Jacques Vandermeulen

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.

PION PRODUCTION IN CENTRAL HIGH-ENERGY NUCLEAR COLLISIONS

Abstract A model for pion production is studied in the context of a cascade calculation. The pions are produced through Δ-resonances which are allowed to decay. The emitted pions are assumed to interact with the other nucleons by forming new Δ -resonances. The time evolution of the pion and Δ-population is studied; it is found that Δs are always more numerous than pions during the sequence of baryon-baryon collisions. The spectrum of the pions is in considerably better agreement with experiment than the one obtained with frozen Δ-isobars. The presence of Δ-resonances appears to be important for the cooling of the pion system. The pion multiplicity is found to deviate from a Poisson distribution. The pion yield is overestimated by at least 25%; this result is discussed within the framework of conventional dynamics.

Possibility of Unusual Antiproton Annihilation on Nuclei

Abstract We give indications in favour of an antiproton annihilation by two nucleons inside a nucleus. The decay is investigated in the frame of a statistical model. It is shown that this process can lead to a substantial increase of the strange particle yield.

N\(\bar N\) Annihilation creates two mesons

AbstractN

Low-energy antiproton annihilation in nuclei

\bar N

Nucleus excitation and deexcitation following p-annihilation at rest

annihilation is assumed to occur exclusively via two-meson production, with nearest threshold dominance. A phenomenological expression for the branching ratios, implementing the rule, contains two parameters. The set ofs- andp-wave (q

Residual mass distribution following p̄-nucleus annihilation

\bar q