A ug 1 99 9 Hadronic freeze - out following a first order hadronization phase transition in ultrarelativistic heavy - ion collisions

We analyze the hadronic freeze-out in ultra-relativistic heavy ion collisions at RHIC in a transport approach which combines hydrodynamics for the early, dense, deconfined stage of the reaction with a microscopic non-equilibrium model for the later hadronic stage at which the hydrodynamic equilibrium assumptions are not valid. With this ansatz we are able to self-consistently calculate the freeze-out of the system and determine space-time hypersurfaces for individual hadron species. The space-time domains of the freeze-out for several hadron species are found to be actually four-dimensional, and differ drastically for the individual hadrons species. Freeze-out radii distributions are similar in width for most hadron species, even though the Ω − is found to be emitted rather close to the phase boundary and shows the smallest freeze-out radii and times among all baryon species. The total lifetime of the system does not change by more than 10% when going from SPS to RHIC energies. 1 Ultra-relativistic heavy ion collisions are the only means available to investigate highly excited dense nuclear matter under controlled laboratory conditions. In such collisions it is sought to recreate a Quark Gluon Plasma (QGP), the highly excited state of primordial matter which is believed to have existed shortly after the creation of the universe in the Big Bang (for recent reviews on the QGP, we refer to [1]). Transport theory has been among the most successful approaches applied to the theoretical investigation of relativistic heavy ion collisions. Microscopic transport models attempt to describe the evolution of the heavy ion reaction from some initial state up to the freeze-out of the newly produced particles on the basis of elementary interactions. The basic constituents in such models are either hadrons [2,3] or partons [4]. At RHIC energies, however, both, partonic and hadronic, degrees of freedom might be equally important and have both to be treated explicitly [5]. However, in such microscopic transport models, the QG-matter to hadron matter transition, i.e. the hadronization stage, has to be modeled in an ad-hoc fashion, whereas hydrodynamic approaches [6–10] incorporate this as a phase transition. This can be done in a consistent way, respecting the laws of thermodynamics (which is not always the case in microscopic transport models). The drawback of hydrodynamics, however , is that in the later reaction stages the basic hydrodynamical assumptions break down. For the freeze-out of the system a decoupling (freeze–out) hyper-surface must be specified (or fine-tuned to existing …