High Density QCD

(Organized by Jean-Paul Blaizot, Luciano Maiani and Alfred Mueller)

January 15-March 9, 2007 with associated ECT* workshop in Trento, 
                         January 8-12, 2007

The Program

With a successful experimental program continuing at Brookhaven and a new
ultra high energy program about to start at CERN, heavy ion collisions are 
offering a rich variety of experimental input for the study of high density 
QCD.  The purpose of this program is to review the present status of high 
density QCD, in its various theoretical and experimental manifestations, 
and to try to understand where progress can be made in the near future.

The main two topics of the program include:  
(i)  equilibrium and non-equilibrium QCD and 
(ii)  QCD evolution near the unitarity limit. 
Our focus is the study of these subjects both as purely theoretical topics 
and as the basis for the phenomenology of high-energy heavy ion collisions 
and small-x physics.  The program has a particular emphasis on how heavy ion 
collisions, especially in their pre-equilibrium stage, and small-x behavior, 
as studied for example at HERA, are related and how one can use information 
from one process to better understand the other process.

Examples of more detailed subjects  which will be part of the program are:

(1)  Equilibration after a heavy ion collision:  The interesting issues are 
the understanding of what are the main mechanisms by which equilibration 
occurs, including the question of whether instabilities and wave turbulence 
play an important role.  It is also a fascinating question as to what extent 
classical gluon dynamics can describe the approach to equilibrium and whether 
the Boltzmann equation can be effectively used.  Key phenomenological issues 
are whether equilibration will be similar at RHIC and CERN and to what extent 
the quasiparticle picture is valid at accessible energies.

(2)  High-energy scattering near the unitarity limit.  This problem is close 
to that of the early stages after a heavy ion collision in that high gluon 
occupancy is important in both cases.  Hard high energy scattering at very 
small x values appears to be a problem which may be close to being solved.  
We hope to critically evaluate this question during our program.  The other 
key issue here is to try and ascertain the role of high occupancy (saturation) 
in the phenomenology at HERA and the possibility for phenomenology in 
proton-proton and proton-ion collisions at the LHC.   Finally, this topic 
seems to have very close formal parallels with reaction-diffusion process which
are widely studied in statistical physics.  We hope to try to better understand
the depth at which these parallels exist and whether there are corresponding 
parallels in observable phenomena which may help to elucidate high density QCD.

(3)  The nature of the quark gluon plasma.  The question whether the QCD plasma
may have unusual properties, for example an unusually small viscosity, is 
currently widely discussed.   Jet propagation as well as quarkonia abundances 
could provide interesting signatures to characterize the physical properties 
of the plasma. We hope to include approaches, such as string theory, in our 
program which can help to elucidate possibly striking features of the QCD 
plasma.

(4)  Relation to experiment.  Our desire is to keep as close as possible to 
phenomenology.   Ideally, we would like to form some long term working 
relations hips between theorists and experimenters. With that in mind we plan 
to have short-term visits, and seminars, by experimenters covering the RHIC, 
HERA and upcoming LHC experimental programs.   Furthermore, in order to help 
make contact with experimenters and to help sharpen the agenda for our program 
an opening conference will be held at the ECT* on January 8-12, 2007 where a 
strong experimental participation is anticipated.