Neutral Pions and Eta 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 mu_B and the chemical freeze-out temperature T_c are deduced from the inclusive neutral pion and eta yields which are augmented by interpolated data on deuteron production. With increasing beam energy mu_B drops from 800 MeV to 650 MeV, while T_c rises from 55 MeV to 90 MeV. For given beam energy 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_th are equal to or slightly lower than T_c, indicative of nearly simultaneous chemical and thermal freeze-out.Comment: 42 pages, 12 figure

Influence of Impact Parameter on Thermal Description of Relativistic Heavy Ion Collisions at GSI/SIS

Attention is drawn to the role played by the size of the system in the thermodynamic analysis of particle yields in relativistic heavy ion collisions at SIS energies. This manifests itself in the non-linear dependence of K+ and K- yields in $AA$ collisions at 1 -- 2 A.GeV on the number of participants. It is shown that this dependence can be quantitatively well described in terms of a thermal model with a canonical strangeness conservation. The measured particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also explained in the context of a thermal model with a common freeze-out temperature and chemical potential. Including the concept of collective flow a consistent picture of particle energy distributions is derived with the flow velocity being strongly impact-parameter dependent.Comment: revtex, 20 figure

Realistic Equations of State for the Primeval Universe

Early universe equations of state including realistic interactions between constituents are built up. Under certain reasonable assumptions, these equations are able to generate an inflationary regime prior to the nucleosynthesis period. The resulting accelerated expansion is intense enough to solve the flatness and horizon problems. In the cases of curvature parameter \kappa equal to 0 or +1, the model is able to avoid the initial singularity and offers a natural explanation for why the universe is in expansion.Comment: 32 pages, 5 figures. Citations added in this version. Accepted EPJ