Characterization of final DEPFET modules for the new Pixel Vertex Detector and commissioning at the Belle II experiment

Sumitted to PubDB: 2025-01-29

Category: Phd Thesis

Tags: TRG

Principal Authors Christian Kiesling, Philipp Leitl
Date 2024-10-04
Belle II Number BELLE2-PTHESIS-2025-002
Abstract The Belle II particle physics experiment is located at the SuperKEKB e+e− collider in Japan. Its research focuses on the CP violation in B meson decays and addresses many open questions in electroweak interactions. The broad physics program also includes the search for new physics beyond the Standard Model of particle physics. An essential component of the Belle II detector for the physics mentioned above, most importantly for analyses of CP violation, is the Pixel Vertex Detector (PXD). The PXD is the innermost tracking device, surrounding the beam pipe, and provides a precise measurement of the decay vertices of B mesons and other weakly decaying particles. The PXD consists of two layers of silicon semiconductor modules with new Depleted P-channel Field Effect Transistor (DEPFET) pixels, used for the first time in a high energy physics experiment. This technology allows for an exceptionally thin active detector design of only 75 µm, achieving 15 µm impact parameter resolution and greater than 99 % detection efficiency. The PXD is an electronic device and must function properly as a subdetector of the larger Belle II electronic detector system. The first part of this thesis was the conduct of a test campaign to verify the electromagnetic compatibility (EMC) of the PXD module design. These were the first EMC measurements for a high energy physics pixel detector and also the first consideration of radiated noise from the beam pipe. After the design and prototyping phase, the module production for the final detector was started. The second part of this thesis was the development of the entire chain of test procedures and a comprehensive quality management including quality assurance software for the characterization and optimization of all production modules. The performance of 75 PXD modules was evaluated, of which 72 % achieved the highest grading after optimization. The third part of this thesis covers the assembly and commissioning of a first reduced version of the PXD at the Belle II experiment in Japan. Optimization of the module performance and operating software continued during operation. Following the occurrence of severe irradiation damage due to beam loss events, an automated detection algorithm was developed. Possible damage mechanisms were investigated and additional protection measures were implemented. The installed PXD successfully operated in the first physics data taking period of the Belle II experiment from 2019 to 2022. It reliably collected data of particular value for lifetime measurements and time-dependent CP violation analyses. During the shutdown period in 2022/23 the first PXD was replaced by PXD2, a complete successor, which will be the innermost tracking detector of Belle II for many years to come.

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