  We show, through analytic arguments, numerical calculations, and comparison
with experimental data, that the ratio of the high-p_T observables v_2/(1-R_AA)
reaches a well-defined saturation value at high p_T, and that this ratio
depends only on the spatial anisotropy of the quark gluon plasma (QGP) formed
in ultrarelativistic heavy-ion collisions. With expected future reduction of
experimental errors, the anisotropy extracted from experimental data will
further constrain the calculations of initial particle production in heavy-ion
collisions and thus test our understanding of QGP physics.
