Towards Understanding Electric Charge Quantization : Search for Sub-millicharged Particles at J-PARC

다국어 초록 (Multilingual Abstract)

The exploration of the dark sector, which constitutes the majority of the universe, and the quantization of electric charge are yet unsolved enigmas in modern physics. The SUB-Millicharge ExperimenT (SUBMET) experiment, going to be carried out at Japan Proton Accelerator Research Complex (J-PARC), endeavors to uncover clues to these mysteries by attempting to detect millicharged particles (mCPs). It represents one of the challenges aimed at understanding the realms beyond the Standard Model of particle physics.

The SUBMET experiment is slated to be conducted on the second basement floor of J-PARC's neutrino monitor (NM) building. It employs modules composed of photomultiplier tubes (PMTs) and scintillators. These modules are designed to directly detect mCPs as they pass through the detector by sensing scintillation light. Four modules are bundled together to form a supermodule. The supermodules are integrated within mechanics that consist of tables and cages, and the entire detector is organized into two layers to facilitate background control.

Also discussed in this dissertation are the high voltage (HV) power supply for the PMTs and monitoring system for the detector's slow control, and the DQM monitoring software. The design and fabrication of a custom data acquisition (DAQ) system based on Domino Ring Sampler 4 (DRS4), dedicated to the SUBMET detector, are further deliberated. The software developed for data analysis is comprehensive, including algorithms for baseline estimation and pulse detection of the PMT signals.

The anticipated backgrounds for the SUBMET experiment include intrinsic PMT dark count rate (DCR) and ambient radiation, as well as backgrounds resulting from cosmic muons and beam-induced events. These backgrounds are predicted through module testing within the laboratory or via computer simulations such as GEANT4. This dissertation also discusses background measurements conducted during a beam-off period at J-PARC, providing real-world observational data.

The SUBMET experiment conducted at J-PARC marks a substantial advance in the quest for mCPs, which are hypothesized components of the dark sector and could shed light on the characteristics of dark matter. The outcomes of this research are poised to lay down a solid foundation for forthcoming explorations into the scarcely charted domain of fractional electric charges, thus enhancing our comprehension of the elemental forces and particles that compose the cosmos, which provides sensitivity to χs (mCPs) with the charge down to $8\times 10^{-5}e$ in $m_\chi < 0.2~\mathrm{GeV/c}^2$ and $10^{-3}e$ in $m_\chi>1.6~\mathrm{GeV/c}^2$. This is the regime largely uncovered by the previous experiments.

국문 초록 (Abstract)

우주의 대부분을 구성하는 암흑 세계의 탐험과 전기 전하의 양자화는 현대 물리학에서 아직 해결되지 않은 수수께끼이다. SUB-Millicharge ExperimenT (SUBMET) 실험은 일본의 Japan Proton Accelerator Research Complex (J-PARC)에서 수행될 예정이며, 미세 전하 입자를 탐지하려는 시도를 통해 이러한 미스터리에 대한 단서를 밝히고자 한다. 이 실험은 입자 물리학의 표준 모델을 넘어서는 영역을 이해하기 위한 도전 중 하나이다.

SUBMET 실험은 J-PARC의 neutrino monitor (NM) 빌딩 지하2층에서 진행될 예정이다. 이 실험은 photomultiplier tube (PMT)와 신틸레이터로 구성된 모듈을 사용한다. 이러한 모듈은 신틸레이터를 통과하는 미세 전하 입자를 직접 검출하기 위해 설계되었다. 네 개의 모듈은 서로 묶여서 슈퍼모듈을 형성한다. 슈퍼모듈은 테이블과 케이지로 구성된 기계 장치에 결합되어 있으며, 전체 검출기는 백그라운드 제어를 용이하게 하기 위해 두 레이어로 구성된다.

이 박사 학위 논문에서는 PMT용 고전압 공급장치와 검출기의 설정 제어 및 DQM 모니터링 소프트웨어를 위한 모니터링 시스템도 논의된다. SUBMET 검출기 전용으로 Domino Ring Sampler 4 (DRS4)를 기반으로 한 맞춤형 data acquisition (DAQ) 시스템의 설계 및 제작에 대해서도 자세히 설명된다. 데이터 분석을 위해 개발된 소프트웨어는 PMT 신호의 기준선 추정과 펄스 탐지를 위한 알고리즘을 포함한다.

SUBMET 실험의 예상 백그라운드에는 내재적인 PMT의 dark count rate (DCR)과 주변 방사선뿐만 아니라 우주선 및 빔에 의한 이벤트로 인한 백그라운드도 포함된다. 이러한 백그라운드는 실험실 내 모듈 테스트 또는 GEANT4와 같은 컴퓨터 시뮬레이션을 통해 예측된다. 이 논문에서는 J-PARC에서 양성자 빔이 켜지지 않은 기간 동안 수행된 백그라운드 측정에 대해서도 논의하며, 실제 관측 데이터를 제공한다.

J-PARC에서 수행될 SBUMET 실험은 미세 전하 입자의 질량 $m_\chi < 0.2~\mathrm{GeV/c}^2$ 영역에서 전하량 $8\times 10^{-5}e$ 부근 및 $m_\chi>1.6~\mathrm{GeV/c}^2$ 영역에서 전하량 $10^{-3}e$ 부근까지 민감도를 확보하여 암흑 세계의 가설적 구성 요소이며 암흑 물질의 특성을 밝힐 수 있는 미세 전하 입자에 대한 탐구에서 중요한 진전을 이룰 것이다. 이 연구의 결과는 미세 전하의 거의 탐험되지 않은 영역으로의 향후 탐험을 위한 견고한 기반을 마련할 것이며, 우주를 구성하는 기본적인 힘과 입자에 대한 우리의 이해를 향상시킬 것이다.

목차 (Table of Contents)

• Abstract i
• 국문초록 iii
• Preface vi
• Acknowledgment ix
• Table of Contents x
• List of Tables xv
• List of Figures xvi
• Abbreviations xxiv
• Nomenclature xxvii
• 1 Introduction 1
• 1.1 Motivation of the Study: Electric Charge Quantization 1
• 1.1.1 The History of the Elementary Charge 1
• 1.1.2 Electric Charge Quantization 2
• 1.2 The Standard Model (SM) 4
• 1.2.1 Beyond the SM 6
• 1.3 Millicharged Particles (mCPs) 9
• 1.4 U(1) Gauge in Dark Sector and Fractional Charge 11
• 1.5 Overview of Existing Experiments 13
• 1.6 Outline of Dissertation 15
• 2 Experimental Site and Principle of Experiment 17
• 2.1 Experimental Site: J-PARC 17
• 2.1.1 Proton Accelerator 17
• 2.1.2 Target and Beam Dump 18
• 2.1.3 Neutrino Monitor Building 18
• 2.2 Meson Production in J-PARC 21
• 2.3 mCP Production in J-PARC 24
• 2.4 Signal Acceptance Rate 28
• 2.5 Scintillation by mCP 28
• 2.6 Acceptance of Photons From Scintillator To PMT 33
• 3 Experimental Components 38
• 3.1 Overview 38
• 3.2 Modules 40
• 3.2.1 Scintillator 40
• 3.2.2 PMT 42
• 3.2.3 Support 45
• 3.2.4 Module Assembly 46
• 3.2.5 Magnetic shielding 47
• 3.3 High Voltage Supply System 48
• 3.3.1 Hardware 48
• 3.3.2 HV Control and Monitoring 50
• 3.3.3 HV Splitter 52
• 3.4 Readout Electronics 54
• 3.4.1 Readout Board 56
• 3.4.2 Trigger and Data acquisition 58
• 3.5 Veto Panels 59
• 3.6 Mechanical Support 60
• 3.6.1 Design 61
• 3.6.2 Weight and seismic analyses 64
• 3.7 On-site Components 65
• 3.7.1 Space 65
• 3.7.2 Power 65
• 4 Backgrounds 83
• 4.1 PMT Dark Current 83
• 4.2 Radiation 85
• 4.3 Beam-induced backgrounds 88
• 4.4 Cosmic backgrounds 89
• 4.5 Summary 91
• 5 Measurement 92
• 5.1 Baseline Estimation 92
• 5.1.1 Asymmetric Least Squares (ALS) Method 94
• 5.1.2 Adaptive Baseline Estimation (ABE) 96
• 5.2 Pulse Finding 98
• 5.3 DCR Measurement 99
• 5.3.1 List of PMTs 99
• 5.3.2 DCR over Time 106
• 5.3.3 DCR Measurement 107
• 5.4 Background Measurement at J-PARC 108
• 6 Discussion 111
• 6.1 Expected Performance and Exclusion Limits 111
• 7 Conclusions 114
• 8 Future Work 116
• 8.1 Cooling to decrease PMT DCR 116
• 8.2 Augment Shield Structure by Lead Blocks 117

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