DIRECT REACTIONS WITH PION-PRODUCTION IN NUCLEUS-NUCLEUS COLLISIONS

Various processes of pion production in nuclear matter at characteristic temperatures T much less than m(pi) are analyzed. It is shown that the pions are produced mainly in NN --> NN(pi) reactions. Taking into account the modification of the NN interaction in nucelar matter, probabilities and differential cross sections of production of pi- mesons in direct NN --> NNpi- reactions are calculated. It is shown that at incident-pion momenta q approximately 2m(pi) processes with pion production via DELTA-resonance intermediate states dominate. From a comparison of the results of the calculation with experimental data on pion production in nucleus-nucleus collisions it is concluded that piN absorption is strong and increases with increasing temperature and density of the nuclear matter. At the initial stage of development of the nuclear fireball the pions are confined inside the system at practically all momenta. Soft (q less-than-or-similar-to m(pi)) pions are emitted at the intermediate stage of expansion of the nuclear fireball, and harder pions are released together with nucleons at the final stage of breakup of the fireball.