Experimental Review on
Lepton Flavor ViolatingTau decays
2008/4 K.Inami Nagoya university
International workshop e+e- collisions from phi to psi PHIPSI08
2008/4 International workshop e+e- collisions from phi to psi 2
Lepton Flavor Violation (LFV) Standard Model (SM) exactly forbids LFV. Extended SM including neutrino mixing term includ
es LFV. Charged Lepton Flavor Violation
can occur through loop diagram.But, extremely tiny branching ratio
below experimental sensitivity Br()<O(10-40) (X.-Y. Pham, EPJ C8(1999)513)
Many new models include LFV vertex naturally. Observation of charged LFV
Clear signal of new physics!
Tau lepton has many possible LFV decay modes. Wide window to probe New Physics effect.
2008/4 International workshop e+e- collisions from phi to psi 3
LFV tau decays Predicted by many new physics models
Normal NP models enhance For some NP parameters, other modes may be
enhanced instead.
Many models allow measurable BR of O(10-
8) within the parameter space.
Reference
SM+ mixing EPJ C8(1999)513 10-40 10-14
SM + heavy Maj R 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 10-10 10-7
2008/4 International workshop e+e- collisions from phi to psi 4
Most famous SM extension LFV are generated through slepton mixing.
Independent parameter for e
SUSY seesaw (J.Hisano et. al.,PRD 60 (1999) 055008)
Achievable BR of O(10-7~-8) if tan~60 and mSUSY~1TeV/c2
SUSY
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When sleptons are heavy (>weak scale),Br() is suppressed.
Higgs in SUSY enhances other LFV modes, such as 3l, l+hadrons, ...
Higgs-mediated MSSM 3 (A.Brignole, A.Rossi, PLB 566 (2003) 217)
Enhanced if tan is large and Higgs mass is small.
SUSY Higgs
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SUSY Higgs Higgs-mediated MSSM (M.Sher, PRD 66 (2002) 057301)
Br() : Br() = 8.4 : 1 Phase space, color factor, mass
MSSM seesaw (E.Arganda, arXiv:0803.2039v1)
Large BR of O(10-7) for , ’,
Need to search for all possible LFV modes To probe unknown physics and discriminate models
2008/4 International workshop e+e- collisions from phi to psi 7
Experiment Recent results obtained by B-factories
High intensity e+e- collider on (4S) KEKB/Belle; World highest luminosity of 1.7x1034/cm2/s
Similar cross-section with BB (~0.9nb) -factory Detectors; Good PID ability and vertex resolution
Lepton ID eff. : ~90% fake rate : ~2% e fake rate : ~0.3 %
2008/4 International workshop e+e- collisions from phi to psi 8
Status of LFV search at spring 2007
Current accumulated luminosity;>700fb-1 at Belle and >500fb-1 at BaBar.
Searches for 3l, lhh modes should be updated.
BelleBelle BaBarBaBar BelleBelle BaBarBaBarUL90(10-7)
Lum(fb-1)
UL90(10-7)
Lum(fb-1)
UL90(10-7)
Lum(fb-1)
UL90(10-7)
Lum(fb-1)
0.5 535 0.7 232 e 0.8 401 1.3 339
e 1.2 535 1.1 232 3l 2-4 87 1-3 92
0.7 401 1.5 339 lhh 2-16 158 1-5 221
’ 1.3 401 1.3 339 V0 2.8-7.7 158
1.2 401 1.5 339 eV0 3.0-7.3 158
e 0.9 401 1.6 339 Ks 0.5 281
e’ 1.6 401 2.4 339 eKs 0.6 281
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LFV analysis Signal extraction
Calc. Minv and E E=Erec-Ebeam
Blinded signal region Event selection study
Estimate backgroundusing sideband data
Open blind and estimate signal yield
Estimate upper limitsby Likelihood fit or number of events in signal region
BG reduction is important to improve the sensitivity.
Find effective ways to use all information
Signal MCSignal region
Background
Minv
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LFV decays; Signal and Background
qq _
2photon process f=leptons,quarks
signal
Only tag side has neutrino(s).
Both sides have neutrino(s).
radiative Bhabha process
e+ e
eee
e
many tracks
e+e-
1 prong tau decay (BR~85%)
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3 leptons; BG reduction
Signal topology
BG events
- conversion veto (e-e+e- and -e+e-) - Electron-veto on the tag-side (e-e+e- and e-+-)
Bhabha() ee(eeee)
Optimized event selection for each mode separately because the BG processes are different.
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lll search; Belle Data; 535fb-1
Efficiency; (6.0-12.5)% NBG; 0.0-0.4 events Nobs; 0 events for all modes Br<(2.0-4.1)×10-8 at 90%C.L.
(Y.Miyazaki et. al., PLB 660 (2008) 154)
eee
data signal
signal region
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lll search; BaBar Data; 376fb-1
Efficiency; (5.5-12.4)% NBG; 0.3-1.3 events
Total BG; 4.2±0.8 Nobs; 6 events in total Br<(3.7-8.0)×10-8 at 90%C.L.
(B.Aubert et. al.,PRL 99 (2007) 251803)
=MinvM
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lV0 search Belle; 543fb-1 data Ten modes are investigated,
l=e, and V0=,K*0,K*0, Main BG: SM decay; Signal Efficiency; 2.5 - 4.9 % Expected NBG; 0.0 - 1.0 events Observed number of events;
1 event for , e, eK*0
0 events for others Br<(5.9-18)×10-8
(Y.Nishio et. al., arXiv:0801.2475)
BaBar; 384fb-1 data Searched for , e
Br<(10-11)×10-8 (B.Aubert et. al., PRL 100;071802 2008)
data signal
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lf0 search Scalar Higgs mediation
No search yet.
Belle; 671fb-1
Use f0
No event is observed. Preliminary result
Br(→lf0)xBr(f0→) f0 : <3.3X10-8, ef0 : <3.4X10-8
signal MC● data
f0
ef0
efficiency
NBG
f0 6.0% 0.11±0.08ef0 5.8% 0.10±0.07
Br(→f0):Br(→):Br (→) = 1.3 : 0.54 :1 (C.H.Chen et.al, PRD74:035010,2006 )
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Current status of LFV search
Sensitivity is improved by factors of 3-10. It comes from the luminosity increase and successful BG reduction.
Search for 3leptons sets most stringent upper limit.
BelleBelle BaBarBaBar BelleBelle BaBarBaBarUL90(10-7)
Lum(fb-1)
UL90(10-7)
Lum(fb-1)
UL90(10-7)
Lum(fb-1)
UL90(10-7)
Lum(fb-1)
0.5 535 0.7 232 e 0.8 401 1.3 339
e 1.2 535 1.1 232 3l 2-40.2-0.4
87535
1-30.4-0.8
92376
0.7 401 1.5 339 lhh 2-16 158 1-5 221
’ 1.3 401 1.3 339 V0 2.8-7.70.6-1.3
158543 1.0() 384
1.2 401 1.5 339 eV0 3.0-7.30.6-1.8
158543 1.1(e) 384
e 0.9 401 1.6 339 f0 0.33 671
e’ 1.6 401 2.4 339 ef0 0.34 671
2008/4 International workshop e+e- collisions from phi to psi 17
Current status of LFV search Experimental results have already ruled out some
parts of the parameter space. Exclude large tan, small SUSY/Higgs mass
Accessing other models and other parameter space
reference
SM+ mixing EPJ C8(1999)513 10-40 10-14
SM + heavy Maj R 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 10-10 10-7
2008/4 International workshop e+e- collisions from phi to psi 18
Int. Luminosity at B-factory; >1300fb-1
~1.2x109 -pairs At super B-factory
>10 times more data BR sensitivity
It depends on background. l; scale as ~1/L
ee is irreducible BG. ~10-8 level at super B-factory
lll, lX0; scale as ~1/L BG can be reduced by lepton-ID
or mass requirement for X0
O(10-9) level at super B-factory
Future prospect
2008/4 International workshop e+e- collisions from phi to psi 19
Summary A data sample of >109 -pairs has been successf
ully obtained by the B-factories.
New searches for 3-prong final state 3l, lV0, lf0 using 400~700 fb-1 of data Better sensitivity than previous analysis
Applied event selection optimized for each mode separately
LFV sensitivities are Br~O(10-8). Exploring some parameter space of New Physics
Many models predict possible BR of O(10-7~-8).
Super B-factory is planned to produce >1010 -pairs. LFV sensitivity will reach Br~O(10-9).
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LFV decays; where we are.
Sensitivity of Br~O(10-8)
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237 fb-1 BaBar data used for this analysis.
(hep-ex: 0607040)
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3 leptons; Results
No signal events No BG events in sideband except for e-e+e-