On the chemical freeze-out criteria in a hot and dense fireball

Intensive investigations of freeze-out criteria in a hot and dense fireball provide important information regarding particle emission from the fireball. A systematic comparison of these proposals is presented here in the framework of a thermodynamically consistent excluded volume model which has been found to describe the properties of hadron gas (HG) quite well. We find that the impact of excluded volume correction is considerably large and the average energy per hadron is 0.9 GeV, $n_b+n_{\bar b}$ stays nearly constant at 0.12/fm$^3$ and the normalized entropy density $s/T^3\approx 6$ in this model. Moreover, these values are independent of the beam or center-of-mass energy and also of the target and beam nuclei. In ideal HG model these quantities show substantial energy dependence. Further we have compared the predictions of various excluded volume models in the precise determination of these criteria and we find that the thermodynamically consistent excluded volume models give the best results. In addition, we find another important criterion that entropy per hadron has a constant value at 6 in our model. We hope that these findings will throw considerable light on the expansion dynamics and the bulk thermodynamic properties of the fireball before chemical freeze-out.