  A model is constructed for the origin of the double-humped structure found in
the di-hahron azimuthal correlation on the away-side in heavy ion collisions.
The parameters in the model are determined by fitting $\Delta\phi$ azimuthal
distribution in central Au+Au collisions at RHIC given by STAR collaboration
when the trigger momentum is in $3-4\rm {GeV/c}$ and the associated particle
momentum is in $1.3-1.8\rm{GeV/c}$. Then we apply our model to the semi-central
and peripheral Au+Au collisions at RHIC and the responding $\triangle \phi$
azimuthal distribution are reproduced respectively. Our results show that the
transverse flow effect plays an essential role in the experimentally observed
di-hadron azimuthal distribution structure. The spread of the hard parton
creation point and initial moving direction is also necessary for the
double-humped structure.

Nuclear Theory