Mass composition of cosmic rays determined by muon fraction with $\varepsilon_{thr.} \geq$ 1 GeV in air showers with energy greater than 5 EeV

TL;DR

This study analyzes muon-to-charged particle ratios in ultra-high-energy air showers to determine cosmic ray composition, finding it mainly consists of protons and helium nuclei up to 10 EeV, with some gamma-ray primaries.

Contribution

It provides new insights into cosmic ray mass composition at energies above 5 EeV using muon fraction analysis and compares data with QGSJETII-04 model predictions.

Findings

Cosmic rays up to 10 EeV are mainly protons and helium nuclei.

Presence of air showers with very low muon content suggests gamma-ray primaries.

Data supports a predominantly light composition at ultra-high energies.

Abstract

The ratio of the number of muons with a threshold of 1 GeV and charged particles at a distance of 600 m from the axis is analyzed. Air showers with energies above 5 EeV and zenith angles with less than 60 degrees are considered. Comparison of experimental data with calculations by the QGSJETII-04 model for various primary nuclei, including a gamma ray, showed that the mass composition of cosmic rays up to energies of $\sim$ 10 EeV mainly consists of protons and helium nuclei, with a small number of heavy nuclei. Data include air showers with a very low muon content, which are assumed to be produced by primary gamma rays.