Observation of the suppression of the flux of cosmic rays above   4x10^19eV
  The energy spectrum of cosmic rays above 2.5x10^18eV, 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 4x10^18eV and 4x10^19eV 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. 
The Pierre Auger Collaboration