2.5. Superheavy Dark Matter
As discussed here by Kolb (2003), it has recently been realized that interesting amounts of superheavy particles with masses ~ 1014±5 GeV might have been produced non-thermally in the very early Universe, either via pre- and reheating following inflation, or during bubble collisions, or by gravitational effects in the expanding Universe. If any of these superheavy particles were metastable they might be able, as seen in Fig. 12, to explain the ultra-high-energy cosmic rays that seem (Takeda et al. 2002; Abu-Zayyad et al. 2002) to appear beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff (Greisen 1966; Zatsepin & Kuzmin 1966). Such models have to face some challenges, notably from upper limits on the fractions of gamma rays at ultra-high energies, but might exhibit distinctive signatures such as a galactic anisotropy (Sarkar 2002). Examples of such superheavy particles are the cryptons found in some models derived from string theory, which naturally have masses ~ 1012±2 GeV and are metastable (like protons), decaying via higher-order multiparticle interactions (Ellis et al. 1990; Benakli et al. 1999). The Pierre Auger experiment (Cronin et al. 2002) will be able to tell us whether ultra-high-energy cosmic rays really exist beyond the GZK cutoff, and, if so, whether they are due to some such exotic top-down mechanism, or whether they have some bottom-up astrophysical origin. Following Auger, there are ideas for space experiments such as EUSO (Petrolini et al. 2002) that could have even better sensitivities to ultra-high-energy cosmic rays.
Figure 12. Calculation of the spectrum of ultra-high-energy cosmic rays that might be produced by the decays (Sarkar 2002) of metastable superheavy particles.