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

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 approx. 10 micrometer for 50 mu thick DEPFET sensors for Belle II, satisfying the vertex detector requirements

Summary and Conclusions of the First DESY Test Beam User Workshop

On October 5/6, 2017, DESY hosted the first DESY Test Beam User Workshop [1] which took place in Hamburg. Fifty participants from different user communities, ranging from LHC (ALICE, ATLAS, CMS, LHCb) to FAIR (CBM, PANDA), DUNE, Belle-II, future linear colliders (ILC, CLIC) and generic detector R&D presented their experiences with the DESY II Test Beam Facility, their concrete plans for the upcoming years and a first estimate of their needs for beam time in the long-term future beyond 2025. A special focus was also on additional improvements to the facility beyond its current capabilities

Status of the BELLE II Pixel Detector

The Belle II experiment at the super KEK B-factory (SuperKEKB) in Tsukuba, Japan, has been collecting $e^+e^−$ collision data since March 2019. Operating at a record-breaking luminosity of up to $4.7×10^{34} cm^{−2}s^{−1}$, data corresponding to $424 fb^{−1}$ has since been recorded. The Belle II VerteX Detector (VXD) is central to the Belle II detector and its physics program and plays a crucial role in reconstructing precise primary and decay vertices. It consists of the outer 4-layer Silicon Vertex Detector (SVD) using double sided silicon strips and the inner two-layer PiXel Detector (PXD) based on the Depleted P-channel Field Effect Transistor (DePFET) technology. The PXD DePFET structure combines signal generation and amplification within pixels with a minimum pitch of $(50×55) μm^2$. A high gain and a high signal-to-noise ratio allow thinning the pixels to $75 μm$ while retaining a high pixel hit efficiency of about $99$. As a consequence, also the material budget of the full detector is kept low at $≈0.21$$\frac{X}{X_0}$ per layer in the acceptance region. This also includes contributions from the control, Analog-to-Digital Converter (ADC), and data processing Application Specific Integrated Circuits (ASICs) as well as from cooling and support structures. This article will present the experience gained from four years of operating PXD; the first full scale detector employing the DePFET technology in High Energy Physics. Overall, the PXD has met the expectations. Operating in the intense SuperKEKB environment poses many challenges that will also be discussed. The current PXD system remains incomplete with only 20 out of 40 modules having been installed. A full replacement has been constructed and is currently in its final testing stage before it will be installed into Belle II during the ongoing long shutdown that will last throughout 2023

Comparison of Supervised and Unsupervised Anomaly Detection in Belle II Pixel Detector Data

Machine learning has become a popular instrument for the identification of dark matter candidates at particle collider experiments. They enable the processing of large datasets and are therefore suitable to operate directly on raw data coming from the detector, instead of reconstructed objects. Here, we investigate patterns of raw pixel hits recorded by the Belle II pixel detector, that is operational since 2019 and presently features 4 M pixels and trigger rates up to 5 kHz. In particular, we focus on unsupervised techniques that operate without the need for a theoretical model. These model-agnostic approaches allow for an unbiased exploration of data, while filtering out anomalous detector signatures that could hint at new physics scenarios. We present the identification of hypothetical magnetic monopoles against Belle II beam background using Self-Organizing Kohonen Maps and Autoencoders. The two unsupervised algorithms are compared to a convolutional Multilayer Perceptron and a superior signal efficiency is found at high background rejection levels. Our results strengthen the case for using unsupervised machine learning techniques to complement traditional search strategies at particle colliders and pave the way to potential online applications of the algorithms in the near future

Mapping the material distribution of a complex structure in an electron beam

The simulation and analysis of High Energy Physics experiments require a realisticsimulation of the detector material and its distribution. The challenge is to describe all activeand passive parts of large scale detectors like ATLAS in terms of their size, position and materialcomposition. The common method for estimating the radiation length by weighing individualcomponents, adding up their contributions and averaging the resulting material distribution overextended structures provides a good general estimate, but can deviate significantly from the materialactually present.A method has been developed to assess its material distribution with high spatial resolutionusing the reconstructed scattering angles and hit positions of high energy electron tracks traversingan object under investigation. The study presented here shows measurements for an extendedstructure with a highly inhomogeneous material distribution. The structure under investigation isan End-of-Substructure-card prototype designed for the ATLAS Inner Tracker strip tracker — aPCB populated with components of a large range of material budgets and sizes.The measurements presented here summarise requirements for data samples and reconstructedelectron tracks for reliable image reconstruction of large scale, inhomogeneous samples, choicesof pixel sizes compared to the size of features under investigation as well as a bremsstrahlungcorrection for high material densities and thicknesses

Effects of gamma irradiation on DEPFET pixel sensors for the Belle II experiment

For the Belle II experiment at KEK (Tsukuba, Japan) the KEKB accelerator was upgraded to deliver a 40 times larger instantaneous luminosity than before, which requires an increased radiation hardness of the detector components. As the innermost part of the Belle II detector, the pixel detector (PXD), based on DEPFET (DEpleted P-channel Field Effect Transistor) technology, is most exposed to radiation from the accelerator. An irradiation campaign was performed to verify that the PXD can cope with the expected amount of radiation. We present the results of this measurement campaign in which an X-ray machine was used to irradiate a single PXD half-ladder to a total dose of 266 kGy. The half-ladder is from the same batch as the half-ladders used for Belle II. According to simulations, the total accumulated dose corresponds to 7–10 years of Belle II operation. While individual components have been irradiated before, this campaign is the first full system irradiation. We discuss the effects on the DEPFET sensors, as well as the performance of the front-end electronics. In addition, we present efficiency studies of the half-ladder from beam tests performed before and after the irradiation

Summary and Conclusions of the First DESY Test Beam User Workshop

On October 5/6, 2017, DESY hosted the first DESY Test Beam User Workshop [1] which took place in Hamburg. Fifty participants from different user communities, ranging from LHC (ALICE, ATLAS, CMS, LHCb) to FAIR (CBM, PANDA), DUNE, Belle-II, future linear colliders (ILC, CLIC) and generic detector R&D presented their experiences with the DESY II Test Beam Facility, their concrete plans for the upcoming years and a first estimate of their needs for beam time in the long-term future beyond 2025. A special focus was also on additional improvements to the facility beyond its current capabilities