Mass Composition of UHECRs from Xmax Distributions Recorded by the Pierre Auger and Telescope Array Observatories

Nicusor Arsene

doi:10.3390/universe7090321

Abstract

In this paper we infer the mass composition of the ultra high energy cosmic rays (UHECRs) from measurements of Xmax 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 Xmax distributions, which we compare with the experimental Xmax 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 (≳70%) by protons and He nuclei in the energy range investigated lgE(eV) = [17.8–19.3] (Auger) and lgE(eV) = [18.2–19.0] (TA). The indirect comparisons between the Xmax 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 (lgE(eV)∼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.

Highlights

The ultra high energy cosmic rays (UHECRs) (E > 1018 eV) are the most energetic particles in the universe produced by the most energetic astrophysical objects
In this paper we infer the mass composition of the ultra high energy cosmic rays (UHECRs) from measurements of Xmax 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 EPOSLHC, QGSJETII-04 and Sibyll 2.1 hadronic interaction models
The reconstructed fractions obtained for both experiments clearly show that the mass composition of primary UHECRs is dominated by light elements (p and He), which present a modulation of their abundances as a function of energy, but keeping the sum roughly constant on the entire energy spectrum

Summary

Introduction

The UHECRs (E > 1018 eV) are the most energetic particles in the universe produced by the most energetic astrophysical objects. It was shown that using only the limited information given by the first two moments of a Xmax distribution may lead, in very particular cases, to a misinterpretation of the mass composition, since different mixes of primary particles can reproduce exactly the same Xmax and σXmax values To avoid such situations, a method was proposed, which uses the entire shape of each Xmax distribution by fitting them with Monte Carlo (MC) templates for four fixed primary species (p, He, N and Fe) obtaining in this way information about fractions of individual nuclei [27].

Monte Carlo Templates

Fitting Fractions of Auger and TA Xmax Distributions

Discussions and Conclusions