A systematic study of direct photon production in heavy ion collisions

TL;DR

This paper provides a theoretical derivation of photon bremsstrahlung in quark-gluon plasma and systematically studies direct photon production in various heavy ion collisions at RHIC energies, highlighting the dominance of initial-state effects at high pT.

Contribution

It offers the first systematic analysis of direct photon production in heavy ion collisions, incorporating a detailed reaction operator approach and considering both initial and final-state effects.

Findings

Landau-Pomeranchuk-Migdal suppression is stronger than previously thought.

Final-state photon contribution is limited to 35% at pT < 5 GeV.

Initial-state cold nuclear matter effects dominate high pT modifications.

Abstract

A theoretical derivation of photon bremsstrahlung, induced by the interactions of an energetic quark in a hot and dense quark-gluon plasma, is given in the framework of the reaction operator approach. For the physically relevant case of hard jet production, followed by few in-medium interactions, we find that the Landau-Pomeranchuk-Migdal suppression of the bremsstrahlung photon intensity is much stronger than in the previously discussed limit of on-shell quarks and a large number of soft scatterings. We present the first systematic study of direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au heavy ion collisions at the Relativistic Heavy Ion Collider at center of mass energies of 62.4 GeV and 200 GeV. We find that the contribution of the final-state photon production at pT < 5 GeV is limited to 35%, and at high transverse momenta, the modification of the direct photon cross section is dominated by initial-state cold nuclear matter effects.