Microscopic study of energy and centrality dependence of transverse collective flow in heavy-ion collisions

Abstract

The centrality dependence of directed and elliptic flow in light and heavy systems of colliding nuclei is studied within two microscopic transport models at energies from $1A$ GeV to $160A$ GeV. The pion-directed flow has negative slope in the midrapidity range irrespective of bombarding energy and mass number of the colliding ions. In contrast, the directed flow of nucleons vanishes and even develops antiflow in the midrapidity range in (semi)peripheral collisions at energies around $11.6A$ GeV and higher. The origin of the disappearance of flow is linked to nuclear shadowing. Since the effect is stronger for a light system, it can be distinguished from a similar phenomenon caused by quark-gluon plasma formation. In the latter case the disappearance of flow due to the softening of the equation of state should be most pronounced in collisions of heavy ions. The centrality dependence of the elliptic flow shows that the maximum in the $〈{v}_{2}(b)〉$ distribution is shifted to very peripheral events with rising incident energy, in accordance with experimental data. This is an indication of the transition from baryonic to mesonic degrees of freedom in hot hadronic matter.