Speaker
Description
The knee-like structure around $4~\rm PeV$ is the most striking feature in the cosmic ray energy spectrum, whose origin remains enigmatic. Recently, the Large High Altitude Air Shower Observatory (LHAASO) provided highly precise measurements of cosmic rays in the knee region. Taking into account LHAASO measurements of the all-particle energy spectrum and the mean logarithmic mass, we propose a novel concept of the total logarithmic mass energy spectrum to characterize the knee. The total logarithmic mass energy spectrum is a weighted energy spectrum where flux contributions of different elements are scaled by their respective logarithmic masses, with heavier nuclei assigned greater weights. By virtue of the total logarithmic mass energy spectrum, we find that proton exerts a predominant role in the formation of the knee. Meanwhile, the energy spectrum of light elements is obtained through mixing the all-particle energy spectrum flux and the total logarithmic mass energy spectrum flux according to the specific weight factors. Based on the acquired energy spectrum of light elements, the case of a mass-dependent knee is ruled out with a significance well above 5$\sigma$ and the rigidity-dependent knee feature is revealed. Moreover, an ankle-like structure stemming from the excess of iron is discovered at $9.7 \pm 0.2_{\rm stat} \pm 0.3_{\rm syst}~\rm PeV$ in the total logarithmic mass energy spectrum.
