0 Transition to resonance - rich matter in heavy ion collisions at RHIC energies

The equilibration of hot and dense nuclear matter produced in the central region in central Au+Au collisions at √ s = 200A GeV is studied within the microscopic transport model UrQMD. The pressure here becomes isotropic at t ≈ 5 fm/c. Within the next 15 fm/c the expansion of the matter proceeds almost isentropically with the entropy per baryon ratio S/A ≈ 150. During this period the equation of state in the (P, ε)-plane has a very simple form, P = 0.15 ε. Comparison with the statistical model (SM) of an ideal hadron gas reveals that the time of ≈ 20 fm/c may be too short to attain the fully equilibrated state. Particularly, the fractions of resonances are overpopulated in contrast to the SM values. The creation of such a long-lived resonance-rich state slows down the relaxation to chemical equilibrium and can be detected experimentally.