Hadron-gamma discrimination from an orbital UHECR observatory

TL;DR

This paper investigates the potential for an orbital ultra-high-energy cosmic ray observatory to distinguish between hadron and photon-induced air showers, focusing on the maximum theoretical discrimination capabilities at extremely high energies.

Contribution

It provides a detailed analysis of the ideal hadron-photon discrimination power for an orbital detector considering LPM and magnetospheric effects, without real-world detection inefficiencies.

Findings

Maximum discrimination power is quantified under ideal conditions.

Photon and hadron showers show distinguishable features at ultra-high energies.

Results inform future design and analysis of orbital UHECR observatories.

Abstract

The identification of very high energy photons is of great importance for the understanding of the origin of extreme energy cosmic rays (EECR). Several can be the sources of high energy photons at Earth. A guaranteed component is the flux of high energy photons expected as a consequence of the interaction of cosmic rays with the cosmic photon background. Another contribution may be expected as by-product at the acceleration sites of protons and nuclei, although such flux should be strongly suppressed for distant sources. On the other hand, top-down scenarios involving the decay of super heavy relic particles or topological defects, even if not currently favored, have as a characteristic signature an increasingly dominant flux of photons at the highest energies. In this work we study the statistical separation between hadron and photon showers at energies where both, LPM effect and magnetospheric interactions are important for the development of the cascades. We consider a detector with the same orbital characteristics as JEM-EUSO, but disregard trigger and reconstruction efficiencies, in order to define the maximum ideal discrimination power attainable.