0 Neutral Pions and η Mesons as Probes of the Hadronic Fireball in Nucleus – Nucleus Collisions around 1 A

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 µ B and the chemical freeze–out temperature T c are deduced from the inclusive π 0 and η yields which are augmented by interpolated data on deuteron production. With increasing beam energy µ B drops from 800 MeV to 650 MeV, while T c rises from 55 MeV to 90 MeV. For given beam energy µ B grows with system size, whereas T c 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 T th are equal to or slightly lower than 1 T c , indicative of nearly simultaneous chemical and thermal freeze–out.