Directed and elliptic flow in heavy-ion collisions from Ebeam=90 MeV/nucleon to Ec.m.=200 GeV/nucleon

Recent data from the NA49 experiment on directed and elliptic flow for Pb+Pb reactions at CERN-SPS are compared to calculations with a hadron-string transport model, the ultrarelativistic quantum molecular dynamics (UrQMD) model. The rapidity and transverse momentum dependence of the directed and elliptic flow, i.e., v₁ and v₂, are investigated. The flow results are compared to data at three different centrality bins. Generally, a reasonable agreement between the data and the calculations is found. Furthermore, the energy excitation functions of v₁ and v₂ from E_beam=90A MeV to E_c.m.=200A GeV are explored within the UrQMD framework and discussed in the context of the available data. It is found that, in the energy regime below E_beam⩽10A GeV, the inclusion of nuclear potentials is necessary to describe the data. Above 40A GeV beam energy, the UrQMD model starts to underestimate the elliptic flow. Around the same energy the slope of the rapidity spectra of the proton directed flow develops negative values. This effect is known as the third flow component (“antiflow”) and cannot be reproduced by the transport model. These differences can possibly be explained by assuming a phase transition from hadron gas to quark gluon plasma at about 40A GeV.