  Two particle correlations within a single jet produced in deeply inelastic
scattering (DIS) off a large nucleus as well as in heavy-ion collisions are
explored. This is performed within the framework of the medium modified
dihadron fragmentation functions. The modification occurs due to gluon
bremsstrahlung induced by multiple scattering. The modified fragmentation
functions for dihadrons are found to follow closely that of single hadrons
leading to a weak nuclear suppression of their ratios as measured by HERMES in
DIS experiments. Meanwhile, a moderate medium enhancement of the near-side
correlation of two high $p_T$ hadrons is found in central heavy-ion collisions,
partially due to trigger bias caused by the competition between parton energy
loss and the initial Cronin effect.

Nuclear Theory