The \( J/\psi \) production mechanism in elementary proton-proton collisions is not yet fully understood and \( J/\psi \) nuclear attenuation in pA collisions, parameterized by \( R_{pA} \equiv \sigma_{pA}/A\sigma_{pp} \) could be one additional observable to disentangle the different models. Understanding J/psi suppression in pA collisions is also a prerequisite before using \( J/\psi \) production in AA collisions as a signal for the quark-gluon plasma.
The physics of \( J/\psi \) nuclear suppression depends crucially on the kinematics considered. When the \( J/\psi \) hadronization time \( t_{hadro} \) is much smaller than the nuclear size \( L\), nuclear suppression arises from the nuclear absorption cross section \( \sigma_{abs} \) of the bound state fully formed in the nucleus. When \( t_{hadro} \gg L \), what propagates through the nucleus is a compact color octet ccbar pair, and \( \sigma_{abs} \) becomes an irrelevant parameter. In this kinematical domain, \( J/\psi \) nuclear suppression data at different collision energies can be explained in a simple model [1, 2] based on partonic energy loss and depending on a single free parameter \( \hat{q} \). In the figure, the E866 data [3] for p-W (left) and p-Fe (right) collisions are compared to energy loss model (see Ref. [1]).
The presence of a large (medium-induced) parton energy loss associated with \( J/\psi \) production can be understood as follows. A large \( x_F \), the gluon radiation required to neutralize the color of the ccbar pair must be soft by energy conservation, and the pair thus remains color octet for a long time. Consequently, at large \( x_F \) the partonic subprocess in \( J/\psi \) hadroproduction looks like small angle scattering of a color charge. The radiation spectrum is similar to the Bethe-Heitler spectrum (more exactly its non-Abelian analog) of an asymptotic charge, and depends on the transverse momentum qperp transferred to the charge. The typical qperp is more important in pA collisions than in pp collisions due to the transverse momentum broadening \( \Delta q_\perp^2 = \hat{q} L \). The resulting medium-induced radiation spectrum is similar to the Bethe-Heitler spectrum with \( q_\perp \) replaced by \( \Delta q_\perp \) [2], and the average energy loss is proportional to the energy of the quarkonium, \( \Delta E \propto E \) . This is the main reason for the rapid increase of the suppression, i.e., decrease of \( R_{pA} \) when \( x_F \) increases.
In summary, the study of Ref. [1] suggests that parton energy loss is a dominant effect in \( J/\psi \) suppression in pA collisions. This scenario will soon be tested in pA collisions at the LHC.
References
[1] F. Arleo and S. Peigné, arXiv:1204.4609 [hep-ph].
[2] F. Arleo, S. Peigné and T. Sami, Phys. Rev. D 83 (2011) 114036.
[3] M. J. Leitch et al. [FNAL E866/NuSea Collaboration], Phys. Rev. Lett. 84 (2000) 3256.