4th Workshop on Mass Origin and Supersymmetry Physics
Mar. 6-8, 2006

Search for the lepton flavor violation
τ decay at the Belle experiment (2)Y. Miyazaki (Nagoya university)
and Belle Collaboration
ContentsIntroduction(Motivation, Analysis method, Belle detector)
Results for LFV τ decay(τ → `KS0 , `V 0, `η/η 0/π 0 and Λπ −)Summary1

0Introduction

Lepton Flavor Violation (LFV) decays have a very small probability in the Standard Model(and proceed only due to neutrino oscillation).
⇓Many extensions of the SM predict LFV decays, enhanced up to the current experimentalsensitivity. (SUSY, Extra dimension and etc.)⇒ Observation of LFV from τ decay would be a clear signature of New Physics.
Previous results
CLEO sensitivities on B ∼O(10−6) for LFV τ decay with ∼ 107 τ τ pairs⇒ The Belle has sensitivities of B ∼ O(10−7 ∼10−8)1Expected branching fraction for LFV τ decay
Reach the level of some New Physics predictions

SUSY
Gauge mediated(MSSM)
Higgs mediated(MSSM)

B(τ"µγ) 3x10-7

B(τ"µee) <1x10-9
B(τ"3µ) <1x10-9~10B(τ"µπ0/η/η') <1x10-9B(τ"µγ) <3x10-7B(τ"µη') <3x10-9B(τ"µη) <5x10-7B(τ"3µ) <1x10-7B(τ"µπ0) <5x10-9B(τ"3µ,3e,µee,eµµ) <O(10-7)
In previous talks, showed
B(τ"l π0,l η,l η',l φ,l Κ*) <O(10-7)−−−B(τ"µγ)<10-9 B(τ"3µ)<10-10 • τ → e γ and µ γ

R-parity-VSO(10) with νRB(τ"eγ)<10-8 B(τ"3e)<10-8B(τ"µγ)<10-9Extra dimension
10−6−710−810−910B(LFV)210−10⇓
will show various LFV decay modes:• τ − → `−KS0• τ − → `−η/η 0/π 0• τ − → `− V 0• τ − → `hh0 and ``0`00• τ − → Λ̄π − and Λπ −Analysis method for LFV τ decay
Procedure for LFV τ decay
1. Select low multiplicity track with a zero net charge
2. Separate into two hemispheres using thrust axis
→ signal and tag
3. Reduce background using PID and kinematic informations→ missing momentum, # of γ ’s etc.
4. Calculate Minv and ∆E→Blind the signal region
5. Estimate the background in signal region
using sideband data
6. Open the blinded regione+
LFV observation or set an upper limits

B(LFV τ decay) <s90%C.L.
2εNτ τe+e− → τ +τ − production
LFV τ decay
generic τ decay

signal side
Complete reconstruction
M inv~ mτ = 1.777 GeVCMτ-τ+missing momentum π +∆E=Erecs/2 ~ 0 GeV

e-v

tag side31-prong decay (85%)(or 3-prongs (15%))

Belle Detector
KEKB:e+(3.5GeV)e−(8 GeV)
√s = 10.58 GeV
σ (τ τ ) ∼ 0.9 nb(σ (B B̄ ) ∼ 1.0 nb)
B-factory is also τ factory!!!

(ACC)

Integrated luminosity:>500/fb collected by Belle detector(> 300/fb for Babar)(TOF)

For lepton IDe efficiency 93%µ efficiency 88%(CDC)

Hadron ID in Belle is obtained by likelihood function from CDC, TOF and ACC. The separation between eachhadron is used by the likelihood ratio as L(i/j ) = Li/(Li + Lj ) where i(j ) is a kind of hadron (π, Kand p)4

X=e or µτ → `KS0 (1)→ `KS0 (where KS0 → π +π −)−1τDataset for this analysis @ 281 fb

Se+ττ+1-prong decay

Event selection• pmiss > 0.4 GeV/c
within the fiducial volumeCM• 10 > Etotal> 5.29GeVCM• cos θtag−mis> 0.0• # of γ in signal side ≤ 1• # of γ in tag side ≤ 2• cos θ`K 0 vs. p`K 0 cut
SS⇒ See plot on the rightRequire cos θ`K 0 <0.14log(p`K 0 − 2.7) + 0.7
SSEff. of cos θ`K 0 vs. p`K 0 cut for each MCSignal 99%
τ τ 0.7%, uds 16%

KsS5π+π-eτ → `KS0 (2)After events selections
ε = 11.8% for eKS0 mode
ε = 13.5% for µKS0 mode
Background:D (∗)± → `±νKS0
π ±KS0In signal region− Expected background0.2 ± 0.2 events− DataNo events in either mode
B(τ → eKS0 ) < 5.6 × 10−8
B(τ → µKS0 ) < 4.9 × 10−8⇓
Set upper limits on branching (Preliminary, hep-ex/0509014)
Improved by a factor of 16 and 19 compared with CLEO
fraction at 90% C.L.
(Previous upper limits: 9.1(9.5)×10−7 for eKS0 (µKS0 ))6τ → `KS0 (3)B(τ → µKS0 ) < 4.9 × 10−8⇒ |λi32λ0i12 |, |λi23λ0i21| < 4.1 × 10−4 (Mν̃ /100GeV/c2)2(i is the generation number.)

λ’ν̃

τ

λsKs (K0)

R-parity violation coupling λλB(τ → eKS0 ) < 5.6 × 10−8⇒ |λi31λ0i12 |, |λi31λ0i21| < 4.5 × 10−4 (Mν̃ /100GeV/c2)2,

τ − could decay into `− KS0 mode via tree-level scalar neutrino (ν̃ ) exchange by the λλ0 couplings.
Using our results, the limits on the λλ0 product as a function of the scalar neutrino mass (Mν̃ ) are given as(J. P. Saha and A. Kundu, PRD 66, 054021 (2002))

10-2.L.

10-3daeBell

10%Ct 90

udelcxe

From τ→µK0 S result
λλ, = (132,112), (233,321)(123,121), (232,212)

-4

From
τ→eK S result,λλ = (123,221), (231,212)(131,112), (133,321)0dµ107-550

75

100125150

175

2002M ∼ν (GeV/c )τ → `η/η 0/π 0 (1)0τ→eη,η→πππ00.250

τ→eπ0

τ→eη’,η’→ππη0.2(GeV)

0.2(GeV)

τ→eη,η→γγ(GeV)(GeV)

0.250-0.250∆E∆E∆E∆E-0.2-0.25-0.2-0.5-0.4-0.5-0.4-0.75

1.72

1.62(GeV/c )τ→µη,η→γγ0.2(GeV)

0.2∆E0-0.2

B < 5.8×10−7

1.80.25

0-0.4

1.7

1.81.9Meη, (GeV/c )2

τ→µπ0-0.25-0.2

1.61.9B < 1.9×10−7

τ→µη,η→πππ001.7

Meπ (GeV/c2)

Meη (GeV/c )B < 4.0×10−7(GeV)

1.8(GeV)Meη∆E-0.61.9

B < 10×10−7τ→µη’,η’→ππη

0.2(GeV)

1.8

∆E

1.7

∆E

1.60-0.2-0.4-0.5-0.41.6

1.7Mµη

1.82(GeV/c )

1.9

1.65

1.7

1.75

Mµη

1.8

1.85

1.62(GeV/c )B < 4.0×10−7
B < 5.8×10−7
Combined B(τ → eη ) < 2.3 × 10−7
Combined B(τ → µη ) < 1.5 × 10−7

1.7Mµπ1.82(GeV/c )

1.9

1.6

1.7Mµη,1.82(GeV/c )B < 1.9×10−7B < 10×10−7
Dataset for this analysis @ 154 fb−1(PLB 622, 218 (2005)) • data, 4 τ τ ,2 uds

8τ → `η/η 0/π 0 (2)In MSSM at large tan β , τ → `η mode has a large braching ratio by Higgs mediated process
Constraint on tan β and MA
100

dedxclu
DF e

80

C60

imer
epx

Belle 95% C.L.
Belle 90% C.L.

his e

ttanβntB(τ → µη ) =!
„«62 4tan β100GeV/c
8.4 × 10−760mA(M. Sher, PRD 66, 057301 (2002))

40200LEP excluded
1001502002250

Comparison with µµ+ µ− and µη 0− enhanced as (ms/mµ)2− color (× 3)− larger phase space than η 0 and 3-body decaymA (GeV/c )(CDF results: pp̄ → h/H/Abb̄ → bb̄bb̄ from RUN I (PRL 86, 4472 (2002))(LEP results: LEP Higgs Working Group)9τ → `V 0
Dataset for this analysis @ 158 fb−1

LFV decay into lepton and vector meson
B < (2.0∼7.7)×10−70.2 τ → e ρ0(Preliminary)0τ- → e- K*(892)0τ- → µ- K*(892)τ- → µ- ρ00

Backgroundfor τ → µV 0− τ τ and udsfor τ → eV 0− τ τ and 2photon(eeuu, eess etc.)∆ E* (GeV)-0.2-0.4*00.2 τ → e K (892)0τ- → e- φ*0

τ- → µ- K (892)0τ- → µ- φ

0-0.2
-0.4
-0.1-0.05 0 0.05 -0.1
0.1 -0.05 0 0.05 0.1 -0.05 0 0.05 0.1 -0.05 0 0.05 0.1
2∆ M (GeV/c )10τ → `hh0
LFV decay into lepton and 2 meson (h, h0 = K ± or π ±)(Including lepton number violation, e.g. τ − → `+h−h0−)
B < (1.8∼8.0)×10−7 (Preliminary)Dataset for this analysis @ 158 fb−1
τ- → e- π+ π-

τ- → e+ π- π-

τ- → e- K+ K-

τ- → e+ K- K-

0.2

00-0.2-0.2-0.40.2

τ- → µ- π+ π-

τ- → µ+ π- π-

τ- → µ- K+ K-

τ- → µ+ K- K-∆ E* (GeV)∆ E* (GeV)

0.2

0.2
0-0.2-0.2-0.4-0.42

τ- → e- π- K+

τ- → e+ π- K-

τ- → µ- π+ K-

τ- → µ- π- K+

τ- → µ+ π- K--0.4

0-0.1 -0.05 0 0.05 0.1 -0.05 0 0.05 0.1 -0.05 0 0.05 0.1 -0.05 0 0.05 0.1

τ- → e- π+ K--0.1-0.05 0 0.05 0.1 -0.05 0 0.05 0.1 -0.05 02∆ M (GeV/c )∆ M (GeV/c )Backgroundτ τ , uds and 2photon
110.05 0.1

τ → Λ̄π and Λπ (1)Search for τ decay with Lepton and Baryon number violation process(τ → Λπ, pKS , pγ , pπ 0 and so on)⇒ Important for cosmology (Baryon Asymmetry Universe)⇒ Sensitive to new physics (SUSY etc.)
We consider two types from τ → Λπ decay• B − L conserving mode⇒ τ − → Λ̄π − (Λ̄ → p̄π +)• B − L violating mode⇒ τ − → Λπ − (Λ → pπ − )We can distinguish between these two modes
using a charge between two pions• B − L conserving mode
⇒ Opposite charge• B − L violating mode
⇒ Same charge

12τ → Λ̄π and Λπ (2)8000Λ candidate
6000

2000

1.105 1.11 1.115 1.12 1.125 1.13
2Mass (GeV/c )(a) signal432
105(b) data

4320-20020m2miss ((GeV/c2)2)(c) uds

432
15-20
020m2miss ((GeV/c2)2)(d) τ+τ-432100-20020m2miss ((GeV/c2)2)13

51pmiss (GeV/c)

01.1

5pmiss (GeV/c)

4000

pmiss (GeV/c)

Event selection• Λ selection using vertex informations
and proton ID• pmiss > 0.4 GeV/c
within the fiducial volumeCM• 10.5 > Etotal> 5.29GeVCM• cos θtag−mis> 0.0• Kaon and Proton veto against tag-sideand π from τ → Λ00π 00• # of γ in signal side ≤ 1• # of γ in tag side ≤ 2• m2miss vs. pmiss cut⇒ See plot on the right2
Require plab
miss > 1.5mmiss − 1pmiss (GeV/c)

Number of events / 0.25 MeV/c2

Dataset for this analysis @ 154 fb−1

-20020m2miss ((GeV/c2)2)τ → Λ̄π and Λπ (3)0.2Background:uds: including real Λτ τ : fake Λ from 3-prongs decay(b)∆E (GeV)(a)∆E (GeV)

After events selections
ε = 11.8% for both modes

0.2

00-0.2-0.2

In signal region− Expected background-0.4-0.4
1.7 ± 0.8 events in both modes
1.725 1.75 1.775 1.8 1.825 2
1.725 1.75 1.775 1.8 1.825 2
Minv (GeV/c )Minv (GeV/c )− Data−−7−−8
1 event for B − L conserving mode B(τ → Λ̄π )) < 1.4 × 10 B(τ → Λπ ) < 7.2 × 10
(B − L conserving)(B − L violating)
0 events for B − L violating mode
(PLB 632, 51 (2006))⇓
These results are the first searches ever performed.
Set upper limits on branching
fraction at 90% C.L.

14

Summary for LFV τ decay

We have searched for various LFV τ decay mode at Belle.
B < O(10−5−10−6) in PDG by CLEO ⇒ B < O(10−7−10−8) by Belle
Updating LFV τ decay using full luminosity and get more stringent upper limits
15

Summary
We have searched for a lepton flavor violating τ decay at Belle.
No observation and sensitivity to lepton flavor violating τ decay branching fraction
is approaching 10−7 ∼ 10−8 at Belle⇒ Improvement of 1−2 orders over CLEO
⇒ Reached the level of some new physics⇒ Provide constraints on the new physics models
We have just updated a search for LFV τ decays⇒ Will show new results on this summer
B-factory is a good τ factory!
Thus, in additional to new physics search in B decay,
we provide sensitivities to new physics
via lepton flavor violation(and precision measurements also) in τ decay{Super B−factory(5∼10/ab)
16