Elucidating the Cosmic-Ray Knee

Marric Stephens

Synopsis

Elucidating the Cosmic-Ray Knee

March 26, 2026• Physics 19, s32

New observations of cosmic rays that distinguish between hydrogen and helium find unexpected complexity in a long-observed spectral feature.

Cosmic-ray energies range from less than 10⁹ eV to over 10²⁰ eV. As energy rises, the cosmic-ray flux drops, declining more steeply beyond a few peta-electron-volts (PeV or 10¹⁵ eV). The origin of this spectral feature, known as the knee, remains a mystery (see Synopsis: Characterizing the “Knee” of High-Energy Cosmic Rays). But now researchers at the Large High Altitude Air Shower Observatory (LHAASO) in China have uncovered a new clue by separating out the cosmic-ray contributions of hydrogen and helium from heavier nuclei [1].

Distinguishing between different cosmic-ray particles in the knee region has been an outstanding challenge for decades. Space-based detectors have excellent discriminatory power, but their modest collecting areas mean that they don’t accumulate statistically useful samples at high energies, where cosmic rays are scarcest. Ground-based detectors must contend with cosmic-ray interactions with the atmosphere, which make identification difficult. Such interactions are less problematic at LHAASO’s altitude of 4410 m. A yearlong survey with the facility’s charged-particle detectors, dedicated muon detectors, and Cherenkov telescopes gathered millions of events. By combining measurements, the researchers determined the hydrogen- and helium-specific spectra over the energy range 0.16–13 PeV.

Previous extrapolations from satellite-borne measurements led some astronomers to the view that the knee is a simple break at a certain energy and that above a higher energy, about 0.1 PeV, helium nuclei outnumber protons. But the LHAASO data yielded an unexpectedly complex picture, with the two species’ spectra showing knees at different energies and crossing over each other twice. LHAASO team member Liping Wang says this behavior suggests that cosmic rays in the knee region acquire their prodigious energies from multiple types of accelerators.

–Marric Stephens

Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.

References

Z. Cao et al. (LHAASO Collaboration), “Precise measurement of the cosmic ray helium spectrum above 0.1 PeV,” Phys. Rev. Lett. 136, 121001 (2026).