Perturbative jet energy loss mechanisms: 
         Learning from RHIC, extrapolating to LHC


High transverse momentum jets are a valuable tool for studying 
the physics of the quark gluon plasma. Within a perturbative 
QCD framework, a partonic jet can lose energy in collisions and 
by radiating additional gluons. The WHDG model combined (D)GLV radiative
loss with a leading log calculation of collisional 
energy loss, thus including the essential physical contribution 
of the collisional loss, with fluctuations given by a gaussian 
width from the fluctuation-dissapation theorem. But there is more 
to the physics of collisions in the QGP. Further study can help
us better understand the mechanisms inducing radiation, as well 
as improving the collisional energy loss model. As the expected number
of (hard) collisions of a jet with the medium is small, a
diffusion /Fokker-Planck formalism is not sufficient: a Poisson
convolution of a single collision spectrum is necessary, taking 
into account the discrete number of collisions. The single 
collision spectrum itself exhibits interesting qualitative 
features, such as a small energy gain contribution, that are neglected
in Bjorken-like calculations. Updated predictions for single particle
inclusive RAA for light and heavy jets for RHIC 
and LHC are presented.