Equation of state and initial temperature of quark gluon plasma at RHIC

TL;DR

This paper uses hydrodynamic models to analyze direct photon data from RHIC collisions, estimating the initial temperature and equation of state of the quark-gluon plasma, and comparing predictions with measurements.

Contribution

It provides a 1+3D hydrodynamic analysis to determine the initial temperature and sound speed of sQGP from photon observables at RHIC.

Findings

Initial temperature of 507+-12 MeV in the fireball center.

Sound speed estimated at 0.36+-0.02.

Photon source size predicted to be larger out of the collision plane.

Abstract

In gold-gold collisions of the Relativistic Heavy Ion Collider (RHIC) a perfect fluid of quarks, sometimes called the strongly interacting quark gluon plasma (sQGP) is created for an extremely short time. The time evolution of this fluid can be described by hydrodynamical models. After expansion and cooling, the freeze-out happens and hadrons are created. Their distribution reveals information about the final state of the fluid. To investigate the time evolution one needs to analyze penetrating probes, such as direct photon observables. Transverse momentum distributions of low energy direct photons were mesured in 2010 by PHENIX, while azimuthal asymmetry in 2011. These measurements can be compared to hydrodynamics to determine the equation of state and the initial temperature of sQGP. In this paper we analyze an 1+3 dimensional solution of relativistic hydrodynamics. We calculate momentum distribution, azimuthal asymmetry and momentum correlations of direct photons. Based on earlier fits to hadronic spectra, we compare photon calculations to measurements to determine the equation of state and the initial temperature of sQGP. We find that the initial temperature in the center of the fireball is 507+-12 MeV, while for the sound speed we get a speed of sound of 0.36+-0.02. We also estimate a systematic error of these results. We find that the measured azimuthal asymmetry is also not incompatible with this model, and predict a photon source that is significantly larger in the out direction than in the side direction.