Hydrodynamic Aspects of Relativistic Heavy Ion Collisions at Rhic

Among the many predictions made for observables of collisions at the Relativistic Heavy Ion Collider (RHIC), hydrodynamic calculations regarding the expected flow characteristics turned out to be surprisingly accurate already a few months after running the new facility. In fact, those predictions were so convincing that they were awarded a bottle of wine in the RHIC prediction competition. That RHIC creates systems which – for the dominant part of their lifetimes – expand according to hydrodynamic principles is of utmost importance: The dynamical evolution is governed by the nuclear equation of state at extreme energies, whose thermodynamic features are thus accessible through collisions in the laboratory. In the following, I will review indications of the strong transverse expansion that can be deduced from the observable final state of central and non-central Au+Au collisions. Thereby, I will pay particular attention to signals of collectivity at mid-rapidity, i.e. in the plane perpendicular to the beam-axis in the center of mass of the reaction (at vanishing pseudorapidity η). Although interesting phenomena are to be expected at η 6= 0, to which I will return later, the midrapidity region is of particular interest for the observation of collectivity: Before the reaction, there is no transverse component of motion. Observing collectivity in the transverse plane at midrapidity must thus be a result of the dynamical evolution of the fireball