Breakdown of kT-factorization and J/Psi production in dA collisions

Abstract

In spite of the sweeping coherence effects in high energy hadron and nuclei collisions, $k_T$-factorization can be recovered for the inclusive gluon production in $pA$ collisions at the leading logarithmic order. In open charm production at RHIC $k_T$-factorization holds numerically with about 10-20% accuracy. This allows to extrapolate the cold nuclear matter effect observed in light and charm meson production in $dA$ collisions to that in $AA$ ones. Unlike the open charm, the breakdown of factorization in $J/ψ$ production is severe. Indeed, already at the lowest order in gluon density the main contribution to the inclusive cross section is proportional to the square of gluon density in the nucleus. As a consequence, one cannot infer the cold nuclear matter effect on $J/ψ$ production in $AA$ collisions from that in $dA$. We present the calculation of $J/ψ$ multiplicity in the framework of the CGC (color glass condensate)/saturation and show that it agrees with the experimental data.