Characterisation, optimisation and performance studies of pixel vertex detector modules for the Belle II experiment

Das Standardmodell der Elementarteilchenphysik beschreibt sehr erfolgreich die fundamentalen Teilchen und ihre Wechselwirkungen. Dennoch bleiben einige Fragen bezüglich Details und Parameter der Theorie bislang unbeantwortet. Einige Phänomene, wie beispielsweise Dunkle Materie oder Quantengravitation, sind bisher nicht, oder nur unzureichend beschrieben. Heutige Hochenergiephysikexperimente erforschen diese neue Physik in Teilchenkollisionen. Das Belle II-Experiment am SuperKEKB e+e−-Beschleuniger in Japan untersucht die Eigenschaften von Teilchenwechselwirkungen mit höchster Präzision, um so die Grenzen der bisherigen Theorie zu erweitern und Parameter des Standardmodells genauer zu bestimmen. Die genaue Beobachtung von zahlreichen Zerfällen von B-Mesonen ermöglicht es, offene Fragen der elektroschwachen Wechselwirkung zu beantworten. Präzisionsmessungen im Belle II Experiment werden insbesondere durch einen Silizium-Pixeldetektor ermöglicht, der sehr nah am Wechselwirkungspunkt der Teilchenkollisionen positioniert ist. Der Pixeldetektor basiert auf der depleted field-effect transistor (DEPFET) Technologie, die hier zum ersten Mal in einem Hochenergiephysikexperiment zum Einsatz kommt. Für den Belle II Pixeldetektor wurden auf dieser Technologie basierende Module produziert. Eine genaue Charakterisierungs- und Optimierungsprozedur für diese Module wurde im Rahmen dieser Dissertation entwickelt. Insgesamt 17 dieser Module wurden diesem Messprogramm im Verlauf dieser Arbeit unterzogen und die Qualifikation jedes einzelnen Moduls für den finalen Detektor wurde geprüft. Detailierte Untersuchungen des Verhaltens der Detektormodule wurden in Teststrahlmessungen durchgeführt. Die Optimierungsprozedur liefert konsistente Resultate zwischen den getesteten Modulen mit Signal-Rausch-Verhältnissen im Bereich von 20 bis 40 bei einem Inpixel-Verstärkungsfaktor von etwa 500 pA/electron. Die intrinsische Ortsauflösung der Detektormodule wurde zu etwa 10 μm gemessen, abhängig vom Einfallswinkel der Teilchen, bei einer Detektionseffizienz von 99.6 %. Das gemessene Verhalten entspricht den Erwartungen des Moduldesigns und die Voraussetzungen an den Belle II Pixeldetektor werden mit dem vorliegenden Moduldesign erfüllt.The Standard Model of particle physics is very successful in describing the fundamental particles and their interactions. Still, some questions regarding the details and input parameters of the theory, as well as regarding to date unsatisfactorily described phenomena are to be answered. Today’s high-energy physics experiments probe this new physics. The Belle II experiment at the SuperKEKB e+e−-collider in Japan explores the precision frontier, measuring the properties of particle interactions at great detail. The precise and abundant study of decays of B-mesons is a particularly good window to seek answers to the open questions of electro-weak interactions. Precise measurements in Belle II are possible in particular with a silicon pixel detector located very close to the interaction region of the electrons and positrons. The pixel detector is based on the depleted field-effect transistor (DEPFET) technology, which is employed for the first time in a high-energy physics experiment. Modules for the Belle II pixel detector were produced. A characterisation and optimisation procedure for these modules was developed in the scope of this thesis. A number of 17 modules have been processed according to this program, and the qualification criteria for the installation in Belle II have been determined. In addition, the performance of detector modules has been evaluated in beam tests. It is demonstrated that the optimisation procedure yields consistent characteristics among the tested modules. Signal-to-noise ratios of 20 to 40 are achieved at an in-pixel amplification factor of about 500 pA/electron in the DEPFET cell. The intrinsic spatial resolution is measured to be in the order of 10 μm, depending on pixel pitch and incidence angles, with a hit efficiency of 99.6 %. The module performance is in good agreement with the design goals and the requirements for the Belle II pixel detector are met

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

Optimization of the front-end read-out electronics for the Belle II DEPFET Sensor

The Belle experiment at the KEKB e$^{+}$e$^{−}$-collider in Tsukuba, Japan yielded an integrated luminosity of 1 ab$^{−1}$. The collected data was used for in depth $B$-physics studies, as well as beyond standard model searches. For even more detailed physics studies and higher sensitivity to new physics contributions, the KEKB collider is being upgraded to SuperKEKB, which is expected to deliver a 40 times higher instantaneous luminosity. Also the Belle detector will be upgraded to Belle II. Besides the upgrade of the Belle-proven sub-detector components, Belle II will be equipped with additional two layers of a silicon pixel detector closest to the interaction point, which will improve vertex resolution. This pixel detector will be based on the DEPFET technology, which allows for very thin sensors to reduce energy loss and multiple scattering effects. For the read-out of the DEPFET pixel matrices, three dedicated front-end chips are being developed. Confirming and studying the functionality of the chip designs is crucial. In this thesis, testing algorithms will be presented and discussed, that were developed for investigating and optimizing the performance of the current designs of the front-end read-out electronics. The software is used to evaluate parameter scans, tune the chip parameters and investigate weaknesses of the existing chip designs, in order to provide feed-back to the chip designers for future design iterations

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

Performance of production modules of the Belle II pixel detector in a high-energy particle beam

The Belle II experiment at the Super B factory SuperKEKB, an asymmetric $e^+ e^−$ collider located in Tsukuba, Japan, is tailored to perform precision B physics measurements. The centre of mass energy of the collisions is equal to the rest mass of the $ϒ(4S)$ resonance of $m_{ϒ(4S)}$ = 10.58 GeV. A high vertex resolution is essential for measuring the decay vertices of B mesons. Typical momenta of the decay products are ranging from a few tens of MeV to a few GeV and multiple scattering has a significant impact on the vertex resolution. The VerteX Detector (VXD) for Belle II is therefore designed to have as little material as possible inside the acceptance region. Especially the innermost two layers, populated by the PiXel Detector (PXD), have to be ultra-thin. The PXD is based on DEpleted P-channel Field Effect Transistors (DEPFETs) with a thickness of only 75 $μ$m. Spatial resolution and hit efficiency of production detector modules were studied in beam tests performed at the DESY test beam facility. The spatial resolution was investigated as a function of the incidence angle and improvements due to charge sharing are demonstrated. The measured module performance is compatible with the requirements for Belle II

Performance of production modules of the Belle II pixel detector in a high-energy particle beam

The Belle II experiment at the Super B factory SuperKEKB, an asymmetric $e^+ e^−$ collider located in Tsukuba, Japan, is tailored to perform precision B physics measurements. The centre of mass energy of the collisions is equal to the rest mass of the $ϒ(4S)$ resonance of $m_{ϒ(4S)}$ = 10.58 GeV. A high vertex resolution is essential for measuring the decay vertices of B mesons. Typical momenta of the decay products are ranging from a few tens of MeV to a few GeV and multiple scattering has a significant impact on the vertex resolution. The VerteX Detector (VXD) for Belle II is therefore designed to have as little material as possible inside the acceptance region. Especially the innermost two layers, populated by the PiXel Detector (PXD), have to be ultra-thin. The PXD is based on DEpleted P-channel Field Effect Transistors (DEPFETs) with a thickness of only 75 $μ$m. Spatial resolution and hit efficiency of production detector modules were studied in beam tests performed at the DESY test beam facility. The spatial resolution was investigated as a function of the incidence angle and improvements due to charge sharing are demonstrated. The measured module performance is compatible with the requirements for Belle II

EUDAQ—a data acquisition software framework for common beam telescopes

EUDAQ is a generic data acquisition software developed for use in conjunction with common beam telescopes at charged particle beam lines. Providing high-precision reference tracks for performance studies of new sensors, beam telescopes are essential for the research and development towards future detectors for high-energy physics. As beam time is a highly limited resource, EUDAQ has been designed with reliability and ease-of-use in mind. It enables flexible integration of different independent devices under test via their specific data acquisition systems into a top-level framework. EUDAQ controls all components globally, handles the data flow centrally and synchronises and records the data streams. Over the past decade, EUDAQ has been deployed as part of a wide range of successful test beam campaigns and detector development applications

Data quality monitors of vertex detectors at the start of the Belle II experiment

The Belle II experiment features a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric e+e− collider at KEK in Tsukuba, Japan. The accelerator completed its first phase of commissioning in 2016, and the Belle II detector saw its first electron-positron collisions in April 2018. Belle II features a newly designed silicon vertex detector based on double-sided strip layers and DEPFET pixel layers. A subset of the vertex detector was operated in 2018 to determine background conditions (Phase 2 operation). The collaboration completed full detector installation in January 2019, and the experiment started full data taking. This paper will report on the final arrangement of the silicon vertex detector part of Belle II with a focus on online monitoring of detector conditions and data quality, on the design and use of diagnostic and reference plots, and on integration with the software framework of Belle II. Data quality monitoring plots will be discussed with a focus on simulation and acquired cosmic and collision data