Cosmic Rays at Extreme Energies

Interest in extremely high energy cosmic rays has increased rapidly in the last decade. More important perhaps is that that interest now extends well beyond the \usual suspects" to include members of the wider astrophysics and high energy physics communities. Factors which have contributed to this development include: a keener appreciation of the diiculties associated with cosmic ray production at these energies; increased conndence in the energy and mass estimates made by the \unconventional" detectors used in ground based cosmic ray research; a realization that cosmic rays at these energies may serve as a unique probe of the extreme astrophysical environments where they are produced, and of the environment between the source or sources and our own galaxy; and lastly, the widespread availability of technologies such as digital wireless telecommunications and GPS-based time synchronization systems which make construction of new more powerful detectors feasible. Cosmic rays were discovered by the Austrian physicist Victor Hess in 1912. EEorts to determine their nature | their composition, source, and spectrum | soon followed. Eighty years have elapsed and these goals have not changed. Now however, detectors cover thousands of square kilometers; the energies of interest exceed 10 18 eV (1 EeV); and the most likely sources, however unlikely they may be, lie far beyond the connnes of our galaxy. In the 1930's Pierre Auger introduced a new era in the study of cosmic rays when he observed \extensive air showers" (EAS) | particle cascades initiated in the upper atmosphere by the interaction of energetic cosmic rays. Auger deduced that the cosmic ray spectrum extended up to and perhaps beyond 10 15 eV: Beginning in the 1960's, air shower detectors were constructed with relatively large areas (i.e., a few square kilometers). These eeorts were motivated in part by the realization that cosmic rays at a few EeV were unlikely to be connned by the Galaxy's magnetic elds. Thus, if sources at these energies were Galactic, the observed ux should exhibit a degree of anisotropy. No clear evidence for anisotropy was found. However, these detectors did observe extraordinarily energetic events | in excess of 10 20 eV ! Unfortunately, even now, little is known with certainty regarding the nature of these extremely high energy (EHE) cosmic rays a. The reason | inadequate data, both in terms of the numbers of events observed and in the detailed characterization of those events. Cosmic rays at these energies …