Chemical equilibrium study in nucleus-nucleus collisions at relativistic energies

Abstract

We present a detailed study of chemical freeze-out in nucleus-nucleus collisions at beam energies of $11.6A$, $30A$, $40A$, $80A$, and $158A\text{GeV}$. By analyzing hadronic multiplicities within the statistical hadronization approach, we have studied the strangeness production as a function of center-of-mass energy and of the parameters of the source. We have tested and compared different versions of the statistical model, with special emphasis on possible explanations of the observed strangeness hadronic phase space undersaturation. We show that, in this energy range, the use of hadron yields at midrapidity instead of in full phase space artificially enhances strangeness production and could lead to incorrect conclusions as far as the occurrence of full chemical equilibrium is concerned. In addition to the basic model with an extra strange quark nonequilibrium parameter, we have tested three more schemes: a two-component model superimposing hadrons coming out of single nucleon-nucleon interactions to those emerging from large fireballs at equilibrium, a model with local strangeness neutrality and a model with strange and light quark nonequilibrium parameters. The behavior of the source parameters as a function of colliding system and collision energy is studied. The description of strangeness production entails a nonmonotonic energy dependence of strangeness saturation parameter ${\gamma }_S$ with a maximum around $30A\text{GeV}$. We also present predictions of the production rates of still unmeasured hadrons including the newly discovered ${\Theta }^{+}\left(1540\right)$ pentaquark baryon.