Speaker
Description
Quarkonia are bound states of charm or beauty quarks with their anti-particle. They have long being considered as unique probes of the quark-gluon plasma (QGP) that is formed in ultra-relativistic heavy-ion collisions. Heavy quarks are produced in the hard scattering processes occurring at the beginning of the collision, while the formation of the bound state is affected by the presence of the QGP, and provides valuable information on it.
In particular, the presence of a deconfined state results in color screening or dissociation mechanisms that suppress the formation of the bound state. On the other hand, the large amount of c-cbar pairs produced during the collision at LHC energies can result in a regeneration of charmonium states.
Quarkonia can also be produced in a well defined phase space region from photon-nucleus or photon-nucleon scattering, which are abundant in ultra-peripheral heavy-ion collisions, and can also occur in nucleus-nucleus collisions with nuclear overlap. In the latter, a measured yield of J/psi compatible with coherent photoproduction opens many questions on the nature of the coherence when the nucleus breaks up.
Comparison with measurements in small systems, such as proton-proton and proton-lead collisions, represent a baseline for interpreting heavy-ion results.
Besides serving as a reference, the measurements in small systems are important on their own since they shed light on the quarkonium production mechanism, that is not fully understood as it has an intrinsic non-perturbative scale.
Polarisation measurements both in small and large systems are particularly powerful to disentangle the different production mechanisms.
Finally, several measurements in high-multiplicity pp or p-Pb events have revealed striking similarities with heavy-ion collisions. In particular, collective effects are observed that can be explained in the presence of multi-parton interactions (MPIs).The measurement of double quarkonium production gives direct access to MPIs, while indirect information can be inferred by the study of the multiplicity dependent production or by the azimuthal correlations between quarkonia and hadrons produced in the same event.
In this talk, an overview of the most recent ALICE results on quarkonium production and polarisation in small and large collision systems will be provided. The comparison with the corresponding available theoretical calculations will be discussed.