Two-particle Direct Photon-Jet Correlation Measurements in PHENIX

TL;DR

This paper reports on measurements of direct photon-hadron correlations in proton-proton and gold-gold collisions at 200 GeV, providing insights into jet suppression, intrinsic transverse momentum, and direct photon production at RHIC.

Contribution

It introduces a novel statistical subtraction method for direct photon-hadron correlations and presents the highest precision measurements of isolated photon yields at RHIC.

Findings

The Au+Au to p+p photon yield ratio, I_AA, aligns with single particle suppression levels.

The intrinsic transverse momentum, sqrt(<k_T^2>), in p+p collisions is approximately 3 GeV.

First direct measurement of prompt photons from jet fragmentation at RHIC.

Abstract

Various 2-particle direct photon-hadron correlation strengths and yields in $p+p$ and $Au+Au$ collisions at sqrtsNN= 200 GeV are presented. The per-trigger yield of direct photon hadron pairs from direct-photon-jet correlations in is obtained by a statistical subtraction of the decay photon pairs from inclusive photon-hadron sample. The decay photon per-trigger yields are estimated from the measured $\pi^0$-hadron by means of a Monte Carlo based calculation which takes into account decay kinematics and detector response. Under the assumption that the suppression is nearly pt independent using a specific averaging scheme, we find an average ratio of Au+Au to p+p per-trigger photon yields, $I_{AA}$, consistent with the single particle suppression level Raa, which can be interpreted as a qualitative confirmation of the basic geometrical picture of jet suppression at RHIC. The application of the event by event photon isolation cuts in $p+p$ results our highest precision measurement yet, and allows for precision studies of the baseline fragmentation function $D(z)$, and well as a variable \pout which is proportional to the apparent intrinsic $k_T$, or non-zero transverse momentum of the original collision partons. With a model dependent extraction method, the average $sqrt(<k_T^2>)$ at this center of mass energy in p+p is found to be in the vicinity of 3 GeV, consistent with analysis of di-hadron (di-jet) correlations. This and the improved precision in the isolated yields warrant further comparison with the baseline perturbative QCD (pQCD) calculations used in the various models of jet energy loss. Finally, we present a unique direct measurement of single prompt photons from jet fragmentation.