Classical initial conditions in high energy nucleus-nucleus collisions
  An iterative proceedure is proposed to compute the classical gauge field produced in the collision of two heavy nuclei at high energy. The leading order is obtained by linearizing the Yang-Mills equations in the light-cone gauge, and provides a simple formula for gluon production in nucleus-nucleus collisions. At this order $k_t-$factorization breaks down. 
Yacine Mehtar-Tani