Quest for Perfection:
The neck and neck race continues

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

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…)

Closing in on “Perfection”

In a recent preprint

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…)

Jet Quenching at LHC: ALICE weighs in

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

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

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