Nonequilibrium Models of Relativistic Heavy-Ion Collisions

Journal of Physics G Nuclear and Particle Physics (Impact Factor: 5.33). 12/2004; DOI: 10.1088/0954-3899/31/6/037
Source: arXiv

ABSTRACT We review the results from the various hydrodynamical and transport models on the collective flow observables from AGS to RHIC energies. A critical discussion of the present status of the CERN experiments on hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A$\cdot$GeV: here the hydrodynamic model has predicted the collapse of the $v_1$-flow and of the $v_2$-flow at $\sim 10$ A$\cdot$GeV; at 40 A$\cdot$GeV it has been recently observed by the NA49 collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as evidence for a first order phase transition at high baryon density $\rho_B$. Moreover, the connection of the elliptic flow $v_2$ to jet suppression is examined. It is proven experimentally that the collective flow is not faked by minijet fragmentation. Additionally, detailed transport studies show that the away-side jet suppression can only partially ($<$ 50%) be due to hadronic rescattering. Furthermore, the change in sign of $v_1, v_2$ closer to beam rapidity is related to the occurence of a high density first order phase transition in the RHIC data at 62.5, 130 and 200 A$\cdot$GeV. Comment: 15 pages, 7 eps figures, to be published in J. Phys. G - Proceedings of SQM 2004

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    ABSTRACT: 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., v⁠and vâ, 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 v⁠and vâ from E{sub beam}=90A MeV to E{sub 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{sub beam}{
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    ABSTRACT: The energy excitation functions of directed flow (v1) and elliptic flow (v2) from Ebeam=90A MeV to Ecm=200A GeV are explored within the UrQMD framework and discussed in the context of the available data. The radial and the elliptic flow of the particles produced in a relativistic heavy-ion collision are intimately connected to the pressure and its gradients in the early stage of the reaction. Therefore, these observables should also be sensitive to changes in the equation of state. To prove this connection, the temporal evolution of the pressure, pressure gradients and elliptic flow are shown. For the flow excitation functions it is found that, in the energy regime below Ebeam≤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. The difference between the data and the UrQMD model can possibly be explained by assuming a phase transition from hadron gas to quark–gluon plasma around Elab=40A GeV. This would be consistent with the model calculations, indicating a transition from hadronic matter to “string matter” in this energy range. Thus, we speculate that the missing pressure might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au + Au (Pb + Pb) collisions already at lower SPS energies.
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