Elliptic and directed flow in heavy ion collisions at GSI-FAIR and CERN-SPS energies

Hannah Petersen

Recent data on Pb+Pb reactions for the directed and elliptic flow from the NA 49 experiment at the CERN-SPS are compared to hadron-string transport model calculations (Ultra-relativistic quantum molecular dynamics model (UrQMD)). The rapidity and the transverse momentum dependence of the directed and elliptic flow, i.e. v1 and v2, are investigated. Since the flow is very sensitive to a change in the centrality of the collision the results are compared to data at three different centrality bins. Generally a reasonable agreement between data and calculations is found. Furthermore, the energy excitation function of the elliptic flow (v2) from GSI-SIS to BNL-RHIC is explored within the UrQMD model and discussed in the context of the available data. It is found that in the energy regime below Ebeam=10 AGeV the inclusion of nuclear potentials is necessary to describe the data. Above 40 A GeV beam energy the UrQMD model starts to underestimate the elliptic flow. This difference can possibly be explained by assuming a phase transition from hadron gas to quark gluon plasma at ~30 GeV.