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

TL;DR

This paper explores the possibility that ultra-high energy cosmic rays can reveal information about the Universe's reheating epoch after inflation, through decays of superheavy dark matter particles potentially detectable via air showers.

Contribution

It introduces a novel instanton-based decay mechanism for superheavy particles as a source of ultra-high energy cosmic rays, linking cosmology and particle physics.

Findings

Ultra-high energy cosmic rays may originate from decays of superheavy dark matter.

Distinct features in the cosmic ray spectrum could differentiate this scenario from others.

The proposed model suggests new particle physics mechanisms involving instantons.

Abstract

We conjecture that the highest energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cut-off 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.