Has the GZK suppression been discovered ?

The energy spectra of ultra-high energy cosmic rays reported by the AGASA, Fly’s Eye, Haverah Park, HiRes, and Yakutsk experiments are all shown to be in agreement with each other for energies below 1020 eV (after small adjustments, within the known uncertainties, of the absolute energy scales). The data from HiRes, Fly’s Eye, and Yakutsk are consistent with the expected flux suppression above 5 × 1019 eV due to interactions of cosmic rays with the cosmic microwave background, the Greisen–Zatsepin–Kuzmin (GZK) suppression, and are inconsistent with a smooth extrapolation of the observed cosmic-ray energy spectrum to energies > 5 × 1019 eV. AGASA data show an excess of events above 1020 eV, compared to the predicted GZK suppression and to the flux measured by the other experiments.  2003 Published by Elsevier Science B.V.