Indications of proton-dominated cosmic-ray composition above 1.6 EeV

Area of Science:

• * Astrophysics
• * Particle Physics

Background:

• * Ultrahigh-energy cosmic rays (UHECRs) provide insights into extreme astrophysical processes.
• * Determining the composition of UHECRs is crucial for understanding their origin and propagation.
• * The energy spectrum of cosmic rays exhibits an 'ankle' feature, suggesting a transition in their sources.

Purpose of the Study:

• * To determine the composition of ultrahigh-energy cosmic rays.
• * To analyze the elongation rate of air showers detected by the High-Resolution Fly's Eye (HiRes) observatory.
• * To constrain theoretical models explaining the cosmic-ray energy spectrum and its features.

Main Methods:

• * Analysis of the depth of air shower maximum (X(max)) for air shower events.
• * Utilizing data collected by the High-Resolution Fly's Eye (HiRes) observatory.
• * Interpretation of results using QGSJET01 and QGSJET-II high-energy hadronic interaction models.

Main Results:

• * Consistent measurement of a constant elongation rate, d/d[log(E)], of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade.
• * Observed elongation rate is consistent with a predominantly protonic composition of cosmic rays.
• * Data constrain models proposing a galactic-to-extragalactic transition as the cause of the energy spectrum ankle.

Conclusions:

• * The composition of cosmic rays at ultrahigh energies appears to be primarily protons.
• * The measured elongation rate provides a key parameter for testing hadronic interaction models.
• * These findings help elucidate the transition of cosmic-ray sources from galactic to extragalactic origins.