  A thermodynamic T-matrix approach for elastic 2-body interactions is employed
to calculate spectral functions of open and hidden heavy-quark systems in the
Quark-Gluon Plasma. This enables the evaluation of quarkonium bound-state
properties and heavy-quark diffusion on a common basis and thus to obtain
mutual constraints. The two-body interaction kernel is approximated within a
potential picture for spacelike momentum transfers. An effective
field-theoretical model combining color-Coulomb and confining terms is
implemented with relativistic corrections and for different color channels.
Four pertinent model parameters, characterizing the coupling strengths and
screening, are adjusted to reproduce the color-average heavy-quark free energy
as computed in thermal lattice QCD. The approach is tested against vacuum
spectroscopy in the open (D, B) and hidden (Psi and Upsilon) flavor sectors, as
well as in the high-energy limit of elastic perturbative QCD scattering.
Theoretical uncertainties in the static reduction scheme of the 4-dimensional
Bethe-Salpeter equation are elucidated. The quarkonium spectral functions are
used to calculate Euclidean correlators which are discussed in light of lattice
QCD results, while heavy-quark relaxation rates and diffusion coefficients are
extracted utilizing a Fokker-Planck equation.
