First results from the MEG/RE12 experiment at PSI

A.M. Baldini 29 sep 2009 Hep-ex:0908.2594v1 18 Aug 2009

2

Most recent + e+ Experiments

Lab. Year Upper limit Experiment or Auth.

PSI 1977 < 1.0 10-9 A. Van der Schaaf et al.

TRIUMF 1977 < 3.6 10-9 P. Depommier et al.

LANL 1979 < 1.7 10-10 W.W. Kinnison et al.

LANL 1986 < 4.9 10-11 Crystal Box

LANL 1999 < 1.2 10-11 MEGA

PSI ~2011 ~ 10-13 MEG

Two orders of magnitude improvement tough experimental challenge! But

several SUSY GUT and SUSY see-saw models predict BRs at the reach of

MEG

3

SUSY-GUT (SUGRA) First contribution: VCKM cLFV

• SUSY SU(5) predictions

BR (e) 10-14 10-

13

• SUSY SO(10) predictions

BRSO(10) 100 BRSU(5) R. Barbieri et al., Phys. Lett. B338(1994) 212

R. Barbieri et al., Nucl. Phys. B445(1995) 215

LFV induced by slepton mixing

Our goal

Experimental limit

combined LEP results favour tan>10

4

SO10

5

Our goal

Experimental limit

VPMNS ( oscillationscLFV

J. Hisano, N. Nomura, Phys. Rev. D59 (1999)

Independent contribution to slepton mixing from masses (see-saw model): V less known tan()=3

0

tan()=1

After SNO After Kamland

-5410R in the Standard Model !!If it is seen it is not SM!

6

Signal and background

e+ +

e = 180°

Ee = E = 52.8 MeV

Te = T

signal e

background

physical

e

e+ +

accidental

e e

ee

eZ eZ

e+ +

7

The sensitivity is limited by the accidental background

2phys.b. RnRnRn μacc.b.μμsig , ,

2eγeγ ΔΔ θt 2

γe ΔΔ EE accBR

Integral on the detector resolutions of the Michel and radiative decay spectra

Effective BRback (nback/R)

The n. of acc. backg events (nacc.b.) depends quadratically on the muon rateand on how well we measure the experimental quantities:

e- relative timing and angle, positron and photon energy

μR

8

Required Performances

Exp./Lab Year Ee/Ee

(%)

E/E

(%)

te (ns)

e

(mrad)

Stop rate

(s-1)

Duty cyc.(%)

BR

(90% CL)

SIN 1977 8.7 9.3 1.4 - 5 x 105 100 3.6 x 10-9

TRIUMF 1977 10 8.7 6.7 - 2 x 105 100 1 x 10-9

LANL 1979 8.8 8 1.9 37 2.4 x 105 6.4 1.7 x 10-10

Crystal Box 1986 8 8 1.3 87 4 x 105 (6..9) 4.9 x 10-11

MEGA 1999 1.2 4.5 1.6 17 2.5 x 108 (6..7) 1.2 x 10-11

MEG 2011 0.8 4 0.15 19 2.5 x 107 100 1 x 10-13

FWHM

BRacc.b. 2 10-14 and BRphys.b. 0.1 BRacc.b. with the following resolutions

Need of a DC muon beam

BR (e) 10-13 reachable

9

Experimental method

1m

e+

Liq. Xe Scin tilla tionDetector

Drift Cham ber

Liq. Xe Scin tilla tionDetector

e+

Tim ing Counter

Stopping TargetThin S uperconducting Coil

M uon Beam

Drift Cham ber

Detector outline1. Stopped beam of 3 107

/sec in a 150 m target

2. Solenoid spectrometer & drift chambers for e+ momentum

3. Scintillation counters for e+ timing

4. Liquid Xenon calorimeter for detection (scintillation)

• Method proposed in 1998: PSI-RR-99-05: 10-14 possibility

• MEG proposal: september 2002: 10-13 goal: A. Baldini and T. Mori spokespersons: Italy, Japan, Switzerland, Russia

10

Detector Construction

SwitzerlandSwitzerlandDrift ChambersBeam LineDAQ

JapanJapanLXe Calorimeter, Spectrometer’s magnet

RussiaRussiaLXe TestsBeam line

ItalyItalye+ counter Trigger LXe Calorimeter

USA(UCI)USA(UCI)Calibrations/Target/DC pressure system

11

Next slides...

1. The PSI E5 beamline

2. The Positron spectrometer

3. The Liquid Xenon calorimeter

4. DAQ

5. The 2008 run

6. Future

12

1) The PSI E5 DC beam

Primary proton beam

• 1.8 mA of 590 MeV/c protons (most intense DC beam in the world)

• 29 MeV/c muons from decay of stop at rest: fully polarized

+

Particles intensity as a function of the selected momentum

13

Beam studies

Optimization of the beam elements:

•Wien filter for /e separation

• Degrader to reduce the momentum stopping in a 150 m CH2 target

• Solenoid to couple beam with COBRA spectrometer

Results (4 cm target): Z-version

• R (total) 1.3*108 +/s• R (after W.filter & Coll.) 1.1*108 +/s• R (stop in target) 6*107 +/s• Beam spot (target) 10 mm /e separation (at collimator) 7.5 (12 cm)

108 /s could be stopped in the target but only 3x107 are used because of accidental background

14

2) The positron spectrometer: COBRA spectrometer

Gradient field Uniform field

COnstant Bending RAdius (COBRA) spectrometer

• Constant bending radius independent of emission angles

• High pT positrons quickly swept out

Gradient field Uniform field

15

The magnet

• Bc = 1.26T current = 359A• Five coils with three different diameters • Compensation coils to suppress the stray field around the LXe detector• High-strength aluminum stabilized superconductor

thin magnet (1.46 cm Aluminum, 0.2 X0)

16

Gradient field

17

The drift chambers

• 16 chamber sectors aligned radially with 10°intervals

• Two staggered arrays of drift cells• Chamber gas: He-C2H6 mixture• Vernier pattern to measure z-

position made of 15 m kapton foils

(X,Y) ~200 m (drift time) (Z) ~ 300 m (charge divisionvernier strips)

goals

18

2 10-3 X0 along positrons trajectory

19

The Timing Counter

• One (outer) layer of scintillator read by PMTs : timing• One inner layer of scintillating fibers read by APDs: trigger (the long. Position

is needed for a fast estimate of the positron direction)•Goal time~ 40 psec (100 ps FWHM)

BC404

5 x 5 mm2

20

TC Final Design

• A PLASTIC SUPPORT STRUCTURE ARRANGES THESCINTILLATOR BARS AS REQUESTED

• THE BARS ARE GLUED ONTOTHE SUPPORT

• INTERFACE ELEMENTS ARE GLUED ONTO THE BARS AND SUPPORT THEFIBRES

• FIBRES ARE GLUED AS WELL

• TEMPORARY ALUMINIUM BEAMS ARE USED TO HANDLE THE DETECTOR DURING INSTALLATION

• PTFE SLIDERS WILL ENSUREA SMOOTH MOTION ALONG THE RAILS

PM-Scintillator Coupler

Scintillator Housing

Scintillator Slab

Main Support

Divider Board

PM

APD

APD F.E. BoardFibers

APD Cooled Support

21

22

• 800 l of Liquid Xe

• 848 PMT immersed in LXe

• Only scintillation light

• High luminosity

• Unsegmented volume

3) The Liquid Xe calorimeter

Liq. Xe

H.V.

Vacuum

for thermal insulation

Al Honeycombwindow

PMT

Refrigerator

Cooling pipe

Signals

fillerPlastic

1.5m

Experimental

checkIn a Large Prototype

23

The liquid xenon calorimeter

24

1st generation R6041Q 2nd generation R9288TB 3rd generation R9288ZA

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

OK!

New (2009) liquid phase purification system completely

made inside the collaboration (F. Sergiampietri)

Xenon purification

LED

PMT Gain

Higher V with light att.

Can be repeated frequently

alpha

PMT QE & Att. L

Cold GXe

LXe

Laser

Laser

(rough) relative timing calib.

< 2~3 nsec

Nickel Generator 9 MeV Nickel γ-line

NaI

Polyethylene

0.25 cm Nickel plate

3 cm 20 cm

quelle

onoff

Illuminate Xe from the back

Source (Cf) transferred by comp air on/off

Proton Acc Li(p,)Be

LiF target at COBRA center

17.6MeV

~daily calib.

Can be used also for initial setup

KBi

TlF

Li(p, 0) at 17.6 MeV

Li(p, 1) at 14.6 MeV

radiative decay

0 - + p 0 + n

0 (55MeV, 83MeV)

- + p + n (129MeV)

10 days to scan all volume precisely

(faster scan possible with less points)

LH2 target

e+

e-

ee

Lower beam intensity < 107

Is necessary to reduce pile-ups

Better t, makes it possible to take data with higher beam intensity

A few days ~ 1 week to get enough statistics

XenonCalibration

Calorimeter energy Resolution and uniformity at Calorimeter energy Resolution and uniformity at 55 MeV by means of55 MeV by means of

Energy Energy resolution on resolution on the calorimer the calorimer Entrance faceEntrance face

RR = 1.5% = 1.5%FWHM = 4.6 FWHM = 4.6 %%

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CW beam line

30

LITHIUM - spectrum +FLUORINE - spectrum

LiF target

Automatic insertion/Extraction from

the experiment center (target)

31

4) DAQ: trigger

Beam rate 108 s-

1

Fast LXe energy sum > 45MeV2103 s-1

g interaction point (PMT of max charge)

e+ hit point in timing counter

time correlation – e+ 200 s-1

angular correlation – e+ 20 s-1

•Uses easily quantities:

energy

•Positron- coincidence in time and direction

•Built on a FADC-FPGA architecture

•More complex algorithms implementable

1 board

2 VME 6U

1 VME 9U

Type2

Type2

LXe inner face

(312 PMT)

. .

. 20 boards

20 x 48

Type1Type1

Type1

16

3

Type2

2 boards

. . .

10 boards

10 x 48

Type1Type1

Type1

16

3

LXe lateral faces

(488 PMT: 4 to 1 fan-in)

Type2

1 board

. . .

12 boards

12 x 48

Type1Type1

Type1

16

3

Timing counters

(160 PMT) Type2Type2

2 boards2 x 48

4 x 48

2 x 48

5 Hz in 2008 data taking

On-line Eγ resolution

σ = 3.8%

45 MeV threshold

(@4 from signal)

55 MeV γ-line from π0-decay

DAQ: readout The Domino Principle

Shift RegisterClock

IN

Out

“Time stretcher” GHz MHz“Time stretcher” GHz MHz

Waveform stored

Inverter “Domino” ring chain0.2-2 ns

FADC 33 MHz

Keep Domino wave running in a circular fashion and stop by trigger Domino Ring Sampler (DRS)

Keep Domino wave running in a circular fashion and stop by trigger Domino Ring Sampler (DRS)

Low cost One “oscilloscope” per channel

34

Liquid xenon: waveforms: 2 digitizers

Trigger@100 MHz

DRS2 @ 500 MHz or 2 GHz

35

4) 2008 run

CW Calibration each three days during 2008 run

- First 3 months physics data taking (september-december 2008)

-DCHs instability on part of the chambers after some months of operation: reduction of efficiency to 30% (now 2009 corrected: new HV distribution system )

-Xe LY increase (now 2009 at the nominal value)

2008

2009

In 2009 xenon scintillation waveforms have the right time decay constant: longer for

gammas

2008 2009

2008 run : 1014 muons stopped in target

RD

RD

RD

RD

RD

Programmed beam

shutdowns

RDCooling system

repair

Air test in COBRA

We also took RMD data once/week at reduced beam intensity

DCH DCH resolutions from 2008 dataresolutions from 2008 data

Tracks with two turns in the spectrometer are used to detetrmine theAngular resolutions

=14 mrad =14 mrad =10 mrad=10 mrad

= 25 mrad= 25 mrad = 18 mrad= 18 mrad

The edge of Michel positrons used todetermine momentum resolution

corecore = 374 keV (60%) = 374 keV (60%) tail1tail1 = 1.06 MeV (33%) = 1.06 MeV (33%)

tail2tail2 = 2 MeV (7%) = 2 MeV (7%)

Timing resolutions from 2008 dataTiming resolutions from 2008 data

Intrinsic timing resolution by using positrons hitting several bars

Photon - positron timing resolution by using Radiative muon decay: physical background

Problems with DRS2 : will improve with DRS4

Tps

Dedicated RMD runs at lower thresholds

Blind analysis: EBlind analysis: E vs vs tte e windowwindow

Sidebands (Sidebands (||TeTe| > 1 ns| > 1 ns ) are used to ) are used to MEASUREMEASURE accidental background accidental background

distributionsdistributions

Radiative decay + In flight positron annihilation + Radiative decay + In flight positron annihilation + resolutionresolution + + pileup: pileup: in agreement with MCsin agreement with MCs

Energy

Likelihood analysis: Likelihood analysis: accidentalsaccidentals + + radiative radiative + signal + signal PDFs to fit data + PDFs to fit data +

Feldman CousinsFeldman Cousins

6.140 Sig

N

Best fit in the signal region

Agreement of 3 different analyses

EE vs E vs Ee+e+

MM

SeeS

f

pTCDCptrackDCAf

)MeV|( )|MeV,( )( 5050

Normalization: measured Michel events simultaneous with the normal MEG trigger

keBRNe )(

)( )( )|()|(

)|(

M

S MtrPsctrackATCetrackMichelTRG

eMEGTRG

f

fNk

me

dove:dove:

pre-scaling 10pre-scaling 1077

-Independent of instantaneous beam rate

- Nearly insensitive to positron acceptance and efficiency factors associated with DCH and TC

90% CL limit90% CL limit

• 90 % C.L. NSig 14.6 corresponds to BR(→e) 3.0 x 10-11

• Computed sensitivityComputed sensitivity 1.3 x 10-11 • Statistical fluctuation 5%Statistical fluctuation 5%• From side bands analysis we expected 0.9 (left) and 2.1 From side bands analysis we expected 0.9 (left) and 2.1

(right) x 10(right) x 10-11-11

• Bad luckBad luck

49

http://meg.psi.chhttp://meg.pi.infn.it

http://meg.icepp.s.u-tokyo.ac.jp

http://meg.psi.chhttp://meg.pi.infn.it

http://meg.icepp.s.u-tokyo.ac.jp

Future prospects

• Re-start of data taking in september, until december (as in 2008)

• Instabilities eliminated: DRS2DRS4 (timing improvement + noise reduction)

• Data taking and trigger efficiencies: 3-5 factor improvement

• Corresponding improvement in sensitivity: 2-4 * 10-12 for 2009 run

• Continue running in 2010 + 2011 for the final (10-13) goal

More details at

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Planning R & D Assembly Data Taking

nownowLoILoI

ProposalProposal

50

P

1

D

DDD

D

D

d

PPd

decay) e isotropic (for factor nSuppressio

cos

1

cos

cos

)cos1)(cos1(cos

(Y. Kuno et al., MEG TN1, 1997 and references)

)cos1( : in H.E.

)cos1( : in H.E.

ePee

Pe

D

P

Det. 2Det. 1

• For suitable geometry big factors can be obtained

• This is not the case for MEG (detailed calculations are necessary )

• In some theories (minimal SU(5) model) the positron has a definite helicity P is less effective

e

51

DRS charge vs Trigger WFM Charge (TC)