1. Status of MEG-I physics analysis
Fabrizio Cei
INFN & University of Pisa
Pisa, 15 September 2014
15/09/2014
Fabrizio Cei

2. Summary of MEG results in 2013
Improved reconstruction algorithms and reconstruction efficiency on both e+ and g sides;
Analyzed 2009/2010 & 2011 samples; total number of 3.6 x 1014 muons stopped on target;
Reached sensitivity of 7.7 x 10-13
Upper limit on m+ e+ g branching ratio: 5.7 x (90% C.L.) factor 4 improvement !
Published in PRL 110 (2013)
NSIG  BR
(normalization factor)
x 10-12
15/09/2014
Fabrizio Cei

3. Analysis strategy Likelihood + Blind (only 2011) analysis
Signal and background
optimization done in sidebands
Timing sidebands
Eg sideband
Events in the blind box ( 0.2%) are hidden up
to the end of optimization procedure (only 2011)
RMD events
15/09/2014
Fabrizio Cei

4. Impact of MEG: t mg vs meg < 2013
Blankenburg et al. Eur.Phys.J. C72 (2012) 2126
Antusch et al. JHEP 0611 (2006) 090 
MEG 2011
excluded
MEG 2011
excluded
q13 recently measured by Daya Bay,
Reno, Double Chooz ... (7 ÷ 10o)
MEG 2011 = PRL 107 (2011)
15/09/2014
Fabrizio Cei

5. t mg vs meg NOW MEG 2013 = PRL 110 (2013) 201801
Blankenburg et al. Eur.Phys.J. C72 (2012) 2126
Antusch et al. JHEP 0611 (2006) 090 
MEG 2013
excluded
More than 150 citations on INSPIRE !
MEG 2013
excluded
MEG 2013 = PRL 110 (2013)
q13 recently measured by Daya Bay,
Reno, Double Chooz ... (7 ÷ 10o)
15/09/2014
Fabrizio Cei

6. Full MEG sample
Number of m+  e+g events = (k factor) x BR (m+  e +g)
BR < 5.7 x 10-13
S = 7.7 x 10-13
( ) 
( )
Double statistics !
Estimated final
sensitivity (toy MC)
 (4 ÷ 5) x 10-13
BR < 2.4 x 10-12
S = 1.6 x 10-12
15/09/2014
Fabrizio Cei

7. 2012 and 2013 analysis
Data 2012/2013 processed with algorithms developed on 2011;
2012:
- resolutions in substantial agreement with that of previous years;
- preliminary normalization factor: 5.29 x 1012 (it was 4.0 x 1012 on 2011);
- optimization almost finished;
2013:
- some problems with DCH/target alignment (work in progress);
- optimization under way.
Unblinding in some months, if no reprocessing will be needed (see next slide)
15/09/2014
Fabrizio Cei

8. Open issues 1) Magnetic field measured in the area in Spring;
Found a problem in measurement machine  measurements repeated
in July and August;
Analysis and computation of new map of magnetic field by splining
algorithms under way;
Next step: comparison of reconstructed positron variables (momentum
and direction) with new and old magnetic field on a sample of data. If no
significant differences will be found, no new reprocessing will be
needed; otherwise … (this would delay the publication of final results).
15/09/2014
Fabrizio Cei

9. Open issues: 2)
Analysis techniques to: - identify candidate events of positron Annihilation in Flight; - reconstruct events where a positron turn in DCH system was missed; were developed. Tested on 2009 data. Expected sensitivity improvement by  15%. Implementation for all years under way.
15/09/2014
Fabrizio Cei

10. Final MEG paper
All MEG results will be published in a single long write-up paper;
Chosen editorial board; already operating;
Several persons will contribute;
Writing of paper will start soon (some sections can be written without
knowing the final results);
Hopefully paper will be sent to a journal in few months.
15/09/2014
Fabrizio Cei

11. Beam polarization measurement
A preliminary measurement of beam polarization was performed in
2010 by using Michel positrons ( events) from 2009 data:
POL = 0.89 ± 0.03, in agreement with calculations;
Analysis under way to improve the statistics by using 2010 and
2011 data ( evts ) and publish a result. Unfortunately,
polarization traces are masked by detector instabilities and not
perfect centering of beam wrt target and DCH system 
dependence of spectrometer acceptance on positron zenith angle
 MC corrections needed (under testing).
However, data well reproduced by MC simulations with large
polarization (including detector instabilities and beam/target
displacements) and totally inconsistent with MC with POL = 0.
(US-DS)/(US + DS) vs energy
for all years together
BLUE: DATA
RED: MC with POL = 0.9
For POL = 0, MC predicts asymmetry consistent
with zero, even including detector effects.
15/09/2014
Fabrizio Cei

12. Conclusions
New limit on m+ e+ g branching ratio set by MEG (5.7 x 10-13)
is a significant constrain for BSM theories.
Sensitivity will be improved by a factor 1.5 when the full MEG
sample (2009 – 2013) will be processed.
New analysis technique for identifying AIF and missing turn events
could further improve the sensitivity by  15%.
Final results will be published in a long MEG summary paper.
Analysis of recent measurements of magnetic field under way.
We are trying to finalize the measurement of beam polarization.
15/09/2014
Fabrizio Cei