Central collisions of gold nuclei are simulated by several existing models and the central net baryon density rho and the energy density eps are extracted at successive times, for beam kinetic energies of 5-40 GeV per nucleon. The resulting trajectories in the (rho,eps) phase plane are discussed from the perspective of experimentally exploring the expected first-order hadronization phase transition with the planned FAIR at GSI or in a low-energy campaign at RHIC.Comment: 11 pages formatted, 17 eps files for 9 figure

We study the phase diagram of a hadronic chiral flavor-SU(3) model. Heavy baryon resonances can induce a phase structure that matches current results from lattice-QCD calculations at finite temperature and baryon density. Furthermore, we determine trajectories of constant entropy per net baryon in the phase diagram.Understanding hot and baryon-dense QCD matter is of central importance in theoretical and experimental heavy-ion physics. Various effective theories for chiral symmetry restoration predict that a line of first-order phase transitions in the plane of quark-chemical potential µ q versus temperature T ends in a critical point as the chemical potential is lowered (see [1] for a review). Presently, the lattice locates that point at T ≈ 160 MeV, µ q ≈ 120 MeV [2]. The Compressed Baryonic Matter (CBM) experiment at GSI FAIR is planned to perform a dedicated experimental effort to detect that line of firstorder phase transitions in relativistic heavy-ion collisions. It is hoped that by varying experimental parameters like the beam energy one could trigger phase transitions of variable strength (latent heat) and perhaps even locate the expected second-order critical point.Models relying exclusively on order-parameter dynamics typically predict significantly lower chiral phase transition temperatures in baryon-dense matter than those found on the lattice (see e.g. Fig. 6 in [1]). As shown by Gerber and Leutwyler some time ago [3], while heavy hadronic states are suppressed by the Boltzmann factor, their contribution to the energy density at high temperature is substantial. This agrees with other studies using a hadron resonance gas approach, which provides a reasonable description of the thermodynamics obtained on the lattice below the critical temperature [4]. Heavy states also reduce [3] the strong dependence of the "critical temperature" (defined via the peak of a suitable susceptibility) on the pion mass obtained in simple models for chiral order parameter dynamics [5]. The lattice indicates a relatively weak dependence of T c on the pion mass [6].In this Letter we investigate the role of heavy hadronic states on the location of the chiral critical point within a non-linear SU (3) L × SU (3) R chiral model [7]. Here, the phase transition at high temperature and baryon density is "driven" by baryonic resonance degrees of freedom, as suggested by the discussion above. We shall show that the model is able to reproduce not only the sketched qualitative phase structure but also the location of the endpoint. The properties of the high mass states (masses and couplings) are important for the actual location of the chiral phase transition line [8] in the plane of T and µ q .Lattice results show that the susceptibility peaks of the chiral condensate and of the Polyakov loop coincide at µ q = 0 [9] which indicates that for small µ q those transition(s) involve a coupling of the chiral dynamics to the gauge fields, see e.g. [10]. However, within a matrix model for Polyakov loops, the effect of µ q > 0 on the critical t...

We study the phase diagram of a generalized chiral SU(3)-flavor model in mean-field approximation. In particular, the influence of the baryon resonances, and their couplings to the scalar and vector fields, on the characteristics of the chiral phase transition as a function of temperature and baryon-chemical potential is investigated. Present and future finite-density lattice calculations might constrain the couplings of the fields to the baryons. The results are compared to recent lattice QCD calculations and it is shown that it is non-trivial to obtain, simultaneously, stable cold nuclear matter.

The measured particle ratios in central heavy-ion collisions at RHIC-BNL are investigated within a chemical and thermal equilibrium chiral SU (3) σ -ω approach. The commonly adopted non-interacting gas calculations yield temperatures close to or above the critical temperature for the chiral phase transition, but without taking into account any interactions. In contrast, the chiral SU (3) model predicts temperature and density dependent effective hadron masses and effective chemical potentials in the medium and a transition to a chirally restored phase at high temperatures or chemical potentials. Three different parametrizations of the model, which show different types of phase transition behaviour, are investigated. We show that if a chiral phase transition occured in those collisions, 'freezing' of the relative hadron abundances in the symmetric phase is excluded by the data. Therefore, either very rapid chemical equilibration must occur in the broken phase, or the measured hadron ratios are the outcome of the dynamical symmetry breaking. Furthermore, the extracted chemical freeze-out parameters differ considerably from those obtained in simple non-interacting gas calculations. In particular, the three models yield up to 35 MeV lower temperatures than the free gas approximation. The inmedium masses turn out to differ up to 150 MeV from their vacuum values.

No abstract