Probing cosmic ray composition with inclined air showers of LHAASO-KM2A

Abstract

The origin of cosmic ray (CR) spectral knee around ∼4 PeV is unknown. Composition studies around the knee can provide information on the CR sources and propagation. LHAASO-KM2A is capable of simultaneously measuring the EM and muonic components in extensive air showers. The triggered events of LHAASO-KM2A are dominated by CRs of higher energies at larger zenith angles because of the increasing atmospheric attenuation. In this work, we use the KM2A observed spectrum of muon-to-electron number ratio to constrain CR composition around the knee. The muon-to-electron number ratio spectra are compared between Monte Carlo (MC) simulation and experiment data over a wide zenith angle range. We report a MC excess at large zenith angles, which sets an upper limit on CR Fe flux at PeVs. Systematic uncertainties of interaction models and atmospheric models are briefly discussed.