Centrality Dependence of Delta-eta, Delta-phi Correlations in Heavy Ion Collisions

TL;DR

This paper reports on two-particle correlation measurements in heavy ion collisions, revealing a long-range 'ridge' structure in central Au+Au collisions at high energies, which diminishes in more peripheral collisions.

Contribution

It provides the first detailed observation of the ridge phenomenon over a broad pseudorapidity range in heavy ion collisions at RHIC energies.

Findings

The ridge extends at least |Delta-eta|=4.

Ridge yield is independent of Delta-eta within the measured range.

Ridge yield decreases in more peripheral collisions.

Abstract

In these proceedings, a measurement of two-particle correlations with a high transverse momentum trigger particle (pT_trig > 2.5 GeV/c) is presented for Au+Au collisions at sqrt(s_NN) = 200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4 < Delta-eta < 2). As in p+p collisions, the near-side is characterized by a peak of correlated partners at small angle relative to the trigger. However, in central Au+Au collisions an additional correlation extended in Delta-eta and known as the 'ridge' is found to reach at least |Delta-eta| = 4. The ridge yield is largely independent of Delta-eta over the measured range, and it decreases towards more peripheral collisions. For the chosen pT_trig cut of 2.5 GeV/c, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.