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

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.