[Paper Review] Calibrating the In-Medium Behavior of Quarkonia

Helmut Satz|arXiv (Cornell University)|Mar 14, 2013

High-Energy Particle Collisions Research3 references4 citations

TL;DR

This paper proposes a model-independent calibration method for quarkonium production in nuclear collisions by comparing $J/\psi$ yields to open charm production rates, using the ratio $R_{AA}(J/\psi)/R_{AA}(c\bar{c})$ as the survival probability. The key result is that at high $P_T$ at the LHC and RHIC, this ratio is unity, indicating no suppression or enhancement; however, at low $P_T$ at RHIC, $J/\psi$ suppression is observed due to reduced $c\bar{c}$ production, suggesting medium effects are kinematically dependent.

ABSTRACT

Quarkonium production has been considered as a tool to study the medium formed in high energy nuclear collisions, assuming that the formation of a hot and dense environment modifies the production pattern observed in elementary collisions. The basic features measured there are the relative fractions of hidden to open heavy flavor and the relative fractions of the different hidden heavy flavor states. Hence the essential question is if and how these quantities are modified in nuclear collisions. We show how the relevant data must be calibrated, i.e., what reference has to be used, in order to determine this in a model-independent way.

Motivation & Objective

• To resolve long-standing ambiguities in interpreting quarkonium suppression in nuclear collisions due to model-dependent parametrizations.
• To establish a reference frame for medium effects that is independent of theoretical assumptions about parton dynamics or binding mechanisms.
• To clarify whether observed $J/\psi$ suppression in heavy-ion collisions arises from medium effects or from modifications in $c\bar{c}$ production.
• To provide a robust, experimental observable—$J/\psi$ survival probability—based on measurable ratios of $J/\psi$ to open charm yields.

Proposed method

• Define the $J/\psi$ survival probability as the ratio $S_{J/\psi} = \left( N_{AA}(J/\psi)/N_{AA}(c\bar{c}) \right) / \left( N_{pp}(J/\psi)/N_{pp}(c\bar{c}) \right) $, which is model-independent.
• Use $R_{AA}$ ratios for $J/\psi$ and open charm ($c\bar{c}$) to compare yields in $AA$ and $pp$ collisions at the same centrality and kinematic conditions.
• Apply the color evaporation model to relate $c\bar{c}$ production to $J/\psi$ yield via a constant $g_{c\bar{c} \to J/\psi}$, assuming this fraction is energy-independent.
• Correlate $J/\psi$ and $c\bar{c}$ production rates in the same $P_T$ and rapidity bins to isolate medium effects from initial-state modifications.
• Utilize experimental data from LHC and RHIC (PHENIX, STAR) to test the method across different kinematic regimes.

Experimental results

Research questions

• RQ1 How can quarkonium production in nuclear collisions be calibrated to extract model-independent information on medium effects?
• RQ2 Is the observed $J/\psi$ suppression in heavy-ion collisions due to medium-induced dissociation or to modifications in $c\bar{c}$ production?
• RQ3 Does the $J/\psi$ survival probability remain constant across different kinematic regions, or does it depend on $P_T$ and rapidity?
• RQ4 To what extent do initial-state effects such as shadowing or energy loss affect the $J/\psi$ to open charm yield ratio?

Key findings

• At high $P_T$ in $pp$ and $AA$ collisions at the LHC, the ratio $R_{AA}(J/\psi)/R_{AA}(c\bar{c})$ is unity, indicating no suppression or enhancement of $J/\psi$ production relative to open charm.
• At low $P_T$ in $Cu-Cu$ collisions at RHIC, $R_{AA}(J/\psi)$ decreases strongly with centrality, while $R_{AA}(c\bar{c})$ remains near unity, indicating a suppression of $J/\psi$ production relative to open charm.
• The $J/\psi$ survival probability $S_{J/\psi}$ is unity at high $P_T$, confirming no medium-induced suppression, but drops to about 25% in the most central collisions at low $P_T$, indicating strong suppression.
• The observed suppression at low $P_T$ is likely due to reduced $c\bar{c}$ production rates in forward rapidity regions, not due to $J/\psi$ dissociation, as $c\bar{c}$ yield suppression is also seen in $pA$ and $AA$ collisions.
• The data from Fermilab $pA$ collisions show a suppression of open charm at forward rapidity, consistent with the kinematic dependence observed in $J/\psi$ suppression at RHIC.