Virtual Journal on QCD Matter

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

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occurred in my daily listing of new posts to nucl-ex and nucl-th. The word “fluctuations(s)” appeared 17 times in the titles and abstracts of papers posted. Among them are

• Probing the Quark-Gluon Phase Transition with Correlations and Fluctuations in Heavy Ion Collisions from the STAR Experiment , which somewhat counter-intuitively shows that forward-backward correlations increase with system size, with centrality  and with center-of-mass energy.
• Multiplicity fluctuations and temperature fluctuations: Tsallis statistics as an “interpolator” between canonical (fixed temperature) and microcanonical (fixed energy) ensembles, leading to a connection between multiplicity fluctuations and fluctuations in transverse momenta.
• Fluctuations in relativistic heavy-ion collisions from the Glauber models: Fluctuations in the initial size from a Glauber Monte Carlo is the “source” for transverse momentum fluctuations (through hydrodynamics).

While not completely clear in my fast reading, it would seem that the latter two papers would predict different correlations between multiplicity and p_T fluctuations- could this be used to discriminate between the suggested mechanisms?

Note: By my count, today there were at least 9 new papers of general interest to “QCD matter”. As far as i can tell, this is not driven by one looming conference proceedings deadline, but seems instead to be another (positive!) fluctuation.

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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.

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A wide variety of thermodynamic and/or statistical techniques have been applied to describe particle spectra and yields in relativistic heavy ion collisions. Not having done  a detailed, ‘statistical’ analysis of the hundreds of such papers, it is still safe to say the vast majority of such calculations do not start from the microcanonical ensemble, that is, they do not incorporate the effects of global energy-momentum conservation.  In Conservation Laws and the Multiplicity Evolution of Spectra at the Relativistic Heavy Ion Collider Chajecki and Lisa study the role of Energy and Momentum Conservation-Induced Constraints (EMCIC’s) on single particle spectra at RHIC.  (In a nice piece of acronym-overloading, this work builds on their previous studies of Energy and Momentum Conservation-Induced Correlations in femtoscopic measurements.) Their studies suggest that the effects of EMCIC’s can lead to surprisingly large shifts in the momentum distributions between low and high multiplicity states. 

There is additional information in this figure that the authors may wish to extract. This low momentum regime is where one expects participant scaling to hold, and this seems to be (roughly) the case for the lowest momentum pions, but is badly violated for the protons. Protons with  p_T < 0.7 GeV/c are suppressed in Au+Au collisions relative to participant scaling; protons with momenta greater than this value are enhanced. The conventional explanation for this is radial flow, which has a larger effect on higher mass particles, and which will lead to a depletion of the low  p_T region. This paper suggests that identical trends can result from EMCIC’s, leading the authors to state “Extracting physics messages from the changing spectra, while ignoring kinematic effects of the same order as the observed changes themselves, seems unjustified.”

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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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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…)

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The one that started it all was the first experimental paper from RHIC. (more…)

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Improving the application of Relativistic Fluid Dynamics to relativistic heavy-ion collisions is an ongoing challenge and currently a very active area of research: recent notable preprints on the topic are centered around two main themes: (more…)

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This post provides a list on historically relevant work on the application of
relativistic fluid dynamics (RFD) to heavy-ion collisions. (more…)

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