Preferential emission of pions in asymmetric nucleus-nucleus collisions.

Abstract

Pion production in Ne on Pb collisions at a beam energy of 800 MeV/nucleon is studied by solving a coupled set of transport equations for the phase-space distribution functions of nucleons, deltas, and pions. The experimentally observed preferential emission of pions away from the interaction zone towards the projectile side in the transverse direction is found to be due to the stronger pion absorption by the heavier target spectator. A model calculation of the pion transverse momentum distribution in the reaction plane agrees with that of the experimental data. The existence of a collective How signature among the final-state baryons of relativistic heavy-ion collisions at beam energies around I GeV/nucleon has been firmly established by the in-plane transverse momentum analysis [l]. Due to the small mass of pions compared to that of baryons, it has been pointed out that the pions might serve as a good probe of any hydrodynamical Row [2]. Moreover, as pions are mainly corning from the decay of b resonances in the relativistic heavy-ion collisions, the remnant of the collective flow carried by 6 resonances might be seen in the final-state pions. Looking for Aow signatures among pions, several groups [2 — 4] have studied the transverse momentum distribution in the reaction plane for pions (average transverse momentum in the reaction plane (p ) versus rapidity y). One of the most striking results from the DIOGENE Collaboration [2] is that the in-plane transverse momentum of pions is always positive even for backward rapidities, for the asymmetric (Ne or Ar)+(Nb or Pb) systems. However, the intranuclear-cascade model predicts values compatible with zero over the whole range of rapidity [2]. The quantum-molecular-dynamics model calculation of the pion transverse momentum distribution [5] indicates that the introduction of the mean field describes some of the experimental effect, but the model predicts less asymmetry than observed experimentally. Therefore, the question whether the experimentally observed preferential emission of pions away from the interaction zone towards the projectile side in the asymmetric nucleusnucleus collisions is due to the collective Bow of pions or due to the shadowing effect of the heavier target spectator has not been resolved. In this article we report on the results of a study using I a hadronic transport model [6— 8]. The preferential emission of pions toward the projectile side in the transverse direction in the reaction Ne+Pb at a beam energy of 800 MeV/nucleon is found to be due to the stronger absorption of pions by the heavier target spectator. The calculated transverse momentum distribution of pions in the reaction plane agrees with that of the experimental data. The calculations were performed by numerically solving a coupled set of transport equations for the phasespace distribution functions of nucleons, deltas, and pions. The transport equation for the particular state b of the baryon (nucleon and delta) reads [7,8] dfb(xp) + V„fb (xp) V„U(x ) V fb— (xp)