Relaxation to equilibrium in relativistic heavy ion collisions

Abstract

Relaxation to equilibrium of hot and dense matter produced in central area of relativistic heavy ion collisions at energies ranging from several AGeV to hundreds AGeV is studied within two Monte Carlo transport models. The analysis was performed for three different areas: (i) fixed cubic cell with volume V = 125 fm³, (ii) fixed asymmetric cell with volume V = 4 x 4 x 1 = 16 fm³, and (iii) expanding cell. In the last case the cell volume follows the growth of the area with uniformly distributed energy. To check whether or not the system is equilibrated, its hadron yields and their energy spectra are compared with those of the statistical model of ideal hadron gas. For all cells and for all collision energies it was found that the matter in the cell was approaching the equilibrium state. The higher the collision energy, the shorter the time of equilibration. The equilibration phase lasts about 10 - 20 fm/c, after that the matter becomes very dilute and the thermal contact between hadrons is lost. Equation of state is well fitted to linear dependence P/ɛ = a = c²_s , where the square of the sonic velocity c²_s increases from 0.12 at E_lab = 11.6AGeV to 0.145 at E_lab = 160AGeV. The characteristic kinks observed in the T - μB phase diagrams are linked to inelastic freeze-out in the expanding fireball.