Pion Squeeze-Out and Flow at 1.15 GeV/Nucleon Au+Au

Abstract

Attaining a better understanding of the hadronic gas phase of nuclear matter has been one of the principal goals of studying heavy-ion reactions in the GeV/nucleon range. In-plane transverse flow is understood as preferential emission of particles on the projectile side or the target side of the reaction plane for velocities higher than or lower than the center of mass velocity respectively. Flow of nucleons has been seen as a signature of the compression generated during the early stage of a heavy-ion collision. The magnitude of the flow can be used to infer the compressibility of nuclear matter at high temperatures and pressures. Squeeze-out, on the other hand, is defined as a preferential emission of particles out of the reaction plane for center-of-mass rapidities. For nucleons, squeeze-out is also understood as an expression of the compression during the early stages of the reaction. It is also natural to use pion production as a probe of the hot, dense stage of a reaction, as the pions are produced during these early phases ^[1,2]. The pions, however, may behave differently than the nucleons, as they are mesons and not baryons and thus interact differently with the surrounding matter. The nucleons mostly experience elastic N-N collisions, while pions generally have inelastic interactions. Also, pions are produced particles and thus may respond differently to the pressure gradients within the nuclear matter. The pions are produced mostly in the dense, high pressure region which is characterized by radial expansion. The nucleons in the pressure gradient region exhibit the maximum flow.