Animé par Johannès Jahan
vendredi 3 juillet 2020 à 14:00
S. Delorme (1), P-B. Gossiaux (1), T. Gousset (1), R. Katz (1)
(1) SUBATECH UMR 6457 (IMT Atlantique, Université de Nantes,IN2P3/CNRS), 4 Rue Alfred Kastler, F-44307 Nantes, France
The Standard Model of particle physics describes the elementary constituants of matter and the interactions between them. Among those constituants, quarks form protons and neutrons that compose atomic nuclei. Due to the strong interaction, quarks stay confined inside composite particles called hadrons (like protons and neutrons), that’s the ordinary state of matter. But under extreme temperature and density conditions, hadronic matter reaches the most extreme state of matter, called Quark-Gluon Plasma (QGP), where quarks are deconfined and can freely evolve.
Quarkonia are boundstates of heavy quark-antiquark pairs and are very interesting probes of the QGP. Indeed, as the medium temperature rises, quarkonia states melt due to the significant screening of the quark interaction, a phenomenon called quarkonia suppression. They can then be seen as a thermometer of the medium. Experimental measurements at collidersrevealed that the variations of the suppression with the various kinematics was more complicated than what theoretical models predicted, pointing out the need for a better theoretical description of quarkonia inside the QGP. In recent years, a lot of work has been done towards a dynamical description of quarkonia inside the QGP, using the open quantum systems formalism. In this framework, one can get a real-time description of a quantum system (here the quarkonium) in interaction with a thermal bath (the QGP) by studying the system reduced density matrix.We investigate the real-time dynamics of a correlated heavy quark-antiquark pair inside the QGP using a quantum master equation previously derived from first QCD principles in .
The novel feature of our approach is to numerically solve the full equation, avoiding to perform so-called semi-classical approximations as was done in  to solve them. The resolution is performed in 1D to lessen the computational cost, nonetheless it is sufficient to gain insight on the dynamics.
-BLAIZOT J.P., ESCOBEDO M.A. (2018), Quantum and Classical Dynamics of Heavy Quarks in a Quark-Gluon Plasma. J. High Energ. Phys.2018, 34
After Stephane’s presentation focused on a more theoretical point of view, I'll present how the quarkonia production is studied in heavy ion collisions with the ALICE experiment. One of these production mechanisms has been highlighted in 2015 by the ALICE collaboration after the observation of an excess in the yield of ⅉ/Ψ at very low transverse momentum (pT<0.3 GeV/c) in the forward rapidity region (2.5<ˠ<4) in peripheral Pb-Pb collisions at sNN‾‾‾‾√=2.76 TeV at the CERN LHC.  The coherent photo-production was proposed as the potential underlying physics mechanism. This mechanism is the main responsible for low-pT ⅉ/Ψ production in ultra-peripheral collisions and is a good probe of the gluon density inside a nucleus. But this mechanism was never observed in more central collisions that are dominated by the hadronic interactions.
If the photo-production is confirmed as the origin of the excess, new results could help extracting the photo nuclear cross section and providing inputs to constrain the gluon distribution inside the nucleus. Furthermore, the ⅉ/Ψ from the coherent photo-production could become a new probe of the Quark and Gluon Plasma.
 ALICE Collaboration, J.Adam et al., "Measurement of an excess in the yield of ⅉ/Ψ at very low pt in Pb-Pb collisions at sNN‾‾‾‾√=2.76 TeV", Phys.Rev.Lett.116(2016)222301, arXiv:1509.08802 [nucl-ex].