Chasing the GZK with HiRes

Abstract

The HiRes Collaboration has recently announced preliminary measurements of the energy spectrum of ultra-high energy cosmic rays (UHECR), as seen in monocular analyses from each of the two HiRes sites. This spectrum is consistent with the existence of the GZK cutoff, as well other aspects of the energy loss processes that cause the GZK cutoff. Based on the analytic energy loss formalism of Berezinsky {\it et al.}, the HiRes spectra favor a distribution of extragalactic sources that has a similar distribution to that of luminous matter in the universe, both in its local over-density and in its cosmological evolution.