Pions and hard photons as probes for nucleus-nucleus collisions

Abstract Pion production in nucleus-nucleus collisions at energies from 20 MeV/u up to 84 MeV/u can be shown to be a largely cooperative effect. The spectra of neutral pions and high energy photons can be understood within the framework of a bremsstrahlung process in contrast to a statistical emission picture, which e.g. underpredicts the pion yield at V20 MeV/u as we have found in a recent experiment.