Muon content of ultra-high-energy air showers: Yakutsk data versus simulations

We analyse a sample of 33 extensive air showers (EAS) with estimated primary energies above $2\cdot10^{19}$ eV and high-quality muon data recorded by the Yakutsk EAS array. We compare, event-by-event, the observed muon density to that expected from CORSIKA simulations for primary protons and iron, using SIBYLL and EPOS hadronic interaction models. The study suggests the presence of two distinct hadronic components, “light” and “heavy”. Simulations with EPOS are in a good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SYBILL, simulated muon densities for iron primaries are a factor of $\sim1.5$ less than those observed for the heavy component, for the same electromagnetic signal. Assuming two-component proton-iron composition and the EPOS model, the fraction of protons with energies $E>10^{19}$ eV is $0.52 ^{+0.19}_{-0.20}$ at 95% confidence level.