A Gap in the Highest Energy Cosmic Ray Spectrum as a
Signature of Unification Scale Physics
Günter Sigl (1),
Sangjin Lee,
David N. Schramm,
Pijushpani Bhattacharjee
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.
G. Sigl, S. Lee, D. N. Schramm, Department of Astronomy and
Astrophysics, Enrico Fermi Institute, University of Chicago, Chicago,
IL 60637-1433, USA, and National Aeronautics and Space
Administration-Fermilab Astrophysics Center, Fermi National
Accelerator Laboratory, Batavia, IL 60510-0500, USA.
P. Bhattacharjee, Indian Institute of Astrophysics, Sarjapur Road,
Koramangala, Bangalore 560 034, India.
(1) To whom correspondence should be addressed.
