A Gap in the Highest Energy Cosmic Ray Spectrum as a Signature of Unification Scale Physics

Abstract

Recent experimental data seem to indicate that there is significant structure in the cosmic ray spectrum above 10¹⁸ electron volts (eV). Besides a dip at ∼5 × 10¹⁸ eV, two events above 2 × 10²⁰ eV have been observed. The implications for the existence of the Greisen-Zatsepin-Kuzmin cutoff, a long-lasting and still open question in cosmic ray physics, are discussed. This cutoff at a few times 10¹⁹ eV, caused by energy losses in the cosmic microwave background, has been predicted to occur in most acceleration models involving extragalactic sources. An acceleration origin of particles above 10²⁰ eV within a few megaparsecs cannot be ruled out yet. However, persistence of the apparent gap in the existing data at a quadrupled total exposure would rule out many acceleration models at the 99 percent confidence level for any source distance. Particles above 10²⁰ eV might then be directly produced by decay from some higher energy scale in contrast to acceleration of charged particles.