Observation of the suppression of the flux of cosmic rays above 4×1019eV

Abstract

The energy spectrum of cosmic rays above 2.5×1018 eV, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index γ of the particle flux, J∝E-γ, at energies between 4×1018 eV and 4×1019 eV is 2.69±0.02(stat)±0.06(syst), steepening to 4.2±0.4(stat)±0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz’min.