  This paper reports analysis on jet-medium interactions and di-jet surface
emission bias at RHIC, based on a new multi-hadron correlation technique called
\texttt{2+1} where back-to-back high $p_{T}$ hadron triggers work as proxy of
di-jets. In contrast with traditional correlations with single triggers, the
associated hadron distributions and spectra at both trigger sides show no
evident modification from d+Au to central Au+Au collisions. This observation
stands for both cases when triggers pairs are symmetric($p_T>$5GeV/$c$ vs.
$p_T>$4GeV/$c$) or asymmetric($E_T>$10GeV/$c$ vs. $p_T>$4GeV/$c$).
