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) eV. It had been anticipated that there woul d be a cutoff in the energy spectrum of primary cosmic rays around 6x10(19) eV induced by the interaction of the particles with the 2.7-K primordial p hotons. However, recent experimental data have established that particles e xist with energies greatly exceeding this. It follows that the sources of s uch particles are probably nearby, on a cosmological scale. However, althou gh the trajectories of such energetic particles through the galactic and in tergalactic magnetic fields may be nearly rectilinear, no astronomical sour ces have as yet been identified. This is the enigma of the highest-energy c osmic 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 obs ervations made by six experiments. The detection methods currently availabl e are described. Special techniques have been developed as particles of 10( 20) eV or higher occur at a rate of only about 1 per km(2) per century. Err ors in measurement are given particular attention. The authors also review the theoretical predictions for a number of candidate sources of cosmic ray s beyond the predicted cutoff. Finally, the four major projects planned to address the question of the origin of UHECRs are briefly described.