We argue that the collinear factorization of the fragmentation functions in high energy hadron and nuclei collisions breaks down at transverse momenta $k_T\lesssim Q_s/g$ due to high parton densities in the colliding hadrons and/or nuclei. We calculate, at next-to-leading order in projectile parton density and to all orders in target parton density, the double-inclusive cross-section for production of a pair of gluons in the scalar $J^{PC}=0^{++}$ channel. Using the low energy theorems of QCD we find the inclusive cross-section for $\pi$-meson production.

• Received 26 February 2007

DOI:https://doi.org/10.1103/PhysRevD.75.074022