Quark Matter Production in Heavy-Ion Collisions

Abstract

The study of high-energy heavy-ion collisions is presently a very active field in experimental particle physics, with the RHIC collider at BNL in operation since summer 2000 and with the ALICE experiment being prepared to study this kind of physics at LHC energies. The first goal of these experimental attempts, which started in 1986, with the AGS and SPS fixed-target programs, is the discovery of the phase transition from confined hadronic matter to decon fined partonic matter. The idea that such a phase transition should exist, between hadronic and quark matter, has been around since the first models of the quark structure of hadrons. It is presently studied in detail in the framework of lattice QCD calculations, which predict its occurrence when the temperature of the system exceeds a critical threshold at around 170 MeV, corresponding to a critical energy density of around 600 MeV/fm³ [1]. Fig. 1 illustrates how the energy density (in units of T ⁴) depends on the temperature of the medium (in units of T _c), increasing by an order of magnitude within a very small temperature range. At the critical temperature, two phenomena should occur: the color degrees of freedom become deconfined and chiral symmetry (spontaneously broken in the hadronic world) gets restored. Both should lead to observable effects, to be looked for in properly designed experiments. The proof of existence of the quark matter phase and the study of its properties are key issues in QCD, for the understanding of confinement and chiral symmetry.