Helena Santos
LIP / FCUL

Abstract: Predictions from lattice QCD and evidence from Pb+Pb collisions data at  LHC Runs I and II, as well as in past experiments at SPS/CERN and RHIC/BNL, suggest that under extreme temperature and density matter undergoes to a phase transition from ordinary hadronic matter to a plasma of quarks and gluons, the QGP. Later this plasma expands and cools resulting in the recombination of quarks into hadrons that reach the detectors carrying the information of the deconfined phase. The QGP has occurred just after the Big Bang, even before anomalies such as matter and anti-matter imbalance and the observation that normal atoms account only for a small fraction of the mass and energy of the universe, have shown up.  The conditions to re-create the primordial universe are ensured by the Heavy Ion Program of LHC, which in Run I and Run II has been providing ultra-relativistic lead ion beams colliding at 2.76 and 5.02 TeV center of mass energy, respectively, as well as p+Pb collisions at 5.02 and 8.16 TeV. ATLAS is strongly committed with the Heavy Ion Program of the LHC. The multi-purpose detector, designed for proton-proton interactions, has shown an excellent performance in p+Pb and Pb+Pb collisions. Results on bulk phenomena, including charged particle measurements and collective flow, and on hard probes, including single muons, high transverse momentum hadrons and jets, will be presented.