Percolation of Color Sources and the Shear Viscosity of the QGP in Central A-A Collisions at RHIC and LHC Energies

TL;DR

This paper uses the Color String Percolation Model to estimate the shear viscosity to entropy ratio of the Quark-Gluon Plasma in heavy-ion collisions at RHIC and LHC energies, indicating a strongly coupled fluid near the phase transition.

Contribution

It provides a novel estimation of the QGP's shear viscosity to entropy ratio using CSPM, connecting experimental data with theoretical models at different collision energies.

Findings

$ ext{eta}/s$ is approximately 0.204 at RHIC and 0.262 at LHC.

QGP behaves as a strongly coupled fluid near the phase transition.

The results are significantly above the AdS/CFT lower bound, supporting strong coupling.

Abstract

The Color String Percolation Model (CSPM) is used to determine the shear viscosity to entropy ratio ($\eta/s$) of the Quark-Gluon Plasma (QGP) produced in Au-Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at RHIC and Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV at LHC. The relativistic kinetic theory relation for $\eta/s$ is evaluated using CSPM values for the temperature and the mean free path of the QGP constituents. The experimental charged hadron transverse momentum spectrum is used to determine the percolation density parameter $\xi$ in Au-Au collisions (STAR). For Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV $\xi$ values are obtained from the extrapolation at RHIC energy. The value of $\eta/s$ is 0.204$\pm$0.020 and 0.262$\pm$0.026 at the CSPM initial temperatures of 193.6$\pm$3 MeV (RHIC) and 262.2 $\pm$13 MeV (LHC) respectively. These values are 2.5 and 3.3 times the AdS/CFT conjectured lower bound $1/4\pi$. We compare the CSPM $\eta/s$ analytic expression with weak coupling (wQGP) and strong coupling (sQGP) calculations. This indicates that the QGP is a strongly coupled fluid in the phase transition region.