Experimental Review on Lepton Flavor Violating Tau decays

1
Experimental Review onLepton Flavor
ViolatingTau decays
International workshop ee- collisions from phi
to psi
PHIPSI08

• 2008/4 K.Inami
• Nagoya university

2
Lepton Flavor Violation (LFV)

• Standard Model (SM) exactly forbids LFV.
• Extended SM including neutrino mixing term
  includes LFV.
• Charged Lepton Flavor Violation
• can occur through loop diagram.
• But, extremely tiny branching ratio
• below experimental sensitivity
• Br(t?mg)ltO(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.

3
LFV tau decays

• Predicted by many new physics models
• Normal NP models enhance t ? mg.
• For some NP parameters, other modes may be
  enhanced instead.
• Many models allow measurable BR of O(10-8) within
  the parameter space.

Reference t?mg t?mmm
SM n mixing EPJ C8(1999)513 10-40 10-14
SM heavy Maj nR 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
mSUGRAseesaw PRD 66(2002)115013 10-7 10-9
SUSY Higgs PLB 566(2003)217 10-10 10-7
4
SUSY

• Most famous SM extension
• LFV are generated through slepton mixing.
• Independent parameter for m?eg
• SUSY seesaw (J.Hisano et. al.,PRD 60 (1999)
  055008)
• Achievable BR of O(10-7-8) if tanb60 and
  mSUSY1TeV/c2

5
SUSY Higgs

• When sleptons are heavy (gtweak scale),
• Br(t?mg) is suppressed.
• Higgs in SUSY enhances other LFV modes, such as
  t?3l, lhadrons, ...
• Higgs-mediated MSSM
• t?3m (A.Brignole, A.Rossi, PLB 566 (2003) 217)
• Enhanced if tanb is large and Higgs mass is small.

6
SUSY Higgs

• Higgs-mediated MSSM (M.Sher, PRD 66 (2002)
  057301)
• Br(t?mh) Br(t?mmm) 8.4 1
• Phase space, color factor, mass
• MSSM seesaw (E.Arganda, arXiv0803.2039v1)
• Large BR of O(10-7) for t?mh, mh, mKK-
• Need to search for all possible LFV modes
• To probe unknown physics and discriminate models

7
Experiment

• Recent results obtained by B-factories
• High intensity ee- collider on U(4S)
• KEKB/Belle World highest luminosity of
  1.7x1034/cm2/s
• Similar cross-section with BB (0.9nb) ?
  t-factory
• Detectors Good PID ability and vertex resolution

Lepton ID eff. 90 m fake rate 2 e
fake rate 0.3
8
Status of LFV search at spring 2007
Belle Belle BaBar BaBar Belle Belle BaBar BaBar
UL90 (10-7) Lum (fb-1) UL90 (10-7) Lum (fb-1) UL90 (10-7) Lum (fb-1) UL90 (10-7) Lum (fb-1)
mg 0.5 535 0.7 232 ep0 0.8 401 1.3 339
eg 1.2 535 1.1 232 3l 2-4 87 1-3 92
mh 0.7 401 1.5 339 lhh 2-16 158 1-5 221
mh 1.3 401 1.3 339 mV0 2.8-7.7 158
mp0 1.2 401 1.5 339 eV0 3.0-7.3 158
eh 0.9 401 1.6 339 mKs 0.5 281
eh 1.6 401 2.4 339 eKs 0.6 281

• Current accumulated luminosity
• gt700fb-1 at Belle and gt500fb-1 at BaBar.
• Searches for 3l, lhh modes should be updated.

9
LFV analysis

• Signal extraction
• Calc. Minv and DE
• DEErec-Ebeam
• Blinded signal region
• ? Event selection study
• Estimate background
• using sideband data
• Open blind and estimate signal yield
• Estimate upper limits
• by 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 MC
Signal region
Background
Minv
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LFV t decays Signal and Background

• ee- ? t t-
• 1 prong tau decay (BR85)

2photon process
tt
fleptons,quarks
signal

• Both sides have neutrino(s).

radiative Bhabha process
e-
e
e-
g
e
e-

• many
• tracks

• Only tag side
• has neutrino(s).

e
11
t?3 leptons BG reduction
BG events
eemm(eeee)
Bhabha(mm)
Signal topology

• g conversion veto
• (e-ee- and m-ee-)
• Electron-veto on the tag-side
• (e-ee- and e-mm-)

• Optimized event selection for each mode
  separately because the BG processes are different.

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t?lll search Belle

• Data 535fb-1
• Efficiency (6.0-12.5)
• NBG 0.0-0.4 events
• Nobs 0 events for all modes
• Brlt(2.0-4.1)10-8 at 90C.L.
• (Y.Miyazaki et. al., PLB 660 (2008) 154)

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t?lll search BaBar

• Data 376fb-1
• Efficiency (5.5-12.4)
• NBG 0.3-1.3 events
• Total BG 4.20.8
• Nobs 6 events in total
• Brlt(3.7-8.0)10-8 at 90C.L.
• (B.Aubert et. al.,PRL 99 (2007) 251803)

Minv-Mt
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t?lV0 search
data
signal

• Belle 543fb-1 data
• Ten modes are investigated,
• le,m and V0r,K0,K0,f,w
• Main BG SM t decayt?pwn,t?pppn
• Signal Efficiency 2.5 - 4.9
• Expected NBG 0.0 - 1.0 events
• Observed number of events
• 1 event for t?mf, ew, eK0
• 0 events for others
• Brlt(5.9-18)10-8
• (Y.Nishio et. al., arXiv0801.2475)
• BaBar 384fb-1 data
• Searched for mw, ew
• Brlt(10-11)10-8
• (B.Aubert et. al., PRL 100071802 2008)

15
t?lf0 search

• Scalar Higgs mediation
• No search yet.
• Belle 671fb-1
• Use f0?pp- (4s)
• No event is observed.
• Preliminary result
• Br(t?lf0)xBr(f0?pp-)
• mf0 lt3.3X10-8, ef0 lt3.4X10-8

Br(t?mf0)Br(t?mmm)Br (t?mh) 1.3 0.54 1
(C.H.Chen et.al, PRD74035010,2006 )
efficiency NBG
mf0 6.0 0.110.08
ef0 5.8 0.100.07
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Current status of LFV search
Belle Belle BaBar BaBar Belle Belle BaBar BaBar
UL90 (10-7) Lum (fb-1) UL90 (10-7) Lum (fb-1) UL90 (10-7) Lum (fb-1) UL90 (10-7) Lum (fb-1)
mg 0.5 535 0.7 232 ep0 0.8 401 1.3 339
eg 1.2 535 1.1 232 3l 2-4 0.2-0.4 87 535 1-3 0.4-0.8 92 376
mh 0.7 401 1.5 339 lhh 2-16 158 1-5 221
mh 1.3 401 1.3 339 mV0 2.8-7.7 0.6-1.3 158 543 1.0(mw) 384
mp0 1.2 401 1.5 339 eV0 3.0-7.3 0.6-1.8 158 543 1.1(ew) 384
eh 0.9 401 1.6 339 mf0 0.33 671
eh 1.6 401 2.4 339 ef0 0.34 671

• Sensitivity is improved by factors of 3-10. It
  comes from the luminosity increase and successful
  BG reduction.
• Search for t?3leptons sets most stringent upper
  limit.

17
Current status of LFV search

• Experimental results have already ruled out some
  parts of the parameter space.
• Exclude large tanb, small SUSY/Higgs mass
• Accessing other models and other parameter space

reference t?mg t?mmm
SM n mixing EPJ C8(1999)513 10-40 10-14
SM heavy Maj nR 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
mSUGRAseesaw PRD 66(2002)115013 10-7 10-9
SUSY Higgs PLB 566(2003)217 10-10 10-7
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Future prospect

• Int. Luminosity at B-factory gt1300fb-1
• 1.2x109 t-pairs
• At super B-factory
• gt10 times more data
• BR sensitivity
• It depends on background.
• t?lg scale as 1/?L
• ee-?tt-g is irreducible BG.
• 10-8 level at super B-factory
• t?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

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Summary

• A data sample of gt109 t-pairs has been
  successfully obtained by the B-factories.
• New searches for 3-prong final state
• t?3l, lV0, lf0 using 400700 fb-1 of data
• Better sensitivity than previous analysis
• Applied event selection optimized for each mode
  separately
• LFV sensitivities are BrO(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 gt1010
  t-pairs.
• ? LFV sensitivity will reach BrO(10-9).

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LFV t decays where we are.

• Sensitivity of BrO(10-8)

21
t?Lp, LK

• 237 fb-1 BaBar data used for this analysis.
• (hep-ex 0607040)

22
t?3 leptons Results

• No signal events
• No BG events in sideband except for e-ee-