RHIC II Science New Directions Working Group
Berndt Mueller
Jamie Nagle
Peter Steinberg



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The charge to the "New Directions" working group is somewhat different than the other groups in the RHIC II Science initiative.

Rather than focus on completing the program begun, and successfully carried out by the originally-designed RHIC accelerator, the goal here is to focus on the "big picture" of how RHIC data fits in with the broader study of the strong interaction in general, and QCD in particular.  The recent few years have been striking, with each succeeding wave of RHIC data forcing us to revise our basic picture of the dynamics relevant to the early stages of heavy ion collisions, and this progress shows no obvious sign of ceasing.  Similarly, advancements on the theory side, from QCD, to effective models (e.g. quark recombination) and from outside the community (e.g. from string theory) are providing new sets of tools as well as entirely new questions.  We may well have sufficient information in hand, both experimentally and theoretically, to at least outline a set of broad questions which may find compelling answers in the data available with the high luminosity in the upgraded detectors for RHIC II.

  Examples of some questions one could ask in this context (from PAS) are:
  • Does the small viscosity needed to describing data at RHIC with hydrodynamics validate Son et al's viscosity bound, or is this "merely" quantum mechanics.  Or, as discussed by Karch et al, does this provide insight into the uncertainty bound via AdS/CFT?

  • Speaking of AdS/CFT, what can we learn about properties of the quark-gluon plasma (finite temperature QCD) from dual gravity theories? Several authors are using these concepts to sucessfully model properties of hadronic physics, so there should be some application in our field as well

  • What are the degrees of freedom of strongly-interacting matter?
    1) Perturbative objects, such as point-like quarks and gluons
    2) Colored bound states, a la Brown and Shuryak
    3) Strong color fields, a la CGC

  • Space-time aspects of the strong interaction, e.g. the conflict between Landau and Bjorken hydrodynamics.  Which is the more relevant scenario and what does it tell us about the strong interacion at asymptotic energies.  What sort of experiments are needed to clarify this issue?

  • Constructing heavy ion collisions: does the factorized parton model, implicit in models such as HIJING, provide a relevant framework?  How can we understand the dynamical generation of parton distributions in the first place?  Similar concerns exist for fragmentation functions, especially with the emergence of quark recombination systematics in the experimental data.

  • What information, if any, is being transported from the initial state by measurements of jet energy loss?  Or are high-pT hadron measurements too "fragile" to tell us anything besides the collision geometry?

  • Heavy quark production and propagation: differences between open and hidden flavored particles.  How can these particles probe the rapid thermalization suggested by other measurements.

  • Parallels with p+p and e+e- reactions have been discussed by several authors recently.  It would be interesting to have frank discussions of how and why heavy ion collisions truly differ from these elementary collisions.  The results may also be interetsing to colleagues at the Tevatron and LHC.