Virtual Journal on QCD Matter

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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