  We study the dijet azimuthal de-correlation in relativistic heavy ion
collisions as an important probe of the transverse momentum broadening effects
of a high energy jet traversing the quark-gluon plasma. We take into account
both the soft gluon radiation in vacuum associated with the Sudakov logarithms
and the jet P_T-broadening effects in the QCD medium. We find that the Sudakov
effects are dominant at the LHC, while the medium effects can play an important
role at RHIC energies. This explains why the LHC experiments have not yet
observed sizable P_T-broadening effects in the measurement of dijet azimuthal
correlations in heavy ion collisions. Future investigations at RHIC will
provide a unique opportunity to study the P_T-broadening effects and help to
pin down the underlying mechanism for jet energy loss in a hot and dense
medium.
