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

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 $v_3^2\{2\}=⟨\cos 3(\ddot{I}{•}_1-\ddot{I}{•}_2)⟩$, where $\ddot{I}{•}_1-\ddot{I}{•}_2$ is the angular difference in azimuth, is studied as a function of the pseudorapidity difference between particle pairs $\hat{I}”\hat{I}\cdot =\hat{I}{\cdot }_1-\hat{I}{\cdot }_2$. Non-zero {\vthree} is directly related to the previously observed large-$\hat{I}”\hat{I}\cdot$ narrow-$\hat{I}”\ddot{I}•$ 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.

7 pages, 4 figures, for submission to Phys. Rev. Lett