  The effect of medium-induced parton energy loss on jet fragmentation is
studied in high-energy heavy-ion collisions. It is shown that an effective jet
fragmentation function can be extracted from the inclusive $p_T$ spectrum of
charged particles in the opposite direction of a tagged direct photon with a
fixed transverse energy. We study the modification of the effective jet
fragmentation function due to parton energy loss in AA as compared to pp
collisions, including $E_T$ smearing from initial state radiations for the
photon-tagged jets. The effective fragmentation function at
$z=p_T/E_T^\gamma\sim 1$ in pA collisions is shown to be sensitive to the
additional $E_T$ smearing due to initial multiple parton scatterings whose
effect must be subtracted out in AA collisions in order to extract the
effective parton energy loss. Jet quenching in deeply inelastic lepton-nucleus
scatterings as a measure of the parton energy loss in cold nuclear matter is
also discussed. We also comment on the experimental feasibilities of the
proposed study at the RHIC and LHC energies and some alternative measurements
such as using $Z^0$ as a tag at the LHC energy.
