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Title: Lepton Flavor Violation: Goals and Status of the MEG Experiment at PSI


1
Lepton Flavor ViolationGoals and Status of the
MEG Experiment at PSI
  • Stefan Ritt
  • Paul Scherrer Institute, Switzerland

2
Agenda
  • Search for m ? e g down to 10-13
  • Motivation
  • Experimental Method
  • Status and Outlook

3
Motivation
  • Why should we search for m ? e g ?

4
The Standard Model
Fermions (Matter) Fermions (Matter) Fermions (Matter)

Quarks u up c charm t top
Quarks d down s strange b bottom

Leptons ne electronneutrino nm muonneutrino nt tauneutrino
Leptons e electron m muon t tau
Bosons

g photon Force carriers
g gluon Force carriers
W W boson Force carriers
Z Z boson Force carriers
Higgs boson
Generation I II III
) Yet to be confirmed
5
The success of the SM
  • The SM has been proven to be extremely successful
    since 1970s
  • Simplicity (6 quarks explain gt40 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

Todays goal is to look for physics beyond the
standard model
CDF
6
Beyond the SM

Find New Physics Beyond the SM
7
Neutron beta decay
  • Neutron b decay via intermediate heavy W- boson

Neutron mean life time 886 s
80 MeV
ne
W -
e-
5 MeV
d
u
  • decay discovery
  • 1934
  • W- discovery
  • 1983

p
n
d
d
u
u
n ? p e - ne
8
New physics in m decay
  • Cant we do the same in m decay?

g
?
m-
e-
? Probe physics at TeV scale with high precision
m decay measurement
9
The Muon
  • Discovery 1936 in cosmic radiation
  • Mass 105 MeV/c2
  • Mean lifetime 2.2 ms

Seth Neddermeyer
ne
W-
e-
Carl Anderson
100
m-
nm
0.014
lt 10-11
led to Lepton Flavor Conservationas accidental
symmetry
10
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

11
LFV and Neutrino Oscillations
  • Neutrino Oscillations ? Neutrino mass ? m ? e
    g possible even in the SM

? LFV in the charged sector is forbidden in the
Standard Model
n mixing
12
LFV in SUSY
  • While LFV is forbidden in SM, it is possible in
    SUSY

10-12
Current experimental limit BR(m ? e g) lt 10-11
13
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

14
History of LFV searches
  • Long history dating back to 1947!
  • Best present limits
  • 1.2 x 10-11 (MEGA)
  • mTi ? eTi lt 7 x 10-13 (SINDRUM II)
  • m ? eee lt 1 x 10-12 (SINDRUM II)
  • MEG Experiment aims at 10-13
  • Improvements linked to advancein technology

cosmic m
10-1
10-2
10-3
10-4
10-5
stopped p
10-6
10-7
m beams
10-6
10-9
stopped m
10-10
10-11
SUSY SU(5) BR(m ? e g) 10-13 ? mTi ? eTi
4x10-16?BR(m ? eee) 6x10-16
10-12
10-13
MEG
10-14
10-15
1940 1950 1960 1970 1980 1990
2000 2010
15
Current SUSY predictions

current limit
MEG goal
tan b
Supersymmetric parameterspace accessible by LHC
  1. J. Hisano et al., Phys. Lett. B391 (1997) 341
  2. MEGA collaboration, hep-ex/9905013

W. Buchmueller, DESY, priv. comm.
16
LFV link to other SUSY proc.

me-LFV
slepton mixing matrix
(g-2)m
g
In SO(10), eEDM is related to m?eg
m-
m-
mEDM
g
R. Barbieri et al., hep-ph/9501334
m-
m-
17
Experimental Method
  • How to detect m ? e g ?

18
Decay topology m ? e g

52.8 MeV
m ? e g
N
g
m
52.8 MeV
180º
EgMeV
10
20
30
40
50
60
e
N
52.8 MeV
  • m ? e g signal very clean
  • Eg Ee 52.8 MeV
  • qge 180º
  • e and g in time

52.8 MeV
EeMeV
10
20
30
40
50
60
19
Michel Decay (100)
  • Three body decay wide energy spectrum

N
52.8 MeV
Theoretical
m ? e nn
EeMeV
N
52.8 MeV
Convoluted withdetector resolution
EeMeV
20
Radiative Muon Decay (1.4)
N
52.8 MeV
m ? e nn g
EgMeV
Prompt Background
21
Accidental Background

Background
m ? e g
g
m ? e nn
m
Annihilation in flight
180º
e
m ? e nn
  • m ? e g signal very clean
  • Eg Ee 52.8 MeV
  • qge 180º
  • e and g in time

Good energy resolution Good spatial
resolution Excellent timing resolution Good
pile-up rejection
22
Previous Experiments
Exp./Lab Author Year DEe/Ee FWHM DEg /Eg FWHM Dteg (ns) Dqeg (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 lt 1.0 ? 10-9
TRIUMF P. Depommier 1977 10 8.7 6.7 - 2 x 105 100 lt 3.6 ? 10-9
LANL W.W. Kinnison 1979 8.8 8 1.9 37 2.4 x 105 6.4 lt 1.7 ? 10-10
Crystal Box R.D. Bolton 1986 8 8 1.3 87 4 x 105 (6..9) lt 4.9 ? 10-11
MEGA M.L. Brooks 1999 1.2 4.5 1.6 17 2.5 x 108 (6..7) lt 1.2 ? 10-11
MEG ? ? ? ? ? ? 10-13
23
How to build a good experiment?

Photon Calorimeter
Muon Beam
Positron Detector
Electronics
24
Collaboration
  • 70 People (40 FTEs) from five countries

25
Paul Scherrer Institute
26
PSI Proton Accelerator
27
Generating muons

Carbon Target
p
m
590 MeV/c2 Protons 1.8 mA 1016 p/s
108 m/s
m
p
28
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 100
Duty cycle 6
29
Muon Beam Line
  • Transport 108 m/s to stopping target inside
    detector with minimal background

Lorentz Force vanishes for given v
Wien Filter
m from production target
30
Results of beam line optimization
Rm 1.1x108 m/s at experiment
e
m
s 10.9 mm
m
31
Previous Experiments
Exp./Lab Author Year DEe/Ee FWHM DEg /Eg FWHM Dteg (ns) Dqeg (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 lt 1.0 ? 10-9
TRIUMF P. Depommier 1977 10 8.7 6.7 - 2 x 105 100 lt 3.6 ? 10-9
LANL W.W. Kinnison 1979 8.8 8 1.9 37 2.4 x 105 6.4 lt 1.7 ? 10-10
Crystal Box R.D. Bolton 1986 8 8 1.3 87 4 x 105 (6..9) lt 4.9 ? 10-11
MEGA M.L. Brooks 1999 1.2 4.5 1.6 17 2.5 x 108 (6..7) lt 1.2 ? 10-11
MEG ? ? ? ? 3 x 107 100 10-13
32
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!

33
Photon Detectors (_at_ 50 MeV)
  • Alternatives to Pair Spectrometer
  • 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

g induced shower
25 cm CsI
CsI
CsI
Liquid Xenon
Density 3 g/cm3
Melting/boiling point 161 K / 165 k
Radiation length 2.77 cm
Decay time 45 ns
Absorption length gt 100 cm
Refractive index 1.57
Light yield 75 of NaI (Tl)
PMT
PMT
PMT
34
Liquid Xenon Calorimeter
  • Calorimeter Measure g Energy, Positionand Time
    through scintillation light only
  • Liquid Xenon has high Z and homogeneity
  • 900 l (3t) Xenon with 848 PMTs(quartz window,
    immersed)
  • Cryogenics required -120C -108
  • Extremely high purity necessary1 ppm H20
    absorbs 90 of light
  • Currently largest LXe detector in theworld Lots
    of pioneering work necessary

g
m
35
LXe g response
  • Light is distributed over many PMTs
  • Weighted mean of PMTs on front face ? x
  • Broadness of distribution ? Dz
  • Position corrected timing ? Dt
  • Energy resolution depends on light attenuation
    in LXe

x
z
36
LXe g response
  • Light is distributed over many PMTs
  • Weighted mean of PMTs on front face ? Dx
  • Broadness of distribution? Dz
  • Position corrected timing? Dt
  • Energy resolution depends on light attenuation
    in LXe

x
z
37
  • Use GEANT to carefully study detector
  • Optimize placement of PMTs according to MC results

38
LXe Calorimeter Prototype
  • ¼ of the final calorimeter was build to study
    performance, purity, etc.

240 PMTs
39
How to get 50 MeV gs?
  • p- p ? p0 n (Panofsky) p0 ? g g
  • LH2 target
  • Tag one g with NaI LYSO

40
Resolutions
  • NaI tag 65 MeV lt E(NaI) lt 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)
  • Position resolution9 mm (FWHM)

FWHM 4.8
FWHM 260 ps
To be improved with refinedanalysis methods
41
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
42
Xenon storage
  • 900L in liquid, largest amount of LXe ever
    liquefied in the world

43
Final Calorimeter
  • Currently being assembled, will go into operation
    summer 07

44
Positron Spectrometer
  • Ultra-thin (3g/cm2) superconducting solenoid
    with 1.2 T magnetic field

high pt track
constant p tracks
e from m?eg
45
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

46
Positron Detection System
  • 16 radial DCs with extremely low mass
  • HeC2H6 gas mixture
  • Test beam measurementsand MC simulation
  • Dp/p 0.8
  • Dq 10 mrad
  • Dxvertex 2.3 mm

FWHM
47
Timing Counter
  • Staves along beam axis for timing measurement
  • Resolution 91 ps FWHM measured at Frascati e- -
    beam
  • Curved fibers with APD readout for z-position

Experiment Size cm Scintillator PMT latt cm FWHMps
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
48
The complete MEG detector
49
MC Simulation of full detector

g
e
Soft gs
TC hit
50
Beam induced background
  • 108 m/s produce 108 e/s produce 108 g/s

Cable ductsfor Drift Chamber
51
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!

52
Sensitivity and Background Rate
  • Aimed experiment parameters

Aimed resolutions
Nm 3 ?107 /s
T 2 ?107 s (50 weeks)
W/4p 0.09
ee 0.90
eg 0.60
esel 0.70
FWHM
DEe 0.8
DEg 5
Dqeg 18 mrad
Dteg 180 ps
Single event sensitivity (Nm T W/4p ee eg
esel )-1 3.6 ? 10-14
Prompt Background Bpr ? 10-17 Accidental
Background Bacc ? DEe Dteg (DEg )2 (Dqeg )2
? 4 ? 10-14 90 C.L. Sensitivity ? 1.3 ? 10-13
53
Current resolution estimates
Exp./Lab Author Year DEe/Ee FWHM DEg /Eg FWHM Dteg (ns) Dqeg (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 lt 1.0 ? 10-9
TRIUMF P. Depommier 1977 10 8.7 6.7 - 2 x 105 100 lt 3.6 ? 10-9
LANL W.W. Kinnison 1979 8.8 8 1.9 37 2.4 x 105 6.4 lt 1.7 ? 10-10
Crystal Box R.D. Bolton 1986 8 8 1.3 87 4 x 105 (6..9) lt 4.9 ? 10-11
MEGA M.L. Brooks 1999 1.2 4.5 1.6 17 2.5 x 108 (6..7) lt 1.2 ? 10-11
MEG 2008 0.8 4.3 0.18 18 3 x 107 100 10-13
54
Current sensitivity estimation
  • Resolutions have beenupdated constantly to
    seewhere we stand
  • Two international reviewsper year
  • People are convinced thatthe final experiment
    canreach 10-13 sensitivity

http//meg.web.psi.ch/docs/calculator/
55
How to address pile-up
  • Pile-up can severely degrade the experiment
    performance!
  • Traditional electronics cannot detect pile-up

Need fullwaveform digitization to reject pile-up
TDC
Discriminator
Measure Time
Amplifier
56
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 ownSwitched Capacitor Array
Chip
57
The Domino Principle
0.2-2 ns
Inverter Domino ring chain
IN
Waveform stored
Out
FADC 33 MHz
Clock
Shift Register
Time stretcher GHz ? MHz
Keep Domino wave running in a circular fashion
and stop by trigger ? Domino Ring Sampler (DRS)
58
The DRS chip
  • DRS chip developed at PSI
  • 5 GHz sampling speed, 12 bits resolution
  • 12 channels _at_ 1024 bins on one chip
  • Typical costs 60 / channel
  • 3000 Channels installed in MEG
  • Licensing to Industry (CAEN) in progress

32 channels input
59
Waveform examples
virtual oscilloscope
pulse shape discrimination

Quantum Step in Technology!
original
Crosstalk removal by subtracting empty channel
firstderivation
Dt 15ns
60
DAQ System Principle
Liquid Xenon Calorimeter
Timing Counter
Drift Chamber
Active Splitter
VME
VME
Trigger Event number Event type
LVDS parallel bus
Trigger
opticallink(SIS3100)
Waveform Digitizing
Busy
Rack PC
Rack PC
GBit Ethernet
Rack PC
Rack PC
Switch
Rack PC
Rack PC
Rack PC
Rack PC
Rack PC
Event Builder
61
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 channelswaveform digitizing
DAQ cluster
62
Monitoring
  • How to keep the experiment stable?

63
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 its not an
    artifact?
  • Need sophisticated continuous calibration!
  • Unfortunately, there is no 52.8 MeVg source
    available

N
52.8 MeV
m ? e g
m ? e nn g
EgMeV
64
Planned Calorimeter Calibrations
  • Combine calibration methods different in
    complexity and energy

Method Energy Frequency
LED pulser few MeV Continuously
241Am source on wire 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 ?
n ? 58Ni
9 MeV
100 mm gold-plated tungsten wire
LED
65
7Li(p,g)8Be Spectrum

7Li (p,?)8Be resonant at Ep 440 keV ?14 keV
?peak 5 mb E?0 17.6 MeV E?1 14.6 MeV
6.1 MeV Bpeak ?0/(?0 ?1) 0.72
Crystal Ball Data
?1
?0
NaI 12x12 ? spectrum
66
CW Accelerator
  • 1 MeV protons
  • 100 mA
  • HV Engineering, Amersfoort, NL

beam spot
p
m p-
67
p0 Calibration
  • Tune beam line to p-
  • Use liquid H2 target
  • p-p ? p0n
  • Tag one g with movable NaI counter
  • Beamline target change take 1 day

NaI
g
q
p0
g
target
68
Midas Slow Control Bus System

BTS Magnet
LXe purifier
LXe storage
LXe cryostat
NaI mover
Beamline
MSCB
COBRA
DC gas system
A/C hut
Cooling water
VME Crates
HV
PC
  • 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
SCS-2000
69
Status and Outlook
  • Where are we, where do we go?

70
Current Status
  • Goal Produce significant result before LHC
  • R D phase took longer than anticipated
  • We are currently in the set-up and engineering
    phase, detector is expectedto be completed end
    of 2007
  • Data taking will go 2008-2010

http//meg.psi.ch
71
What next?

Will we findm ? e g ?
72
Polarized MEG
  • m are produced already polarized
  • Different target to keep m polarization
  • Angular distribution of decays predicteddifferent
    ly by different theories(compare Wu experiment
    for Parity Violation)

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
73
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
Background
S. Yamada _at_ SUSY 2004, Tsukuba
74
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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