High $p_T$ Azimuthal Asymmetry in Non-central A+A at RHIC

The high $p_T>3$ GeV azimuthal asymmetry, $v_2(p_T)$, in non-central nuclear collisions at RHIC is shown to be a sensitive measure of the initial density distribution of the produced quark-gluon plasma. A generalization of the GLV non-abelian energy loss including Bjorken 1+1D expansion as well as important kinematic constraints is used. Unlike the low $p_T<2$ GeV hydrodynamic region, the pattern of azimuthal asymmetry and suppression at high $p_T$ provides an experimental tool to measure the {\em initial conditions} at RHIC.