ALICE dévoile les mystères du J/ψ

L’analyse des premiers résultats de l’expérience ALICE auprès du LHC semble indiquer qu’on pourra bientôt se prononcer définitivement sur le sort du J/ψ dans un plasma chaud quark–gluon. Le nombre énorme de charges de couleur libres (quarks et gluons) pourrait affaiblir la liaison du quark et de l’antiquark dans le J/ψ, si bien que celui-ci finirait par disparaître (il serait éliminé). La théorie montre que la probabilité de cette dissociation dépend de la température du plasma quarks–gluons, de sorte que l’observation de cette élimination du J/ψ revient à placer un thermomètre dans la matière elle-même.

The J/ψ meson, a bound state of a charm (c) and an anticharm (c) quark, is unique in the long list of particles that physicists have discovered over the past 50 years. Found almost simultaneously in 1974 – at Brookhaven, in proton–nucleus collisions, and at SLAC, in e+e collisions – this particle is the only one with two names, given to it by the two teams. With a mass greater than 3 GeV it was by far the heaviest known particle at the time and it opened a new field in particle physics, namely the study of "heavy" quarks.

The charm quark and its heavier partners, the bottom and top quarks (the latter discovered more than 20 years later, in 1995), have proved to be a source of both inspiration and problems for particle physicists. By now, thousands of experimental and theoretical papers have been published on these quarks and the production, decay and spectroscopy of particles containing heavy quarks have been the focus of intense and fruitful investigations.

However, despite a history of almost 40 years, the production of the J/ψ itself still represents a puzzle for QCD, the standard theory of strong interactions between quarks and gluons. On the one hand, the creation of a pair of quarks as "heavy" as charm (mc ≈ 1.3 GeV/c2) in a gluon–gluon or quark–antiquark interaction is a process that is "hard" enough to be treated in a perturbative way and therefore well understood by theory. On the other hand however, the binding of the pair is essentially a "soft" process – the relative velocity of the two quarks in a J/ψ is "only" about 0.5 c – and this proves to be much more difficult to model.

J/ψ production

About fifteen years ago, the results obtained at Fermilab’s Tevatron collider first showed a clear inconsistency with the theoretical approach adopted at the time to model J/ψ production, the so-called colour-singlet model. This unsatisfactory situation led to the formulation of the more refined approach of nonrelativistic QCD (NRQCD), which brought a better agreement with data. However, other quantities such as the polarization of the produced J/ψ, i.e. the extent to which the intrinsic angular momentum of the particle is aligned with respect to its momentum, were poorly reproduced. This uncomfortable situation also arose partly because of controversial experimental results from the Tevatron, where the CDF experiment’s results on polarization from Run1 disagreed with those from Run2. Considerable hope is therefore placed on the results that the LHC can obtain for this observable (more on this later).

The J/ψ and its suppression can be seen as a thermometer in the medium created in the collision.

Nevertheless, despite these unresolved mysteries surrounding its production, the J/ψ has an important "application" in high-energy nuclear physics and more precisely in the branch that studies the formation of the state of (nuclear) matter where quarks and gluons are no longer confined into hadrons: the quark–gluon plasma (QGP). If such a state is created, it can be thought of as a hot "soup" of coloured quarks and gluons, where colour is the "charge" of the strong interaction. In the usual world, quarks and gluons are confined within hadrons and colour cannot fly over large distances. However, in certain situations, as when ultrarelativistic heavy-ion collisions take place, a QGP state could be formed and studied. Indeed, such studies form the bulk of the physics programme of the ALICE experiment at the LHC.

The J/ψ is composed of a heavy quark–antiquark pair with the two objects orbiting at a relative distance of about 0.5 fm, held together by the strong colour interaction. However, if such a state were to be placed inside a QGP, it turns out that its binding could be screened by the huge number of colour charges (quarks and gluons) that make up the QGP freely roaming around it. This causes the binding of the quark and antiquark in the J/ψ to become weaker so that ultimately the pair disintegrates and the J/ψ disappears – i.e. it is "suppressed". Theory has shown that the probability of dissociation depends on the temperature of the QGP, so that the observation of a suppression of the J/ψ can be seen as a way to place a "thermometer" in the medium itself.

Such a screening of the colour interaction, and the consequent J/ψ suppression, was first predicted by Helmut Satz and Tetsuo Matsui in 1986 and was thoroughly investigated over the following years in experiments with heavy-ion collisions. In particular, Pb–Pb interactions were studied at CERN’s Super Proton Synchrotron (SPS) at a centre-of-mass energy, √s, of around 17 GeV per nucleon pair and then Au–Au collisions were studied at √s=200 GeV at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC).

As predicted by the theory, a suppression of the J/ψ yield was observed with respect to what would be expected from a mere superposition of production from elementary nucleon–nucleon collisions. However, the experiments also made some puzzling observations. In particular, the size of the suppression (about 60–70% for central, i.e. head-on nucleus–nucleus collisions) was found to be approximately the same at the SPS and RHIC, despite the jump in the centre-of-mass energy of more than one order of magnitude, which would suggest higher QGP temperatures at RHIC. Ingenious explanations were suggested but a clear-cut explanation of this puzzle proved impossible.

At the LHC, however, extremely interesting developments are expected. In particular, a much higher number of charm–anticharm pairs are produced in the nuclear interaction, thanks to the unprece¬dented centre-of-mass energies. As a consequence, even a suppression of the J/ψ yield in the hot QGP phase could be more than counter-balanced by a statistical combination of charm–anticharm pairs happening when the system, after expansion and cooling, finally crosses the temperature boundary between the QGP and a hot gas of particles. If the density of heavy quark pairs is large enough, this regeneration process may even lead to an enhancement of the J/ψ yield – or at least to a much weaker suppression with respect to the experiments at lower energies. The observation of the fate of the J/ψ in nuclear collisions at the LHC constitutes one of the goals of the ALICE experiment and was among its main priorities during the first run of the LHC with lead beams in November/December 2010.

The ALICE experiment is particularly suited to observing a J/ψ regeneration process. For simple kinematic reasons, regeneration can be more easily observed for charm quarks with low transverse-momentum. Contrary to the other LHC experiments, both detector systems where the J/ψ detection takes place – the central barrel (where the J/ψ→e+e decay is studied) and the forward muon spectrometer (for J/ψ→μ+μ) – can detect J/ψ particles down to zero transverse momentum.

As the luminosity of the LHC was still low during its first nucleus–nucleus run, the overall J/ψ statistics collected in 2010 were not huge, of the order of 2000 signal events. Nevertheless, it was possible to study the J/ψ yield as a function of the centrality of the collisions in five intervals from peripheral (grazing) to central (head-on) interactions.

Clearly, suppression or enhancement of a signal must be established with respect to a reference process. And for such a study, the most appropriate reference is the J/ψ yield in elementary proton–proton collisions at the same energy as in the nucleus–nucleus data-taking. However, in the first proton run of the LHC the centre-of-mass energy of 7 TeV was more than twice the energy of 2.76 TeV per nucleon–nucleon collision in the Pb–Pb run. To provide an unbiased reference, the LHC was therefore run for a few days at the beginning of 2011 with lower-energy protons and J/ψ production was studied at the same centre-of-mass energy of Pb–Pb interactions.

The Pb–Pb and p–p results are compared using a standard quantity, the nuclear modification factor RAA. This is basically a ratio between the J/ψ yield in Pb–Pb collisions, normalized to the average number of nucleon–nucleon collisions that take place in the interaction of the two nuclei and the proton–proton yield. Values smaller than 1 for RAA therefore indicate a suppression of the J/ψ yield, while values larger than 1 represent an enhancement.

The results from the first ALICE run are rather striking, when compared with the observations from lower energies (figure 1). While a similar suppression is observed at LHC energies for peripheral collisions, when moving towards more head-on collisions – as quantified by the increasing number of nucleons in the lead nuclei participating in the interaction – the suppression no longer increases. Therefore, despite the higher temperatures attained in the nuclear collisions at the LHC, more J/ψ mesons are detected by the ALICE experiment in Pb–Pb with respect to p–p. Such an effect is likely to be related to a regeneration process occurring at the temperature boundary between the QGP and a hot gas of hadrons (T≈160 MeV).

The picture arises from these observations is consistent with the formation, in Pb–Pb collisions at the LHC, of a deconfined system (QGP) that can suppress the J/ψ meson, followed by a hadronic system in which a fraction of the charm–anticharm pairs coalesce and ultimately give a J/ψ yield larger than that observed at lower energies. This picture should be clarified by the Pb–Pb data that were collected in autumn 2011. Thanks to an integrated luminosity for such studies that was 20 times larger than in 2010, a final answer on the fate of the J/ψ inside the hot QGP produced at the LHC seems to be within reach.

ALICE is also working hard to help solve other puzzles in J/ψ production in proton–proton collisions, in particular by studying, as described above, the degree of polarization. A first result, recently published in Physical Review Letters, shows that the J/ψ produced at not too high a transverse momentum are essentially unpolarized, i.e. the angular distribution of the decay muons in the J/ψ→μ+μ process is nearly isotropic (figure 2). Theorists are now working to establish if such behaviour is compatible with the NRQCD approach that up to now is the best possible tool for understanding the physics related to J/ψ production.

In conclusion, a particle that has been known for almost half a century continues to be a source of inspiration and progress. However, even if particle and nuclear physicists working at the LHC are confident of being able finally to understand its multifaceted aspects, the future often brings the unexpected. So stay tuned and be ready for surprises.