Ultra High Energy Cosmic Ray Protons: Signatures and Observations

The status of the Greisen-Zatsepin-Kuzmin (GZK) cutoff and pair-production dip in Ultra High Energy Cosmic Rays (UHECR) is discussed. They are the features in the spectrum of protons propagating through CMB radiation in extragalactic space, and discovery of these features implies that primary particles are mostly extragalactic protons. The spectra measured by AGASA, Yakutsk, HiRes and Auger detectors are in good agreement with the pair-production dip, and HiRes data have strong evidences for the GZK cutoff. The Auger spectrum, as presented at the 30th ICRC 2007, agrees with the GZK cutoff, too. The AGASA data agree well with the beginning of the GZK cutoff at E < ∼ 8 × 10 eV, but show the excess of events at higher energies, the origin of which is not understood. The difference in the absolute fluxes measured by different detectors disappears after energy shift within the systematic errors of each experiment.