Master Thesis

Development of Radiative Decay Counter for
ultimate sensitivity of MEG II experiment
訳
究極感度の MEG II 実験のための輻射崩壊同定用カウンターの開発

Shota Nakaura
Supervisor: Prof.Wataru OOTANI
Department of Physics, School of Science,
The University of Tokyo
January 5, 2016

Abstract
In order to search for the charged lepton flavour violating decay (cLFV) µ+ → e+ γ,
the Mu to E Gamma (MEG) experiment started data-taking in 2008. The publishedresult [1] is consistent with the background-only hypothesis and set upper limit on
the branching ratio:
B(µ+ → e+ γ) < 5.7 × 10−13
For achieving a better sensitivity, the MEG experiment will be upgraded to MEG II
which will improve the sensitivity by one order of magnitude and start to take data
in 2016.
In MEG II, the major background source will be the accidental overlap of apositron from normal muon decay and a random γ-ray. Therefore, the rejectionof the accidental background events plays an important role. One of the major
background sources is the Radiative Muon Decay (RMD). The Radiative Decay
Counter (RDC) identifies the background γ-ray from RMD by detecting the associated low momentum positron. Simulation studies suggest that the RDC will
improve the sensitivity by about 30 % and MEG II could reach the ultimate sensitivity of 4.0 × 10−14 . The details of the detector will be discussed in this thesis.

Contents
1 Introduction

1

2 Theoretical Background
2.1 Muon Physics in the Standard Model . . . . . . . . . . . . . . . .
2.1.1 Decay Mode . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 µ → eγ in the Standard Model . . . . . . . . . . . . . . .
2.2 Muon Physics Beyond the Standard Model . . . . . . . . . . . . .
2.2.1 Grand Unified Theory . . . . . . . . . . . . . . . . . . .
2.2.2 Hierarchy Problem and Supersymmetry . . . . . . . . . .
2.2.3 See-Saw Mechanism . . . . . . . . . . . . . . . . . . . .
2.2.4 µ → eγ as a Probe of New Physics . . . . . . . . . . . .

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3
4
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6
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7
8
9

3 MEG II Experiment
3.1 MEG Experiment . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Signal and Backgrounds . . . . . . . . . . . . . . . . . .
3.1.2 Beam and Target . . . . . . . . . . . . . . . . . . . . . .
3.1.3 Positron Spectrometer . . . . . . . . . . . . . . . . . . .
3.1.4 Gamma-ray Detector . . . . . . . . . . . . . . . . . . . .
3.1.5 Data Acquisition System . . . . . . . . . . . . . . . . . .
3.2 Upgrade Experiment: MEG II . . . . . . . . . . . . . . . . . . .
3.2.1 Key Points in the Upgrade . . . . . . . . . . . . . . . . .
3.2.2 µ+ Beam and Target . . . . . . . . . . . . . . . . . . . .
3.2.3 The Positron Spectrometer . . . . . . . . . . . . . . . . .
3.2.4 The Gamma-ray Detector . . . . . . . . . . . . . . . . .
3.2.5 Background Tagging Detector . . . . . . . . . . . . . . .
3.2.6 New DAQ board . . . . . . . . . . . . . . . . . . . . . .

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4 Radiative Decay Counter (RDC)
4.1 Mechanism of background identification . . . . . . . . . . . . . .
4.2 Simulation Study on Sensitivity Improvement . . . . . . . . . . .
4.3 Detector Design . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Upstream RDC . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Downstream RDC . . . . . . . . . . . . . . . . . . . . .

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i

CONTENTS

ii

4.4

Beam Test with a Prototype . . . . . . . . . . . . . . . . . . . . .
4.4.1 Downstream part . . . . . . . . . . . . . . . . . . . . . .
4.4.2 Upstream part . . . . . . . . . . . . . . . . . . . . . . . .

43
43
43

5 Downstream Counter Optimization
5.1 Scintillator Selection . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 Plastic Scintillator . . . . . . . . . . . . . . . . . . . . .
5.1.2 Inorganic Scintillator . . . . . . . . . . . . . . . . . . . .
5.1.3 Self-radiation as energy calibration source . . . . . . . . .
5.2 SiPM Selection . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Basic Properties of SiPMs . . . . . . . . . . . . . . . . .
5.2.2 Energy Resolution . . . . . . . . . . . . . . . . . . . . .
5.3 Estimation of the Current through the MPPC . . . . . . . . . . . .
5.3.1 Afterglow . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 MPPC Grouping for the Plastic Scintillators . . . . . . . . . . . .

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6 Counter Construction
6.1 Timing Counter Part . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3 Time resolution . . . . . . . . . . . . . . . . . . . . . . .
6.2 Calorimeter Part . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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7 Test of the Constructed Detector
7.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Data acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3 Energy Calibration . . . . . . . . . . . . . . . . . . . . . . . . .
7.4 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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78
79

List of Figures
1.1

List of particles in the Standard Model [2] . . . . . . . . . . . . .

1

2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8

Energy spectrum of the electrons from Michel decay . . . . . . .
Energy spectrum of gamma-rays from RMD . . . . . . . . . . . .
µ → eγ decay in the Standard Model . . . . . . . . . . . . . . . .
one-loop contributions to Higgs mass . . . . . . . . . . . . . . .
one of the diagrams contributing to µ → eγ decay process in BSM
Predicted B(µ+ → e+ γ) in SU(5) SUSY GUT model [4] . . . . .
Predicted B(µ+ → e+ γ) in SU(10) SUSY GUT model [4] . . . . .
Correlation between B(µ → eγ) and B(τ → µγ) with SUSY seesaw [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5
5
6
8
10
10
11

3.1 µ+ → e+ γ decay from stopped muon . . . . . . . . . . . . . . . .
3.2 physics background . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 accidental background . . . . . . . . . . . . . . . . . . . . . . .
3.4 A schematic view of the MEG experimental apparatus . . . . . .
3.5 The 590 MeV proton ring cyclotron . . . . . . . . . . . . . . . .
3.6 πE5 channel and the MEG beam transport system . . . . . . . . .
3.7 µ+ stopping target . . . . . . . . . . . . . . . . . . . . . . . . . .
3.8 Proton cyclotron facility and the beam lines at the PSI . . . . . . .
3.9 Layout of the COBRA . . . . . . . . . . . . . . . . . . . . . . .
3.10 Picture of the COBRA . . . . . . . . . . . . . . . . . . . . . . .
3.11 COBRA z-direction magnetic field . . . . . . . . . . . . . . . . .
3.12 Concept of COnstant projected Bending RAdius spectrometer . .
3.13 Schematic view of drift chambers . . . . . . . . . . . . . . . . .
3.14 Picture of drift chambers . . . . . . . . . . . . . . . . . . . . . .
3.15 Sectional view of a drift chamber . . . . . . . . . . . . . . . . .
3.16 Concept of positron z-reconstruction . . . . . . . . . . . . . . . .
3.17 Plastic scintillation bars for time measurement. . . . . . . . . . .
3.18 Plastic scintillation fibers for z-position measurement . . . . . . .
3.19 R9869 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.20 Side view of the liquid xenon detector . . . . . . . . . . . . . . .
3.21 Top view of the liquid xenon detector . . . . . . . . . . . . . . .

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24
25
25

iii

11

LIST OF FIGURES

iv

3.22 DRS chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.23 DRS4 evaluation board [14] . . . . . . . . . . . . . . . . . . . . 25
3.24 Conceptual view of the DRS principle . . . . . . . . . . . . . . . 26
3.25 Concept of the upgrade . . . . . . . . . . . . . . . . . . . . . . . 27
3.26 MEG II experimental apparatus . . . . . . . . . . . . . . . . . . . 28
3.27 MEG II muon stopping target prototype . . . . . . . . . . . . . . 28
3.28 One of the hyperbolic mesh ground planes . . . . . . . . . . . . . 29
3.29 Drift chamber mockup . . . . . . . . . . . . . . . . . . . . . . . 29
3.30 Picture of a single counter . . . . . . . . . . . . . . . . . . . . . 30
3.31 Six SiPMs series-connected on a PCB . . . . . . . . . . . . . . . 30
3.32 Half of the downstream detector (128 coutners) . . . . . . . . . . 30
3.33 Position resolutions as a function of the first photon conversion depth 31
3.34 γ-ray entrance window in the MEG . . . . . . . . . . . . . . . . 31
3.35 γ-ray entrance window in the MEG II . . . . . . . . . . . . . . . 31
3.36 Ultraviolet sensitive MPPC (S10943-3186) . . . . . . . . . . . . 32
3.37 Collected scintillation light distribution of pileup photon events . . 33
3.38 Simplified scheme of the DAQ board . . . . . . . . . . . . . . . . 34
3.39 The WaveDREAM board . . . . . . . . . . . . . . . . . . . . . . 34
3.40 Crate to place the WaveDREAM board . . . . . . . . . . . . . . . 34
4.1 Amount of background gamma rays in MEG and MEG II . . . . .
4.2 An example of a background event identified by RDC . . . . . . .
4.3 Concept of the RDC . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 The time diﬀerence between LXe and RDC . . . . . . . . . . . .
4.5 Energy deposit in the downstream RDC . . . . . . . . . . . . . .
4.6 Distribution of the Nsig upper limit . . . . . . . . . . . . . . . . .
4.7 The sensitivity dependence on the downstream RDC performance
4.8 MEG II detector with the full RDC . . . . . . . . . . . . . . . . .
4.9 Upstream RDC design . . . . . . . . . . . . . . . . . . . . . . .
4.10 Upstream RDC prototype . . . . . . . . . . . . . . . . . . . . . .
4.11 Downstream RDC design . . . . . . . . . . . . . . . . . . . . . .
4.12 Timing Counter Part and Calorimeter Part of the RDC . . . . . . .

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42

5.1
5.2

45

5.3
5.4
5.5
5.6
5.7
5.8
5.9

Emission spectrum of BC-418 & BC-420 [15] . . . . . . . . . . .
Photon detection eﬃciency vs wavelength for S13360 series MPPC[23] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Decay scheme of 176 Lu to 176 Hf . . . . . . . . . . . . . . . . . .
The energy spectrum of LYSO self-radiation [20] . . . . . . . . .
Setup for LYSO self-radiation rate measurement . . . . . . . . . .
Measured energy spectrum of the LYSO self-radiation . . . . . .
Surface of the SiPM . . . . . . . . . . . . . . . . . . . . . . . . .
SiPM structure . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conceptual picture of the crosstalk and the after-pulse [21] . . . .

45
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49
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49

LIST OF FIGURESv

5.10 Setup to measure the energy resolution . . . . . . . . . . . . . . .
5.11 Observed 60 Co and self-radiation energy spectrum . . . . . . . . .
5.12 Energy resolution as a function of the overvoltage . . . . . . . . .
5.13 Average positron energy deposit times hit rate in each crystal . . .
5.14 Charge distribution of 60 Co and the self-radiation from LYSO. . .
5.15 Measured spectrum of the radiation from 90 Sr . . . . . . . . . . .
5.16 Waveforms of photons from crystals . . . . . . . . . . . . . . . .
5.17 Monitored current though the MPPC attached on crystal . . . . .
5.18 Circuit model of series-connected SiPMs. Each gray area illustrates each SiPM. The red and the blue regions correspond to a
fired pixel and inactive pixels respectively, that is, only one pixel in
SiPM1 is fired in this figure. . . . . . . . . . . . . . . . . . . . .
5.19 Single photon waveforms simulated with SPICE (Simulation Program with Integrated Circuit Emphasis). (a) only one SiPM isplaced in the circuit. (b) three SiPMs are connected in series asshown in Fig.5.18. This simulation was conducted by Kohei Yoshidawith parameters of AdvanSiD [26] which is one type of SiPMs. . .
5.20 Setup for the I-V curve measurement. . . . . . . . . . . . . . . .
5.21 I-V curves of three MPPCs in the same group . . . . . . . . . . .

50
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55

6.1 Components of the timing counter part . . . . . . . . . . . . . . .
6.2 Design of the short PCB . . . . . . . . . . . . . . . . . . . . . .
6.3 Twelve short PCBs . . . . . . . . . . . . . . . . . . . . . . . . .
6.4 Connection of chassis ground and MPPC ground . . . . . . . . .
6.5 A capacitor attached on the PCB . . . . . . . . . . . . . . . . . .
6.6 Three scintillation bars with six MPPCs . . . . . . . . . . . . . .
6.7 Four long PCBs . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.8 Picture of the conductive epoxy CW2400 . . . . . . . . . . . . .
6.9 MPPC alignment jig . . . . . . . . . . . . . . . . . . . . . . . .
6.10 The shortest counter. . . . . . . . . . . . . . . . . . . . . . . . .
6.11 Setup for the performance test of the scintillator bar counters . . .
6.12 Measured ∆T distribution . . . . . . . . . . . . . . . . . . . . . .
6.13 Position dependence of the timing resolution of the scintillator bars
6.14 12 plastic scintillators assembled in the frame. . . . . . . . . . .
6.15 Mechanism to press the MPPCs onto LYSO crystals . . . . . . . .
6.16 Ten MPPCs soldered on the PCB . . . . . . . . . . . . . . . . . .
6.17 MPPC pressed by a spring . . . . . . . . . . . . . . . . . . . . .
6.18 LYSO crystal with a reflective film . . . . . . . . . . . . . . . . .
6.19 Side view of the LYSO part . . . . . . . . . . . . . . . . . . . . .
6.20 ROHACELL foam and the Carbon fiber layers . . . . . . . . . . .
6.21 The frame for the LYSO crystals . . . . . . . . . . . . . . . . . .
6.22 76 crystals covered with a reflector film and placed in the frame .
6.23 The support and alignment plates . . . . . . . . . . . . . . . . . .

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56

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57

LIST OF FIGURES
6.24 A Derlin plate . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.25 The cabling for calorimeters. . . . . . . . . . . . . . . . . . . . .
6.26 The frame and the springs . . . . . . . . . . . . . . . . . . . . . .
6.27 Downstream RDC placed on the edge of the COBRA magnet . . .

vi
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70

7.1 The combined counters . . . . . . . . . . . . . . . . . . . . . . . 71
7.2 Total cross sections of gamma-rays in carbon . . . . . . . . . . . 72
7.3 Conceptual drawing of the event . . . . . . . . . . . . . . . . . . 72
7.4 Decay scheme of 88 Y . . . . . . . . . . . . . . . . . . . . . . . . 73
7.5 Event displays of the simulation . . . . . . . . . . . . . . . . . . 73
7.6 Edep.in the crystals vs in the plastic scintillators . . . . . . . . . . 74
7.7 Gamma-rays interacted in the plastic scintillator and in the crystal
74
7.8 The detector with Y-88 . . . . . . . . . . . . . . . . . . . . . . . 75
7.9 Channels used in the test . . . . . . . . . . . . . . . . . . . . . . 75
7.10 Test setup for measurement of the gamma-ray’s total absorption
process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
7.11 Bias and amplifier circuit diagram applied for the plastic scintillator 76
7.12 Bias and readout circuit diagram applied for the LYSO crystals . . 76
7.13 Signal pulses from the plastic scintillator and the central LYSO crystal 77
7.14 Height in the LYSO crystal vs height in the plastic scintillator . . . 78
7.15 Setup for the energy calibration of the plastic scintillator using Sr-90 78
7.16 Measured energy deposit of beta-ray in the plastic scintillator . . . 79
7.17 Simulated energy deposit of beta-ray in the plastic scintillator . . . 79
7.18 Energy spectrum of the self-radiation and gamma-rays from Y-88 . 79
7.19 The sum of the energy deposits . . . . . . . . . . . . . . . . . . . 80

List of Tables
2.1
2.2

Muon decay modes and their branching ratios [18] . . . . . . . .
Particles in the MSSM . . . . . . . . . . . . . . . . . . . . . . .

4
8

3.1

Properties of liquid xenon . . . . . . . . . . . . . . . . . . . . . .

23

5.1
5.2
5.3

Basic Properties of BC-418 & BC-422 [15] . . . . . . . . . . . .
Inorganic Scintillator Properties . . . . . . . . . . . . . . . . . .
Four types of MPPC used for the test. They have a 3×3 mm2 active
area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44
46
50

Properties of CW2400 [24] . . . . . . . . . . . . . . . . . . . . .

61

6.1

vii

Chapter 1
Introduction
Particle physics is a field of study that explains what elementary constituents ofmatter are and what determines their behavior. Since J.J. Thomson discovered anelectron as the first elementary particle in 1897, people have sought for a bettermodel to describe our world.
The Standard Model, which has been developed since the beginning of the 20th
century, is one of the most successful theories which explain physical phenomena.
It contains three types of particles: spin- 12 fermions (quarks and leptons), spin1 gauge bosons and spin-0 Higgs boson. The particles in the standard model issummarized in Fig.1.1.

Fig. 1.1: List of particles in the Standard Model [2]1CHAPTER 1. INTRODUCTION

2

When particles interact with each other, we say that there is a force between
them. Currently four forces (strong, weak, electromagnetic and gravity) are considered to be fundamental. The Standard Model describes strong, weak and electromagnetic forces as SU(3)C × SU(2)L × U(1)Y symmetry and its spontaneous
breaking. Gravity is not included in this model because it is negligibly small compared with the other forces. The discovery of the Higgs boson in 2012 strongly
supported the model’s validity since the particle was the last piece to be filled.
Although the Standard Model has been successful in explaining so many experimental results, there are still many unanswered questions and unnaturalnesses,
such as dark matter and the hierarchy problem. It suggests that the Standard Modelis not a final theory but just a low-energy eﬀective theory. In this situation, new theories which replace the Standard Model at high energy scales have been proposed.
They are called "physics Beyond the Standard Model " or "new physics". There aremany promising candidates such as Grand Unified Theory (GUT) and SUperSYmmetry (SUSY). Charged Lepton Flavor Violation (cLFV) is a good probe to test
new physics because the predicted branching ratios of cLFV decays depend on themodel. Many of new physics models predict experimentally accessible branchingratios of the µ → eγ, the µ → eee, and the µ − e conversion while these processes
are almost forbidden in the Standard Model. Confirming the presence of these processes implies the obvious existence of new physics and that is why they are called"golden channels".

Chapter 2Theoretical Background
Muon was discovered in 1936 and found to be a replica of the electron with aheavier mass (105.7 MeV). It is classified as the second-generation charged lepton.
Here we discuss the properties and the connection with new physics.

2.1 Muon Physics in the Standard Model
Interactions in the Standard Model are summarized in the form of a Lagrangian.
Leptons do not have color charge so they don’t have any strong interaction term.
The quantum numbers of the second-generation Leptons are shown in this table:- SECOND GENERATION lepton

T3

Y

Q

µ−L
µ−R

- 12
0

-1
-2

-1
-1

νµ

1
2

-1

0

Here we consider tree-level terms (i.e. without loops) related to the muon in the
Lagrangian.
L = eµγ λ µAλg− √ (ν µL γ λ µ L Wλ+ + µL γ λ ν µL Wλ− )
2
)
{
(
}
√
1
′
2
λ
λ
2
− g 2 + g 2 µL γ − + sin θW µ L + µR γ sin θW µ R Z λ0
2
mµ
−
µµH
(2.1)
v
The respective terms correspond to the interaction via photon, W boson, Z boson,and Higgs boson.
3

CHAPTER 2. THEORETICAL BACKGROUND

4

Table 2.1: Muon decay modes and their branching ratios [18]Modeµ− → e− ν e ν µµ− → e− ν e ν µ γµ− → e− ν e ν µ e+ e−

Fraction≃ 100 %
(1.4 ± 0.4) %(3.4 ± 0.4) × 10−5µ− → e− νe ν µµ− → e− γµ− → e− e+ e−µ− → e− 2γ< 1.2 %
< 5.7 × 10−13< 1.0 × 10−12< 7.2 × 10−11

Confidence Level

90 %
90 %
90 %
90 %

2.1.1 Decay Mode
As given in the second line of (2.1), the muon decay process in the Standard Modelis described by V-A interaction (i.e. only left-handed muons interact via a chargedweak current interaction). The interaction is expressed as
][
]
GF [
L Fermi = √ ν e γ λ (1 − γ5 ) µ eγλ (1 − γ5 )νe
2

(2.2)

and its diﬀerential decay rate is]
m5µ G2F 2 [
d2Γ
(3
−
2x)
±
P
cos
θ
(2x
−
1)
=
x
µ
e
dx d cos θ e 192π 3(2.3)

Here G F is the Fermi coupling constant, x is defined as x = Ee /Weµ (here Weµ
is the maximum electron energy: Weµ = max(Ee ) = (m2µ + m2e )/2m µ ), Pµ is thedegree of muon polarization, and θ e is the angle between muon spin and positron
momentum. The dominant process: µ− → e− νe ν µ is called Michel decay. As
mentioned later, the energy spectra of electrons from the Michel decay and of the
gamma-ray from Radiative Muon Decay (RMD:µ− → e− ν e ν µ γ) are important todefine the experimental requirements. The two spectra are shown in Fig.2.1, 2.2.

CHAPTER 2. THEORETICAL BACKGROUND

Fig. 2.1: Energy spectrum of theelectrons from Michel decay

2.1.2

5

Fig. 2.2: Energy spectrum of
gamma-rays from RMDµ → eγ in the Standard Model

The Standard Model forbids the charged Lepton Flavor Violation. However, theneutrino oscillation can induce the µ → eγ decay. Here we introduce a leptonmixing matrix, called Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix.
Ue1 Ue2 Ue3*U.
UPM N S = µ1 Uµ2 Uµ3 +/,Uτ1 Uτ2 Uτ3 1
00c13
0 s13 e−iδ c12 s12 0*+*
1
s23 +/ *.−s12 c12 0+/= .0 c23 s23 / . 0iδ
c13 - , 0
0 1,0 −s23 c23 - ,−s13 e 0

(2.4)

(2.5)

where ci j = cos θi j , si j = sin θi j and θi j is a mixing angle. With the neutrino
mixing, the µ+ → e+ γ decay can be induced as shown Fig.2.3 but the calculatedbranching ratio is negligibly small:2∆mi1
3α ∑ ∗
U Uei 2
B(µ → eγ) =32π i=2,3 µiMW2≃ 10−54(2.6)

Here, Ui j is the PMNS matrix and ∆mi2j are the mass squared diﬀerences between
the neutrinos. The sensitivity of the experiment today is O(10−13 ) and the decay
has not been observed.

CHAPTER 2. THEORETICAL BACKGROUND

6

Fig. 2.3: µ → eγ decay in the Standard Model

2.2 Muon Physics Beyond the Standard Model
The Standard Model so excellently explains almost all the experimental observations. However, there are still many unsatisfied points for it to be the final theory.
We will briefly discuss the main problems in the Standard Model and hypotheses
which try to solve them in this section.

2.2.1 Grand Unified Theory
While the Standard Model successfully unified electromagnetic and weak interaction based on the SU(2)L × U(1)Y group, the magnitudes of coupling constants are
diﬀerent and independent. This is obviously unsatisfactory and the next step is toextend the gauge group and try to unify all coupling constants. The Grand Unified
Theory (GUT) is a model which tries to unify electroweak interaction and strong
interaction, assuming the SU(3)C × SU(2)L × U(1)Y group is a part of a larger
group G (i.e. G ⊃ SU(3)C × SU(2)L × U(1)Y ). The simplest group is SU(5) so
it can be considered the minimal extension of the Standard Model. It puts the 15
fermions of the first generation into 5-plet and 10-plet:
0 ucb −urc ug d g
d gc*.
*. d c +/0 ugc ur dr +//..
.. rc //
ψ5 = . d b / , ψ10 = .
(2.7)
0 ub d b ///..
.. − //
0 e+ /e0 -L,, ν -LThe superscription C indicates the charge conjugation. This expression enables us
to explain the charge quantization and to predict the value of the electroweak mixing angle (i.e. sin2 θW ). However, predictions of the minimal SU(5) is inconsistentCHAPTER 2. THEORETICAL BACKGROUND

7

with the experimental result such as the proton decay search at KAMIOKANDE.
The unified coupling constant cannot be got with minimal SU(5). Although theminimal SU(5) model is already excluded, by introducing new physics the experimental phenomena can be explained and the unification of coupling constants canalso be obtained.

2.2.2 Hierarchy Problem and Supersymmetry
If we assume the Standard Model is a low energy eﬀective theory, a certain energy
scale Λ up to which the Standard Model is valid exists. The scale is called cut-oﬀ
and considered to be much larger than the electroweak scale, such as 1015 GeV(GUT scale) or 1019 GeV (Planck scale).
Although the gauge symmetry implies zero-mass gauge bosons and the chiralsymmetry accounts for the small mass of fermions, there is no symmetry to protect
the mass of the Higgs boson. As a result, the Higgs mass gets a large quantum
correction and can easily reach the cut-oﬀ scale. Actually the observed Higgs masssquared m2h = 2µ2 has the quadratic divergence Λ2 :
2µ2 = 2µ20 − cΛ22µ20 is the bare mass-squared, and c is a constant. In order to get a 102 GeV Higgs
mass on the left hand, the two terms on the right hand should cancel out. AssumingΛ ∼ 1015 GeV and c ∼ α ∼ 10−2 , it requires a fine tuning of the bare parameter
2µ2−26 . This is quite unnatural and it is diﬃcult toat the precision of ( cΛ2 ) ∼ 10
recognize the Standard Model as the final theory.
The application of supersymmetry (SUSY) to the Standard Model is an attractive solution. The SUSY is a symmetry which connects particles whose spin is
diﬀerent by 12 . The SUSY transformation is given by
Q|Fermion⟩ = |Boson⟩(2.8)

Q† |Boson⟩ = |Fermion⟩(2.9)

where Q is a SUSY generator or supercharge. Since supersymmetry is independentof the other internal symmetries, the connected pair of particles carry the same
quantum numbers except for the spin. In addition, the diﬀerence of spin statistics
gives the opposite sign to the Feynman diagram with a loop. As the result, thequadratic divergence which derives from Fig. 2.4a and Fig. 2.4b will cancel out.
Although SUSY is an attractive idea, it should be broken. That is because
we have never observed bosons which are the partners of quarks or leptons withthe same mass. Due to the SUSY breaking, the Higgs mass has a logarithmic
2
divergence MSU
SY log Λ. MSU SY is the energy scale where the SUSY breaking takes
22
place. If we assume MSU SY is suﬃciently small (αMSUSY ≤ MW → MSU SY ≤1TeV), we are still free from the divergence.

CHAPTER 2. THEORETICAL BACKGROUND

8

Table 2.2: Particles in the MSSM
Particlequark
leptongluon
W bosonZ bosonHiggs boson

Spin
1/2
1/2
1
1
1
0

(a) radiative correction to the Higgs
mass from top quark loop

Superpartner Spinsquark0slepton0
gluino
1/2
wino
1/2
bino
1/2
Higgsino
1/2(b) correction from a loop of the superpartner of top quark (stop)

Fig. 2.4: one-loop contributions to Higgs mass
In the Minimal Supersymmetric extension of the Standard Model (MSSM), we
require two Higgs potentials H1 , H2 and they are expressed as( )v1 1(2.10)
H1 = √2 (0)v2 0H2 = √(2.11)
2 1
in a vacuum with SU (2) × U (1) breaking. tan β is defined as the ratio of the two
Higgs vacuum expectation values: tan β = v2 /v1 .

2.2.3 See-Saw Mechanism
Although the neutrino mass is strictly zero in the Standard Model, the observationof neutrino oscillation strongly suggests the non-zero neutrino mass. There arethree possible types of neutrino mass: Pure Dirac, Pseudo-Dirac and Seesaw. The
first case has only Dirac masses, and the second case also includes small Majorana
masses. The third Seesaw is the extreme scenario where Majorana masses are much

CHAPTER 2. THEORETICAL BACKGROUND

9higher than Dirac ones. The most general mass term is expressed as
1
1
L m = − m R (νR ) c νR − m L (νL ) c νL − m D νR νL + h.c.
2
(2
)(
)
)
(
1
m
m
ν
L
D
L
= − (νL ) c νR
+ h.c.
m D m R (νR ) c
2(2.12)

νR and νL are right- and left-handed neutrinos. If we assume that m L = 0 andm D ≫ m R , the mass matrix becomes()
0 mD(2.13)Mν =mD mR
and its eigenvalues m s ,ma are
ms ≃ m R(2.14)

m2D
mR(2.15)

ma ≃From the two eigenvalues, we obtainm s · ma ≃ m2D(2.16)

This relation means that if the mass m R is much bigger than m D (weak scale), ma
gets much smaller than m D , which could explain why the neutrino mass is verysmall.

2.2.4µ → eγ as a Probe of New Physics

As mentioned above, the Standard Model forbids cLFV process and the µ → eγ
decay cannot be experimentally observed. On the other hand, new physics likethe supersymmetric grand unified theory (SUSY-GUT) or supersymmetric seesaw
model (SUSY-seesaw) introduce large cLFV and the rate of the µ → eγ reaches
an observable level through loop diagrams such as the one in Fig.2.5. TeV-scaleparticles carry flavor violating couplings and the branching ration of µ → eγ israther enhanced.

CHAPTER 2. THEORETICAL BACKGROUND

10

Fig. 2.5: one of the diagrams contributing to µ → eγ decay process in BSM
B(µ+ → e+ γ) as a function of the right-handed selectron mass mHe R in SU(5)SUSY- GUT and SU(10) SUSY - GUT model are shown in Fig.2.6, 2.7. The
gaugino mass M2 is set as SUSY input parameters.

Fig. 2.6: Predicted B(µ+ → e+ γ) in SU(5) SUSY GUT model [4]CHAPTER 2. THEORETICAL BACKGROUND

11

Fig. 2.7: Predicted B(µ+ → e+ γ) in SU(10) SUSY GUT model [4]In Fig. 2.8, the correlation of branching ratios for two cLFV processes in SUSYseesaw are shown. Considering that the neutrino mixing angle is θ 13 ∼ 9◦ , therelevant region is confined to the magenta areas only.

Fig. 2.8: Correlation between B(µ → eγ) and B(τ → µγ) with SUSY seesaw [5]The µ → eγ decay rate of O(10−14 ) is the level the MEG II experiment canreach. As explained, the µ → eγ process has no background from the Standard
Model. That is, the observation of µ → eγ means the obvious existence of new
physics.

Chapter 3
MEG II Experiment
The first search for the "golden" channel µ+ → e+ γ began in 1947 with cosmic
muons. From the 1970s, the high-intensity µ+ beams became available. In 1999
the MEGA experiment used muons from the surface muon beam at the Los Alamos
Meson Physics Facility and set the upper limit of B < 1.2 × 10−11 (90% C.L.)[19].
The Mu to E Gamma (MEG) experiment at the Paul Scherrer Institute (PSI) started
data taking in 2008 and set the limit B < 5.7 × 10−13 (90% C.L.) by analyzing
the first half of data [1]. The MEG experiment will be upgraded and will aim at
a sensitivity of 4 × 10−14 . In this chapter, the MEG experiment will be reviewed
and the detail of the upgrade experiment (called MEG II) will be discussed in thischapter.

3.1 MEG Experiment
An experiment to search for cLFV process µ+ → e+ γ with a high sensitivity was
proposed by four Japanese and one Russian institutions, and the research committee in the PSI approved it and the MEG experiment started in 1999 [8]. Now
the MEG collaboration consists of about sixty physicists from Japan, Switzerland,
Italy, Russia and the USA. In the PSI, the world’s most intense DC muon beam isavailable. This powerful muon beam enables us to search for rare decays such asµ+ → e+ γ.

3.1.1 Signal and Backgrounds
Properties of µ → eγ
The muon beam is stopped on a target. The signal event from stopped muons isbeautifully simple :• a two-body decay into a positron and gamma-ray

12

CHAPTER 3. MEG II EXPERIMENT

13• the two particles have an equal energy of 52.8 MeV (half of the muon mass)• the two particles are emitted time-coincidently• the two particles are emitted with an angle of 180◦ between themFig. 3.1: µ+ → e+ γ decay from stopped muon

Therefore, the basic idea of the MEG experiment is to precisely measure the positronenergy Ee , the gamma-ray energy Eγ , the relative timing t eγ between them, and theopening angle Θeγ . The expected number of signal events (Nsig ) is expressed as
Nsig = R µ × T × B × Ω × ϵ γ × ϵ e × ϵ s(3.1)

where R µ is the muon stopping rate, Ω is the solid angle covered by the gamma-ray
and positron detectors, T is the measurement time, the ϵ γ and ϵ e are eﬃciencies ofthese detectors, and ϵ s is the eﬃciency of the selection criteria.
Background Events
There are two major backgrounds in the µ+ → e+ γ search. One is a physics
background from a Radiative Muon Decay (RMD: µ+ → e+ νe ν µ γ), where thepositron and gamma-ray are emitted back-to-back and two neutrinos carry oﬀ smallenergy (Fig.3.2). The other is an accidental overlap of a positron from normal muon
decay and a random gamma-ray. The uncorrelated gamma-ray mainly comes froman RMD or a positron annihilation in flight (AIF). The accidental background isdominant and it is determined by the detector resolutions of the MEG experiment.
The expected number of accidental background events (Nacc ) is expressed as2
× ∆t eγ × T
Nacc ∝ R2µ × (∆E)γ2 × ∆Pe × (∆Θ)eγ(3.2)

where ∆E, ∆Pe , ∆Θ and ∆t eγ are the experimental resolutions for the photon energy, the muon momentum, the opening angle, and the relative timing.

CHAPTER 3. MEG II EXPERIMENT

Fig. 3.2: physics background14

Fig. 3.3: accidental backgroundµ → eγ Search Requirements
While we need high-rate muon beam to get enough statistics, the high-rate stopped
muon decay (> 107 /sec) increases the number of the accidental background events
more than the one of signal events as we can see in (3.1) and (3.2). From a pointof view of the accidental background suppression, a DC muon beam is better thana pulsed beam.
As can be seen in Fig.2.1, the energy spectrum of positrons from Michel decay
have a its maximum around the signal region. It means that the positron spectrometer has to measure high positron hit rates (∼ 107 − 108 /sec). While the improvementof the positron energy resolution is not eﬃcient for reducing accidental background
events, a photon detector with a high energy resolution can strongly suppress theaccidental background events since the energy spectrum of background gammaray from RMD falls rapidly for the signal region as shown in Fig.2.2. Summarizing
them, we need
• a high intensity DC muon beam
• a positron spectrometer which can measure high-rate positrons precisely• a gamma-ray detector which has a high energy resolution
In addition to the good resolutions of detectors, an active tag of background events
can be another possibility to further improve the search sensitivity. Based on thisconcept, the MEG II experiment employs a new detector, radiative decay counter(RDC), which will be later described in detail.
MEG Experimental Apparatus Overview
The MEG experiment, whose schematic view is shown in Fig.3.4, is designed tomeet the requirements above. A high intensity muon beam in the π-E5 beam line
at PSI is brought to the stopping target in the MEG detector at 3 × 107 µ+ stops/sec.
Emitted Gamma-rays are measured by a liquid xenon scintillation detector. Positrons
are observed by a spectrometer composed of low material drift chambers and scintillation counters which is placed in a gradient magnetic field.

CHAPTER 3. MEG II EXPERIMENT15

The coordinate system (x, y, z) is defined as follows. The origin is the centerof the spectrometer magnet where the stopping target is located. The z-axis is the
direction along the µ+ beam axis and the y-axis points vertically upward.

Fig. 3.4: A schematic view of the MEG experimental apparatus

3.1.2 Beam and Target
Surface µ+ beam
The PSI runs a 590 MeV proton cyclotron with a maximum current of 2.3 mA (1.36
MW), as shown in Fig.3.5.

CHAPTER 3. MEG II EXPERIMENT

16

Fig. 3.5: The 590 MeV proton ring cyclotron which has four accelerator cavities(gray) and eight sector magnets (green)
The π-E5 channel extracts pions and muons produced in the target E in the
primary proton beam (Fig.3.8). The extracted muons for the MEG experiment areso called "surface muons" and they are produced on the surface of the production
target. Surface muons have a mean momentum of 28 MeV/c with a spread of 5-7%
(FWHM).
The beam from π-E5 is brought to the MEG detector by a beam transport system shown in Fig.3.6. There are two main roles of the system: reducing the amountof beam contamination and adjusting the muon momentum. The system consistsof Triplet I (first quadrupole triplet), a Wien filter, Triplet II (second quadrupole
triplet) and a beam transport solenoid (BTS). The Triplet I couples to the π-E5
channel. The Wien filter separates muons from contamination positrons by 7.5σ
(12 cm) with crossing magnetic field (133 G) and electric field (195 kV). Triplet
II refocuses the beam at the collimator system. The magnetic field of the BTS is
designed to minimize the beam spot size on the stopping target by adjusting theoscillation of the beam profile. In the BTS a muon momentum degrader made of a
300 µm Mylar film is placed in order for muons to stop on the target eﬃciently.

CHAPTER 3. MEG II EXPERIMENT

17

Fig. 3.6: Schematic view of the πE5 channel and the MEG beam transport systemµ+ stopping target
The muon stopping target is made of a polyethylene-polyester film with 205 µm
thickness (Fig.3.7). It has six holes of 10 mm diameter to monitor the target position. It has an angle of 20.5◦ with respect to the beam axis in order to obtain agood stopping power while keeping the multiple scattering of the positron and the
production of background gamma-rays due to positron annihilation in flight to aminimum.

Fig. 3.7: µ+ stopping target

CHAPTER 3. MEG II EXPERIMENT

18

Fig. 3.8: The cyclotron facility and the beam lines at the PSI. The MEG experimentis carried out in πE5 which is located at the center.

CHAPTER 3. MEG II EXPERIMENT

19

3.1.3 Positron SpectrometerTo fulfill the requirement of the MEG experiment, the positron spectrometer mustsatisfy these conditions:• stable operation under high positron hit rate
• good resolution of positron momentum, direction and timing
In order to suppress multiple coulomb scattering of positron and suppress gammaray production from position annihilation in flight, we also require• a low mass tracker
The spectrometer consists of three parts: a superconducting solenoidal magnet,
drift chambers, and timing counters.
Constant projected Bending Radius (COBRA) magnet
The COBRA magnet is thin enough to allow the signal gamma-ray traverse it andreach the LXe gamma-ray detector placed outside the magnet. To meet the first requirement, a thin wall superconducting magnet was developed (Fig.3.9, Fig.3.10).
The step structure solenoid is composed of five coils with three diﬀerent radii andgenerates a gradient magnetic field as can be seen in Fig.3.11.

Fig. 3.9: Layout of the COBRA

Fig. 3.10: Picture of the COBRA

CHAPTER 3. MEG II EXPERIMENT

20

Fig. 3.11: COBRA z-direction magnetic field
The gradient magnetic field enables positrons with the same momentum to have
trajectories with a constant projected bending radius regardless of their emission
angles, as seen in Fig3.12(d). Thanks to this property, the positrons can be sweptout more quickly from the sensitive detector volume, as can be seen in Fig.3.12(a)and Fig3.12(c). Accordingly, the drift chambers can be positioned in a narrow
radial region corresponding to high momentum positrons only, and it leads to a
reduced positron hit rate without losing signal eﬃciency.

Fig. 3.12: Concept of COnstant projected Bending RAdius spectrometer. (a) and(b) show trajectories of positrons with a uniform magnetic field. (c) and (d) are theones with COBRA.

CHAPTER 3. MEG II EXPERIMENT

21

Drift Chambers
A widespread device used to measure a charged particle track is the drift chamber.
In the MEG, positron trajectories are measured with sixteen drift chambers as canbe seen in Fig.3.13 and Fig.3.14. Because the tracking ability is limited by multiple
coulomb scattering, the gas filled inside the chambers are a light gas mixture ofhelium and ethane (1:1). The low-mass chamber (∼ 1.7 × 10−3 X0 ) is also helpful
for suppressing background gamma production from positron annihilation.

Fig. 3.13: Schematic view of drift
chambers

Fig. 3.14: Picture of drift chambers

Each chamber has a two-layer structure and the drift cells are staggered (Fig.3.15).
The chamber walls are made of 12.5 µm polyimide films with aluminum deposition
and they work as cathodes. The cathode of the cell consists of four strips with a zigzag pattern (Fig.3.16). Since the anode wires are all mounted along the z-direction,
the positron’s hit z-position is reconstructed using the charge information. While
the z-position is roughly estimated by the charge observed at both ends of the anode wire, a more precise z-position reconstruction is provided by the ratio of theinduced charges of the four strips.

Fig. 3.15: Sectional view of a drift chamber

CHAPTER 3. MEG II EXPERIMENT

22

Fig. 3.16: Concept of positron z-reconstruction
Timing Counters
While the drift chamber shows a good performance as a tracking device, it does nothave enough time resolution for the MEG experiment, that is why there is an additional detector for timing measurements, the so-called timing counter system. It
consists of 15 plastic scintillation bars and 128 plastic scintillation fibers as shownin Fig.3.17 and Fig.3.18. The former are used for precise time measurement of ∼50
psec, and the latter are used for z-position measurement of the positron hit. Twosets of the timing counter systems are installed at the upstream and downstreamsides of the drift chambers.

Fig. 3.17: Plastic scintillation bars
for time measurement.

Fig. 3.18: Plastic scintillation fibers
for z-position measurement

CHAPTER 3. MEG II EXPERIMENT

23

3.1.4 Gamma-ray Detector
As with the positron spectrometer, a gamma-ray detector plays an important rolefor µ+ → e+ γ detection. One of main features of the MEG experiment is that over
800 litter liquid xenon is used as the scintillation detector. The scintillation light isread out by photomultiplier tubes.
Liquid Xenon
The basic properties of liquid xenon is summarized in the following table.
Table 3.1: Properties of liquid xenon
Atomic Number
54
Atomic Weight
131
Density at 161.35 K
2.978 g/cm3
Radiation length
2.77 cm
Critical Energy
10.5 MeV
Mollier radius
4.1 cmBoiling point
165 K
Melting point
16 1K
wavelength
178±14 nm W ph for electrons 21.6 eV [10]Decay time (recombination)
45 nsec [11]Decay time (fast components) 4.2 nsec [11]Decay time (slow components) 22 nsec [11]Absorption length>100 cm
scatter length
29 cm to 50 cm

As a scintillation medium, liquid xenon has good characteristics:• high light yield• fast decay time
• short radiation length• uniformity
Thanks to these advantages, a compact gamma-ray detector which has good energy
and time resolutions can be realized.

CHAPTER 3. MEG II EXPERIMENT

24

When a particle passes through liquid xenon, Xe∗ (excited state) or Xe+ (ion) is
produced. In either case, the scintillation process is considered to be de-excitationof dimers (Xe∗2 ). In the ion case, the process is
Xe∗ + Xe → Xe+2
Xe∗2 → 2Xe + hν

(3.3)
(3.4)

On the other hand, the ion emits photon through following process:Xe+ + Xe∗ → Xe+2
Xe+2 + e− → Xe∗∗ + Xe
Xe∗∗ → Xe∗ + heatXe∗ + Xe + Xe → Xe∗2 + Xe
Xe∗2 → 2Xe + hν

(3.5)
(3.6)
(3.7)
(3.8)
(3.9)

The feature of the photons from the emission processes is useful to achieve a goodenergy resolution because photons from Xe∗2 cannot be absorbed by Xe.
Photomultiplier tube
Since the scintillation light of liquid xenon has a peak at 175 nm [12], the photomultiplier tubes must be sensitive to the vacuum ultraviolet. The MEG collaboration and Hamamatsu Photonics developed a 2-inch VUV-sensitive photomultiplier,R9869 (Fig.3.19), which has a quartz window instead of normal glass one. Thanks
to the new type of window, 80 % of the scintillation light can pass.

Fig. 3.19: R9869
Detector design
Over 800 litter liquid xenon fill a C-shaped cryostat shown in Fig.3.20. 846 photomultiplier tubes surround the active volume and they collect the scintillation light.
It covers approximately 10 % of the solid angle seen by the muon stopping target.

CHAPTER 3. MEG II EXPERIMENT

Fig. 3.20: Side view of the liquid
xenon detector

25

Fig. 3.21: Top view of the liquid
xenon detector

3.1.5 Data Acquisition System
The MEG experiment records the waveforms from all the detectors to obtain the
precise information such as the charge, the time, the baseline, and the pileup witha fast digitizer called Domino Ring Sampler (DRS) [13]. The DRS has been developed at the PSI and the latest version is called "DRS4". Each DRS4 chip (Fig.3.22)
has eight data readout channels.

Fig. 3.22: DRS chipFig. 3.23: DRS4 evaluation board [14]

CHAPTER 3. MEG II EXPERIMENT

26

Fig.3.24 shows the principle of the DRS. Each channel of the DRS has 1024 capacitive sampling cells. The sampling speed is determined by the series-connected
inverters and the highest speed is 5 GHz. Due to the rotating signal, the sampling
frequency continuously runs on the inverter chain and it is called "domino wave".
When an external trigger signal comes, the domino wave stops and the waveform
stored is read out with a shift resister at lower frequency and digitized by an external
12 bit FADC .

Fig. 3.24: Conceptual view of the DRS principle
The data was acquired with a sampling speed of 1.6GHz for the liquid xenon
detector and timing counter and 0.8GHz for the drift chambers.

3.2 Upgrade Experiment: MEG II
We achieved a sensitivity of 7.7 × 10−13 with the first half of the data in the MEG
experiment. To achieve a higher sensitivity, more statistic is necessary. However,
we do not expect a significant improvement by just increasing the statistics because
of increasing background events in the signal region. Accordingly, we will require• higher intensity of muon beam
• detectors able to operate with the increased hit rate
• detectors with higher energy, time, angular resolutions
for the upgrade experiment, MEG II.

3.2.1 Key Points in the Upgrade
The improvement in the upgrade is summarized as follows (numbers refer to thenumbers shown in Fig.3.25)• the increased number of stopping muons on the target (1)CHAPTER 3. MEG II EXPERIMENT

27• the thinner stopping target (2)• a new positron tracker with smaller radiation length, larger acceptance andhigher resolutions (3,4)• the gamma-ray detector with with higher resolutions and a better detection
eﬃciency (6,7)• active tagging of background events (8)• integration of splitter, trigger and DAQ

Fig. 3.25: Concept of the upgradeFig.3.26 illustrates the schematic view of the MEG II experimental apparatus.

CHAPTER 3. MEG II EXPERIMENT

28

Fig. 3.26: MEG II experimental apparatus

3.2.2µ+ Beam and Target

A thinner target with a smaller slant angle (15◦ ) with respect to the muon beam
will be used to minimize the positron multiple scattering and the generation of the
background-gamma and the muon stopping rate will be 7 × 107 muons/sec. Since
there was a target deformation in the MEG experiment, the study on the target
material is ongoing. The candidates are polyethylene (140 µm), beryllium (90µm) and scintillator film (130 µm). Fig.3.27 shows MEG II muon stopping target
prototype, used in the pre-engineering run 2015.

Fig. 3.27: MEG II muon stopping target prototype

CHAPTER 3. MEG II EXPERIMENT

29

3.2.3 The Positron Spectrometer
Drift chamber
The MEG II will use a completely new single volume drift chamber. It has 1200
wires with 8◦ stereo angles (Fig.3.28). The expected resolutions are σ Ee+ of 130
keV, σθ of 5.3 mrad and σϕ of 3.7 mrad. The chamber is designed such that it islong enough for positrons not to hit massive materials and stop there before entering
the timing counters. As a result, the positron eﬃciency will be improved from 40% to 90 %. The mockup chamber is shown in Fig.3.29.

Fig. 3.28: One of the hyperbolic meshground planes

Fig. 3.29: Drift chamber mockup

Timing counter
Although the time resolution in the MEG timing counter was expected to be ∼ 40
psec, a worse resolution of 65 psec was achieved because of the actual experimental
conditions such as the electronics time jitter. The timing counter in the upgrade
experiment consists of 512 pixelated counters with multiple SiPM readout. The
time resolution of a single counter of ∼ 75 psec is achieved. Fig.3.30 and Fig.3.31
are the pictures of the single counter.

CHAPTER 3. MEG II EXPERIMENT

30

Fig. 3.30: Picture of a single counterFig. 3.31: Six SiPMs are series connected on a PCB and attached on bothends of a counter

A single positron hits several counters and the overall timing resolution√ improves depending on the number of hit counters (Nhit ) approximately as 1/ (Nhit ).
From simulations, the average Nhit is estimated to be about nine with the optimized
counter layout. In the beam test, an excellent timing resolution of 30 psec wasachieved using multiple counter hits. Fig.3.32 shows 128 counters which werebeing installed to the inside of the COBRA magnet.

Fig. 3.32: Half of the downstream detector (128 coutners)

CHAPTER 3. MEG II EXPERIMENT

31

3.2.4 The Gamma-ray Detector
The performance of the MEG gamma ray detector is limited by the non-uniformityof photomultipliers coverage as shown in Fig.3.33. Especially the events with ashallow interaction point show bad resolutions.

Fig. 3.33: Position resolutions as a function of the first photon conversion depth.
The red markers are resolutions in the MEG experiment. The blue ones are expected resolutions in the MEG II experiment.
To improve the uniformity, 246 photomultipliers on the gamma entrance window will be replaced with 4092 SiPMs in the upgrade experiment. The number of
the SiPMs are determined by the area of the gamma-ray entrance window.

Fig. 3.34: Gamma-ray entrance window in the MEG experimentFig. 3.35: Gamma-ray entrance window in the MEG II experiment (CG)The MPPCs should be sensitive to the vacuum ultraviolet as the photomultipliers, and have fast responses. The MEG II collaboration and Hamamatsu Photonics
have developed MPPCs which have a high photon detection eﬃciency (∼20 %) forCHAPTER 3. MEG II EXPERIMENT

32

liquid xenon scintillation light and a fast decay time (∼ 50 nsec). Fig.3.36 shows
the MPPC. It has four independent sensor chips (6×6 mm2 ) and the chips are connected in series to reduce the capacitance. The basic properties of SiPMs will begiven in the following chapters.

(a) Picture of the MPPC(b) Schematic view of the MPPC

Fig. 3.36: Ultraviolet sensitive MPPC (S10943-3186)
In Fig.3.37, event displays of pileup photons in the MEG and the MEG II
gamma detector are shown. Thanks to the higher granularity of scintillation readout, the imaging power is improved and two photons are identified eﬀectively in the
upgraded detector. Accordingly, background photons from high energy positronannihilation will be reduced.

CHAPTER 3. MEG II EXPERIMENT(a) Scintillation light distributions in the
MEG liquid xenon detector

33(b) Scintillation light distributions in the
MEG II liquid xenon detector

Fig. 3.37: Collected scintillation light distribution of pileup photon events

3.2.5 Background Tagging Detector
In the MEG II experiment, a new detector for active background tagging will be
introduced. The details are discussed in the next chapter.

3.2.6 New DAQ board
The MEG II experiment needs more DAQ and trigger channels. In addition to thespace problem, the new drift chamber adopts the timing technique which requires
the higher bandwidth. To fulfill the requirements, a new DAQ board called WaveDREAM was proposed.
The basic idea of the WaveDREAM is to merge the waveform digitizing technology with the trigger while improving the bandwidth. To realize the better bandwidth, a new front-end was designed and tested as can be seen in Fig.3.38, Fig.3.39
and Fig.3.40. In addition to them, the SiPM power supply is also included in the
WaveDREAM.

CHAPTER 3. MEG II EXPERIMENT

34

Fig. 3.38: Simplified scheme of the DAQ board

Fig. 3.39: The WaveDREAM board

Fig. 3.40: Crate to place the WaveDREAM boardChapter 4
Radiative Decay Counter (RDC)The MEG II experiment will use a twice µ+ beam rate and the influence of theaccidental background will be more serious. To further improve the ultimate sensitivity of MEG II, we will introduce a new detector, the "Radiative Decay Counter(RDC)", which actively tags the accidental background events. The basic idea andthe detector design will be discussed in this chapter.

4.1 Mechanism of background identification
As explained before, the dominant background in µ+ → e+ γ search are the accidental background. The background gamma-ray mainly comes from a Radiative Muon
Decay (RMD: µ+ → e+ νe ν µ γ) or a positron Annihilation In Flight (AIF) as can beseen in Fig.4.1. The rate of the AIF gets smaller thanks to the low mass tracker inthe MEG II. However, the amount of the gamma-ray from the RMD will not benefit
from the improved detector because the amount of background gamma-rays is determined by the branching fraction of RMD. Therefore, an active identification of
background gamma rays from RMD is needed to further improve the experiment’s
sensitivity.

Fig. 4.1: The amount of background gamma rays (Eγ > 48 MeV) in the MEG experiment and the MEG II experiment.
35

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)

36

When a high energy gamma ray (> 48 MeV) is emitted from an RMD, a low
momentum positron (2-5 MeV typically) is emitted with a high probability. Such
low momentum positrons have a so small bending radius that they are out of theacceptance of the MEG tracker. By installing a new positron detector, RDC, on
the central axis of the solenoid, an identification of background gamma rays from
RMDs is possible (Fig.4.2 and Fig.4.3). The RDC will identify 54 % of the total
background gamma rays and will improve the sensitivity by 28 %.

Fig. 4.2: An example of a background event identified by RDC

Fig. 4.3: Concept of the RDC
The RDC measures the positron hit time by plastic scintillation detectors witha fast response and checks time coincidence with the gamma ray measured in theliquid Xenon detector. As can be seen in Fig.4.4, the time diﬀerence between thepositron’s hit in the RDC and the gamma-ray’s hit in the liquid Xenon detector hasa peak with a FWHM of ∼ 6 nsec due to the positron’s time-of-flight from the muon

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)37

stopping target to the RDC. Therefore, the time resolution of 100 psec which has
already been obtained in the lab test is enough for the RMD detection.

Fig. 4.4: The time diﬀerence between the positron’s hit in the downstream RDC andthe gamma-ray’s hit in the liquid Xenon detector (red line). The time resolution of
100 psec is included. The black line is the accidental positron hit from Michel
decay.
In addition to the hit time, the downstream RDC will be able to measure theenergy deposit. Positrons which hit the RDC come not only from RMD but also
from Michel decay. The positrons from Michel decay have a larger momentum(Fig.4.5). Hence, the signal-to-background ratio in the RMD-tagging is improvedby measuring the positron energy deposit.

Fig. 4.5: Energy deposit in the downstream RDC. Blue: positrons from Michel
decay. Red: positrons from RMD with > 48 MeV energy gamma ray.

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)38

4.2 Simulation Study on Sensitivity Improvement
The sensitivity dependent on the time and energy resolutions of the RDC was estimated. Here, the experiment sensitivity is defined as the median upper limit ofan ensemble of pseudo experiments with the background-only hypothesis. Fig.4.6
illustrates the distribution of Nsig upper limit at 90 % C.L. where Nsig is the numberof signal events.

Fig. 4.6: Distribution of the upper limit of the 7999 pseudo experiments. The time
resolution of the upstream, the one of the downstream, and the energy resolution
are assumed to be 500 psec, 100 psec, and 8 % respectively.
The downstream RDC with the time resolution of 100 psec and the energy
resolution of 8 % improves the sensitivity by 16 %. Fig.4.7 shows the expected
sensitivity as a function of the RDC downstream resolution. The improvement due
to the upstream RDC which has the time resolutions of 500 psec is included. As
can be seen in the figures, the sensitivity is flat around time resolution of 100 psecor the energy resolution of 8 % which were already obtained in the lab test.

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)(a) Sensitivity as a function of the time resolution of the downstream RDC

39(b) Sensitivity as a function of the energy
resolution of the downstream RDC

Fig. 4.7: The sensitivity as a function of the downstream RDC performance. The
blue bands are the sensitivity of the MEG II experiment without the RDC

4.3 Detector Design
Since the RDC will be set on the central axis of the solenoid (or the µ+ beam axis),
it should satisfy these requirements:• compact enough to be placed in the limited space• operational in the magnetic field• cope with MHz order positron hits
• a high eﬃciency• does not generate background gamma rays
To meet the first and the second requirement, SiPMs are used for scintillation lightcollection because they are compact and not aﬀected by magnetic fields.
The RDC consists of the beam upstream and downstream parts. The detector
radius is about 9 cm, which is large enough to detect most of the low momentum
positrons from RMD when the associated γ-ray has an energy above 48 MeV. The
upstream and downstream parts will be placed on both COBRA endcaps (Fig.4.8).

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)

40

Fig. 4.8: Schematic view of MEG II detector with the full RDC. The RDC detectors
are placed inside both COBRA endcaps.

4.3.1 Upstream RDC
To minimize the eﬀect on the µ+ transport, the upstream detector is designed to bethin. It is made of 704 squared scintillation fibers with 250 µm thickness as shownin Fig.4.9.

Fig. 4.9: Upstream RDC design
The scintillation fibers will be glued for the detector to be flat and they will bebundled at both ends and read out by SiPMs.
For the prototype, fibers with a thickness of 265-270 µm were glued (Fig.4.10).
A plastic mylar film with 12 µm thickness will be attached as support structure.

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)41

The ineﬃciency of the detector, which will aﬀect the RMD performance, is causedby the SiPM recovery time, the fiber clad and the low light yield of the fiber. For
example, the ineﬃciency due to the recovery time of 50 nsec can be estimated to
be about 3 % for the individual fiber placed around the center where the hit rate is
0.6 MHz. The detailed study of these eﬀects is ongoing.

(a) 64 fibers are bundled and bent by 90
degrees due to space limitation.

(b) Picture of the glued fiber surface
taken with a microscope

Fig. 4.10: Upstream RDC prototype

4.3.2 Downstream RDC
Since the downstream detector has nothing to do with the muon beam transportation, a more massive detector can be mounted in the downstream part. The downstream RDC consists of 12 plastic scintillator bars for the hit time measurement
and 76 LYSO crystals for the energy deposit observation (Fig.4.11 and Fig.4.12).
The plastic scintillator bars have a thickness of 5 mm and the their length ranges
from 7 to 19 cm. The width of the bars is 1 or 2 cm. The bars placed near the beamaxis have the smaller width to cope with the high hit rate. The size of the LYSO
crystal is 2×2×2 cm3 . MPPCs are used for the scintillation light readout.

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)

42

Fig. 4.11: Downstream RDC design(a) Plastic scintillator counters(b) LYSO countersFig. 4.12: (a) Timing Counter part and (b) Calorimeter Part of the Downstream
RDC

CHAPTER 4. RADIATIVE DECAY COUNTER (RDC)

43

4.4 Beam Test with a Prototype
4.4.1 Downstream partWe demonstrated in a beam test using a prototype detector that RMD events canbe identified by the RDC. A clear RMD time peak was observed in the RDCliquid xenon coincidence trigger. After getting the results, the installation of thedownstream part was approved by the MEG II collaboration and the construction
has started.

4.4.2 Upstream part
The eﬀect on the beam properties due to the upstream RDC was simulated. The
muon stopping eﬃciency is decreased by 3 % (from 71.9 % to 69.8 %) with the
RDC. With regards to the beam spread, sigma X increases by 7 % (from 1.11 cm
to 1.19 cm) and sigma Y by 5.9 % (from 1.07 cm to 1.13 cm).
The investigation of the eﬀect with the muon beam using a mockup detector
is still ongoing and the MEG II collaboration will decide whether it should be accepted or not based on the result. Preliminary results show that the eﬀects on the
beam are small.

Chapter 5
Downstream Counter Optimization
5.1 Scintillator Selection
Since the RDC is placed on the µ+ beam axis and the emitted positrons spiral out
in the beam axis direction due to the COBRA magnet, the positron hit rate in the
RDC is high. The detector should be optimized to have a fast response to cope withthe positron rate.

5.1.1 Plastic Scintillator
Plastic scintillators are the most widely used particle detection device. They havean excellently fast response of O(nsec) and are suitable for the timing counter devices. For small-size scintillators, BC-418 and BC-422 from Saint-Gobain (Table
5.1) are the best candidates.
Table 5.1: Basic Properties of BC-418 & BC-422 [15]Light Output, %Anthracene
Rise Time, (ns)Decay Time (ns)
Pulse width, FWHM, (ns)Wavelength of Max. Emission, nm

BC-418
67
0.5
1.4
1.2
391

BC-422
55
0.35
1.6
1.3
370

The time resolution of the small plastic scintillators largely depends on the number of detected photons. In addition to the light yield, the detection eﬃciency forthe scintillation light is also important. MPPCs, used for readout as will be described later, have a peak in the detection eﬃciency around 450 nm (Fig.5.2), so
BC-418 is advantageous as can be seen in Table 5.1 and Fig.5.1.
44

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

45

Fig. 5.1: Emission spectrum of BC-418 & BC-420 [15]Fig. 5.2: Photon detection eﬃciency vs wavelength for S13360 series MPPC [23]Considering the properties, we decided to use BC-418 for the RDC timing
counter part.

5.1.2 Inorganic Scintillator
In order to achieve a good energy resolution, high photon statistics are necessary.
For the RDC detector calorimeter part, the scintillator should have a fast decay
time to reduce pileup events. From these points of view, GSO and LYSO are goodcandidates. Table. 5.2 shows basic properties on various inorganic scintillators.

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

46

Table 5.2: Inorganic Scintillator Properties
Inorganic scintillator
Density (g/cm3 )Attenuation length (cm)Decay time (ns)Wave length (nm)
Relative light yield

NaI BGO
3.67 7.13
2.6 1.11
230 300
415 480
100 7-12

LSO GSO LYSO
7.40
6.71
7.1
1.14
1.38
1.12
40
30-60
41
420
430
420
40-75
20
70-80

5.1.3 Self-radiation as energy calibration source
Lutetium yttrium orthosilicate (LYSO) contains the radioactive isotope 176 Lu. The
abundance of 176 Lu in Lutetium is 2.6 % and it has a long life time of 3.6 × 1010
years as can be seen in Fig.5.3. We measured the rate of the self-radiation with

Fig. 5.3: Decay scheme of
176 Lu to 176 Hf

Fig. 5.4: The energy spectrum of LYSO selfradiation [20]a prototype detector and checked if it can be used for the energy calibration. The
setup is the following Fig.5.5:

Fig. 5.5: Setup for LYSO self-radiation rate measurement

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION47

The rate was measured with a scaler. In parallel, waveforms were aquired with
the DRS to know the energy. The discriminator and the DRS have a common
threshold level. The measured rate was 2.1 kHz for energies > 250 keV as shownin Fig.5.6. The rate is large enough for the self-radiation to be used as calibration
source.

Fig. 5.6: Measured energy spectrum of the LYSO self-radiation
On the other hand, the self-radiation can be a background source through theaccidental pileup. Here we consider the pileup ratio due to the self-radiation. Assuming that the pulse length ∆T is 500 nsec, the fraction of the pileup events dueto the self-radiation f sel f = 2kHz is roughly estimated to be:
2 × ∆T × f sel f = 10−3

(5.1)

This small value will not aﬀect the performance significantly. Considering theresult and the high light yield, we decided to use LYSO.

5.2 SiPM Selection
One type of popular photon sensitive devices in particle physics experiments is
Silicon Photomultipliers (SiPM). SiPMs have several advantages:• high photon detection eﬃciency• high gain
• good time resolution• compact

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

48• insensitive to magnetic fields
For the RDC construction, Multi-Pixel Photon Counters (MPPCs) developed
by HAMAMATSU Photonics are used. We have studied several MPPC models
and optimized the RDC. In this section basic properties of the SiPMs are describedfirst and the optimization process will be discussed.

5.2.1 Basic Properties of SiPMs
A SiPM is a multi-pixelated avalanche photodiode operating in "Geiger mode".
The Geiger mode is a condition where the reverse voltage applied on a SiPM ishigher than the breakdown voltage and even weak light can cause a discharge.
The basic idea of SiPMs is to precisely count the number of incoming photons.
Structure & Principle
If an incident photon is absorbed, an electron-hole pair is generated (Fig.5.7 andFig.5.8). The electron drifts in the depletion layer and is accelerated by a highelectric field of O(105 ) V/cm. The electron acquires suﬃcient energy to produce
further electron-hole pairs through impact ionization and triggers the avalanche.
This avalanche stops when the current flows through the resistor which is attachedon each cell ("pixel") and the voltage drops below the breakdown voltage.
Photon Detection Eﬃciency
The absorption of incident photons and the generation of photoelectrons are important processes since they determine the photon detector eﬃciency. It is expressedas Photon Detection Eﬃciency (PDE) and the PDE is defined as
PDF = ϵ × QE × Ptrigger(5.2)

where ϵ is the fill factor (the fraction of the active area), QE is the quantum eﬃciency (probability for the photon entering the active area to create a electron-holepair) and Ptrigger is the probability for the carrier to trigger avalanche.
Gain
Thanks to the avalanche process of the carrier in the electric field, the amount ofpulse charge is large enough to be measured. The degree of the multiplication isexpressed as the gain. The gain is defined as
G=

Q
e

(5.3)

where Q is the pulse charge and e is the elementary charge. The gain is roughly
proportional the pixel size which corresponds to the capacitance of each cell.

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

49

Fig. 5.7: Surface of the SiPM

Fig. 5.8: SiPM structure
Crosstalk and After-pulsing
A triggered pixel can induce an avalanche in neighboring pixels and this phenomenon is called crosstalk. A carrier from the primary avalanche can be trapped
and released after quenching. Since this delayed release can trigger another avalanche,
it is called after-pulsing. These parasitic avalanches are the limitation factor the
SiPM performance.

(a)

(b)

Fig. 5.9: Conceptual picture of the crosstalk and the after-pulse [21]

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

50

5.2.2 Energy Resolution
To distinguish the RMD positron from the Michel decay positron, a calorimeterwith a good energy resolution is needed. While the energy resolution is determined
mainly by the crystal light yield, the properties of photon sensors can also aﬀect
the performance. The overvoltage dependence of the energy resolution was studied
for four types of MPPCs [22] as shown in Table. 5.3.
Table 5.3: Four types of MPPC used for the test. They have a 3 × 3 mm2 active area
MPPC type
S12572-015P
S12572-025P
S12572-050P
S13360-3050PE

pixel size µm number of pixels fill factor %
15
40000
53
25
14400
65
50
3600
62
50
3600
74

CT suppress
without
without
without
with

For the study, 60 Co is used as a gamma source and an MPPC is attached to a
LYSO crystal whose size is 2 × 2 × 2 cm3 . Optical grease is used for the coupling.
The data is acquired by self-triggered condition and contains the eﬀect of the 176 Lu
decay (see Fig.5.1.3).

Fig. 5.10: Setup to measure the energy resolution
In Fig.5.11, an example of the observed energy spectrum is shown. The photopeaks are fitted with gaussians. Here we define the energy resolution R as
R=

σ
Eγ

where Eγ and σ are the mean and the width of the photo-peak respectively.

(5.4)

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

51

Fig. 5.11: Observed 60 Co and self-radiation energy spectrum. The two photo-peaks
at 1.17 MeV and 1.33 MeV are fitted with gaussian functions. The backgroundfloor is fitted with an exponential function.
The measured energy resolution for each kind of MPPC is shown in Fig.5.12.
As the applied voltage increases, the energy resolution gets better thanks to thehigher PDE. However, the optical crosstalk probability also gets large at high overvoltage and it limits the improvement due to the fluctuation of the number of thefired pixels.

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

(a)

(b)

(c)

(d)

52

Fig. 5.12: Energy resolution as a function of the overvoltage for four types of
MPPCs
The required energy resolution is 8 % at 1 MeV and all four types of MPPCs
satisfy the condition. We have chosen the MPPC considering other factors.
S12572-050P and S13360-3050PE have the largest pixel size and the waveform
for high energy particles can be deformed due to the saturation of the pixels. The
deformation can make it diﬃcult to separate piled up pulses by waveform analysis. S12572-015P has the smallest gain due to the pixel size and shows the worst
signal-to-noise ratio. This can be a problem when the MPPC accumulates radiation
damage and the noise level gets worse.
Therefore, S12572-025P has a good balance and we decided to use this type forthe calorimeter part.

5.3 Estimation of the Current through the MPPC
For the HV power supply, the current limit is one of the important factors to determine the operation range. While the current limit of the HV power supply for the
MEG II experiment is 100 µA by default, the RDC requires a larger current limit
due to the high positron hit rate. Therefore, the estimation of the current throughCHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

53

the MPPCs is necessary.
First, the positron average energy deposit and hit rate for all crystals were estimated by a simulation. Fig.5.13 shows the energy deposit per seconds in the
crystals.

Fig. 5.13: Average positron energy deposit times hit rate in each crystal [GeV/sec]
The energy deposit in the crystals around beam axis is large (∼5 TeV/sec) because of the MHz order hit rate.
Second, the charge value at the 1.13 MeV photo-peak of 60 Co measured withan MPPC coupled to a LYSO crystal was calculated (Fig.5.14). The current foreach crystal was estimated as (the charge) / (1.13 MeV) × (mean energy deposit)× (hit rate). The current ranges from 80 µA to 400 µA. The overvoltage is set toabout 5 V which is larger than the actual operation voltage and the current should
be smaller than in this test’s result.
Third, as a cross check, we measured the current increase due to exposure to
90 Sr and the corresponding hit rate (Fig.5.15). The voltage is supplied and thecurrent is measured by a picoammeter. From the dose ratio of measured and simulated hits, the current in the actual run is estimated to be 80 µA to 400 µA and isconsistent with the 60 Co result.

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

Fig. 5.14: Charge distribution of 60 Co
and the self-radiation from LYSO.

54

Fig. 5.15: Measured spectrum of theradiation from 90 Sr

As a result, the HV power supply for the RDC should reach 1 mA instead of
100 µA and it will be developed to meet this requirement.

5.3.1 Afterglow
When testing the crystal properties, the current through the attached MPPC significantly fluctuated for some crystals and it seemed that weak light from the crystal
reached the MPPC even if it was not exposed to a radioactive source (Fig.5.16).
Also, the current increased after leaving the crystal in room light. After exposing
the crystal to the room light over 10 hours, we measured the current as a functionof time for the crystal with the highest current and it showed an exponential decay
with a time constant of ∼ 90 minutes (Fig.5.17). It turned out that it was causedby an afterglow of LYSO crystal. The aferglow may aﬀect the detector’s performance such as the energy resolution due to the photon number fluctuation and weare currently studying the eﬀect.

(a)

(b)

Fig. 5.16: Waveforms of photons (a) from the crystal with small afterglow and(b) from the crystal with large afterglow. The significant number of photons wereobserved even if the crystals were not exposed to the radioactive source.

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

55

Fig. 5.17: The current through the MPPC attached to the LYSO crystal with thelargest afterglow was monitored for 3 hours with a Picoammeter, after exposing
the crystal to the room light over 10 hours. The light-shielding started at time= 0.
The overvoltage applied to the MPPC was about 4 V.

5.4 MPPC Grouping for the Plastic Scintillators
Since the light yield of plastic scintillators is small, the gain of the attached MPPCs
should be large to obtain the suﬃciently good signal-to-noise ratio. Therefore, thetype of S13360-3050PE which has a 50×50 µm2 pixel size and a 3×3 mm2 sensor
area with the crosstalk suppression was chosen for the plastic scintillators.
In addition to the area covered by the attached MPPCs, the connection of the
MPPCs also aﬀects the counter performance because the pulse shape, especially thetail, depends on the sensor capacitance. In order to realize a good time resolution,
the series connection is the best way because of the smallest capacitance. Fig.5.18
illustrates the model circuit of the SiPMs connected in series where only one pixelis activated, the bias circuit for the SiPMs and a signal amplifier circuit. As can beseen in Fig.5.19, the signal waveform of the series-connected SiPMs gets sharper
than the waveform of the single SiPM.

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

56

Fig. 5.18: Circuit model of series-connected SiPMs. Each gray area illustrates each
SiPM. The red and the blue regions correspond to a fired pixel and inactive pixels
respectively, that is, only one pixel in SiPM1 is fired in this figure.

(a)

(b)

Fig. 5.19: Single photon waveforms simulated with SPICE (Simulation Programwith Integrated Circuit Emphasis). (a) only one SiPM is placed in the circuit. (b)three SiPMs are connected in series as shown in Fig.5.18. This simulation was
conducted by Kohei Yoshida with parameters of AdvanSiD [26] which is one typeof SiPMs.
For the proper operation of the series-connected MPPCs, the MPPCs in thesame connection should have similar properties. Especially the shape of the I-V
curve should be the same for all MPPCs in series. In order to group the MPPCs,

CHAPTER 5. DOWNSTREAM COUNTER OPTIMIZATION

57

the I-V curves of all of MPPCs were measured as can be seen in Fig5.20 and 5.21.
Since the current through the MPPCs are common in the series connected circuit,the overvoltages for the MPPCs which have similar shapes of the I-V curve areautomatically adjusted to the same values. Hence, the MPPCs with similar I-V
curves are selected to be used in the same connection.

Fig. 5.20: Setup for the I-V curvemeasurement.

Fig. 5.21: I-V curves of three MPPCs
in the same group. The plot of diﬀerent MPPCs is drawn in diﬀerent colors.

To measure the I-V curve, the applied voltage ranges from 48 V to 62 V insteps of 0.25 V. The current through the MPPC is automatically measured using
a picoammeter with a labview software program. To reduce the fluctuation, thecurrent is obtained 30 times at each voltage point and the corresponding average isused as the representative. The temperature is controlled by a thermostat chamber.
The overvoltage applied to the MPPCs for the operation will be about 2.5 V.
The MPPCs were sorted according to the current at 2.5 V and the MPPCs weregrouped according to the order obtained by the sorting procedure.

Chapter 6Counter Construction
Since the MEG II experiment aims at starting data taking in 2016, completing thedetectors is one of the most important tasks. As mentioned above, the installation of
the downstream RDC was approved by the MEG collaboration and the production
has started. In this chapter, the construction of the plastic scintillator and the LYSO
part will be separately discussed.

6.1 Timing Counter Part
The timing counter part of the RDC has 12 plastic scintillator bars and 48 MPPCs
for the readout. To increase the acceptance for the scintillation light, several MPPCs are attached on one the scintillator plate. The number of attached MPPCs is
two for a 1 cm wide scintillator and three for the 2 cm wide one as can be seen in
Fig.6.1.

Fig. 6.1: Enlarged figure of Fig.4.12(a). Components are illustrated.
58

CHAPTER 6. COUNTER CONSTRUCTION

59

6.1.1 PCB Layout
The Print Circuit Boards (PCBs) are designed to have multi-layers to be compact
and to suppress pick-up noise. The signal and ground lines are on diﬀerent layers.
In addition to the two layers, copper layers are set on the both sides for shielding(Fig.6.2). The short and long PCBs are shown in Fig.6.3 and Fig.6.7.

Fig. 6.2: Design of the short PCB

Fig. 6.3: Twelve short PCBs
In our experience, connecting the ground of a MPPC with the chassis groundis eﬀective on reducing noise. In order to avoid slow noise due to ground loops, acapacitor can be inserted between the grounds. The grounding via the capacitor canbe used as an option during the experiment depending on the actual noise condition(Fig.6.4, 6.5).

CHAPTER 6. COUNTER CONSTRUCTION

Fig. 6.4: The connection of the chassis ground and the MPPC ground. A
capacitor is inserted between them.

60

Fig. 6.5: A capacitor attached on the
PCB

While the short PCB hosts the three MPPCs for one wide scintillator bar, six
MPPCs for three narrow scintillators are put on the long PCB because of the space
limitation as can be seen in Fig.6.6. In order to avoid the optical crosstalk among
the scintillator bars, a light shielding film is inserted between the counters.

Fig. 6.6: Three scintillation bars with six MPPCs attached to the long PCB

Fig. 6.7: Four long PCBs

6.1.2 AssemblySome types of SiPMs are known to be damaged by heating. For a safer treatment, the MPPCs are glued on the PCBs with conductive epoxy CW2400 (Fig.6.8),
whose properties are given in Table.6.1.

CHAPTER 6. COUNTER CONSTRUCTION

61

Table 6.1: Properties of CW2400 [24]Volume Resistivity
Operation Temperature
Pot time
Typical thickness<0,001 Ohm-cm-91 to 100 ◦ C
8-10 min at 25 ◦ C
5 mil = 127 µm

Fig. 6.8: Picture of the conductive epoxy CW2400
For the MPPC alignment, polyoxymethylene and acrylic jigs were prepared asshown in Fig.6.9(a) and Fig.6.9(b). The acrylic parts are oil coated to avoid glue
adhering (Fig.6.9(c)).

(a)

(b)

(c)

Fig. 6.9: MPPC alignment jig
MPPCs on the PCB and the plastic scintillators are coupled with optical cement(Fig.6.10). The counter is covered with aluminized mylar as a reflector and a black
sheet for light shielding.

CHAPTER 6. COUNTER CONSTRUCTION

62

Fig. 6.10: The shortest counter.

6.1.3 Time resolution
The performance for each counter was checked with the following setup: The
counter was irradiated by beta-rays from 90 Sr and the trigger condition is a hit
in a reference counter with time resolution of 30 psec (Fig.6.11). The scintillator size of the reference counter is 5 × 5 × 4 mm3 and a MPPC is attached forthe readout. Here, the time resolution is defined as the sigma of the distribution∆T = (T0 + T1 )/2 − TRC where Ti and TRC are the times measured by the SiPMs
attached to the scintillator and by the reference counter respectively (Fig.6.12). All
counter showed a time resolution of less than 90 psec which does not include thetime resolution of the reference counter.
The position dependence of the time resolution was studied for four counterswith diﬀerent lengths as can be seen in Fig.6.13.

CHAPTER 6. COUNTER CONSTRUCTION

Fig. 6.11: Setup for the performance
test of the scintillator bar counters63

Fig. 6.12: Measured ∆T distribution

Fig. 6.13: Position dependence of thetiming resolution of the scintillator
bars
After studying every counter’s performance, all of the counters have been assembled in the RDC frame. Each counter has been aligned with small aluminumparts (Fig.6.14).

CHAPTER 6. COUNTER CONSTRUCTION

64

Fig. 6.14: 12 plastic scintillators assembled in the frame.

6.2 Calorimeter Part
The calorimeter part has 76 LYSO crystals and one MPPC is attached to each crystal. While the MPPCs for the plastic scintillators are glued with optical cement, theMPPCs are not glued to the LYSO crystals for ease of maintenance. Instead, theMPPCs are pressed onto the LYSO crystals with non-magnetic springs as can beseen in Fig.6.15. For this purpose, a part of the PCB for the signal transmission ismade of a flex-PCB.

Fig. 6.15: Mechanism to press the MPPCs onto LYSO crystals

CHAPTER 6. COUNTER CONSTRUCTION

65

6.2.1 PCB Layout
As mentioned above, the PCB for the calorimeter part is partially flexible and the
MPPCs attached on the PCB are pressed by springs onto LYSO crystals as can beseen in Fig.6.16 and Fig.6.17.

Fig. 6.16: Ten MPPCs soldered on the PCB. The brown branches are flexible parts.

Fig. 6.17: MPPC pressed by a spring

6.2.2 Assembly
The crystals are covered with a 65 µm-thick reflective film which has a window for
the coupling with the MPPC.

Fig. 6.18: LYSO crystal with a reflective film

CHAPTER 6. COUNTER CONSTRUCTION

66

Since the LYSO crystals are pushed by the MPPCs with the spring, a support
structure is necessary to keep the crystals in position (Fig.6.19). In order to provide
enough support while reducing the amount of material, a support plate made of
Rohacell and carbon fiber is inserted (Fig.6.20 and Fig.6.21).

Fig. 6.19: Side view of the LYSO partFig. 6.20: Sandwiched structure
made of ROHACELL foam and thecarbon fiber layers

Fig. 6.21: The frame for the LYSO
crystals. The white screws attached
the side of frame are used for the
crystal alignment

All of the LYSO crystals are mounted inside the frame as shown in Fig.6.22.
For z-direction alignment of the crystals, a thin Derlin plate is placed between the
crystals and the support plate.

CHAPTER 6. COUNTER CONSTRUCTION

67

Fig. 6.22: 76 crystals covered with a reflector film and placed in the frame.
On the other side of the crystals, a SUS plate for the support and a Derlin plate
for the MPPC alignment are attached (Fig.6.23).

Fig. 6.23: The support and alignment plates. The white blocks in the frame are thedummy crystals for assembly tests
The two plates have 76 windows and the MPPCs can reach the crystals’ surface
as can be seen in Fig.6.24. Optical grease is used for the coupling.

CHAPTER 6. COUNTER CONSTRUCTION

68

Fig. 6.24: A Derlin plate and four sets of the PCBs with MPPCs. The crystal
surface can be seen from the lower windows
The cables from the PCBs are bundled on one side and come out through eightholes in the light-shielding cover. Because of the space limitation for the cabling,some cables are attached on the PCB vertically and are bent by 90 degrees as canbe seen in Fig 6.25.

Fig. 6.25: The cabling for calorimeters.
The frame with springs to press the MPPCs is mounted as shown in Fig.6.26.
We could observe signals from all channels.

CHAPTER 6. COUNTER CONSTRUCTION

69(a) Back view of the MPPCs pushed by springs(b) Side view of the MPPCs pushed by springs

Fig. 6.26: The frame and the springs to attach MPPCs on the LYSO crystals
Items to be improved
During the construction, we found that the frame for the RDC downstream isslightly magnetic although it is made of non-magnetic stainless steel. It is said
that some types of stainless steel become magnetic during machining. Considering
this fact, we will produce a frame made of carbon fiber instead of stainless steel.
Also, some MPPCs did not touch the crystal surface properly. The reflectorof the crystals and the polyoxymethylene plate for the MPPC alignment will be
improved.

6.3 Installation
The downstream RDC was not installed in the beam area although a mockup of thepositron spectrometer and a part of the main positron timing counter were installed
and tested in the MEG II detector with a muon beam in the pre-engineering run
in 2015. In the downstream COBRA endcap as shown in Fig.6.27, installation ofa beam monitoring system and a target monitoring system is also considered inaddition to the RDC. Therefore, the downstream RDC installation will be doneCHAPTER 6. COUNTER CONSTRUCTION

70

after defining the installation scheme for all the items.

Fig. 6.27: Downstream RDC placed on the edge of the COBRA magnet. The
counter in the drawing is set in parking position. The design is preliminary and thedetails have not been fixed yet.

Chapter 7
Test of the Constructed Detector
The timing counter part and the calorimeter part of the RDC had been separately
studied. The next step was checking the compatibility by taking data with the twoparts. The two parts were combined and set on the aluminum profiles.

Fig. 7.1: The combined counters
The constructed downstream RDC was tested in the lab with the gamma-ray
source Y-88. The goal of the test is to check these items:• whether all components in the constructed detector work
• the calibration method of each counter71CHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

72

The dominant interaction process of MeV oder gamma-ray in a plastic scintillator
was the Compton scattering as shown in Fig.7.2.

Fig. 7.2: Total cross sections of gammarays in carbon [16]. The gamma-raysaround 1 MeV mainly interact throughthe Compton scattering.

Fig. 7.3: The event where theelectron is scattered in the plastic scintillator and the scatteredgamma-ray loses all energy in
LYSO crystals

Events where the scattered electron is observed in the plastic scintillator andthe scattered gamma-ray loses all energy in LYSO crystals as can be seen in Fig.7.3
enables us to reconstruct the incident gamma-ray energy. The constructed counter
test uses the total absorption events.

7.1 Simulation
We simulated the energy deposit in the gamma-ray from Y-88 with GEANT4. The
gamma-ray source was set at a distance of 1 cm from the center of plastic scintillator.
Since the dominant process of 88 Y is the emission of two gamma-rays whoseenergy are 0.9 MeV and 1.8 MeV as can be seen in Fig.7.4, the gamma-rays withthe two energy were produced in each event.

CHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

73

Fig. 7.4: Decay scheme of 88 Y
One million events where the gamma-rays enter the plastic scintillator placedin the center were produced. Fig.7.9a shows the RDC detector of the simulation.

Fig. 7.5: RDC detector of the simulation. The green and blue parts are plastic
scintillators and LYSO crystals respectively. The support structure made of ROHACELL and carbon fiber plates is placed between the two parts although it is not
drawn. The images of the two gamma-rays emit simultaneously and Y-88 are also
drawn.
Fig.7.6 shows the energy deposit in the detectors when a gamma-ray is scatteredin the plastic scintillator. It suggests that we are able to see the correlation between
the energy deposits in the plastic scintillators and the LYSO crystals.

CHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

74

Fig. 7.6: Energy deposit in the crystals vs Energy deposit in the plastic scintillators.
The hit in the plastic scintillators is required.
The two horizontal line can be understood as the events where one of the simultaneously emitted gamma-rays scatters oﬀ an electron in the plastic scintillator andthe other is observed in the crystal, as shown in Fig.7.7.

Fig. 7.7: The event where one gamma-ray interacts in the plastic scintillator andthe other gamma-ray loses all its energy in the crystal

7.2 Data acquisition
Circuit configuration
The constructed detector was placed in the thermostat chamber and the radioactive
source Y-88 was set at a distance of 1 cm from the plastic scintillators as shown inCHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

75

Fig.7.8. The lead collimator was also placed between the source and the detector.
The thickness of the collimator is 1 cm and it has the rectangular hole whose sizeis 2×1 cm2 .

Fig. 7.8: The detector with Y-88 inside the thermostat chamber. The temperature
was kept at 30 ◦ C
Because of the limitation of the electronics, one plastic scintillator and nine
LYSO crystals were used for the DAQ. In this chapter, ch1 and ch2 is defined asshown in Fig.7.9 (b).

(a)

(b)

Fig. 7.9: (a) 76 LYSO and 24 plastic scintillator channels (b) 9 LYSO and 2 plastic
scintillator Channels used in the test

CHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

76

The trigger condition is a coincidence between the plastic scintillator and thecentral crystal. The signals from the plastic scintillator is amplified with an amplifier developed at PSI with a gain of 10. The details of the circuit configuration are
illustrated in Fig.7.10, Fig.7.11 and Fig.7.12.

Fig. 7.10: Test setup for measurement of the gamma-ray’s total absorption process

Fig. 7.11: Bias and amplifier circuit diagram applied for the plastic scintillator

Fig. 7.12: Bias and readout circuit diagram applied for the LYSO crystals

CHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

77

Observed signal
Fig.7.13 shows the typical signal pulses from the plastic scintillator and the LYSO
crystal.

Fig. 7.13: Signal pulses from the plastic scintillator and the central LYSO crystal.
The scatter plot of the pulse heights in the plastic scintillator and in the LYSO
crystal is shown in Fig.7.14. In order to minimize the hit position dependence ofthe pulse height in the plastic scintillator, the geometric mean of the pulse heightsof the signals from both ends is used. We can see events with a clear correlation
between the signals from the two detectors as expected.

CHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

78

Fig. 7.14: The height in the LYSO crystal vs the height in the plastic scintillator.
height[ch1] > 0.05 mV and height[ch2] > 0.05 mV are chosen as the analysis cut.

7.3 Energy Calibration
Plastic scintillator
In the actual MEG II run, the energy scale of the plastic scintillators will be calibrated by using a Landau distribution for Michel positrons passing through thescintillator. We tested the calibration for the scintillator in the lab by using Sr-90.
The setup is shown in the Fig.7.15. The source was placed at the same position as
for Y-88. The trigger condition is a hit in the reference counter.

Fig. 7.15: Setup for the energy calibration of the plastic scintillator using Sr-90

Fig.7.16 shows the observed Landau distribution. The energy scale was calibrated by comparing this with the distribution in a simulation (Fig.7.17) where theCHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

79

peak position is 0.9MeV.

Fig. 7.16: Measured energy depositof beta-ray in the plastic scintillator

Fig. 7.17: Simulated energy depositof beta-ray in the plastic scintillator

LYSO crystal
The energy scale for the crystal part can be calibrated with the data of the selfradiation as discussed in section 5.1.3. In addition to the peak of self-radiation, thephoto-peaks of 0.9 MeV and 1.8 MeV were also available for the calibration in thistest with Y-88 as can be seen in Fig.7.18.

Fig. 7.18: Energy spectrum of the self-radiation and gamma-rays from Y-88

7.4 Result
We selected the events with a correlation between the energy deposits in the two
detectors as shown in Fig. 7.19(a). Fig.7.19(b) shows the sum of the energies ofCHAPTER 7. TEST OF THE CONSTRUCTED DETECTOR

80

the two detectors for the selected events where the peak position is 1.8 MeV as
expected.

Fig. 7.19: (a) Measured energy deposit in the center LYSO crystals vs one in theplastic scintillator (b) The sum of the energy deposits. The analysis cut: ECr y +EPS < 2.1&ECr y + EPS > 1.6&ECr y − EPS < 1.1&ECr y − EPS > 0.65 where
ECr y and EPS are the energy deposit in the center LYSO crystal and the plastic
scintillator respectively.
Summarizing this chapter, the data with both LYSO crystal and plastic scintillator was successfully taken and the events with a correlation between the energy
deposits in the two detectors were observed. We demonstrated that the energy sum
between the two detectors worked properly after the calibrations.

Summary and Prospect
Summaryµ+ → e+ γ decay has been an important probe to test the new physics and the MEG
II experiment to search for the decay aims at starting an engineering runt in 2016.
To further improve the sensitivity of the MEG II experiment, the RDC will be newly
introduced.
The downstream RDC was approved by the MEG II collaboration and the construction has started. We decided to use LYSO as the calorimeter part and the SiPM
for the readout was selected.
The construction has finished and the performances of the timing counter partand the calorimeter part were separately checked in the laboratory. The measured
time and energy resolutions were good enough to obtain the sensitivity improvement expected with the RDC.
The constructed detector was tested in the lab. The data with both plastic scintillator and LYSO crystals was successfully taken and the calibration method foreach counter worked properly.
The study on the muon beam profile with the upstream RDC is ongoing. The
preliminary results suggest that the the eﬀects on the beam profile and rate aresmall.
ProspectDuring the downstream RDC construction, we found some problems in the parts
such as the magnetisation of the frame. After solving these problems and checking
all of channels, the downstream RDC will be installed in the beam area. We will
improve the sensitivity of MEG II using the RDC and try to discover the golden
channel, µ+ → e+ γ.

81

Acknowledgment
Here, I would like to express my thanks to all of the people who have given me the chance to join
the MEG II experiment.
First of all, I would like to express the deepest appreciation for the great support from Prof.
Wataru Ootani. Although I faced more diﬃculties in entering the university than most of the otherstudents, I feel that my eﬀort has paid oﬀ because the two years in the Tokyo university and the Paul
Scherrer Institute have been the best period of my life.
Second, I thank Prof. Toshinori Mori, the chairperson of the experiment, for letting me join theexperiment.
Third, I would like to express my appreciation to the MEG II collaborators. Dr. Ryu Sawada,
who has proposed the Radiative Decay Counter, supported my research very strongly. Without his
persistent help, this thesis would not have been possible. Dr. Yuki Fuji introduced me to the RDC
and gave me basic knowledge about research while working together. Dr. Toshiyuki Iwamoto gave
me a lot of advice about work in Switzerland. Dr. Yusuke Uchiyama helped my research by giving
his incisive comments. Dr. Kei Ieki always comes to the oﬃce early and leaves for home late and I
have learned a lot from his attitude toward his work. Dr. Malte Hildebrandt and Dr. Peter-RaymondKettle have kindly taken care of us and gave us a lot of information not only about physics but also
about Europe. Dr. Angela Papa have often given us chances to discuss the fiber detector’s properties
with her group and sometimes we enjoyed lunch or (Italian) coﬀee together. Dr. Emanuele Ripiccini
gave us a lot of opportunities to enjoy our life, such as organizing a weekend party in the guesthouse.
I especially would like to express my gratitude to Ms. Giada Rutar who is the best Swiss friend of
us Japanese students. I enjoyed the Swiss life with Mr. Daisuke Kaneko, Ms. Miki Nishimura, Mr.
Shinji Ogawa, Mr. Kohei Yoshida, Mr. Naoya Shibata, Mr. Mitsutaka Nakao, Mr. Ryoto Iwai, Mr.
Souichi Tanaka, Mr. Hiroyuki Nagashima, Mr. Naoyuki Sumi, Mr. Kyohei Noguchi and Ms. Aya
Morishita. Especially, always Daisuke made people laugh, Miki made the mood bright, Shinji madethings quick, Kohei and Naoya gave us many laughs, Mitsutaka scolded one of his seniors, Ryoto
told us many dangerous stories. They gave me many cherished memories.
Fourth, I want to thank the staﬀs and the secretaries of the International Center for Elementary
Particle Physics (ICEPP). In particular I learned many important things, such as how Kansai people
should survive in Tokyo, from Mr. MasaHERO Yamatani who will win a nobel prize in the future.
Finally, I would like to express my gratitude to my parents, Masafumi and Mayumi, for their
big support.
January 2016
Tokyo
Shota Nakaura

82

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