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…)
The large value of charged multiplicity dNch/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.
Today the ALICE Collaboration posted the first analyses of data from Pb+Pb collisions at the LHC:
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.
Three new preprints address the question of what it takes to describe the measured elliptic flow in Au+Au collisions at RHIC in the framework of a partonic Boltzmann equation: (more…)
The discovery of the “ridge” by the STAR Collaboration has proven stubbornly resistant to quantitative theoretical analysis. (See Berndt Mueller’s “Theorists Confront the Ridge” for a description of two recent attempts.) The discovery turns out to be nearly as resistant to historical analysis, at least for this outside observer. (more…)
Over the past 1-2 years some confusion has entered the field as to what exactly constitutes the elliptic flow scaling law predicted by the parton recombination models. (more…)
The recent post on the AMO competition for perfect fluidity offers a wonderful opportunity to compare and contrast the techniques of condensed matter physics versus those of relativistic heavy ion physics. (more…)
In April of 2005 Brookhaven National Laboratory announced that scientists at RHIC had created the most ideal liquid ever observed in nature. Now, 3 years later, RHIC is facing competition to this claim from strongly interacting ultra-cold Fermi gases. (more…)
The detailed dependence of the elliptic flow parameter v2 on the particle masses and transverse momentum has provided strong evidence for the applicability of hydrodynamics for describing the bulk motion of final-state particles at RHIC. (For a partial list of references, see the Hydro Primer elsewhere in this journal.) Equally intriguing have been various scaling patterns observed in the flow data: The separate curves v2(pT,m) for different mass particles are substantially more alike when studied versus “transvese kinetic energy” KET = mT -m. Plotting v2 as as a function of KET collapses the data into one curve for KET < 1 GeV, while showing two distinct branches for mesons and baryons for larger values of KET. These two branches collapse into one curve when both v2 and KET are divided by the quark content nq (2 for mesons, 3 for baryons). (more…)
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