19 april 2006fabrizio cei1 the liquid xenon calorimeter of the meg experiment fabrizio cei infn and...

19 April 200619 April 2006 Fabrizio CeiFabrizio Cei 11

The Liquid Xenon The Liquid Xenon Calorimeter of the MEG Calorimeter of the MEG

ExperimentExperiment

Fabrizio CeiFabrizio Cei

INFNINFN and and Universita’ Universita’ didi PisaPisa

Incontri di Fisica delle Alte Energie – Incontri di Fisica delle Alte Energie – IFAE 2006IFAE 2006

Pavia, 19-21 April 2006Pavia, 19-21 April 2006


OutlineOutline

• Brief overview of the Brief overview of the MEG experiment;MEG experiment;• The The Liquid XenonLiquid Xenon scintillation scintillation

calorimeter;calorimeter;• The The Calorimeter (Large) Prototype;Calorimeter (Large) Prototype;• Measured and expected Measured and expected performances;performances;• Status of Status of calorimeter preparation;calorimeter preparation;• Conclusions.Conclusions.


Overview of the MEG Overview of the MEG experimentexperiment


The MEG Experiment at The MEG Experiment at PSIPSI

Search for Search for Lepton Lepton FlavourFlavour Violating decay Violating decay e e

The Paul Scherrer The Paul Scherrer InstituteInstitute

The The most powerful continuous most powerful continuous machinemachine in the world; in the world;

Proton energyProton energy 590 MeV590 MeV;; Power 1.1 MWPower 1.1 MW;; Nominal operation currentNominal operation current 1.8 1.8

mA.mA.


The MEG CollaborationThe MEG Collaboration

~ 40 FTEs~ 40 FTEs


The The e edecay – 1) decay – 1)

• ForbiddenForbidden in the in the Standard Model of Standard Model of electroweak interactionselectroweak interactions because of the because of the conservation of lepton family numbersconservation of lepton family numbers..

• With With massive neutrinosmassive neutrinos (we know that (we know that mm > > 0 !0 !) and ) and mixingmixing, , eeis allowed but at a is allowed but at a negligible levelnegligible level (relative probability (relative probability ~ 10~ 10-55-55))

2

222

3

5227.1

sin2sin2 192

eW

F

M

mmG


The The e edecay – 2)decay – 2)

• All All SM extensionsSM extensions enhance the rateenhance the rate through mixing in the high energy through mixing in the high energy sector.sector.

e0

e

2emSUSYSUSY

SUSYBR( )e

4

5 2

SUS2

Y

2100 GeV

10 tanem

mm

≈ 10-12

Predictions in the Predictions in the range 10range 10-12-12 10 10-15-15

SM Background negligible SM Background negligible clear evidence for physics beyond the clear evidence for physics beyond the standard modelstandard model


Historical perspectiveHistorical perspective

BR < 0.1BR < 0.1

Present limitPresent limit1.2 x 101.2 x 10-11-11

Improvements in Improvements in physicsphysics linked with improvements in linked with improvements in technology.technology.


ee: Signal and : Signal and BackgroundBackground

ee++ ++

ee = = 180°180°

EEee == E E == 52.852.8 MeVMeV

TTee = = TT

Signal ee

Background

physicalphysical

ee

ee++ ++

accidentaaccidentall

ee ee

ee ee

eZ eZ eZeZ

ee++ ++

Accidental backgroundAccidental backgrounddominant in the signal region dominant in the signal region


ee: required : required performancesperformances

Exp./Lab Year Ee/Ee

(%)

E/E

(%)

te (ns)

e

(mrad)

Stop rate

(muons s-1)

Duty cyc.(%)

BR

(90% CL)

SIN SIN 19719777

8.78.7 9.39.3 1.41.4 -- 5 x 105 x 1055 100100 3.6 x 103.6 x 10-9-9

TRIUMFTRIUMF 19719777

1010 8.78.7 6.76.7 -- 2 x 102 x 1055 100100 1 x 101 x 10-9-9

LANLLANL 19719799

8.88.8 88 1.91.9 3737 2.4 x 102.4 x 1055 6.46.4 1.7 x 101.7 x 10--

1010

Crystal BoxCrystal Box 19819866

88 88 1.31.3 8787 4 x 104 x 1055 (6..9)(6..9) 4.9 x 104.9 x 10--

1111

MEGAMEGA 19919999

1.21.2 4.54.5 1.61.6 1717 2.5 x 102.5 x 1088 (6..7)(6..7) 1.2 x 101.2 x 10--

1111

MEGMEG 20020077

0.80.8 44 0.150.15 1919 2.5 x 102.5 x 1077 100100 1 x 101 x 10-13-13

The The sensitivity is limited bysensitivity is limited by the the accidental backgroundaccidental background

BR (BR ( e e 10 10-13-13 allowed, but needed allowed, but needed 14

eγ2eγ

2γeμacc 103ΔtΔθΔEΔERBR

FWHMFWHM

Need of a DC beamNeed of a DC beam

Some of them Some of them already fulfilled !already fulfilled !


MEG Experiment MEG Experiment Layout Layout

1m

e+

Liq . Xe Scin tilla tionDetector

Drift C ham ber

L iq . Xe Scin tilla tionDetector

e+

Tim ing C ounter

Stopping TargetThin S uperconducting Coil

M uon Beam

Drift C ham ber

• Muon beam stoppedMuon beam stopped in in aa 150 150 mm target.target.

• Liquid Xenon Liquid Xenon calorimetercalorimeter (800 l, (800 l, 850 PMTs)850 PMTs) for photon for photon detection detection using using scintillation light: scintillation light: fast fast responseresponse (~ 20 ns) and (~ 20 ns) and high light yieldhigh light yield (~ 0.8 (~ 0.8 NaI).NaI).

• (Thin wall) solenoidal (Thin wall) solenoidal spectrometerspectrometer & & drift drift chamberschambers forfor positron positron momentum momentum measurement. measurement.

• Scintillation countersScintillation counters for positron timing.for positron timing.

Easy signal selection for Easy signal selection for ++ decaying at decaying at restrest

ee++ ++ EEee == E E == 52.852.8 MeVMeV

ee = = 180°180°


Energy release in LXeEnergy release in LXe

Positron trackPositron track

Hits on TCHits on TC

One MC One MC eventevent


The Liquid Xenon The Liquid Xenon CalorimeterCalorimeter


The Calorimeter – 1)The Calorimeter – 1)

Liq. Xe

H.V.

Vacuum

for thermal insulation

Al Honeycombwindow

PMT

Refrigerator

Cooling pipe

Signals

fillerPlastic

1.5m

Measurement of Measurement of energy, energy, direction direction andand timing timing

Liquid Xenon propertiesLiquid Xenon properties

Density 2.95 g/cm3

Boiling and melting points 165 K, 161 K

Energy per scintillation photon 24 eV

Radiation length 2.77 cm

Decay time 4.2 ns, 22 ns, 45 ns

Scintillation light wave length 175 nm

Scintillation light absorption length > 100 cm

Attenuation length (Rayleigh scattering) 30 cm

Refractive index 1.74

Experimental Experimental checkcheck


The Calorimeter – 2)The Calorimeter – 2)

Homogeneous Homogeneous 0.8 m0.8 m33 volume of volume of Liquid Liquid XenonXenon - - pulse tube refrigeratorpulse tube refrigerator - - 67 cm < r < 108 cm67 cm < r < 108 cm - - |cos(|cos()| < 0.35; |)| < 0.35; || < 60| < 60oo 10 10 %% Only Only scintillation lightscintillation light;; Read by Read by 846 PMTs (846 PMTs (HamamatsuHamamatsu)):: - 2 inches diameter;- 2 inches diameter; - Maximum - Maximum photocathodic coveragephotocathodic coverage on the on the photon entrance face photon entrance face 43 % 43 %; ; - Immersed in Liquid Xenon;- Immersed in Liquid Xenon; - Low temperature (- Low temperature (165 165 ooKK);); - - Quartz windowQuartz window for matching with for matching with scintillation light wavelength (scintillation light wavelength (178 nm178 nm);); Thin entrance wall;Thin entrance wall; Waveform digitizingWaveform digitizing @ 2 GHz for @ 2 GHz for pile-up rejection.pile-up rejection.


PMT R & D history - 1)PMT R & D history - 1)

The MEG calorimeter will work in an The MEG calorimeter will work in an intense intense background background environment. Photons will be produced by several sources environment. Photons will be produced by several sources ((muon radiative decay, bremsstrahlung, positron annihilation, muon radiative decay, bremsstrahlung, positron annihilation, neutron capture in Xenon and materials surrounding the detectorneutron capture in Xenon and materials surrounding the detector …). …).

We estimated that We estimated that the light level due to the backgroundthe light level due to the background would would correspond to correspond to a few a few A anodic currentA anodic current for a for a PMT gain G = 10PMT gain G = 1066..

At so high rates one expects At so high rates one expects undesired undesired behavioursbehaviours because of because of increasing of photocathode resistivity at low temperaturesincreasing of photocathode resistivity at low temperatures;;

changes in amplification when the average anodic current changes in amplification when the average anodic current becomes comparable with the PMT base currentbecomes comparable with the PMT base current ( (50 50 AA). ).


PMT R & D history – 2)PMT R & D history – 2)11stst generation: generation: R6041QR6041Q 22ndnd generation: generation: R9288TBR9288TB

Photocathode: Photocathode: Rb-Cs-SbRb-Cs-Sb K-Cs-K-Cs-SbSb Material to reduce Material to reduce surface resistivity: surface resistivity: Mn layerMn layer Al Al stripsstripsQ.E. @ 165 Q.E. @ 165 ooK: K: ~ 5 %~ 5 % ~ 15 ~ 15 %%

Photon high rate simulated by Photon high rate simulated by high frequencyhigh frequency (tens of kHz) (tens of kHz) LED pulsingLED pulsing

ONONONON OFFOFF

OFFOFF

OverlinearityOverlinearity(hidden in the (hidden in the previous plot previous plot by Q.E. drop)by Q.E. drop)

http://meg.web.psi.ch/gallery/view_photo.php?set_albumName=album04&id=aaa


PMT R & D history – 3)PMT R & D history – 3)33rdrd generation: generation: R9869QR9869Q

Photocathode:Photocathode: K-Cs-Sb K-Cs-Sb Material to reduce Material to reduce surface resistivity: surface resistivity: Al strips (doubled)Al strips (doubled)Q.E. @ 165 Q.E. @ 165 ooK: K: ~ 15 %~ 15 %

Doubling the Al strips Doubling the Al strips produces a produces a better better stabilization of stabilization of resistivity at low resistivity at low temperatures.temperatures.

Insertion of Insertion of Zener Zener diodesdiodes in the last two in the last two stages of base stages of base amplification chain amplification chain removes removes overlinearityoverlinearity.


Xenon purity – 1)Xenon purity – 1)Energy resolutionEnergy resolution strongly depends on strongly depends on scintillation light absorptionscintillation light absorption:: - - reduced number of photoelectronsreduced number of photoelectrons;; - - loss of uniformity (combined with loss of uniformity (combined with Rayleigh scatteringRayleigh scattering))..

Xenon almost transparentXenon almost transparent to its own scintillation light, but to its own scintillation light, but possible contaminants can be very opaquepossible contaminants can be very opaque … …


Xenon purity – 2)Xenon purity – 2)We developed a We developed a purification purification systemsystem to reduce to reduce impurities impurities below ppbbelow ppb. Xenon is circulated in . Xenon is circulated in liquid phase (liquid phase (100 l/hour100 l/hour by means by means of a Barber-Nicols cryogenic fluid of a Barber-Nicols cryogenic fluid pump) and pump) and water vaporwater vapor is removed is removed by a by a purifier cartridgepurifier cartridge filled with filled with molecular sievesmolecular sieves..

Purification performancesPurification performances

Purification system tested Purification system tested and improved by means of and improved by means of the Calorimeter Prototype.the Calorimeter Prototype.

(old system, new one is much faster)(old system, new one is much faster)


The Calorimeter The Calorimeter PrototypePrototype

(Large Prototype)(Large Prototype)


The Large Prototype - 1)The Large Prototype - 1)

Presently the Presently the largestlargest Liquid Liquid Xenon calorimeter in the Xenon calorimeter in the world:world: 40 x 40 x 50 cm40 x 40 x 50 cm33;; ~ 70 liters of Liquid Xenon~ 70 liters of Liquid Xenon 228 PMTs228 PMTs (types (types R6041R6041 & & R9288R9288, , not the newest ones);not the newest ones); MeasurementsMeasurements:: - cryogenic and long term - cryogenic and long term operation;operation; - absorption length;- absorption length; - - energyenergy, , timingtiming and and positionposition resolution.resolution. Operating conditions similar Operating conditions similar to to that of that of final detectorfinal detector..


The Large Prototype – 2)The Large Prototype – 2)

LEDs-source

--sources sources and and

LEDsLEDs for for PMTPMT

calibrationcalibration

and and

monitoringmonitoring

Seen from insideSeen from inside


The Large Prototype - 3)The Large Prototype - 3) Home-made Home-made Polonium Polonium alpha –sourcesalpha –sources mounted on mounted on 50 micron tungsten wires50 micron tungsten wires (to be replaced by (to be replaced by commercial commercial Am sourcesAm sources, specifically , specifically developed by developed by SORAD Ltd.SORAD Ltd.);); ~ 50 Bq for each source~ 50 Bq for each source;; First application of this type First application of this type of sources; preprint submitted of sources; preprint submitted to NIM.to NIM. Already used for Q.E. Already used for Q.E. determinationdetermination..


Measured Performances Measured Performances of the Calorimeterof the Calorimeter


Measurement of absorption Measurement of absorption lengthlength

cm 40 Ray

By using By using alpha sourcesalpha sources (on walls and on wires) is possible to give a (on walls and on wires) is possible to give a lower lower limit of the limit of the Xenon absorption lengthXenon absorption length absabs and an estimate of the and an estimate of the light yieldlight yield..

absabs > 95 cm (95 % C.L.) > 95 cm (95 % C.L.) Light Yield ~ 37500 scintillation photons/MeV (0.9 NaI)Light Yield ~ 37500 scintillation photons/MeV (0.9 NaI)


Measurement of energy Measurement of energy resolutionresolution

Charge exchange reactionCharge exchange reaction

--p p 00nn

Liquid Hydrogen targetLiquid Hydrogen target to to maximize photon flux;maximize photon flux; 00 Frame: Frame: monochromatic monochromatic spectrumspectrum LAB Frame: LAB Frame: flat spectrum; flat spectrum; Back-to-back configurationBack-to-back configuration:: EE = 55, 83 MeV = 55, 83 MeV;; Even Even a modest collimationa modest collimation ( ( 5 5oo)) guarantees a guarantees a sufficientsufficient monochromaticity monochromaticity ((E E 0.3 MeV 0.3 MeV));; Need of an Need of an opposite side opposite side detectordetector (a (a NaI array NaI array withwith LYSO LYSO preshowerpreshower).).


Experimental setupExperimental setup

H2 target LYSO

Eff 14 %

NaI

beam

S1LPEff (S1&& LP) 88 %


Energy spectra in NaI & Energy spectra in NaI & LPLP

83 MeV 83 MeV 55 MeV correlation 55 MeV correlation

129 MeV line from 129 MeV line from --p p n n(LXe sensitive to 9 MeV (LXe sensitive to 9 MeV neutrons)neutrons)


Energy resolution @ 55 Energy resolution @ 55 MeVMeV

Event selectionEvent selection::

LP && S1 && (NaI + LYSO)LP && S1 && (NaI + LYSO);; 83 MeV line in NaI + LYSO 83 MeV line in NaI + LYSO

65 MeV < E65 MeV < ENaI + LYSONaI + LYSO < 95 MeV < 95 MeV;; No saturated PMTsNo saturated PMTs;; Collimator: r < 4 cmCollimator: r < 4 cm..

FWHM: FWHM: E/E = (4.9 E/E = (4.9 0.4) % 0.4) %

CollimatorCollimator

PMTsPMTs


Measurement of timing Measurement of timing resolutionresolution

high gainnormal gain

110 psec 103 psec

LXe – LYSO timing difference @ 55 MeVLXe – LYSO timing difference @ 55 MeV

tt LYSOLYSO BeamBeamNormal gain Normal gain 110110 ƟƟ 6464 ƟƟ 6161 = = 6565 High gain High gain 103103 ƟƟ 6464 ƟƟ 6161 = = 5353

FWHM =FWHM =153 ps153 ps

125 ps125 ps )ps5540:(measured (depth) 22222 sczt z z 1 1 2 cm 2 cm


Status of Calorimeter Status of Calorimeter PreparationPreparation


• PMTsPMTs: : 850 PMTs under850 PMTs under testing in PSI and in Pisatesting in PSI and in Pisa LXe facility (3 LXe facility (3 4/day); 4/day);

• CryostatCryostat underunder constructionconstruction; delivery ; delivery at PSI in spring 2006;at PSI in spring 2006;

• Gas SystemGas System: : almost almost readyready

in in E5 area at PSI.E5 area at PSI.


PMT mountingPMT mounting


ConclusionsConclusions


The MEG experiment is expected to start The MEG experiment is expected to start engineering runs in 2006engineering runs in 2006;; Experimental tests with sub-detectors showed that Experimental tests with sub-detectors showed that

many many of the needed resolutions were already fulfilledof the needed resolutions were already fulfilled;; For LXe calorimeter, we obtained an absorption lengthFor LXe calorimeter, we obtained an absorption length absabs > 100 cm > 100 cm, an energy resolution , an energy resolution E/E < 5 % E/E < 5 % @ 55 MeV and a timing resolution of @ 55 MeV and a timing resolution of ~ 150 ps FWHM~ 150 ps FWHM;;We successfully used We successfully used alpha sources mounted on wiresalpha sources mounted on wires for for calibrationcalibration and and monitoringmonitoring of the detector of the detector (the first application of these sources);(the first application of these sources); The The calorimeter buildingcalorimeter building and the and the PMT testing andPMT testing and calibrationcalibration are in advanced state. are in advanced state.


The MEG web pageThe MEG web page

Please, Please, visit it !visit it !


Backup slidesBackup slides


Comparison with LHCComparison with LHC

MEG

“Supersymmetric parameter space accessible by LHC”

“Supersymmetric parameter space accessible by LHC”

W. Buchmueller, DESY, priv. comm.

MEGA

R&D

19992000200120022003200420052006200720082009

Engineering

Data

Plans

• Data taking from 2007 on to reach 10-13 sensitivity (90% CL)

• Obtain a “significant” result before the LHC era

• Eventually reach 10-14 during LHC era

Plans

• Data taking from 2007 on to reach 10-13 sensitivity (90% CL)

• Obtain a “significant” result before the LHC era

• Eventually reach 10-14 during LHC era


Connection with Connection with --oscillationsoscillations –1) –1)

MEGMEG

Experimental Experimental boundbound

Largely favouredLargely favoured and confirmed by and confirmed by KamlandKamland

Additional contributionAdditional contribution toto slepton mixingslepton mixing fromfrom VV2121, matrix element , matrix element responsible responsible forfor solar neutrino deficit solar neutrino deficit. (. (J. Hisano & N. Nomura, Phys. Rev. J. Hisano & N. Nomura, Phys. Rev. D59D59 (1999) (1999) 116005)116005)

All All solar solar experimentsexperiments combinedcombined

tan(tan() = ) = 3030OO

tan(tan() = 0) = 0OO

AfterAfterKamlandKamland


Connection with Connection with --oscillations –2)oscillations –2)

Correlation Correlation betweenbetween BR (BR (ee && sinsin22((1313)) (still unknown !)(still unknown !)

In these models,In these models,BR (BR (ee is one is one of the most of the most sensitive sensitive tool to measuretool to measure sinsin22((1313))..

Sensitivity of futureSensitivity of futurelong-baseline long-baseline experimentsexperimentsA. Masiero et al., hep-ph/0407325A. Masiero et al., hep-ph/0407325

1010-11-11

1010-14-14


MC SimulationMC Simulation GeometryGeometry: full simulation of : full simulation of calorimeter structurecalorimeter structure, internal , internal

and external and external vesselsvessels, , PMT holdersPMT holders and and honeycombhoneycomb;; Scintillation light trackingScintillation light tracking: :

- - decay curvedecay curve and and wavelength spectrumwavelength spectrum of LXe scintillation; of LXe scintillation;- - absorptionabsorption and and Rayleigh scatteringRayleigh scattering in Liquid Xenon; in Liquid Xenon;

- - FresnelFresnel and and total reflectiontotal reflection on PMT quartz window and on PMT quartz window and PMT holders;PMT holders; - PMT quartz window - PMT quartz window transmittancetransmittance;; OutputsOutputs:: - - Energy depositEnergy deposit, , positionposition and and timingtiming in Liquid Xenon; in Liquid Xenon; - - Waveform outputWaveform output: hit timing of scintillation photons for : hit timing of scintillation photons for each PMT with digitizer binning. each PMT with digitizer binning. (( ～～ 8 x 108 x 104 4 photoelectrons photoelectrons @ 50 MeV).@ 50 MeV).


Calibration techniques - Calibration techniques - 1)1)

The required performances of the detector demand The required performances of the detector demand multiplemultiple and and complementarycomplementary calibration and monitoringcalibration and monitoring methods. methods.

Alpha sourcesAlpha sources: : Q.E. determination, LXe optical properties,Q.E. determination, LXe optical properties, energy scale stability, permanently installed within the calorimeterenergy scale stability, permanently installed within the calorimeter. . ButBut: no good energy reference, useless for timing;: no good energy reference, useless for timing; -lines from neutron capture in Ni-lines from neutron capture in Ni ( (EE = 9 MeV = 9 MeV): ): absolute energy absolute energy scale, light yield stability, usable very frequentlyscale, light yield stability, usable very frequently.. ButBut: useless for timing & Q.E., low sensitivity to optical properties;: useless for timing & Q.E., low sensitivity to optical properties; Charge exchange reactionCharge exchange reaction: : --p p 00nn (followed by (followed by 00 decay in two decay in two gamma’s): gamma’s): energy scale determination, absolute timing and position energy scale determination, absolute timing and position calibration, simultaneous calibration of the whole apparatus.calibration, simultaneous calibration of the whole apparatus. ButBut: difficult to use frequently, hardware modifications needed, : difficult to use frequently, hardware modifications needed, useless for Q.E., low sensitivity to optical properties; useless for Q.E., low sensitivity to optical properties;


Calibration techniques - Calibration techniques - 2)2)

Cockroft-Walton Proton acceleratorCockroft-Walton Proton accelerator based on based on

- - Resonant cross section at EResonant cross section at Epp = 440 keV = 440 keV ( (peakpeak = 6 mbarn, = 6 mbarn, 15 keV); 15 keV);

- - Main methodMain method: : simultaneous calibration of the whole apparatus,simultaneous calibration of the whole apparatus,

useful for absolute energy scale and monitoring, frequently usableuseful for absolute energy scale and monitoring, frequently usable..

MeV 17.6 Be Li 84

73 p


Alpha sources in LXeAlpha sources in LXe

MeasuredMeasured SimulatedSimulated

100 m thicktungsten wire

50 m thick goldplate clippedaround the wire


Radioactive Background in Radioactive Background in LPLP

-trigger-trigger withwith 5 5 10 1066 gaingain;;

Geometrical cutsGeometrical cuts to to exclude exclude -sources-sources;;

Energy scale: Energy scale: -source-source 208208Tl (2.59±0.06) MeVTl (2.59±0.06) MeV 4040K (1.42±0.06) MeVK (1.42±0.06) MeV Other lines ?? Other lines ??

uniform uniform on theon the front face; front face; few 10 minfew 10 min ( (withwith non-non-

dedicated triggerdedicated trigger);); nice calibration nice calibration forfor low low

energy energy ’s’s..

4040KK (1.461 MeV) (1.461 MeV)

208208TlTl (2.614 (2.614 MeV)MeV)

Never seen before !Never seen before !


Measurement of position Measurement of position resolutionresolution

(40 MeV gamma beam with (40 MeV gamma beam with 1 mm 1 mm collimatorcollimator))

Reconstruction by (localized) Reconstruction by (localized) weighted average methodweighted average method


SensitivitySensitivity

T = 2.6 T = 2.6 •• 10 107 7 ss

RR = 0.3 = 0.3 •• 10 1088 ++/s/s

Detector parametersDetector parameters /4/4 = 0.09 = 0.09

ee 0.9 0.9

selsel (0.9) (0.9)33 = 0.7 = 0.7

0.60.6

SignalSignal NNsigsig = BR = BR •• T T •• R Rmm •• /4/4 •• e e selsel Single Event Single Event SensitivitySensitivity SES = 1/(T SES = 1/(T •• R R •• /4/4 •• e e selsel ) ) 4 4 • • 1010-14-14

Correlated Correlated BackgroundBackground BRBRcorrcorr 3 3 •• 10 10-15-15

Accidental Accidental BackgroundBackground BRBRaccacc R R•• EEee •• ( (EE22 •• ( ())22 •• ttee 3 3 • • 1010-14-14

Upper Limit @ 90 % Upper Limit @ 90 % C.L.C.L. BR BR (( e e )) 1 1 • • 1010-13-13

DiscoveryDiscovery 4 events4 events ( (P = 2 P = 2 •• 10 10-3-3) correspond to ) correspond to BR =BR = 2 2 • • 1010-13-13

Cuts @ Cuts @ 1.4 FWHM1.4 FWHM

19 april 2006fabrizio cei1 the liquid xenon calorimeter of the meg experiment fabrizio cei infn and...

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