Collective flow and long-range correlations in relativistic heavy ion collisions

TL;DR

This paper analyzes dihadron correlation data from heavy ion collisions, demonstrating that the long-range ridge-like correlations are consistent with collective flow effects, but also discusses potential pitfalls in background subtraction methods.

Contribution

It clarifies the role of collective flow in ridge-like correlations and highlights issues with the ZYAM background subtraction method.

Findings

Long-range correlations are consistent with collective flow.

Discrepancies arise from background subtraction methods.

ZYAM method can introduce pitfalls in data interpretation.

Abstract

Making use of recently released data on dihadron correlations by the STAR collaboration, I analyze the long-range ("ridge-like") part of these data and show that the dependence on both transverse momentum as well as orientation with respect to the event plane are consistent with correlations expected from only collective flow. In combination with previously analyzed centrality-dependent data, they provide strong evidence that only collective flow effects are present at large relative pseudorapidy. In contrast, by analyzing a "background subtracted" signal, the authors presenting the new data concluded that the ridge-like part of the measured correlation could not in fact be entirely generated from collective flow of the medium. I explain the discrepancy and illustrate some pitfalls of using the ZYAM prescription to remove flow background.