Combinatorial Kalman Filter and High Level Trigger Reconstruction for the Belle II Experiment
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ETP-KA/2018-23 Combinatorial Kalman Filter and High Level Trigger Reconstruction for the Belle II Experiment Zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften von der Fakultät für Physik des Karlsruher Institut für Technologie (KIT) genehmigte Dissertation von M.Sc. Nils Lennart Braun aus Bruchsal Tag der mündlichen Prüfung: 21. Dezember 2018 Referent: Prof. Dr. Michael Feindt Korreferent: Prof. Dr. Florian Bernlochner KIT – Die Forschungsuniversität der Helmholtz Gesellschaft www.kit.edu 3 cbn This document is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0): https://creativecommons.org/licenses/by-nc/4.0/deed.en Contents 1 Introduction7 2 Experimental Setup9 2.1 From Accelerated Electrons to Final State Particles..............9 2.2 From Digital Detector Data via the Trigger to the Storage Drives...... 19 2.3 Belle Analysis Software Framework 2...................... 23 2.4 Time Schedule and Performed Tests...................... 26 3 Foundations 27 3.1 Track Reconstruction............................... 27 3.2 Track Finding Concepts at Belle II....................... 29 3.3 Track Fitting................................... 38 3.4 Mathematical Foundations............................ 45 4 Fast Reconstruction for the High Level Trigger 47 4.1 Introduction.................................... 48 4.2 Processing Time Studies............................. 52 4.3 Principles of FastReco .............................. 70 4.4 Multiprocessing Using the ØMQ Library.................... 79 4.5 Summary..................................... 86 5 Event Timing 87 5.1 The Role of the Event Time........................... 88 5.2 Event Timing Using the CDC Information................... 89 5.3 Flight Time Estimation............................. 90 5 6 Contents 5.4 Event Time Extraction Using the Drift Length Information......... 94 2 5.5 Event Time Extraction Using χ Information................. 97 5.6 Event Time Extraction Using Hit Information................. 102 5.7 Combination of the Methods.......................... 103 5.8 Summary..................................... 110 6 Combinatorial Kalman Filter 111 6.1 Motivation..................................... 112 6.2 Principles..................................... 112 6.3 Application of the CKF at Belle II....................... 114 6.4 CKF from CDC to SVD - an Example..................... 116 6.5 CKF for SVD Hit Attachment.......................... 120 6.6 Merging of CDC and SVD Tracks........................ 134 6.7 CKF for PXD Hit Attachment......................... 146 6.8 Performance in Phase 2............................. 159 6.9 Summary..................................... 169 7 Conclusion 173 8 Bibliography 177 Townsfolk: What a puzzle to the rest of us is Belle Belle: Oh, isn’t this amazing? It’s my fav’rite part because — you’ll see Here’s where she meets Prince Charming But she won’t discover that it’s him ’til chapter three! Beauty and the Beast Alan Menken, Howard Ashman INTRODUCTION 1 Since people have first observed their surroundings, they have been trying to grasp the inner structures that drive the world around them. Mankind has developed complex theories that describe and predict the universe – from the macroscopic theories of galaxy cluster formations to the microscopic theory of particle physics. One of the most successful results of this cross-generational effort is the Standard Model of particle physics, which describes the matter and its interactions through forces discovered so far. The current theory of the Standard Model is however known to not answer all open questions of particle physics. The measured imbalance between the observed amount of matter and antimatter, the missing explanation for dark matter and dark energy or the incompatible theories of general relativity and particle physics are just some of the pending issues of the model. Large experiments were invented and run to solve those mysteries. Ever-growing international collaborations with experts from multiple fields work on precise detectors, huge accelerator complexes, complex software and difficult data processing to understand the inner secrets of particle physics. One of the most promising new experiments at the high-precision frontier is the Belle II experiment [1] currently being commissioned at the SuperKEKB accelerator in Tsukuba, Japan. The Belle II detector is a general-purpose high-energy particle physics detector − + built around the collision point of the e e produced by the accelerator. The intermediate Υ(4S) resonance decaying into only two B mesons offers a perfect laboratory for a variety of precision measurements of the Standard Model and searches for new physics, e.g. meson spectroscopy, searches for rare decays, lepton-flavour violations or dark-matter searches. For performing these measurements with a high precision, large data samples are needed. 7 8 1 Introduction Therefore, the SuperKEKB accelerator was upgraded to be able to deliver an unprecedented, 35 −1 −2 world-record breaking instantaneous luminosity of L = 8 × 10 cm s , which is 40 times higher than was achieved at its predecessor, KEKB. Due to the large increase in the collision rate and the demanding needs of a high-precision measurement, the detector hardware and the reconstruction software were upgraded or entirely rebuilt. The increased luminosity is, however, accompanied with difficult challenges to hardware and software. As the large amount of recorded data cannot be stored to disk, a fast and reliable online trigger system is used to suppress physically uninteresting decays. Strong processing time requirements are imposed on the software part of this trigger – the high level trigger (HLT) – as well as constraints on the output rate and the efficiencies for important decay channels. For this thesis, the software used for the HLT decision was developed from scratch. The first studies shown in this work confirm whether it is feasible to run the offline reconstruction also on the online software trigger, which has major benefits compared to using a different, dedicated reconstruction. A replacement for the legacy multiprocessing framework based on the open-source library ØMQ with beneficial new features was implemented in this thesis for the first time. A basic ingredient for many reconstruction steps is a precise knowledge of the event collision time T0, which cannot be deduced from the accelerator revolution frequency alone, as the readout windows of the detectors are too large. The estimation of this value needs to be performed as exact and reliable as possible, since many detector reconstructions depend on it. In this work, a combined time extraction method based on the drift time information of the central drift chamber (CDC) was developed and studied. One of the most important steps in both the online and offline event reconstruction is the search for the charged decay products using the tracking detectors. Tracking is the only reconstruction part able to extract the parameters of all charged final state particles with a high precision. The increased instantaneous luminosity comes with a drastically enlarged beam-induced background, especially in the innermost silicon-based tracking detectors, which imposes major challenges to the tracking. To address these issues, a combinatorial Kalman filter (CKF) was introduced into the Belle II software through the work of this thesis. It enables a wide range of applications during track finding and is used for attachment of hits and tracks in the pixel detector (PXD) and the strip-vertex detector (SVD). The obtained improvements to the vertex resolution are crucial in allowing a larger range of physics channels to be studied and a higher precision in all measurements. The Belle II detector is introduced in Chapter2 with a focus on the tracking subdetectors and the trigger setup required for this thesis. The foundations behind the development of tracking algorithms and the concept applied at Belle II are described in Chapter3. In Chapter4, the detailed processing time measurements for the HLT reconstruction as well as the developed trigger setup are discussed together with the new implementation for the multiprocessing framework. The first-developed precise event time determination required for a successful data acquisition is described in Chapter5 together with its performance. The combinatorial Kalman filter developed during this work is presented in Chapter6. Its large beneficial impact on the resolution of the reconstruction and other important figures of merit as well as its performance on the first recorded data is outlined in the chapter. The thesis ends with conclusions on the performed work in Chapter7. There is someone in the attic Building a strange machine Never really seen him But I think he works all day The Attic – In Flames Anders Fridén, Björn Gelotte EXPERIMENTAL SETUP 2 The task of the Belle II experiment is to search new physics in particle decays at unprece- dented high precision. The following chapter summarizes the basic concepts how this task is performed and which techniques are used for this. The information given in this chapter is described in more detail in various other places. A reference with a general overview is included in each section. As a first step in analyzing particle decays, resonances which decay into those particles have to be produced artificially. This is done with the help of the large accelerator complex SuperKEKB, which is described in Section 2.1.1. Afterwards in Section 2.1.2,