A pot of gold at the end of the cosmic "raynbow"?

We critically review the common belief that ultrahigh energy cosmic rays are protons or atomic nuclei with masses not exceeding that of iron. We find that heavier nuclei are indeed possible, and discuss possible sources and acceleration mechanisms for such primaries. We also show detailed simulations of extensive air showers produced by ``superheavy'' nuclei, and discuss prospects for their detection in future experiments.Comment: Talk to be presented at the International Symposium on Very High Energy Cosmic Ray Interactions X

Footprints of Super-GZK Cosmic Rays in the Pilliga State Forest

High energy cosmic ray data collected by the SUGAR Collaboration in the Pilliga State Forest have been re-analyzed using up-to-date shower simulations. Complete, inclined angle, Monte Carlo simulations reveal 2 events with energies in excess of $7\times 10^{19}$ eV at 95% CL, independent of the choice of hadronic interaction model and of chemical composition of the primary. An additional 3 events, with mean energy \agt 7\times 10^{19} eV, were also re-analyzed in the same manner. A lower bound on the flux at the high end of the spectrum, as observed in the Southern sky, is presented on the basis of our analysis.Comment: 7 pages revtex, 7 figure