Clustering of color sources and the shear viscosity of the QGP in heavy ion collisions at RHIC and LHC energies
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Regular Article - Experimental Physics
Clustering of color sources and the shear viscosity of the QGP in heavy ion collisions at RHIC and LHC energies J. Dias de Deus1 , A.S. Hirsch2 , C. Pajares3 , R.P. Scharenberg2 , B.K. Srivastava2,a 1
CENTRA, Instituto Superior Tecnico, 1049-001 Lisbon, Portugal 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 2
Received: 24 May 2012 / Revised: 2 July 2012 / Published online: 28 August 2012 © The Author(s) 2012. This article is published with open access at Springerlink.com
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 (λmfp ). 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 √ sN N = 200 GeV (RHIC) and 262.2 ± 13 MeV in central √ Pb + Pb collisions at sN N = 2.76 TeV (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.
1 Introduction The observation of the large elliptic flow at RHIC in noncentral heavy ion collisions suggest that the matter created is a nearly perfect fluid with a very low shear viscosity [1–4]. Recently, attention has been focused on the shear viscosity to entropy density ratio η/s as a measure of the fluidity [5–8]. The observed temperature averaged η/s, based on viscous hydrodynamics analyses of RHIC data, are suggestive of a strongly coupled plasma [9, 10]. The effect of the bulk viscosity is expected to be negligible. It has been a e-mail:
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conjectured, based on infinitely coupled super-symmetric Yang–Mills (SYM) gauge theory using the correspondence between Anti de-Sitter(AdS) space and conformal field theory (CFT), that the lower bound for η/s is 1/4π and is the universal minimal viscosity to entropy ratio even for QCD [11]. However, there are theories in which this lower bound can be violated [12]. In this work, we use the color string percolation model (CSPM) [13, 14] to obtain η/s as a function of the temperature above and below the hadron to QGP transition. The measured η/s values are for Au + Au √ collisions at sN N = 200 GeV at RHIC and for Pb + Pb √ collisions at sN N = 2.76 Te
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