Observation of the suppression of the flux of cosmic rays above 4x1019eV

Abstract

The energy spectrum of cosmic rays above 2.5x1018eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the flux, J ~ E-gamma, at energies between 4x1018eV and 4x1019eV is 2.69+-0.02(stat)+-0.06(syst), steepening to 4.2+-0.4(stat)+-0.06(syst) at higher energies, consistent with the prediction by Greisen and by Zatsepin and Kuz'min.