Energy and system-size dependence of two- and four-particle $v_2$ measurements in heavy-ion collisions at RHIC and their implications on flow fluctuations and nonflow

Abstract

We present STAR measurements of azimuthal anisotropy by means of the two- and four-particle cumulants $v_2$ ($v_2\{2\}$ and $v_2\{4\}$) for Au+Au and Cu+Cu collisions at center of mass energies $\sqrt{s_{{}_{\mathrm{NN}}}}=62.4$ and 200 GeV. The difference between $v_2\{2{\}}^2$ and $v_2\{4{\}}^2$ is related to $v_2$ fluctuations ($\ddot{I}{ƒ}_{v_2}$) and nonflow $(\hat{I}{´}_2)$. We present an upper limit to $\ddot{I}{ƒ}_{v_2}/v_2$. Following the assumption that eccentricity fluctuations $\ddot{I}{ƒ}_{\hat{I}}\mu$ dominate $v_2$ fluctuations $\frac{\ddot{I}{ƒ}_{v_2}}{v_2}\approx \frac{\ddot{I}{ƒ}_{\hat{I}}\mu }{\hat{I}}\mu$ we deduce the nonflow implied for several models of eccentricity fluctuations that would be required for consistency with $v_2\{2\}$ and $v_2\{4\}$. We also present results on the ratio of $v_2$ to eccentricity.

15 pages, 12 Figures