Heavy-ion collisions at ultra-relativistic energies offer the unique possibility to study hadronic matter under extreme conditions of density and temperature. Under these conditions, hadronic matter is predicted to undergo a phase transition into a new state of matter, the quark-qluon plasma, where quarks and gluons are free to move over a large volume compared to the physical size of hadrons. A second phase transition, that of chiral symmetry restoration, where masses drop to zero, is predicted to take place under similar or even identical conditions.
In a central collision between two nuklei at relativistic energies one can distinguish three main stages: first, as the nuclei start to overlap, hard scattering process between the partons inside nucleons take place redistributing the orginal beam energy in internal degrees of freedom. The time scale for this stage is ~1 fm/c. This leads to a system consisting of heated and compressed excited matter occuoying a volume which is assumed to be the Lorentz contracted volume of the colliding nuclei (~100 fm³). This is the second stage where, if the temperature and density reach high enough values, a deconfined system of quarks and gluons might be formed, the quark-gluon plasma (QGP). Present calculation of lattice QCD set the critical temperature for deconfinement to T_c~200 MeV. Even if T_c is not reached a gas of hadrons (mainly pions) at unusually high temperature and density id formed, usually referred to as the fireball. Typically it will live for a period of a few fm/c before undergoing a fast expansion and cooling, giving rise to the last stage of interaction. The QGP hadronizes if it was formed, the resulting hadron gas further expands and cools down till it becomes decoupled, its constituents not interacting anymore among themselves and making their way to be detected by the experiment.

High Energy Group is actively involved in NA49, NA61, WA98 and ALICE experiments.

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