Virtual Journal on QCD Matter

Following shortly after my previous journal entry on recent progress in tightening the upper bound on the shear viscosity-to-entropy density ratio (η/s) of the quark-gluon plasma, the competition has struck back. In a letter appearing in Science today

• Universal Quantum Viscosity in a Unitary Fermi Gas

Cao et al. report results from new precision measurements of the η/s ratio in an ultracold gas of trapped fermionic ⁶Li atoms in the universal regime as a function of temperature. (more…)

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In a recent preprint

• 200AGeV Au+Au collisions serve a nearly perfect quark-gluon liquid

Song, Bass, Heinz, Hirano, and Shen present systematic comparisons of RHIC data on elliptic flow in Au+Au collisions with a hybrid model of the reaction dynamics. They conclude that the kinematic shear viscosity ν = 4πη/s in the quark-gluon plasma phase of the reaction does not exceed 2.5 on average, compared with the lower KSS bound of 1. (more…)

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The 21st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions, colloquially referred to as Quark Matter 2009, took place in Knoxville, Tennessee from March 30th to April 4th, 2009. (more…)

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The success of ideal Relativistic Fluid Dynamics (RFD) in describing hadron spectra and elliptic flow at RHIC has led to a strong interest in the transport coefficients of QCD, in particular the shear- and bulk-viscosity as well as the shear-viscosity over entropy-density ratio η/s. Of course ideal RFD assumes η to be zero, which is unphysical, but its success has hinted at a very small value for η/s. The purpose of this post is to review what we currently know about η/s of QCD matter and put the different approaches used to study this quantity into context.

(more…)

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Over the past few months a new initiative, called Theory-Experiment Collaboration for Hot QCD Matter (TECHQM), has taken shape, which potentially will have great impact on how our community arrives at scientific conclusions on the nature and properties of hot and dense QCD matter created at the RHIC and LHC facilities. (more…)

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