by A. Andronic & E. Scomparin . Published: 13 October 2012

PWG-DQ (Dileptons and Quarkonia) was represented at QM’12 by a plenary talk (E. Scomparin) and 3 parallel talks (R. Arnaldi, H. Yang, and I.C. Arsene), addressing the nuclear modification factor (RAA) and elliptic flow (v2 ) of J/ψ with the complete Pb-Pb data sample of ALICE. A first measurement, albeit statistics-limited, of ?(2S) was also presented. Several posters were addressing J/ψ and low-mass dilepton production in pp collisions. Even though some of the results were already ”previewed” at the Hard Probes conference in Cagliari in June, they generated a large interest at QM.

Figure 1: Centrality dependence of the nuclear modification factor and elliptic flow of inclusive J/? measured with ALICE in three rapidity ranges.

The J/ψ measurement down to pT=0 is an unique capability of ALICE, both for the Central Barrel and for the Muon Spectrometer. Already seen with the first ALICE Pb-Pb data [1], the inclusive J/ψ production is at LHC distinctly different than at RHIC. Less nuclear suppression (i.e. a larger RAA value) is seen at LHC both at forward rapidity and at mid- rapidity. The centrality dependence of the nuclear suppression factor in three rapidity ranges is shown in Figure 1, hinting to an enhanced J/? production from forward to mid-rapidity.

The ALICE measurement in the Muon Spectrometer benefits from a larger integrated luminosity collected with the dimuon trigger and allows a differential study of RAA and the measurement of elliptic flow. Our data, presented in Figure 2, shows an enhanced production at low-pT compared to high-pT, a trend which is, again, different than the one seen at RHIC. Our measurement of elliptic flow as a function of pT provides a tantalizing hint of finite J/ψ flow.

Figure 2a: Transverse momentum dependence of the nuclear modification factor for centrality classes 0-20% and 40-90% measured with ALICE (symbols).The hashed bands show, for a transport model calculation, the overall production, while the open ones indicate the QGP regeneration component.

Figure 2b: Transverse momentum dependence of elliptic flow for the centrality class 20-60% of inclusive J/? measured with ALICE

These observations support the picture of J/? melting in the QGP stage [2], followed by subsequent regeneration in QGP or at chemical freeze-out. Both the statistical hadronization [3] and transport [4, 5] models reproduce the data. Both models assume thermalization of charm quarks in QGP. Corroborating data and model calculations, we can conclude that J/? production is a probe of QGP as proposed in the seminal paper by Matsui and Satz [2]. Current transport model calculations [4, 5] predict that about half of the low-pT J/ψ yield in central Pb-Pb collisions at ?sNN = 2.76 TeV is produced by the recombination of charm quarks in QGP, while the rest is due to primordial production. Within the statistical hadronization model [3], the charmonium states become probes of the phase boundary between QGP and hadron phase. This extends with a heavy quark the family of quarks employed for the determination of the hadronization temperature (via the conjectured connection to the chemical freeze-out temperature extracted from fits of statistical model calculations to hadron abundances).

It is important to emphasize that the two competing models discussed above, despite providing similar predictions, have rather different underlying physics. While in the statistical model the hadronization is a process in which all quark flavors take part concurrently, in the kinetic model J/ψ survives as a hadron in the hot medium dominated by deconfined gluons and light quarks. In the statistical model all charmonium states are generated exclusively at hadronization, while in the kinetic model only at most half of the J/ψ yield originates from deconfined charm and anti-charm quarks. Discriminating the two pictures implies providing an answer to fundamental questions related to the fate of hadrons in a hot medium. High-quality J/? data in Pb-Pb collisions over broad ranges in pT and y and a precision measurement of other charmonium states (?(2S) and possibly also ?c) could allow this and are among the objectives of the ALICE Upgrade proposal. Meanwhile, soon our measurements in p-Pb collisions could allow better constraints to the models.


[1] ALICE collaboration, Phys. Rev. Lett. 109 (2012) 072301. [2] T. Matsui, H. Satz, Phys. Lett. B 178 (1986) 416. [3] P. Braun-Munzinger, J. Stachel, Phys. Lett. B 490 (2000) 196. [4] Y. Liu, Z. Qu, N. Xu, P. Zhuang, Phys. Lett. B 678 (2009) 7. [5] X. Zhao, R. Rapp, Nucl. Phys. A 859 (2011) 114.

Further reading

E. Scomparin, Results on quarkonia from ALICE
R. Arnaldi, Jpsi and psi (2S) production in Pb-Pb collisions with the ALICE Muon spectrometer at the LHC
H. Yang, Elliptic flow of J/psi at forward rapidity in Pb-Pb Collisions at 2.76 TeV with the ALICE experiment
I.C. Arsene, J/psi production at mid-rapidity in Pb-Pb collisions at 2.76 TeV