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Studies on the Neural Z-Vertex-Trigger for the Belle II Particle Detector

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Studies on the Neural Z-Vertex-Trigger for the Belle II Particle Detector Studies on the neural z-Vertex-Trigger for the Belle II Particle Detector Fernando Abudin´en M¨unchen2014 Studien zu dem neuronalen z-Vertex-Trigger f¨ur den Belle II Teilchendetektor Fernando Abudin´en Masterarbeit an der Fakult¨atf¨urPhysik der Ludwig{Maximilians{Universit¨at M¨unchen vorgelegt von Fernando Abudin´en aus Barranquilla, Kolumbien M¨unchen, den 28.01.2014 Gutachter: Prof. Dr. Jochen Schieck Contents Zusammenfassung viii Abstract ix 1 Introduction 1 2 Physics Motivation 3 2.1 The Violation CP Symmetry . .4 2.1.1 Parity Transformation . .4 2.1.2 Charge Conjugation . .5 2.1.3 CP Transformation . .5 2.2 The Six-Quark Model and the CKM Matrix . .8 2.2.1 The Cabibbo Matrix . .8 2.2.2 The CKM Matrix . .9 2.2.3 The Wolfenstein Parametrization and the Unitarity Triangles . 11 2.3 Flavoured Neutral Mesons . 14 2.3.1 Quantum-Mechanical Description of Flavour Oscillation . 14 2.3.2 Types of CP Violation . 17 2.4 CP Violation in the B-Sector . 20 2.4.1 B Mesons phenomenology . 20 2.4.2 Flavour Oscillation of Neutral B Mesons . 21 2.4.3 Time Dependent CP Violation . 23 2.4.4 Time Dependent CP Violation Measurement . 24 2.5 Achievements of Belle . 28 2.5.1 ¯b ccs¯ Transitions and the Golden Chanel B J= K0 ...... 28 ! ! S 2.5.2 ¯b sss¯ Transitions and B φK0 .................. 30 ! ! S 2.6 New Physics in Belle2 . 32 2.6.1 New Physics in ¯b sss¯ Decays . 32 ! 3 The Belle II Experiment 34 3.1 The SuperKEKB Accelerator and the IR design . 34 3.2 The Belle II Detector . 36 3.2.1 The Coordinate System . 37 vi CONTENTS 3.2.2 Beampipe . 38 3.2.3 Pixel Detector PXD . 38 3.2.4 Silicon Vertex Detector SVD . 40 3.2.5 Central Drift Chamber CDC . 40 3.3 The Background . 44 3.3.1 Touschek Scattering . 44 3.3.2 Beam-Gas Scattering . 45 3.3.3 Synchrotron Radiation . 45 3.3.4 Radiative Bhabha Scattering . 46 3.3.5 Electron-Positron Pair Production via two Photon Process . 47 3.3.6 The z-Background-Distribution in Belle . 48 3.4 Trigger System . 49 3.4.1 The Track Segment Finder (TSF) . 51 3.4.2 2D and 3D CDC Trigger . 52 3.5 Track Parametrization . 53 3.6 Event Simulation . 55 4 The neural z-Vertex Trigger 58 4.1 Overview of the neural z-Vertex Trigger . 58 4.2 The Multi-Layer Perceptron MLP . 59 4.2.1 MLP Software Implementation . 61 4.2.2 The Choice of the Activation Function . 61 4.2.3 Training Process and Cost Function . 62 4.2.4 Overfitting Effect and Stop of the Training Process . 63 4.2.5 Testing Process . 64 4.3 Hardware Implementation of the neural z-Vertex-Trigger . 65 5 Experimental Results 66 5.1 Estimation of the θ-Binwidth . 66 5.2 Track Parameters for charged Particles . 71 5.3 Estimation of the pT -Binwidth . 72 5.4 Estimation of the φ-Binwidth and Symmetry Check . 74 5.5 Estimation of the total Net-Hedgehog Size . 78 5.6 Drift time and Random Offset . 79 5.7 Optimization of the Input Layer Size . 81 5.8 Optimization of Hidden-Layer Parameter . 85 5.9 The Effect of Background and of a light (x; y)-Displacement . 87 6 Conclusion and Outlook 93 A The Belle II Vertex Detector 95 A.0.1 Readout and Cooling System of the PXD . 95 A.0.2 Readout and Cooling System of the SVD . 97 Table of Contents vii Acknowledgments 112 Zusammenfassung Der Elektron-Positron Teilchenbeschleuniger KEKB und der dazugeh¨origeBelle Detektor in Tsukuba, Japan werden zur Zeit verbessert. Bisher hielt KEKB mit einer Luminosit¨at von 2:11 1034 cm−2s−1 den Weltrekord. Der neue Beschleuniger, SuperKEKB, wird eine 40 mal h¨ohereLuminosit¨atals· bis jetzt erreichen. In dem neuen Belle II Detektor wird ein hochaufl¨osenderPixel-Vertex-Detektor eingebaut. Damit werden eine h¨ohereDaten- produktion und eine bessere Vertexauf¨osungerreicht, was das Unitarit¨asdreieck weiter einschr¨anken wird. Infolgedessen wird die Suche nach neuer Physik pr¨azisiert. Auf Grund der neuen Strahlparameter wird sich die Belastung durch ungewollte Stre- ungsprozesse, z.B. Touschek-Streung, um ca. einen Faktor 30 erh¨ohen. Die gestreuten Teilchen produzieren Untergrundsignale im Detektor. Da die enorme Datenproduktion des Detektors jegliche Daten¨ubertragungskapazit¨at¨ubersteigt, muss der Untergrund im Voraus gefiltert werden. Von Belle ist bekannt, dass diese Untergrundereignisse nicht vom Kollisionspunkt ausge- hen. Eine sehr gute M¨oglichkeit, den Untergrund zu reduzieren, w¨aredie Spuren geladener Teilchen zu identifiziern und Ereignisse zu verwerfen die nicht vom Kollisionspunkt stam- men. F¨ureine ausreichende Untergrundunterdr¨uckung ist eine Aufl¨osungvon ungef¨ahr 2 cm erforderlich. Da die Entscheidung in Echtzeit erfolgen muss, ist nicht ausreichend Zeit f¨urherk¨omliche Spurrekonstruktion. Die vielverspechendste Methode ist bisher ein neuronaler z-Vertex Trigger, bestehend aus mehreren Multi Layer Perzeptronen (MLP). Die bisher durchgef¨uhrtenExperimente basierten auf idealisierten Simulationsereignissen innerhalb sehr begrenzter Detektorbereiche mit einzelnen Teilchenspuren ohne Untergrund. In dieser Arbeit wurden die MLP f¨urbestimmte Bereiche des Detektors optimiert und das Netz mit realistischeren Daten inklusive Untergrund trainiert und getestet. Die Anzahl der MLP, die notwendig sind, um den ganzen Detektor abzudecken, konnte anhand der Ergebnisse auf 2 106 abgesch¨atztwerden. Die niedrigste und die h¨ochste Aufl¨osung,die unter Anwendung∼ von· simulierten Ereignissen mit Untergrund erreicht wurden, sind σ = 1:70 0:02 cm max ± σ = 0:85 0:04 cm. min ± Somit wurde gezeigt, dass auch in der Anwesenheit von Untergrund der neuronale z-Vertex Trigger in der Lage ist, die gefordete Aufl¨osungvon weniger als 2 cm zu erreichen. Abstract The electron-positron particle collider KEKB together with its Belle detector in Tsukuba, Japan are currently being upgraded. So far, KEKB held the world record luminosity of 2:11 1034 cm−2s−1. The new collider, SuperKEKB, will reach a 40 times larger luminosity than· its predecessor. Additionally, a new high-resolution Pixel-Vertex-Detector will be built into the new Belle II detector. In consequence, a higher data production and a better vertex resolution will be achieved. This will constrain the unitarity triangle and the search of new physics. Because of the new beam parameters, the load due to undesired scattering process, e.g. Touschek scattering, will be larger by a factor 30. The scattered particles produce back- ground signals inside the detector. Since the enormous data production of the detector exceeds any data transference capacity, the background has to be filtered out in advance. From the Belle experiment, it is known that these background events do not come from the collision point. Therefore, a good possibility to reduce the background could be to identify the tracks of charged particles and to reject events which do not stem from the collision point. For a sufficient background suppression, a resolution of about 2 cm is needed.Since the decision has to me made in real time, there is not enough time for usual track reconstruction. The most promising method up to now has been a neural z-vertex trigger composed by several multi layer perceptrons (MLP). Previous experiments were based on idealized simulated events within narrow detector re- gions using single particle tracks, but without background. In this work, the MLP has been optimized for specific detector regions. The networks were trained and tested with more realistic data samples including background. On the basis of the achieved results, the total amount of required MLPs to cover the whole detector could be approached to 2 106. The lowest and the highest resolution reached, using simulated events under the presence∼ · of background, is σ = 1:70 0:02 cm max ± σ = 0:85 0:04 cm. min ± Thus it could be shown that, also under the presence of background, the neural z-vertex trigger is capable to reach the required resolution of less than 2 cm. x Abstract Chapter 1 Introduction High luminosity colliders and high resolution detectors are required to provide the necessary amount of data, with sufficient accuracy for the search of new physics. B factories are a special kind of collider which produce large amounts of so-called B mesons. B mesons are particles that allow to study a particular violation of symmetry, the Charge-Parity violation, which plays a crucial role in understanding why there is much more matter than antimatter in our universe. The KEKB collider in the locality of Tsukuba, Japan belongs to this kind of collider. It operated at the highest Luminosity ever achieved of 2:11 1034 cm−2s−1 before being temporarily suspended in 2010. At the present time, the KEKB· collider and its detector Belle are under an upgrade process. The new SuperKEKB factory will achieve a luminosity of 80 1034 cm−2s−1. In addition, the resolution of the new Belle II detector will be improved through· the insertion of a new pixel vertex detector in its innermost part. Nevertheless, not all the signals that will be registered by the detector belong to interesting physics events: particle detectors have also to deal with background signals caused by scattered particles. In the case of SuperKEKB, the new beam parameters which, on the one hand are required to reach the design luminosity, lead on the other hand to a higher load on scattered particles. Since the data production rate of Belle II will exceed all possible transference capacities at the current level of technological advance, it would not be possible to store all the detected events. Therefore, the background noise needs to be suppressed as much as possible.
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