Centrality Dependence of Two-Particle Correlations in Heavy Ion Collisions

TL;DR

This paper investigates two-particle correlations in heavy ion collisions using PHOBOS data, revealing a maximum cluster size in peripheral collisions, the presence of a ridge structure, and clarifying flow subtraction effects.

Contribution

It provides a detailed analysis of cluster sizes and correlation structures across different collision centralities and system sizes, including the first observation of a ridge at high transverse momentum.

Findings

Cluster size reaches a maximum in peripheral collisions.

A ridge structure extends over large pseudorapidity at high pT.

Quantitative understanding of flow subtraction parameters.

Abstract

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 sqrt(s_NN)=200GeV. 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>2.5GeV 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.