The Highest - Energy Cosmic Rays

The unexpected discovery of the cosmic microwave background by Arno Penzias and Robert Wilson in 1965 is now the centerpiece of our understanding of the Big Bang and the subsequent evolution of the universe. (See PHYSICS TODAY, November 1997, page 32.) The discovery also set off something of a race to verify one of its implications for cosmic rays. In 1966, Kenneth Greisen (Cornell University) pointed out that the most energetic cosmic-ray particles would be affected by interaction with the ubiquitous photons of this microwave background. Greisen predicted that, if cosmic-ray sources were far enough away from us and if their energy spectrum extended beyond 10 eV then the ultrahigh-energy protons and nuclei would interact inelastically with the backgound radiation. The threshold for this energy sapping interaction is the onset of pion photoproduction. Greisen predicted that a smooth power-law cosmic-ray energy spectrum would therefore be abruptly cut off near 5 x 10 eV The same effect was independently predicted by G. T. Zatsepin and V A. Kuzmin in the Soviet Union. From the first, this Greisen-Zatsepin-Kuzmin (GZK) cutoff has been much sought after by cosmic-ray physicists looking at the highest energies. (See the box on page 32.) Unfortunately for the observers, the cosmic-ray flux at these energies is only about one event per century per square kilometer of detector array. Therefore, our progress in understanding the high-energy end of the cosmicray spectrum has been painfully slow. Nonetheless, in 1991 and 1993, two very different experiments observed events that appear to be clearly beyond the predicted cutoff. The 1991 event, which still holds the record, had an incident energy of 3.2 x 10 eV far beyond anything we can produce with an accelerator. It's a single particle or nucleus with the kinetic energy of a well-hit tennis ball. A number of new experiments and experimental proposals are in the works to further investigate this energy region. We examine here the implications of the energy spectrum measurements on our understanding of the origin of cosmic rays, and we discuss the newer experimental methods that extend and confirm the previous measurements.