Status and Prospects of
MEG II experiment
Satoru Kobayashi
UTokyo, ICEPP (D3)

Flavor Physics Workshop
09/29/2021

1Outline• Introduction• Charged Lepton Flavor Violation• 𝜇 → 𝑒𝛾 search• MEG II experiment• Engineering run 2021
• Status• Schedule• SummarySatoru Kobayashi

09/29/2021

Flavor Physics Workshop

2++MEG II - Search for 𝜇 → 𝑒 𝛾 MEG result (2016)
𝐵𝑟 𝜇 → 𝑒𝛾 < 4.2 × 10−13 (90% C.L.)×2 intensity muon beam×2 resolution
×2 efficiency~60
physicists

MEG II plan
𝐵𝑟 𝜇 → 𝑒𝛾 ~ 6 × 10−14(90% C.L. sensitivity)• Upgrade from MEG experiment• Intensity frontier experiment• High intensity muon beam at Paul Scherrer Institute (PSI),Switzerland
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

3Charged Lepton Flavor Violation
12

Quarks3
Discovered

udtbcse MEG μ

B-factory

Neutrinosνe~O(103)TeV

Undiscovered

Charged Leptons

τDiscoveredνμντ
Dipoleinteraction

Four-fermion
interaction
Journal of the Physical Society of Japan 85, 091002 (2016)• Rates are too small (𝐵𝑟 𝜇 → 𝑒𝛾 ~10−54 ) in the SM.
• Large enhancement is predicted by new physics.
• High energy scale beyond LHC is indirectly accessible.
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

4Charged Lepton Flavor Violation
Synergy with B-anomalies

Muon cLFVMEG II (PSI)COMET (J-PARC)
DeeMe(J-PARC)
Mu2e (FermiLab)
Mu3e (PSI)

Tau cLFV
Belle II (KEK)
LHCb (LHC)ATLAS (LHC)
CMS (LHC)
Phys. Rev. D 97 (2018) 015019• 𝜇 → 𝑒𝛾 search has a sizable synergy with recentanomalies / results such as muon g-2 and R(K*),
R(D(*)) from B-factory.
• Through Leptoquark(LQ) models for example.
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

5μ→eγ SearchSignal
νμγe+γμ+μ+•••

Accidentalbackground

Same Timing
Same Energy
Opposite direction

νee+Michel positron

Accidental gamma:• Annihilation in flight• Radiative muon decay• 𝜇 → 𝑒𝛾 decay: two-body decay• Main background: accidental• Positron from Michel decay + accidental gamma-ray.

• Key: Precise measurement of positron + gamma-ray todiscriminate signal and BG.
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

6

MEG II Detectors
1.8% energy resolution
2.5 mm position resolution

4092 VUV-MPPC

668 PMT35 ps resolution

512 counters

12000 wires
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

7

Goal of Engineering run 2021
Primary Goal:
Start a short but first physics data acquisition

Requirements:• Stable operation of all detectors in beam.
• Test of full electronics + DAQ system• Monitor of LXe MPPC PDE radiation damage• Positron tracking analysis.
• Trigger commissioning

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

8Engineering run 2021 - Electronics ~ July• The full readout electronics was installed in the firsthalf of 2021.
• DAQ: stable with a reasonable DAQ rate 5-10 Hz.
• 1.4 GSPS, 100 MB/s
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

9Engineering run 2021 - LXe July ~ AugustGamma-ray event display with pileups# of readout

MPPCPMTTotal

2020

640

364

1004

2021

4092

668

4760

Orange: 2020 readout• 99% of photosensors are working well.
• 28 MPPC & 27 PMT are not working due to shortcircuit / HV supply

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

10Engineering run 2021 - LXe August ~ September• Noise contribution to energy resolution: totally negligible.
• 37 keV : 0.08% of 52.8 MeV (Signal energy)• Radiation damage of MPPC PDE is monitored through thebeam time.
• Deterioration rate: roughly consistent with previous measurements.
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

11Engineering run 2021 - LXe August ~ September•

Calibration measurement with external gamma-ray source is set up.
•••

17.6 MeV gamma-ray from Li target + proton beam.
9 MeV gamma-ray from capture of thermalized neutrons.

Uniformity over the entrance face is being studied for the first time.

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

12Engineering run 2021 – CDCH conditioningCurrent Readout under muon beam
Conditioning

August ~ September

Stableoperation

2020 readout• Drift chamber is stabilized by conditioning under beam.
• O2 cleaning with high HV (Working point + 50V)

Now both positron and gamma detectors are ready!
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

13Working proof 1: Cosmic-ray
Cosmic-ray

SeptemberLXe
COBRA magnet

CDCHpTCPattern 1
Pattern 2• Cosmic-ray data are useful for time & position alignment.
• Time coincidence between LXe and pTC is observed withcosmic-ray.

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

14

Working proof 1: Cosmic-ray
September

LXe

Cosmic-ray
LXe
COBRA

Drift circles

CDCHpTC

CDCH
track

pTC

Pattern 1
Pattern 2• We have a cosmic-ray track with many drift
circles in CDCH.
Position & Time alignment between detectors is getting ready.
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

15

Working proof 2: Positron Tracking
Just in progressPositron energy spectrum••

TC and CDCH z position correlation

Positron energy spectrum mainly from Michel decaypTC cluster and CDCH track are geometricallycorrelated
Positron tracking study with the real dataset is under development.

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

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Working proof 3: Trigger Development
Just in progress
Coincidence Trigger

Opening Angle Trigger

Gamma-ray energy

50 nsBlack: w / direction matching
Red: w /o direction matching

25 ns

Opening Angle

Time difference• Trigger commissioning is in progress.
• Coincident + back-to-back + high-energy positron &gamma-ray
Data collection with MEG trigger is coming soon!!
Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

17

Schedule2022

2021
AugDetector
setup

Sep

Oct

Detector test Trigger
In beamsetup

NovDecJan

Feb

Accelerator
shutdown

Datacollection
CEX runMPPCannealing• We will start data collection for a month.
• The first physics data acquisition with MEG II setup.

• After the muon beam time, we will measure the timing& energy resolution of LXe.
• Toward a long physics run in 2022, we will recover
MPPC PDE radiation damage by thermal annealing.

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

18Summary• MEG II experiment is an intensity frontier
experiment that searches for 𝝁 → 𝒆𝜸 decay.
• Projected sensitivity: Br(𝜇 → 𝑒𝛾)~6 × 10−14.

• The commissioning of all detectors with the full
readout electronics is in progress.
• We will start the first physics data acquisition
very soon.
• For one month with beam intensity 3 × 107 𝜇/𝑠.

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

19Liquid Xenon gamma-ray detector (LXe)
4092 VUV-MPPC
x2
improvement668 PMT

Concept

900 L liquid xenon volume + VUV-sensitive photosensors

UpgradeReplaced 216 PMTs with 4,092 VUV-MPPCs on the entrance face

Achievement

Acceptable energy & good position resolution(1.8% / 2.5 mm).

Issue

Radical decrease of MPPC PDE under high intensity muon beam.

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

20

pixelated Timing Counter (pTC)

512 counters

M. Nishimura PhD thesis

Concept

Fast plastic scintillator + SiPM series readout

Achievement

Good timing resolution (35 ps).

Issue

Radiation damage of SiPM
• Solution: cooling & replacement

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

21

Cylindrical Drift Chamber (CDCH)

Concept

Ultra-low mass cylindrical stereo wire chamber

Achievement

Optimization of gas mixture + conditioning• He:iC4H10 (90:10) + 1.2% 2-propanol + 0.5% O2

Issue

Wire-breaking & aging• Dry wire volume + extraction of broken wires

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

22

Engineering run 2021 – pTC & CDCH -2020 readout• Full readout

Satoru Kobayashi

2020

2021

pTC128512

CDCH1281152• Tracking study withfull coverage is inprogress.
09/29/2021

Flavor Physics Workshop

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𝜇 → 𝑒𝛾 search History

𝐵𝑟 𝜇 → 𝑒𝛾 < 4.2 × 10−13

Satoru Kobayashi

09/29/2021

Flavor Physics Workshop

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