  High $p_T$ jets are known to be strongly modified by the dense, strongly
interacting medium created in heavy-ion collisions. The jet signal, extracted
from two particle $\Delta\phi$ correlation, shows a systematic evolution of
these modifications as function of $p_T$ and centrality. At intermediate $p_T$,
both near side and away side correlations are modified. But the modifications
are much stronger at the away side, resulting in a characteristic cone type of
structure in central Au + Au collisions. The robustness of cone structure is
strengthened by studying the jet shape as function of angle relative to the
reaction plane. As one increase the $p_T$ for BOTH hadrons, the cone structure
seems to be filled up, and a peak structure appears on the away side. However,
the interpretation of these results require careful separation of medium effect
and surface bias.
