Thermal photon measurements at PHENIX

TL;DR

This paper reports measurements of low momentum direct photons in relativistic heavy-ion collisions at RHIC, revealing an excess with high azimuthal anisotropy and increasing effective temperature, with no clear system size dependence.

Contribution

It provides new experimental data on direct photon production and properties across different collision systems and energies at RHIC.

Findings

Observation of excess direct photons above prompt production.

Large azimuthal anisotropy of photon emission.

Effective temperature increases with photon transverse momentum.

Abstract

Photons are emitted at all stages of relativistic heavy-ion collisions and do not interact with the medium strongly. With access to the versatility of RHIC, measurements of low momentum direct photons are made possible across different system size and beam energies. An excess of direct photons, above prompt photon production from hard scattering processes, is observed for a system size corresponding to $dN_{ch}/d\eta$ of 20-30, with a large azimuthal anisotropy and a characteristic dependence on collision centrality. After subtracting the prompt photon component, the inverse slope of the spectrum is continuously increasing with the effective temperature ranging from 250 MeV/c at $p_{T}$ of 1-2 GeV/c to about 400 MeV/c at 2-4 GeV/c. Within the experimental uncertainty, there is no indication of a system size dependence of the inverse slope. In this proceeding, results from Au+Au collisions from the PHENIX experiment will be presented.