Equilibration in relativistic nuclear collisions. A Monte Carlo calculation

A relativistic Monte Carlo calculation of the nucleus-nucleus interaction in the GeV range is presented. The interaction process is described as a sequence of classical, binary, on-shell baryon-baryon collisions. Pion production is introduced via the formation of Δ-resonances. The latter are given a definite mass and a lifetime against pion emission larger than the collision time. They are, however, assumed to scatter or disappear in collisions with nucleons. At the end of the collision process, they are allowed to decay. The model is used to study the equilibration during a head-on collision between two 40Ca nuclei. The system is found to be compressed up to a time 6–8 fm/c and to decompress very rapidly. The final nucleon and pion momentum distributions are not completely thermalized. They are, however, tentatively described by effective temperatures. The rapidity distributions show larger temperatures than the perpendicular momentum distributions. Also, nucleon temperatures are generally larger than pion temperatures. The theoretical transverse temperatures and the pion multiplicities agree fairly well with the experimental data. The role of the delta particles is investigated. It is shown that the delta production quickens the equilibration process by transforming longitudinal kinetic energy into mass energy. Furthermore, it favours high compression of the system. Non-central collisions are studied. The results are consistent with the concept of geometrical separation between participant and spectator nucleons. However, our model predicts more transparency than the so-called fireball model. The participant part is shown to decompress very rapidly, while the spectator parts are slightly kicked off for intermediate impact parameters. Finally, some results are shown for the case of a head-on collision of a 20Ne projectile by a 60Ni target. A strong shock wave propagates into the target for several fm/c, after which all the matter is dispersed.

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