Centrality-dependent direct photon pt spectra in Au+Au collisions at RHIC

TL;DR

This paper models the centrality-dependent transverse momentum spectra of direct photons in Au+Au collisions at RHIC using hydrodynamics and energy loss, matching experimental data and analyzing source contributions.

Contribution

It provides a comprehensive hydrodynamic framework to describe direct photon spectra across centralities, incorporating parton energy loss and source competition, which improves understanding of photon production mechanisms.

Findings

Photon spectra agree with PHENIX data across broad $ ext{p}_T$ range.

Parton energy loss reduces fragmentation and jet photon contributions by about 40%.

Photon $R_{AA}$ is centrality independent at high $ ext{p}_T$ within 5% accuracy.

Abstract

We calculate the centrality-dependence of transverse momentum ($\pt$) spectra for direct photons in Au+Au collisions at the RHIC energy, based on a realistic data-constrained (3+1) dimensional hydrodynamic description of the expanding hot and dense matter, a reasonable treatment of the propagation of partons and their energy loss in the fluid, and a systematic study of the main sources of direct photons. The resultant $\pt$ spectra agree with recent PHENIX data in a broad $\pt$ range. The competition among the different direct photon sources is investigated at various centralities. Parton energy loss in the plasma is considered for photons from fragmentation and jet photon conversion, which causes about 40% decrease in the total contribution. In the high $\pt$ region, the observed $R_{AA}$ of photons is centrality independent at the accuracy of 5% based on a realistic treatment of energy loss. We also link the different behavior of $R_{AA}$ for central and peripheral collisions, in the low $\pt$ region, to the fact that the plasma in central collisions is hotter compared to peripheral ones.