Beam Energy Dependence of the Third Harmonic of Azimuthal Correlations in Au+Au Collisions at RHIC

TL;DR

This study analyzes the third harmonic of azimuthal correlations in Au+Au collisions across a range of energies, revealing insights into the presence of quark-gluon plasma and its properties at different collision energies.

Contribution

It provides the first systematic measurement of the third harmonic $v_3^2 ext{(2)}$ over a wide energy range, highlighting its persistence at low energies and its scaling behavior.

Findings

$v_3^2 ext{(2)}$ persists down to 7.7 GeV in central collisions.

$v_3^2 ext{(2)}$ is consistent with zero in peripheral collisions at low energies.

A minimum in scaled $v_3^2 ext{(2)}$$ occurs near 20 GeV.

Abstract

We present results from a harmonic decomposition of two-particle azimuthal correlations measured with the STAR detector in Au+Au collisions for energies ranging from $\sqrt{s_{NN}}=7.7$ GeV to 200 GeV. The third harmonic $v_3^2\{2\}=\langle \cos3(\phi_1-\phi_2)\rangle$, where $\phi_1-\phi_2$ is the angular difference in azimuth, is studied as a function of the pseudorapidity difference between particle pairs $\Delta\eta = \eta_1-\eta_2$. Non-zero {\vthree} is directly related to the previously observed large-$\Delta\eta$ narrow-$\Delta\phi$ ridge correlations and has been shown in models to be sensitive to the existence of a low viscosity Quark Gluon Plasma (QGP) phase. For sufficiently central collisions, $v_3^2\{2\}$ persist down to an energy of 7.7 GeV suggesting that QGP may be created even in these low energy collisions. In peripheral collisions at these low energies however, $v_3^2\{2\}$ is consistent with zero. When scaled by pseudorapidity density of charged particle multiplicity per participating nucleon pair, $v_3^2\{2\}$ for central collisions shows a minimum near {\snn}$=20$ GeV.