Systematic studies of the centrality dependence of soft photon production in Au+Au collision with PHENIX

TL;DR

This paper presents systematic measurements of soft photon production and flow in Au+Au collisions at RHIC, providing insights into the properties of quark-gluon plasma and testing different photon emission models.

Contribution

It offers the first detailed centrality dependence of soft photon yield and flow coefficients in Au+Au collisions, helping to distinguish between competing production mechanisms.

Findings

Soft photon yield increases with collision centrality.

Elliptic flow $v_2$ of soft photons is significant and centrality-dependent.

Triangular flow $v_3$ measurements provide additional constraints.

Abstract

Since the earliest days of Heavy Ion Physics thermal soft photon radiation emitted during the reaction had been theorized as a smoking gun signal for formation of a quark-gluon plasma and as a tool to characterize its properties. In recent years the existence of excess photon radiation in heavy ion collisions over the expectation from initial hard interactions has been confirmed at both RHIC and LHC energies by PHENIX and ALICE respectively. There the radiation has been found to exhibit elliptic flow $v_2$ well above what can currently be reconciled with a picture of early emission from a plasma phase. During the 2007 and 2010 Au+Au runs PHENIX has measured a high purity sample of soft photons down to $p_T>0.4\,\text{GeV}/c$ using an external conversion method. We present recent systematic studies by PHENIX from that sample on the centrality dependence of the soft photon yield, and elliptic and triangular flow $v_2$ and $v_3$ in Au+Au collisions which fill in the experimental picture and enable discrimination of competing soft photon production scenarios.