The muon content of atmospheric air showers and the mass composition of cosmic rays

Cosmic rays are messengers from outer space holding the answer to the Universe's deepest mysteries: What are they exactly? Where do they come from? How do they accelerate to such energies, and how are the laws governing their interactions? At the highest energies, these unubiquitous particles can only be detected through the showers of secondary particles that they generate in their interactions with the Earth's atmosphere. Large ground-based observatories, like the Pierre Auger Observatory, detect these extensive air showers and attempt to reconstruct as much information of the primary cosmic ray as possible. In particular, the number of secondary muons is a key observable because it is directly related to the atomic mass number of the primary that generated them. Understanding the mass composition as a function of the energy would shed light on various open questions strongly linked to the origin of cosmic rays. The Pierre Auger Observatory has dedicated scintillators buried underground to directly detect muons, the Underground Muon Detector (UMD).
This thesis is devoted to the accurate determination of the muon content of air showers and to the study of its composition implications. ... mehrWe analyze direct muon measurements of air showers with energies between $10^{17.22}\,$eV and $10^{19.46}\,$eV from two experiments. At lower energies we extensively analyze UMD data, which we complement at higher energies with measurements from the Akeno Giant Air Shower Array (AGASA). For the UMD data, we develop new methods that significantly improve the estimation of the muon number. These methods also allow for the reconstruction of the muon signal as a function of time with an unprecedented time resolution, opening the door to the reconstruction of new composition-sensitive observables. As a direct application, we study the measured lateral distribution of muons and the models that attempt to describe it.
Furthermore, we analyze the mass composition implications of the UMD and AGASA data. The composition interpretation of the data can only be inferred by a comparison against air-shower simulations. We therefore simulate single-proton, single-iron, and mixed composition scenarios based on the three newest-generation high-energy hadronic interaction models. To better compare the results against those of other experiments, we compute the so called $z$-values, a scale of the muon content in data relative to that of proton and iron simulations. The combined results offer a picture consistent with other experiments: the unexpectedly heavy composition constitutes evidence of a muon deficit in air-shower simulations that increases with the energy. These results can help to improve the high-energy hadronic interaction models, which in turn would improve the precision of the inferred mass composition.