Approach to equilibrium in high energy heavy ion collisions

Thomas Epelbaum

IPhT

Wed, Jun. 18th 2014, 14:30

Amphi Claude Bloch, Bât. 774, Orme des Merisiers

My thesis deals with the theory of the early stages of a heavy ion collision. Just after such a collision, the matter produced -- called the Quark-Gluon-Plasma (QGP) -- has been shown to be far out of thermal equilibrium. One would like to know whether the QGP thermalizes, and what is the typical time scale for this. Proving that the QGP thermalizes would also justify from first principles the hydrodynamical treatment of the subsequent evolution of a heavy ion collision.
After having recalled some essential theoretical concepts, my thesis addresses these questions in two different theories. par In a first part, we study a scalar field theory. Starting from an out of equilibrium initial condition, one studies the approach to equilibrium in a fixed volume or in a one-dimensional expanding system. In both cases, clear signs of thermalization are obtained. These results are derived thanks to the classical statistical approximation (CSA), that includes contributions beyond the Leading Order perturbative calculation. par In a second part, the Color Glass Condensate -- a quantum chromodynamics effective theory well suited to describe the early life of the QGP -- is used to treat more realistically the approach to thermalization in heavy ion collisions. After having derived some analytical prerequisites for the application of the CSA, the numerical simulations performed with the Yang-Mills equations show evidences of an early onset of hydrodynamical behavior of the QGP: the system becomes isotropic on short time scales, while the shear viscosity over entropy ratio is very small, which is characteristic of a quasi perfect fluid.

Contact : lbervas