1. MEG Experiment Data Analysis: a status report
Fabrizio Cei
INFN & University of Pisa
On behalf of the MEG Collaboration
CNS1, Roma, Dec 3rd 2012
03/12/2012
Fabrizio Cei

2. Outline Brief remind of the MEG experiment; Analysis strategy;
Results ;
Advances in analysis;
Re-analysis of 2009 data;
Sensitivity & sideband analysis for 2011 data;
Other analyses;
Perspectives.
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Fabrizio Cei

3. Brief remind of the MEG Experiment
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4. Goal and signal - back-to-back topology; Search for m  eg LFV decay.
Target sensitivity BR  wrt normal muon decay.
Previous Upper Bound: BR  1.2 x (MEGA, 2001)
Signature:
- back-to-back topology;
- energy equally shared
between e+ and g;
- simultaneous emission.
e+  g
qeg = 180°
Ee = Eg = 52.8 MeV
Te = Tg
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5. (radiative muon decay, RMD)
Background
accidental
  e n n
  e g n n
ee  g g
eZ  eZ g
physical
m  e g n n
(radiative muon decay, RMD)
e  g n
e+ + n
RRD = Rm* BR(m → enng)  0.1 Racc g
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6. The Detector World’s most intense continuous muon beam
pE5 (up to 108 stopped m+/s);
Photon detection by a Liquid Xenon calorimeter;
Positron detection by a Drift Chamber
spectrometer for momentum and by
scintillation bars for timing measurements.
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Fabrizio Cei

7. Summary of MEG performances
2009
2010
2011
Gamma Energy (%)
1.9% (w > 2 cm)
1.7% (w > 2 cm)
Gamma Position (mm)
5 (u,v),6(w)
5 (u,v), 6 (w)
Positron Momentum (%)
0.59 (core 80%)
0.61 (core 79%)
0.59 (core 86%)
Positron Angle (mrad)
6.7 (F,core), 9.4 (Q)
7.2 (F, core), 11.0 (Q)
6.5 (F,core), 10.6 (Q)
Vertex Position (mm)
1.5 (Z), 1.1 (Y)
2.0 (Z), 1.1 (Y)
1.9 (Z), 1.3 (Y)
Gamma-Positron timing (ps)
146 (core)
126 (core)
133
Gamma efficiency (%) 58 59
Trigger efficiency (%) 92 95
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Fabrizio Cei

8. Analysis strategy: blind + likelihood
Signal and bkg optimization
done in sidebands
Timing sidebands
Eg sideband
Events in the blind box ( 0.2%) are hidden
up to the end of optimization procedure
RMD events
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9. MEG likelihood analysis
The most dangerous
bck is measured !
Likelihood function
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10. PDF’s
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11. Normalization
where:
107 pre-scaling
factor
TRG = 22: Michel events trigger (only DCH track required)
TRG = 0: MEG events trigger
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12. Results
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13. Data sets for 2009 & 2010 Muons on target 2009 Muons on target 2010
Statistics for 2010 about twice that for 2009
Stable detector conditions
(LXe, DC, TC)
Optimized degrader
Improved electronics timing
Slightly lower DC efficiency
because of higher noise.
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14. Sensitivity evaluation
Upper limit averaged over a sample of 1000 toy MCs, generated with 0 signal
events and using background rates and spectra measured in sidebands.
Confirmed by applying the analysis procedure to events falling in timing and
angular sidebands.
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Fabrizio Cei

15. Unbinned maximum likelihood fit 2009
Three independent
analyses performed by
different groups gave
consistent results.
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16. Unbinned maximum likelihood fit 2010
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17. Combination 2009-2010 Factor five improvement wrt previous limit !
Published result:
PRL 107,171801,
(2011)
Factor five
improvement
wrt previous
limit !
Sensitivity 1.6 x 10-12; difference due to 2009 fluctuation.
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Fabrizio Cei

18. Advances in analysis
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19. New Kalman filter
A new Kalman filter was developed recently, with important features:
More accurate measurement model;
Track model based on GEANE package  well linked with the GEANT-based
MC simulation of the detector;
Accurate extrapolation to the Timing Counter bars  stricter correlation
between DCH and TC information;
Use of timing information from TC to refine hit position  refine track fitting;
More precise evaluation of track length (important for relative timing
resolution) and uncertainties on kinematic variables;
Compatibility with a (possible) active target.
Several tests performed. General results:
15% higher efficiency of track reconstruction (5% for Ee > 50 MeV);
Resolutions consistent with the old code;
Lower high energy tail in positron energy spectrum.
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Fabrizio Cei

20. New vs Old algorithm 1) Ee Difference of reconstructed positron
variables for positrons
reconstructed by both
the OLD and the NEW
algorithm.
Shown XNEW – XOLD
No significant biases
observed. Ee
feg
qeg
Teg
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21. New vs Old algorithm 2) Relative timing resolution 
Positron energy spectrum:
Black = OLD
Red = NEW
5% higher efficiency and reduced tails 
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22. New waveform reconstruction algorithm
Pile-up important at high rate  needed a pile-up identification algorithm.
Difficult to identity on single PMT waveform because of fluctuations  needed to sum waveforms of several (all) PMTs.
Simple charge sum and/or light distribution not enough:
- difficult to extract energy of main pulse;
- shape depends on interaction depth.
New strategy:
- operate a first-order event reconstruction;
- use timing and position reconstructions and PMT calibration constants to
correctly weigh the single PMT waveforms and perform the sum;
- fit the sum waveform with a multi-pulses template and identify as
“main pulse” that closest in time with the event time;
- remove other pulses and determine the energy of the main pulse.
Final results: - signal reconstruction efficiency increases from 58 to 63%.
- reduced dependence of energy spectra on event depth.
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Fabrizio Cei

23. Pileup elimination 1) Original sum waveform
Fitted with 3-peak template
Black line: main pulse
Blue dashed lines: removed pulses
Start of integration window to
determine main pulse energy.
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Fabrizio Cei

24. Pileup elimination 2) 55 MeV photons from p0 decay
BKG event by event comparison
NEW-OLD
Better reconstruction
of shallow events
After applying pileup elimination
algorithm, pileup flagged events look
very similar to no pileup events.
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25. Analysis of 2009 data with improved algorithms In progress
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26. Sensitivity & Normalization
Sensitivity with toy MC: BR = 3.1 x 10-12
Consistent with timing sidebands: BR = (3 ÷ 3.2) x 10-12
Previous result was BR = 3.3 x (7% improvement,
consistent with expectations)
Normalization factor:
k = 1.19 x (Michel)
k = 1.18 x (RMD)
Previous calculation was 1.1 x 1012
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27. Event ranking Events ranked according to the variable
Independent analyses have compatible event ranking (many events in common with limited re-shuffling).
Ranking not used in likelihood fit.
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28. The top score event Magenta: BKG Red: RD Green: Signal Blue: Total
OLD Algorithms pdf’s
NEW Algorithms pdf’s
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29. Event distributions Lines: 1, 1.64, 2 s contours
51 MeV < Eg < 55 Mev &&
52 MeV < Ee < 55 MeV
Qeg > 178o && |Teg| < 280 ps
Lines: 1, 1.64, 2 s contours
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30. Top ranked events: New vs Old
Small displacements
Black: Old ranking, events 1-8
Red: New ranking, events 1-8
Blue Lines: Connection between events present in both lists.
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31. Signal region likelihood fit; Confidence Level curve; Limits; ...
Analysis in progress:
Signal region likelihood fit;
Confidence Level curve;
Limits;
...
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32. 2010 re-analysis re-processed with new algorithms.
2010 data sample ( 2 larger than 2009 sample)
re-processed with new algorithms.
Pdf preparation under way.
Complete re-analysis starting soon:
- sensitivity;
- normalization;
- sideband checks;
- signal region fit.
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33. Sensitivity & sideband analysis for 2011 data
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34. Data 2011 Number of stopping muons in 2011.
Data statistics more than doubled:
2011 > ( )
1.85 x 1014
Smooth running conditions (except for a cryo-plant damage in November);
Improved LXe calibration (NaI replaced by BGO);
Higher DAQ Efficiency and live time (> 95 %);
Resolutions/efficiencies comparable or better than that of previous years.
All data processed with old and new algorithms for further checks.
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35. Normalization 2011
Confirmed by an independent calculation based on Muon Radiative Decay events
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36. Sensitivity 2011
Two independent evaluations (different analysis groups, different pdf’s).
Median = 
using k factor
Sensitivity = 1.26 x 10-12
1.35 x 10-12
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Fabrizio Cei

37. Example of sideband fit
We are ready for unblinding 2011 data and plan to do it after finishing checks on 2009/2010 samples
Sideband fit agrees
with sensitivity
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38. Future perspectives
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39. Data collected in 2012
Higher number of collected muons than in 2011; still 20 days of DAQ.
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40. Detector status Presented on Nov. 30th . No significant deterioration
since end of Sep. 
If we will have
beam time, we
plan to run for
three months
during 2013
without opening
the detector.
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Fabrizio Cei

41. Projected sensitivity
Stopped m+ per year
Evaluated by toy
MC and likelihood
analysis
Expected final
sensitivity:
 5 x 10-13
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Fabrizio Cei

42. Other searches 1)
Study of RMD events and measurement of RMD branching ratio and
Michel parameters. Paper in preparation.
Dots: data
Black Line:
theoretical
prediction
with Pol = 0.9
Grey boxes:
uncertainties
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Fabrizio Cei

43. Other searches 2)
Measurement of muon beam polarization with Michel e+. Paper in preparation.
Search for more exotic muon decays (mediated by pseudo-scalar particles
or with massless Majorons)
Best fit:
P = 0.89  0.03
RED: DATA
BLUE: MC,POL = 0 q q
RED: DATA
BLUE: MC,POL = 1
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Fabrizio Cei

44. Summary
The analysis of MEG experiment data is going on to have shortly a new publication, with a larger sample of data and higher quality results.
2009 Data: reprocessed with new algorithms; analysis under way;
2010 Data: reprocessed with new algorithms; analysis close to start;
2011 Data: ready for unblinding after conclusions of analysis;
Combined result to be presented at the Winter Conferences and published;
2012 Data: sample larger than 2011  expected relevant contribution to the final result.
2013: Three months of data taking planned ( 0.5 x 2012 statistics).
Expected final sensitivity  (5 ÷ 6) x at the end of DAQ.
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45. On demand
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Fabrizio Cei

46. Background m  e g n n accidental   e n n physical   e g n n
ee  g g
eZ  eZ g
Racc  (Rm)2 * (DQ)2 * (DEg)2
* DT * DEe 
O Muon rate to be used is
a trade off between
expected number of
signal events and
background level;
O Used 3 x 107 m+/s;
O Sensitivity is limited by
accidental background;
O High resolution
detectors and a
continuous muon beam
are mandatory.
physical
m  e g n n
(radiative decay, RMD)
e  g n
e+ + n
RRD = Rm* BR(m → enng)
 0.1 Racc g
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47. Part of
Total  60 physicists
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Fabrizio Cei

48. Overview of calibration system
Needed to ensure:
Required precision;
Long term detector stability;
Continuous checks for a
detector based on
innovative technology (LXe).
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Fabrizio Cei

49. A sideband event vs pdf’s
Magenta: BKG
Red: RD
Green: Signal
Blue: Total
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Fabrizio Cei