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

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

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 me g

(BR 10-54)

m e g is an extremely powerfull test for

New Physics models

meg predicted by several models ( mainly (but not only)

SUSY GUTS) @ rates just below the MEGA results

MSSM with

large tanb

Just one recent example

G.Isidori

…

B physics constraints

Signal and background

e+ m+ g

qeg = 180°

Ee = Eg = 52.8 MeV

Te = Tg

signal m e g

background

Physical: RMD

m e g n n

e+ m+ g n

n

Accidental (Dominant)

m e n n

m e g n n

ee g g

eZ eZ g

e+ m+ n

n

g

1/10 of the accidental

one

We have to measure positrons and photons kinematic variables in the most

precise way in order to reduce backgrounds

4(5

) M

easu

red

vari

ab

les

1m

e+

Liq. Xe ScintillationDetector

g

Drift Chamber

Liq. Xe ScintillationDetector

e+

g

Timing Counter

Stopping Target

Thin Superconducting Coil

Muon Beam

Drift Chamber

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)

Detector outline

Part of

One half of the collaborators...

PDF= Probabilty

Distribution Function

Signal

Radiative Bkg

Accidental Bkg

MEG Physics analysis

Blind analysis

9

Michel positrons

Mott scattering device

double turn tracks

PDFs

g variables

+ 4.4 g from

Am/Be source,

9 MeV g from n

capture (n

generator) in Ni

0.3%

Straight line fit

Linearity

g

+ Mott scattering device

e+ variables

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

MEG Physics runs

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

BR =0 probability = 8%

Nsig best value = 3.4

Sensitivity = 3.3 x 10-12

0.17 <9.6x

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

Sensitivity = 1.6 x 10-12

MEG result

Systematic errors included (2% effect on UL)

Larger contributions from relatice angle offset, correlation in

positron kinematical variables, normalization

1.7 .96 3.2

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 meg

• 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