Neutral pions and η mesons as probes of the hadronic fireball in nucleus-nucleus collisions around1AGeV

Abstract

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 ${\ensuremath{\mu}}_{B}$ and the chemical freeze-out temperature ${T}_{c}$ are deduced from the inclusive ${\ensuremath{\pi}}^{0}$ and \ensuremath{\eta} yields which are augmented by interpolated data on deuteron production. With increasing beam energy ${\ensuremath{\mu}}_{B}$ drops from 800 MeV to 650 MeV, while ${T}_{c}$ rises from 55 MeV to 90 MeV. For given beam energy ${\ensuremath{\mu}}_{B}$ 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 ${T}_{\mathrm{th}}$ are equal to or slightly lower than ${T}_{c},$ indicative of nearly simultaneous chemical and thermal freeze-out.