by Livio Bianchi. Published: 22 June 2012

Dr Livio Bianchi - INFN University di Torino

The J/psi meson still defies a simple and complete theoretical interpretation. Since it is composed of heavy quarks, its formation and evolution involves different QCD energy regimes and any kind of calculation on charmonium observables implies assumptions about the factorization of perturbative and non-perturbative effects. On this assumption rely all the proposed theoretical models such as the Color Singlet Model (CSM) and the Non-Relativistic QCD (NRQCD). However none of these models provides a completely satisfactory description of different measurements.

The advent of the LHC allows the study of quarkonia resonances in hadronic collisions occurring at a more than three times higher center-of-mass energy compared to Tevatron, thus offering the opportunity to test theoretical models in a new regime. More particularly, ALICE studies J/psi production down to pt=0 in a wide rapidity range: at mid-rapidity (y<0.9) through the di-electron decay channel, and at forward rapidity (2.5 < y < 4) exploiting the di-muon decay channel.

J/psi polarization parameters as a function of the transverse momentum in the helicity and Collins-Soper reference frames

First results on inclusive production cross sections in pp collisions at 7 and 2.76 TeV were delivered and in the first case, when compared with data from the other LHC experiments, a very good agreement was found. Cross sections at the lower energy center-of-mass collisions couldn’t be compared with other measurements since ALICE was the only experiment performing this study: this was done in order to obtain a suitable reference for heavy ion J/psi measurements, in particular for the extraction of the nuclear modification factor.

The extraction of the non-prompt component in the inclusive J/psi yields, i.e. J/psi coming from the decay of B hadrons, was performed at mid-rapidity (where the vertex of secondary decay can be detected through the use of the Inner Tracking System), providing a complementary measurement with respect to the other LHC experiments and reaching pt = 1.3 GeV/c.

ALICE is also the first LHC experiment being able to perform a study on the pt dependence of the inclusive J/psi polarization at forward rapidity. The study was carried out considering the full angular distribution of muons coming from the decay of the charmonium state. The result is that no significant polarization is observed in the pt region from 2 to 8 GeV/c and the comparison with theoretical model predictions show no perfect agreement with both NRQCD and CSM, with the first one slightly favored.

J/psi yield as a function of the charged particle multiplicity densities at mid-rapidity

The last interesting result obtained by ALICE is that, in pp collisions at 7 TeV, the number of J/psi candidates increases steeply when considering events with high multiplicity. Even if the effect is naively expected, the magnitude (up to 4-5 sigma) at the highest multiplicities considered (four times the average observed multiplicity) is quite large and, moreover, is not reproduced by Pythia 6.4 with the Perugia-0 tuning. Further studies on this topic are ongoing, also considering open charm production as a function of the charged particle multiplicity.

The J/psi saga is still open and attracts the debate of an active and fervent physics community. ALICE’s results, as those of the other LHC experiments, are contributing to the better understanding of quarkonium hadroproduction, which is a good benchmark for the a better knowledge of the different energy regimes of QCD. More and more results, also obtained exploiting higher statistics data samples, will be delivered soon…stay tuned!!