The following article is from the September 2011 issue of the CERN Courier.
The ALICE collaboration has measured the nuclear modification (RAA) factor of J/? mesons down to transverse momentum (pT) equal to zero, in lead-lead collisions at the LHC. The 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.
The J/? RAA factor is larger at the LHC than that measured in gold-gold collisions at lower centre-of-mass energy by the PHENIX collaboration at the RHIC collider (Brookhaven National Laboratory, USA). No significant dependence with the centrality of the collision is observed, while the PHENIX experiment observed higher suppression in the most central collisions. These observations hint at recombination of charm and anti-charm quarks in the quark gluon plasma (QGP) formed in heavy ion collisions at LHC energies.
The first Pb-Pb collisions delivered by the LHC in November 2010 at unprecedented centre of mass energies are being exploited by the ALICE collaboration to characterize the QGP. The ALICE detector has been especially conceived for measurements in heavy-ion collisions and is able to study QGP via comprehensive measurements of hadron abundances and correlations as well as thermal photons.
At LHC energies, new mechanisms of charmonium production in the QGP could occur. Perturbative QCD calculations have predicted that a large amount of charm quarks, around 50 c-cbar pairs, is produced per central Pb-Pb collisions at ?sNN=2.76 TeV. The charm quarks will then coexist with the QGP during its dynamical evolution like Brownian particles. A number of dynamical transport models predict that charm and anti-charm quarks could combine in later stages leading to an enhancement of the charmonium production in the most central Pb-Pb collisions.
The ALICE apparatus detects charmonium down to pT=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 since the recombination of charm and anti-charm quark is expected to be the main production mechanism for charmonium at low p sub>T (pT<m). Different rapidity domains allow for studying QGP with different charm densities.
In the Quark Matter conference held in Annecy, France, in May 2011, the ALICE collaboration presented the measurement of J/? production in lead-lead collisions at 2.76 TeV. In particular, the RAA factor was studied as a function of collision centrality. The RAA factor appears to show very little dependence on centrality, a trend different to that observed at lower energies. The RAA factor for central and mid-central collisions is larger at the LHC compared to the RHIC.
CERNNuclear modification factor of the J/? in Pb-Pb collisions at 2.76 TeV measured by the ALICE collaboration.
Complementarily, the ATLAS and CMS collaborations have measured smaller J/? RAA factor at high pT (pT<6.5 GeV). These observations contrast with the expectations from the mechanism of colour-screening dissociation of charmonium in the QGP, and hint at recombination of charm and anti-charm quarks in the QGP as the main mechanism for J/? production in central lead-lead collisions at LHC energies. Analysis of J/? production as a function of the pT and rapidity are ongoing and will shed light on this topic very soon.
Further information can be found in the talk Quarkonium production measurements with the ALICE detector at the LHC.