Pion production and source effects in intermediate energy heavy ion collisions

The study of pion production in heavy ion collisions was mainly motived by the observation of an enhancement of yield from a source of hot or/and dense nuclear matter. At subthreshold incident energies the background of incoherent nucleon-nucleon processes is expected to be strongly reduced, then any collective signal should be better seen. In the data no significant deviation of the cross-sections was observed, excepted may be for the lowest incident energies. An overview of this subject was already presented at the last RIKEN-IN2P3 symposium and in this paper, the author focuses on a particular aspect of charged pions spectra from which one can expect to draw some information about the collision dynamics in the region below 100 MeV per nucleon. It was in one of the first experiment of π production in Ne+NaF collisions at BEVALAC, that a large difference between π and π+ was observed at an angle of zero degrees. The ratio π-/π+ showed a narrow maximum as function of momentum for those pions having a velocity close to the beam. This peak was clearly visible for incident energy above 200 MeV per nucleon, where the projectile velocity better matched the experimental pion acceptance. This effect was easily interpreted in term of the Coulomb interaction between the pions and the fragments of the projectile travelling along the beam axis. A complete theory was developed, which was used to determine the projectile charge after the collision; this was confirmed experimentally in coincidence experiments. Nearly at the same time (more than ten years ago) it was suggested by the authors of reference, working on pion production in heavy ion collisions at higher incident energies, that it might be possible to get some geometrical picture of the collision via the same coulomb process