Development and validation of a model for the response of the Belle II vertex detector

Abstract

The future super flavour factory SuperKEKB with its detector system Belle II offers precision physics measurements to test the Standard Model or probe undiscovered phenomena. The physics goals of Belle II require a very precise detection of the decay point, or vertex, of B mesons from their low momentum decay products. A novel, two layer vertex detector composed of very thin, high resolution silicon pixel detectors based on Depleted Field Effect Transistors (DEPFET) is in production for Belle II. A realistic and experimentally validated simulation for DEPFET pixel detectors is a crucial tool to optimize the resolution of the vertex detector well before construction. In this thesis, a detailed detector simulation for the response of thin DEPFET pixel detectors to charged particles is presented. The detector simulation provides a description of the straggling of particles in silicon, the drift, diffusion and collection of the signal and the response of the read-out electronics. The model yields a precise prediction of the spatial resolution of the detector, given design parameters such as the pixel size, the sensor thickness and the electronics noise. The second part of this thesis is devoted to the results of measurements of the response of DEPFET detector prototypes to a beam of charged particles in test beam lines at CERN and DESY. New methods for the calibration, tracking and alignment of the EUDET/AIDA beam telescope are presented, that allow a quantitative determination of the signal distribution, hit efficiency and spatial resolution. Comparison of the results with the predictions of the response model yields good agreement. The validated model predicts a spatial resolution of ~10 micron for 50 micron thick DEPFET sensors for Belle II, satisfying the vertex detector requirements.