Nuclear Particle Astrophysics (NPA) Seminar: Audrey Francisco, CERN, “Quarkonium States as a Probe of the QGP at LHC”

Extreme temperature and energy densities produced in ultra-relativistic heavy-ion collisions at the Large Hadron Collider provide an unique opportunity to study the properties of matter. A phase transition of the hadronic matter to a deconfined medium of quarks and gluons, the Quark-Gluon Plasma (QGP) is predicted by Quantum Chromodynamics and considerable theoretical and experimental efforts have been invested to study its properties. Among the prominent probes of the QGP, heavy quarks play a crucial role since they are created in primary hard-scattering processes, before the QGP formation, and their number is conserved throughout the partonic and hadronic phases of the collision. Bound states of heavy quarks — quarkonium (charmonium for ccbar
and bottomonium for bbbar) provide remarkable probes of the medium. Two antagonist mechanisms are required to reproduce experimental observations: a sequential suppression of the quarkonium states, early suggested as a
signature of the QGP, and quarkonium regeneration by recombination of deconfined quarks. However theoretical predictions carry large uncertainties and many unknows remain. The momentum space azimuthal anisotropy of charmonium production (referred as elliptic flow), provides important information on regeneration mechanisms and charm thermalisation in the medium. Recent observations show significative flow coefficients in Pb-Pb and
p-Pb collisions that are not reproduced by theory, reassessing our understanding of charmonium production in the medium.