The MEG II Drift Chamber and the Search for Rare Muon Decays: Performance and Implications for Recent Experimental Results

The MEG~II experiment at the Paul Scherrer Institut searches for the charged Lepton Flavor Violating decay $\mu^+ \to e^+\gamma$, a golden channel to test the validity of the Standard Model and to explore possible signatures of New Physics. In this thesis, we report the results of the MEG~II analysis of the dataset collected during the 2021–2022 physics runs, achieving the highest sensitivity to date on the $\mu^+ \to e^+\gamma$ branching ratio, $\mathcal{S} = 2.2 \times 10^{-13}$. No evidence for the $\mu^+ \to e^+\gamma$ decay was observed, and a new upper limit on the branching ratio was established: \[ \text{BR}(\mu^+\to e^+\gamma) \leq 1.5 \times 10^{-13} \ (90\% \ \text{C. L.}) \] which is twice as stringent as the previous limit. A central contribution to this achievement was the calibration of the experiment’s tracking detector—a single-volume cylindrical drift chamber—using 2022 data, along with the optimization of its performance through updates to the positron reconstruction algorithms, leading to a 10\% improvement in resolution. Systematic uncertainties related to tracker alignment were also significantly reduced thanks to a novel tracking algorithm for cosmic ray events. This thesis details the candidate’s work on these developments and their impact on the final analysis.