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MEG Experiment. m ->e g branching ratio. Present limit: 1.2x10 -11. LFV process Forbidden in the SM Sensitive to SUSY-GUT, SUSY-seesaw etc. Our goal : Br( m ->e g )>10 -13 ~10 -14. Clear 2-body kinematics. 52.8 MeV. 180 °. 52.8 MeV. - PowerPoint PPT Presentation
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MEG Experiment
Motivation & Event Signature
Only allowed after KamLAND
LFV process Forbidden in the SM Sensitive to SUSY-GUT, SUSY-seesaw etc. Our goal : Br(->e)>10-13~10-14
52.8 MeV
52.8 MeV
180°
Clear 2-body kinematics
->e branching ratioPresent limit:1.2x10-11
Michel decay (μ +→ e + νeνμ) + random γ Background Rate ~ 10-
14
Radiative muon decay (μ +→ e + νeνμγ) Background Rate < 10-
14
MEG Experiment & DetectorApproved in 1999,at Paul Scherrer Institut, in Switzerland
Physics run in 2006Initial goal at 10-13, finally to 10-14
+ beam : World’s most intense DC Beam 108 + /s
detector : 800liter liquid xenon scintillation detector with 830 PMTs
e+ detector : solenoidal magnetic spectrometer with a gradient magnetic field (COBRA)
COBRAXe Detector
Drift Chamber
TimingCounter
262cm
252cm
Paul Scherrer InstitutPaul Scherrer Institut
ExperimentalHall
• The most powerful machine in the world.• Proton energy: 590MeV• Nominal operation current: 1.8mA.• Max > 2.0mA possible.
Small Prototype
Small PrototypeSmall Prototype
• 32 2-inch PMTs surround the active volume of 2.34 liter
• -ray sources of Cr,Cs,Mn, and Y
• source for PMT calibration• Operating conditions
– Cooling & liquefaction using liquid nitrogen
– Pressure controlled
– PMT operation of 1.0x106 gain•Proof-of-Principle Experiment
•PMT works in liquid xenon?
•Light yield estimation is correct?
•Simple setup to simulate and easy to understand.
•Proof-of-Principle Experiment
•PMT works in liquid xenon?
•Light yield estimation is correct?
•Simple setup to simulate and easy to understand.S.Mihara et al. IEEE TNS 49:588-591, 2002
S.Mihara et al. IEEE TNS 49:588-591, 2002
Small PrototypeSmall PrototypeEnergy resolutionEnergy resolution
• Results are compared with MC prediction.1. Simulation of int. and energy de
position : EGS4
2. Simulation of the propagation of scint. Light
EGS cut off energy : 1keV
Rayleigh Scattering Length: 29cm
Wph = 24eV
Small PrototypeSmall PrototypePosition and Timing resolutionsPosition and Timing resolutions
• PMTs are divided into two groups by the y-z plane
– g int. positions are calculated in each group and then compared with each other.
– Position resolution is estimated as
sz1-z2/√2
• The time resolutionis estimated bytaking the difference
between two groups. • Resolution improves
as ~ 1/√Npe
Large Prototype
Large Prototype
• 70 liter active volume (120 liter LXe in use)– 372x372x496 mm3: 17.3 R.L. in depth
• Development of purification system for xenon
• Total system check in a realistic operating condition:
– Monitoring/controlling systems• Sensors, liquid N2 flow control, refrigerator operat
ion, etc.– Components such as
• Feedthrough,support structure for the PMTs, HV/signal connectors etc.
– PMT long term operation at low temperature• Performance test using
– 10, 20, 40MeV Compton beam– 60MeV Electron beam– from 0 decay
TERAS Beam
• Electron beam (TERAS, Tsukuba in Japan)– Energy: 764MeV– Energy spread: 0.48%(sigma)– Divergence: <0.1mrad(sigma)– Beam size: 1.6mm(sigma)
• Laser photon– Energy: 1.17e-6x4 eV (for 40MeV)– Energy spread: 2x10-5 (FWHM)– Divergence: unknown– Beam size: unknown
Compton Spectrum
•(E-Ec/2)2+(Ec/2)2
Collimator size
10MeV
20MeV40MeV
Energy Spectrum Fitting
• Principle…
E Npe
Convolution of
Compton Spectrum
Response Function
Suppose Compton Spectrum around the edge
(E-Ec/2)2+Ec2/4Detector Response Function
Gaussian with Exponential tailf(x) = N*exp{t/2(t/2-(x-x0)}, x<x0+t N*exp{-1/2((x-x0)/)2}, x>x0+t
ConvolutionIntegration +/- 5
E~1.9%
1. TERAS Beam TestInverse Compton Energy : 10, 20, 40 MeV Incident Position : Detector CenterEstimate Energy Resolution using Compton Edge
1.6% in
40MeVEnergy
40MeVVertex distribution
3.8mm in
0 Beam Test at PSI
180°
Opening angle selection of two ’s monochromatic
- (at rest) + p -> 0 + n, 0(28MeV/c) ->
Concept
54.9MeV
82.9MeV
Energy (MeV)
175°
Opening angle (deg)
Ene
rgy
(MeV
)
155°
55M
eV80
MeV
170°
• Requiring
FWHM = 1.3 MeV
• Requiring > 175o
FWHM = 0.3 MeV0
Beam Test Setup
H2 target+degrader
beam
LPNaI
LYSO
Eff ~14%
S1Eff(S1xLP)~88%
8x8NaI115cm 115cm
-
n LP Xe
0 Beam Test at PSI
- (at rest) + p -> 0 + n, 0(28MeV/c) -> + monochromatic calibration of around 52.8MeV - + p -> n(8.9MeV) + (129MeV) linearity check & neutron response
LP X
e to
tal c
harg
e
NaI Energy (MeV)
-p->n0, 0->2
-p->n To get Energy Resolution Select 0 events Select NaI energy Select incident position in Xe detector Remove too shallow and too deep events
Energy ResolutionsE
Xe[
Nph
]
= 1.23 ±0.09 %FWHM=4.8 %
55 MeV
σ = 1.00±0.08 % FWHM=5.2%
83 MeV
CEX 2004
83 MeV to Xe
55 MeV to Xe
Right is a nice function of gamma energy
PSI 2003TERAS 2003alpha
Energy Resolution vs Energy
Position Reconstruction
• Localized Weight Method
• Projection to x and y directions.
• Peak point and distribution spread
•Position reconstruction using the selected PMT
Examples of Reconstruction
(40 MeV gamma beam w/ 1 mm collimator)
Position ReconstructionResolution
Reconstruction of the event depth
• Using event broadness on the inner face
• Necessary to achieve good timing resolution
3 cm
Liq. Xe
Liq. Xe
14 cm
(a)
(b)
05
1015
2025
3035
0
10
20
30
40
50
0
2000
4000
6000
8000
10000
05
1015
2025
3035
0
10
20
30
40
50
0
200
400
600
800
1000
1200
1400
1600
1800
52.8MeV
52.8MeV
• D: depth
parameterMC simulation Data
Previous Test
This Test
Short abs
Long abs
D=
D
DD
DD
D: 20~100 0~25cm
Energy Resolution vs. Depth Parameter
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
30 40 50 60 70 80 90
Sigma2 threshold
Resolution (%)
Number of Photoelectrons
D
• For incident at the detector center
• D > 35, 45, 55….85• Resolution: < 2% in sigma
except shallow events (D<45).
Nfpmt(0.5)
• Another depth parameter: Nfpmt(0.5)– Sort front-face PMTs in the order of th
e distance to the x-y weighted mean position.
– Sum up PMT output in that order.– Stop summing when the sum exceeds the h
alf of Qsum[Front]. Number of PMTs used in the SUM.
1.0
0.5
QPMT
NPMT
Shallow
Deep
Timing/Z Resolution
• Improving Z resolution is essential to improve timing resolution.
• Intrinsic timing resolution can be evaluated by comparing left and right parts of the detector.
– <T> = (TLTR)/2
XenonNaI S1
LYSO tLP - tLYSO
-
3 cm
Liq. Xe
Liq. Xe
14 cm
(a)
(b)
05
1015
2025
3035
0
10
20
30
40
50
0
2000
4000
6000
8000
10000
05
1015
2025
3035
0
10
20
30
40
50
0
200
400
600
800
1000
1200
1400
1600
1800
52.8MeV
52.8MeV
Left
Right
TL
TR
Absolute timing, Xe-LYSO analysis55
MeV
high gainnormal gain
110 psec 103 psec
LYSO Beam L-R depth reso.
110 64 61 = 65 = 56 33 psec
103 64 61 = 53 = 43 31 psec
No
rma
l g
ain
Hig
h
ga
in
A few cm in Z
Timing ResolutionTiming Resolution• Estimated using Electron Beam
(60MeV) data
• Resolution improves in proportion to 1/sqrt(Npe).
• For 52.8 MeV ~ psec + depth resolution.
• QE improvement and wave-form analysis will help to achieve better resolution.
(Visit “The DRS chip” by S.Ritt)
T
imin
g R
esol
utio
n (p
sec)
104 4x104
45 MeV Energy deposit by 60 MeV electron injection
52.8MeV
(nsec)
=75.62.0ps=75.62.0ps
Number of Photoelectron
<TL>-<TR>
PMT Development
Photomultiplier R&D• Photocathode
– Bialkali :K-Cs-Sb, Rb-Cs-Sb• Rb-Cs-Sb has less steep increase of sheet resistance
at low temperature• K-Cs-Sb has better sensitivity than Rb-Cs-Sb
– Multialkali :+Na• Sheet resistance of Multialkali dose not change so m
uch.• Difficult to make the photocathod, noisy
• Dynode Structure– Compact– Possible to be used in magnetic field up to 100G
• Metal channel Uniformity is not excellent
Ichige et al. NIM A327(1993)144
PMT (HAMAMATSU R6041Q)
FeaturesFeatures 2.5-mmt 2.5-mmt quartzquartz window window Q.E.: Q.E.: 6%6% in LXe (TYP) in LXe (TYP) (includes collection eff.)(includes collection eff.)
Collection eff.: 79% (TYP)Collection eff.: 79% (TYP) 3-atm 3-atm pressure proofpressure proof Gain: Gain: 101066 (900V supplied TYP) (900V supplied TYP) Metal Channel DynodeMetal Channel Dynode thin and compathin and compactct TTS: 750 psec (TYP)TTS: 750 psec (TYP) Works stablyWorks stably within a fluctuation of 0.5 % a within a fluctuation of 0.5 % at 165K t 165K
57 mm
32 mm
1st generation R6041Q 2nd generation R9288TB 3rd generation R9869
228 in the LP (2003 CEX and TERAS)
127 in the LP (2004 CEX)
111 In the LP (2004 CEX) Not used yet in the LP
Rb-Sc-Sb
Mn layer to keep surface resistance at low temp.
K-Sc-Sb
Al strip to fit with the dynode pattern to keep surface resistance at low temp.
K-Sc-Sb
Al strip density is doubled.
4% loss of the effective area.
1st compact version
QE~4-6%
Under high rate background,
PMT output reduced by 10
-20% with a time constant of
order of 10min.
Higher QE ~12-14%
Good performance in high rate BG
Still slight reduction of output in very high BG
Higher QE~12-14%
Much better performance in very high BG
PMT Development Summary
MotivationUnder high rate background, PMT output (old Type PMT, R6041Q) reduced by 10-20%.This output deterioration has a time constant (order of 10min.): Related to the characteristics of photocathode whose surface resistance increases at low temperature.
Rb-Sc-Sb + Mn layer used in R6041QNot easy to obtain “high” gain. Need more alkali for higher gain.Larger fraction of alkali changes the characteristic of PC at low temp.
So, New Type PMTs, R9288 (TB series) were testedunder high rate background environment.
K-Sc-Sb + Al strip used in R9288Al strip, instead of Mn layer, to fit with the dynode pattern
Confirmed stable output. ( Reported in last BVR)But slight reduction of output in very high rate BG
Add more Al Strip
Al Strip PatternLow surface resistance
R9288 ZA series
Yasuko HISAMATSU MEG VRVS Meeting @PSI June 2004
Gas Purification
Possible Contaminants
• Residue Gas Analysis– Vacuum level
• LP Chamber 2.0x10-2Pa• Analyzing section 2.0x10-3Pa
HeH2O CO/N2
O2
CO2Xe
H2O is the most dangerous for Xe scintillation light
Purification System• Xenon extracted from the chamber is
purified by passing through the getter.• Purified xenon is returned to the chamber
and liquefied again.• Circulation speed 5-6cc/minute
Gas return
To purifier
Circulation pump
Purification SystemPurification System• Xenon extracted from the
chamber is purified by passing through the getter.
• Purified xenon is returned to the chamber and liquefied again.
• Circulation speed 5-6cc/minute
• Enomoto Micro Pump MX-808ST-S– 25 liter/m
– Teflon, SUS
Gas return
To purifier
Circulation pump
How much water contamination?
Measured/MCSimulated data
Before purification: ~10 ppm
After purification: ~10 ppb
Purification Performance• Xenon Detector Large
Prototype• 3 sets of Cosmic-ray trigger
counters• 241Am alpha sources on the
PMT holder• Stable detector operation for
more than 1200 hours
Cosmic-ray events events
Xenon Purifier
• Attenuation of Sci light– Scintillation light emission from an excited molecule
• Xe+Xe*Xe2*2Xe + h
– Attenuation• Rayleigh scattering Ray~30-45cm
• Absorption by impurity
Absorption Length• Fit the data with a function :
A exp(-x/ abs)• abs >100cm (95% C.L)
from comparison with MC.• CR data indicate that abs >
100cm has been achieved after purification.
Liquid Phase Purification
Liquid-phase Purification Performance
In ~10 hours, λabs ~ 5m
PET vs MEG
• 511 keV vs 52.8 MeV– Smaller number of photo-electron– Shallower detector
• Read out from back vs from front– Smaller light collection– Efficient detection
• Small amount of material in front• Efficient fiducial volume
• Mild requirement on resolutions
The End