Measurements of All-Particle Energy Spectrum and Mean Logarithmic Mass of Cosmic Rays from 0.3 to 30 PeV with LHAASO-KM2A

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 $3.67\pm 0.05\pm 0.15\mathrm{PeV}$. 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.

Figure 1

The LHAASO-KM2A all-particle energy spectrum flux multiplied by $E^{2.75}$ as a function of energy. The gray shadow band represents the estimated systematic uncertainties. The results of the HAWC [38], CASA-MIA [15], Tibet-III [6], KASCADE [11], IceCube/IceTop [9, 10] are also plotted for comparison. The error bars represent the statistical uncertainties.

Figure 2

The $⟨\ln (A)⟩$ (red dots) from experimental data as a function of cosmic rays energy. The error bars show the statistical uncertainties. The gray shadow band represents the combined systematic uncertainties. The results of CASA-MIA [14] and Tunka-133 [44] are also plotted for comparison. The shadow bands represent the systematic uncertainty for KASCADE [11, 45] and IceCube/IceTop [9] experiment, respectively. The curves are the $⟨\ln (A)⟩$ which are calculated according to the cosmic rays composition models.

Figure 3

Upper: The all-particle energy spectrum flux multiplied by $E^{2.75}$ as a function of energy. Bottom: The $⟨\ln (A)⟩$ of cosmic rays as a function of energy. The error bars show the statistical uncertainties and the gray shadow band represents the estimated combined systematic uncertainties. The blue solid curves indicate the fit of Eq. (4) to the data.