# Relativistic and nuclear matter

## Our research work is focused on the theoretical description of matter in extreme conditions, when relativistic and quantum effects are relevant. Applications include nuclear collisions at high energy and the Quark Gluon Plasma compact stars, cosmology and any phenomena where matter is at local thermodynamic equilibrium in a relativistic regime.

Statistical mechanics and fluid dynamics in the relativistic regime feature intriguing conceptual theoretical problems, which remain hidden in the non-relativistic limit. The very definition of otherwise familiar concepts in classical physics such as temperature and velocity of a fluid, becomes non-trivial. A quantum and relativistic consistent formulation of hydrodynamics has been recently drawn much attention, especially in the context of the Quark Gluon Plasma phenomenology.

Quark Gluon Plasma and relativistic heavy ion collisions

In very high energy collisions of heavy nuclei (currently ongoing at the LHC collider at CERN) quarks and gluons are released from their hadronic bounds and a new state of matter is formed, the so-called Quark-Gluon Plasma, at astoundingly large values of temperature (several TeraKelvin), pressure and density, by far the largest ever reached in a terrestrial laboratory, in a domain where relativistic effects are prevailing. The transition from hadronic matter, where protons, neutrons and other hadrons, are individual particles to the Quark-Gluon Plasma phase is a definite prediction of the theory of strong interactions, Quantum Chromo-Dynamics (QCD) and it is supported by numerical calculations (lattice QCD). In high energy collisions of heavy nuclei, the plasma lives only for 10-22 sec because its rapid expansion cools it and gets it back to the hadronic phase.

We investigate the properties of the QCD plasma by studying

*Spin and polarization in the Quark Gluon Plasma*

Colliding nuclei at very high energy

QCD phase diagram

Hadronization in high energy collisions

In high energy collisions of elementary particles or nuclei many hadrons are produced in the final state, even though the quantum field theory of strong interactions (QCD) has quarks and gluons as fundamental degrees of freedom. This phenomenon (confinement) is related to the nature of QCD as a strongly interacting non-perturbative theory at large distances (1 fm) and small energy scale (around 1 GeV). The process of hadron formation at large distance scale is called *hadronization *and it cannot presently be computed with *ab initio* QCD calculations, neither analytically nor numerically.

An intriguing feature of hadronization is that particle multiplicities and spectra, in the low momentum region, can be succesfully reproduced to a high level of accuracy with an equilibrium statistical mechanical calculation (Statistical Hadronization Model). The statistical equilibrium/thermalization is an emergent feature of QCD in the strong non-linear regime which has not been fully understood yet.