Virtual Journal on QCD Matter

An eight-week program entitled “Quantifying the Properties of Hot QCD Matter” was held at the Institute for Nuclear Theory in Seattle during May-July 2010 (Program 10-2a). For details of the program, see the workshop website, including the agenda and online slides of all talks.

Each week was devoted to discussion in a specific area, as follows:

• New results from LHC and RHIC
• Initial conditions, small x
• Viscous hydrodynamics I
• Viscous hydrodynamics II
• Jet quenching
• Strong versus weak coupling
• Heavy flavor and quarkonium
• Electromagnetic probes

The linked document contains a list of questions and challenges assembled by the organizers of the program, compiled at the end of each week. No attempt has been made to unify the presentation of topics or to resolve conflicting statements, and only minor editing has been applied. This document should be regarded as a snapshot of the field of hot QCD matter research in relativistic heavy ion collisions in summer 2010, as seen through the biased lenses of the six organizers (Brian Cole, Ulrich Heinz, Peter Jacobs, Yuri Kovchegov, Berndt Müller, and James Nagle.

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Here is an interesting Twitter application, initiated by Peter Steinberg of Entropy Bound fame: a Twitter and/or RSS feed listing arXiv postings of interest to the QCD Matter community. Try it!

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As is probably all too clear, I am returning to the Virtual Journal after a very long absence. Expect much more frequent posts from me, and much less formal posts (in the first person in most cases) in the future.

At the beginning of this month, I gave four lectures at the National Nuclear Physics Summer School hosted this year by Michigan State University. The interactions with the students reminded me how much I like teaching and research, ironically at the same time as I begin administrative duties that will have none of the former and little of the latter. (Posting to the Virtual Journal is one of the methods by which I intend to remedy this.)

The organizers of the Summer School did a very nice job creating an interactive environment. After each lecture (questions were encouraged during the lecture, but you know how that goes) the students were assigned to 4-5 person groups for discussion, and then were asked to fill out a simple questionaire:

• Things that were clear.
• Things that were not clear
• Specific Questions:

The lecturers were then asked to address the questions in a separate session the same day, using the chalkboard rather than the projector. I can’t speak for the students, but this was tremendous fun for me, and I like to think it helped in understanding (one observation in support of this was the full attendance at these sessions). The questions were both humbling (“Gee, I thought I explained that clearly”) and stimulating. Below are a few of my favorites:

• Why does lattice QCD not predict expected number of degrees of freedom?
• Is T_c = 170 MeV some settled value or is there some wiggle room?
• Where does thermodynamics get put into QCD?
• How does matter behave at the critical point?
• Why do you use gold nuclei?
• What is a color superconductor?
• Why are people using Bjorken energy density when it appears incorrect based on observed rapidity distribution?
• What is the basis for the bag model?
• What are the hard and soft regimes and how do they connect?
• How is a perfect fluid different from a superfluid?
• What are the definitions of jet and of associated particles?

My only regret is that I did not get to address all of the questions in the discussion section. As you can see, each of these and the many others I received are worthy of a mini-lecture rather than a  one sentence answer.

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The 21st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions, colloquially referred to as Quark Matter 2009, took place in Knoxville, Tennessee from March 30th to April 4th, 2009. (more…)

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This is the 6th post in our series in which we have asked a number of leading scientists in our field to identify the 3-5 most important challenges which the field of hot and dense QCD matter theory has to address (click for the previous posts by Larry McLerran, Carsten Greiner, Nu Xu, Dmitri Kharzeev and Tetsuo Hatsuda).

Krishna Rajagopal (Center for Theoretical Physics, MIT):

1. Is there a critical point in the μ_B < 500 MeV region of the QCD phase diagram? If yes, one needs to determine the μ_B at which it is located with an understood systematic error of 100 MeV. This needs to be done both experimentally, via measurements made during a RHIC beam energy scan, and theoretically, via lattice calculations.
2. 1. Is the Quark-Gluon Plasma at T ~ 3 T_C similar in its properties to the Quark-Gluon Plasma at T ~ 1.5 T_C ? In particular, is it just as liquid-like, or will there start to be some evidence of the existence of quasiparticles?
   2. What properties of the Quark-Gluon Plasma are there which can be either calculated on the lattice or determined from experimental data which quantify the liquid-ness of the Quark-Gluon Plasma? η/s is one such, are there others?
   3. We have learned that v₂ is sensitive to the initial profile of the almond as well as to the value of η/s. Is there an observable which has a different sensitivity to these two kinds of effects and thus can be used to break the present degeneracy? A yes answer to this question only counts if the observable is measurable with small enough error bars and calculable with at least the same degree of reliability that v₂ can be calculated in terms of η/s and initial conditions using viscous hydro calculations now coming on line. If a yes answer is found, it then becomes fair to ask whether the combination of experimental measurements and theoretical calculations can be used to determine η/s with an understood systematic error that is, say, at the 20% level.
   4. Does the bulk viscosity of the Quark-Gluon Plasma rise dramatically in a narrow window of temperature around T_C? Is this responsible for chemical freezeout in heavy ion collisions at low baryon number density?
3. 1. Is there a rigorous factorization theorem for parton energy loss, valid in the limit of high parton energy E? By this I mean a rigorous way of organizing the calculation into a part which can reliably be done with perturbative QCD and a part which describes properties of the strongly coupled medium. A yes answer to this question requires that it be understood how, by dint of sufficient effort, one could reliably compute to higher order in either alpha or 1/E. A yes answer also requires that the functions which describe properties of the strongly coupled medium be well-defined beyond perturbation theory.
   2. Even without an affirmative answer to (3a), what is the best way to use jet measurements in collisions at RHIC and LHC energies, combined, to determine qhat for the Quark-Gluon Plasma at T=1.5 T_C and 3 T_C with a systematic error that is understood?
4. 1. Do we have any theoretical methods other than AdS/CFT with which to do reliable calculations of dynamical properties of any strongly coupled gauge theory plasma which has no quasiparticles of any sort?
   2. How large are the 1/N_c² corrections to any of the quantities of interest to our field that have been calculated in N=4 SYM theory using the AdS/CFT correspondence?
   3. Less difficult than (4b), how do these quantities change as one makes the gauge theory more QCD-like in various other ways while staying at N_c→∞?
5. Is it possible to use LHC data and RHIC data combined to determine whether charmonium and bottomonium mesons are bound at the temperatures achieved in LHC and RHIC collisions? If charmonium (bottomonium) mesons are bound at rest for the temperatures achieved in RHIC (LHC) collisions, can evidence be found of their dissociation if they are moving through the plasma with a high enough velocity?

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This is the 5th post in our series in which we have asked a number of leading scientists in our field to identify the 3-5 most important challenges which the field of hot and dense QCD matter theory has to address (click for the previous posts by Larry McLerran, Carsten Greiner, Nu Xu and Dmitri Kharzeev).

Tetsuo Hatsuda (Theoretical Hadron Physics Group, University of Tokyo):

• What is the state of matter at temperatures between T_C and 2 T_C, and how it is related to the weakly interacting QGP at high enough temperature?
• What is the “direct” evidence of the QCD phase transition (or rapid crossover) and T_C (or pseudo critical temperature) in relativistic heavy ion collisions?
• What is the exact location of the critical point(s) in the QCD phase diagram?
• What is the state of matter at baryon densities between
  ρ₀ and 10 ρ₀ relevant to the interior of neutron stars?

To answer the above questions, the following immediate challenges need to be addressed:

• Determination of T_C (or the pseudo critical temperature)
  with a few % accuracy right at the physial u,d,s masses in lattice QCD. Dynamical fermions having good chiral properties must be used.
• Calculation of spectral functions of q-qbar and qqq states in all spin-flavor channels below and above T_C in full QCD simulations.

Also we need to develop new theoretical approaches such as

• a non-equilibrium formulation of lattice field theory and its
  numerical techniques.
• method(s) to simulate QCD at high baryon density.
• testing of theories of QCD at high baryon density in collaboration with cold atom experiments.

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This is the 4th post in our series in which we have asked a number of leading scientists in our field to identify the 3-5 most important challenges which the field of hot and dense QCD matter theory has to address (click for the previous posts by Larry McLerran, Carsten Greiner and Nu Xu).

Dmitri Kharzeev (Brookhaven National Lab, Nuclear Theory Group Leader):

• What are the dynamical degrees of freedom in quark-gluon matter in the vicinity of the deconfinement phase transition? Is a quasi-particle description applicable in this region at all? What is the role of extended field configurations of QCD in the plasma? Does topology play a role? if yes, what are the experimental signatures?
• How does the transition from the initial gluon fields to a thermalized plasma occur? Is the plasma really fully thermalized? How to describe a real-time dynamics of non-Abelian gauge fields at strong coupling?
• What are the transport properties of the quark-gluon plasma? How do we extract, with a reasonable accuracy, the transport coefficients of plasma (e.g. shear and bulk viscosities) from the data? What is the microscopic origin of the perfect liquid behavior?
• What is the true mechanism of parton energy loss? Is it perturbative? If not, how to describe it? At what transverse momentum (centrality, mass number, …) does the transition from strong to weak coupling occur?

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This is the 3rd post in our series in which we have asked a number of leading scientists in our field to identify the 3-5 most important challenges which the field of hot and dense QCD matter theory has to address (click for the previous posts by Larry McLerran and Carsten Greiner).

Nu Xu (Lawrence Berkeley Lab & spokesperson of the STAR collaboration):

The Holy Grail for high-energy nuclear collisions is the creation of a new form of matter with partonic degrees of freedom. In the past seven years, experimental results have demonstrated that a hot and dense system has been created in heavy ion collisions at RHIC. The system also shows a strong collectivity that has developed at the early partonic stage of the collision:

• Quark gluon plasma and color glass condensate at RHIC? The Perspective from the BRAHMS experiment.
• The PHOBOS perspective on discoveries at RHIC.
• Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC collisions.
• Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration.
• RHIC Theory White papers: Nucl. Phys. A750, p1-169 (2005).
• Partonic flow and phi-meson production in Au + Au collisions at s(NN)**(1/2) = 200-GeV (and references therein)

Strictly speaking, the term MATTER signifies a state in equilibrium. However, in my view, the question of local thermalization in high-energy nuclear collisions has not yet been answered. We do not know when (and how) the system reaches equilibrium nor how long the system remains in equilibrium during its evolution. This question remains an important challenge to both theory and experiment in the era of LHC and RHC II. The future development of theory depends on the answer to this question. Here I give two examples:

• In LGT calculations (see e.g. here), the equilibration is an intrinsic assumption in the calculation. In order to perform a comparison of the data with the lattice results, one must have an adequate understanding of the equilibrium of the system;
• recently, there have been intense discussions on the application of the AdS/CFT duality to QCD and high-energy nuclear collisions (see .e.g here). Again, in these calculations, thermal equilibrium of the medium is assumed. Without a clear knowledge about the nature of the equilibrium of the medium created in high-energy nuclear collisions, the comparison between data and the results from above calculations are meaningless.

The progress of experiments also depends on the answer to the question of thermalization. At both SPS (with the NA61 experiment) and RHIC, there are experimental plans to search for the tri-critical point and/or the phase transition in the QCD phase diagram. The whole concept of phase implies the underlying equilibration of the matter. The understanding of the QCD phase diagram will not be complete if one does not have the control over thermalization in heavy ion collisions.

The second challenge is the process of hadronization in high-energy nuclear collisions. This question address the issues related to how the system, may it be may or not be in equilibrium, is transferred from a parton dominated to a hadron dominated state. Quark coalescence or recombination models have been proposed in analyzing the RHIC data (see the Parton Recombination Primer). However, the microscopic dynamics of the model are not fully understood. For example, we do not know when (and how) the gluon degrees of freedom disappear in the hadronization process. We also do not know when this approach should stop and the ‘normal’ fragmentation should take over at the high transverse momentum region.

I think these are the two most important issues in our field. Many other questions are all linked to these two. In the near future, experimentally, the heavy quark upgrade and Energy Scan Program at RHIC and the LHC heavy ion results will shed lights on both questions.

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With the advent of the RHIC II and LHC programs, our field is entering a new discovery phase – it is therefore timely to take note of where we are with respect to our current state of knowledge of QCD matter and what the greatest theoretical challenges will be for the next few years.

This is the 2nd post in our series in which we have asked a number of leading scientists in our field to identify the 3-5 most important challenges which the field of hot and dense QCD matter theory has to address (click here for the previous post by Larry McLerran)

Carsten Greiner (Goethe University, Frankfurt):

• Development of a transport code for the deconfined phase of relativistic heavy-ion collisions which including mean fields, i.e. a code which treats soft and hard QCD phenomena consistently.
• Development of a full microscopic theory of hadronization across all energy/momentum scales (i.e. including formation of bulk matter, parton recombination, fragmentation and heavy flavor hadrons).
• Develop a fundamental understanding of the theory of dissipative relativistic hydrodynamics.
• Develop a transport formulation for the Glasma (i.e. how to describe the dynamics of a CGC-like initial state).
• If we consider AdS/CFT to be the right tool to derive non-perturbative QCD quantities – can this approach be extended to a true non-equilibrium setup beyond the extraction of transport coefficients for hydro?

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We have asked Scott Pratt, one of the pioneers of the application of HBT to relativistic heavy-ion collisions and a leading expert on the topic to contribute this primer to the Virtual Journal. Scott’s early work related to HBT can be found in:

• Pion Interferometry for Exploding Sources
• Pion Interferometry of Quark-Gluon Plasma

(more…)

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