ar X iv : n uc l - th / 0 51 20 51 v 1 1 3 D ec 2 00 5 “ RHIC serves the perfect fluid ” – Hydrodynamic flow of the QGP ∗

The bulk of the hot and dense matter created at RHIC behaves like an almost ideal fluid. I present the evidence for this and also discuss what we can learn about the transport properties of the quark-gluon plasma (QGP) from the gradual breakdown of ideal fluid dynamic behavior at large transverse momenta, lower beam energies, larger impact parameters, and forward rapidities. 1 The QCD Equation of State and ideal fluid dynamics With relativistic heavy-ion collisions one explores the phase diagram of strongly interacting bulk matter in the regime of high energy density and temperature. Lattice QCD (LQCD) tells us [1] that for zero net baryon density QCD matter undergoes a phase transition at T cr = 173 ± 15 MeV from a color-confined hadron resonance gas (HG) to a color-deconfined quark-gluon plasma (QGP). The critical energy density e cr ≃ 0.7 GeV/fm 3 [1] corresponds roughly to that in the center of a proton. At the phase transition, the normalized energy density e/T 4 rises rapidly by about an order of magnitude over a narrow temperature interval ∆T < ∼ 15 − 20 MeV, whereas the pressure p/T 4 (which is proportional to the grand canonical thermodynamic potential) is continuous and rises more gradually (Fig. 1). Both seem to saturate at about 80-85% of the Stefan-Boltzmann value for an ideal gas of noninteracting quarks and gluons, the energy density more quickly (at about 1.2 T cr), the pressure more slowly. Above about 2 T cr , the lattice data follow the Equation of State of an ideal gas of massless particles, e = 3p. For many years this observation has been interpreted as lattice QCD support for the hypothesis of a weakly interacting, perturbative QGP. The recent RHIC data taught us that this interpretation was quite wrong. The first part of the title of this talk, which was lifted from a coffee mug nowadays distributed by Brookhaven National Laboratory to their guests, alludes to this exciting discovery.