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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Not to be outdone, the CMS Collaboration posted today its own circumstantial evidence of jet quenching in Pb+Pb collisions at the LHC. As in yesterday’s ATLAS publication, the evidence is based on the observation of strongly asymmetric di-jets. In one example, shown on the CMS web page, the trigger jet has a total transverse momentum of 205 GeV, while the back-jet has only 70 GeV momentum. Several other pairs with somewhat smaller, but still large transverse momentum asymmetry are also shown. The collaboration promises a comprehensive publication with these and similar results soon after the completion of the heavy ion run.

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The large value of charged multiplicity  dN_ch/dη = 1584 observed by ALICE in the 5% most central collisions, and the large value of elliptic flow they report in minimum bias collisions both suggest that the demise of the strong coupling paradigm at the LHC have been greatly exaggerated. I attribute this in part to “the tyranny of asymptotic freedom”, that is, the beauty of the running coupling constant in QCD has led to some wishful thinking about how fast a logarithmic term can vary. There is also the enormous appeal and simplicity of the “classical QGP” described as a nearly free, massless Boltzmann gas. But the data from RHIC, and now these first ever so exciting results from LHC, have taught us that there is also a wealth of fundamental physics and new phenomena in the strongly-coupled regime that is so much more interesting than a non-interacting gas!

At the same time there has indeed been a demise, in fact several, in this case of the various predictions of (mostly) lower multiplicity densities than the value of ~1600 reported by ALICE. I was one of those who would have guessed the lower values of 1200-1300, but since that was only a guess, the surprise for me was a pleasant one. Ironically, earlier on the day this value was announced in Andrea Dainese’s talk at the LHCC , I had read with great interest this paper Hadron production at the LHC: Any indication of new phenomena by Levin and Rezaeian , describing in a very accessible fashion their predictions for LHC multiplicities based on saturation physics. There is also a nice discussion of the extensions incorporated in their approach over the KLN model, so it is more than a little puzzling that the simpler KLN model (at least in one of its various instantiations) better describes the Pb+Pb data at the LHC.

(See also Berndt Mueller’s post for a more quantitative discussion of what we learn from the first two ALICE papers on Pb+Pb collisions at the LHC.)

(*) The title of this post is drawn from one of the loveliest songs in the American jazz idiom.

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Today the ALICE Collaboration posted the first analyses of data from Pb+Pb collisions at the LHC:

• Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
• Charged-particle multiplicity density at mid-rapidity in central Pb-Pb collisions at sqrt(sNN) = 2.76 TeV

(more…)

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