jeudi 31 mars 2016 à 12:30
Amphi G. Besse
The aim of ultra-relativistic heavy-ion collisions is the study of nuclear matter at high temperature and pressure where quantum chromodynamics predicts the existence of a deconfined state of nuclear matter, the Quark-Gluon Plasma (QGP). Quarkonia are produced at the early stages of the collision via the hard scattering of partons. They travel through the formed mediumand are affected by it, making them ideal probes of the QGP. According to the color-screening model, measurement of the in-medium dissociation probability of the different quarkonium states could provide an estimate of the system temperature. Since the charm quark density produced in hadronic collisions increases with energy, recombination of cc pairs in the QGP or at the phase transition is predicted to give rise to a sizable J/ψproduction at LHC energies, which is likely to partially compensate the J/ψsuppression due to color-screening in the QGP.
In the first part of the seminar, I will focus on the Run1 results from the ALICE Collaboration on J/ψproduction in Pb-Pb collisions, in the dimuon decay channel, at forward rapidity. I will present the nuclear modification factor and elliptic flow measurements as well as the first intriguing measurement of an excess in the yield of J/ψat very-low transverse momentum, in peripheral Pb-Pb collisions. I will explain why this new measurement could possibly become a new probe of the QGP. Recently, the LHCb collaboration joined the heavy ion physics program of the LHC. LHCb is the unique experiment at the LHC which can operate in fixed target mode in addition to the collider mode, thanks to its System for Measuring Overlap with Gas (SMOG). In the second part of the seminar, I will present the status of the analyses of the proton-gas and Lead-gas data taking of the end of 2015, as well as the outcomes from the first LHCb Pb-Pb data taking period. Finally, I will briefly present the AFTER@LHC project, a proposal for a future multipurpose fixed-target experiment using the multi-TeV proton or heavy ion beams of the LHC. The experiment aims at studying the large x frontier for particle and astroparticle physics, spin physics and heavy ion physics. I will show first simulations studies on expected quarkonium production yields.