[Paper Review] Was the Highest Energy Cosmic Ray a Photon?

P. Homola, Markus Risse+6 more|arXiv (Cornell University)|Jul 16, 2004

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper re-evaluates the hypothesis that the highest-energy cosmic ray ever observed (the Fly's Eye event) was a photon, using advanced Monte Carlo simulations that include geomagnetic precascading and LPM effects. It finds the photon hypothesis cannot be excluded at 1.5σ significance, suggesting UHE photons remain a viable primary candidate and highlighting the importance of dual-hemisphere observatories like Pierre Auger for definitive photon identification.

ABSTRACT

The hypothetical photonic origin of the most energetic air shower detected by the Fly's Eye experiment is discussed. The method used for the analysis is based on Monte Carlo simulations including the effect of precascading of ultra-high energy (UHE) photons in the geomagnetic field. The application of this method to data expected from the Pierre Auger Observatory is discussed. The importance of complementing the southern Auger location by a northern site for UHE photon identification is pointed out.

Motivation & Objective

• To reassess the photonic origin hypothesis of the highest-energy cosmic ray event recorded by the Fly’s Eye experiment.
• To improve upon prior analyses by incorporating accurate geomagnetic field modeling and updated cross-section data for UHE photon precascading.
• To evaluate the feasibility of identifying UHE photons at the Pierre Auger Observatory, particularly through complementary observations in both hemispheres.
• To quantify the sensitivity of future UHE experiments to photon fluxes and to set upper limits on photon contributions to the UHE cosmic ray flux.

Proposed method

• Employed detailed Monte Carlo simulations using CORSIKA and a custom code (PRESHOWER) to model UHE photon propagation and air shower development.
• Incorporated the effect of geomagnetic field-induced precascading (pair production) for UHE photons before atmospheric entry, accounting for energy-dependent conversion probabilities.
• Simulated extensive air showers (EAS) induced by photons and their secondary particles, including LPM suppression effects in the electromagnetic cascade.
• Compared simulated shower profiles (especially Xmax depth) with the observed Fly’s Eye event data to compute the probability of a photon primary.
• Used statistical methods to estimate confidence levels for excluding photon fractions in the UHE cosmic ray flux, assuming a given measurement uncertainty (ε).
• Evaluated the directional and energy-dependent preshower formation at both the southern (Auger South) and northern (Auger North) observatory sites, considering differences in geomagnetic field strength and orientation.

Experimental results

Research questions

• RQ1 Can the photonic origin of the Fly’s Eye record event be excluded based on updated simulations including precascading and LPM effects?
• RQ2 How does the accuracy of the geomagnetic field model affect the probability of a photon primary for the Fly’s Eye event?
• RQ3 What is the expected sensitivity of the Pierre Auger Observatory to UHE photon primaries, and how does it depend on detector location?
• RQ4 How do the differing geomagnetic conditions at Auger South and Auger North influence the preshower effect and thus photon identification?
• RQ5 Can a dual-hemisphere observation strategy significantly improve the confidence in identifying UHE photon signals?

Key findings

• The photonic origin hypothesis for the Fly’s Eye event cannot be excluded at the 1.5σ significance level, indicating it remains a viable explanation.
• The discrepancy between simulated and observed shower maximum depth (Xmax) is within 1.5σ, suggesting the data are consistent with a photon primary despite prior conclusions to the contrary.
• The analysis corrects a numerical error present in a widely cited reference (Erber, 1995), which previously led to an overestimated exclusion of the photon hypothesis.
• At Auger South, the higher preshower threshold (due to weaker magnetic field) allows better event-by-event separation of unconverted photons from hadronic showers.
• At Auger North, the stronger magnetic field (0.54 G vs. 0.25 G at South) lowers the preshower onset threshold to 20–30 EeV, enabling detection of directional anisotropies in shower observables.
• With 30 events and ε=5%, a photon fraction exceeding 14% in the UHE flux could be excluded at 95% confidence level; with 50 events, the limit rises to 10%.