System Size, Energy, Pseudorapidity, and Centrality Dependence of Elliptic Flow

TL;DR

This Letter presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions using the PHOBOS detector at the Relativistic Heavy Ion Collider, finding that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects.

Abstract

This paper presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.

Figures (5)

• Figure 1: v$_2$ vs. $\eta$ for Cu-Cu collisions at $\sqrt{s_{_{\rm NN}}} =$ 62.4 and 200 GeV using the hit-based analysis. The boxes show the 90% C.L. systematic errors and the bars represent the 1-$\sigma$ statistical errors. Previously published 200 GeV Au-Au data (without error bars) is shown for comparison.
• Figure 2: v$_2$ vs. $|\eta|-$y$_{\rm beam}$ for Cu-Cu collisions at $\sqrt{s_{_{\rm NN}}} =$ 62.4 and 200 GeV from the hit-based analysis. Only 1-$\sigma$ statistical errors are shown.
• Figure 3: v$_2$ vs. N$_{\rm part}$ for Cu-Cu collisions at $\sqrt{s_{_{\rm NN}}} =$ 62.4 and 200 GeV. The boxes show the 90% C.L. systematic errors and the lines represent the 1-$\sigma$ statistical errors. The results from two analysis methods are shown, as discussed in the text. v$_2$ is shown for $|\eta| < 1$ and $0 < \eta < 1$ for the hit-based and track-based methods, respectively.
• Figure 4: The average eccentricity defined in two ways, ($\langle \varepsilon_{\rm std} \rangle$ and $\langle \varepsilon_{\rm part} \rangle$), as described in the text, vs. N$_{\rm part}$ for simulated Cu-Cu and Au-Au collisions at $\sqrt{s_{_{\rm NN}}} =$ 200 GeV. The bands show the 90% C.L. systematic errors.
• Figure 5: (a) v$_2$ (unscaled) vs. N$_{\rm part}$, (b) v$_2$/$\langle \varepsilon_{\rm std} \rangle$ vs. N$_{\rm part}$, and (c) v$_2$/$\langle \varepsilon_{\rm part} \rangle$ vs. N$_{\rm part}$, for Cu-Cu and Au-Au collisions at $\sqrt{s_{_{\rm NN}}} =$ 62.4 and 200 GeV. 1-$\sigma$ statistical error bars are shown. v$_2$ is shown in $|\eta| < 1$ for the hit-based method.