Determination of the proton-to-helium ratio in cosmic rays at ultra-high energies from the tail of the $X_{\max }$ distribution

Abstract

We present a method to determine the proton-to-helium ratio in cosmic rays at ultra-high energies. It makes use of the exponential slope, $\Lambda$, of the tail of the $X_{\max }$ distribution measured by an air shower experiment. The method is quite robust with respect to uncertainties from modeling hadronic interactions and to systematic errors on $X_{\max }$ and energy, and to the possible presence of primary nuclei heavier than helium. Obtaining the proton-to-helium ratio with air shower experiments would be a remarkable achievement. To quantify the applicability of a particular mass-sensitive variable for mass composition analysis despite hadronic uncertainties we introduce as a metric the `analysis indicator' and find an improved performance of the $\Lambda$ method compared to other variables currently used in the literature. The fraction of events in the tail of the $X_{\max }$ distribution can provide additional information on the presence of nuclei heavier than helium in the primary beam.

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Cited by in corpus (4)

• Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers
• The spectra and composition of Ultra High Energy Cosmic Rays and the measurement of the proton-air cross section
• Lower limit on the ultra-high-energy proton-to-helium ratio from the measurements of the tail of $X_{\max }$ distribution
• On the efficiency of the evaluation of the primary cosmic ray composition using lateral distributions of air shower electromagnetic component