Indications of Proton-Dominated Cosmic Ray Composition above 1.6 EeV

TL;DR

This study analyzes ultra-high energy cosmic rays using airshower data from HiRes, indicating a predominantly protonic composition above 1.6 EeV and constraining models of cosmic ray origin and spectrum features.

Contribution

It provides the first detailed composition analysis above 1.6 EeV using Xmax measurements, supporting a proton-dominated composition and challenging certain galactic-extragalactic transition models.

Findings

Consistent elongation rate of 47.9 g/cm^2/decade between 1.6 and 63 EeV.

Data favor a protonic composition when interpreted with QGSJET models.

Results constrain models explaining the cosmic ray spectrum ankle.

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 constant elongation rate d<Xmax>/d(log(E)) of 47.9 +- 6.0 (stat.) +- 3.2 (syst.)g/cm^2/decade for energies between 1.6 EeV 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 EeV.