Indications of Proton-Dominated Cosmic-Ray Composition above 1.6 EeV

We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum ($X_{\max }$), for air shower events collected by the High-Resolution Fly’s Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate $d⟨X_{\max }⟩/d[\log (E)]$ of $47.9\pm 6.0(\mathrm{stat})\pm 3.2(\mathrm{syst})\mathrm{g}/{\mathrm{cm}}^2/\mathrm{\text{decade}}$ for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at $4\times {10}^{18}\mathrm{eV}$.