Observations and implications of the ultrahigh-energy cosmic rays

The authors define “ultrahigh-energy cosmic rays” (UHECRs) as those cosmic rays with energies above ${10}^{18}\mathrm{eV}.$ It had been anticipated that there would be a cutoff in the energy spectrum of primary cosmic rays around $6\times {10}^{19}\mathrm{eV}$ induced by the interaction of the particles with the 2.7-K primordial photons. However, recent experimental data have established that particles exist with energies greatly exceeding this. It follows that the sources of such particles are probably nearby, on a cosmological scale. However, although the trajectories of such energetic particles through the galactic and intergalactic magnetic fields may be nearly rectilinear, no astronomical sources have as yet been identified. This is the enigma of the highest-energy cosmic rays. The paper reviews the history of research in this energy regime and critically assesses the observational results on the energy spectrum, arrival directions, and composition of the primary cosmic rays based on observations made by six experiments. The detection methods currently available are described. Special techniques have been developed as particles of ${10}^{20}\mathrm{eV}$ or higher occur at a rate of only about 1 per ${\mathrm{km}}^2$ per century. Errors in measurement are given particular attention. The authors also review the theoretical predictions for a number of candidate sources of cosmic rays beyond the predicted cutoff. Finally, the four major projects planned to address the question of the origin of UHECRs are briefly described.