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

There are multiple techniques to determine the chemical composition of the ultrahigh-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 1018.0 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 Λ measured by the Pierre Auger Observatory and the Telescope Array experiment we derive the 68% CL constraints on the proton-to-helium ratio p/He>7.3 and p/He>0.43 for 1018.0