mardi 19 avril 2016 à 16:00
Amphi G. Besse
Since the 70’s, physicists have discovered that nucleons are made of elementary particles called quarks and gluons. These particles are ruled by the theory of quantum chromodynamics which explains that quarks are confined within the boundaries of hadrons, like nucleons. This theory also predicts the existence of a deconfined state of matter under extreme conditions of temperature and density called the Quark and Gluon Plasma (QGP). In nature, those conditions were presumably fulfilled some microseconds after the Big Bang. Experimentally, we expect to observe the formation of a QGP in high-energy ultra-relativistic heavy-ions collisions.
Many observables are available to observe and characterise such a state of matter, and the production yield of the J/ψ meson is one of the most powerfull. This yield should be reduced by the presence of the deconfined state of matter, due to a mechanism called color screening.
In my presentation, I will present results on the ongoing analysis of the inclusive production of J/ψ in Pb-Pb collisions at center-of-mass energy √sNN = 5 TeV, detected with the ALICE muon spectrometer at the LHC during the 2015 data taking (run 2). The first data sample recorded in 2011 (run 1 with √sNN = 2.76 TeV) exhibits a clear J/ψ suppression compare to expectations in the absence of nuclear matter. However, this suppression is less pronounced at low pT compare to lower energy experiment (Figure 1 2 ), which is probably due to a new mechanism called recombination. The increased beam energy should provide a better understanding of the suppression/regeneration mechanism, and consequently on the charateristics of the deconfined state of matter.
1. T. Matsui ans H. Satz, Phys. Lett. B 178, 416 (1986)
2. The ALICE collaboration, Centrality, rapidity and transverse momentum dependence of J/ψ suppression in
PbPb collisions at √sNN = 2.76 TeV, Physics Letters B. Vol.734, P. 314 ?327