New Challenges to Hydrodynamics from Azimuthal Anisotropy at RHIC

Abstract

This paper presents $v_{4}/v_{2}^2$ ratio as a function of transverse momentum ($p_{t}$), pseudorapidity ($η$) and collision centrality in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). It is found that $v_{4}/v_{2}^2$ is larger than Hydrodynamic calculations, the centrality and transverse dependence of this ratio can not be fully described by Hydrodynamics, and the pseudorapidity dependence is opposite to what one expects from Hydrodynamics. The $p_{t}$ dependence of $v_{1}$ is also presented. It is found that $v_{1}(p_{t})$ for $|η|<1.3$ changes sign, and two possible explanations of the sign change are discussed.