Shear viscosity to entropy density ratio of a relativistic Hagedorn resonance gas

Article history: Received 16 October 2009 Received in revised form 22 December 2009 Accepted 8 January 2010 Available online 16 January 2010 Editor: W. Haxton The new state of matter produced at Relativistic Heavy Ion Collider reveals a strongly coupled quark– gluon plasma with an extremely small shear viscosity to entropy density ratio η/s. We calculate the η/s of an equilibrated hadron matter characterized by a relativistic hadron resonance gas with a Hagedorn mass spectrum that grows exponentially with the hadron mass. We find with increase in temperature of the system the η/s value decreases due to rapid increase in the multiplicity of massive resonances. In the vicinity of the critical temperature for deconfinement transition, the minimum value of η/s in the Hagedorn resonance gas is found to be consistent with the current estimates for a strongly coupled quark–gluon plasma. © 2010 Elsevier B.V. All rights reserved.