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
strong interaction, e.g. the conflict between Landau and Bjorken
hydrodynamics. Which is the more relevant scenario and what does
tell us about the strong interacion at asymptotic energies. What
of experiments are needed to clarify this issue?
- Constructing heavy
collisions: does the factorized parton model, implicit in models such
as HIJING, provide a relevant framework? How can we understand
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
propagation: differences between open and hidden flavored
How can these particles probe the rapid thermalization suggested by
- Parallels with p+p
reactions have been discussed by several authors recently. It
interesting to have frank discussions of how and why heavy ion
collisions truly differ from these elementary collisions. The
may also be interetsing to colleagues at the Tevatron and LHC.