MEG測定器

KEK測定器開発室セミナー @ KEK,筑波
24/November/2009
ICEPP

Yusuke UCHIYAMA
On behalf of MEG collaboration

Theme

μ+ → e+ γ search experiment, MEG started physics datataking in 2008.
In this talk, we report the detector and measurement
techniques used in MEG.

Contents








09/11/24

Introduction− Subject and purpose
Overview of MEG
Performance of detector
Waveform Analysis
Conclusion

KEKDTP seminar/Yusuke UCHIYAMA

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Introduction

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Subject of research

Lepton-flavor violating muon decay : μ → e γ−

cLFV : Forbidden in SM−

Out of experimental reachwith finite ν mass (BR<10-50)−

Clear probe to new physics beyond SM

μ→eγ decay

Normal muon decay(Michel decay)
Lepton flavors are conserved
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Physics Motivation

Large BR is predicted with many new Pysics−−

SUSY-GUT, SUSY-seesaw ,,,Possibility from just below current limit.
Current exp. limit : 10-11ex)SU(5) SUSY-GUT: 10-15~10-13,
SO(10): 10-13~10-11,
SUSY-seesaw: >10-14
 Large tanβ → larger BRLimit by MEGA

MEG

Connection to other physics−−−

cLFV : µ-e Conversion, τ-LFV (τ→lγ,etc) ..
g-2, EDM
LHC (direct search)

J. Hisano and D. Nomura, 1998

m2e e m2e  m2e m2l = m2 e m2  m 2 
Mixing through slepton
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m

2
 e

KEKDTP seminar/Yusuke UCHIYAMA

m

2
 

m

2
 


5

Position of the MEG ExperimentCurrent experimental upper limit :−Br(μ→eγ) < 1.2x10-11 (1999, MEGA@LAMPF)

Target：down to a sensitivity of 10-13

In 2008, started physics data taking ~2011? → MEG upgrade ??

No other experiments (nor future program)
Other cLFV searchµ-e conversion~300 times smaller BRCurrent U.L.~10-13(@PSI)
Future exp. ~10-16

COMET @J-Parc
mu2e @Fermilabτ-LFV
5～10 years

Many different modesBR ~ O(103-5)xBr(µ)
To conduct these experiments is importantCurrent U.L.~10-7~-8 (B-factories)
independent with MEG results
Future program : superBComplementary with LHC−

09/11/24

To be a pioneer ofcoming New physics era !

Possibility of SUSY particles discovery at the beginning of LHC
KEKDTP seminar/Yusuke UCHIYAMA

6μ→eγ Search

Need a large number of muon−−

High rate experiment
Use positive muon (μ+)
Prevent from forming muonic atoms

μ+→e+γ signal：a positron and a gamma−

Clean 2 body decay

at restBoth at 52.8MeV (monochromatic),
 Back-to-back, Time coincidence

Backgrounds−−

09/11/24

Radiative muon decay (prompt BG)
 Rapid decrease of phase space in signal region
 We can control with reasonable resolutions
Accidental overlap of uncorrelated e+ and γ (accidental BG)
 Source of γ: radiative decay, e+ AIF, Bremsstrahlung, CR

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Accidental Background

Accidental BG limits the experiment−

BG rate is proportional to the instant beam rate → DC beam is the best

Bacc = Rµ・fe0・fγ0・(δω/4π)・(2δt)
Back to backTime overlap(Linear to time resolution)(quadratic to angular resolution)=Nµ (DC beam)γ BG Spectrum

Signale+ BG Spectrum

SignalMichel e+0

Ee+ (/52.8MeV)High rate e+
09/11/24

Radiative decay
10.9

Eγ (/52.8MeV)

1γ energy measurement is most important

KEKDTP seminar/Yusuke UCHIYAMA

8Requirements for μ→eγ experimentHigh intensity DC μ+ beam>107/sec

High rate tolerable detectors
All of >107/sec μ+ generate e+ Pileup of γs become a source of high energy BGHigh resolution detectors
γ energy measurement is most important Angle and time measurements are also effective

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9The MEG Experiment

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10MEG ExperimentWorld's most intense DC μ+ beam @PSI (Switzerland)

MEG detectors−−

Positron spectrometer
Liquid xenon γ -ray detector

Started physics data taking in autumn 2008
πE5 beam line @PSI
~60 collaboratorsµ + beamγ
COBRA SC magnet
LXe γ -ray detector

e+Drift chambers
Timing counters

proposal
1st result
planning
R&D
Assembly E.R.
Data taking
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 200909/11/24

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MEG ExperimentWorld's most intense DC μ+ beam @PSI (Switzerland)

MEG detectors−−

Positron spectrometer
Liquid xenon γ -ray detector

Started physics data taking in autumn 2008
πE5 beam line @PSI
~60 collaborators
U-Tokyo,PSI UCIrvine INFN & U
Waseda, JINR
+ Dubna,Pisa,µ beamKEK
Rome
BINP Novosibirsk
Genova,Pavia,Lecce

COBRA SC magnet
LXe γ -ray detector

e+Drift chambers
Timing counters

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121.2MW proton Ring-Cyclotron at PSI
590MeVMax current 2.2mA

cf.
MEGA used pulsed beam
6% duty cycle
Instant intensity 2.6x108
average 1.3x107
MEG
Duty cycle 100%
Instant＝ave 3x107 (2008)

Provides world's most intense DC muon beam
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'Surface muon' Beam Transport System
MEG DetectorSurface μ: µ produced from π at rest on the surface of prod. target−−

Through the beam transport system−Extract at 175° from the primary p beam
Low momentum(29MeV/C) with small variance μ+ beam

Separate e+・degrade・tune beam profile

3x107μ+/sec stop on target−−

10mm spot size
200μm polyethylene film target , placed at 20.5°slant angle from beam-axis

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Suppression of scatter & BG VS stopping power
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14MEG Detector

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Liquid Xenon Detector

900 liter liquid xenon−

Scintillation medium
High light yield (75% of NaI(Tl))
 Fast response (τdecay=45ns)
 High stopping power (X0=2.8cm)
 No self-absorption
 Uniform, no-aging
−Active volume ~800lΩ/4 π=11%846 PMTs

Challenges
Vacuum ultraviolet (178nm)
 Low temperature (165K)
 Need high purity


−




No segmentation

Measure energy,position,time atonce
Identify pileup events−−−

Light distribution
time distribution
waveform

2007年完成
09/11/24

50cm
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Prototype / R&D

Verified performance with prototype detector−

Energy resolution @55MeV
−

σup = 1.23%, FWHM = 4.8%

Time resolution @55MeV


σt = 65 ps

Various R&D, obtained a lot of know-how necessary of the final detector

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Prototype / R&D

Verified performance with prototype detector−

Energy resolution @55MeV
−

σup = 1.23%, FWHM = 4.8%

Time resolution @55MeV


σt = 65 ps

Various R&D, obtained a lot of know-how necessary of the final detector
Now came back to KEKstart 2nd life

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Cryostat
2 layers of vacuum-tight cryostat
Thin window for γ entrance face

Inner vessel

Entrance window with honeycomb structure
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PMT Installation
2”PMT developed for MEG
 Quartz window for VUV K-Cs-Sb photocathode Al strips on photocathode Metal-channel dynode Zener diode at last steps of Bleeder

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20Completed in2007
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The first ton-scale LXe detector in the worldin useKEKDTP seminar/Yusuke UCHIYAMA

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Slow Control System

Temp(in)Temp(out)

Press

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Positron Spectrometer

A spectrometer efficiently measure 3x107 high rate e+
Measure e+ momentum・emission angle・µ+ decay time&position
with high resolution

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“COBRA” Magnet
Superconducting solenoid form highly gradient magnetic field−

Center 1.27 T → edge 0.49 T
Field on beam-axis

COBRA magnet
center

09/11/24

Compensation coils

Other featuresThin coil<0.2X0。 to transmit γ-ray(85%)
Rapid switch on/off
stabilize ~30min (cooling with GM ref.)Step structure solenoid
Two compensation coils suppress fringe field（no return yoke）
Low B around LXe detector
for PMT <50Gauss
24
KEKDTP seminar/Yusuke UCHIYAMASpecially Gradient B-Field

Uniform B-field
Gradient B-fielde+ quickly swept out

Same momentum⇒same radius(COnstant Bending RAdius)

Enable measurement in high rate
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Drift Chamber(DCH)

16 modules−−

Target−He
atmosphere

Arranged concentrically(10.5°interval)
2 layers per 1 module

Chamber gas−−

He:ethane(50:50)
Pressure control

Ultra low mas chamber

Drift chamber

09/11/24

Outside He atmosphere

KEKDTP seminar/Yusuke UCHIYAMA−

Multiple-scattering limits
the performance−

To suppress γ BG source−

In total along e+ trajectory~2.0 x 10-3 X0

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DCH Designr-dir

Beam-dir (z-axis)r-dir

2 layers staggered by half cell
9 drift-cells in 1 layer
Open-frame structure
Form cell only with cathode foils

z-dir

09/11/24

12.5μm cathode foil
Vernier pattern → z reconstruction

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09/11/24

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Timing Counter

Hit timing counter one turn after
exit of DCH. Measure hit timing
Two layers of plastic scintillater
arrays−Outer：Scintillation bars
4x4x80cm3, BC404
 15bars concentrically(10.5°interval)
 Fine-mesh PMT at two ends
 High precision timemeasurement
Beam-dir(z-axis)−Inner：Scintillating fiber
5x5mm2
 128 fibers along z-dir.
 Readout by APD
 Hit pattern → trigger

90cm

Not used in 2008 analysis
Defects in APD readout

z-dir
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Data Acquisition
Multi-threading

MIDAS system
 Frontend control
 Event building Logging Online database Slow control
 History monitoring Web interface

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TriggerFPGA-FADC architecture−100MHz FADC on VME
boards

MEG trigger−

γ energy−e+-γ coincidencee+-γ direction match(back-to-back)

−






In addition, 10 trigger
types are mixed in normal
data taking−

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Max output PMT in LXe
TC hit position

Calibration, normalization

Beam rate

3x107s-1

Fast LXe Q sum(>40MeV)

2x103s-1

Time coincidence

100s-1

Direction match

10s-1
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Readout Electronics


Record waveform from all sub-detectors (no ADC,TDC)−DRS chip (Domino Ring Sampler)−Up to 5GSPS, 1024cell, 8ch/chip

Sampling speed : 1.6GHz for LXe&TC, 500MHz for DCH

−

100ns

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Calibration


55MeV high-energy γ from π 0 decay−

Evaluation of resolutions (energy, position, timing)−

Calibrate energy scale−

Use same beamline as μ+−

Take some time for setup (~5days)−

Conducted at beginning & end of physics run.
More BG than normal μ+ run.

π- p → π0 n → γ γ n
LH2

γ

γ

NaI







17.6 MeV γ from Li(p,γ)Be reaction−

Lower energy (1/3)−

Uniformity, light yield monitor−

MEG dedicated p-beamline (opposite side)−

Easy to switch (~20min)−

3 times per week, regular calibration

μ Michel decay−

π − beamµ+ b ea mp beamCalibrate e+ (DCH＆TC)

μ radiative decay−

Tag back-to-backTime calibration

LED, α source

09/11/24 PMT calibration−

Cockcroft-Walton accelerator
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PMT Calibration


LED
−
−



PMT gain calibration
Time offset calibration

Alpha
−
−

PMT Q.E. calibration
LXe attenuation length measurement

Am source on wire
09/11/24

Reconstructed α event

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Variation of LXe light yield

Lower than expected
Recover by purification
Decrease by (possible) leakConfirmed we can monitor light yield using
several kinds of daily calibration.

We decided to continue purification during data taking(gas phase:continuously, liquid phase:intermittently(beam shutdown))

Correction of light yield
Monitor Li(p,γ)Be 17.6MeV line

purification

Measured energy scale

Finally, keep overall
energy scale
uncertainty under
0.4%

Pulse shape was alsochanging 9月

12月

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Xenon System
High pressurestorage tankGas phase purifier

200W pulse-tube
refrigerator

Detector
Getter

１０００l liquid dewar
Liquid phase purifier
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Xenon System
Gas phase purifier

High pressurestorage tank

200W pulse-tube
refrigerator

Detector

１０００l liquid dewar
Liquid phase purifier
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Xenon System:Liquefaction/Transfer
High pressurestorage tankGas phase purifier

Liquefaction
200W pulse-tube
refrigerator

Liquid transfer

Detector
Getter

１０００l liquid dewar
Liquid phase purifier
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Xenon System:Purification System
高圧ガス貯蔵タンク

Gas phase purifier

Gas phase
purification

200W pulse-tube
refrigerator

Detector

N2,
O 2,
H2O,
etc.

absorption

Liquid phase
purification

O2
GetterH2O

１０００l liquid dewar
Liquid phase purifier
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Xenon ScintillationDe-excitation process (fast)−

Xe + Xe* → Xe2* → 2Xe + hνRecombination process (slow)−

Xe+ + Xe → Xe2+
Xe2+ + e- → Xe** + Xe
Xe** → Xe* + heat−

Xe + Xe* → Xe2* → 2Xe + hν−−

Absorption

Quench Xe * + N
2
2 → 2Xe + N2
Xe2* + O2 → 2Xe + O2
(two-body collision)

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Quench Xe * + N
2
2 → 2Xe + N2
Xe2* + O2 → 2Xe40+ O2(two-body collision)Light yield & pulse shapeFurther purification during shut down−

Waveform

Whole volume passed gas
purification system (getter).
2008/92008/12
2009/7

Jul/09Dec/08
Decay time
2008
2009

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DCH Discharge Problem


DCH frequently discharged−−It happened also in 2007
engineering run.
−
−
−



Repaired in maintenance period
At the beginning of 2008, every
chamber could be operated
We thought we could fix the problem...,

Helium permeated into HV line slowly ...

In 2008, after a few months−

Inside magnet is filled with pure-He.
DCH-outside is exposed in He
atmosphere. (HV line)

Gradually some chambers starts todischarge again.

Finally, out of 32 planes−−

09/11/24

18 planes were operational
Only 12 planes worked at nominal
voltage

Degradation of e+ measurement(efficiency・resolution)

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Solution for the Discharge problem








Exhaustively investigated weak
points for all HV connections.
HV soldering spot on PCB and HV
via on PCB are suspicious.
Discharge was reproduced at Lab.
test finally after many trial.
Solutions−

New design of PCB
−

09/11/24

Separate layers for HV and GND
completely− (3 → 4 layers)

Potting HV soldering spot withepoxy

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After modification

Two chambers with new HV PCB
into “Aquarium” to see long term
operation with He/Ethane insideand pure-He outside and nominal
HV
16 chambers are mounted on the
support structure inside the “Hecabin”. Signal check with nominal
HV

Helium cabin

Aquarium
09/11/24

Stable operation for ~7 months

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Reconstruction&
Performance
In 2008

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Gamma energy I
Energy response map (before corr.)

Reconstruction−−

Sum of PMT outputs
Correction of non-uniformity (collection efficiency)−relative scaleUse 17.6MeV γ from Li(p,γ)Be reaction− It illuminate the detector uniformly.

Treatment of shallow events

Low resolution at shallow part−−−Shower escapeLarge variation of photon collection, Photon leakage Detector entrance face
Saturation of signal(dyn.range of elec.)

But want to use for statistics.
 Recovered saturation using waveform Correct photon collection efficiency by
calculating solid angleε∼30% recover

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intermediate
In front of a PMT
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Gamma energy IIRecover of pileup events
Not discard pileup events, but use with unfolding.
Improve efficiency−−−

ε ~8% gain

ID pileup → reconstruct energy using region without pileup → replace
PMT outputs for pileup region with estimated charge → then normal
reconstruction

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Gamma energy IIRecover of pileup events
Not discard pileup events, but use with unfolding.
Improve efficiency−−−

ε ~8% gain

ID pileup → reconstruct energy using region without pileup → replace
PMT outputs for pileup region with estimated charge → then normal
reconstruction

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Resolution map

Gamma energy III

π 0 55MeV−

Evaluate energy resolution as a response to 55MeV
Evaluate res for all over the entrance face−

Average res (averaged over the event distri. in MEG run)−

Peak at 1.7%

σ up=2.0% for deep(>2cm), 3.0% (1~2cm), 4.2% (0~1cm)
Determine energy scale
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Gamma energy IVUsing γ spectrum of μ decay (side-band)−

Check those correction, resolution and energy scale
Fit spectrum with the expected
one from MC
 Parameters Energy scale Resolution
 Fraction of pileup

Pileup component

Resolution
π0 resultμ-fit result

Well consistent → validation of analysis
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Positron Tracking:pattern recognition
Raw Hit dist.
@3x107

Select hits with time and z
info.
Clustering, connecting

Find track candidates

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Positron Tracking:Kalman Filter








Reconstruct e+ trajectory by
track fitting with Kalman-filter
Extrapolate the track up/down totiming counter / target.
Reconstruct momentum・
emission angle・vertex ontargetReconstruct ToF to timing
counter

Extend to TC
Back to Target

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Positron momentum

Evaluate momentum
response (resolution) by
fitting kinematical edge
(52.8MeV) of Michel
spectrum

Data
Fit


=

−

Momentum response function

09/11/24

Response function：triple
Gaussian−Core = 374keV (60%)
Tail = 1.06MeV (33%), 2.00MeV (7%)

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Positron emission angle?


Evaluate angularresolution using 2 turn
events−See difference of angle
between reconstruction
with each turnσθ = 18 mrad
σφ = 10 mrad

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Muon decay vertexReconstruct μdecay vertex as a point crossing e+ track and target
plane
Evaluate resolution with−−

09/11/24

Using holes on target
Using 2 turn events
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σx = 4.5 mm
σy = 3.2 mm
55

Gamma position


Reconstruction：Fit with solid angle

Evaluate resolution−−−

π 0 run with Pb bricks
Shadow of slits gives resolution and bias
Results σ=4.5~5mm, bias(RMS)=0.7mm
xy




−
−

project

Compared with MC:− Reduce systematic.
− 1.8mm worse than MC
 QE uncertainty

Detailed study with MC−

Event distribution with Pb brick

Data

MC

Take in the difference with data
Resolution variation by the relative position to PMTShape of the response Double Gaussian

σxy~5mm(position dependent)

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Opening AngleNot possible to reconstruct direction of gamma−



Direction of the line b/w μ vertex and γ interaction point

Combined resolution

09/11/24: σθeγ = 20.6 mrad,

KEKDTP seminar/Yusuke UCHIYAMA

σφeγ = 13.9 mrad
57

Gamma timing IReconstruction−

Subtract scinti photon propagation time from PMT hit time.

−

Combine a lot of measurement by different PMTs (~150PMTs) (χ2 fit).

π 0→ γ γ−−Time difference b/w the reference counter
Results
Gaussian σ = 78ps @55MeV, 61ps @83MeV
t




γ

γ
Reference counter

Better resolution at December (high light yield).
− σ =68ps @55MeVt

σt=80ps @52.8MeV(This value is not used directly.)

Energy dependence

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Gamma timing IICorrection by μ radiative decay−−

Change of pulse shape as improvement of purity
Observed drift of t0

purification
Low intensity RD run 24h /1week Better S/N, better
precision of t0
Correct with linear function−

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Stability after the correction<20 psKEKDTP seminar/Yusuke UCHIYAMA

59Time resolutionteγ ：time difference b/w e+ and γ
time on target−

e+：TC measurement, subtract ToF
from track length−

γ：LXe interaction time, subtract
ToF

Observe RD peak in normal datataking−Correct small dependence of γenergyσteγ = 148 ± 17 ps

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Gamma efficiency
Detection efficiency−

π 0 2γ：NaI single trigger
ConsistentMC
within 5%
μ data single spectrum
Calculate position dependent efficiency with MC
Multiply with e+ event distribution−

In analysis region of 46 < Eγ < 60MeV

−
−
−
−





γ

?

γ

tag
NaI

εdet = 66 %Analysis efficiency−

Inefficiency (pileup, CR cut)9%εγ = (60 ± 3) %09/11/24

Interact with some material before
active volume

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Positron efficiency0.4

e+ detection efficiency−

εe+ = εDCH x ADCH-TC
εDCH : tracking efficiency
 A
DCH-TC : DCH-TC matchingprobability. Make inefficiency

if e+ interacts with material
and annihilates or changes its
direction largely.

εe+ decreased graduallyduring the run−

DCH discharge problem

Expectation (full DCH)：~40% (= 80x50)

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Waveform Analysis

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Domino Ring Sampler
0.2-2 ns

Inverter “Domino” ring chainIN
Waveform
stored

Clock

Out
FADC
33 MHz

Shift Register“Time stretcher” GHz → MHz

Switched Capacitor Arrays
−
−
−
−

−
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High speed sampling
Low power consumption
Low cost
High channel density
No precise timing

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

FADC

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32 channels input

40 MHz
12 bit
FADC

USB adapter
board

Version 3

Version 2

General purpose VPC board built at PSI

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CalibrationNon-linear response in amplitude & time
Calibrate the response−−

Measure response to reference voltages
Measure response to sine wave

Not constant sampling intervals (but fixed over time)

Synchronization among chip by a reference clock−−−

Trigger system distributes a global reference clock (20MHz)
Each chip digitizes the clock
Clock analysis (offline)
Global synchronization
 Event-by-event time calibration
Synchronization precision
σ~40 psec

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What is the merits?
Pileup identification

Particle identification (PSD)



Noise
−
−
−



Can investigate noise (online oscilloscope)
Event-by-event baseline subtraction
Additional noise reduction

Precise waveform analysis in offline−−−

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Digital filter
Various timing algorithms
Fitting waveform

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Online Display

Re-binned
For data size reduction

Sum
Raw waveform
Fitting

Digital filter

Frequency spectrum

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Baseline

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Charge IntegrationUse high-pass filter−−

Remove low freq
noise
Make pulse sharp

original
waveform
template fit

Narrow integration
window
 Low noise Robust to pileup

integrationarea(50ns)

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after optimized highpass FIR filter

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69Time Extraction
digital constant fraction−−−

Eliminate time-walk effectParameter adjustable
Interpolate sample pointsLinear or cubic

Template fitting−−

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Maximal usage of sample points
Robust to noise

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Coherent noise subtraction

Estimate coherent noise using baseline region

Sum waveform

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Pedestal

Chargeaverage

integration

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Coherent noise subtraction

Coherent noise
subtraction using
no hit channel

Signal from drift chamber

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Cross talk removal
Signal from timing counter

subtract

Hit
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Hit
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Pulse Shape Discrimination

Alpha
Gamma



PSD by−09/11/24

Q/A, pulse width, decay time

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Digitizer upgrade

DRS2 → DRS4−

Many modifications are applied from the experience with DRS2
−−

DRS2 have been used since 2004

Replaced all DRS2 with DRS4 in September
But not yet full performance
Eliminate temperature drift
 Linearity improve (upto 1 V) Differential input
 Timing accuracy (?)
 Double cell (twice sampling speed or twice window)−It takes longer than expected to install
Completely new system
 Several problems to debug

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Conclusion

MEG実験は2008年秋、物理データ取得を開始。
RUN2008ではMEG最初の3ヶ月分のデータをとった。



3 ton LXe detector の実用化に世界初成功。安定に運転している。



SCAを用いた高速波形取得。波形解析手法を開発。



検出器の解析手法を確立。
−

RUN2008を一通り解析し結果を出した。
RUN2008のsensitivity : 1.3 x 10-11
実際のデータからのupper limit : Br(μ→eγ) < 3.0 x 10-11 @90% C.L.
(preliminary)

今年はこの4倍の統計をためる。(11月頭から物理ラン再開)−

これに応じてsensitivityの向上−

sensitivityの詳細は今年の検出器の性能に依存する(現在、校正・評価中)が性能向上も見込めるため今年も統計で制限されるだろう。
http://arxiv.org/abs/0908.2594.

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Thank you.

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