First measurement of the fluctuations in the muon content of air showers at ultra-high energy

First measurement of the fluctuations in the number of muons in air showers at ultra-high energies.

In a recent publication, the Pierre Auger Collaboration presented the first measurement of the fluctuations in the number of muons in air showers at ultra-high energies. This observation provides new insights into the origin of the deficit of muons in air shower simulations that was found in the previous measurement of the average muon content of ultra-high energy air showers.

2021 05 muon fluctutations1

Figure 1: Correlation between the muon content, measured with the surface detector array, and the energy, measured with the fluorescence detectors.

The new measurement was carried out using so-called golden hybrid events that were collected in the 14 years since 2004. Golden hybrid events are air shower events that are simultaneously detected by the fluorescence detector and the surface detector of the Pierre Auger Observatory and for which, in addition, the quality of the detection in each sub-detector is good enough to successfully reconstruct the energy of the shower.

By selecting the subsample of events with zenith angle larger than 60 degrees (inclined air showers), the measurement with the surface detector can be interpreted as a measurement of the muons in the air showers. Electrons and photons, which usually make up a significant fraction of the signals at the surface, in the case of inclined showers are absorbed in the atmosphere. The number of muons that arrive at the ground for a given air shower is closesly related to the primary energy, i.e. N_mu = a E^b, but using the second, independent measurement of the energy from the fluorescence detector, muon content and primary energy can be separated (see figure 1).

2021 05 muon fluctutations2

Figure 2: Relative fluctuations in the number of muons.

The fluctuations in the number of muons esentially are obtained by subtracting the detector resolution of the FD and SD from the fluctuations around the average number of muons (solid line in figure 1). The resulting relative fluctuations as a function of the energy are shown in figure 2. Note that the measurement falls within the range that is expected from current hadronic interaction models for pure proton and pure iron primaries (red and blue lines in figure 2). Taking the observed mass composition into account (grey band in figure 2), it is found that the observed fluctuations match the expectation. This is in contrast to what is found in the average number of muons. Here the predictions taking into account composition are much below the observed number of muons. For primary energies of 10^19 eV this situation is summarized in figure 3, which directly compares relative fluctuations and the average.

2021 05 muon fluctutations3

Figure 3: Relative fluctuations vs. average number of muons at 10^19 eV.

The finding that fluctuations are consistent with model predictions, while the average is not, means that the necessary increase in the average number of muons in the simulations may be due to a small effect accumulating over many generations or a very particular modification of the first interaction that changes N_mu without changing the fluctuations.


Related paper:

Measurement of the fluctuations in the number of muons in extensive air showers with the Pierre Auger Observatory
The Pierre Auger Collaboration, Phys. Rev. Lett. 126, 152002 (2021)
[] [doi: 10.1103/PhysRevLett.126.152002]