Abstract
The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between 2times 10^{17} and 2times 10^{18} eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at 10^{{17.5}}, {mathrm{eV}} and 10^{{18}}, {mathrm{eV}} . We find that, for the models to explain the data, an increase in the muon density of 38%pm 4% (12%)pm {}^{21%}_{18%} for EPOS-LHC, and of 50% (53%)pm 4% (13%)pm {}^{23%}_{20%} for QGSJetII-04, is respectively needed.
Highlights
More than a hundred years after the discovery of cosmic rays (CRs), their origin remains unknown
The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision
The studies presented in this work are the final results of the engineering array phase of the underground muon detectors (UMD)
Summary
More than a hundred years after the discovery of cosmic rays (CRs), their origin remains unknown Because of their low intensity, CRs above ∼ 1014 eV can only be studied through the detection of the showers of particles, known as extensive air showers (EASs), which they create in the atmosphere. The interpretation of Xmax and of the number of muons, and in general of every EAS observable sensitive to mass, relies on the comparison of their measured values with those predicted by EAS simulations. These resort to hadronic interaction properties at very high energies and in phase-space regions not well covered by accelerator experiments. Any additional insight in the evolution of composition-sensitive parameters as a function of energy provides new light on the hadronic processes at energies above those which are achievable with current particle accelerators
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