1. The MEG Experiment at PSI: a sensitive search for m eg decay
Fabrizio Cei
INFN and University of Pisa
XV Incontri sulla Fisica delle Alte Energie
Lecce, April 2003

2. Outline Physics motivations: The   e signature:
SUSY indications;
Connection with neutrino oscillations.
The   e signature:
Signal & Background.
The experimental setup:
The muon beam;
The positron spectrometer;
The timing counter;
The Liquid Xenon e.m. calorimeter;
Trigger & Electronics.
Conclusions:
Sensitivity;
Time profile.
Lecce, April 24, 2003
Fabrizio Cei

3. Physics Motivations
In the Standard Model with massive Dirac neutrinos Lepton Flavour Violation processes (as m  eg, t  eg, m  eee,
m  e) are predicted at immeasurably small levels
However, Super Symmetric Theories predict such processes at much more reasonable rates.
Since the SM background is negligible, processes like m  eg are clear evidences for Super Symmetry.
Problem: are such rates experimentally observable ?
Lecce, April 24, 2003
Fabrizio Cei

4. SUSY Indications Some predictions:
LFV processes especially sensitive to SSM grand unified theories (SUSY-GUT).
LFV induced by finite slepton mixing through radiative corrections.
Some predictions:
SUSY SU(5) BR (m  eg)   10-13
SUSY SO(10) BRSO(10)  100 BRSU(5)
(R. Barbieri et al., Phys. Lett. B338(1994) 212
R. Barbieri et al., Nucl. Phys. B445(1995) 215)
Lecce, April 24, 2003
Fabrizio Cei

5. Predictions of m  eg BR in SU(5) Models
Experimental Bound
J. Hisano et al.,
Phys. Lett. B391 (1997) 341
Goal of MEG Experiment
Small (< 10) tan b values
are highly disfavoured by
recent combined LEP data.
(ALEPH, DELPHI, L3 & OPAL
Collaborations, hep-ex/ )
Lecce, April 24, 2003
Fabrizio Cei

6. Connection with n-oscillations
Additional contribution to slepton mixing from V21, matrix element responsible
for solar neutrino deficit. (J. Hisano & N. Nomura, Phys. Rev. D59 (1999) )
Our goal
Experimental bound
tan(b) = 30O
tan(b) = 0O
Largely favoured
and confirmed by Kamland
All solar n experiments combined
Lecce, April 24, 2003
Fabrizio Cei

7. Previous m  eg Searches
Other LFV searches
Lab.
Year
Upper limit
Experiment or Authors
PSI
1977
< 1.0  10-9
A. Van der Schaaf et al.
TRIUMF
< 3.6  10-9
P. Depommier et al.
LANL
1979
< 1.7  10-10
W.W. Kinnison et al.
1986
< 4.9  10-11
Crystal Box
1999
< 1.2  10-11
MEGA
~2005
~ 10-13
MEG
The MEG experiment aims to gain two orders of magnitude in the upper limit (not a simple experimental challenge!).
MEG
Lecce, April 24, 2003
Fabrizio Cei

8. The MEG Collaboration Lecce, April 24, 2003 Fabrizio Cei
INFN & Lecce University G. Cataldi, C. Chiri, P. Creti, F. Grancagnolo, M. Panareo, S. Spagnolo
INFN & Pisa University A. Baldini*, C. Bemporad, F.Cei, M.Grassi, F. Morsani, D. Nicolo’, R. Pazzi, F. Raffaelli, F. Sergiampietri, G. Signorelli
INFN & Pavia University A.de Bari, P. Cattaneo, G. Cecchet, G. Nardo’, M. Rossella
INFN & Genova University S. Dussoni, F. Gatti, D. Pergolesi, R. Valle
INFN Roma I D. Zanello
ICEPP, University of Tokyo T. Mashimo, S. Mihara, T. Mitsuhashi, T. Mori*, H. Nishiguchi, W. Ootani, K. Ozone, T. Saeki, R. Sawada, S. Yamashita
KEK, Tsukuba T. Haruyama, A. Maki, Y. Makida, A. Yamamoto, K. Yoshimura
Osaka University Y. Kuno
Waseda University T. Doke, J. Kikuchi, H. Okada, S. Suzuki, K. Terasawa, M. Yamashita, T. Yoshimura
PSI, Villigen J. Egger, P. Kettle, M. Hildebrandt, S. Ritt
Budker Institute, Novosibirsk L.M. Barkov, A.A. Grebenuk, D.G. Grigoriev, B, Khazin, N.M. Ryskulov
* Spokepersons
Lecce, April 24, 2003
Fabrizio Cei

9. (muon radiative decay)
Signal and Background
Background
Signal
  e g
“Accidental”
  e n n
  e g n n
ee  g g
eZ  eZ g
“Prompt”
m  e g n n
(muon radiative decay)
e+ + g
e+ + g n
e+ + n
qeg = 180°
Ee = Eg = 52.8 MeV
te = tg g
Lecce, April 24, 2003
Fabrizio Cei

10. Experimental Strategy
Use a high-intensity muon beam and
m-decay at rest;
Measure g time, energy and angle by using a fast & high-resolution e.m. calorimeter;
Measure e+ momentum by using a high-resolution spectrometer;
Measure e+ time by using fast counters (scintillator bars);
Use a trigger scheme based on the e+-g coincidence.
Lecce, April 24, 2003
Fabrizio Cei

11. A simulated event 52.8 MeV photon 52.8 MeV positron
Lecce, April 24, 2003
Fabrizio Cei

12. Required Performances
Experimental sensitivity limited by spill-in of background (especially accidental) into signal region  high resolution measurements are needed. The accidental BR is
To obtain BR (m  eg)  we must have BRacc  3•10-14 and this requires:
FWHM
Exp./Lab
Year
DEe/Ee
(%)
DEg /Eg
Dteg (ns)
Dqeg
(mrad)
Stop rate
(s-1)
Duty cycle (%) BR
(90% CL)
SIN
1977
8.7
9.3
1.4 -
5 x 105
100
3.6 x 10-9
TRIUMF 10
6.7
2 x 105
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
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
2005
0.8 4
0.15 19
2.5 x 107
1 x 10-13
Lecce, April 24, 2003
Fabrizio Cei

13. Detector Building Switzerland Russia Italy Japan Drift Chambers,
Readout & DAQ
Russia
Manpower,
Muons transport solenoid
Italy
e+ counter (Ge + Pv), Trigger (Pi, Le),
Splitter (Le),
LXe Calorimeter (Pi, Le)
Japan
LXe Calorimeter, Superconducting Solenoid
Lecce, April 24, 2003
Fabrizio Cei

14. The Paul Scherrer Institute
Experimental Hall
The most powerful machine
in the world;
Proton energy 590 MeV;
Nominal operation current 1.8 mA
Lecce, April 24, 2003
Fabrizio Cei

15. The Muon Beam It exists;
It provides continuous > 108 +/s (with e+ contamination) on 5x5 mm2;
Two separate configurations of the pE5 beam line (U & Z);
Muon momentum 29 MeV/c.
Primary proton beam
Lecce, April 24, 2003
Fabrizio Cei

16. Beam Line Tests Goal: optimize the beam elements in order to achieve:
a) a good m/e separation  mass selection device (Wien filter);
b) a good coupling between beam and spectrometer  solenoidal magnet;
c) a high intensity of stopping muons in a spot  5x5 mm2  degrader to reduce the muon momentum before a 150 mm target.
Lecce, April 24, 2003
Fabrizio Cei

17. Results Rm (after filter) m/e separation
Rm (total)
Rm (after filter)
m/e separation
Spot size sV 6.5 mm, sH 5.5 mm (U)
Lecce, April 24, 2003
Fabrizio Cei

18. The Positron Spectrometer
COnstant Bending RAdius (COBRA) spectrometer
Constant bending radius independent of emission angles
Gradient field
Uniform field
High pT positrons quickly swept out
Gradient field
Uniform field
Lecce, April 24, 2003
Fabrizio Cei

19. Magnetic Field Longitudinal Profile (R = 0) Radial Profile
Needed < 50 G: OK)
Lecce, April 24, 2003
Fabrizio Cei

20. Solenoids and Coils Central coil Bc = 1.26 T current = 359 A;
Five coils with three different diameters;
Compensation coils to suppress the stray field around the Liquid Xenon detector;
High-strength aluminium stabilized superconductor
 thin magnet (1.46 cm Aluminium, 0.2 X0);
“Crash” Tests completed;
Winding
Magnet ready to be shipped at PSI within this year.
Lecce, April 24, 2003
Fabrizio Cei

21. (X,Y) ~ 200 mm (drift time); (Z) ~ 300 mm; (t) ~ 10 ns
Positron Tracker
17 chamber sectors aligned radially with
10° intervals and 20 wires each;
Two staggered arrays of drift cells;
Chamber gas: He-C2H6 mixture (50/50) to reduce multiple scattering;
Vernier pattern made of 15 mm kapton foils to measure z-position by charge division.
(X,Y) ~ 200 mm (drift time); (Z) ~ 300 mm; (t) ~ 10 ns
Lecce, April 24, 2003
Fabrizio Cei

22. Drift Chambers R & D First results with a small-size prototype
@ Tokyo university (90Sr source,
no magnetic field)
Full-size PSI; test in November
(cosmic muons & 90Sr source, magnetic field);
Improved vernier strips structure (uniform resolution);
Summary of Drift Chamber simulation.
Lecce, April 24, 2003
Fabrizio Cei

23. Positron Timing Counter
BC404 scintillator
Two layers of scintillation counters read by PMTs at right angles with each other.
Outer: mainly timing measurement Inner: mainly trigger information
Goal: timing resolution 100 ps FWHM
Lecce, April 24, 2003
Fabrizio Cei

24. Timing Counter R & D
Timing resolution measurement: COsmic Ray TESt Facility (CORTES)
4 small counters + 8 MSGC + 1 scintillator bar (1 cm thick)
Measured resolution
t improves as ~1/√Npe  2 cm thick
to get 100 ps FWHM resolution
New design in progress; expected full simulation of geometry and light collection and prototype tests in October/November; final design and building in 2004.
Lecce, April 24, 2003
Fabrizio Cei

25. Liquid Xenon Calorimeter
Liq. Xe
H.V.
Vacuum
for thermal insulation
Al Honeycomb
window
PMT
Refrigerator
Cooling pipe
Signals
filler
Plastic
1.5m
800 l of Liquid Xenon equipped with  800 PMTs;
Homogeneous detector;
Only scintillation light;
Large light yield (~ NaI).
Liquid Xenon properties
Experimental
check
Lecce, April 24, 2003
Fabrizio Cei

26. LXe Calorimeter Performances
Full MC simulation of Liquid Xenon behaviour and calorimeter response.
Position resolution:
corresponding to:
angular resolution
(practically independent of light absorption length).
Z-coordinate resolution (depth of interaction point; important for timing
resolution  preliminary measurements later):
Energy resolution strongly depends on optical properties of Liquid Xenon
reconstruction algorithms needed to correct for non homogeneities.
Lecce, April 24, 2003
Fabrizio Cei

27. Energy Reconstruction
FWHM(E)/E (%)
FWHM  4.1 %
Asymptotic value FWHM  2.5 % for labs 
labs = 100 cm
Lecce, April 24, 2003
Fabrizio Cei

28. Liquid Xenon Calorimeter Prototype
The Large Prototype (LP)
40 x 40 x 50 cm3;
228 PMTs, 100 litres Liquid Xenon
(the largest in the World).
Purposes:
Test cryogenic operations on a
long term and on a large volume;
Measure the Liquid Xenon properties;
Check the reconstruction methods;
Measure the Energy, Position and
Timing resolutions with:
a) Cosmic rays;
b) -sources;
c) 60 MeV eˉ from KSR storage ring;
d) 40 MeV  from TERAS
Compton Backscattering;
e) e+ and 50 MeV  from p° at PSI.
Planned this year
Lecce, April 24, 2003
Fabrizio Cei

29. The LP from “inside” FRONT face
LEDs
-source
FRONT face
-sources and LEDs for PMT calibration and monitoring
Lecce, April 24, 2003
Fabrizio Cei

30. Measurement of LXe Optical Properties
First tests showed a number of scintillation photons much lower than expected.
It improved with Xenon purification: Oxysorb + gas getter + re-circulation (took time).
There was a strong absorption due to contaminants (mainly H2O, ~ 3 ppm).
March 2002
Now
labs> 1 90% C.L.
Lecce, April 24, 2003
Fabrizio Cei

31. Improvement of labs with time
We observed an improvement both in comparison with MC simulation and GXe data
No Monte Carlo !!
Increase with time
Lecce, April 24, 2003
Fabrizio Cei

32. Radioactive Background in LP
-trigger with 5106 gain;
Geometrical cuts to exclude -sources;
Energy scale: -source
208Tl (2.59±0.06) MeV
40K (1.42 ± 0.06) MeV
Other lines ??
uniform on the front face;
few 10 min (with non-dedicated trigger);
nice calibration for low energy ’s.
40K (1.461 MeV)
208Tl (2.614 MeV)
Never seen before !
Lecce, April 24, 2003
Fabrizio Cei

33. Timing Resolution Measurements
our goal
Dt = (Dtz2 + Dtsc2)1/2 = ( )1/2 ps = 100 ps (FWHM)
Dtz Time-jitter due to g interaction point (MC: )
Dtsc Scintillation time and photon statistics
Measurement of Dtsc2 with 60 MeV electron Kyoto Sincrotron Ring
(N.B. Electron energy spectrum degraded
by materials in front of the detector)
52.8 MeV
weighted average of the PMT TDCs time-walk corrected;
Dtsc vs number of photoelectrons;
MeV is ok;
new PMT with improved QE: 5 10 %
5 %
10 %
Lecce, April 24, 2003
Fabrizio Cei

34. PMT characterization: Cryogenic Test Facility in Pisa
Full design and mechanical
drawings completed;
Cryostat delivered at the beginning of april;
Orders made for dry UHV
pumping group, leak detector,
UHV components, cryogenic
bottle, PMT’s …; some of them
already received;
Laboratory in preparation.
Lecce, April 24, 2003
Fabrizio Cei

35. Trigger Based on simple quantities:  energy (QSUM);
1 board
2 VME 6U
1 VME 9U
Type2
LXe inner face
(312 PMT)
. . .
20 boards
20 x 48
Type1 16 3
2 boards
10 boards
10 x 48
LXe lateral faces
(488 PMT: 4:1 fan-in)
12 boards
12 x 48
Timing counters
(160 PMT)
2 x 48
4 x 48
Based on simple quantities:
 energy (QSUM);
e+ -  coincidence
in time and direction;
LXe & Timing Counter
information (no DC).
Built on a FADC-FPGA
architecture;
More complex algorithms
implementable.
Lecce, April 24, 2003
Fabrizio Cei

36. Trigger Performances Trigger Status
Beam rate s-1
Fast LXe energy sum > 45 MeV 2103 s-1
g interaction point (PMT of max charge)
e+ hit point in timing counter
time correlation g – e+ 200 s-1
angular correlation g – e+ 20 s-1
Trigger Status
Design and simulation of type1 board completed;
Prototype board delivered by late spring.
Lecce, April 24, 2003
Fabrizio Cei

37. Prototypes delivered in autumn
Readout Electronics
Waveform digitising for all channels;
Custom domino sampling chip PSI;
Cost per DSC ~ 1 US$;
2.5 GHz sampling 40 ps timing resolution;
Sampling depth 1024 bins;
Readout similar to trigger.
Prototypes delivered in autumn
Lecce, April 24, 2003
Fabrizio Cei

38. Sensitivity T = 2.6 • 107 s Rm = 0.3 • 108 m+/s Detector parameters
W/4p = 0.09
ee  0.9
esel  (0.9)3 = 0.7
eg  0.6
Signal
Nsig = BR • T • Rm • W/4p • ee esel eg
Single Event Sensitivity
SES = 1/(T • Rm • W/4p • ee esel eg )  4 • 10-14
Correlated Background
BRcorr  3 • 10-15
Accidental Background
BRacc  Rm • DEe • (DEg)2 • (Dqeg)2 • Dteg  3 • 10-14
Upper 90 % C.L.
BR (m  e g)  1 • 10-13
Discovery
4 events (P = 2 • 10-3) correspond to BR = 2 • 10-13
1.4 FWHM
Lecce, April 24, 2003
Fabrizio Cei

39. Conclusions and Time Schedule
The experiment may provide a clean indication of New Physics (SUSY);
Measurements and detector simulation make us confident that
we can reach a SES of 4 • for meg ( BR  10-13);
Final prototypes will be ready and measured within November 2003:
Large Prototype for photon energy, position and timing resolutions;
Full scale Drift Chamber;
-Transport and degrader-target.
Experiment approved by INFN-CSN1 at beginning of April.
Tentative time schedule
Planning
R & D
Assembly
Data Taking
Now
LoI
Proposal
Revised
document
Lecce, April 24, 2003
Fabrizio Cei

40. Lxe Calorimeter Calibration
p-p  p0n
p0 (28 MeV/c)  g g
54.9 MeV < E(g) < 82.9 MeV q Eg
Requiring q > 170o
FWHM = 1.3 MeV
Requiring q > 175o
FWHM = 0.3 MeV Eg p0 g Eg p-
170o
1.3 MeV for q>170o
0.3 MeV for q>175o
54.9 MeV
82.9 MeV g
175o
Am-Be g source 4.43 MeV
p-p  g n
E(g) = MeV q Eg
Lecce, April 24, 2003
Fabrizio Cei

41. LXe Calorimeter Calibration (2)
m  egnn
Ee > 0.85; Eg > 0.8; qeg > 120o
Crystal box
PRD 38 (1988) 2077
te-tg
150 ps 1
5 ~ 6
0.3 ~ 0.4
108 m/s
Accidental background
R.Tribble’s talk at
Univ. of Tokyo Oct. 1999
107 m/s
Signal 1/10
Background <1/100
Accidental background
Lecce, April 24, 2003
Fabrizio Cei