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 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 ␥ S with a maximum around 30A GeV. We also present predictions of the production rates of still unmeasured hadrons including the newly discovered ⌰ + ͑1540͒ pentaquark baryon.

Hadron production in central nucleus–nucleus collisions at chemical freeze-out

“…To minimize such a possible bias, we use ratios constructed from all measured hadron species avoiding, as much as possible, the repetition of any particular yield. Another possibility is to fit the measured hadron yields [20,21], which implies an extra parameter, the fireball volume. This method is free of the bias discussed above, but is subject of another bias, arising, for instance, whenever two measured yields (i.e.…”

“…The results for this case (presented in Table 4 in the Appendix) show a similar pattern as in case of the fits using ratios. We note that poor quality of the fits at SPS energies characterizes other recent analyses within the thermal model [20,69]. Table 1 Summary of the results of the fits using mid-rapidity data at top SPS energy ( √ s N N =17.3 GeV).…”

“…This makes the model a unique tool in the attempt to quantify from the experimental side the features of the phase diagram of hadronic matter [17,18,19]. Recent analyses over a broad energy range [20,21] have contributed in the on-going efforts to understand the chemical freeze-out criteria [1,10,11,22,23,24] in the phase diagram. It has been pointed out within a thermal model analysis that scanning the energy one encounters a transition from baryon-to meson-dominated freeze-out [25], with its associated fingerprint on the characteristics of hadron yields, also evidenced earlier [26].…”

“…The introduction of a γ S factor is an attempt to model this situation (see e.g. [27,20]). It is established that γ S ≃ 1 for central collisions at RHIC [8,9].…”

Recent results from the NA49 experiment

“…Especially the Ξ and Ω production is substantially underestimated and the maximum in the K + /π + ratio is not reproduced. The statistical hadron gas models [10,11], on the other hand, provide a better overall description of the measurements. However, the introduction of an energy dependent strangeness under-saturation factor γ S is needed [11], in order to capture the structures in the energy dependence of most particle species (K + , K − , φ , Ξ).…”