  Pair correlations between large transverse momentum neutral pion triggers
(p_T=4--7 GeV/c) and charged hadron partners (p_T=3--7 GeV/c) in central
(0--20%) and midcentral (20--60%) Au+Au collisions are presented as a function
of trigger orientation with respect to the reaction plane. The particles are at
larger momentum than where jet shape modifications have been observed, and the
correlations are sensitive to the energy loss of partons traveling through hot
dense matter. An out-of-plane trigger particle produces only 26+/-20% of the
away-side pairs that are observed opposite of an in-plane trigger particle. In
contrast, near-side jet fragments are consistent with no suppression or
dependence on trigger orientation with respect to the reaction plane. These
observations are qualitatively consistent with a picture of little near-side
parton energy loss either due to surface bias or fluctuations and increased
away-side parton energy loss due to a long path through the medium. The
away-side suppression as a function of reaction-plane angle is shown to be
sensitive to both the energy loss mechanism in and the space-time evolution of
heavy-ion collisions.
