BelleSearch for Lepton Flavor Violatingτ decays at BelleK.Inami(Nagoya univ.)
2010/2/23

KEKB and Belle
KEKB: e+(3.5 GeV) e‐(8GeV)
σ(ττ)~0.9nb,σ(bb)~1.1nb
A B‐factory is also a τ‐factory!
Integrated luminosity: >1000 fb‐1⇒>9x108 τ‐pairs(6~8x108 for this analysis)

Belle Detector:
Good track reconstruction
and particle identifications

Lepton efficiency:90%
Fake rate : O(0.1) % for eO(1)% for μ22

Lepton Flavor Violation
Lepton flavor violation (LFV) in charged lepton sector
Many extensions of the SM predict LFV decays.
Their branching fractions are enhanced as high as current
experimental sensitivity⇒Observation of LFV is a clear signature of New Physics (NP)Tau lepton : the heaviest charged lepton‐ Opens many possible LFV decay modes which depend onNP models
K0
K0

R‐parity violation

Higgs‐mediation LFV

3

Event Selection
5‐1 (3‐1) prong events for lKsKs (lKs and 3leptons)

Signal side:π+ π− µ‐Kse‐τ‐ Ks
τ+π+π−

Select events with low multiplicity
and separate two sides using thrust‐ Signal (charged tracks from LFV)
‐ Tag (generic 1‐prong decay)

e+Tag‐side:νGeneric 1‐prong decay(Br(τÆ1‐prong+ν)~85%)

Reduce background eventsusing PID, kinematical information
optimize the event selection
for each mode separately
4Analysis method
Signal side:π+ π −
Ks
e‐

τ− Ks
τ+

After event Signal Extraction using
particles in signal sideselectionµ‐π+π−e+Blind analysis⇒Blind signal region
Estimate number of BG
using sideband data

M inv =E 2signal − p 2signal∆ E = E CMsignalCM− E beam

Signal region :90% elliptical region
including signal MC

Signal MC∆E~ 0 GeV

Minv ~ τ mass5

Optimization of event selection
To find the LFV signature
we optimize the selection criteria to obtain a goodsensitivity for the signal discovery, not for a lower UL.
To state 99% C.L. evidence― Need 2events for NBG~0.1― Need 4events for NBG~0.5
Æ Diff. of effective efficiency is 2.

Number of observed event, N99obs.
which we need for 99% CL evidence,as a function of Expected of BG, NBG

4Unless the efficiency drops significantly,
we set the criteria to reduce NBG as
much as possible.

0.5
20.1

6

Recent analysis• τÆlll• τÆlKs• τÆlf0• τÆlhh’• τÆlγSimple

Dominant BGµ: ττ and qq with π mis-ID
e: QED processes

Difficulty of reducing the BG

Hard– BG reduction with• Particle ID, Invariant mass cut– Optimize for each final state individually• Introduce intelligent variables (likelihood, neural netetc.)7τÆ3leptons[EPS2009,Preliminary]• Data: 782fb‐1– Prev.: 543fb‐1• No event is found in
the signal region.
• Remaining BG;
Bhabhae+e‐Æe+e‐µ+µ‐• B<(1.5‐2.7)x10‐8– Improved the UL
along with theluminosity fromprevious Belle resultModee−e+e−µ− µ+ µ−e−µ+µ−µ−e+e−µ− e + µ−e−µ+e−

ε (%)
6.0
7.6
6.1
9.3
10.1
11.5

NBGEXP
0.21+‐0.15
0.13+‐0.06
0.10+‐0.04
0.04+‐0.04
0.02+‐0.02
0.01+‐0.01

σsyst (%)
9.8
7.4
9.5
7.8
7.6
7.7

UL (x10‐8)
2.7
2.1
2.7
1.8
1.7
81.5τÆlKs and lKsKs• Accessible in R‐parity violation[PRD66:054021,2002]• Data: 671fb‐1• Remaining BG:
Fake lepton + real Ks from e+e‐Æ qq• No events in signal region

at 90%CL• B(τÆlK0s) < (2.3‐2.6) x 10‐8
• B(τÆlK0sK0s) < (7.1‐8.0)x10‐8⇒ improve in a factor of (31‐43) from CLEO

[PRD66:071101R,2002]

9τÆlf0[PLB672:317,2009]• Accessible level in Higgs mediation[PRD74:035010,2006]‐1• Data: 671fb• f0(980)Æπ+π− Æ Mass restriction reduces BG significantly.
• Remaining BG:e+e‐Æqq and e+e‐qq• B(τÆlf0)xB(f0 Æπ+π−)<(3.2‐3.4)x10‐8f0(980)10τÆlhh’• Data: 671fb‐1• Dominant BG:τÆπππν with mis‐ID, e+e‐Æqq• B<(3.3‐16)x10‐8arXiv:0908.3156 [hep-ex]

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10

10

10−

10µ−γeγµ−−π00e −πµ−ηe− ηµ−η′e− η′µ − KSe KS
e−−e++e−−e+µ−µ−
e −µ +µ−µ+e−e−µ− e+ e−µ −µ+µ−e+π−π−
e−π+π−µ+π−π−µππ
e−−π+−K−+
e+π−K−eπKe−+K+−K−−e −K+K−µ−π−K+µ+π−K−µπKµ−+K+−K−−µ−K K
e − KS K Sµ KS KSe−−f00µfe−−ρ0*e−K–*
eK−e− φe− ω0µ− ρ*
µ−K
–*
µK
µ−−φµ –ω
– pγ0
p– π−
Λπ−–Λπ−ΛK−
ΛK

Upper limit of BR for LFV τ decays

LFV results
Belle

lγlP0
lKs
lhh’
BaBar

lf0
CLEO-5-6-73leptons

lV0-8• Reach the sensitivity of O(10‐8)12Effect to physics models• Experimental results have already ruled out someparts of the parameter space.
– Exclude large tanβ, small SUSY/Higgs massreferenceτÆµγ

τÆµµµSM+ ν mixing

PRD45(1980)1908, EPJ C8(1999)513-40
10Undetectable
10-14

SM + heavy Maj νR

PRD 66(2002)034008

10-9

10-10

Non-universal Z’

PLB 547(2002)252

10-9

10-8

SUSY SO(10)PRD 68(2003)033012

10-8

10-10

mSUGRA+seesaw

PRD 66(2002)115013

10-7

10-9

SUSY Higgs

PLB 566(2003)217

10-1010-7– Accessing other models and other parameter space13

Future prospects
UL of BR

• In super B‐factory, Nτ willbe >1010.
• Sensitivity depends on
BG level.
– Recent improvement ofthe analysis(BG understanding,intelligent selection)
Æ Improve achievablesensitivity

1010-6

CLEOτ→µγτ→µη
τ→µµµ-7

B factories

10(Belle, BaBar)-8• B(τÆµµµ)~O(10‐10)

Old estimation
New estimation

at 50ab‐1• Improvement of BG
reduction is important.
– Beam BG– Resolution

10Super B factory-910-3

10-2

10-1

110-1
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Luminosity (ab )Summary• Search for LFV τ decays using ~109 τ decays– 48 modes are investigated.

• No evidence is observed yet.
• Upper limits on branching ratio around O(10‐8)– B(τÆµµµ)<2.1x10‐8, B(τÆµKs)<2.3x10‐8, etc.
– Exploring some new‐physics parameters space.
– Optimization for BG reduction is important.

• Plan– Finalize LFV search with full data set– Hadronic decay• Decay structure for hadronic decay with Kaon

– Rare decay, CPV decay, EDM etc.
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Luminosity
Peak Luminosity 2.1x1034cm‐2 s‐1

Skew sextupole⇒ World record!!!
Integrated luminosity: >1000 fb‐1⇒>9x108 τ‐pairs(6~8x108 for this analysis)[/nb/s]17