(Non)thermal aspects of charmonium production and a new look at J/ψ suppression

Abstract

To investigate a recent proposal that J/ψ production in ultra-relativistic nuclear collisions is of thermal origin we have reanalyzed the data from the NA38/50 collaboration within a thermal model including charm. Comparison of the calculated with measured yields demonstrates the non-thermal origin of hidden charm production at SPS energy. However, the ratio ψ ′ /(J/ψ) exhibits, in central nucleus-nucleus collisions, thermal features which lead us to a new interpretation of open charm and charmonium production at SPS energy. Implications for RHIC and LHC energy measurements will be discussed.The suppression of J/ψ mesons (compared to what is expected from hard scattering models) was early on predicted [1] to be a signature for color deconfinement. Data for Sinduced collisions exhibited a significant suppression but systematic studies soon revealed that such suppression exists already in p-nucleus collisions and is due to the absorption in (normal) nuclear matter of a pre-resonant state consisting, e.g., of a color singlet ccg state that is formed on the way towards J/ψ production. The situation has been summarized in [2][3][4].The newest data for Pb+Pb collisions now exhibit clear evidence for anomalous absorption beyond the standard nuclear absorption expected for such systems. The most recent results are summarized in [5][6][7]. The observed anomalous suppression is not explained in conventional models where the charmonia are broken up by interactions with co-movers as discussed in [5]. For a discussion of the present status of J/ψ suppression and its understanding in terms of phenomenological models see [8].However, it was recently conjectured [9,10] that J/ψ production is of thermal origin and exhibits no direct connection to color deconfinement. Since the charmonia are heavy mesons with masses much larger than any conceivable temperature, thermal production would be a big surprize. On the other hand, substantial evidence now exists [11][12][13][14][15][16][17] that hadron production (other than charmonia) in ultra-relativistic nuclear collisions proceeds through a state of chemical equilibrium near or at the phase boundary between hadron

Ultrarelativistic Nucleus–Nucleus Collisions and the Quark–Gluon Plasma

“…Taking into account the measured dependence of the relative yield of ψ ′ to J/ψ on centrality in Pb-Pb collisions at SPS energy, seen in Fig. 14, the model appears appropriate for N part >100, for which the ratio approaches the thermal value [69,68].…”

The Muon Spectrometer of the ALICE experiment

“…These heavy quarks will be embedded in a matter mainly formed by gluons and light quarks (u,d,s) and could behave like Brownian particles [ 11], therefore their transverse momentum (p T ) and rapidity distributions will probe the properties of the surrounding matter. In addition, the study of the heavy quark bound states will allow to probe the medium via the Debye screening [ 12], the gluon dissociation of quarkonia [ 13], statistical recombination of heavy quarks [ 14] and/or statistical hadronization [ 15]. Additionally, high p T heavy quarks will also probe the surrounding hadronic medium via heavy-quark matter interaction [ 18].…”

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“…At LHC energies, in addition to the originally proposed J/ψ suppression due to colour screening, the production of J/ψ due to (re-)combination of charm quarks is suggested as an additional mechanism in two theoretical approaches: the statistical hadronisation at the confinement phase boundary [2,3] and transport models [4,5,6]. The latter assume destruction and production of charmonium states predominantly during the deconfined stage of the system evolution in addition to the primordial production.…”