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Transcript
New results of the MEG experiment: search
for m+e+g with sensitivity to BR 10-13
A. Baldini INFN Pisa: PANIC
July 28th 2011
The Standard Model of particle
interactions was explicitily built on the
non observation of this decay (Lepton
Flavour)
Even with the introduction of the recently
discovered neutrino masses SM predicts
the non observability of me g
(BR 10-54)
m e g is an extremely powerfull test for
New Physics models
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
~2012
~ 10-13
MEG
meg predicted by several models ( mainly (but not only)
SUSY GUTS) @ rates just below the MEGA results
…
MSSM with
large tanb
Just one recent example
B physics constraints
G.Isidori
Signal and background
signal
meg
background
4(5) Measured variables
Physical:
RMD
e+ m+
g
megnn
n
qeg = 180°
Ee = Eg = 52.8 MeV
Te = Tg
e+ m+
n
g
1/10 of the accidental
one
Accidental
(Dominant)
menn
megnn
ee  g g
eZ  eZ g
n
n
e+ m+
g
We have to measure positrons and photons kinematic variables in the most
precise way in order to reduce backgrounds
Detector outline
Liq. Xe Scintillation
Detector
Liq. Xe Scintillation
Detector
Thin Superconducting Coil
g
Stopping Target
Muon Beam
e+
g
Timing Counter
e+
Drift Chamber
Drift Chamber
1m
1.
Stopped pE5 beam of 3 107
m /sec in a 150 mm target
2.
Solenoidal spectrometer &
drift chambers for e+
momentum (DCH)
3.
Scintillation counters for
e+ timing (TC)
4.
Liquid Xenon calorimeter
for g detection (LXe)
(scintillation)
Part of
One half of the collaborators...
MEG Physics analysis
PDF= Probabilty
Distribution Function
Signal
Radiative Bkg
Accidental Bkg
Blind analysis
9
PDFs
Michel positrons
Mott scattering device
double turn tracks
g variables
0.3%
+ 4.4 g from
Am/Be source,
9 MeV g from n
capture (n
generator) in Ni
Linearity
Straight line fit
g
e+ variables
+ Mott scattering device
MEG Physics runs
2008:
3 months of data taking with low efficient Drift Chambers (DCH) system and
varying xenon light yield as a function of (gaseous) purification
Sensitivity = 1.3 x 10-11, 90%UL = 2.8 x 10-11
Published : NPB 834 (2010) 1
2009 and 2010
DCH system stable (HV distribution system problem corrected) + Xenon
purified and stable. .
2010: twice the statistics of 2009
Sensitivity: average UL obtained by 100 Toy MC experiments generated
with no signal
2009 and 2010 data sets
Slightly worse DC noise
Conditions
Twice the statistics of 2009 data
2009 preliminary results
presented at ICHEP 2010
Recent analysis improvements
Better understanding of the spectrometer
• alignement by using cosmic rays tracks and millipede algorithm (CMS)
• relative spectrometer-target aligned by using reconstructed holes in the target
• geometrical correlation between positron momentum and angular error taken
into account
Better evaluation of the relative alignement of spectrometer and
calorimeter
Usage of the background measured in the sidebands in the
likelihood
FC statistical method + profile likelihood for estimating the
confidence intervals
Relative alignement of g
and e+
Spectrometer – target alignement
Update of 2009 data
Events in the signal region: 2009 updated
2009 likelihood analysis
0.17
<9.6x
Sensitivity = 3.3 x 10-12
BR =0 probability = 8%
Nsig best value = 3.4
2010 data
Events in the side bands
Events in the signal region
2010 data unblinded on July 5° 2011
2010
Sensitivity = 2.2 x 10-12
2009 +2010
Summary of analysis results
3.2
1.7
.96
MEG result
Sensitivity = 1.6 x 10-12
Systematic errors included (2% effect on UL)
Larger contributions from relatice angle offset, correlation in
positron kinematical variables, normalization
Profile likelihood curves
not directly used for computing
confidence intervals
Summary
• 8% probability statistical fluctuations happen!
•The combined analysis of 2009 and 2010 data is consistent with a null result
and gives an UL (90% CL) of 2.4 x 10-12 : a constraint 5 times better than the
previous best one to the existence of meg
• Paper on the arXive containing the results shown here:
http://xxx.lanl.gov/abs/1107.5547
• 2011 data taking started (noise problem in DC cured)
• MEG will continue to run this and next year to reach a sensitivity of few times x
10-13
Backup slides
BR<1.5 x 10-11 90%CL