Measurements of soft and intermediate $p_T$ photons from hot and dense matter at RHIC-PHENIX

TL;DR

This paper reports measurements of direct photons in proton-proton and gold-gold collisions at RHIC, revealing an excess in Au+Au collisions indicative of hot, dense matter with temperatures above the QGP critical point.

Contribution

First measurement of direct photon spectra in this energy range using the virtual photon method at PHENIX, providing insights into the initial temperature of quark-gluon plasma.

Findings

Significant excess of direct photons in Au+Au collisions over scaled p+p results.

Hydrodynamical models suggest initial temperature exceeds QGP critical temperature.

Data from d+Au collisions will help evaluate nuclear effects.

Abstract

The measurements of direct photons in $1.0 < p_{T} < 5.0 $GeV/$c$ for p+p and Au+Au collisions have been made through the virtual photon method at PHENIX. The fraction of the virtual direct photon component to the inclusive $e^{+}e^{-}$ yield is determined by a shape analysis using the $e^{+}e^{-}$ mass spectra in $m_{ee} < 300 $MeV/$c^{2}$ for $1.0 < p_{T} < 5.0 $GeV/$c$. The real direct photon spectra in p+p and Au+Au collisions are obtained from the virtual direct photon fractions, and a significant excess over a binary-scaled p+p result is seen in Au+Au collisions. Hydrodynamical models which reproduce the Au+Au result indicate the initial temperature of the matter is higher than the critical temperature of QGP. The d+Au data taken in 2008 are promising to evaluate the contribution of the nuclear effects because of its large statistics.