Cosmogenic photons as a test of ultra-high energy cosmic ray composition

TL;DR

This paper proposes using cosmogenic photon flux measurements as an independent method to determine the composition of ultra-high energy cosmic rays, potentially confirming or challenging current hadronic interaction model-based conclusions.

Contribution

It introduces a novel observational test based on cosmogenic photons to infer cosmic ray composition, offering a cross-check to existing methods.

Findings

Photon flux is significantly suppressed if cosmic rays are heavy nuclei.

Future observatories can differentiate between proton and heavy nuclei dominance.

This method provides an independent constraint on cosmic ray composition.

Abstract

Although recent measurements of the shower profiles of ultra-high energy cosmic rays suggest that they are largely initiated by heavy nuclei, such conclusions rely on hadronic interaction models which have large uncertainties. We investigate an alternative test of cosmic ray composition which is based on the observation of ultra-high energy photons produced through cosmic ray interactions with diffuse low energy photon backgrounds during intergalactic propagation. We show that if the ultra-high energy cosmic rays are dominated by heavy nuclei, the flux of these photons is suppressed by approximately an order of magnitude relative to the proton-dominated case. Future observations by the Pierre Auger Observatory may be able to use this observable to constrain the composition of the primaries, thus providing an important cross-check of hadronic interaction models.