Pionic fusion ‐Coherent pion production in nuclear collisions‐

The coherent production of pions in a collision of two complex nuclei has recently been observed with a number of different target projectile combinations, mostly near the physical threshold. Such reactions are highly interesting for mainly two reasons: the total free energy of the entrance channel is converted into a fast pion; the two incident nuclei undergo a fusion process to form a bound state of the united nucleus (’’ultra cold fusion’’). Such a pionic fusion reaction is difficult to understand microscopically on the basis of our conventional knowledge about nuclear collisions. Its cooperative nature raises two new types of questions: What is the mechanism that couples the kinetic energy of the entrance channel to the pion field, avoiding the usually observed thermalisation process? What are the selection rules for different entrance channel fragmentations that determine the population of a specific state of a given final nucleus? Such a reaction can also be used as a tool to investigate specific ph...