${\pi }^0$ Azimuthal Anisotropy in Au+Au Collisions at $\sqrt{s_{NN}}=39-200$ GeV from PHENIX: Collision Energy and Path-Length Dependence of Jet-Quenching and the Role of Initial Geometry

The azimuthal anisotropy of high $p_T$ particle production in heavy ion collisions is a sensitive probe of the jet quenching mechanism. Recent PHENIX measurements for Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV show a ${\pi }^0$ $v_2$ signal that exceeds the pQCD energy loss calculations up to $p_{T} \sim 10$ GeV/c, challenging the traditional perturbative picture of the energy loss process. Here, we present an update and details of that measurement, as well as new high $p_T$ measurements at $\sqrt{s_{NN}}=62$ and 39 GeV. These measurements not only provide an important constraint for understanding the path-length dependence of jet energy loss and the role of initial collision geometry, but also allow a search for the onset of jet quenching as $\sqrt{s_{NN}}$ is varied.