Nuclei composition discrimination study based on Cherenkov image of air shower

Purpose In ground-based cosmic ray experiments, the Cherenkov image detected by imaging air Cherenkov telescopes contains crucial information about the longitudinal development of Extensive Air Showers, which can be used for composition discrimination between different nuclei.
Methods Proton and iron showers were simulated to study the composition discrimination in the energy range from 100 TeV to ~ 10 PeV with a zenith angle of 45°. A new variable, namely θ_x^max, was introduced from the longitudinal development of Cherenkov photons, which represents the position at which the angular distribution of Cherenkov photons detected by telescope reached maximum.
Results Comparing to the shower maximum of Cherenkov photons (X_Cer^max), it was found that below 1 PeV, θ_x^max has better composition discrimination ability compared to X_Cer^max, while they have similar composition discrimination ability above PeV.
Conclusions For nuclei composition identification, θ_x^max performed better than X_Cer^max, especially in low energies (below the PeV range). This is due to that θ_x^max is suffering less statistical fluctuations compared to X_Cer^max. Meanwhile, θ_x^max is less affected by the statistics of Cherenkov photons compared to X_Cer^max.