Kavli Affiliate: Li Xin Li
| First 5 Authors: The LHAASO Collaboration, Zhen Cao, F. Aharonian, Q. An, A. Axikegu
| Summary:
We present the measurements of all-particle energy spectrum and mean
logarithmic mass of cosmic rays in the energy range of 0.3-30 PeV using data
collected from LHAASO-KM2A between September 2021 and December 2022, which is
based on a nearly composition-independent energy reconstruction method,
achieving unprecedented accuracy. Our analysis reveals the position of the knee
at $3.67 pm 0.05 pm 0.15$ PeV. Below the knee, the spectral index is found to
be -$2.7413 pm 0.0004 pm 0.0050$, while above the knee, it is -$3.128 pm
0.005 pm 0.027$, with the sharpness of the transition measured with a
statistical error of 2%. The mean logarithmic mass of cosmic rays is almost
heavier than helium in the whole measured energy range. It decreases from 1.7
at 0.3 PeV to 1.3 at 3 PeV, representing a 24% decline following a power law
with an index of -$0.1200 pm 0.0003 pm 0.0341$. This is equivalent to an
increase in abundance of light components. Above the knee, the mean logarithmic
mass exhibits a power law trend towards heavier components, which is reversal
to the behavior observed in the all-particle energy spectrum. Additionally, the
knee position and the change in power-law index are approximately the same.
These findings suggest that the knee observed in the all-particle spectrum
corresponds to the knee of the light component, rather than the medium-heavy
components.
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