Subthreshold pion production in nucleus-nucleus collisions

Abstract The production of neutral pions in nucleus-nucleus collisions has been observed at projectile energies down to 20 MeV/u; at such low energies the production of a pion requires a cooperative action of manu nucleons. Apparently the necessary conversion of energy out of the relative motion takes place in the early stage of the collision before thermalization. This fast process results in bremsstrahlung-like pion spectra and angular distributions. A non-equilibrium emission is suggested as well by the excitation function as by the dependence on target and projectile mass; the latter also indicates non-isotropic reabsorption of the emitted pions. The systematic data available presently form a rather complete basis for a consistent description of the pion production from nuclear reactions.