In-medium charmonium production in proton-nucleus collisions

We study charmonium production in proton-nucleus (p-A) collisions focusing on final-state effects caused by the formation of an expanding medium. Toward this end we utilize a rate equation approach within a fireball model as previously employed for a wide range of heavy-ion collisions, adapted to the small systems in p-A collisions. The initial geometry of the fireball is taken from a Monte-Carlo event generator where initial anisotropies are caused by fluctuations. We calculate the centrality and transverse-momentum dependent nuclear modification factor (R-pA) as well as elliptic flow (v(2)) for both J/ and (2S) and compare them to experimental data from RHIC and the LHC. While the R(pA)s show an overall fair agreement with most of the data, the large v(2) values observed in p-Pb collisions at the LHC cannot be accounted for in our approach. While the former finding generally supports the formation of a near thermalized QCD medium in small systems, the discrepancy in the v(2) suggests that its large observed values are unlikely to be due to the final-state collectivity of the fireball alone.