Pion production in intermediate energy nucleus-nucleus collisions

Pion production in heavy-ion collisions is reviewed while placing emphasis on recent subthreshold experiments. The data are compared with predictions of NN collision and statistical models. The study of collision dynamics together with pion energy spectra suggests that the production mechanism at energies above 60 A MeV can be understood by either a microscopic description such as BUU, or by a statistical picture reflecting centrality. As for below 60 A MeV, the importance of high momentum components in the momentum distribution and of the limiting temperature of hot nuclear matter are discussed from respective points of view. (orig.)