  The radiative energy loss of fast partons traveling through the quark-gluon
plasma (QGP) is commonly studied within perturbative QCD (pQCD).
Nonperturbative (NP) effects, which are expected to become important near the
critical temperature, have been much less investigated. Here, we utilize a
recently developed $T$-matrix approach to incorporate NP effects for gluon
emission off heavy quarks propagating through the QGP. We set up four cases
that contain, starting from a Born diagram calculation with color-Coulomb
interaction, an increasing level of NP components, by subsequently including
(remnants of) confining interactions, resummation in the heavy-light scattering
amplitude, and off-shell spectral functions for both heavy and light partons.
For each case we compute the power spectra of the emitted gluons, heavy-quark
transport coefficients (drag and transverse-momentum broadening, $\hat{q}$),
and the path-length dependent energy loss within a "QGP brick" at fixed
temperature. Investigating the differences in these quantities between the four
cases illustrates how NP mechanisms affect gluon radiation processes. While the
baseline perturbative processes experience a strong suppression of soft
radiation due to thermal masses of the emitted gluons, confining interactions,
ladder resummations and broad spectral functions (re-)generate a large
enhancement toward low momenta and low temperatures. For example, for a 10 GeV
charm quark at 200 MeV temperature, they enhance the transport coefficients by
up to a factor of 10, while the results smoothly converge to perturbative
results at sufficiently hard scales.
