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Department of Physics and Astronomy Department of Physics and Astronomy University of Heidelberg Master thesis in Physics submitted by Lennart Huth born in Buchen 2014 Development of a Tracking Telescope for Low Momentum Particles and High Rates consisting of HV-MAPS This Master thesis has been carried out by Lennart Huth at the Physikalisches Institut under the supervision of Herrn Prof. Dr. André Schöning Development of a Tracking Telescope for Low Momentum Particles and High Rates consist- ing of High-Voltage Monolithic Active Pixel Sensors Physics beyond the Standard Model (SM) of particle physics motivates the search for the charged lepton flavor violating decay m+ ! e+e−e+ by the Mu3e experiment. This decay is suppressed with a branching ratio below 10−54 within the SM. Detecting this decay would be a clear sign for new physics beyond the SM. Reaching the aimed sensitivity of better than one in 1016 m+− decays in a reasonable time requires excellent momentum and vertex resolution for background suppression at high decay rates O(109µ/s). The maximum energy of 53 MeV of the decay particles results in a multiple scattering limited vertex and momentum resolution requiring a detector with little material. These requirements will be fulfilled by a pixel detector consisting of 50 µm thin High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) with a time resolution of better than 20 ns. A low momentum particle tracking telescope was developed to test the detector components and the data acquisition (DAQ) to perform a first integration test of the Mu3e detector and to use it at the Paul-Scherrer-Institue (PSI). The development of the mechanics and DAQ and the results of two testbeam campaigns is presented: It is shown that the readout concept of the telescope is working and that the sensors can be synchronized. It also can handle high particle rates O(600 kHz). The mechanical alignment can be carried out with a high precision O(100 µm). Furthermore it is shown that the particle track reconstruction works for 5 GeV electrons with thick sensors of 2 of radiation length. All measurements are supported by additional simulations. h Entwicklung eines Strahlteleskops für niederenergetische Teilchen und hohe Raten basierend auf hochspannungsbetriebenen monolithischen aktiven Pixelsensoren Physik jenseits des Standardmodells (SM) der Teilchenphysik motiviert die Suche nach dem geladenen Lepton-Zahl verletzenden Zerfall m+ ! e+e−e+ durch das Mu3e Experiment. Dieser ist im SM mit einem Verzweigungsverhältnis von weniger als 10−54 unterdrückt und dessen Beobachtung wäre daher ein klares Zeichen für neue Physik. Um die angestrebte Sensitivität von weniger als einem in 1016 m+ − Zerfällen in einer angemessenen Zeit zu erreichen, wird eine hohe Zerfallsrate von O(109µ/s) sowie exzellente Vertex-, Impuls- und Zeitauflösung zur Untergrundunterdrückung benötigt. Durch die geringe maximale Energie von 53 MeV der Zerfallsteilchen ist die Energie- und Impulsauflösung von Vielfachstreeung dominiert. Dies erfordert einen dünnen Detektor. Um diese Anforderungen zu erfüllen, wird der Pixeldetektor von Mu3e aus dünnen, bei Hochspannung betriebenen monolithischen aktiven Pixelsensoren (HV-MAPS) gebaut, die am Physikalischen Institut der Universität Heidelberg charaktersiert werden. Um die Detektorkomponenten und die Datennahme des Mu3e-Detektors zu testen, sowie einen ersten Integrationstest durchzuführen und es am PSI einzusetzen, wurde ein Strahlteleskop aus HV-MAPS realisiert. Die mechanische Konstruktion, das Datennahmesystem und die Analyseprogramme werden vorgestellt. Außerdem wird der Einsatz bei zwei Strahlzeiten und deren Ergebnisse erläutert. Es wird gezeigt, dass es möglich ist ein synchronisiertes Teleskop aus HV-MAPS zu betreiben, welches hohe Teilchenraten O(600kHz) verarbeiten kann. Außerdem ist es möglich das System mechanisch mit einer Präzision von O(100 µm) auszurichten. Des weiteren wird gezeigt, dass die Spurrekonstruktion trotz der noch ungedünnten Sensoren, d.h. mit 2 Strahlungslänge, gute Resultate für Elektronen mit einem Impuls von 5 GeV liefert. Die Ergebnisseh der Strahlzeiten lasssen sich anhand von Simulationen verifizieren. Contents Contents 6 I Introduction 11 1 The Standard Model of Particle Physics 15 1.1 Muon Decays . 17 2 Mu3e - Search for New Physics at the High Rate Frontier 19 2.1 Experimental Situation . 19 2.2 The Mu3e Experiment . 20 2.2.1 Signal Decay . 21 2.2.2 Backgrounds . 21 2.2.3 Requirements . 22 2.2.4 Experimental Concept . 23 2.2.5 Sub Detectors . 23 2.2.6 Readout Concept . 25 2.2.7 Muon Beam . 26 3 Particle Interaction and Detection 29 3.1 Particle Interactions with Matter . 29 3.2 Multiple Coulomb Scattering . 30 3.3 Particle Tracking Detectors . 31 3.3.1 Silicon Pixel Detectors . 32 3.3.1.1 MAPS . 33 3.3.2 HV-MAPS . 33 3.4 Data Transfer and Readout . 34 3.4.1 Field Programmable Gate Arrays (FPGA) . 34 3.4.2 Signal Transmission . 35 4 Particle Track Reconstruction 37 4.1 Coordinate System . 37 4.2 Track Model . 37 4.3 Straight Track Reconstruction without Multiple Scattering . 38 4.4 Straight Track Reconstruction with Correlations . 40 7 8 Contents II Setup & Measurements 41 5 The MuPix Telescope 43 5.1 Motivation . 43 5.2 Telescope Concept . 44 5.3 Mechanics . 46 5.3.1 PCB Holders . 47 5.4 MuPix Prototypes . 49 5.4.1 Sensor Electronics . 49 5.4.2 Hit Storage . 51 5.4.3 MuPix Readout . 51 5.4.4 Special Characteristics of the Prototypes . 53 5.5 Electronics and Cabling . 53 5.5.1 Stratix IV Development Board . 54 5.5.2 The MuPix Test Board . 54 5.5.3 HSMC Adapter Cards . 55 5.5.4 LVDS Cards . 56 6 Data Acquisition 59 6.1 DAQ-Concept . 59 6.2 Telescope Readout Firmware . 60 6.3 FPGA Time Information and Synchronization . 61 6.4 Readout Software . 65 6.4.1 Threads: . 67 6.5 The MainWindow . 68 6.6 Online Monitoring . 72 7 Simulations 73 7.1 Event Generation . 73 7.2 Track Reconstruction Algorithm Structure . 76 7.3 Validation of Event Generation . 77 7.4 Validation of the Track Reconstruction . 80 7.5 Comparison of the Track Reconstruction Methods . 81 7.6 Reconstruction Efficiency and Background Rejection . 82 8 DESY T22 Testbeam 87 8.1 Setup . 88 8.2 Data Taking & Performance . 90 8.3 Analysis . 95 8.4 Hot Pixel Removal . 95 8.5 Telescope Alignment . 96 8.5.1 Timing and Synchronization . 97 8.5.2 Track Reconstruction . 98 8.6 Summary . 101 9 PSI pM1 Testbeam 103 9.1 Setup . 103 9.2 Performance . 105 Contents 9 9.3 Analysis . 107 9.3.1 Hot Pixel Removal . 107 9.3.2 Timing Analysis . 108 III Conclusions 113 10 Summary & Outlook 115 10.1 Summary . 115 10.2 Outlook . 116 IV Appendix 119 A Track Fitting 121 B Gray Code Converter 123 C Digital to Physical Address Conversion 125 D FPGA Communication 127 E DAC Settings 131 List of Figures 132 List of Tables 135 Part I Introduction 11 Introduction One challenge in modern physics is the search for new physics beyond the Standard Model (SM) of particle physics. This search can be done in different ways. On the one hand, one can increase particle energies and try to directly produce new heavy particles. This idea is followed by experiments at large accelerators like the Large Hadron Collider (LHC) or the planned International Linear Collider (ILC). On the other hand, one can go to high particle rates to find very rare decays, which are forbidden or suppressed down to an unobservable level in the SM. This research can be done with relatively small accelerators, which deliver a high number of low momentum particles, like the High Intensity Proton Accelerator (HIPA) at the Paul-Scherrer-Institute (PSI) in Switzerland. Both research fields require new detector concepts. For completeness, it should be also mentioned that the search for new physics can also be done by searching for dark matter. The Mu3e experiment aims to find the decay of a muon into three electrons, which is strongly suppressed in the SM or to give an upper limit on the branching ratio of this decay in the order of 10−16. Therefore, an intense muon beam of 2 · 109 muons per second is needed to perform the experiment on an appropriate timescale. The currently existing beamline at PSI provides up to 108 muons per second. A new High intensity Muon Beamline (HiMB) could provide the required rate of 2 · 109 muons per second. In order to be sensitive to such a rare decay, one has to build a detector with very high momentum resolution, excellent vertex reconstruction and low noise to suppress background below the aimed sensitivity level. The material budget in the active region has to be minimized to reduce multiple Coulomb scattering, which limits the vertex and momentum resolution at low particle momenta. The last important issue is to have an excellent timing to reduce combinatorics. This can be achieved by the use of a novel silicon pixel design, the so called High Voltage Monolithic Active Pixel Sensor (HV-MAPS). The main advantages of this tech- nology are the fast pixel readout, which reduces the dead time of the detector, and the fact that the electronics of the chip is directly implemented in the active area. Therefore no additional separate electronic layer is required, which would introduce additional material. Due to the thin active area the chip can be thinned down to less than 50 µm. The high event multiplicity additionally requires very precise timing in the order of 100 ps. This requirement can be met by scintillating fibers and tiles, which are also 13 14 included in the detector design. All new devices have to be tested, characterized and specified for the usability in the Mu3e experiment. Therefore, the MuPix HV-MAPS prototypes are characterized in the lab at the Physikalisches Institut (PI) in Heidelberg and in test beams at DESY in Hamburg, at CERN and at PSI in Villigen.
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