MEG II backgroundDominant background is accidental coincidences of e and γ.l Background e : Michel decay (μ àeνν )l Two sources of background γ

1. Radiative muon decay (RMD, μàeγνν )

2. positron annihilation in flight (AIF, eeàγγ).

μ+γ

e+180�

Performance evaluation of RPCl Measurement setup

ü A simplified detector setup for performance evaluation(Single layer 200μm gap, 3cm × 3cm plate size, 10MΩ/sq DLC resistivity).

ü Material budget was 0.3% X0à To be reduced with readout pad

improvementl Performance

ü Rate capability: At least 0.1MHz/cm2,but not a complete resultà To be remeasured

ü Detection efficiency: 23%à To be improved with larger

gap distanceü Timing resolution: better than 360ps

à Already good enough

References: [1] A.M. Baldini et al. Eur. Phys. J. C 78: 380, 2018 [2] A. M. Baldini et al. Eur. Phys. J. C, 76: 434, 2016

Development of ultra-low material RPC for background identification in MEG II experiment

Atsushi Oya1 , Kei Ieki1, Atsuhiko Ochi2, Rina Onda1, Wataru Ootani1University of Tokyo1 ,Kobe University2

Abstract An ultra-low material Resistive Plate Chamber (RPC) is being developed to suppress γ-ray background in MEG II experiment. It is required to detect low energy positrons associated with radiative muon decays, which is a major source of γ-ray background, under a harsh environment with a high intensity muon beam passing through it. The sensitivity of the experiment is expected to improve by 10% with this detector. R&D studies of this detector are presented.

MEG II experimentMEG II, an upgraded experiment from MEG will search for BSM via lepton flavor violating muon decay μà eγ with 6×10%&' branching fraction sensitivity [1], which improves the sensitivity of the MEG experiment by one order of magnitude.

1

Background identification detectors: Detectors to suppress background γ from RMD 3

4Resistive Plate Chamber(RPC)A hopeful candidate of the upstream detector is Resistive Plate Chamber with electrodes based on Diamond Like Carbon (DLC).

l RPC: Electric field is applied between two resistive electrodesØ Ionizations from charged particles produce avalanches in the gas gap

l Electrodes made of DLC sputteredKapton filmü DLC has mixed structure of sp2 bond

and sp3 bond of carbonü The advantages of DLC

1. Low material budget2. Adjustable surface resistivity

ü Readout pad is implemented at the top and the bottom of the detectorü Pillars to control gas gap distance

l MEG II designü ~4 layers (at maximum) to satisfy the requirement of low material

2

Summary and conclusionl In order to further improve the experimental sensitivity of the MEG II, we

are developing RPC for the background identification detector based on DLC sputtering technology.

l The measured performance looks promising, but still needs further design optimization to meet the requirements.

MEG II signall 180 relative angle for e and γl same emission timing of e and γl Both of e and γ have 52.8 MeV energy

Difficulty of upstream detectorl Must be operated under high intensity muon beam (21MeV/c, 100MHz

in total, 4MHz/cm2 at the center, 60 week data taking) passing through the detector1. Low material budget (< 0.1% of ()) so as not to degrade the beam2. Radiation hardness and rate capability

upstream detector will be installed hereDevelopment is underwayà This study

downstream detector: already developed

5

Contact: Atsushi Oya , atsushi@icepp.s.u-tokyo.ac.jp

Chapter 3

Radiative Decay Counter

The Radiative Decay Counter (RDC) is the new detector in the MEG II experiment whichis able to improve the sensitivity by identifying significant part of the background photons fromRMD. In this chapter, the detail of the RDC is described.

3.1 Principle of background identification

The concept of the RDC is illustrated in Figure 3.1. As previously mentioned, a positronemitted from the target follows a trajectory along the gradient magnetic field, which is producedby the COBRA magnet. When a high energy photon is emitted from RMD, a low momen-tum positron of typically 2-5 MeV is also emitted. This positron does not enter the positronspectrometer but it is swept away along the beam axis. The bending radius of these positronsare smaller than 6 cm when the energy of the gamma-ray is greater than 48 MeV. Therefore,the background photons from RMD can be identified by detecting the time-coincident low mo-mentum positrons on the beam axis. The detectors can be installed at both upstream anddownstream of the muon stopping target. Figure 3.2 shows the expected hit timing di↵erenceof the RDC and the photon detector. The timing peak in the red line is corresponding to theRMD events. The spread of the 6 ns (FWHM) mainly comes from the fluctuation of the time-of-flight of positrons. According to the simulation result, 41% of total background photons canbe identified by installing two RDC detectors and thus the sensitivity is improved by 22%.

Figure 3.1: Schematic view of MEG II detectors

19

Motivation of our studyl The aim of this study is to develop the upstream background identification

detector, improving the sensitivity by 10 %

RMD events emitting ~52.8 MeV photon is accompanied by 1-5MeV positron à By detecting this positron, RMD is identified

readoutKapton (50μm)

DLC (0.1μm)pillar (gas gap)

+q-q

+HV

-HV

E-field

drift

drift

structure of RPC

chemical structure of DLCresistive plate made of DLC film

0 00 2 )

(4 0

Requirement to MEG II design1. Timing resolution of 1ns2. 90% positron detection

efficiency in totalà ~40% detection efficiency

for single layer

μ+e+�

�Michel

Accidental BG

From RMD or AIF

γ

μ+

γ

e+�

�1-5MeV whenthe γ is energetic

0 500 1000 1500curent of HV [nA]

0

100

200

300

400

500

600

700

800

310×

rate

[Hz]

trigger rate [Hz] vs Current of HV[nA]trigger rate [Hz] vs Current of HV[nA]

0 1000 2000 3000Xray intensity [a.u]

0

100

200

300

400

500

600

700

800

310×

rate

[Hz]

A]µtrigger rate [Hz] vs Current of Xray[ A]µtrigger rate [Hz] vs Current of Xray[

0 1000 2000 3000Xray intensity [a.u]

0

200

400

600

800

1000

1200

1400

curr

ent o

f HV

[nA

]

A]µcurrent of HV [nA] vs Current of Xray[ A]µcurrent of HV [nA] vs Current of Xray[

current b/w gap vs Xray intensity

0.1MHz/cm2 equivalent intensity

Rate capabilityØ Measured using intense Xray

0 3 6 0

l 8 3 3 6 3 0ü 3 3

5- 4- 3- 2- 1- 0 1 2 3 4 5time [ns]

0

100

200

300

400

500

coun

t [a.

u]

constant fraction timehistcftime

Entries 9090Mean 0.6657- Std Dev 0.4573

constant fraction time

0 3 3

Timing resolutionØ Measured using Sr90 β ray

Detection time distribution

Acknowledgement: This work was supported by JSPS KAKENHI Grant Number JP26000004.

schematic view of DLC film

0.02- 0 0.02 0.04 0.06 0.08 0.1 0.12height[V]

1

10

210

310coun

t [a.

u]

heighthistnoiseheight

Entries 41590Mean 0.002993Std Dev 0.002135

hist of height for signal region

hist of noise height

height

Pulse height spectra

Detection efficiencyØ Measured using Sr90 β ray

200μm gap, 1650V

filled withR134a based gas

incomplete count-ratemeasurement with ORTEC142IH amplifier (it was slow)

measured with 2GHzbandwidth ~40dBamplifier

measured with ~40dB amplifier

360ps including reference counter