Study on chemical equilibrium in nucleus - nucleus collisions at relativistic energies

We present a detailed study of chemical freeze-out in nucleus-nucleus collisions at beam energies of 11.6, 30, 40, 80 and 158A GeV. By analyzing hadronic multiplicities within the statistical hadronization approach, we have studied the chemical equilibration of the system as a function of center of mass energy and of the parameters of the source. Additionally, we have tested and compared different versions of the statistical model, with special emphasis on possible explanations of the observed strangeness hadronic phase space under-saturation. One of the main results of the study of high energy A-A collisions is a surprising success of the statistical-thermal models in reproducing essential features of particle production (see for example [ 1, 2]). In this paper we study nucleus-nucleus collision within the statistical model in the energy range that is believed [ 3] to cover the threshold for creation of Quark-Gluon Plasma (QGP) in the early stage of Pb-Pb collisions. The main idea of the SHM is that hadrons are emitted from regions at statistical equilibrium. No hypothesis is made about how statistical equilibrium is achieved; this can be a direct consequence of the hadronization process. In a single collision event, there might be several clusters with different collective momenta, different overall charges and volumes. However, Lorentz-invariant quantities like particle multiplicities are independent of clusters momenta.