Abstract
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 (RpA) as well as elliptic flow (v2) for both J/ψ and ψ(2S) and compare them to experimental data from RHIC and the LHC. While the RpAs show an overall fair agreement with most of the data, the large v2 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 v2 suggests that its large observed values are unlikely to be due to the final-state collectivity of the fireball alone.
Highlights
JHEP03(2019)015 d-Au collisions at RHIC and more precisely established at the LHC, especially at backward rapidity where the light-hadron multiplicity is the highest
We study charmonium production in proton-nucleus (p-A) collisions focusing on final-state effects caused by the formation of an expanding medium
Show only the CNM effects, bounded by the anti-/shadowing obtained from EPS09-LO and EPS09-NLO calculations [47, 48] for both charmonia and open charm; as for the RHIC case, the centrality dependence of shadowing is mimicked by a nuclear absorption-type behavior, while for anti-shadowing we employ a parameterization of the pertinent lines shown in figure 3 of ref
Summary
The kinetic-rate equation approach developed in refs. [32, 33] is based on a spacemomentum integrated Boltzmann equation, dNΨ(τ ) dτ. To schematically account for the effects of quantum evolution in the early stages of the charmonium evolution, we utilize formation times, τΨform, for the different states (J/ψ, ψ(2S), χc(1P )) that are assumed to have a range of 1-2 fm to reflect uncertainties associated with their binding energies Their effect is rather small in semi-/central AA collisions, but becomes augmented in small systems due to shorter fireball lifetimes. Much like for different centralities in AA collisions, one can expect significant variations in the kinetic-freezeout temperature as a function of multiplicity in small systems: for a smaller total entropy the criterion that the mean-free-path is comparable to the fireball size (or inverse expansion rate) is reached at a larger particle density (or temperature). The cross section inputs will be specified in the respective sections
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