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 (${\sigma }_{v_2}$) and nonflow $({\delta }_2)$. We present an upper limit to ${\sigma }_{v_2}/v_2$. Following the assumption that eccentricity fluctuations ${\sigma }_{\epsilon }$ dominate $v_2$ fluctuations $\frac{{\sigma }_{v_2}}{v_2}\approx \frac{{\sigma }_{\epsilon }}{\epsilon }$ 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

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