Abstract: $J/\\psi$ production: intrinsic vs extrinsic transverse momentum and its influence on shadowing effects E. G. Ferreiro, F. Fleuret, J. P. Lansberg, A. Rakotozafindrabe

Gluon distribution (PDF) within a nucleon is known to be modified by the nuclear environnement, thus leading to shadowing effects on the $J/\\psi$ production through gluon fusion in p+A or A+A collisions. The nuclear ratio of the PDF is a function of $x$, the nucleon momentum fraction carried by the gluon, and $Q2$, the energy scale of the process. By taking the $J/\\psi$ transverse momentum~$p_T$ into account when evaluating $x_1$, $x_2$ and $Q2$, we investigate the influence of~$p_T$ on the shadowing. However, there is an on-going debate on the way the $J/\\psi$ acquires its~$p_T$: either initial gluons carry an intrinsic transverse momemtum and the latter is subsequently transferred to the $J/\\psi$ ($2 \\to 1$~process), or the $p_T$ comes from the emission of a recoiling outgoing gluon ($2 \\to 2$~process). On the average, initial gluons involved in these processes originate from different hence resulting in different shadowing effects!\ . We present our results in d+Au, Cu+Cu and Au+Au collisions at RHIC energy, obtained in the framework of a Glauber Monte Carlo model, and where the cold nuclear matter effects --~\{\\it shadowing and nuclear -- have been introduced~\\cite\{our-paper, For the $2 \\to 1$~process, is computed from the $J/\\psi$ rapidity and~$p_T$ spectra extracted from p+p data measured by the PHENIX experiment. For the $2 \\to 2$~process, we consider the partonic cross-section given which satisfactorily describes the aforementioned $J/\\psi$ spectrum down to $p_T\\sim0$.\ G.~Ferreiro, F.~Fleuret, A.~Rakotozafindrabe, arXiv:0801.4949 [hep-ph].\ J.~P.~Lansberg, F.~Fleuret, A.~Rakotozafindrabe, \\textit\{in and J.~P.~Lansberg, \\textit\{Phys. Rev. \{\\bf (2008) 032006.