1. New Results from the MEG Experiment
Jeanine Adam on behalf of the MEG Collaboration
Zurich PhD Seminar 2011
29th August 2011 / ETH Zurich

2. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
Contents
MEG Experiment
Introduction
Detector
Calibrations
Data Taking
Run Overview
Detector Performance
Analysis
2009
2010
Alternatives
08/29/2011
Zurich PhD Seminar 2011 / Jeanine Adam /

3. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
08/29/2011
Zurich PhD Seminar 2011 / Jeanine Adam /

4. Introduction: Goal and Signature
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Introduction: Goal and Signature
Goal: Search for the lepton flavor violating decay with a sensitivity of in branching ratio (current upper limit by MEGA: 1.2 · )
An observation of this decay will reveal new physics beyond the SM!
Signature of signal event ( decay at rest):
and emitted back-to-back
Each and carries an energy of 52.8 MeV
Coincident in time
Background
Radiative muon decay:  → e   
Precise measurements of kinematic variables for photon and positron are crucial!
Accidental coincidence:  → e   + 
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Zurich PhD Seminar 2011 / Jeanine Adam /

5. Introduction: MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Introduction: MEG Experiment
International collaboration (~80 physicists)
MEG is located at the Paul Scherrer Institute (PSI)
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Zurich PhD Seminar 2011 / Jeanine Adam /

6. Detector: Beam and Target
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Detector: Beam and Target
Beam
Low momentum muons (28 MeV/c)
Wien filter ( / separation)
Superconducting beam transport solenoid (BTS) with degrader
Stopping rate of 3 × 10 7 muons/sec
Target
205 m polyethylene foil
Slanted angle of 20.0°
Holes (r = 5 mm) to check vertex reconstruction
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Zurich PhD Seminar 2011 / Jeanine Adam /

7. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Detector: Overview
Photon: Liquid xenon scintillation detector (position, timing, energy)
Positron: COBRA positron spectrometer (position, timing, energy)
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Zurich PhD Seminar 2011 / Jeanine Adam /

8. Detector: Spectrometer
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Detector: Spectrometer
COBRA Magnet
Superconducting main magnet
Gradient magnetic field (B = 0.49 – 1.27 Tesla)
Timing Counters
2 x 15 plastic scintillator bars read out by PMTs
2 x 256 scintillation fibers read out by APDs
Drift Chamber System
16 dc modules aligned radially to beam axis
Low-mass construction (12.5 m thick Kapton foils with 250 nm Al deposition, He:C2H6 gas mixture)
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Zurich PhD Seminar 2011 / Jeanine Adam /

9. Detector: LXe Calorimeter
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Detector: LXe Calorimeter
World’s largest liquid xenon detector
Filled with 900 liter of LXe (T = K)
Scintillation light is picked up by 846 PMTs
High purity at sub-ppm level to avoid scintillation light absorption due to impurities (water, oxygen)
Calibration of LXe detector 2 x week (total 6 hours):
LED → PMT gain calibration
Alpha → PMT quantum efficiency calibration
Cockcroft-Walton → Uniformity, stability
Neutron Generator → Stability (beam off and on)
Special calibration runs:
CEX ( , ) → Energy scale, uniformity
Cosmics → Alignment
...
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Zurich PhD Seminar 2011 / Jeanine Adam /

10. Calibrations: PMT Calibrations
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Calibrations: PMT Calibrations
PMT Gain Calibrations:
Several LEDs inside the LXe detector
Can be flushed with different intensities
Fit correlation between variance and mean of charge distributions to calculate gain of each PMT
PMT QE Calibrations:
25 241Am sources (known positions) inside the detector
Compare MC simulation with data to calculate QE of each PMT
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Zurich PhD Seminar 2011 / Jeanine Adam /

11. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Calibrations: Cockcroft-Walton Proton Accelerator
1 MeV Cockcroft-Walton proton accelerator and a Li2B4O7 crystal target are used to excite the reactions Li(p,)Be and B(p,)C
Reactions produce gamma lines of 4.4 / 11.7 / 17.6 MeV
Check stability of the detector and correct non-uniformity of energy response
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Zurich PhD Seminar 2011 / Jeanine Adam /

12. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
Introduction
Detector
Calibrations
Calibrations: Neutron Generator
Since 2010 a pulsed neutron generator is used to obtain neutrons with good S/N ratio
The thermalized neutrons are captured in Nickel producing a 9 MeV photon emission
Data taking is possible during beam off / on periods:
Similar spectra
Very stable performance during 2010
NG data is used in 2011 as standard monitor tool
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Zurich PhD Seminar 2011 / Jeanine Adam /

13. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
Data Taking
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Zurich PhD Seminar 2011 / Jeanine Adam /

14. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
Run Overview
Performance
Run Overview
Run 2008
First MEG physics data taking period (3 months)
Result published in Nuclear Physics B 834 (2010) 1
Improvements for 2009 / 2010
Stable drift chamber HV and Xenon light yield situation
New drift chamber alignment (Millipede)
Better understanding of positron correlations
New / improved LXe calibration methods
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Zurich PhD Seminar 2011 / Jeanine Adam /

15. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
Run Overview
Performance
Run Overview
Run 2009
Physics data taking DAQ time total: 35 days Muons on target: · 10 14
Run 2010
Optimized beam (adjusted degrader)
Improved time resolution situation
Physics data taking DAQ time total: 56 days Muons on target: · 10 14
Twice the statistics of 2009 data
2009
PSI accelerator
maintenance work
2010
CEX run
PSI accelerator
maintenance work
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Zurich PhD Seminar 2011 / Jeanine Adam /

16. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
Run Overview
Performance
Detector Performance
2009
2010
Gamma Energy (%)
1.9
Gamma Timing (ps) 96 67
Gamma Position (mm)
5 (u,v) / 6 (w)
Gamma Efficiency (%) 58 59
e Timing (ps)
107
e Momentum (keV)
310
330
e Theta (mrad)
9.4
11.0
e Phi (mrad)
6.7
7.2
e Vertex (mm)
1.5 (z) / 1.1 (y)
2.0 (z) / 1.1 (y)
e Efficiency* (%) 40 34
e-gamma timing (ps)
146
122
Trigger Efficiency (%) 91 92
* e efficiency = DC-TC matching efficiency x DC detection efficiency
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17. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
Analysis
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Zurich PhD Seminar 2011 / Jeanine Adam /

18. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
2009
2010
Alternatives
Analysis
Blind box analysis, i.e. optimization of analysis and background studies are done in sidebands
Likelihood fit:
Parameters: Number of signal, radiative muon decay and accidental background events
Observable:
Determination of upper limit by Feldman & Cousins
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Zurich PhD Seminar 2011 / Jeanine Adam /

19. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
2009
2010
Alternatives
Analysis 2009 Data
Nsig = 3.4 (+6.6 / -4.4)
UL = 9.6 · (90% CL)
LL = 1.7 · (90% CL)
Total
Accidental
Radiative
Signal
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Zurich PhD Seminar 2011 / Jeanine Adam /

20. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
2009
2010
Alternatives
Analysis 2010 Data
Nsig = (+5.0 / -1.9)
UL = 1.7 · (90% CL)
LL = ---
Total
Accidental
Radiative
Signal
08/29/2011
Zurich PhD Seminar 2011 / Jeanine Adam /

21. Zurich PhD Seminar 2011 / Jeanine Adam / jeanine.adam@psi.ch
MEG Experiment
Data Taking
Analysis
2009
2010
Alternatives
Analysis Summary
Nsig (best fit)
BR LL (90% CL)
BR UL (90% CL)
2009
3.4
1.7 ·
9.6 ·
2010
-2.2
---
1.7 ·
-0.5
2.4 ·
MEG Result
data is consistent with “no signal” hypothesis
Fitting was done by 3 analysis groups with different parameterizations, statistical approaches, ... with consistent results
08/29/2011
Zurich PhD Seminar 2011 / Jeanine Adam /

22. Analysis Alternatives
MEG Experiment
Data Taking
Analysis
2009
2010
Alternatives
Analysis Alternatives
Reduce 5 dim observable to a one dimensional discriminant variable:
2 different kind of analysis:
Cut analysis (minimize sensitivity)
Likelihood fit (same procedure as for the 5 dim observable)
Advantages:
Fast analysis
Fast toyMC production
Lsig, LBG, LRMD are the signal, accidental background and RMD likelihoods
Likelihood Fit
2009
08/29/2011
Zurich PhD Seminar 2011 / Jeanine Adam /

23. Analysis Alternatives
MEG Experiment
Data Taking
Analysis
2009
2010
Alternatives
Analysis Alternatives
Results of the Rsig cut analysis and the Rsig likelihood fit:
Both the Rsig cut analysis and the likelihood fit results are consistent with the results of the three analysis groups
The Rsig cut analysis result is very close to the official MEG result
Use it for checks and estimations which couldn’t be done until now (too time consuming)
UL (90% CL)
Official Analysis
Rsig Cut Analysis
Rsig Likelihood Fit
2009 UL
9.6 ·
1.1 ·
2010 UL
1.7 ·
2.8 ·
1.8 · UL
2.4 ·
3.2 ·
2.6 ·
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Zurich PhD Seminar 2011 / Jeanine Adam /

24. Summary and Conclusion
MEG Experiment
Data Taking
Analysis
Summary and Conclusion
The MEG experiment is searching for the LFV decay aiming a sensitivity of ~10 -13
Physics data taking started in 2008
data are consistent with “no signal” hypothesis
New physics is now constrained by a 5x tighter upper limit:
MEG is taking more physics data this and next year to reach ~ sensitivity
08/29/2011
Zurich PhD Seminar 2011 / Jeanine Adam /