Nuclei composition discrimination study based on Cherenkov image of air shower

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.

Proton and iron showers were simulated to study the composition discrimination in the energy range from \(\sim 100\,\text {TeV}\) $\sim 100\text{TeV}$ to \(\sim 10\,\text {PeV}\) $\sim 10\text{PeV}$ with a zenith angle of \(45^{\circ }\) ${45}^{\circ }$ . A new variable, namely \(\theta _{x}^\text {max}\) ${\theta }_x^{\text{max}}$ , 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.

Comparing to the shower maximum of Cherenkov photons ( \(X_\text {Cer}^\text {max}\) $X_{\text{Cer}}^{\text{max}}$ ), it was found that below 1 PeV, \(\theta _{x}^\text {max}\) ${\theta }_x^{\text{max}}$ has better composition discrimination ability compared to \(X_\text {Cer}^\text {max}\) $X_{\text{Cer}}^{\text{max}}$ , while they have similar composition discrimination ability above PeV.

For nuclei composition identification, \(\theta _{x}^\text {max}\) ${\theta }_x^{\text{max}}$ performed better than \(X_\text {Cer}^\text {max}\) $X_{\text{Cer}}^{\text{max}}$ , especially in low energies (below the PeV range). This is due to that \(\theta _{x}^\text {max}\) ${\theta }_x^{\text{max}}$ is suffering less statistical fluctuations compared to \(X_\text {Cer}^\text {max}\) $X_{\text{Cer}}^{\text{max}}$ . Meanwhile, \(\theta _{x}^\text {max}\) ${\theta }_x^{\text{max}}$ is less affected by the statistics of Cherenkov photons compared to \(X_\text {Cer}^\text {max}\) $X_{\text{Cer}}^{\text{max}}$ .