Hydrodynamic Approaches to Relativistic Heavy Ion Collisions

First data reported by the STAR Collaboration at RHIC [1] has a significant meaning that the observed large magnitude of elliptic flow for charged hadrons is consistent with hydrodynamic predictions [2]. This suggests that large pressure possibly in the partonic phase is built at the early stage (τ ∼ 0.6 fm/c) in Au+Au collisions at sNN = 130 and 200 GeV. This situation at RHIC is in contrast to that at lower energies such as AGS or SPS where hydrodynamics always overpredicts the data [3]. Moreover, this also suggests that the effect of the viscosity in the QGP phase is remarkably small and that the QGP is almost a perfect fluid [4]. Hadronic transport models are very good to describe experimental data at lower energies, while they fail to reproduce such large values of elliptic flow parameter at RHIC (see, e.g., Ref. [5]). So the importance of hydrodynamics is rising in heavy ion physics. After the first STAR data were published [1], other groups at RHIC have also obtained the data concerning with flow phenomena [6]. Contrary to the success of the hydrodynamics to describe the elliptic flow, many dynamical models including hydrodynamics cannot reproduce the HBT radii [7, 8, 9]. It is known as the “HBT puzzle”.