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

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.

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

“…Moreover, the conclusions depend on the details of the analyses as well as the hadronic shower model used, see e.g., Ref. [914]. Thus additional higher statistics measurements are needed to resolve this picture.…”

Ultra-high energy cosmic rays: origin and propagation

“…Figure 5.2 shows the energy spectrum predicted by the models B0nG-M1 and B0nG-M2. Despite the fact that the overall energy spectrum is well reproduced by both models, B0nG-M1 presents a dominant proton component of the arriving flux at the highest energies, which is in tension with previous results related to mass composition[91] pointing to a mixed composition of the flux at the end of the spectrum. For example, figure5.3 shows the results found in[81] regarding the X max distributions.…”