This master thesis describes the measurement of the π0 reconstruction efficiency correc- tion using τ-pair events from the Belle II experiment. The review part of the thesis begins with an overview of the Standard Model and the Belle II experiment, its detector system and the simulation software. This is followed by the description the τ-pair events and the method used for measuring the π0 reconstruction efficiency correction. The central part focuses on the event selection and the extraction of the signal yields. The results present the measured values of the average and momentum dependent π0 reconstruction efficiency correction, an improved π0 selection optimized for τ-lepton analyses, and a dedicated study on photon timing selection requirements. 1

This thesis is focused on the alignment studies of the vertex detector during first years of the Belle II detector operation and the first prompt sin 2ϕ1 analysis using the first rel- evant data collected by the detector. Firstly, the Belle II detector and the SuperKEKB accelerator is introduced. Secondly, the software framework and tools operation is ex- plained, then the alignment procedure and developed validation procedures are described in detail. Fourth section reports about the first years of the detector operation. Next three sections are related to different alignment studies during thee different periods of the vertex detector operation: the Phase 2, VXD Commissioning and beginning of the Phase 3 early. The last section covers the time-dependent CP Violation and mixing measurements performed using the data collected prior to the 11en of May 2020. 1

The Belle II detector is designed to collect data from the high luminosity electron-positron (e$^+$e$^-$) collisions of the SuperKEKB collider. It will explore the flavour sector of particle physics through precision measurements. The backgrounds of particles induced by the electron and positron beams will be much higher than in any previous \epem collider. It is important that these backgrounds be well understood in order to ensure appropriate measures are taken to protect the Belle II detector and minimize the impact of the backgrounds. In February 2016 electron and positron beams were circulated through the two 3 km vacuum pipe rings without being brought into collision during `Phase I' of SuperKEKB commissioning. Beam backgrounds were measured using Belle II's commissioning detector, BEAST II. BEAST II is composed of several small subdetectors, including helium-3 thermal neutron detectors. The BEAST II thermal neutron detector system and results from its Phase I running are presented in this dissertation. The Phase I experiment studies beam-gas interactions, where beam particles collide with residual gas atoms in the beampipes, and beam-beam interactions, where beam particles interact with each other. Simulations of these two types of backgrounds were performed using the Strategic Accelerator Design (SAD) and GEometry And Tracking (GEANT4) software packages. A method to account for the composition of the gas in the beampipes was developed in order to correctly analyse the beam-gas component of the background. It was also determined that the thermal neutron rates in the data on the positron beam were 2.18$^{+0.44}_{-0.42}$ times higher than the simulation of beam-gas interactions and 2.15$^{+0.34}_{-0.33}$ times higher for beam-beam interactions. The data on the electron beam were 1.32$^{+0.56}_{-0.36}$ times higher for beam-gas interactions and 1.91$^{+0.54}_{-0.48}$ time higher for beam-beam interactions. The impact of these studies on Belle II is discussed. / Graduate / samdejong86@gmail.com

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This project contributes to the research and development studies towards successful commissioning of the SuperKEKB electron-positron collider. This accelerator and storage rings complex aims at delivering the high-luminosity collisions of beams of electrons and positrons needed for the Belle II experiment. Such beams produce parasitic radiation—called “machine-induced backgrounds”, or simply “beam backgrounds” — that have detrimental effects on the experimental apparatus performance and durability. The Beast II effort is dedicated to measuring the beam backgrounds, and aims at testing the predictive power of the background models that were used in various phases of the Belle II design. A second objective is to ensure that the environment is safe for the detector prior to installing it around the beam lines. A major component of beam backgrounds consists of electromagnetic radiation. This study focusses on measuring this radiation at the location of the Belle II electromagnetic calorimeter. The measurements were achieved by placing scintillator crystals at positions representative of the Belle II calorimeter crystals that are the closest to the beam lines, and comparing the data with predictions for different operating parameters of the accelerator. Different phenomena related to machine backgrounds were observed: vacuum scrubbing, the electron-cloud effect, injection-related noise, beam-gas scattering and Touschek losses. Studies on the positron ring showed average background levels 13.5 +/- 3.5 times larger than simulation, whereas that ratio reached O(100-1000) for the electron ring. In the latter, the large uncertainty on the pressure measurements and the gas constituents limit the predictive power of the measurements. Radiation shields were also designed, fabricated, delivered and installed in the detector to protect the electromagnetic calorimeter from radiation coming from the beam lines. / Graduate

This master thesis describes development and analysis of simulation, reconstruction and alignment for tests of Belle II vertex detector sensors as well as alignment procedure for the vertex detector itself. The first goal is to develop and test software tools which would allow analysis of sensor tests within common Belle II software framework. The second goal is to prepare the alignment chain for the vertex detector, being essential component of the detector calibration. First part of the thesis describes the Belle II experiment and its vertex detector, as well as the experimental beam test of the sensors and the common software framework. The theory behind the used alignment procedure utilizing Millepede II for alignment and General Broken Lines for track fitting is explained. Second part of the thesis then summarizes software tools developed or used and the results reached with these tools with emphasis on the alignment.

This thesis is about the alignment of vertex semiconductor detectors of the Belle II experiment. It is divided into several sections. The first part introduces the Belle II experiment as a new generation B- factory, with its ambitious plans of data taking and physical analysis. The second part follows with a more detailed description of Belle II detector and software tools designed for their alignment and calibration. The central part of the thesis describes physical processes used for calibration of the vertex detector, connected with the search of an optimal solution for run-time monitoring and calibration of the detector. We also tested the effect of misalignment on the Belle II tracking software. The following part gives results of the study of misalignment effects on physical observables related to analysis of selected physical channels. The last part describes the development of a data quality monitoring tool for the tracking system. The tools has to provide a run-time diagnostic of misalignment and miscalibration by monitoring the precision and accuracy of reconstruction of physical observables. Powered by TCPDF (www.tcpdf.org)

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A fundamental assumption of our universe has been that equal amounts of matter and antimatter were created during the Big Bang. Since the world we live in is made entirely of matter, one either needs to question this assumption, or explain how matter was enriched with respect to antimatter. This is a puzzle that scientists all over the world are trying to solve. Particle accelerator experiments like Belle II in Tsukuba, Japan, enable precision studies of the heavier siblings of protons, for example charmed baryons. Since these baryons are unstable, they offer a unique tool to understand the matter-antimatter-asymmetry of the universe: by comparing the decay-patterns of the baryons and antibaryons, we look for tiny differences in the interactions that, if they exist, can have led to the matter-antimatter imbalance. The project aims to examine the capacity of the Belle II generator, which is a crucial part of analysis in experimental particle physics. The aim of this project is also to develop a toolkit for estimating decay-parameters from baryon decay distributions: one single-step process and one multi-step process, and compare statistical properties of the estimators in order to see which one yields a more precise estimation.

In preparation for the large backgrounds expected to be present in the Belle II detector from the SuperKEKB e+e- collider, the radiation hardness of several large (5 x 5 x 30 cm3) thallium doped cesium iodide (CsI(Tl)) scintillation crystals are studied. The crystal samples studied consist of 2 spare crystals from the Belle experiment using PIN diode readout and 7 spare crystals from the BaBar experiment using photomultiplier tube readout. The radiation hardness of the scintillation properties of the CsI(Tl) crystals was studied at accumulated 1 MeV photon doses of 2, 10 and 35 Gy. At each dose, the longitudinal uniformity of the crystals light yield, scintillation decay times, time resolution and energy resolution was measured. As the Belle II collaboration is considering an upgrade to pure CsI crystals if CsI(Tl) does not satisfy radiation hardness requirements, the scintillation properties of a pure CsI scintillation crystal were also measured and compared to the CsI(Tl) crystal measurements. In addition to experimental work, Monte Carlo simulations using GEANT4 were written to compare ideal pure CsI and CsI(Tl) crystals and to study the effects of radiation damage on the performance of the Belle II electromagnetic calorimeter. / Graduate