The success of hydrodynamical approaches in relativistic heavy ion reactions is considered as the evidence of a very early local thermalization of colliding system. On the other hand, several theoretical studies suggest that there exist a so-called pre-thermalization behavior, where the thermodynamical relations become valid long before the real thermal equilibrium is attained, so that the hydrodynamic description becomes possible without attaining the strict local thermal equilibrium. Such a situation has also long been claimed by Tsallis in his non extensive statistics where the thermodynamical relations can be fulfilled with a power, not exponential distribution. Here we discuss a possible origin for the emergence of power law spectra and the relation with the pre-thermal behavior of the system.

Université Fédérale de Rio de Janeiro, Brésil