Neutral pions and η mesons as probes of the hadronic fireball in nucleus-nucleus collisions around 1A GeV

Chemical and thermal freeze-out of the hadronic fireball formed in symmetric collisions of light, intermediate-mass, and heavy nuclei at beam energies between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated, isospin-symmetric ideal hadron gas with grand-canonical baryon-number conservation. For each collision system the baryochemical potential μB and the chemical freeze-out temperature Tc are deduced from the inclusive π0 and η yields which are augmented by interpolated data on deuteron production. With increasing beam energy μB drops from 800 MeV to 650 MeV, while Tc rises from 55 MeV to 90 MeV. For given beam energy μB grows with system size, whereas Tc remains constant. The centrality dependence of the freeze-out parameters is weak as exemplified by the system Au+Au at 0.8A GeV. For the highest beam energies the fraction of nucleons excited to resonance states reaches freeze-out values of nearly 15%, suggesting resonance densities close to normal nuclear density at maximum compression. In contrast to the particle yields, which convey the status at chemical freeze-out, the shapes of the related transverse-mass spectra do reflect thermal freeze-out. The observed thermal freeze-out temperatures Tth are equal to or slightly lower than Tc, indicative of nearly simultaneous chemical and thermal freeze-out