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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. Motivation. Why should we search for m  e g ?.

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lepton flavor violation goals and status of the meg experiment at psi

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

Stefan Ritt

Paul Scherrer Institute, Switzerland

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

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motivation

Motivation

Why should we search for m  e g ?

the standard model
The Standard Model

Generation I II III

*) Yet to be confirmed

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the success of the sm
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

CDF

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beyond the sm

High Energy Frontier

  • Produce heavy new particles directly
  • Heavy particles need large colliders
  • Complex detectors
  • High Precision Frontier
  • Look for small deviations from SM (g-2)m , CKM unitarity
  • Look for forbidden decays
  • Requires high precision at low energy
Beyond the SM

Find New Physics

Beyond the SM

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neutron beta decay
Neutron beta decay
  • Neutron b decay via intermediate heavy W- boson

Neutron mean life time:

886 s

~80 MeV

ne

W -

e-

~5 MeV

u

d

  • decay discovery:

~1934

W- discovery:

1983

p

n

d

d

u

u

n  p+ + e - + ne

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new physics in m decay
New physics in m decay
  • Can’t we do the same in m decay?

g

?

m-

e-

 Probe physics at TeV scale with high precision m decay measurement

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the muon
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

< 10-11

led to Lepton Flavor Conservationas “accidental” symmetry

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lepton flavor conservation
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

g

W-

m-

e-

nm

ne

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

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lfv in susy

g

g

W-

m-

e-

m-

e-

nm

ne

LFV in SUSY
  • While LFV is forbidden in SM, it is possible in SUSY

≈ 10-12

Current experimental limit: BR(m e g) < 10-11

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lfv summary
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 – 108m/s DC beam needed
    • ~ 2 years measurement time
    • excellent background suppression

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history of lfv searches

m→ e g

mA→ eA

m→ eee

History of LFV searches
  • Long history dating back to 1947!
  • Best present limits:
    • 1.2 x 10-11 (MEGA)
    • mTi → eTi < 7 x 10-13 (SINDRUM II)
    • m → eee < 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

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

10-12

10-13

MEG

10-14

10-15

1940 1950 1960 1970 1980 1990 2000 2010

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current susy predictions

ft(M)=2.4 m>0 Ml=50GeV 1)

Current SUSY predictions

current limit

MEG goal

tan b

“Supersymmetric parameterspace accessible by LHC”

  • J. Hisano et al., Phys. Lett. B391 (1997) 341
  • MEGA collaboration, hep-ex/9905013

W. Buchmueller, DESY, priv. comm.

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lfv link to other susy proc

g

m-

e-

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-

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experimental method

Experimental Method

How to detect m  e g ?

decay topology m e g
Decay topology m  e g

52.8 MeV

m e g

N

g

52.8 MeV

m

180º

Eg[MeV]

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

Ee[MeV]

10

20

30

40

50

60

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michel decay 100

n

m

n

e

Michel Decay (~100%)
  • Three body decay: wide energy spectrum

N

52.8 MeV

Theoretical

m  e nn

Ee[MeV]

N

52.8 MeV

Convoluted withdetector resolution

Ee[MeV]

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radiative muon decay 1 4

g

n

m

n

e

Radiative Muon Decay (1.4%)

N

52.8 MeV

m  e nn g

Eg[MeV]

“Prompt” Background

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accidental background

g

g

n

m

n

n

n

m

e

e

“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

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how to build a good experiment
How to build a good experiment?

Photon Calorimeter

Muon Beam

Positron Detector

Electronics

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collaboration
Collaboration
  • ~70 People (40 FTEs) from five countries

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paul scherrer institute

Proton Accelerator

Swiss Light Source

Paul Scherrer Institute

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psi proton accelerator
PSI Proton Accelerator

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generating muons
Generating muons

Carbon Target

p+

m+

590 MeV/c2 Protons

1.8 mA = 1016 p+/s

108m+/s

m+

p+

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muon beam structure
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 %

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muon beam line

-

x

x

x

m+

x

x

x

e+

+

Muon Beam Line
  • Transport 108m+/s to stopping target inside detector with minimal background

Lorentz Force vanishes for given v:

Wien Filter

m+ from production target

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results of beam line optimization
Results of beam line optimization

Rm ~ 1.1x108m+/s at experiment

e+

m+

s ~ 10.9 mm

m+

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previous experiments31
Previous Experiments

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the mega experiment
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
Photon Detectors (@ 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:

PMT

PMT

PMT

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liquid xenon calorimeter

H.V.

Refrigerator

Signals

Cooling pipe

Vacuum

for thermal insulation

Al Honeycomb

Liq. Xe

window

PMT

filler

Plastic

1.5m

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: -120°C … -108°
  • Extremely high purity necessary:1 ppm H20 absorbs 90% of light
  • Currently largest LXe detector in theworld: Lots of pioneering work necessary

g

m

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lxe g response
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

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lxe g response36
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

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slide37

Use GEANT to carefully study detector

  • Optimize placement of PMTs according to MC results

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lxe calorimeter prototype
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
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
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)
  • Position resolution:9 mm (FWHM)

FWHM = 4.8%

FWHM = 260 ps

To be improved with refinedanalysis methods

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lessons learned with prototype
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

GXe pump

(10-50L/min)

Heat exchanger

GXe storage tank

Getter+Oxysorb

Cryocooler

LN2

(100W)

LN2

Cryocooler

(>150W)

Liquid pump

(100L/h)

Purifier

1000L storage dewar

LXe Calorimeter

Liquid circulating

purifier

Xenon storage
  • ~900L in liquid, largest amount of LXe ever liquefied in the world

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final calorimeter
Final Calorimeter
  • Currently being assembled, will go into operation summer ‘07

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positron spectrometer

Homogeneous Field

Gradient Field

(COnstant-Bending-RAdius)

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

high pt track

constant |p| tracks

e+ from m+e+g

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drift chamber
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
Positron Detection System
  • 16 radial DCs with extremely low mass
  • He:C2H6 gas mixture
  • Test beam measurementsand MC simulation:
    • Dp/p = 0.8%
    • Dq = 10 mrad
    • Dxvertex = 2.3 mm

FWHM

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timing counter
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

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the complete meg detector
The complete MEG detector

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mc simulation of full detector
MC Simulation of full detector

g

e+

“Soft” gs

TC hit

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beam induced background
Beam induced background
  • 108m/s produce 108 e+/s produce 108g/s

Cable ductsfor Drift Chamber

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detector performance
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
Sensitivity and Background Rate
  • Aimed experiment parameters:

Aimed resolutions:

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

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current resolution estimates
Current resolution estimates

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current sensitivity estimation
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/

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how to address pile up
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

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waveform digitizing
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
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)

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the drs chip
The DRS chip
  • DRS chip developed at PSI
  • 5 GHz sampling speed, 12 bits resolution
  • 12 channels @ 1024 bins on one chip
  • Typical costs ~60 € / channel
  • 3000 Channels installed in MEG
  • Licensing to Industry (CAEN) in progress

32 channels input

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waveform examples
Waveform examples

“virtual oscilloscope”

pulse shape discrimination

Quantum Step in Technology!

original:

Crosstalk removal by subtracting empty channel

firstderivation:

Dt = 15ns

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daq system principle
DAQ System Principle

Liquid Xenon Calorimeter

Timing Counter

Drift Chamber

Active Splitter

VME

VME

LVDS parallel bus

Trigger

Event number

Event type

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

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daq system
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

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monitoring

Monitoring

How to keep the experiment stable?

long time stability
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 MeVg source available

N

52.8 MeV

m  e g

m  e nn g

Eg[MeV]

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planned calorimeter calibrations
Planned Calorimeter Calibrations
  • Combine calibration methods different in complexity and energy:

n 58Ni

9 MeV

100 mm gold-plated tungsten wire

LED

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7 li p g 8 be spectrum
7Li(p,g)8Be Spectrum

7Li (p,)8Beresonant 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 12”x12”  spectrum

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cw accelerator
CW Accelerator
  • 1 MeV protons
  • 100 mA
  • HV Engineering, Amersfoort, NL

beam spot

p+

m+p-

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p 0 calibration
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

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m idas s low c ontrol b us system
Midas Slow Control Bus System

BTS Magnet

Beamline

LXe purifier

LXe storage

LXe cryostat

NaI mover

MSCB

DC gas system

A/C hut

Cooling water

VME Crates

HV

COBRA

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

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status and outlook

Status and Outlook

Where are we, where do we go?

current status

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

R&D

Set-up

Engineering

Data

Taking

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

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what next

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
What next?

Will we findm e g ?

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polarized meg
“Polarized” MEG
  • m are produced already polarized
  • Different target to keep m polarization
  • Angular distribution of decays predicteddifferently by different theories(compare Wu experiment for Parity Violation)

Detector acceptance

SU(5) SUSY-GUTA = +1

SO(10) SUSY-GUTA 0

MSSM with nRA = -1

Y.Kuno et al.,

Phys.Rev.Lett. 77 (1996) 434

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expected distribution
Expected Distribution
  • A = +1
  • B (m+ e+ g) = 1 x 10-12
  • 1 x 108m+/s
  • 5 x 107 s beam time (2 years)
  • Pm = 0.97

Signal +Background

Background

S. Yamada @ SUSY 2004, Tsukuba

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conclusions
Conclusions
  • The MEG Experiment has good prospectives to improve the current limit for m  e gby 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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