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 2·1019 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 ∼ 1.5 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 > 1019 eV is 0.52 −0.20 at 95% confidence level. Number of muons in extensive air showers (EAS) is used as an estimator of the primary composition of ultra-high-energy cosmic rays detected by surface arrays. Precision of muon-based composition studies is limited by their sensitivity to hadronic interaction models which incorporate extrapolation of experimental data to kinematic regions never tested in a laboratory experiment. On the other hand, the responses of the ground-based cosmic-ray detectors to different EAS components are different, and variations in the muon content may affect the relation between the signal recorded on the ground and the inferred energy