Pion spectra in equilibrium models of nuclear collisions.

Abstract

We calculate pion spectra within three simple models of pion production in relativistic heavy ion collisions. All three models are based on particle freeze-out from thermal and chemical equilibrium. In the first one, nucleons, pions, and \ensuremath{\Delta} resonances freeze out at a fixed temperature and density. The spectra are a superposition of thermal pions and pions from the decay of the \ensuremath{\Delta} resonance. In the second model, we assume that pions are radiated from a hot, spherical source of nuclear matter, which loses energy because of this radiation. In the last calculation, we employ a hydrodynamic model of the expansion process to determine the pion spectra produced by a sequential freeze-out at fixed density and different times and temperatures. Only this last model gives a shape of the pion distribution similar to the experimental one. It is found that three different effects contribute to the concave shape of the spectra in this model.