The main goal of heavy-ion collisions at relativistic energies is to produce a hot and dense medium at an energy density greater than 1 GeV/fm3. It is expected that in such extreme conditions a new state of matter, the Quark-Gluon Plasma (QGP), can be formed, consisting of deconfined quarks, antiquarks and gluons with partially restored chiral symmetry. Exploring properties of the created medium is crucial for understanding the physics of dense systems. Evolution models of the universe assert that the whole matter was in the QGP state about 10 ms after the Big Bang, and such systems may also be present in the core of neutron stars.

Like Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in the past, now, the LHC collider has opened a new era in heavy ion physics. At the Large Hadron Collider relativistic lead (Pb) ions collide at the largest energy ever obtained in the laboratory of 5 TeV per pair of interacting nucleons. It is by a factor of 25 larger than the energy obtained at the RHIC BNL collider. At such hight energies new features of Quark-Gluon Plasma have been uncovered. ATLAS detector is an excellent tool to systematically study many aspects of nucleus-nucleus collisions, like charged particles multiplicities, charged particles densities, elliptic flow and higher order flow harmonics, collision centrality, charged particle spectra, jet quenching or quarkonia suppression. Our Group has been involved in the ATLAS experiment heavy-ion program since its very beginning, i.e. since 2003. (see history). At present we are analysing vast amounts of experimental data collected by the ATLAS detector(see presentation and publicution section for more details). In particular members of the Heavy-Ion Kraków Group are interested in studing the collective phenomena of produced particles, like the flow effects.