Thermal photons at PHENIX experiment

Abstract

Direct photons are a unique probe to study the properties of the medium created in heavy ion collisions. In particular low pT direct photons are of great importance since one expects that they are predominantly of thermal origin. In A+A systems PHENIX has observed a large yield of low pT direct photon that are emitted with a significant azimuthal anisotropy with respect to the reaction plane (v2). The mechanism responsible for the large yield and large v2 is not understood yet. Following recent evidence for collective behavior of charged particle production from small systems like p+A, d+Au, and 3He+Au, PHENIX has made systematic measurements of direct pho- tons with different collision energies and system configurations. It has been found that the low pT direct photon yield dNγ/dη is proportional to (dNch/dη)α. This behavior holds for beam energies measured both at RHIC and at the LHC in large-on-large systems, while data from small systems suggest an onset of QGP formation at low dNch/dα. In this talk, I will report recent measurements of thermal photon and related observables.