Degree of Chemical Non-equilibrium in Central Au-Au Collisions at RHIC energies

Abstract

We investigate the difference between hadron resonance gas (HRG) calculations for chemical freeze-out parameters at fully and partly chemical equilibria. To this end, the results are compared with the particle ratios measured in central Au-Au collisions at a wide range of nucleon-nucleon center-of-mass energies, \hbox{$\sqrt{s_{NN}}=7.7-200 $GeV} as offered by the STAR experiment. We restrict the discussion to STAR, because of large statistics and overall homogeneity of STAR measurements (one detector) against previous experiments. We find that the matter produced at these energies is likely in fully chemical equilibrium, which is consistent with recent lattice QCD results. The possible improvements by partial chemical equilibrium ($\gamma_S\neq 1$) are very limited. We also discuss these results with the ones deduced from $\phi/\pi^-$ and $\Omega^-/\pi^-$ ratios. These hadron ratios are sensitive to the degree of chemical equilibrium. Accordingly, the conclusion that the matter produced reaches fully chemical equilibrium in central Au-Au at RHIC energies is confirmed.