First Identification and Precise Spectral Measurement of the Proton Component in the Cosmic-Ray `Knee’

Kavli Affiliate: Zhuo Li

| First 5 Authors: The LHAASO Collaboration, Zhen Cao, , ,

| Summary:

We report the first high-purity identification of cosmic-ray (CR) protons and
a precise measurement of their energy spectrum from 0.15 to 12 PeV using the
Large High Altitude Air Shower Observatory (LHAASO). Abundant event statistics,
combined with the simultaneous detection of electrons/photons, muons, and
Cherenkov light in air showers, enable spectroscopic measurements with
statistical and systematic accuracy comparable to satellite data at lower
energies. The proton spectrum shows significant hardening relative to
low-energy extrapolations, culminating at 3 PeV, followed by sharp softening.
This distinct spectral structure – closely aligned with the knee in the
all-particle spectrum – points to the emergence of a new CR component at PeV
energies, likely linked to the dozens of PeVatrons recently discovered by
LHAASO, and offers crucial clues to the origin of Galactic cosmic rays.

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