The high energy frontier of the Pierre Auger Cosmic Ray Observatory Paul Sommers, Professor of Physics, Penn State University

The Auger Observatory, now operating in Argentina, measures ultra-high energy (UHE) cosmic rays mostly in the energy range 1018-1020 eV. Results have already settled some important questions. The "GZK suppression" in the energy spectrum has been confirmed along with evidence for detectable extragalactic sources closer than 100 Mpc. Upper limits on UHE photons now exclude most exotic top-down production scenarios. Upper limits on UHE neutrinos have also been obtained. Unexpected properties of measured air shower cascades have raised questions about hadronic interactions at extremely high energies and about the nuclear mass composition of the primary cosmic rays. These open questions, together with the quest to identify and study nearby sources of trans-GZK cosmic rays, motivate the construction of a much larger detector in the northern hemisphere.