High intensity beam lines simulation and optimization for MEG-II and Mu3e experiments at PSI

This thesis work focuses on the high intensity beam line (named PiE5) which provides 28 MeV/c momentum surface muons to the MEG-II and the Mu3e experiments hosted at Paul Scherrer Institute. They both search for charged Lepton Flavour Violation decay channels, respectively μ+ → e+ γ and μ+ → e+ e− e+, aiming at discovering new physics beyond the Standard Model (SM) by revealing deviations from the SM predictions. For the Mu3e experiment an innovative Compact Muon Beam Line (CMBL) has been realized to match severe spatial constraints in the experimental hall and to guarantee a high particle transmission rate. During the 2023 Mu3e beam line commissioning, we optimized the muons propagation throughout the entire line and released the final settings for the Mu3e phase I. MEG-II has also an innovative and very complex line, ending up with a special beam transport solenoid to match the gradient field of the last magnet where the MEG-II detector apparatus is built around and inside. As part of my work, we performed all the beam tuning for the MEG-II physics run 2024. Finally, as a conclusive part of my thesis work, I set up the simulations of the full beam lines serving MEG-II and Mu3e taking into account, for each present magnetic element, its contributions to the magnetic field multipole expansion up to the dodecapole order. To do so, I made use of a recently-released beam lines simulation software, consisting of a collection of Python packages and named Xsuite: among the many functionalities it provides, it gives users the possibility to carry on the tuning of beam lines’ magnets even during beam lines operational phases (thanks to its very fast computing algorithms), as well as to simulate the radiation-matter interactions that occur when particles encounter components such as moderators and collimators.