Lead nuclei, accelerated and collided by the LHC recreate the state of matter which existed when the Universe was few microseconds old. This matter, composed of quark and gluons, is hundred thousand times hotter than the core of the Sun and acts as a fluid whose viscosity is lower than of a super-cold helium. Expanding and cooling down, this matter converts back into hadrons, emitting dozens of thousands of particles. Understanding the nature of such matter and its evolution represents a challenge both to the theory and experiment. The ATLAS detector provides an excellent opportunity to perform complex and detailed studies of this fascinating object, measuring its bulk propertied and its response to penetrating probes. Study of particle correlations: differential and integrated anisotropic flow, and event-by-event fluctuations tell us about the initial geometry of the interactions and help to understand how this geometry evolves into the final state. The new ATLAS results directly show that the geometric shape of the medium affects the jets emerging from it. The new ATLAS studies of the jets suppression and jet fragmentation are essential to understand how the energetic partons are interacting with the medium. Recent high precision measurements of the boson production in heavy ion collisions and first results on the boson-jet correlations are important steps towards quantitative understanding of the parton energy loss mechanism.
