π0 azimuthal anisotropy in Au+Au collisions at √(sNN)=39-200GeV 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 √(sNN)=200GeV show a π0v2 signal that exceeds the pQCD energy loss calculations up to pT∼10GeV/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 pT measurements at √(sNN)=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 √(sNN) is varied.