A GAP IN THE HIGHEST ENERGY COSMIC-RAY SPECTRUM AS A SIGNATURE OF UNIFICATION SCALE PHYSICS

Recent experimental data seem to indicate that there is significant st ructure in the cosmic ray spectrum above 10(18) electron volts (eV). B esides a dip at similar to 5 x 10(18) eV, two events above 2 10(20) eV have been observed. The implications for the existence of the Greisen -Zatsepin-Kuzmin cutoff, a long-lasting and still open question in cos mic ray physics, are discussed. This cutoff at a few times 10(19) eV, caused by energy losses in the cosmic microwave background, has been p redicted to occur in most acceleration models involving extragalactic sources. An acceleration origin of particles above 10(20) eV within a few megaparsecs cannot be ruled out yet. However, persistence of the a pparent gap in the existing data at a quadrupled total exposure would rule out many acceleration models at the 99 percent Confidence level f or any source distance. Particles above 10(20) eV might then be direct ly produced by decay from some higher energy scale in contrast to acce leration of charged particles.