Chemical equilibrium study in nucleus-nucleus collisions at relativistic energies

TLDR

The study investigates chemical freeze‑out in nucleus‑nucleus collisions at beam energies from 11.6A to 158A GeV and predicts production rates for unmeasured hadrons, including the Θ⁺(1540) pentaquark. Using the statistical hadronization framework, the authors compare a baseline model with an extra strange‑quark nonequilibrium parameter to three alternative schemes—two‑component, local strangeness neutrality, and separate strange‑light quark nonequilibrium—to describe hadron yields. Analysis of hadronic multiplicities reveals that midrapidity yields overestimate strangeness, the strangeness saturation parameter γ_S exhibits a nonmonotonic energy dependence peaking near 30A GeV, and the data challenge the assumption of full chemical equilibrium.

Abstract

We present a detailed study of chemical freeze-out in nucleus-nucleus collisions at beam energies of $11.6A$, $30A$, $40A$, $80A$, and $158A\phantom{\rule{0.3em}{0ex}}\text{GeV}$. 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 ${\ensuremath{\gamma}}_{S}$ with a maximum around $30A\phantom{\rule{0.3em}{0ex}}\text{GeV}$. We also present predictions of the production rates of still unmeasured hadrons including the newly discovered ${\ensuremath{\Theta}}^{+}(1540)$ pentaquark baryon.