The air shower simulation framework CORSIKA 8. Development and first applications to muon production

Tools to accurately simulate extensive air showers are a key asset for the understanding of ultra-high energy cosmic rays. In this thesis, the Monte Carlo air shower simulation framework CORSIKA 8 is presented. CORSIKA 8 constitutes a next-generation code that aims to combine new functionality with a high level of flexibility and modularity. Notable aspects include the ability to freely combine an arbitrary number of physical processes and to setup simulation environments consisting of several media, including custom atmospheric models. A special feature is the possibility to inspect the complete lineage of particles, which allows linking particles on ground with any of their preceding generations. After describing the foundations of Monte Carlo shower simulations, I explain the architecture of CORSIKA 8 in depth. Focusing on the hadronic and muonic shower components, results obtained with CORSIKA 8 and other simulation codes are compared with each other. Even when using the same hadronic interaction models, a number of differences are observed, in particular regarding low-energy interactions, which have a considerable impact on the lateral distribution of muons at kilometre-scale distances up to a factor of two and more. Making use of the lineage technique, I study the phase space of hadronic interactions in order to quantify the importance for muon production and compare the results with the Heitler-Matthews toy model. At high energies ($\sqrt{s}$ ≳ 500 GeV) particle production in the forward region is confirmed to be especially important, while the central region becomes relevant at low energies ($\sqrt{s}$ ≲ 50 GeV) in particular for muons at large distances. Additionally, I study the impact of modified hadronic interactions on air shower observables. Modified hadron-air cross-sections mainly affect the longitudinal development, causing a larger shift of the maximum muon production depth than of the shower maximum. Artificially increased ρ${}^0$ production, on the other hand, can greatly increase the number of muons with only small impact on other observables. Finally, I also consider the possibility of large multiplicity boson production in the first interaction and study its phenomenology in air showers with a simple toy model. Within the scope of this thesis, I developed the foundations of the CORSIKA 8 framework. Based on the studies that have become possible with CORSIKA 8, I point out some new opportunities towards an improved understanding of muons in air showers.