Extension of the Cosmic-Ray Energy Spectrum Beyond the Predicted   Greisen-Zatsepin-Kuz'min Cutoff
  The cosmic-ray energy spectrum above 10^{18.5} eV is reported using the updated data set of the Akeno Giant Air Shower Array (AGASA) from February 1990 to October 1997. The energy spectrum extends beyond 10^{20} eV and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff. 
M. Takeda, N. Hayashida, K. Honda, N. Inoue, K. Kadota, F. Kakimoto,   K. Kamata, S. Kawaguchi, Y. Kawasaki, N. Kawasumi, H....