PHENIX photons and dileptons

TL;DR

The PHENIX experiment measured photons and dileptons in various collisions, revealing excess yields in heavy-ion collisions that suggest new phenomena beyond simple nuclear effects, providing insights into the early collision stages.

Contribution

This paper presents new measurements of electromagnetic probes in different collision systems, highlighting excess yields in Au+Au collisions not attributable to known nuclear effects.

Findings

Dilepton yield excess in Au+Au collisions at low pT and centrality.

Large enhancement of direct photons in Au+Au over scaled p+p.

Excess cannot be explained by nuclear effects based on d+Au results.

Abstract

Electro-magnetic probes such as dileptons and photons are strong probes to investigate the thermodynamical state of the early stages of collisions since they leave the system unscathed. The PHENIX experiment has measured both photons and dileptons in p+p, d+Au and Au+Au collisions. An excess of dilepton yield over the expected hadronic contribution is seen in 0.2-0.8\,GeV/$c^2$ in Au+Au collisions, which is prominent in lower $p_T$ and most central. Direct photons are measured through their internal conversion to electron pairs. We saw a large enhancement in Au+Au collisions over p+p yield scaled by the number of binary collisions. It turned out from the latest results on d+Au collisions that this enhancement is not explainable by a nuclear effect.