Thermal photons at PHENIX experiment

Veronica Canoa Roman

doi:10.1051/epjconf/202023502001

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.

Highlights

Direct photons are one of the probes to study the properties of a hot and dense QCD medium
The primary photon contributor in the low pT region below 3GeV/c is the thermal photons from the medium that keep the thermodynamic information from when they are generated
Comparing the data with scaled pQCD calculations to Ncoll, we can find a clear excess over the scaled p+p yield in the low pT region, as it has been seen in A+Au at 200 GeV

Summary

Introduction

Direct photons are one of the probes to study the properties of a hot and dense QCD medium. The primary photon contributor in the low pT region below 3GeV/c is the thermal photons from the medium that keep the thermodynamic information from when they are generated. The contribution of these photons to the yield have an exponential shape, in analogy to the black body radiation. At the PHENIX experiment, it was measured the direct photon yield and the flow in Au-Au collisions at 200 GeV. It was found a large yield and a large v2. The details can be found in [1]

Direct photons in large systems

Direct photons in small systems

Direct photons scaling

Conclusions