Revisiting the Cosmic Ray knee: new insights from LHAASO

Recently, LHAASO has performed precise measurements of the all-particle spectrum, the mean logarithmic mass and the proton spectrum of cosmic rays at energies around the knee with more elemental spectra expected in the near future. These datasets for the first time bridge direct and indirect measurements and provide an unprecedented opportunity to test models of CR acceleration and propagation in the Galaxy. In this work, we develop a phenomenological model of CR spectrum and composition, in which CRs experience rigidity-dependent effects during their propagation in the Galaxy. We conclusively find that a single population of sources with propagation effects can't plausibly explain both elemental and all-particle spectra around the knee. In contrast, a two-population model that includes an additional light contribution at the knee provides a consistent description of both. We find that model requires a relatively high but plausible energy scale shift of $\sim15\%$ for LHAASO.