Equation of state of resonance-rich matter in the central cell in heavy-ion collisions at s=200A GeV

The equilibration of hot and dense nuclear matter produced in the central cell of central Au+Au collisions at s=200A GeV is studied within a microscopic transport model. The pressure in the cell becomes isotropic at t~5 fm/c after beginning of the collision. Within the next 15 fm/c the expansion of matter in the cell proceeds almost isentropically with the entropy per baryon ratio S/A≅150, and the equation of state in the (P,ɛ) plane has a very simple form, P=0.15ɛ. Comparison with the statistical model of an ideal hadron gas indicates that the time t~20 fm/c may be too short to reach the fully equilibrated state. Particularly, the creation of long-lived resonance-rich matter in the cell decelerates the relaxation to chemical equilibrium. This resonance-abundant state can be detected experimentally after the thermal freeze out of particles.