Pion production and collective flow effects in intermediate energy nucleus-nucleus collisions

摘要

Invoking the formation of a fireball we analyze the experimental data on pion multiplicity from La + La collision at E-lab(kin) = 1.35 GeV/A and its kinetic energy spectrum. lab We discuss the freeze-out scenario, the possible role of the delta and strange particles and whether the hadronic repulsion (leading to an excluded volume effect) plays any role at the thermochernical and the hydrodynamical freeze-out stage. The importance of collective flow is realized in explaining the pion spectrum at this collision energy. The analysis also provides evidence of the presence of strange particles and the delta resonance in partial chemical equilibrium, at an initial chemical freeze-out stage when the hydrodynamic flow has not developed to have any significant effect. At the final "hydrodynamical freeze-out" we do not find any evidence of the deltas and the strange particles when the thermal temperature has dropped considerably. The importance of the collective flow vis a vis the resonance decay contribution is also discussed in explaining the pion kinetic energy spectra. Further, all the model parameters are determined uniquely, unlike previous analysis, thus avoiding any arbitrariness.