Evidence for charmonium generation at the phase boundary in ultra-relativistic nuclear coll.pdf

a r X i v : n u c l - t h / 0 7 0 1 0 7 9 v 2 1 7 J u l 2 0 0 7 Evidence for charmonium generation at the phase boundary in ultra-relativistic nuclear collisions A. Andronic a, P. Braun-Munzinger a,b, K. Redlich c, J. Stachel d aGesellschaft fu?r Schwerionenforschung, GSI, D-64291 Darmstadt, Germany bTechnical University Darmstadt, D-64289 Darmstadt, Germany cInstitute of Theoretical Physics, University of Wroc law, PL-50204 Wroc law, Poland dPhysikalisches Institut der Universita?t Heidelberg, D-69120 Heidelberg, Germany Abstract We investigate the transition from suppression to enhancement of J/ψ mesons produced in ultra-relativistic nuclear collisions in the framework of the statisti- cal hadronization model. The calculations are confronted with the most recent data from the RHIC accelerator. This comparison yields first direct evidence for genera- tion of J/ψ mesons at the phase boundary. Based on the success of this approach we make specific predictions for LHC energy. Twenty years ago the J/ψ meson was proposed [1] as a crucial observable for the di- agnosis of the Quark-Gluon Plasma (QGP) produced in ultra-relativistic nucleus-nucleus collisions. Since then this probe has been the focus of intense experimental and theoretical efforts. In the course of these, it was recently realized [2] that even complete J/ψ melting in the QGP via Debye screening [1] could lead to large J/ψ yields due to production at the phase boundary (hadronization). Predictions using the corresponding statistical hadronization model (SHM) [2,3] met with initial success when compared to data. In a different approach, based on the kinetic model [4], the J/ψ production is described via dy- namical melting and (re)generation over the whole temporal evolution of the QGP [5,6,7]. Transport model calculations [8,9] were also performed. We note that, in general, the gen- eration can only take place effectively if the charm quarks reach thermal (not chemical) equilibrium and are free to trav