Why is image larger than predicted by hydrodynamics? Original Research Article

The second and fourth Fourier harmonics of the azimuthal distribution of particles, v2 and v4, have been mesured in Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC). The ratio v4/2(v2) is significantly larger than predicted by hydrodynamics. Effects of partial thermalization are estimated on the basis of a transport calculation, and are shown to increase the ratio by a small amount. We argue that the large value of v4/2(v2) seen experimentally is mostly due to elliptic flow fluctuations. However, the standard model of eccentricity fluctuations is unable to explain the large magnitude of v4/2(v2) in central collisions.