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 Joint CATHIE-INT mini-program: Quarkonium in Hot Media - from QCD to Experiment
 (June 16 - 26, 2009)

  Reported by Nora Brambilla, Dmitri Kharzeev, Peter Petreczky, Helmut Satz, Antonio Vairo, and Ramona Vogt
  Date posted August 3, 2009

The mini program "Quarkonium in Hot Media: from QCD to Experiment" was held at the INT in Seattle on June 16-26, 2009 with 32 participating researchers. The program focused on different topics relevant for quarkonium production in a hot medium, including the properties of quarkonium at high temperatures; the role of cold nuclear matter effects on quarkonium production in nucleus-nucleus collisions; and the interpretation of the available experimental data in heavy-ion collisions.

One of the aim of this program was to enhance the exchange of ideas between theorists studying quarkonia in a high-temperature environment using different approaches, e.g. ab-initio calculations (lattice QCD and pQCD) and phenomenological models (potential models, sum rules etc. The applicability of potential models with screening has been extensively discussed and scrutinized with an emphasis on the importance of thermal broadening of the quarkonium states. In particular, it has become clear that, in the weak-coupling limit, the dominant effect is thermal broadening rather than screening, as suggested by Matsui and Satz. There were several fruitful discussions on how to understand the temperature dependence of the Euclidean correlators calculated in lattice QCD in terms of effective theories (pNRQCD) and weak-coupling expansion in the short-distance regime. An understanding of these issues is crucial for quantitative results on quarkonium properties in the thermal deconfined medium.

It is important to understand the role of cold nuclear matter effects to interpret the quarkonium yields in heavy-ion collisions. In the past, cold nuclear matter effects were studied in terms of the nuclear absorption cross section σabs extracted from proton-nucleus (pA) data. The absorption was interpreted as resulting from interactions of the small QQ pair with nuclear fragments after the collision. Recently it has become clear, however, that there are other cold nuclear matter effects, including shadowing and initial-state energy loss. Therefore the analysis of cold nuclear matter effects in terms of σabs should be considered as only an effective description. The absorption cross section depends on kinematic variables such as √s, xF etc. Its value also strongly depends on the nuclear parton distribution functions (nPDF) used in the analysis. Clearly a more differential approach in studying cold nuclear matter effects should be adopted.

A further aim of the mini-program was the clarification of some outstanding issues, including separation of initial- and final-state nuclear matter effects. Several data driven analysis were presented at the meeting. In particular, the interplay of different effects was studied by H. Wöhri, C. Lourenco, R. Vogt and P. Faccioli during the workshop. One of the outcome was the determination of σabs using the newest nuclear shadowing parametrization, EPS09. The kinematics dependence of σabs was discussed by H. Wöhri and C. Lourenco. The question of which part of the cold nuclear matter effects extracted from the pA data should be used in the heavy-ion data analysis was addressed.

Figure 1: The nuclear modification factor for the J/ψ yield at RHIC and SPS as function of ετ.

The physical interpretation of the cold nuclear matter effects strongly depends on the quarkonium production mechanism which is not yet well understood. Therefore, the role of polarization in constraining the quarkonium production mechanism was discussed by J.-P. Lansberg, P. Faccioli and C. Lourenco.

Another significant outcome of the program was the development of a comparative measaure of anomalous J/ψ suppression (relative nuclear modification), taking cold nuclear matter effects into account, at both RHIC and the SPS. This new comparison arose from a collaborative effort during the workshop by A.D. Frawley, R. Granier de Cassagnac, A. Linden Levy, C. Lourenco, H, Satz, R. Vogt and H. Wöhri. In addition, R. Arnaldi and E. Scomparin from Torino University, members of the NA60 Collaboration measuring J/ψ at the SPS, remotely joined this effort. The suppression factor can be presented in a number of ways, including as a function of the number of nucleons participating in the collision, Npart, and the energy density, ε, of the resulting fireball. It was found that the most suitable variable for comparing of relative anomalous suppression at RHIC and the SPS is instead ετ, where τ is an effective thermalization time. In terms of ετ, while the suppression patterns are qualitatively similar, there is clearly more anomalous suppression at RHIC than at the SPS.