Ideal hydrodynamics and elliptic flow at SPS energies: Importance of the initial conditions

Abstract

The elliptic flow excitation function calculated in a full (3+1)d hybrid Boltzmann approach with an intermediate hydrodynamic stage for heavy ion reactions from GSI-SIS to the highest CERN-SPS energies is discussed in the context of the experimental data. In this study, we employ a hadron gas equation of state to investigate the differences in the dynamics and viscosity effects. The specific event-by-event setup with initial conditions and freeze-out from a non-equilibrium transport model allows for a direct comparison between ideal fluid dynamics and transport simulations. At higher SPS energies, where the pure transport calculation cannot account for the high elliptic flow values, the smaller mean free path in the hydrodynamic evolution leads to higher elliptic flow values. In contrast to previous studies within pure hydrodynamics, the more realistic initial conditions employed here and the inclusion of a sequential final state hadronic decoupling provides results that are in line with the experimental data almost over the whole energy range from $E_{\mathrm{lab}}=2-160A$GeV. Thus, this new approach leads to a substantially different shape of the $v_2/\epsilon$ scaling curve as a function of $(1/Sd{N}_{ch}/dy)$ in line with the experimental data compared to previous ideal hydrodynamic calculations. This hints to a strong influence of the initial conditions for the hydrodynamic evolution on the finally observed $v_2$ values, thus questioning the standard interpretation that the hydrodynamic limit is only reached at RHIC energies.

6 pages, 5 figures

References in corpus (7)

• Fully integrated transport approach to heavy ion reactions with an intermediate hydrodynamic stage
• Space-time evolution of bulk QCD matter
• Mass ordering of differential elliptic flow and its violation for phi mesons
• Importance of Granular Structure in the Initial Conditions for the Elliptic Flow
• (3+1)-Dimensional Hydrodynamic Expansion with a Critical Point from Realistic Initial Conditions
• Directed and elliptic flow in heavy ion collisions from $E_{\mathrm{beam}}=90$ MeV/nucleon to $E_{c.m.}=200$ GeV/nucleon
• Effects of a phase transition on HBT correlations in an integrated Boltzmann+Hydrodynamics approach

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