Direct photon production in d+A and A+A collisions at RHIC

TL;DR

This paper systematically investigates direct photon production in various d+A and A+A collisions at RHIC energies, analyzing cold nuclear matter effects and medium-induced photon processes, and compares simulations with experimental data.

Contribution

It provides a comprehensive analysis of cold nuclear matter effects and medium-induced photon production, clarifying their roles in direct photon yields at RHIC.

Findings

Cold nuclear matter effects dominate high $p_T$ photon suppression.

No evidence found for large medium-induced photon enhancement.

Current data cannot constrain all nuclear matter effects.

Abstract

Direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au collisions at center of mass energies $\sqrt{s}=62.4$ GeV and 200GeV at RHIC is systematically investigated. We study the jet quenching effect, the medium-induced photon bremsstrahlung and jet-photon conversion in the hot QGP. We account for known cold nuclear matter effects, such as the isospin effect, the Cronin effect, shadowing and cold nuclear matter energy loss. It is shown that at high $p_T$ the nuclear modification factor for direct photons $R_{AA}^{\gamma} (p_T) < 1$ is dominated by cold nuclear matter effects and there is no evidence for large cross section amplification due to medium-induced photon bremsstrahlung and jet-photon conversion in the medium. Comparison of numerical simulations to experimental data also rules out large Cronin enhancement and incoherent photon emission in the QGP but the error bars in the current experimental data cannot provide further constraints on the magnitudes of other nuclear matter effects.