Lower limit on the ultra-high-energy proton-to-helium ratio from the measurements of the tail of $X_{\mathrm{max}}$ 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 $10^{18.0} \mbox{eV}$ to $10^{19.3} \mbox{eV}$ is estimated using the tail of the distribution of the depth of the shower maximum $X_{\mathrm{max}}$. Using the exponential decay scale $\Lambda$ measured by the Pierre Auger Observatory and the Telescope Array experiment we derive the 68\%\,CL constraints on the proton-to-helium ratio $\mathrm{p}/\mathrm{He} > 7.3 $ and $\mathrm{p}/\mathrm{He} > 0.43 $ for $10^{18.0} < E < 10^{18.5}$ eV and $10^{18.3} < E < 10^{19.3}$ eV correspondingly. It is shown that 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.