Approach for composition measurement of cosmic rays using the muon-to-electron ratio observed by LHAASO-KM2A

Abstract

Composition measurement of cosmic rays (CRs) around the knee of the CR energy spectrum is crucial for studying the processes of particle acceleration and propagation of Galactic CRs. The Square Kilometer Array (KM2A) of Large High Altitude Air Shower Observatory (LHAASO) can provide precise measurement of the muonic and electromagnetic (em.) components in CR-induced extensive air showers, and hence a good chance to disentangle the CR composition. Here we propose an approach of decomposing CR compositions with the number ratio between muons and em. particles ($N_{\mu}$/$N_{\rm e}$) observed by LHAASO-KM2A: we reconstruct the energy spectra of individual CR compositions by fitting $N_{\mu}$/$N_{\rm e}$ distributions in each reconstructed energy bin using the template shapes of $N_{\mu}$/$N_{\rm e}$ distributions of individual CR compositions based on Monte Carlo (MC) simulation. We evaluate the performance of this approach with MC tests where mock data of LHAASO-KM2A are generated by MC simulation. We show that the input composition model can be well recovered in this approach, independent of the CR composition model adopted in the MC simulation for the template distributions. The uncertainties of the reconstructed spectra at < 20 PeV, mainly limited by simulation statistics, are $\le$ 7% for proton, He, and Fe groups, and $\le$ 8% and $\le$ 16% for CNO and MgAlSi groups, respectively.