recent lfv results on tau lepton from bellephipsi11.inp.nsk.su/talks/phipsi11_hayasaka.pdf•belle...

Recent LFV results on

tau lepton from Belle

K.Hayasaka (KMI, Nagoya Univ.)

20/Sep/2011 1PhiPsi11@BINP

Introduction

• Lepton flavor violation (LFV) in charged leptons

⇒negligibly small probability in the Standard Model (SM)

even including neutrino oscillations:

Observation of LFV is a clear signature of New Physics (NP)

– Many extensions of the SM predict LFV decays.

• These branching fractions could be enhanced as high as current

experimental sensitivity. (~10-8)

• Tau lepton = The heaviest charged lepton

• Expected strong coupling to NP

• Many possible LFV decay modes


m (or e)t

W-

g

ntnm (or ne)

t LFV search

= ideal probe to NP

54

2

2

2

10)( -

W

SMm

mBr n

gt

Introduction(2)

By searching for various kinds of t LFV,

we can distinguish between NP models.

• tgℓhh’

• t-→ℓ-h+h’-: usual LFV

Higgs mediated model

• t-→ℓ+h-h’-: lepton number violating

Majorana n model

• tgLh

• t-→Lh-: B-L cons. SUSY GUT model

• t-→Lh-: B-L viol. ??? New model ???


We have updated tgℓhh’ and tgLh search with Belle full data sample!

Ernesto Arganda, Maria J. Herrero

and Jorge Portolés, JHEP06(2008)079

KEKB/Belle


tts tt ,s

Belle finalized data takingon Jun.30,2010.total:>1ab-1

U(4S):711fb-1

U(5S):121fb-1

U(3S):3.0fb-1

U(2S):24fb-1

U(1S):5.7fb-1

Off-resonance:

87fb-1

U(4S)

off-resonance

t

5

Analysis method

CM

beam

CM

signal EEE -

2

signal

2

signal pEM -inv

0~E GeV

invM ~ t mass

signal region~±3s in resolution

(blind region)

Signal MC

Signal Extraction

Blind analysis

⇒Blind signal region

Estimate BG level

using sideband data and MC

If no excess is found,

set upper limits @ 90%CL

using the Cousins-Highland (CH)

frequentist counting method

e+e- →t+t-

clean environment

e e

t

t

m

generic 1-prong

decay

p

p

Search for t→ℓhh’14 modes are investigated (h,h’=p± and K±)• t-→ℓ-h+h’-: 8 modes (lepton flavor violation)• t-→ℓ+h-h’-: 6 modes (lepton number violation)

Current upper limits

• Belle Br<(4.4-16)x10-8 @ 671fb-1

• BaBar Br<(7-48)x10-8 @ 221fb-1update with 854fb-1

@Belle

K+

e- or m-

n

p-

t-

t+

e- e+

p+p-

n

p-

t-

t+

e- e+

n

signal generic tt BG

misid as m-

misid as K+

g

p-

n

p0t-

t+

e- e+

g

e+e-

misid as K+

n

Missing momentum can help to reject this kind of BGs since signal has n only on tag side.

Also,

qq events

can be BG

6

BG rejection for t→ℓhh’

20/Sep/2011 PhiPsi11@BINP 7

To reduce tt and qq BG

• mpK mode

⇒ mmiss2 -- pmiss correlation

• ehh’, mpp and mKK modes

⇒ mmiss2 selection

2d selection

•mpK mode

Dominant BG is from t→pppn with a pp

combination misidentified as Km.

⇒MmpK is shifted into the t-mass signal region

while the original Mppp will be below the

t-mass due to the missing neutrino.

Assign ppp mass for selected events

⇒ Mppp>1.52 GeV/c2

dataqq MCtt MCmKp MC

To reduce tt BG

75% of eff. is kept while 75% of BG is rejected.



Results for t→ℓhh’

8

予想される背景事象(0.1-1.5)events

Set upper limits @90%CL:

Br(t→ℓhh’)< (2.0-8.4)x10-8

(preliminary)

most sensitive results

Improve our previous results

by a factor of 1.8 on the average

1 event : in m+p-p- and m-p+K-

no events: in other modes

⇒ no significant excess

In the signal region

(preliminary)

20/Sep/2011 PhiPsi11@BINP

Search for tgLh/Lh


4 modes are searched for. (h=p or K)• t-→Lh-: (B-L) conserving decay• t-→Lh-: (B-L) violating decay

Current upper limits

• Belle Br<(7.2-14)x10-8 @ 154fb-1

• BaBar Br<(5.8-15)x10-8 @ 237fb-1update with 906fb-1

@Belle

(no search for LK on Belle)

p-

n

t-

t+

e- e+L

pp-

signal p-

n

t-

t+

e- e+Ks0

p-p+

misid as L

e- e+

p-

L

pp-

p0

gg

pp0

gg

Generic tt BG qqBG

n


BG rejection for tgLh/Lh

To reduce tt BG including KS0

⇒ reconstruct KS0 and reject events

that are likely to be KS0

qq MC

85% of eff. is kept while

75% of Ks0 BG events is rejected.

To reduce qq BG including L

⇒ reject events with a proton in tag side

(due to BN conservation, the events

including a L tend to have baryon on

tag side.)

A third of qq BG events are rejected

while a loss of eff. is negligibly small.

Results for tgLh/Lh


no candidate events are found

⇒ no significant excess

In the signal region

0

0

0

0

2.3

2.2

2.2

2.1

Set upper limits@90%CL:

Br(t-gLp-)<2.8x10-8

Br(t-gLK-)<3.1x10-8

Br(t-gLp-)<3.0x10-8

Br(t-gLK-)<4.2x10-8

(B-L) cons.

(B-L) viol.

most sensitive results

Around (2-3)x improvement

from the previous BaBar results

(preliminary)

(preliminary)

Upper Limits on t LFV Decay


before

summer

Updated modes

New Upper Limits on t LFV Decay


after

summer

Update using full data samples will be finalized soon!

Reach upper limits around 10-8 ~100x more sensitive than CLEO

Future prospect on t LFV

• Belle-II will collect

~8x1010 tau leptons. (=50ab-1)

• Sensitivity depends on BG level.

Recent improvement

of the analysis

(BG understanding,

intelligent selection)

Improve achievable sensitivity

B(tgmg)~O(10-9) and

B(tgmmm)~O(10-10) at 50ab-1

Improvement of BG reduction

is important.– Beam BG

– Signal resolution


Summary• Belle completed operation with a 1000fb-1 data sample,

which contains ~109 tau-pairs. This is the world’s largest t data sample.

• There are many kinds of t LFV including lepton number (L) violation

or (B-L) violation, i.e., baryon number – lepton number.

• By adding more data and studying the dominant BGs and optimizing

the analyses to suppress these BGs, we have significantly improved

t LFV upper limits.

• Almost all t LFV modes have been investigated with the 1000fb-1 data

sample. The last remaining ones are tgeg/mg.

• Belle-II will collect ~1011 tau leptons

and the sensitivity of O(10-9 - -10) will be achieved for t LFV decays.


Br(t→ℓhh’)< (2.0-8.4)x10-8 Br(tgLh)<(2.8-4.2)x10-8

The most sensitive results for these modes.(preliminary)

@90%CL

recent lfv results on tau lepton from bellephipsi11.inp.nsk.su/talks/phipsi11_hayasaka.pdf•belle...

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