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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Yesterday, the ALICE Collaboration weighed in with a jet quenching result of their own. In their preprint

• Suppression of Charged Particle Production at Large Transverse Momentum in Central Pb-Pb Collisions at √s_NN = 2.76 TeV

the ALICE physicists analyze a quantity much more familiar from the RHIC experiments: the suppression of large-p_T hadrons in nuclear collisions relative to binary collision scaled p+p reactions. Three aspects are remarkable:

1. The maximal suppression factor (1/R_AA ≈ 7) at p_T = 7 GeV is about 40% stronger than that seen at RHIC;
2. R_AA rises strongly with increasing p_T, reaching 0.35 at 20 GeV;
3. The shape of R_AA(p_T) in the intermediate range 1 GeV ≤ p_T ≤ 7 GeV is almost identical to that observed at RHIC.

The lower minimum of R_AA in spite the much flatter slope of the hadron spectrum indicates a much larger energy loss of the leading parton. It will be interesting to see whether the energy loss coefficient scales like the charged particle multiplicity between RHIC and LHC.

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