Open Heavy Flavour

Quarkonium

Quarkonium was proposed as a probe of the QCD matter formed in relativistic heavy-ion collisions more than two decades ago. A familiar prediction, quarkonium suppression due to color-screening of the heavy-quark potential in deconfined QCD matter [1], has been experimentally searched for at the SPS and RHIC heavy-ion facilities. A less familiar prediction was also suggested at the same time: quarkonium enhancement due to recombination of charm and anti-charm quarks, if there is a significant density of charm quarks created in the initial stages of the collisions [2].

The NA50 experiment at SPS reported the observation of J/? and ?’ suppression in central heavy-ion collisions [3]. This observation was also reported for proton- induced reactions [4]. Quarkonium absorption in cold nuclear matter (CNM) has been hypothesized as the mechanism responsible for quarkonium suppression at mid-rapidity in pA collisions. A model based on Glauber theory, has been developed to estimate the magnitude of quarkonium suppression in central heavy-ion collisions due to quarkonium absorption in CNM. The magnitude of J/? suppression due to dissociation in hot QCD matter was then measured to be about 40% (which means that 60% of the initial produced J/? survives the hot QCD matter formed in central Pb-Pb collisions at SPS). One has to consider that the measured J/? production consists of prompt-J/? (? 65%) and decay-J/? from higher resonances like ?c and ??. The observed J/? suppression would be compatible with the suppression of these resonances. The NA60 experiment measured J/? production in pA collisions at the same CM collision energy as the heavy-ion measurements. A new evaluation of the quarkonium absorption in CNM was performed and a first attempt to consider the parton shadowing was addressed. The outcome of this analysis is that only about 20-30% of the suppression in the most central Pb-Pb collisions at SPS energies is due to dissociation in hot QCD matter [5].

The PHENIX experiment at RHIC has reported the observation of J/? suppression in central Au-Au collisions at 200 GeV? (10 times higher than the maximum energy in the CM at SPS) [6]. Deuteron-gold collisions have been utilized to measure CNM effects at RHIC energies [7]. As of today, only parton-shadowing and quarkonium- absorption mechanisms have been considered and the d-Au J/? results are not fully understood. As a consequence, J/? suppression due to dissociation in hot QCD matter is roughly estimated to be 40-80% in central Au-Au collisions at RHIC energies. This result would suggest higher suppression than that observed at SPS. In addition, the large rapidity coverage of the PHENIX experiment enables measurements at large rapidity, showing that the suppression is larger than that observed at mid-rapidity. This is a very intriguing experimental observation, and it has not yet been understood whether its origin is due predominantly to hot or cold nuclear matter effects. Finally, the STAR experiment has measured a smaller suppression at high transverse momentum (pT ? 5
GeV?/c) at mid-rapidity [8] although the experimental errors remain large.

The LHC collider has opened a new energy regime for the study of quarkonium in heavy-ion collisions. At these energies (15 times higher than the maximum energy in the CM at RHIC, and 150 times higher than that at SPS), on average one J/? particle is expected to be produced in every central Pb-Pb collision, together with about 50-100 cc ? quark pairs. As suggested in 1988 [2], under these conditions the charm quark yield per unit of rapidity could be large enough to enhance the charmonium production in later phases of the hot QCD-matter dynamical evolution, in particular when the energy density is low enough to enable the charmonium bound state to be formed [9, 23]. At LHC, J/? is abundantly produced, allowing for detailed studies of its production, such as azimuthal asymmetry, polarization, and RAA and their dependence on rapidity and transverse momentum. The nuclear modification factor RAA is defined as the ratio of the yield measured in nucleus-nucleus (AA) to that expected on the basis of the proton- proton yield scaled by the number of binary nucleon-nucleon collisions in the nucleus- nucleus reaction. Bottomonium resonances are being studied at RHIC and LHC and are complementary to the charmonium experimental observations at RHIC and at LHC. In particular, the bottom rapidity density at LHC is expected to be similar to that of charm at RHIC, and color-screening of the ?(2S) resonance should be similar to that of the J/?. The first results on the J/? RAA at LHC down to pT=0 will be presented
in this talk. More detailed description of this analysis can be found in [10].

The baseline of quarkonium studies in heavy-ion collisions at LHC, as in the previous studies at lower energies, requires the study of quarkonium production in proton-proton and proton-nucleus collisions. The LHC provides huge proton-proton luminosities and should provide pPb collisions in the future. Quarkonium studies in proton-proton collisions at this new energy regime might allow further insight to understand its production mechanisms. Furthermore, it has been argued that high- multiplicity pp collisions at LHC could lead to the formation of high energy density QCD matter, similar to heavy ions collisions [12]. Indeed, the charged particle multiplicity reached in pp collisions at the LHC is similar to that measured in semi-peripheral Cu-Cu at 200 GeV? [13, 14].

Open questions:
- Cold nuclear matter effect at SPS, RHIC and LCH energies;
- Study of J/psi and psi' in heavy ion collisions at LHC energies
- Study of upsilon in heavy ion collisions at RHIC and LHC energies
- Study of chi_c production in heavy ion collisions
- ...


[1] T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986).
[2] B. Svetitsky, Phys. Rev. D37, 2484 (1988).
[3] NA50 coll. (B. Alessandro et al.), Eur. Phys. J. C39, 335 (2005); Eur. Phys. J. C49, 559 (2007).
[4] NA50 coll. (B. Alessandro et al.,), Eur. Phys. J. C48, 329, (2006).
[5] R. Arnaldi for the NA60 coll., Nucl. Phys. A830, 345c (2009), arXiv:0907.5004v2.
[6] PHENIX coll. (A. Adare et al.), Phys. Rev. Lett. 98, 232301 (2007), nucl-ex/0611020;
arXiv:1103.6269v1 (2011).
[7] PHENIX coll. (A. Adare et al.), arXiv:1010.1246v1 (2010).
[8] H. Masui and Z. Tang for the STAR coll., in these proceedings.
[9] P. Braun-Munzinger and J. Stachel, Phys. Lett. B490, 196 (2000); A. Andronic et al., Phys. Lett.
B571, 36 (2003); A. Andronic et al., Phys. Lett. B652, 259 (2007).
[12] K. Werner et al.,Phys. Rev. C83, 044915 (2011), arXiv:1010.0400v1; K. Werner et al., Phys. Rev.
Lett. 106, 122004 (2011), arXiv:1011.0375v2.
[13] ALICE coll. (K. Aamodt et al.), Eur. Phys. Jour. C68 345 (2010). [14] PHOBOS coll. (B. Alver et al.), Phys. Rev. C 83, 024913 (2011).