by Roberta Arnaldi. Published: 16 June 2017

Roberta Arnaldi, of INFN-Torino, Italy, provides a summary of the seminar she gave at CERN last May on the new results of quarkonium analyses performed by ALICE, using p-Pb and Pb-Pb collisions data from LHC Run 2.

Fig 1: J/y RAA as a function of centrality measured by the PHENIX experiment (at RHICH energies) and by ALICE at √sNN = 2.76 and 5.02 TeV.

 

A quarkonium is a bound state of a heavy quark and its own antiquark. The study of these states, in particular their suppression or enhanced production, can provide important information on the quark-gluon plasma (QGP). The high colour density achieved in ultra-relativistic nucleus-nucleus collisions is, in fact, expected to screen the binding energy between quarks, inducing a suppression of quarkonium states. Since the various quarkonium resonances are characterized by different binding energies, this suppression happens in “sequential” order, from the melting of the more loosely bound states, at lower temperatures, to that of the tightly bound ones when higher temperatures are reached.

Ever since quarkonia were suggested as QGP signatures for the first time in 1986, the production of charmonium and bottomonium (a bound-state of c-c- and b-b- quarks, respectively) has been investigated at all heavy-ion accelerator facilities. The suppression of the J/y meson (the ground state of charmonium) was first observed at the SPS in the 1990, at a centre-of-mass energy (√sNN) of 17 GeV. The observation was followed, at the beginning of the new century, by RHIC’s measurements at √sNN = 200 GeV and already from the first results it was obvious that a scenario based just on quarkonium melting by colour screening was too simplistic. Many other effects, in fact, have to be taken into account in the interpretation of the results.

First of all, as the energy of the collisions increases, the number of c and c- and quarks in the medium increases too, leading to the possibility of forming quarkonia by quark recombination. The most striking evidence of the role played by this mechanism is provided by the ALICE results on J/y production, at low transverse momentum, in Pb-Pb collisions at √sNN = 2.76 TeV. In spite of the higher energy density reached at LHC, the J/y production is less suppressed with respect to RHIC energies; this is measured through the nuclear modification factor (RAA), shown in Fig.1, where the quarkonium yields in Pb-Pb are compared to the yields in pp, scaled by the number of binary collisions.  Recent results from Pb-Pb collisions at √sNN  = 5.02 TeV confirm the observation, with a much increased precision. Furthermore, J/y formed by recombination should inherit the flow acquired by charm quarks in the medium. A complementary measurement of a non-zero J/y elliptic flow, now performed with very good accuracy by ALICE at √sNN  = 5.02 TeV, confirms the importance of this additional production mechanism.

Further insight on quarkonium behaviour can be inferred from the study of bottomonium states. Due to the still limited amount of b and  quarks, even at LHC energies, these resonances are hardly affected by recombination, contrarily to what happens to charmonium. Their suppression, observed in Pb-Pb collisions both at √sNN = 2.76 and 5.02 TeV reflects, therefore, the melting itself. ALICE has measured a stronger suppression for the Υ(2S) state, with respect to the more tightly bound Υ(1S), an observation suggestive of the sequential melting.

Fig 2:  J/y RAA as a function of rapidity measured by ALICE at √sNN = 8.16 TeV, compared to theoretical calculations.

 

Finally, quarkonium production is also modified by cold nuclear matter effects not related to the formation of a QGP. These effects are usually studied in pA collisions. Results obtained by ALICE in p-Pb collisions at √sNN = 5.02 and 8.16 TeV show a strong modification of the J/y production which can be understood in terms of nuclear shadowing and/or energy loss (Fig. 2). However, the size of these effects cannot account for the suppression observed in AA collisions, confirming that its origin is due to hot matter effects.

ALICE is now just at the beginning of the quarkonium analyses of the Run2 data. A new bunch of high precision results is very soon expected and it will complement the large wealth of results obtained so far!