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The JEM-EUSO mission
Yoshiyuki Takahashi and the JEM-EUSO Collaboration 2009 New J. Phys. 11 065009 (21pp)   doi: 10.1088/1367-2630/11/6/065009

Ultrahigh-energy cosmic rays: a window on postinflationary reheating epoch of the Universe?

Abstract. It is conjectured that the highest energy cosmic rays, E>E_GZK, where E_GZK~5*10¹⁹ eV is the Greisen-Zatsepin-Kuzmin cutoff energy of the cosmic-ray spectrum, may provide a unique window on a very early epoch of the Universe-namely, that of reheating after inflation-provided that these cosmic rays are due to the decays of parent superheavy long-lived X particles. These particles may constitute a considerable fraction of cold dark matter in the Universe. It is argued that the unconventionally long lifetime of the superheavy particles, which must be in the range 10¹⁰-10²² yr, might require novel particle physics mechanisms of their decays, such as instantons. A toy model illustrating the instanton scenario is proposed. In our scenario, the expected generic features of ultrahigh-energy extensive air showers are similar to those of other top-down scenarios. However, some properties of the upper part of the cosmic-ray spectrum make the instanton scenario distinguishable, at least in principle, from other ones

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