collisions, where kt-factorization is recovered, while the latter property apparently breaks down in the case of nucleus–nucleus collisions.