Transcript Slide 1

Lepton Flavor Violation:
Goals and Status of the MEG
Experiment at PSI
Stefan Ritt
Paul Scherrer Institute, Switzerland
Agenda
Search for m  e g down to 10-13
• Motivation
• Experimental Method
• Status and Outlook
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Motivation
Why should we search for m  e g ?
The Standard Model
Fermions (Matter)
u
Leptons
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t
g
up
charm
top
photon
d
s
b
g
down
strange
bottom
ne
nm
nt
electron
neutrino
muon
neutrino
tau
neutrino
e
m
t
electron
muon
tau
I
II
III
gluon
W
W boson
Z
Z boson
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Force carriers
Quarks
Generation
c
Bosons
Higgs*
boson
*) Yet to be confirmed
4
The success of the SM
• The SM has been proven to be extremely successful since 1970’s
• Simplicity (6 quarks explain >40 mesons and baryons)
• Explains all interactions in current accelerator particle physics
• Predicted many particles (most prominent W, Z )
• Limitations of the SM
• Currently contains 19 (+10) free parameters such as particle
(neutrino) masses
• Does not explain cosmological observation
such as Dark Matter and Matter/Antimatter
Asymmetry
Today’s goal is to look for
physics beyond the standard
model
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CDF
5
Beyond the SM
Find New Physics
Beyond the SM
High Energy Frontier
High Precision Frontier
• Produce heavy new particles directly
• Heavy particles need large colliders
• Complex detectors
• Look for small deviations from SM
(g-2)m , CKM unitarity
• Look for forbidden decays
• Requires high precision at low energy
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Neutron beta decay
Neutron b decay via intermediate heavy W- boson
~80 MeV
ne
W-
e-
Neutron mean
life time:
886 s
~5 MeV
n
u
d
u
d
d
u
p
n  p+ + e - + ne
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b decay discovery:
~1934
W- discovery:
1983
7
New physics in m decay
Can’t we do the same in m decay?
m-
g
?
e-
 Probe physics at TeV scale with high precision m decay measurement
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The Muon
Seth Neddermeyer
• Discovery: 1936 in cosmic radiation
ne
• Mass: 105 MeV/c2
• Mean lifetime: 2.2 ms
m   e n en m
W-
Carl Anderson
≈ 100%
m-
m   e n en m g
m   e g
e-
nm
0.014
< 10-11
led to Lepton Flavor Conservation
as “accidental” symmetry
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Lepton Flavor Conservation
• Absence of processes such as m  e g led to concept of
lepton flavor conservation
• Similar to baryon number (proton decay) and lepton
number conservation
• These symmetries are “accidental” because there is no
general principle that imposes them – they just “happen”
to be in the SM (unlike charge and energy conservation)
• The discovery of the failure of such a symmetry could
shed new light on particle physics
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LFV and Neutrino Oscillations
Neutrino Oscillations  Neutrino mass 
m  e g possible even in the SM
g
W-
m-
nm
ne
e-
 LFV in the charged sector is
forbidden in the Standard Model
n mixing
mn4
BR( m  e g )  4  10-60
SM
mW
-
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LFV in SUSY
• While LFV is forbidden in SM, it is possible in SUSY
g
Wmn4
BR( m  e g )  4  10-60
SM
mW
-
m-
nm
e-
ne
-
mm2~~e
g
m~
m-
e~
~ 0
4
e-
me2m  100 GeV 
-5
tan2 b ≈ 10-12
BR( m  e g )  10


2
SUSY
m  mSUSY 
Current experimental limit: BR(m  e g) < 10-11
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LFV Summary
• LFV is forbidden in the SM, but possible in SUSY (and many other
extensions to the SM) though loop diagrams ( heavy virtual SUSY
particles)
• If m  e g is found, new physics beyond the SM is found
• Current exp. limit is 10-11, predictions are around 10-12 … 10-14
• First goal of MEG: 10-13
• Later maybe push to 10-14 ($$$)
• Big experimental challenge
• Solid angle * efficiency (e,g) ~ 3-4 %
• 107 – 108 m/s DC beam needed
• ~ 2 years measurement time
• excellent background suppression
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History of LFV searches
cosmic m
• Long history dating back to 1947!
10-1
• Best present limits:
m→eg
mA → eA
m → eee
10-2
• 1.2 x 10-11 (MEGA)
10-3
• mTi → eTi < 7 x
10-4
10-13
(SINDRUM II)
• m → eee < 1 x 10-12 (SINDRUM II)
• MEG Experiment aims at 10-13
• Improvements linked to advance
in technology
10-5
stopped p
10-6
10-7
m beams
10-6
stopped m
10-9
10-10
10-11
SUSY SU(5)
BR(m  e g) = 10-13

mTi  eTi = 4x10-16

BR(m  eee) = 6x10-16
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10-12
10-13
MEG
10-14
10-15
1940
1950
1960
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1970
1980
1990
2000
2010
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Current SUSY predictions
ft(M)=2.4 m>0 Ml=50GeV
1)
current limit
MEG goal
tan b
“Supersymmetric parameterspace
accessible by LHC”
1)
2)
J. Hisano et al., Phys. Lett. B391 (1997) 341
MEGA collaboration, hep-ex/9905013
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W. Buchmueller, DESY, priv. comm.
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LFV link to other SUSY proc.
me-LFV
mm2~~e
m~
m-
e~
mm2~m~
m~
m-
m~e2m~
mm2~m~
m~t2m~
m~e2t~ 

2
mm~t~ 

mt~2t~ 
g
m~
m-
~ 0
mEDM
 m~e2~e
 2
 mm~~e

2

m
~
 t ~e
e-
~ 0
(g-2)m
slepton mixing matrix:
g
de
m
2
m~m~
m~
In SO(10), eEDM is related to meg:
10-27 e  cm
g
 1.3 sin 
BR(m  eg )
10-12
m~
R. Barbieri et al., hep-ph/9501334
m26 June '07
~
0
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Experimental Method
How to detect m  e g ?
Decay topology m  e g
meg
52.8 MeV
N
g
m
52.8 MeV
180º
10
20
30
40
50
60
Eg[MeV]
N
e
52.8 MeV
m
•
•
•
→ e g signal very clean
Eg = Ee = 52.8 MeV
qge = 180º
e and g in time
52.8 MeV
10
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30
40
50
60
Ee[MeV]
18
Michel Decay (~100%)
Three body decay: wide energy spectrum
N
52.8 MeV
Theoretical
n
m  e nn
m
n
Ee[MeV]
e
N
52.8 MeV
Convoluted with
detector resolution
Ee[MeV]
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Radiative Muon Decay (1.4%)
g
N
n
52.8 MeV
m  e nn g
m
n
e
Eg[MeV]
“Prompt” Background
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“Accidental” Background
meg
g
Background
g
g
n
m  e nn
m
m
n
e
Annihilation
in flight
180º
e
e
n
m
m  e nn
n
m
•
•
•
→ e g signal very clean
Eg = Ee = 52.8 MeV
qge = 180º
e and g in time
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Good energy resolution
Good spatial resolution
Excellent timing resolution
Good pile-up rejection
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Previous Experiments
Exp./
Lab
Author
Year
Ee/Ee
%FWHM
Eg /Eg
%FWHM
teg
(ns)
qeg
(mrad)
Inst. Stop
rate (s-1)
Duty
cycle
(%)
Result
SIN
(PSI)
A. Van der
Schaaf
1977
8.7
9.3
1.4
-
(4..6) x 105
100
< 1.0  10-9
TRIUMF
P.
Depommier
1977
10
8.7
6.7
-
2 x 105
100
< 3.6  10-9
LANL
W.W.
Kinnison
1979
8.8
8
1.9
37
2.4 x 105
6.4
< 1.7  10-10
Crystal
Box
R.D. Bolton
1986
8
8
1.3
87
4 x 105
(6..9)
< 4.9  10-11
MEGA
M.L. Brooks
1999
1.2
4.5
1.6
17
2.5 x 108
(6..7)
< 1.2  10-11
?
?
?
?
?
?
~ 10-13
MEG
How can we achieve a quantum step in detector technology?
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How to build a good experiment?
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Collaboration
~70 People (40 FTEs) from five countries
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Paul Scherrer Institute
Proton Accelerator
Swiss Light Source
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PSI Proton Accelerator
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Generating muons
Carbon Target
p+
m+
MeV/c2
590
Protons
1.8 mA = 1016 p+/s
108 m+/s
m+
p+
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Muon Beam Structure
Muon beam structure differs for different accelerators
Pulsed muon beam, LANL
DC muon beam, PSI
Instantaneous rate much higher in pulsed beam
Duty cycle: Ratio of pulse width over period
Duty cycle: Ratio of pulse width over period
Duty cycle: 6 %
Duty cycle: 100 %
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Muon Beam Line
Transport 108 m+/s to stopping target inside detector with minimal background
-
Lorentz Force vanishes for given v:
x
x
x
x
x
x
m+
e+

   !
F  q  ( E  v  B)  0
+
Wien Filter
m+ from production target
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Results of beam line optimization
Rm ~ 1.1x108 m+/s at experiment
e+
m+
s ~ 10.9 mm
m+
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Previous Experiments
Exp./
Lab
Author
Year
Ee/Ee
%FWHM
Eg /Eg
%FWHM
teg
(ns)
qeg
(mrad)
Inst. Stop
rate (s-1)
Duty
cycle
(%)
Result
SIN
(PSI)
A. Van der
Schaaf
1977
8.7
9.3
1.4
-
(4..6) x 105
100
< 1.0  10-9
TRIUMF
P.
Depommier
1977
10
8.7
6.7
-
2 x 105
100
< 3.6  10-9
LANL
W.W.
Kinnison
1979
8.8
8
1.9
37
2.4 x 105
6.4
< 1.7  10-10
Crystal
Box
R.D. Bolton
1986
8
8
1.3
87
4 x 105
(6..9)
< 4.9  10-11
MEGA
M.L. Brooks
1999
1.2
4.5
1.6
17
2.5 x 108
(6..7)
< 1.2  10-11
?
?
?
?
3 x 107
100
~ 10-13
MEG
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The MEGA Experiment
• Detection of g in pair spectrometer
• Pair production in thin lead foil
• Good resolution, but low efficiency (few %)
• Goal was 10-13, achieved was 1.2 x 10-11
• Reason for problems:
• Instantaneous rate 2.5 x 108 m/s
• Design compromises
• 10-20 MHz rate/wire
• Electronics noise & crosstalk
• Lessons learned:
• Minimize inst. rate
• Avoid pair spectrometer
• Carefully design electronics
• Invite MEGA people!
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Photon Detectors (@ 50 MeV)
• Alternatives to Pair Spectrometer:
g induced shower
• Anorganic crystals:
– Good efficiency, good energy resolution,
poor position resolution, poor homogeneity
– NaI (much light), CsI (Ti,pure) (faster)
• Liquid Noble Gases:
– No crystal boundaries
– Good efficiency, resolutions
Liquid Xenon:
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25 cm CsI
Density
3 g/cm3
Melting/boiling point
161 K / 165 k
Radiation length
2.77 cm
Decay time
45 ns
Absorption length
> 100 cm
Refractive index
1.57
Light yield
75% of NaI (Tl)
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CsI CsI
PMT PMT PMT
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Liquid Xenon Calorimeter
• Calorimeter: Measure g Energy, Position
and Time through scintillation light only
• Liquid Xenon has high Z and homogeneity
Refrigerator
• Extremely high purity necessary:
1 ppm H20 absorbs 90% of light
• Currently largest LXe detector in the
world: Lots of pioneering work necessary
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Signals
Cooling pipe
• ~900 l (3t) Xenon with 848 PMTs
(quartz window, immersed)
• Cryogenics required: -120°C … -108°
H.V.
Vacuum
g
Liq. Xe
for thermal insulation
Al Honeycomb
window
m
PMT
Plasticfiller
1.5m
34
LXe g response
• Light is distributed over many PMTs
• Weighted mean of PMTs on front face
x
• Broadness of distribution
 Dz
1000 0
8000
6000
52.8 MeV
g
4000
2000
3 cm
0
• Position corrected timing
 Dt
50
40
30
20
Liq. Xe
10
0
• Energy resolution
depends on light attenuation in LXe
x
0
5
10
15
20
25
30
35
(a)
z
180 0
160 0
140 0
120 0
100 0
52.8 MeV
g
800
600
400
200
14 cm
0
50
40
30
20
Liq. Xe
10
0
0
5
10
15
20
25
30
35
(b)
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LXe g response
• Light is distributed over many PMTs
• Weighted mean of PMTs on front face
 x
• Broadness of distribution
 z
1000 0
8000
6000
52.8 MeV
g
4000
2000
3 cm
0
• Position corrected timing
 t
50
40
30
20
Liq. Xe
10
0
• Energy resolution
depends on light attenuation in LXe
x
0
5
10
15
20
25
30
35
(a)
z
180 0
160 0
140 0
120 0
100 0
52.8 MeV
g
800
600
400
200
14 cm
0
50
40
30
20
Liq. Xe
10
0
0
5
10
15
20
25
30
35
(b)
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• Use GEANT to
carefully study
detector
• Optimize
placement of PMTs
according to MC
results
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LXe Calorimeter Prototype
¼ of the final calorimeter was build to study performance, purity, etc.
240 PMTs
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How to get 50 MeV g’s?
p- p  p0 n (Panofsky)
p0  g g
• LH2 target
• Tag one g with NaI & LYSO
•
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Resolutions
• NaI tag:
65 MeV < E(NaI) < 95 MeV
• Energy resolution at 55 MeV:
(4.8 ± 0.4) % FWHM
• LYSO tag for timing calib.:
260 150 (LYSO) 140 (beam)
= 150 ps (FWHM)
FWHM = 4.8%
• Position resolution:
9 mm (FWHM)
To be improved with refined
analysis methods
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FWHM =
260 ps
40
Lessons learned with Prototype
• Two beam tests, many a-source and cosmic runs in Tsukuba, Japan
• Light attenuation much too high (~10x)
• Cause: ~3 ppm of Water in LXe
• Cleaning with “hot” Xe-gas before filling did not help
• Water from surfaces is only absorbed in LXe
• Constant purification necessary
• Gas filter system (“getter filter”) works, attenuation length can be
improved, but very slowly (t ~350 hours)
• Liquid purification is much faster
First studies in 1998, final detector ready in 2007
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Xenon storage
~900L in liquid, largest amount of LXe ever liquefied in the world
GXe pump
(10-50L/min)
Heat exchanger
GXe storage tank
Getter+Oxysorb
Cryocooler
(100W)
LN2
LN2
Cryocooler
(>150W)
Liquid pump
(100L/h)
Purifier
LXe Calorimeter
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Liquid circulating
purifier
1000L storage dewar
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Final Calorimeter
Currently being assembled, will go into operation summer ‘07
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Positron Spectrometer
Ultra-thin (~3g/cm2) superconducting solenoid with 1.2 T magnetic field
Homogeneous Field
high pt track
constant |p| tracks
Gradient Field
(COnstant-Bending-RAdius)
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e+ from m+e+g
44
Drift Chamber
• Measures position, time and curvature of positron tracks
• Cathode foil has three segments in a vernier pattern  Signal ratio on vernier
strips to determine coordinate along wire
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Positron Detection System
• 16 radial DCs with
extremely low mass
• He:C2H6 gas mixture
• Test beam measurements
and MC simulation:
• q = 10 mrad
• xvertex = 2.3 mm
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FWHM
• p/p = 0.8%
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Timing Counter
•
Experiment
Size [cm]
Scintillator
PMT
latt [cm]
FWHM[ps]
G.D. Agostini
3 x 15 x 100
NE114
XP2020
200
280
T. Tanimori
3 x 20 x 150
SCSN38
R1332
180
330
T. Sugitate
4 x 3.5 x 100
SCSN23
R1828
200
120
R.T. Gile
5 x 10 x 280
BC408
XP2020
270
260
TOPAZ
4.2 x 13 x 400
BC412
R1828
300
490
R. Stroynowski
2 x 3 x 300
SCSN38
XP2020
180
420
BELLE
4 x 6 x 255
BC408
R6680
250
210
MEG
4 x 4 x 90
BC404
R5924
270
90
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•
•
Staves along beam axis for
timing measurement
Resolution 91 ps FWHM
measured at Frascati
e- - beam
Curved fibers with APD
readout for z-position
47
The complete MEG detector
Liq. Xe Scintillation
Detector
Liq. Xe Scintillation
Detector
Thin Superconducting Coil
g
Stopping Target
Muon Beam
e+
g
Timing Counter
e+
Drift Chamber
Drift Chamber
1m
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MC Simulation of full detector
g
e+
“Soft” gs
TC hit
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Beam induced background
108 m/s produce 108 e+/s produce 108 g/s
Cable ducts
for Drift Chamber
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Detector Performance
• Prototypes of all detectors have been built and tested
• Large Prototype Liquid Xenon Detector (1/4)
• 4 (!) Drift Chambers
• Single Timing Counter Bar
• Performance has been carefully optimized
• Light yield in Xenon has been improved 10x
• Timing counter 1 ns  100 ps
• Noise in Drift Chamber reduced 10x
• Detail Monte Carlo studies were used to optimize material
• Continuous monitoring necessary to ensure stability!
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Sensitivity and Background Rate
Aimed experiment parameters:
Aimed resolutions:
Nm
3 107 /s
T
2 107 s (~50 weeks)
W/4p
0.09
Ee
0.8%
ee
0.90
Eg
5%
eg
0.60
qeg
18 mrad
esel
0.70
teg
180 ps
FWHM
Single event sensitivity (Nm • T • W/4p • ee • eg • esel )-1 = 3.6  10-14
Prompt Background
Bpr  10-17
Accidental Background Bacc  Ee • teg • (Eg )2 • (qeg )2  4  10-14
90% C.L. Sensitivity  1.3  10-13
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Current resolution estimates
Exp./
Lab
Author
Year
Ee/Ee
%FWH
M
Eg /Eg
%FWHM
teg
(ns)
qeg
(mrad)
Inst. Stop
rate (s-1)
Duty
cycle
(%)
Result
SIN
(PSI)
A. Van der
Schaaf
1977
8.7
9.3
1.4
-
(4..6) x 105
100
< 1.0  10-9
TRIUMF
P.
Depommier
1977
10
8.7
6.7
-
2 x 105
100
< 3.6  10-9
LANL
W.W.
Kinnison
1979
8.8
8
1.9
37
2.4 x 105
6.4
< 1.7  10-10
Crystal
Box
R.D. Bolton
1986
8
8
1.3
87
4 x 105
(6..9)
< 4.9  10-11
MEGA
M.L. Brooks 1999
1.2
4.5
1.6
17
2.5 x 108
(6..7)
< 1.2  10-11
MEG
2008
0.8
4.3
0.18
18
3 x 107
100
~ 10-13
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Current sensitivity estimation
• Resolutions have been
updated constantly to see
where we stand
• Two international reviews
per year
• People are convinced that
the final experiment can
reach 10-13 sensitivity
http://meg.web.psi.ch/docs/calculator/
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How to address pile-up
• Pile-up can severely degrade the experiment performance!
• Traditional electronics cannot detect pile-up
TDC
Amplifier
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Discriminator
Need full
waveform digitization
to reject pile-up
Measure Time
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Waveform Digitizing
• Need 500 MHz 12 bit digitization for Drift Chamber system
• Need 2 GHz 12 bit digitization for Xenon Calorimeter + Timing
Counters
• Need 3000 Channels
• At affordable price
Solution: Develop own
“Switched Capacitor Array” Chip
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The Domino Principle
0.2-2 ns
Inverter “Domino” ring chain
IN
Waveform
stored
Clock
Shift Register
Out
FADC
33 MHz
“Time stretcher” GHz  MHz
Keep Domino wave running in a circular fashion and
stop by trigger  Domino Ring Sampler (DRS)
26 June '07
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The DRS chip
• DRS chip developed at PSI
• 5 GHz sampling speed, 12 bits resolution
• 12 channels @ 1024 bins on one chip
32 channels input
• Typical costs ~60 € / channel
26 June '07
• 3000 Channels installed in MEG
• Licensing to Industry (CAEN) in progress
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Waveform examples
“virtual oscilloscope”
pulse shape discrimination
original:
Crosstalk removal by subtracting empty channel
first
derivation:
t = 15ns
26 June '07
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DAQ System Principle
Liquid Xenon Calorimeter
Drift Chamber
Timing Counter
Active Splitter
VME
VME
Trigger
Event number
Event type
optical
link
(SIS3100)
Waveform
Digitizing
Trigger
Busy
Rack PC
Rack PC
Rack PC
Rack PC
Rack PC
Switch
Rack PC
Rack PC
Rack PC
Rack PC
Event Builder
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DAQ System
• Use waveform digitization
(500 MHz/2 GHz) on all channels
• Waveform pre-analysis directly in
online cluster (zero suppression,
calibration) using multi-threading
• MIDAS DAQ Software
• Data reduction: 900 MB/s  5 MB/s
• Data amount: 100 TB/year
2000 channels
waveform digitizing
26 June '07
DAQ cluster
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Monitoring
How to keep the experiment stable?
Long time stability
• Especially the calorimeter needs to run stably over years
• Primary problem: Gain drift of PMT might shift background event into
signal region
• If we find m  e g , are we sure it’s not an artifact?
• Need sophisticated continuous calibration!
• Unfortunately, there is no 52.8 MeV
g source available
N
52.8 MeV
meg
m  e nn g
Eg[MeV]
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Planned Calorimeter Calibrations
Combine calibration methods different in complexity and energy:
Method
Energy
Frequency
LED pulser
~few MeV
Continuously
241Am
5.6 MeV a
Continuously
n capture on Ni
9 MeV g
daily
p+  7Li
17.6 MeV g
daily
p0 production on LH2
54 – 82 MeV g monthly ?
source on wire
100 mm gold-plated
tungsten wire
26 June '07
n  58Ni
9 MeV
LED
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7Li(p,g)8Be
Spectrum
7Li
(p,g)8Be
resonant at Ep= 440 keV
=14 keV speak = 5 mb
Eg0 = 17.6 MeV
Eg1 = 14.6 MeV
6.1 MeV
Bpeak g0/(g0+ g1)= 0.72
Crystal Ball Data
g1
NaI 12”x12”
26 June '07
g0
g spectrum
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CW Accelerator
• 1 MeV protons
• 100 mA
• HV Engineering,
Amersfoort, NL
beam spot
p+
m+ p-
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p0 Calibration
• Tune beam line to p• Use liquid H2 target
•
NaI
g
p0
q
p-p  p0n
• Tag one g with movable
NaI counter
• Beamline & target
change take ~1 day
g
target
26 June '07
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Midas Slow Control Bus System
BTS Magnet
Beamline
LXe purifier
LXe storage
LXe cryostat
NaI mover
MSCB
DC gas system
•
•
•
•
A/C hut
VME Crates
HV
All subsystems controlled by same MSCB system
All data on tape
Central alarm and history system
Also used now at: mSR, SLS, nEDM, TRIUMF
HVR
26 June '07
Cooling water
COBRA
PC
SCS-2000
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Status and Outlook
Where are we, where do we go?
Current Status
• Goal: Produce “significant” result before LHC
• R & D phase took longer than anticipated
http://meg.psi.ch
• We are currently in the set-up and
engineering phase, detector is expected
to be completed end of 2007
• Data taking will go 2008-2010
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
26 June '07
R&D
Set-up
Engineering
Data
Taking
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What next?
Will we find
meg?
Yes
No
• Improve experiment from
10-13 to 10-14:
• Denser PMTs
• Second Calorimeter
• Carefully check results
• Be happy 
• Most extensions of the SM (SUSY,
Little Higgs, Extra Dimensions)
predict m  e g
 More experiments needed
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“Polarized” MEG
•
m are produced already polarized
• Different target to keep m polarization
• Angular distribution of decays predicted
differently by different theories
(compare Wu experiment for
Parity Violation)
1  APm cosqe
dN(m   eg )
 BR(m   eg ) 
d cosqe
2
Detector acceptance
SU(5) SUSY-GUT
A = +1
SO(10) SUSY-GUT
A0
MSSM with nR
A = -1
Y.Kuno et al.,
Phys.Rev.Lett. 77 (1996) 434
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Expected Distribution
•
•
•
•
•
A = +1
B (m+ e+ g) = 1 x 10-12
1 x 108 m+/s
5 x 107 s beam time (2 years)
Pm = 0.97
Signal +
Background
S. Yamada
@ SUSY 2004, Tsukuba
26 June '07
Background
Particle Colloquium Heidelberg
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Conclusions
• The MEG Experiment has good prospectives to improve the current
limit for m  e g by two orders of magnitude
• Pushing the detector technologies takes time
• The experiment is now starting up, so expect exciting results in
2008/2009
http://meg.psi.ch
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