Abstract:
The equilibration of hot and dense nuclear matter produced
in the central region in central Au+Au collisions at
$\sqrt{s}=200$ AGeV is studied within the microscopic
transport model UrQMD. The pressure here becomes isotropic
at $t \approx 5$ fm/c. Within the next 15 fm/c the expansion
of the matter proceeds almost isentropically with the
entropy per baryon ratio $S/A \approx 150$. During this
period the equation of state in the $(P,\epsilon)$-plane has
a very simple form, $P=0.15 \epsilon$. Comparison with the
statistical model (SM) of an ideal hadron gas reveals that
the time of $\approx 20$ 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.