search for lepton flavor violating decays t - l - p 0 , l - h , l - h ’ at belle

Search for Lepton Flavor violating decays l , l , l ’ at Belle

Yuji Enari Nagoya University

Mt. Tsukuba

KEKBBelle collaboration

Seminar@Fermi Lab, 2005.6.19 Search for LFV decays at Belle, Y. Enari 2

Contents1. KEKB-Belle experiment2. Lepton Flavor violation3. Analysis4. Discussion and conclusion


1. KEKB-Belle experiment

e : 8GeV e : 3.5GeV

s = 10.58 GeV

Lpeak = 1.58 1034cm-2s-1

Cross section　　　： 0.9 nb BB : 1.0 nb

KEKB : Asymmetric energy e+e　Collider

KEKB is　-Factory

約１ 5H ｚで対が生成される。

Ldt = 450 fb-1World Record

Continuous injection mode

LER: 1.7A

HER: 1.2A


History of integrated luminosity

Continuous injection

2003 Summer: SVD/CDC inner partreplacement

Ldt = 154fb-1

Ldt = 450fb-1

Ldt = 244fb-1


1.2 Belle detectorKLM RPC+Fe, sandwich structure

CDCsmall cell +He/C2H6

ECLCsI(Tl) 16X0

ACC n=1.015~1.030 S/K ~ 3 　@ p < 3.5GeV/c

SVD3 lyr. DSSD

TOF Scinti + FM-PMT TOF = 100ps, S/K ~ 2 　@ p<1.4GeV/c

SC solenoid 1.5T8 GeV e

3.5 GeV e

Size ： 7m×7m×7m 、　 Weight : 2000t

Cylindrical Coordinate ：e　Beam axis as ｚ axis.

E resol.E /E = 1.8% @E=1GeV


1.2 Performance of Lepton IDEf

ficie

ncy

(%)

Fake

rate

(%)

Electron IDECL : (a) E/p　　　　 (b) XECL-Xtrk

　　　　 (c) E9/E25

Muon IDKLM: (a) # of Hit layer (b) Shower Shape

CDC: (d) dE/dx

ACC: (e) Nphoton

~90%

~1.5%

~92.5%

~0.25%


Small cell drift chamber installation (2003. Aug.)

• SVD upgrade (3Layer 4 layer)• To fit CDC configuration for SVD upgrade.• To reduce hit rate of Cathode part.

Layer

Hit

rat e

/wire

(kH

z) HER 1.1ALER 1.5AL=9.6x1033cm-1s-1

MainInner

Cathode

Partial cross section of CDC (50 layer.)


Small cell drift chamber• Replace Cathode chamber (1cm-cell 3 layers w/ cathode r~8cm) to Small cell chamber (5mm-cell / 2 layers r~10cm).

Small cell Installation


Alignment after installation

• Measure following variable using cosmic track. – rotation angle 　– shift length x,y

for both forward and backward in each layer.

of endplate

Result: Large y~200m were observed in Inner layers at forward.

Main partInnerSmall cell


Tracking performance after installation (CDC)

Make alignment and calibration constants.pt resolutionspatial resolution

Small cell

Normal cell

= 120 m

= 100 m

di-muon data sample.

pt/pt = 1.5%

CDC tracking system work well with having good resolution.

( before)


Search for Lepton Flavor Violating decays.


In Standard Model, there is no Lepton Flavor Violation.Each lepton number is strongly conserved.

Recently neutrino oscillation has been observed.Although take into account neutrino oscillation, expected branching ratio of LFV in charged sector is B < 10-40.

2. Lepton Flavor Violation

レプトンフレーバー保存則　レプトンの世代を超える遷移を禁止

How strong is Lepton number? (Simple motivation)

If we find LFV, Direct evidence of New physics.


LFV in BSM

If our sensitivity is O(10-7), reach MSSM level.

If m ~ 1 eV ： B(→LFV)<10

B(→LFV)


Minimal Super-symmetric Standard Model

• Higgs mediated process

（１） Phase space　　 → 2 body decay is bigger.（２） Color effect.　　 → 3 times bigger.（３） Mass difference.　　 → B / B （ ms/m ） 2

B　→　　is 8 times bigger

than B　→　

　　　　

A0:CP-odd neutral Higgs boson.

M. Sher, Phys.Rev.D66 05731(2002)K.S.Babu PRL,89,241802 (2002)A. Dedes, et.al. PLB 549,2442 (2002)


Purpose of this analysis.

We search for LFV tau decay using pair events of e+e annihilation process, as evidence beyond the Standard Model.

Signal modes ：　　　　　　　　　 l 　　　　　　　　　　　　　M0

lepton pseudo-scalar meson

　　　e　

　　　e　

　　　e　’

　　　　

　　　　

　　　　’


Previous experimental results• Using ee → 　process

Data ： 4.26106 eventsEfficiency ： → : 2.9% →0: 6.8% Background ： → : 0ev. 　→ 0: 0ev./0.5ev. (obs./exp.)

Number of events

MSSM

MARKII

ARGUS

CLEO

90%

C.L

. U. L

. on

Br

？ This analysis N = 137

M

B() = 99%B() = 39%


3. Analysis

Procedure1 ． Selection criteria

- High signal efficiency- BG suppression

2 ． Signal extraction- e/　and　’- set signal box- BG estimation

3 ． Determine branching ratio

（→　 add subdecay mode for M0 ．）

Blind Analysis method To avoid making vias into results,we shaded around signal region at very first level until end of BG estimation.


Additional mode.

3.1 Signal event reconstruction

　 signal side ( e　　　　　　’) + tag side (1 trk + n　+ missing.)

(e+)

・　 recon. modes for　　　’

22.7%

39.4%

B MmodeType

　　→1 － 3

　　→1 － 1

1 － 1 　　→ 98.8%

1 － 3 　　’→ 17.5%

tag side: 1 prong　　　　　　　 decay

signal side ：　 1or 3 prong

Br(1 prong) = 84 %


3.2 BG suppression 　 Level-1Bhabha, process

2 photon process

ee ~ 120 nb = 1 nb

eexx ~ 20 nbFeature: Large Cross section Low Pt. particles escape to Beam pipe.　 Rejection: pt, missing momentum towards acceptance.

Feature ： Huge Cross section　　　 No missing momentum　　　↓ 　 Rejection: e/ veto in tag side

Total Energy

signal : 　 5% loss : →1/45ee : →1/4900

These events account for 95%. 　　　　　　　→ Reject at first level.

→ Rej. Factor : 1/10000


3.2 BG suppression Level-2 　

pair process

qq process

= 0.9 nb

qq = 4.2 nbFeature ： Multi prong　 include ’ 。

Feature: Similar topology with signal Include e ・　　 Having large missing p.

sig : 2 % loss　 : 67% rej.uds: 65% rej.

Opening angle of l 　 and M0.

veto for ’s Mtag < 1.777 GeV/c2

　

Rej. factor : 1/2900 ．

→ Rej. Factor : 1/83000


3.2 BG suppression 　

• Hard to reject events : Main BG.

Level-1 Level-2Data ----- 　

Bhabha 120nb 　　nb 　

2photon nb 　 qq 4.2nb 　　nb 　

Need additional criterion to reject 　process

←　　 :Can be reject.


3.2 BG suppression － newly applying cut －　 Signal event : 　 are only in tag side. 　process : are in both tag and signal side.

　 Having effective missing mass: 　 mmissing become large.

black: signal MC Blue: generic

Signal event : 17% loss process 　 : 95% reject.

2

Signal ev. 　process

Apply this cut :


3. Analysis

Procedure1 ． Selection criteria

- High signal efficiency- BG suppression

2 ． Signal extraction- e/　and　’- set signal box- BG estimation

3 ． Determine branching ratio

Blind region also explained.


3.3 Signal extraction – e/　and　’ –

109ev. 100±5.9

背景事象の構成　　レプトンの誤識別：　全体の 80 ％　　（ , をと間違える）　

Lepton ID requirement for track.Invariant mass distribution.

　　　　　　　　

Data: 57ev. MC : 48.1±4.0

0

-5　< M　 < 3

77ev. 81.6 ± 8.4

’

-3 < M ’ < 3

Good agreement are seen for sideband events.

Data: MC :


3.3 Signal event extraction

(a) Inv. mass Ml should be tau mass.　　　 Ml= E2

l p2l

　

(b) energy El should have beam energy. E = El – Ebeam

*

→ 　信頼度の高い測定を行うため　　　　　　　ブラインド・アナリシスを適用．

±10

±10

　　

low=25±2 MeV/c2

hi=14±2 MeV/c2

low=57±4 MeV

hi=35±1 MeV

signal region ：　 Elliptical shape which having efficiency of 90%. Shape is determined by minimize the area.　　 S/N is improved 1.5 times than sqare shape

Blind region: 0.5< E < 0.5(GeV), 　 5 < Ml< 5

Blind region

signal event ： Ml M=m, E=0


3.4 Background property(153.8, 　　 428, 　 232fb-1)

- BG of muon modes are larger than electron modes. The fake rate of lepton ID.- There is no peaking BG around signal region.


3.4 Background Estimation

Using sideband events. - Projection in each axis. - Fit BG events by

Gaussian and 3rd polynomial - Normarized by sideband events.

　　b0 　　 =Func

Difference ： 0.30 take account as systematic error

　　b0 　　 =Flat

Try two pattern for BG function.

Flat distribution.


3.5 Signal efficiency and BG events S/N

High signal efficiencyHigh BG suppression If B = 　, S/N=3~12 ．　　　　　　→　 3 observation can be achieved.

Open Blind.

are obtained


3.6 Results 　－ Observed events ー5 ev.

1 ev.

1 ev.

0 ev. 0 ev. 0 ev.

0 ev. 0 ev.

BG expect 0.2±0.2 0.2±0.2 0.0±0.4 0.3±0.2

BG expect 3.9±1.2 0.6±0.4 3.0±0.8 0.9±0.4

# of observed events are consistent with estimation ．→ 　 Set upper limit on Branching ratio ．


3.7 Systematic uncertainty

• Concerning to s0

　　　→ BG estimation Difference of two pattern - If b0＜ 1, set b0=0 in order to obtain conservative limit.

B =

s0

2 N　B0　effB =

Total: 30 ~ 66 %

• Concerning to sensitivityM0 recon. 2.0~4.0%Tracking 2.0%0 veto 0 or 5.5%e ID 1.0% ID 2.0%Trigger 0.5~0.7%Beam BG 2.1~2.3%MC model 0.5%MC stat. 0.8~1.7% Total: 4.2 ~ 8.6%

N 1.4%B0 0.7~3.4%


Upper limit on branching ratio （ 90 ％ C.L.)

•Upper limit evaluation (POisson Limit Estimator program)– Frequentist approach.

Systematic uncertainty: using the Gaussian error.J. Conrad et al., PRD 012002 (2003)


4. Discussion

First search result!

Efficiency ：　 Add M0 recon. mode　　　→ Gain 1.5~1.7MSSM Higgs mediatedR-parity-V

Improve in efficiency and BG suppression from previous result.　　→ Reach 10-7 level which is expected from MSSM.

Ldt # of BGCLEO 4.7 0.5→16Belle 154 　　　 3.0 ratio 32 　　 1/5

CLEO


Constraint on tan vs mA

This limit is comparable with Energy frontier experiment, Tevatron

In tan> 50,mA > 110GeV/c2


Summary• Search for LFV decays 　e, , e, , e’, ’ using 137

106 　pair events which are accumulated at KEKB-Belle experiment.

• Achievement ：　 Improve sensitivity by 64 (max.)（１） # of : Using 32 times larger statistics.（２） Efficiency : Add recon. M0 mode ． → Gain 1.5~1.7

→ First search for ’ mode.（３） BG 　 : 　 Newly missing cut apply →~ 5 times improve.（４） Reliability 　 : Blind analysis

Apply two pattern in BG estimation.

• Result ： No excess in signal region. Set 90% C.L. upper limit on branching ratio ．　　 The limits reach 10　level　, put constraint on MSSM ．

　　　　　　　　　　 In tan> 50, mA > 110GeV/c2

　→ Comparable sensitivity with Tevatron ．


Backup


(1) # of pair (N)

(2) Signal detection efficiency (eff)

　　　　　

(3) Background

　　　　 Suppression factor:1/108

　　　　 BG estimation.

Remarkable points

s0

2 N ・ eff ・ BM0B = BM0 : Branching ratio of M0

s0 : # of signal eventsSensitivity:

　→ Add reconstruction mode for M0

→ 　 Reliability

Event selectioncriteria is

most important.


For future, Super B-factory project.


Status of LFV search in BelleNew result of this summer.

CLEOBabar

Belle Published Preliminary


Systematic uncertainty


Signal MC model dependence.


Status for mu to e transition　 e

　 eX < 1011~12


Belle collaboration.• ~ 400 physicists from 59 institutions, 13 regions. • ~ 100 scientific papers were published.


Belle detector.• General purpose detector.• Super conducting Magnet. (1.5T)

Belle control room


XT Curve & Max. Drift Time

Small cell

Normal cell


運動量測定 : 軌跡測定から pt/pt ~ 0.19pt 0.30/ %

光子 : 軌跡を伴わない ECL のヒット　　 E /E = 1.8%@E=1GeV

CDC

ACC

ECL

TOF

r 平面

e+e 　→　　　→　・　　　　　　　　・　　　　　　　　　　　　　


We are here.

Official Goal

Installation of Crab Cavity

• 550 /fb will be reached before the Crab Cavity.• The Official Goal (=1 /ab) will be achieved before Summer 2007.


Installation of Crab Cavity

• Shutdown for 18-26 months in 2009-2010 for upgrade.• 0.6 /ab/month in 2020.

We are here.Shutdown

SuperKEKB


Motivation - 　　l　　l　　l　’ - • In MSSM at large tan　region　　　　may have　large branching ra

tio by Higgs mediated process.

• Correlation between other mode– Br( 3l ) 　Br(　) ~ 0.2 – Br()　Br(　) ~ 0.006– Br(’)　Br(　) ~ 0.004

M. Sher, Phys.Rev.D66 05731(2002)K.S.Babu PRL,89,241802 (2002)A. Dedes, et.al. PLB 549,2442 (2002)

A.Brignole and A Rossi, hep-ph/0404211

The ratio could be a few times larger or smaller. It depend on radiative correction.

Our sensitivity reach this level.We can put constraint on tan and mA

backup


2.1 Selection criteria ( 1-3 prong + ’)– 1-3 prong + n2

E > 0.1 GeV , barrel and endcapptrk>0.06GeV/c barrel.ptrk >0.1GeV/c endcap.

net charge=0. dzrms < 1cm, drrms < 1cm to reject junk track.signal side: eID > 0.9 + 2trk(+,–).

– Missing momentum towards tag side.– pmiss vs. mmiss

2 2-D cut.– 0.5 < cos’ < 0.92– -3 < M < 3

using thrust axis, divide 2 hemisphere

for tag side, mtag < 1.777 GeV/c2

’

e/


2.2 Selection criteria (1-1 prong + )– 1-1 prong + n　 2

signal side: eID > 0.9 + 2 . Pe>0.7GeV/c, E > 0.22GeV– 0 veto for ’s tag side:– NOT (e)– Mtag < 1.777 GeV/c2

Kinematical cut.– Missing momentum towards tag side.– pmiss vs. mmiss

2 2-D cut.– 0.5 < cos’ < 0.92– -5 < M < 3

e/


3.0 過去の実験

• ee → で生成されたレプトンを使用

信号領域に検出事象はなかった．　　　→検出効率３～５％．背景事象の抑制率は 1/106 ．

対事象の数

MSSM からの予想

MARKII

ARGUS

CLEO

崩壊分岐比の上限値

DELPHI

？

search for lepton flavor violating decays t - l - p 0 , l - h , l - h ’ at belle

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