Nuclei composition discrimination study based on Cherenkov image of air shower

PurposeIn 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.
MethodsProton 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.
ResultsComparing to the shower maximum of Cherenkov photons ( X _Cer ^max), it was found that below 1 PeV, θ _x ^maxhas better composition discrimination ability compared to X _Cer ^max, while they have similar composition discrimination ability above PeV.
ConclusionsFor nuclei composition identification, θ _x ^maxperformed better than X _Cer ^max, especially in low energies (below the PeV range). This is due to that θ _x ^maxis suffering less statistical fluctuations compared to X _Cer ^max. Meanwhile, θ _x ^maxis less affected by the statistics of Cherenkov photons compared to X _Cer ^max.