ALICE goes in search of charmonium in the quark–gluon plasma

The ALICE collaboration has measured the nuclear modification (R_AA) factor of J/Ψ mesons down to a transverse momentum (p_T) equal to zero, in lead–lead (Pb–Pb) collisions at √s_NN=2.76 TeV, delivered by the LHC in November 2010. The results, presented at the Quark Matter 2011 conference (Heavy ions in Annecy), hint at the recombination of charm and anticharm quarks in the quark–gluon plasma (QGP) formed in heavy-ion collisions at LHC energies.

The ALICE detector was conceived especially for measurements in heavy-ion collisions and is able to study QGP via comprehensive measurements of hadron abundances and correlations as well as of thermal photons. At LHC energies, new mechanisms of charmonium production in the QGP could occur. QCD calculations have predicted that a large number of charm quarks, around 50 c-c pairs, should be produced per central lead–lead collision at √s_NN=2.76 TeV. These charm quarks would then coexist with the QGP during its dynamical evolution, like Brownian particles. A number of dynamical transport models predict that c and c quarks could then combine in later stages, leading to an enhancement of charmonium production in the most central Pb–Pb collisions.

ALICE detects charmonium down to p_T=0 in two different rapidity domains: |y|<0.9 in the dielectron channel and 2.5<y<4 in the dimuon channel. The detection at low transverse momentum is crucial because the recombination of the charm and anticharm quarks is expected to be the main production mechanism for charmonium at low p_T (p_T<3 GeV/i>c). The different rapidity domains allow for the study of QGP with different charm densities.

In particular, ALICE has studied the nuclear modification factor, R_AA, as a function of collision centrality for J/Ψ mesons. R_AA is defined as the ratio of the yield measured in nucleus–nucleus (AA) collisions 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. The results from ALICE indicate that the J/Ψ R_AA factor appears to show little dependence on centrality (see figure), a trend that is different from that observed at lower energies. The factor for central and mid-central collisions is larger at the LHC than was measured at lower centre-of-mass energy in gold–gold collisions in the PHENIX experiment at the Relativistic Heavy Ion Collider, Brookhaven. In complementary studies, the ATLAS and CMS collaborations at the LHC have measured a smaller J/ΨR_AA factor at high p_T (p_T>6.5 GeV/c).

These observations contrast with expectations from the dissociation of charmonium through the mechanism of colour-screening in the QGP. They hint instead at the recombination of charm and anticharm quarks in the QGP as the main mechanism for J/Ψ production in central Pb–Pb collisions at LHC energies. ALICE’s analysis of J/Ψ production as a function of the p_T and rapidity continues and should shed light on the topic soon.