A report by the WHISP working group on the combined analysis of muon data at cosmic-ray energies above 1 PeV

Different measurements of the muon content in extensive air showers at energies between $1$ PeV and a few EeV have revealed discrepancies with the predictions of high-energy hadronic interaction models. Measurements cover several zenith angles, distances from the core, and muon energy thresholds from some hundreds of MeV to approximately tens of GeV. One of the most puzzling anomalies is the muon excess in air showers, which is observed particularly at very-high primary energies in some experiments. An updated combined analysis of several air-shower experiments has been carried out to investigate the presence of this discrepancy in the shower data. For this purpose, the energy scales of the experiments were cross-calibrated using a reference cosmic-ray energy spectrum. Comparisons with the predictions of several post-LHC hadronic interaction models are performed. Below $100$ PeV, we observed that the data is found between the predictions of the hadronic interaction models for protons and iron nuclei. At higher energies, some experiments shows an overall increment of the shower muon content with respect to the model predictions, which grows with the primary energy. Other experiments, however, seems to be in agreement with the simulations, while one experiment, seems to show a deficit in the measured data at ultra-high energies.

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