Centrality Dependence of Two-Particle Correlations in Heavy Ion Collisions

Data from the PHOBOS detector have been used to study two-particle correlations over a broad range of pseudorapidity. A simple cluster model parameterization has been applied to inclusive two-particle correlations over a range of centrality for both Cu+Cu and Au+Au collisions at √ sNN = 200 GeV. Analysis of the data for Au+Au has recently been extended to more peripheral collisions showing that the previously-observed rise in cluster size with decreasing system size eventually reaches a maximum value. Model studies have been used to quantify the significant effect of limited detector acceptance on the extracted cluster parameters. In the case of Au+Au, correlations between a trigger particle with pT > 2.5 GeV and inclusive associated particles have also been studied. These reveal the presence of a ‘ridge’ at small relative azimuthal angle which extends with roughly constant amplitude out to the largest relative pseudorapidity studied. The large phase-space coverage of the PHOBOS detector has enabled a quantitative understanding of the so-called ‘ZYAM’ parameter used in the subtraction of the contribution of elliptic flow to these triggered correlations. Studies of single-particle distributions extracted using data from ultra-relativistic heavy ion collisions have provided a wealth of information concerning the global properties of these interactions. Two-particle correlations can provide more detailed information, in particular probing the characteristics of the particle production process itself. The PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC) has been used to study correlations in which both particles in a pair are chosen from the inclusive distribution [1, 2] as well as those in which inclusive particles are correlated with a trigger particle selected to have pT > 2.5 GeV [3]. Inclusive correlations were analyzed using a simple cluster model parameterization for particles emitted in collisions of Au+Au and Cu+Cu at √ sNN = 200 GeV [2]. The average number of emitted particles (cluster size, Keff) and their spread in pseudorapidity (cluster width, δ) were extracted for interactions over a range of centrality. It was found that the cluster sizes are quite large and increase with decreasing system size. Comparing results for Au+Au and Cu+Cu, similar cluster sizes are observed for collisions at the same fraction of the total inelastic cross section, as opposed to systems with the same number of participants. The left panel of Fig. 1 shows data for the cluster sizes versus centrality in Au+Au which have recently been extended to more peripheral collisions. The rise in cluster size is seen to saturate and possibly even begin to decrease in the most peripheral collisions. Continuation of this trend might result in values for the cluster size close to that seen for p+p collisions which is shown by the gray band in the figure. Preprint submitted to Nuclear Physics A September 14, 2009 0 σ / σ 10.2 0.4 0.6 0.8 1.0 ef f ∞ |η| K