Eur. Phys. J. C (2012) 72: 2123
https://doi.org/10.1140/epjc/s10052-012-2123-x

Regular Article - Experimental Physics

Clustering of color sources and the shear viscosity of the QGP in heavy ion collisions at RHIC and LHC energies

¹ CENTRA, Instituto Superior Tecnico, 1049-001, Lisbon, Portugal
² Department of Physics, Purdue University, West Lafayette, IN, 47907, USA
³ Departamento de Fisica de Particulas, Universidale de Santiago de Compostela and Instituto Galego de Fisica de Atlas Enerxias (IGFAE), 15782, Santiago, de Compostela, Spain

^* e-mail: brijesh@purdue.edu

Received: 24 May 2012
Revised: 2 July 2012
Published online: 28 August 2012

Abstract

We present our results on the shear viscosity to entropy ratio (η/s) in the framework of the clustering of the color sources of the matter produced at RHIC and LHC energies. The onset of de-confinement transition is identified by the spanning percolating cluster in 2D percolation. The relativistic kinetic theory relation for η/s is evaluated using the initial temperature (T) and the mean free path (λ _mfp). The analytic expression for η/s covers a wide temperature range. At T∼150 MeV below the hadron to QGP transition temperature of ∼168 MeV, with increasing temperatures the η/s value drop sharply and reaches a broad minimum η/s∼0.20 at T∼175–185 MeV. Above this temperature η/s grows slowly. The measured values of η/s are 0.204±0.020 and 0.262±0.026 at the initial temperature of 193.6±3 MeV from central Au+Au collisions at (RHIC) and 262.2±13 MeV in central Pb+Pb collisions at (LHC). These η/s values are 2.5 and 3.3 times the AdS/CFT conjectured lower bound 1/4π but are consistent with theoretical η/s estimates for a strongly coupled QGP.