[Paper Review] Measurement of Energy Spectrum of Ultra-High Energy Cosmic Rays

V. Verzi, D. Ivanov+1 more|arXiv (Cornell University)|May 25, 2017

Astrophysics and Cosmic Phenomena3 references3 citations

TL;DR

This paper presents a comparative analysis of the ultra-high energy cosmic ray (UHECR) energy spectrum measured by the Telescope Array (TA) and Pierre Auger Observatory, using extensive air shower detection via fluorescence and surface detectors. It reports a consistent suppression in the flux above ~5×10¹⁹ eV, supporting the GZK cutoff, and highlights systematic uncertainties in energy reconstruction and composition interpretation due to hadronic interaction models.

ABSTRACT

Ultra-High Energy Cosmic Rays (UHECRs) are charged particles of energies above $10^{18}$ eV that originate outside of the Galaxy. Because the flux of the UHECRs at Earth is very small, the only practical way of observing UHECRs is by measuring the extensive air showers (EAS) produced by UHECRs in the atmosphere. This is done by using air fluorescence detectors and giant arrays of particle detectors on the ground. The Pierre Auger Observatory (Auger) and Telescope Array (TA) are two large cosmic ray experiments which use such techniques and cover 3000 km$^2$ and 700 km$^2$ areas on the ground, respectively. In this paper, we present the UHECR spectrum reported by the TA, using an exposure of 6300 km$^2$ sr yr accumulated over 7 years of data taking, and the corresponding result of Auger, using 10 years of data with a total exposure exceeding 50000 km$^2$ sr yr. We review the astrophysical interpretation of the two measurements, and discuss their systematic uncertainties.

Motivation & Objective

• To compare the energy spectra of ultra-high energy cosmic rays (UHECRs) measured by the Telescope Array (TA) and Pierre Auger Observatory.
• To assess systematic uncertainties in energy reconstruction, particularly those arising from hadronic interaction models and detector calibration.
• To evaluate the astrophysical implications of the observed spectrum features, including the GZK cutoff and composition evolution at high energies.
• To enable future full-sky energy spectrum measurements by combining data from TA and Auger with improved exposure and sensitivity.

Proposed method

• Measurement of extensive air showers (EAS) using fluorescence detectors (FD) and surface detectors (SD) at TA and Auger.
• Energy reconstruction via simulation-based calibration of shower development, using Monte Carlo models for hadronic interactions.
• Comparison of energy scales between TA and Auger using overlapping energy ranges and common shower reconstruction techniques.
• Incorporation of auxiliary facilities, such as TALE and Infill detectors, to extend energy threshold and improve low-energy spectrum coverage.
• Use of Xmax (shower maximum) measurements to infer cosmic ray composition and test propagation models.
• Combination of FD and SD data in AugerPrime and TA×4 upgrades to enhance muon and composition sensitivity at the flux suppression region.

Experimental results

Research questions

• RQ1 What is the shape of the UHECR energy spectrum as measured by TA and Auger, and how do they compare across the full energy range?
• RQ2 To what extent do systematic uncertainties in energy scale and hadronic interaction models affect the interpretation of the GZK cutoff?
• RQ3 How does the inferred chemical composition of UHECRs influence the observed spectrum features, such as the ankle and suppression?
• RQ4 Can the observed flux suppression at ~5×10¹⁹ eV be explained by GZK physics, or does it require alternative models of acceleration or propagation?
• RQ5 How will future upgrades (TA×4 and AugerPrime) improve the precision and sky coverage of UHECR energy spectrum measurements?

Key findings

• The TA and Auger experiments both observe a flux suppression at energies above ~5×10¹⁹ eV, consistent with the GZK cutoff prediction.
• The energy spectrum measured by TA, with an exposure of 6300 km² sr yr over 7 years, shows a suppression consistent with Auger’s result from 10 years of data and 50,000 km² sr exposure.
• The two experiments report consistent Xmax values, indicating agreement on shower development, but infer different mass compositions due to differing hadronic interaction models and Monte Carlo procedures.
• Systematic uncertainties in the energy scale are estimated at ~10–20%, primarily driven by hadronic interaction model extrapolations beyond accelerator energies.
• The inclusion of a trend toward heavier nuclei at the highest energies suggests the spectrum break may arise from source-level rigidity-dependent acceleration cutoffs rather than purely GZK propagation effects.
• Future upgrades, including TA×4 and AugerPrime, will extend sky coverage to ~6000 km² and improve composition sensitivity in the flux suppression region, enabling full-sky energy spectrum mapping.