Azimuthal Anisotropy at the Relativistic Heavy Ion Collider: The First and Fourth Harmonics

TLDR

The study reports the first observations of directed flow (v1) and fourth harmonic (v4) in the azimuthal distribution of particles relative to the reaction plane in Au+Au collisions at RHIC. Both measurements exploit the large elliptic flow (v2) generated at RHIC. From the correlation of v2 with v1, v2 is found to be positive (in‑plane), the integrated v4 is roughly ten times smaller than v2, and upper limits are set for the sixth and eighth harmonics.

Abstract

We report the first observations of the first harmonic (directed flow, ${v}_{1}$) and the fourth harmonic (${v}_{4}$), in the azimuthal distribution of particles with respect to the reaction plane in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at the BNL Relativistic Heavy Ion Collider (RHIC). Both measurements were done taking advantage of the large elliptic flow (${v}_{2}$) generated at RHIC. From the correlation of ${v}_{2}$ with ${v}_{1}$ it is determined that ${v}_{2}$ is positive, or in-plane. The integrated ${v}_{4}$ is about a factor of 10 smaller than ${v}_{2}$. For the sixth (${v}_{6}$) and eighth (${v}_{8}$) harmonics upper limits on the magnitudes are reported.