Evidence for Proton-Dominated Cosmic Ray Composition above 1.6 EeV

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  • 21 October 2009
Abstract
We report studies of ultra-high energy cosmic ray composition via analysis of depth of airshower maximum (Xmax), for airshower events collected by the High Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a predominantly protonic composition of cosmic rays at energies above 1.6 EeV, when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. We measure a mean Xmax at 10 EeV of 770.2 +- 2.1 (stat.) +- 4.2 (syst.) g/cm^2, and an elongation rate dXmax/d(log(E)) of 47.5 +- 5.9 (stat.) +- 2.8 (syst.) g/cm^2/decade. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum "ankle" at 4 EeV.

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