The many-body physics of relativistic non-Abelian gauge theories
plays an important role in current heavy ion collision experiments,
in the astrophysics of compact stars, as well as in cosmology.
Within the program, the methods available for studying
such systems were discussed on a broad spectrum, ranging from analytic
effective field theory techniques, through numerical lattice Monte
Carlo simulations, all the way to string-theory inspired dualities.
Specific focal points were:
scrutinizing recent advances in the non-perturbative
study of systems with a finite baryon density; the issue of how fast
thermalization can take place; as well as mapping the status of
perturbative and non-perturbative particle production rate
computations, both with and without resonance enhancement.

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RESEARCH HIGHLIGHTS

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QCD phase diagram at finite density
**
As is well known, the Boltzmann weight in the QCD path integral
is complex at non-zero chemical potential, which poses a formidable
challenge for practical simulations. There are ongoing efforts
to evade this problem, via the use of imaginary chemical potential,
the strong coupling expansion, or the density of states method.
A relatively recent alternative uses complex Langevin dynamics.
The program brought together various experts in the field, and allowed
for intense discussions on the pros and cons of various approaches.
It is hoped that studies in simplified systems, some of them
of academic nature but others actually relevant for strongly interacting
systems, help to suggest avenues for progress.

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Thermalization
**
Given some values for thermodynamic potentials and transport coefficients,
the macroscopic evolution of a system and the propagation of various probe
particles can be studied via hydrodynamics. One focus of the meeting was
on the values of these transport coefficients and on how close to
"ideal" the hydrodynamic evolution really is. Another focus was on the
route to equilibrium in a heavy ion collision, from the initially very
nonequilibrium initial conditions. We had a review of the picture in
strong coupling, and a vigorous discussion of some competing scenarios
and open questions in the case that the energy density is so high that
weak coupling methods should apply. The way that over-occupied plasmas
approach equilibrium was at least partly clarified. In particular it
became clear during the meeting that a Bose-Einstein condensate does not
play a major role in these dynamics, and is probably in any case not a
well defined notion.

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Towards systematic determination of
transport coefficients and spectral functions
**** **
The dynamical properties of a many-body system, particularly
the rates at which various types of particles are being produced,
are encoded in various gauge-invariant spectral functions and their
zero-frequency limits, known as transport coefficients. It would
be of utmost importance to develop controlled non-perturbative
methods for the determination of these quantities, starting from
Euclidean correlators computed via Monte Carlo simulations.
Moreover, effective field theory techniques or computations
in idealized but related theories may help to develop a generic
understanding of the qualitative shape of the spectral functions.
The meeting had some intensive discussions about the strengths and
limitations of various methods for the analytic continuation of
Euclidean data to obtain spectral functions. In addition applications
to QCD problems of current importance, particularly heavy quark
correlators, as well as to cosmology, in which case resummed
perturbative computations could suffice, were considered.