Pion spectra in equilibrium models of nuclear collisions

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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 Δ resonances freeze out at a fixed temperature and density. The spectra are a superposition of thermal pions and pions from the decay of the Δ 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.