Ultra - High Energy Cosmic Rays : a Window to Post - Inflationary Reheating Epoch of the Universe ?

We conjecture that the highest energy cosmic rays, E > E GZK , where E GZK ∼ 5 · 10 19 eV is the Greisen–Zatsepin–Kuzmin cutoff energy of cosmic ray spectrum, may provide a unique window into the very early epoch of the Universe, namely, that of reheating after inflation, provided these cosmic rays are due to decays of parent superheavy long-living X-particles. These particles may constitute a considerable fraction of cold dark matter in the Universe. We argue that the unconventionally long lifetime of the superheavy particles, which should be in the range of 10 10 − 10 22 years, might require novel particle physics mechanisms of their decays, such as instantons. We propose a toy model illustrating the instanton scenario. Generic expected features of ultra-high energy extensive air showers in our scenario 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.