e μ+ →e - Infn

villaolmo.mib.infn.it

e μ+ →e - Infn

1

!e μ + e + decay from # MEG expe$ment:

results from # fir& physics run

Angela Papa

INFN and University of Pisa

11( ICATPP Villa Olmo 5-9 October 2,9


2

Outline

Motivations

The event signature

The MEG experimental set-up

The detector performances and first results

Monitoring and calibration data

MEG event selection

Likelihood analysis

PDFs construction

Result

Conclusion


Why μ + e +

Fundamental physical process

Very sensitive tool to investigate physics beyond Standard Model

1 st experiment

(Pontecorvo&Hincks)

Universality of

Fermi interaction

(...elettroweak theory...)

The best upper limit

MEGA experiment (2001)

B.R. =1.2 x 10 -11 @ 90% C.L.

Flavour Physics

MEG expected sensitivity:

B.R. = 2 x 10 -13 @ 90% C.L.

3

R&D

Set-up

Engineering

Data

Taking

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010


4

Motivations

(*) function of SUSY-GUT parameters:

Ref. L. Cabibbi et al. hep-ph:0605139

Neutrino oscillation: established fact

P νl →ν l ′ = |〈ν l ′|ν l 〉| 2 =


V

∣ li V ∗

i

l ′ ie −i(m2 i /2E i)/L


2

≠0

non-vanishing value of al least one neutrino mass

non-vanishing value of at least one of non-diagonal matrix element

Standard Model with massive neutrinos (Dirac)

Γ(µ → eγ) = ≈ G2 F m5 µ

192π 3

(

α

∆m

2π sin2 2θ sin 2 2 L

4E

B(µ + → e + γ) ≈ 10 −54

)

SU(5) SUSY-GUT or SO(10) SUSY-GUT

10 −14 < B(µ + → e + γ) < 10 −11 (*)


5

Event signature and background

Segnature muon decay at rest

e +

µ +

γ

Positron and gamma

in timing coincidence,

moving collinearly back-to-back,

with their energies equat to ~ 52.8 MeV

B ~ 10 -13

Correlated background

e + ν

µ +

ν

γ

Positron and gamma

in opposite directions,

two neutrinos with a small amount of energy

B ~ 10 -15

Accidental background

e + µ +

γ

ν

ν

Timing coincidence between

Michel positron and gamma from

radiative muon decay or positron

annihilation in flight

B ~ 10 -14


6

Experimental set-up

Muon Beam

Photon detection

LXe calorimeter (energy,direction and time)

Positron detection

COBRA magnet + Drift chamber (momentum and direction)

Timing counter (time)

e +

µ +

γ


7

Paul Scherrer Institut

The most intense continuous muon beam in the world

1.2 MW PROTON CYCLOTRON


8

“Surface muon” beam and target

Pure muon beam at low momentum

small straggling and good identification of the muon decay region

Reducing contaminants (beam particles other than muons)

Reducing the beam momentum to stop muons in a thin target

Target


9

Superconducting magnet

Field gradient solenoid

low momentum positrons swept away, without hitting the chambers

monocromatic positron with the projected trajectory radius independent of the

emission angle


Low mass Drift Chambers

Only 2x10 -3 X0 along track

16 drift chamber sectors, each made by two drift chambers, with staggered layers of wire

planes

good e + momentum and direction mesurements

good time resolution (useful for the e + trajectory reconstruction)

12.5 um of thick cathode foils with a Vernier pattern structure

Chamber gas: He:C2H6 mixture (50%:50%) in He environment (inside COBRA)

TRANSVERSE

COORDINATES

( t drift )

LONGITUDINAL

COORDINATES

( charge division

+Vernier)

Expected Resolusions

(FWHM, MC simulation)

@ 52.8 MeV

10

∆p e +/p e + = 0.7 − 0.9%

∆θ e

+ = 9 − 12mrad


Timing Counters

Two sectors of 15 scintillator bars read out by PMTs, placed at each end of spectrometer

the best e + timing measurement

a fast estimation of the e + emission angle φ (for triggering)

256 optical fibers read out by APDs

a fast determination of the e + impact point z (for triggering)

R5964 PMTs Hamamatsu

S8664-55 Hamamatsu APDs (5x5 mm)

BCF20 optical fiber (5x5 mm)

BC404 plastic scintillator (4x4x90 cm)

11

Expected Resolutions

(FWHM, MC simulation)

@ 52.8 MeV

∆t = 100ps


LXe calorimeter

A large homogeneous calorimeter using only scintillation light

very good resolutions for photon energy, direction and time measurements

Cooling pipe

Refrigerator

H.V.

Signals

Vacuum

for thermal insulation

- Volume: 0.8 m 3 LXe

- 846 PMTs immersed in LXe

- thin entrance wall

(honeycombe structure)

- Photocathodic coverage 40%

- Solid angle coverage 10%of 4π

Liq. Xe

PMT

Al Honeycomb

window

µ

Rapid and high light yield

scintillator

• τ = 4, 22 and 45 ns

• 40000 ph/MeV

12

Plasticfiller

1.5m

Expected Resolusions

(FWHM, MC simulation)

@ 52.8 MeV

∆E γ /E γ = 4.5%

∆t γ = 115ps

∆θ θ,φ ≈ 16 − 18mrad


13

Trigger and DAQ

Flexible and efficient trigger system, to select the candidate events, using fast

detectors only

FADC digitization at 100 MHz

online selection algorithms implemented into FPGAs

Domino Ring Sampler (DRS) chip for excellent pile-up rejection with a full

waveform digitization (> 100 MHz)

all 1000 PMTs signals (LXe and TC) digitize at 2GHz

all 3000 DC channels (anodes and cathodes) digitize at 500 MHz

Type2


14

Calibration methods

Several complementary and redundant methods to calibrate and monitor all detectors

the ONLY way to ensure that required performances are reached and maintained during the time

XEC spectrum

300 charge_6

Entries 15580

Mean 9586

RMS 2405

250

p0 229.6 ±

6.1

p1 1.249e+04 ±

12

p2 452.7 ±

12.3

p3 1.466 ±

0.090

200

p4 1.074e+04 ±

45

p5 1230 ±

33.9

150

100

50

0

0 2000 4000 6000 8000 10000 12000 14000

qsum2

Physical process Particle/energy (MeV) Frequence

π − (p, n)π 0

π 0 → γγ

π 0 → γe + e −

CEX e/γ = 55, 83 year -month

C-W accelerator γ = 4.4, 11.7, 17.6 weak

3 7 Li (p,γ) 48 Be

5 11 B (p,γ) 6

12

C

Radioactive source

241

Am - Am/Be γ = 4.4; α = 5.4 day


15

The 2008 physics run

MEG run detectors started up after

the winter accelerator shut down

Physics run started in September

after a long calibration run using pion

charge-exchange reaction (CEX) in

the summer

Special runs were frequentely

conducted to monitor and calibrate

the detectors (CW, RMD) DURING

physics run

Another CEX calibration run was

performed in December


16

The 2008 physics run:

Detector performances


17

LXe light-yield monitoring

Xenon purification in gas and liquid

CW run (Lithium)


18

LXe energy and timing:

CEX run

σ =1.54 ± 0.06%

FWHM = 4.55 ± 0.2%

! t (ps)

LXe time resolution

160

140

120

100

∆t = 105 ± 1 ps

(FWHM)

@ 54.9 MeV

80

60

40

20

0 10 20 30 40 50 60 70 80 90

!10

100

Nphe

3

Efficiency(from CEX run) : 61%


TC and LXe-TC timing

INTRINSIC TIME RESOLUTION

Michel run

µ → eνν

(two adiacent bar hit by the same e + )

TC-LXE RELATIVE TIME

RESOLUTION

Radiative muon decay (RMD) run

µ → eννγ

19

Special run with lower

trigger thr. (E g > 27 MeV)


DCH momentum:

σ core ~ 374 keV

Positron energy scale and resolution are evaluated

by fitting the kinematical edge of the Michel

positron spectrum at 52.8 MeV

Sigma’s of core and tail components:

core = 374 keV(60%)

tail = 1.06 MeV (33%)

2.00 MeV (7%)

GIVEN THE FEWER HITS-ON-TRACK DUE TO HV PROBLEM

STILL NOT AT THE DESIDERATED LEVEL

(σ e ~ 250keV)

DCH angular resolution:

σ ϑ ~ 18 mrad

σ φ ~ 10 mrad

Angular resolutions extracted from the

difference of two track segments, when the

track has two turns in the spectrometer

20

STILL FAR FROM THE DESIGN VALUES

IMPROVEMENT IN DCH Z RESOLUTION FOR 2009

RUNS FORESEEN


21

DCH instability

DCH started to show frequent HV trips after 2-3 months of operation

reduced positron efficiency and resolution

The DCH instability uncertainty cancels out in the analysis

The DC modules have now been modified and showed no problem; two of

them have been successfully operated for 6 months

DURING 2008 PHYSICS RUN


22

The 2008 physics run:

first results


23

Data sample:

9.5 x 10 13 muons stopped on the target


24

MEG event selection

MEG trigger

E g > 40 MeV &|Δt eg |< 10 ns &|Δϕ | < 7.5 0

Pre-selected events

Al least one track detected by DCHs

smaller relative time LXe-TC

(~16% original triggered sample)

Side-boxes

Events used for

optimizing

algorithms and

background studies

Blinding box

Events saved in

separated hidden

files


Likelihood analysis

A candidate μ→eγ event is characterized by 5 kinematical variables:

Eg, Ee, teg, ϑeg, ϕeg

Likelihood function is built in terms of Signal S, radiative Michel

decay RMD and background BG number events and their probability

density function PDFs (S,R and B):

L(N sig ,N RMD ,N BG )= N N obs

e −N

N obs !

N∏

obs

i=1

[

Nsig

N S + N RMD

N

R + N BG

N B ]

Signal PDF:

is the product of the PDFs for the 5 kinematical variables Eg, Ee, teg, ϑeg, ϕeg

RMD PDF:

is the product of the theoretical PDF ( correlated Eg, Ee, ϑeg, ϕeg ) folded with detector

response, and the measured teg PDF (same of signal one)

BG PDF:

is the product of the background spectra of the 5 kinematical variables Eg, Ee, teg, ϑeg, ϕeg,

precise measured in the side-bands

25


26

Two examples

teg PDF from RMD

with sigma = 147 ps

(here 40 ≤ Eg ≤ 45 MeV)

Eg BG PDF from side-band events

black: data

green: RMD

blue: AIF

(+ resolution+pile up)


27

Signal region

vs PDFs:

e + energy

Legend (*):

Black: data

γ energy

Red: Signal PDF

Blue: RMD PDF

Green: BG PDF

Θeg relative angle

(*)Note:

All curves normalized to the event number


Analysis schemes

(*) Phys. Rev. D57(1998) 3873

The 90% confidence levels are calculated by 3 indipendent likelihood fitting tools, all

based on the Feldman-Cousins approach (*)

All results are consistent

1 ST SCHEME

uses an a-priori estimates of NRMD and NBG

A likelihood ratio LR table is costructed as a function of NS

The 90% confidence level for BR comes from the LR for experimental data vs tabulated

values

2 ND -3 RD SCHEME

extract NS, NRMD and NBG by likelihood fit on the observed events in the signal

region, with two indipendent algorithms

90% confidence level of NS comes (NS NRMD)-plane, with NBG fixed

BR from the LR ordering technique

28


29

Results 0 ≤ Ns ≤ 14.7 (best fit in the signal region)

Main systematic uncertainties

- gamma energy scale (≤ 0.6%)

- positron energy scale (≤ 1%)

- positron angular direction(≤ 0.35%)


30

Normalization

BR(µ + → e + γ)

is calculated by the 90% C.L. normalizing the upper limit in NS to the

Michel positrons (same cuts) assuming

BR(µ + → e + ν e ν µ ) ≈ 1

The method is indipendent of the istantaneous beam rate and of the instability associated

to the acceptance and efficiency of DCH and TC

+ +

+ +

N e

= BR( µ → e γ ) ⋅ k = BR(

µ → e γ )/

SES

γ

k


f


+

⎡ ε(

TRG = 0|

e γ ) ⎤



( TRG 22|

track em

TC)

×

⎣ε

= ∩ ∩ ⎦

S

≡ N

eνν

× ⎢ ⎥ ×

+

fM



A(

γ |

track)

⋅ε(

γ )

⋅ Psc(

22)

f

f

S

M

≡ A( DC ) ⋅ε(

trackp , > 50MeV|

DC ) ⋅ε(

TC|

p > 50MeV)

≡ K

M

e

e

S

Signal to Michel

relative efficiency

(data/MC)

TRG = 0 : MEG Trigger

TRG =22: Special Trigger fo Michel positrons

k = 4.7 x 10 11 (+- 10%)

Prescaling factor = 10 7


31

Upper limit on BR(μ + →e + ϒ)

Ns ≤ 14.7 corresponds to:

BR(µ + → e + γ) ≤ 3 × 10 −11

This includes systematics effects evaluations

It is about two times worse then expected sensitivity

On the teg side-bands we find:

BR(µ + → e + γ) ≤ (0.9 − 2.1) × 10 −11

RESULTS AVAILABLE: arXiv 0908.2594


MEG-like event

GAMMA PATH

HITS ON CHAMBERS

HITS INSIDE XEC

POSITRON TRACK

HITS ON TC


33

Conclusion

Detector

LXe performance improving over run, able to calibrate it successfully!

already at the desiderated light yield in 2009 run

DCH instabilities largely reduced MEG sensitivity

low efficiency and worse resolution

Now DCH layers refurbished and proved to work for run 2009

Additional improvements are anticipated from the Timing counters, trigger

and digitizing system

Analysis

developed a full analysis method with 3 indipendent schemes

MEG is ready to re-start data-taking in this week

- expecting a sensitivity of few 10 -12 by the end of 2009

- two more years of data-taking to reach 10 -13 sensitivity


Back-up slides

34


35

Results

1 st scheme: Ns ≤ 18.1 (no fit)

2 th scheme: 0 ≤ Ns ≤ 14.7 (best fit in the signal region)


36

Results

3 th scheme: 0 ≤ Ns ≤ 17 (best fit in the signal region)


MEG history

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

R&D

Set-up

Engineering

Data

Taking

The best upper limit

MEGA experiment (2001)

B.R. =1.2 x 10 -11 @ 90% C.L.

MEG expected sensitivity: B.R. = 2 x 10 -13 @ 90% C.L.

• Beyond Standard Model framework search for

• LFV (into B and K physics)

• g-2

• neutrinos

• LHC

• Dark matter

37

More magazines by this user
Similar magazines