Master thesis defense by Molly Hjort Jensen

Title: Performance study of the dual-readout calorimeter in a multi-particle environment 

Abstract: The Future Circular Collider (FCC) study has proposed a new highest-luminosity high-energy electron-position (FCC-ee) collider. This would allow for unprecedented precision measurements running at among other the Z pole (91.2 GeV). For this collider, the IDEA detector concept including a dual-readout (DR) calorimeter was developed. This fibre-based sampling calorimeter measures scintillation and Cerenkov radiation simultaneously, such that the degree of non-compensation on an event-by-event basis can be determined. The application of Silicon Photomultipliers (SiPMs) enables individual fibre readout leading to high granular segmentation in the lateral direction. However, it posses no longitudinal segmentation. This calorimeter technique promises easy separation of electromagnetic and hadronic showers as well as great energy resolution for isolated particles. Conversely, this thesis attempts to evaluate whether it is possible to disentangle signals in a multi-particle environment in the absence of longitudinal separation of the IDEA-like DR calorimeter. The performance of which is quantified here by the success of decay mode classification, the energy resolution of individual final states, and the reconstructed pi0 mass. For this purpose, tau decay modes offer an interesting case study. The tau lepton exhibits a distinct signature and tau polarisation measurements allow for an direct determination of the weak mixing angle. Specifically, 25,000 tau leptons in the e+e -> Z -> tau+tau- process were generated and decayed using Pythia8 and Tauola++. Interaction between tau lepton final state particles and dual-readout calorimeter block was simulated in Geant4. Following data validation, a clustering algorithm was used to cluster the 2D calorimeter signals generated by the showers. Subsequently, machine learning was used for particle identification and energy regression. The reconstruction of resolved pi0 yielded a mass of m = 134.4 MeV with a resolution of 7.4%. Finally, the overall decay mode classification accuracy was 76.0 %, while for the hadronic channels only, it was 63.1 % (75.0 %) including (excluding) the class R.