Medium-induced parton energy loss inγ+jet events of high-energy heavy-ion collisions

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 $\mathrm{AA}$ as compared to $\mathrm{pp}$ 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 $\mathrm{pA}$ 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 $\mathrm{AA}$ 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 Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies and some alternative measurements such as using $Z^0$ as a tag at the LHC energy.