Particle ratios from AGS to RHIC in an interacting hadronic model

Abstract

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.

Impact of baryon resonances on the chiral phase transition at finite temperature and density

“…At small temperatures and high chemical potentials the phase transition is driven by the rapid increase of the vector density of the baryon resonances, especially the ∆s, leading to a second minimum in the These results show that the simultaneous description of the phase diagram and nuclear matter saturation gives strong constraints on the model. Even more constraints may appear if also neutron stars are considered [59]. Since many different models with different equations of state give a good description of nuclear matter saturation and neutron stars, such additional constraints are urgently needed -although there is still considerable uncertainty in the lattice calculations.…”

“…In addition, it is possible to make contact between the theory of the phase diagram and experimental observables, e.g. by determining in one model the chemical freeze-out temperature in relativistic heavy ion collisions -as obtained from fits to the measured particle ratios -and the phase transition temperature [60,61].…”

“…These results show that the simultaneous description of the phase diagram and nuclear matter saturation gives strong constraints on the applied model. Even more constraints may appear if also neutron stars are considered [60]. Several models give a good description of nuclear matter saturation or neutron star properties and different models yield a reasonable description of the LQCD results on the phase diagram.…”

Volume Effect of Bound States in Quark-Gluon Plasma

“…The influence of particle volume is widely discussed in the Van der Waals excluded volume model (VdW), [5,6] to describe hadronic gas at high density. In QGP, the number density is very high so that the ball-like bound states would be quite close to each other as discussed by J.W.…”