Lower limit on the ultra-high-energy proton-to-helium ratio from the measurements of the tail of Xmax distribution

Abstract

There are multiple techniques to determine the chemical composition of the ultra-high-energy cosmic rays. While most of the methods are primarily sensitive to the average atomic mass, it is challenging to discriminate between the two lightest elements: proton and helium. In this paper, the proton-to-helium ratio in the energy range from 1018.3 eV to 1019.3 eV is estimated using the tail of the distribution of the depth of the shower maximum Xmax. Using the exponential decay scale measured by the Telescope Array experiment we derive the constraint on the proton-to-helium ratio p/He > 0.43 (68% CL). The result is conservative with respect to the admixture of heavier elements. We evaluate the impact of the hadronic interaction model uncertainty. The implications for the astrophysical models of the origin of cosmic rays and the safety of the future colliders are discussed.