Direct Photons at RHIC

TL;DR

This paper reports on measurements of direct photon production at RHIC, providing insights into the quark-gluon plasma and testing QCD predictions through innovative virtual photon detection methods.

Contribution

It introduces a novel approach to measure thermal photons via low invariant mass electron-positron pairs and presents new experimental data from RHIC Year-7.

Findings

Significant excess of direct photons observed in Au+Au collisions

Measured direct photon yields exceed NLO pQCD predictions

Enhanced azimuthal anisotropy data obtained for direct photons

Abstract

Direct photons are ideal tools to investigate kinematical and thermodynamical conditions of heavy ion collisions since they are emitted from all stages of the collision and once produced they leave the interaction region without further modification by the medium. The PHENIX experiment at RHIC has measured direct photon production in p+p and Au+Au collisions at 200 GeV over a wide transverse momentum ($p_T$) range. The $p$ + $p$ measurements allow a fundamental test of QCD, and serve as a baseline when we try to disentangle more complex mechanisms producing high $p_T$ direct photons in Au+Au. As for thermal photons in Au+Au we overcome the difficulties due to the large background from hadronic decays by measuring "almost real" virtual photons which appear as low invariant mass $e^+e^-$ pairs: a significant excess of direct photons is measured above the above next-to-leading order perturbative quantum chromodynamics calculations. Additional insights on the origin of direct photons can be gained with the study of the azimuthal anisotropy which benefits from the increased statistics and reaction plane resolution achieved in RHIC Year-7 data.