  The di-jet asymmetry --- the measure of the momentum imbalance in a di-jet
system --- is a key jet quenching observable. Using the event generator \jewel
we show that the di-jet asymmetry is dominated by fluctuations both in
proton-proton and in heavy ion collisions. We discuss how in proton-proton
collisions the asymmetry is generated through recoil and out-of-cone radiation.
In heavy ion collisions two additional sources contribute to the asymmetry,
namely energy loss fluctuations and differences in path length. The latter is
shown to be a sub-leading effect. We discuss the implications of our results
for the interpretation of this observable.
