Constraints on the spread of nuclear masses in ultra-high-energy cosmic rays based on the Phase I hybrid data from the Pierre Auger Observatory

TL;DR

This paper uses hybrid data from the Pierre Auger Observatory to constrain the nuclear mass spread in ultra-high-energy cosmic rays and tests the consistency of hadronic interaction models across a broad energy range.

Contribution

It applies a correlation analysis to hybrid data with advanced models, extending previous constraints on cosmic-ray composition near the ankle.

Findings

Excluded pure and proton-helium compositions at 5σ near the ankle.

Tested and constrained post-LHC hadronic interaction models.

Extended analysis over a wider energy range.

Abstract

We present an analysis of the correlation between the depth of the maximum of air-shower profiles and the signal in water-Cherenkov stations in events registered simultaneously by the fluorescence and surface detectors of the Pierre Auger Observatory. The analysis enables us to place constraints on the spread of nuclear masses in ultra-high-energy cosmic rays with a minor impact from the experimental systematic uncertainties and uncertainties in air-shower simulations. Due to this unique feature, the correlation analysis has previously allowed us to exclude all pure and proton-helium compositions near the ankle in the cosmic-ray energy spectrum at 5$\sigma$ confidence level. The same property makes the correlation analysis an effective tool for testing the consistency of predictions of the post-LHC hadronic interaction models, including their latest versions such as EPOS LHC-R, QGSJet-III-01, Sibyll${}^\bigstar$ and Sibyll 2.3e. In this work, the correlation analysis using the Phase I hybrid data from the Pierre Auger Observatory is presented. The analysis uses the newest generation of hadronic interaction models and covers an extended energy range around the ankle in the cosmic-ray spectrum.