An improved measurement of $v_2$ for $\pi^0$ in a broad range of $p_T$ and centrality is presented. By combining $v_2$ with the $R_{ AA}$, we provide new insights on jet-medium interactions. We show that current pQCD energy loss models cannot describe the suppression of the $\pi^0$ as a function of the angle with respect to the reaction plane. Our result could help to resolve the factor of 4 differences in the predicted transport coefficients among these models. Alternatively, it may suggest that non-perturbative effects associated with the strongly coupled QGP are important, and new theoretical developments are needed to fully understand the jet medium interactions.
Categories: nucl-ex