Αn important probe into the properties of the QGP is the study of the so-called quarkonia, that give access to the early stages of the collision.
Quarkonia are bound states of heavy flavour quarks (charm or bottom) and their antiquarks. Two types of quarkonia have been extensively studied: charmonia, which consist of a charm quark and an anti-charm, and bottomonia made of a bottom and an anti-bottom quark. A third type of quarkonium consisting of a top and anti-top quark has not been observed yet and probably it does not exist. Because of its high mass the top quark decays through the electroweak force before forming any bound state.
Since the seminal work of Tetsuo Matsui and Helmut Satz in 1986 the behaviour of the quarkonia in high temperature QCD was considered a signal for the deconfinement phase and the formation of the QGP. Charmoniuma suppression was extensively studied already at lower energies (SPS and RHIC) and a suppression exceeding the one expected due to cold nuclear matter was observed.
The idea behind this proposal seemed rather straightforward. Charm and anticharm quarks in the presence of the Quark Gluon Plasma, in which there are many free colour charges, are not able to see each other any more and therefore they cannot form bound states. The “melting” of quarkonia into the QGP manifests itself in the suppression of the quarkonium yields compared to the production without the presence of the QGP.
The mechanism seems apparently simple. As the temperature increases so does the colour screening resulting in greater suppression of the quarkonium states as it is more difficult for charm – anticharm or bottom – antibottom to form new bound states. At very high temperatures no quarkonium states are expected to survive. Instead, they melt into the plasma.
Quarkonium sequential suppression is therefore considered as a QGP thermometer. States with different masses have different sizes and they are expected to be screened and dissociated at different temperatures. In fact this is due to the fact that the longer the size, the weaker the binding. This for example can explain why it is harder to melt a J/ψ compared to Ψ(2S).
The search for quarkonia suppression as a QGP signature started experimental immediately after the publication of the paper by Matsui and Satz. This was almost 25 years ago. First at SPS (NA50, NA60 experiments) then at RHIC (Phenix and Star) and now at LHC. It turned out that the picture that we described above was too simplified and other processes should be taken into account in the study of the formation/dissociation of quarkonia in the hot QGP. Two main reasons further complicate this picture (although we will briefly mention a third one).
First of all, as the collision energy increases, so does the number of charm-anticharm quarks that can form bound states. Experimental evidence suggests that a balancing effect of recombination of charm-anticharm pairs appears at high temperatures. For example, ten pairs of charm-anticharm quarks were measured at RHICH, a number that is increased by a factor of ten at the LHC. This points to the fact that the new mechanism that contributes in the production of the J/ψ becomes more and more important in the high-energies regime. The second complication arises from the fact that the suppression of charmonium states was also observed in proton-lead collisions, in which Quark Gluon Plasma is not formed. Recent experimental evidences suggest that the observed suppression in proton-nucleus collisions (pA) is due to cold nuclear matter effects. Grasping the wealth of experimental results requires understanding the medium modification of quarkonia and disentangling hot and cold-matter effects. Today there is a large amount of data available from RHIC and LHC on charmonium and bottomonium suppression.
An important experimental observable of nuclear matter effects is the nuclear modification factor, namely the RAA, which is the observed particle yield in nucleus-nucleus collisions (AA) relative to scaled proton-proton collisions (pp), where QGP formation is not expected.
If the nuclear modification factor is larger than one, there is enhancement of the production in AA of charmonia while a RAA<1 indicates suppression.
Cold nuclear matter effects can be distinguished in initial state effects and final state effects. Initial state effects are due to the partonic structure of the colliding nuclei. Final-state effects are related to the formed quarkonia and affect them after they are fully formed.
Nuclear shadowing is one of the most important initial cold nuclear matter effects. It stems from the fact that when the nucleons are bound to form a nucleus, the distribution of the partons is different compared to a free nucleon. In that sense, it is necessary to measure how the partons’ distribution changes inside a nucleus.
The second important initial state effect is the energy loss known also as the Cronin effect. A parton can radiate low energy gluon and go through multiple elastic scattering collisions before the collision that leads to the production of a c-cbar pair.
Finally, a well-known final state effect is the break up of quarkonia by collisions with nucleons. The formed c-cbar pair interacts while crossing the nucleus and can be destroyed. This phenomenon becomes more important if quarkonia with much higher parton densities are formed.
It should be noted that energy loss depends on the density of gluons which is higher in QGP compared to normal nuclear matter. This fact should be taken into account when proton-lead collisions are compared to lead-lead.
ALICE studies Pb-Pb collisions, where QGP is expected and very recently also p-Pb collisions which are crucial for the determination of cold nuclear matter effects. By studying charmonium suppression and enhancement ALICE tries to distinguish between effects due to the formation of the QGP and those from cold nuclear matter. As we discussed in order to understand the hadronization processes that take place in heavy flavours we have to study carefully the different factors that affect quarkoniuma production .
Cynthia Hadjidakis from IPN Orsay and Roberta Arnaldi from INFN -Torino have recently presented ALICE results in this field in two different seminars.
Cynthia delivered a talk on “ALICE results on quarkonium hadroproduction” in the Heavy Ion Forum in the beginning of June at CERN. Cynthia presented measurements from the J/ψ (mid- and forward rapidity), Upsilon and psi’ (forward rapidity) in Pb-Pb collisions which were compared to results from pp collisions at the same energy. In her presentation she also presented the first results from p-Pb collisions on J/ψ production at forward rapidity. (Cynthia’s presentation can be found: here)
Roberta Arnaldi gave an LHC seminar with title “J/psi production in proton-nucleus collisions at ALICE: cold nuclear matter really matters” on June 18. After a short introduction on the main concepts and on results from lower energy experiments, the first results on J/psi production in p-Pb collisions at sqrt(s_NN) = 5.02 TeV from the ALICE experiment were discussed. They included the study of the nuclear modification factors R_pPb at forward and backward rapidity, down to zero transverse momentum, as well as of the ratio of the forward and backward J/psi yields, differentially in y and pT. In the last few slides Roberta also compared results with predictions from theoretical models as well as to preliminary results from other LHC experiments. (You can view her presentation here) .
The author would like to thank Cynthia Hadjidakis and Roberta Arnaldi for their ardent support.