Measurements of the variance of the logarithmic mass of cosmic rays from 0.3 to 30 PeV with LHAASO-KM2A

The Large High Altitude Air Shower Observatory (LHAASO) has accomplished comprehensive and precise measurements of very-high-energy cosmic ray air showers. For the first time, the logarithmic shower muon number has been employed to derive the mean logarithmic mass (ln A) of cosmic rays, while fluctuations in the logarithmic shower muon number have been utilized to construct the variance of the logarithmic mass (σln A²). The analysis provides a clear depiction of the cosmic ray mass composition evolution with energy and explicitly illustrates the energy-dependent trend of σln A². The reliability of this parameterization method has been validated by employing simulations based on three different hadronic interaction models (QGSJET-II-04, EPOS-LHC, and SIBYLL 2.3d) with various cosmic ray compositions. The results demonstrate excellent consistency between the simulated input values of ln A and σln A² and those derived from the parameterization, thus confirming the robustness and accuracy of this approach.