Direct photons are a unique probe to study the properties of the medium created in heavy ion collisions. Low transverse momentum ( ) direct photons are of special interest since thermal photons are supposed to be dominant, while at high direct photons come from initial hard scattering (pQCD). PHENIX has observed a large excess of direct photon yield as well as large azimuthal anisotropy at low in Au+Au collisions at the c.m.s energy per nucleon pair GeV. The mechanism to produce a large direct photon yield with a large elliptic anisotropy ( ) is not well understood yet. PHENIX has made systematic measurements of direct photons with different collision energies and system configurations. It has been found that direct photon yield is proportional to charge particle multiplicity . This behavior holds for beam energies measured both at RHIC and at the LHC in large systems. This scaling suggests that there is a transition from p+p to A+A system which could be understood with the analysis of smaller systems like p+Au and d+Au.
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