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)1/2 = 200 A GeV is studied within the microscopic transport model UrQMD. The pressure here becomes isotropic at t approx. 5 fm c-1. Within the next 15 fm c-1 the expansion of the matter proceeds almost isentropically with the entropy per baryon ratio S/A approx. 150. During this period the equation of state in the (P,ε)-plane has a very simple form, P=0.15ε. Comparison with the statistical model (SM) of an ideal hadron gas reveals that the time of approx. 20 fm c-1 may be too short to attain the fully equilibrated state. In particular, the fractions of resonances are overpopulated in contrast with 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. (author)