A first measurement of the muon content of air showers using hybrid measurements combining radio and particle detection is presented. For inclined air showers with zenith angles above 60°, the water-Cherenkov detector (WCD) of the Pierre Auger Observatory performs an almost pure measurement of the muonic component, whereas the Auger Engineering Radio Array (AERA) allows reconstructing the electromagnetic energy independently using the radio emission of the air shower.
The analysis of more than six years of AERA data shows a deficit of muons predicted by all current-generation hadronic interaction models for energies between 4 EeV and 20 EeV. This deficit, already observed in previous analyses of Auger, is now confirmed for the first time with radio data. This analysis is limited by low statistics of only 59 high-quality events due to the small area of AERA of 17 km² and the high energy threshold of 4 EeV originating from the WCD reconstruction.
With the AugerPrime Radio Detector currently being deployed, this analysis can be extended to the highest energies to allow for in-depth tests of hadronic interaction models with large statistics.