Thesis defense of Jean-Marco Alameddine

The observation of cosmic messenger particles provides unique insights into astrophysical processes. As high-energy nuclei or photons reach the Earth's atmosphere, a particle cascade called extensive air shower is initiated. By detecting secondary shower particles, the energy, direction, and identity of the initial messenger particle can be reconstructed. This task heavily relies on an accurate simulation of particle cascades. A major challenge in this context is the muon puzzle – a significant, as yet unresolved muon deficit in air shower simulations compared to experimental observations. To overcome limitations of existing shower simulation codes, the next-generation simulation framework CORSIKA 8 is currently under development. This work describes the implementation of the Monte Carlo simulation code PROPOSAL as the first electromagnetic and muonic interaction model in CORSIKA 8. PROPOSAL describes muon interactions with maximal precision, minimizing possible systematic uncertainties in the description of the muonic shower component in CORSIKA 8. The code structure of PROPOSAL is modularized, and a description of electromagnetic processes is implemented. An interface between CORSIKA 8 and PROPOSAL is written, which is validated by comparisons with previous CORSIKA versions. Notably, these validations reveal that CORSIKA 8 shows a 5% increase of muons in hadronic showers. As this number is insufficient to solve the muon puzzle, inaccuracies in muon propagation are ruled out as a possible cause. Furthermore, this work allows for the first physics-complete shower simulations with CORSIKA 8, which is a crucial step toward its first release. For the scientific community, CORSIKA 8 is going to be a powerful tool for the simulation and investigation of particle cascades, especially to further understand and solve the muon puzzle.

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