Low $p_T$ direct photon production at RHIC measured with PHENIX

TL;DR

This paper reports measurements of low transverse momentum direct photon production in heavy-ion collisions at RHIC, revealing large yields and azimuthal anisotropy consistent with thermal emission from quark-gluon plasma, but challenging current theoretical models.

Contribution

It provides new experimental data on direct photon production at various energies and system sizes, highlighting features that are difficult to reproduce with existing theories.

Findings

Large direct photon yields in systems with high multiplicity

Azimuthal anisotropy of direct photons consistent with thermal emission

Inverse slope parameter shows no system size dependence

Abstract

PHENIX has used the versatility of RHIC to map out low $p_{T}$ direct photon production as function of collision system size and beam energy. For systems with a size corresponding to a $dN_{ch}/d\eta$ larger than 20-30, we observe a large yield of direct photons, a large azimuthal anisotropy with respect to the reaction plane, and a characteristic centrality dependence of $dN_{\gamma}/dy \propto (dN_{ch}/d\eta)^{\alpha}$, with $\alpha \sim$ 1.2. \\ In this proceeding, we will present new results from Au+Au collisions at $\sqrt{s_{NN}}$ = 39, 62.4 and 200 GeV. After subtracting the prompt photon component, the inverse slope for the $p_{T}$ range from 1-2 GeV/$c$ is 250 MeV/$c$, but increases to about 400 MeV/$c$ for the range from 2 to 4 GeV/$c$. Within the experimental uncertainty, there is no indication of a system size dependence of the inverse slope. Furthermore, the system size dependence of the yield, expressed through the power $\alpha$, remains independent of $p_{T}$ over the entire observed range from 1 to 6 GeV/$c$. Like the large yield and azimuthal anisotropy, these features, while qualitatively consistent with the emission of thermal photons from the quark gluon plasma, elude a quantitative description through theoretical model calculations.