Precise Measurement of the Cosmic Ray Helium Spectrum above 0.1 PeV

Area of Science:

• Astrophysics
• Particle Physics
• Cosmic Ray Physics

Background:

• The composition of cosmic rays, particularly the relative abundance of protons and helium nuclei at high energies, is crucial for understanding their origins and acceleration mechanisms.
• Previous measurements have provided insights into cosmic ray spectra, but a detailed understanding of the helium spectrum and its interplay with the proton spectrum at PeV energies remains incomplete.

Purpose of the Study:

• To measure the cosmic ray helium energy spectrum in the 0.16–13 PeV range using data from the Large High Altitude Air Shower Observatory (LHAASO).
• To compare the measured helium spectrum with the proton spectrum to understand their relative contributions and spectral changes at PeV energies.
• To investigate the implications of these spectral features for Galactic cosmic ray sources.

Main Methods:

• The helium energy spectrum was derived by subtracting the proton spectrum from the combined light component (proton and helium) spectrum.
• Observations were conducted using the Large High Altitude Air Shower Observatory (LHAASO).
• A consistent energy scale was maintained for both proton and helium spectrum measurements.

Main Results:

• The cosmic ray helium spectrum exhibits a significant hardening around 1.1 PeV, followed by a softening around 7 PeV, indicative of a helium 'knee.'
• In the LHAASO energy range, the helium spectrum is softer than the proton spectrum, leading to protons becoming the dominant component at approximately 0.7 PeV.
• A second spectral crossing occurs around 5 PeV, where helium nuclei again become the dominant component over protons.

Conclusions:

• The observed features in the helium and proton spectra provide critical data for models of Galactic cosmic ray acceleration and propagation.
• The energy-dependent changes in the relative abundance of protons and helium nuclei suggest distinct or evolving source populations and/or propagation effects.
• These findings necessitate a re-evaluation of current models to accurately describe the origin and composition of high-energy cosmic rays.