Muons in air showers at the Pierre Auger Observatory: Mean number in highly inclined events

Understand

We present the first hybrid measurement of the average muon number in air showers at ultra-high energies, initiated by cosmic rays with zenith angles between $62^\circ$ and $80^\circ$.

• The measurement is based on 174 hybrid events recorded simultaneously with the Surface Detector array and the Fluorescence Detector of the Pierre Auger Observatory.
• The muon number for each shower is derived by scaling a simulated reference profile of the lateral muon density distribution at the ground until it fits the data.
• A $10^{19}$~eV shower with a zenith angle of $67^\circ$, which arrives at the Surface Detector array at an altitude of 1450 m above sea level, contains on average $(2.68 \pm 0.04 \pm 0.48\,\mathrm{(sys.)}) \times 10^{7}$ muons with energies larger than 0.3 GeV.