What A Difference a Day Makes (*)

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!Predictions for dN_ch/d\eta excluded by ALICE

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.

First LHC Heavy Ion Data

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


An Interesting Fluctuation…

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

While not completely clear in my fast reading, it would seem that the latter two papers would predict different correlations between multiplicity and pT 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.

What do we know about the shear-viscosity of QCD matter?

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.


A Conservative Approach

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. 

The key plot in this well-written paper is Figure 3, which shows the ratio of the yields in  p+p collisions to central Au+Au collisions as a function of pT, compared to curves calculated on the basis of energy-momentum conservation alone. Even light particles such as pions show 50% effects in the low pT region 0.2 to 0.7 GeV/c; the effect is much larger (a factor of ~5) for protons in the same transverse momentum regime.   The structure is consistent with one’s naive expectations- the presence of a larger ‘reservoir’ in the Au+Au case makes it easier to access high transverse momentum than in p+p collisions. 

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  pT < 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  pT 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.”


In reading this paper, I was reminded of a very clever analysis by  T.T. Chou, C.N. Yang and E. Yen: Single Particle Momentum Distribution At High-Energies And Concept Of Partition Temperature . These authors noted that energy-momentum conserving delta-function in the microcanonical ensemble ‘inevitably’ leads to an exponential distribution of single-particle energies, with an exponential slope they labeled the ‘partition temperature’ (and took pains to note was not necessarily a real temperature). They applied this idea to the analysis of the rapidity distributions measured by UA5 in proton-antiproton collisions at center-of-mass energy 540 GeV. After introducing another, independent, ansatz, i.e., that confinement leads to  an exponential distribution in transverse momentum, they obtained a striking good description of how “phase space” considerations (aka EMCIC’s) described the systematic variation of the pseudo-rapidity distributions with collision multiplicity.     


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…)

A New Angle on the Ridge

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…)

The One That Started It All

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

Recent Hydro Developments

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…)

Hydro Primer

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