  We study the evolution and saturation of the gluon distribution function in
the quark-gluon plasma as probed by a propagating parton and its effect on the
computation of jet quenching or transport parameter $\hat{q}$. For thermal
partons, the saturation scale $Q^2_s$ is found to be proportional to the Debye
screening mass $\mu_D^2$. For hard probes, evolution at small $x=Q^2_s/6ET$
leads to jet energy dependence of $\hat{q}$. We study this dependence for both
a conformal gauge theory in weak and strong coupling limit and for (pure gluon)
QCD. The energy dependence can be used to extract the shear viscosity $\eta$ of
the medium since $\eta$ can be related to the transport parameter for thermal
partons in a transport description. We also derive upper bounds on the
transport parameter for both energetic and thermal partons. The later leads to
a lower bound on shear viscosity-to-entropy density ratio which is consistent
with the conjectured lower bound $\eta/s\geq 1/4\pi$. We also discuss the
implications on the study of jet quenching at the BNL Relativistic Heavy Ion
Collider and the CERN Large Hadron Collider and the bulk properties of the
dense matter.
