ETD

• bartender
• cad
• fers
• frozen spin
• geant4
• muedm
• readout
• scintillating fibers
• sputtering
• tracker

This thesis focuses on the search for the muon electric dipole moment (EDM), a fundamental observable directly connected to time-reversal (T) violation and, assuming CPT invariance, also to CP violation. Within the Standard Model, the expected value of the muon EDM is extremely small and far below present experimental sensitivity. Therefore, any non-zero measurement would represent clear evidence of physics beyond the Standard Model. The work is carried out in the framework of the MuEDM experiment, which proposes an innovative approach based on the Frozen Spin technique and the use of an intense continuous muon beam.
The first part of the thesis introduces the theoretical framework of EDM searches, discussing the relation between electric dipole moments, CP violation, and the matter–antimatter asymmetry observed in the Universe. Particular attention is devoted to the Frozen Spin technique, in which the spin precession induced by the anomalous magnetic moment of the muon is almost completely canceled through a suitable combination of electric and magnetic fields. In this configuration, a possible EDM would generate a slow spin precession out of the orbital plane, observable through the time evolution of the decay positron asymmetry.
The thesis then presents the experimental requirements of the detector developed for the MuEDM experiment. The system must operate under vacuum and inside a strong magnetic field while reconstructing low-energy positrons with high spatial and timing precision. To satisfy these constraints, a lightweight and highly granular tracker based on scintillating fibers was designed.
A significant part of the work is devoted to the study of scintillating fibers and their optical properties. The mechanisms of scintillation light production, photon trapping through total internal reflection, and light transport inside optical fibers are discussed. To investigate these effects, a dedicated Geant4 simulation was developed to model the propagation of optical photons inside a single scintillating fiber. The simulation includes the optical properties of the polystyrene core and PMMA cladding, scintillation spectra, absorption lengths, and timing characteristics. The results demonstrate the crucial role of the cladding in ensuring stable light transport and provide a quantitative estimate of the photon collection efficiency.
The thesis also describes the design and construction of the Cylindrical Helix Tracker (CHeT), the tracking detector developed for the experiment. The detector consists of six concentric cylindrical layers of scintillating fibers arranged in a stereo geometry, enabling three-dimensional track reconstruction. The tracker geometry was optimized to achieve the required spatial, timing, and momentum resolutions while remaining compatible with the strict mechanical constraints of the experimental apparatus.
The readout system is based on Silicon Photomultipliers (SiPMs), selected for their high gain, fast timing response, compact dimensions, and insensitivity to magnetic fields. Dedicated electronic boards and modular structures were developed to interface the fiber bundles with the FERS acquisition system. Monte Carlo simulations combined with reconstruction algorithms based on the GenFit framework and supported by a dedicated helix pre-fit were used to evaluate the expected detector performance. The results demonstrate that the system can reconstruct positron trajectories with sufficient precision to satisfy the experimental requirements.
Another important contribution of the thesis concerns the optimization of scintillating fibers for operation inside the detector. During assembly, the fibers are embedded in Stycast resin, producing optical losses due to the reduced refractive index contrast between the cladding and the external medium. To mitigate this effect, aluminum sputtering was investigated as a method to deposit a thin reflective coating on the fibers. Dedicated simulations comparing naked and aluminized fibers show that the metallic coating significantly improves the light collection efficiency in the presence of the resin.
The thesis also documents the practical implementation of the sputtering procedure. Several engineering and mechanical challenges related to coating thousands of fibers uniformly were addressed by developing dedicated mounting and fixing strategies compatible with the geometrical limitations of the sputtering chamber.
The final part of the work is devoted to the characterization of the FERS acquisition system and the experimental validation of the detector response. Different acquisition modes were studied in terms of charge, timing, and counting performance. Experimental measurements using scintillating fibers coupled to SiPMs allowed the validation of the readout chain and the evaluation of detector stability under realistic operating conditions.
Finally, the thesis presents a tracker pre-commissioning campaign together with the integration of the front-end electronics into the Monte Carlo simulation framework. The objective is to develop a realistic digital model of the experiment capable of connecting detector physics, electronics response, and reconstruction algorithms within a unified simulation chain.