Enabling next-generation particle cascade simulations, Implementing the Propagation Software PROPOSAL as an Interaction Model for CORSIKA 8

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.