Direct observation of dijets in central Au+Au collisions with STAR

TL;DR

This paper reports the first direct observation of back-to-back dijets in central Au+Au collisions at RHIC, revealing how high-pT hadron correlations are affected by the dense medium, and providing insights into parton energy loss.

Contribution

It presents the first direct measurement of dijet correlations in central Au+Au collisions, showing the emergence of back-to-back peaks at high pT thresholds and constraining models of in-medium energy loss.

Findings

Back-to-back dijet peaks are observed at high pT thresholds.

Medium modifications of dijet fragmentation functions are analyzed.

Results support models of parton energy loss in dense QCD matter.

Abstract

We present first results of azimuthal (Delta phi) correlations of high transverse momentum (p_T) hadrons from the high-statistics Au+Au dataset collected at RHIC in 2004. Previous correlations measurements observed the disappearance of back-to-back (Delta phi ~ pi) correlations in central Au+Au collisions, supporting the emerging picture of parton energy loss in dense QCD matter. In the present analysis, increasing the p_T thresholds reveals a narrow back-to-back dijet peak which emerges above the decreasing combinatoric background. We study the possible medium modification of the dijet fragmentation function and compare to models, to place constraints on the nature and magnitude of in-medium energy loss.