Charged pion anisotropy in relativistic nucleus-nucleus collisions

Abstract The momentum distributions with respect to the reaction plane for charged pions emitted in Ne-nucleus and Ar-nucleus collisions have been measured with the 4 π detector Diogene at the Saturn synchrotron. The reaction plane is estimated from the baryon momenta, using a standard technique for flow analysis. The pions exhibit a non-zero flow. For asymmetric systems, there is a preferential emission in the direction of the lighter (projectile) nucleus. Such a behaviour contradicts the predictions of INC calculations and is underestimated by QMD calculations which include mean field effects. It can be interpreted with a simple geometrical model in terms of absorption by the spectator nuclear matter.