First proof-of-principle measurement of muons in inclined air showers using radio and water Cherenkov detectors
Understanding the particle composition of ultra-high-energy cosmic rays is a major challenge in astroparticle physics. A key observable is the number of muons in extensive air showers, which provides insight into hadronic interactions at energies far beyond those accessible in terrestrial accelerators. In a new study, the Pierre Auger Collaboration demonstrates for the first time that radio measurements from the Auger Engineering Radio Array (AERA) can be used, together with the water-Cherenkov detector (WCD) array, to estimate the muon content in highly inclined air showers.
Showers initiated by ultra-high-energy cosmic rays develop as cascades of secondary particles in the atmosphere. For inclined air showers, most of the electromagnetic component is absorbed before reaching the ground, leaving mainly muons for detection with the WCD. In contrast, AERA provides an independent estimate of the energy in the electromagnetic cascade of the air shower using the radio emission of electrons and positions. In this work, we used 40 high-quality events in the zenith angle between 65° and 80° recorded in about 10 years of data. Figure 1 shows the correlation between the muon content estimator from the WCD and the energy estimator determined by AERA.
Figure 1: Measured muon content estimator, N19, as a function of the energy estimator, √ Srad. For each measured event, the reconstructed estimators and their uncertainties are shown by the gray data points. The black profile denotes the average for each energy bin, the y-uncertainty is given by the uncertainty of the mean.
For an interpretation of the measured muon content we obtained predictions of the muon content from full Monte-Carlo simulations using CORSIKA and three different hadronic interaction models. In Figure 2 we normalize the muon estimator by the energy estimator to compensate for the expected energy dependence based on theoretical models. We find that data are compatible with the prediction of hadronic interaction models for iron nuclei. However, previous analyses in the energy range of this work indicate a mean composition between proton and nitrogen. While the current data match the prediction for an iron primary, a lighter composition would also be compatible given the current systematic uncertainties. The presented result, based on the novel radio detection technique, is in broad agreement with previous Auger analyses using different detector combinations, in which a deficit of muons in simulations was reported.
Figure 2: Normalized muon content as a function of energy estimator. The predictions for different hadronic interaction models are denoted by the colored
lines for protons and iron primaries. Vertical error bars indicate the statistical uncertainty of each data point, while square brackets indicate the systematic
uncertainty of the measurement. The diagonal offsets represent the correlated effect of systematic shifts in the energy estimator.
This proof-of-principle study opens a new avenue for hybrid detection of cosmic-ray air showers, demonstrating that radio arrays can complement traditional surface detectors. Future studies using the recently deployed AugerPrime Radio Detector and the 750m grid of the WCD will allow high statistics measurements and extend the energy range to the highest energies testing hadronic interaction models in great detail.
Related Paper:
Measuring the muon content of inclined air showers using AERA and the water-Cherenkov detectors of the Pierre Auger Observatory
The Pierre Auger Collaboration, Phys. Rev. D 112 (2025) 123042
[arxiv.org/abs/2507.02558] and data at [doi: 10.5281/zenodo.15784484]
