Recent Result of theMEG Experiment
Satoshi MIHARA
Seminar at Kyoto Univ.
1/Dec/2010

1

Outline• Introduction• MEG History• 2009 Run• 2009 Data Analysis and Result• New cLFV experiments at J-PARC• Summary2

Important Remark• All results presented in this presentation
are preliminary3µ→ｅγ Introduction• Muon discovery in 1937
• Order of 10 improvementin 50 years• Current best limit set by
MEGA collaboration•BR(µ→ｅγ) < 1.2×10-11@ 90%C.L.

• Strong physics motivation• Neutrino oscillation• SUSY GUT4

Signal and Background• Signal• Gamma and positron with 52.8MeV• Back to back
• Time coincidence5Signal and Background• Prompt background - Radiative muon decay• Gamma and positron < 52.8MeV• Any angle < 180• Time coincidenceo6Signal and Background• Accidental background• Gamma and positron < 52.8MeV• Any angle• Random
7

Signal and Background• Accidental background dominates in MEG• DC muon beam is necessary• Good detector resolution is crucial tosuppress the background
8

PSI Surface MuonBeam910Injection Energy72 MeVExtraction Energy

590 MeV

Extraction Momentum

1.2 GeV/c

Energy spread (FWHM)

ca. 0.2 %Beam Emittanceca. 2π mm×mrad

Beam Current

2.0 mA DC

Accelerator Frequency

50.63 MHzTime Between Pulses

19.75 ns

Bunch Width

ca. 0.3 ns

Extraction Losses

ca. 0.03%

19.75 nsｐππ

11µµ

MEG Detector• Beam Transport System• Liquid Xenon Gamma-ray Detector• Positron Spectrometer12Liquid Xenon GammaLiquidXenon
Detector
ray Detector
liter literliquidliquid
xenonxenon
900•900846 2” PMTs (Hamamatsu R9869)
photomultipliers
submersed in• 846immersed
in LXe directly!
!

"liquid!

•!
!
!

!

•Hamamatsu
R9869
Good uniformity( homogeneous,liquid )High light output ( ~75% of NaI )Uses only scintillation light
Short decay time ( 45ns )information 3
High density (3g/cm )• High light output
Short scintillation wavelength ~ 175nm
Short decay time
• Quartz
window for PMT
High density
Low165K• temperature"!

•!

pulse tube cryocooler
developed bytoKEK
Purification
system implemented
Purification
remove Hlike
O, H
O22,O,
N2 Oetc.
< 10ppb
removetoimpurity
N22 and2"

13

Positron Spectrometer• COBRA magnet• Drift chamber system• Timing counter

14

MEG History
1999

Proposal...

2007 Dec.
Engineering run
2008 Sep.-Dec. 1st physics data acquisition
2009
Analysis of 2008 data
Hardware upgradeNov.-Dec. 2nd physics data acquisition
Dec.Analysis of 2009 data2010 Jul.3rd physics data acquisition15

2008 Result Summary• NP B834(2010)
1-12• Sensitivity:
1.3×10-11• 90% C.L. upperlimit: 2.8×10-11• Toy MC study→ 5%16

2009 Run17

Review 2009 Run• Successfully finished 1st MEG
physics run in 2008

• However - Major issues to beinvestigated before starting
2009 run• DC HV stability (Hediffusion problem)• LXe light yield (unexpectedimpurity contamination)

18

Situation Spring 2009•••

Back to “Square One”•

Total detector DISMANTLED for
Maintenance/Repair/Improvement during
shutdown 2008/2009

DC••

Dismantled all modules
New anode-prints+wires+extensive test inthe labLXe•••

Exchange the suspicious LN2 cooling pipe
New NEG pump installation
New purifier tower installation
19Further ImplementationSprig 2009
•Update from DRS2 to DRS4••

Differential I/P••

On-board timing calibration•Internal clock &synchronization
3.2 GSPS possible

••

DRS2

signal

XEC1.6GHz
DC 0.7GHz

Fix the “ghost pulse”
problem

PLL

Inverter Chainclock
Channel 0
Channel 1
Channel 2

trigger

Channel 3
Channel 4

DRS4Channel 5
Channel 6
Channel 7

signal20

Channel 8

Run 2009 Summary• 8½ weeks physics DAQ
rbeamchtrifD• 25th/Oct - 22nd/Dec• 93 TB data on disk!

2008"Discharge problem reduced efficiency andresolution for positron measurement
were worse! ε ~14%(~1/3), σ , σθφEThe problem was long term exposure tohelium, fixed before physics run in 2009

•LXe
Lightyieldas
expected•
2009
oved α/γ separation
Good
•impr
Efficiency(30~40%) and resolutions
2008
Discharge problem reduced efficiency and•Precise calibrations
resolution for positron measurement

Drift chamber
"!

!

"!

"

"

"

!

ε ~14%(~1/3), σE, σθφ were worse
The problem was long term exposure tohelium, fixed before physics run in 2009!

"2009
•DCEfficiency(30~40%) and resolutions improved•HV instability solved•Ran with “full efficiency”!

"

21>6?6!"#$%&'()*&++,+-.&+/0/.&%1)123&453&"678953.&%1923&&&&&&&&&&&&&&&&&:63;2*(<2&!=1B

2009 Data Analysis

22and Monitoring
LXeCalibration
Calibration!

••••

PMT gain monitored by LED, QE by α!"#"$#%&
Detector

Light yield monitoring ( CW, CR, AmBe etc.)
PMT Gainby LED & QE by α!

!

Cockcroft-Walton proton accelerator'&%#%()*%(muonLight yield by CW, CR, Am-Be"

17.6MeV γ by Li(p,γ)Be reaction

Cockcroft-Walton proton accelerator at the" Light yield monitoring & σ at 17.6MeV
rear end of the experimental area E•2008 physics run and shutdown:

17.6 MeV γ through Li(p, γ)Be reaction gaseous purification to increase light yield+//Energy calibrationby CEX••+//C

π-p→ π0n, π0→γγPurification••

During shutdown

55-MeV - 83-Me Gamma ray
Close to our 52.8 MeV signal

Light yield became as much as expected
And decay time of γ waveform changed

2009 physics run: no purification
Light yield monitoring: < 1% level

Checkby the RMD edge !"#$%&'()*&++,+-.&+/0/.&%1)123&453&"678953.&%1923&&&&&&&&&&&&&&&&&:63;2*(<2&!=1>6?6+@A'()*A+/0/23

BPositron Calibration• Calibration using cosmic ray

events triggered by scintillation
counters located outside
COBRA• Resolutions evaluated using
residuals of two turn tracks• Momentum• Angle• φ and θreconstructed by ★
(1st turn)reconstructed by ★
(2nd turn)24

Performance Summary•

Resolutions insigma• 2009performance ispreliminary• Further

improvement
foreseen after
detailed
calibration

2008

2009

published

Preliminary

Gamma Energy (%)
Gamma Timing (psec)
Gamma Position (mm)Gamma Efficiency (%)
e+ Timing (psec)
e+ Momentum (%)
e+ Efficiency (%)
e+ Angle (mrad)e+-gamma Timing (psec)
Muon Decay Point (mm)Trigger Efficiency (%)

2.0 (w>2cm)
80
5(u,v)/6(w)
63
<125
1.6
1410(φ)/18(θ)
148
3.2(R)/4.5(Z)66

2.1 (w>2cm)
> 67
←
58
←
0.74 (core)~40%
7.1(φ core)/11.2(θ)
142 (core)
3.3(R)/3.4(Z)83.5Stopping Muon Rate (Hz)DAQ Time / Real Time (days)

3×107
48/78

2.9×107
35/43

Sensitivity

1.3×10-11

6.1×10-1225

MEG Data Analysis
Principle• Blind analysis• E -T• Likelihood analysis• Probability Density Function (PDF)
γeγfrom data

26

Data Sampleth)Analysis box (~10σ wid48 ≤ Eγ ≤ 58 MeV
50 ≤ Ee ≤ 56 MeV| Teγ | ≤ 0.7 nsd| φeγ |, | θeγ | ≤ 50 mra

27•Analysis box (containing0.2% data ) was blinded
during calibration andoptimization of physics
analysis•Side band data (16%) tostudy background•

Michel positrons for positrondetector response study•

RMD with low gamma
energy to evaluate timingresolutions

Likelihood Analysis
L(N SIG ,N RMD ,N BG ) =••••

N

N obs

exp
N obs!

"N N obs

N SIG
N RMD
N BG
#[ N S + N R + N B]
i=1

Nobs= NSIG + NRMD + NBG
Nsig, NRMD and NBG are evaluated based on the maximum likelihood
analysis method•Input: Eγ, Ee, Teγ, Relative angles (φ,θ)

Three independent likelihood analysis tools are employed to check possible
systematic effects
PDF evaluated (mostly) from data•

Except RMD

28

Normalization• The normalization factor is evaluated fromthe number of observed Michel positrons•

k=(1.0±0.1)×1012• BR = N /k
SIG

29

Gamma PDF• Signal PDF• CEX π data,
055MeV• Background PDF• Sideband data
48!

50!

52!

54!

Eγ (MeV)!

30

56!

58!

Positron PDF• Signal PDF• Measured resolution• Background PDF• Sideband data
31

Relative Time/Angle
PDFs• Relative Time
• Signal PDF from
RMD

• Relative Angle• Signal PDF frommeasured resolutions• Flat distributions asbackground PDFs

32

Sensitivity
Consistent with evaluation with sideband data fitting: 4-6×10-12NPoeg

atsit
iiv

ve

T

eT

eγeγ

sidsideb

eb

an

andd

•
•

Average 90% C.L. estimated with toy MC with null signal is 6.1×10-1233

Unblinding

34

Likelihood Fit Resultyr
ani
miPler

35

Accidental BG
RMDSignalTotal
Dashed lines for 90% CLUL for NSIG

Summary ofLikelihood Analysis• N limit = 14.5 @ 90% C.L.
• varies between 12 to 14.5 depending on the analysis• N =0 in the 90% C.L. region• varies between 20% to 60% depending on theSIG

SIGanalysis• N best fit = 3.0• varies between 3.0 to 4.5 depending on the analysis
SIG36

Event Distribution
after unblinding
Prelim

Pr
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ym

in
ar

y37

Eγ = 52.25 MeV
Ee+ = 52.84 MeV
ΔΘ = 178.8 degrees
ΔT = 2.68 x 10-11 s

Event Display38

Further Check•

High quality e+ track category events (59%)

PrelimPr
in

eli

arym

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Consideration• Improved upper limit on Br(µ→ｅγ)• 1.5×10 at 90% C.L. (previous result 2.8×10 )• Toy MC/Sideband C.L. evaluation, 4-6×10• cf. MEGA limit 1.2×10
-11

-11

-12-11• Events around the signal region do not disappear
by selecting high quality tracks

40

Prospects•

2010

Expected Detector/Analysis
improvement

Preliminary•

Improve of synchronizationof DRS4 provides betterσeγ•

Noise reduction andelectronics modification ofDC•

Better calibration withmonochromatic positron
Mott scattering•

Refinement of LXe analysis

41

Gamma Energy (%)
Gamma Timing (psec)
Gamma Position (mm)Gamma Efficiency (%)
e+ Timing (psec)
e+ Momentum (%)
e+ Efficiency (%)
e+ Angle (mrad)e+-gamma Timing (psec)
Muon Decay Point (mm)Trigger Efficiency (%)

1.5 (w>2cm)
675(u,v)/6(w)
58
90
0.7
40%
8(φ)/8(θ)
120
1.4(R)/2.5(Z)94

Stopping Muon Rate (Hz)DAQ Time / Real Time (days)

3×107
95/117

Sensitivity

1.8×10-12

DAQ Prospects• 2010 DAQ restarted at the end of July
• 3 years DAQ until the end of 2012
• Final goal sensitivity ~ a few×10-1342

What should we doafter MEG?
Short introduction to mu-e conversion searchexperiments at J-PARCμàeγ and μ-e conversion• If μàeγ exits, μ-e conv. must be• Even if μàeγ is not observed, μ-e conv may be– Loop vs Tree– Searches at LHC

What is a μ-e Conversion ?
1s state in a muonic atom

Neutrino-less muon
nuclear capture(=μ-e conversion)

nucleusμ + (A, Z) à e- + (A,Z)μ−

muon decay in orbit

lepton flavorschanges by one unit−−eμ à ν νnuclear muon capture−μ + ( A, Z) à ν μ + ( A, Z − 1)- N)Γμ(NeàB(μ N à e- N) =Γ (μ N à ν N' )

mu-e conversion searchexperiment at J-PARC
COMET 10-16 for 2 years running• Proton Beam• pàπàμ• The Muon Source• Proton Target• Pion Capture• Muon Transport• The Detector• Muon Stopping Target• Electron Transport• Electron Detection

Mu2e, competing experiment at FNALDeeMeMasaµ-e electrons may directly
coming from a production target→ DeeMe•An electron analogue of the surface muon.
•Experiment could be very simple, quick and low-cost.

DeeMeDeeMeAnother m-e conversion search at J-PARC••••

Mu-e conversion electron directly
comes from the target?
1010 muon stops/sec/MWTransport 105MeV/c delayedelectrons
Expected reach (crude)• New beam line (150msr)•10 for Al (2x10 sec)
• cf SINDRUM II limit: 7x10
Capture rate0.08 → 0.60( Al )Pion capture
Pion decay – muon production
Muon stopping target

e spectrometer

New μ beam line

8x10-13 for C (107 sec)
2x10-13 for Al (107 sec)
-14

targetkicker• D2 beam line (40msr)••(C)

3GeV proton

7π--13µ-D2Graphite target
with water coolingSummary•

•

You!

MEG

MEG is not at the edge of a cliff yet!

•2 months DAQ in 2009 with stable detector
operation•

Preliminary result from 2009 data••••

Sensitivity : 6.1×10-12
90% C.L. upper limit:1.5×10-11
NSIG=0 is in the 90% C.L. region

3 years DAQ until the end of 2012

Two new experiments searching for mu-econversion at J-PARC to confirm MEG result•COMET and DeeMe49

cliff

Event Distribution
after unblindingPrelim

Pr
in

eli

arym

inary50