π0 azimuthal anisotropy in Au+Au collisions at from PHENIX: collision energy and path-length dependence of jet-quenching and the role of initial geometry

The azimuthal anisotropy of high pT particle production in heavy ion collisions is a sensitive probe of the jet quenching mechanism. Recent PHENIX measurements for Au+Au collisions at show a π0 v2 signal that exceeds the pQCD energy loss calculations up to , 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 pT measurements at 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 is varied.