Direct Photon Production and Azimuthal Anisotropy at Low Transverse Momentum measured in PHENIX

TL;DR

The PHENIX experiment observed a significant excess of low-$p_{T}$ direct photons with large azimuthal anisotropy in Au+Au collisions at 200 GeV, challenging existing theoretical models and suggesting thermal radiation from the medium.

Contribution

The study introduces a new photon identification technique that enhances statistics and reduces uncertainties, providing more precise measurements of direct photon production and anisotropy.

Findings

Large excess of low-$p_{T}$ direct photons in Au+Au collisions

Significant azimuthal anisotropy of the excess photons

Development of a new conversion photon identification method

Abstract

The PHENIX experiment discovered a large excess of low-$p_{T}$ direct photons in Au+Au collisions at 200 GeV compared to reference p+p collisions, which has been attributed to thermal radiation from the medium produced in the collisions. At the same time the excess photons show a large azimuthal anisotropy, expressed as Fourier coefficients $v_{2}$ and $v_{3}$. These surprising results have not yet been fully described by theoretical models. We will present the results obtained from real photons in the electromagnetic calorimeter and photons converted on the outer shell of the Hadron Blind Detector. PHENIX has also developed a new technique to identify conversion photons without assuming the radius where the conversion happened. This method greatly increases the available statistics and reduces systematic uncertainties.