Mass Composition of UHECRs from $X_{\rm max}$ Distributions Recorded by the Pierre Auger and Telescope Array Observatories

TL;DR

This study analyzes the mass composition of ultra high energy cosmic rays using $X_{max}$ distribution data from Pierre Auger and Telescope Array, revealing a dominance of protons and helium nuclei and good dataset compatibility.

Contribution

It introduces a method to compare $X_{max}$ distributions from two major observatories, enhancing understanding of UHECR composition and data consistency.

Findings

UHECRs are mainly protons and helium nuclei in the studied energy range.

Good compatibility between Auger and TA datasets, especially above the ankle.

Further data analysis needed at lower energies for better harmonization.

Abstract

In this paper we infer the mass composition of the ultra high energy cosmic rays (UHECRs) from measurements of $X_{\rm max}$ distributions recorded at the Pierre Auger (2014) and Telescope Array (TA) (2016) Observatories, by fitting them with all possible combinations of Monte Carlo (MC) templates from a large set of primary species (p, He, C, N, O, Ne, Si and Fe), as predicted by EPOS-LHC, QGSJETII-04 and Sibyll 2.1 hadronic interaction models. We use the individual fractions of nuclei reconstructed from one experiment in each energy interval to build equivalent MC $X_{\rm max}$ distributions, which we compare with the experimental $X_{\rm max}$ distributions of the other experiment, applying different statistical tests of compatibility. The results obtained from both experiments confirm that the mass composition of the UHECRs is dominated ($\gtrsim$$70\%$) by protons and He nuclei {in the energy range investigated $\lg E (\rm eV)$ = [17.8--19.3] (Auger) and $\lg E \rm (eV)$ = [18.2--19.0] (TA).} The indirect comparisons between the $X_{\rm max}$ distributions recorded by the two experiments show that the degree of compatibility of the two datasets is good, even excellent in some high energy intervals, especially above the ankle ($\lg E (\rm eV) \sim 18.7$). However, our study reveals that, at low energies, further effort in data analysis is required in order to harmonize the results of the two experiments.