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  3. Precise Measurement Of The Cosmic Ray Helium Spectrum Above 0.1 Pev.
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  3. Precise Measurement Of The Cosmic Ray Helium Spectrum Above 0.1 Pev.

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Precise Measurement of the Cosmic Ray Helium Spectrum above 0.1 PeV.

Zhen Cao1,2,3, F Aharonian3,4,5,6, Y X Bai1,3

  • 1State Key Laboratory of Particle Astrophysics and Experimental Physics Division and Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, China.

Physical Review Letters
|April 11, 2026

View abstract on PubMed

Summary
This summary is machine-generated.

Cosmic ray helium spectra measured by LHAASO reveal a hardening and softening, indicating a helium "knee." Protons and helium nuclei swap dominance in the cosmic ray composition at specific energies, impacting our understanding of Galactic cosmic ray sources.

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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.