Chemical equilibrium study in nucleus-nucleus collisions at relativistic energies

We present a detailed study of chemical freeze-out in nucleus-nucleuscollisions at beam energies of 11.6, 30, 40, 80 and 158A GeV. By analyzinghadronic multiplicities within the statistical hadronization approach, we havestudied the strangeness production as a function of centre of mass energy andof the parameters of the source. We have tested and compared different versionsof the statistical model, with special emphasis on possible explanations of theobserved strangeness hadronic phase space under-saturation. We show that, inthis energy range, the use of hadron yields at midrapidity instead of in fullphase space artificially enhances strangeness production and could lead toincorrect conclusions as far as the occurrence of full chemical equilibrium isconcerned. In addition to the basic model with an extra strange quarknon-equilibrium parameter, we have tested three more schemes: a two-componentmodel superimposing hadrons coming out of single nucleon-nucleon interactionsto those emerging from large fireballs at equilibrium, a model with localstrangeness neutrality and a model with strange and light quark non-equilibriumparameters. The behaviour of the source parameters as a function of collidingsystem and collision energy is studied. The description of strangenessproduction entails a non-monotonic energy dependence of strangeness saturationparameter gamma_S with a maximum around 30A GeV. We also present predictions ofthe production rates of still unmeasured hadrons including the newly discoveredTheta^+(1540) pentaquark baryon.Comment: 36 pages, 14 figures. Revised version published in Phys. Rev. C: title changed, one paragraph added in section 2, other typos correcte