Precise Measurement of the Cosmic Ray Helium Spectrum above 0.1 PeV

TL;DR

This study measures the cosmic ray helium spectrum above 0.1 PeV, revealing spectral features like a hardening at 1.1 PeV and a softening at 7 PeV, with implications for cosmic ray sources.

Contribution

It provides the first detailed measurement of the helium spectrum above 0.1 PeV, showing spectral hardening and softening, and compares it with the proton spectrum to understand cosmic ray composition.

Findings

Helium spectrum hardens at ~1.1 PeV and softens at ~7 PeV.

Proton overtakes helium as the dominant component below 0.7 PeV.

Helium overtakes protons again above 5 PeV, indicating multiple spectral crossings.

Abstract

We report a measurement of the cosmic ray helium energy spectrum in the energy interval 0.16 -- 13~PeV, derived by subtracting the proton spectrum from the light component~(proton and helium) spectrum obtained with observations made by the Large High Altitude Air Shower Observatory~(LHAASO) under a consistent energy scale. The helium spectrum shows a significant hardening centered at $E \simeq$ 1.1~PeV, followed by a softening at $\sim$ 7 PeV, indicating the appearance of a helium `knee'. Comparing the proton and helium spectra in the LHAASO energy range reveals some remarkable facts. In the lower part of this range, in contrast to the behavior at lower energies, the helium spectrum is significantly softer than the proton spectrum. This results in protons overtaking helium nuclei and becoming the largest cosmic ray component at $E \simeq$ 0.7 PeV. A second crossing of the two spectra is observed at $E \simeq$ 5 PeV, above the proton knee, when helium nuclei overtake protons to become the largest cosmic ray component again. These results have important implications for our understanding of the Galactic cosmic ray sources.