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Search for Lepton Flavor Violation in Tau Decays at BABAR
Olga Igonkina University of Oregon
August 30, 2004

Introduction BABAR Detector Search for Search for Å

LF V tau decay s Olga Igonkina


Introduction

Lepton are are are

Flavor Violating forbidden in the allowed but very very sensitive to

(LFV ) tau decays classical Standard Model; small in SM with neutrino mixing; non-SM physics.

Å
-

-

Å
+

-

W





Å -

Å

-



-

Å H
0



-

~
-

-

-

W

Å

-



The limits on LFV before BABAR/Belle :
B ( Å ) < 1.1 ç 10 B ( ÅÅÅ) < 1.9 ç 10
-6

at 90% CL [CLEO, 13.8 fb at 90% CL [CLEO, 4.8 fb

-1

]

-6

-1

]

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Theory Grab-bag
Model SM + lepton CKM SM + left-h. heavy Dirac neutrino SM + right-h. heavy Ma jorana neutrino SM + left and right-h. neutral singlets MSSM + right-h. heavy Ma jorana neutrino MSSM + seesaw left-right SUSY SUSY S O(10) SUSY-GUT SUSY + neutral Higgs SUSY + Higgs triplet gauge mediated SUSY breaking MSSM+universal soft SUSY breaking MSSM+nonuniversal soft SUSY breaking Non universal Z (technicolor) two Higgs doublet III seesaw with extra dimensions ... See also E. Ma hep-ph/0209170 for a review
(Some of the numb ers are guesses on base on given publication. For precise information, please, consult the reference)

10-40 < 10-18 < 10-9 10-8 10-10 10-7 10-10 10-8 10-8 10-10 10 10 10 10 10 10
-8 -7 -10 -9 -15 -11





10-14 < 10-18 < 10-10 10-9 10-9 10
-10

10 10 10 10 10 10

-10 -7 -9 -6 -8 -17

- 10

-7

Ref. hep-ph/9810484 SJNP25(1977)340 PRD66(2002)034008 PRD66(2002)034008 hep-ph/0306195 hep-ph/0206110 hep-ph/0306195 hep-ph/0209303 hep-ph/0307393 hep-ph/0304081) hep-ph/0209170 hep-ph/0310148 hep-ph/9911459 hep-ph/0305290 PLB547(2002)252 hep-ph/0208117 hep-ph/0210021

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BABAR Experiment
Instrumented flux return EM calorimeter 1.5 T Magnet e+ (3.1 GeV) Drift chamber e- (9.0 GeV)

Cherenkov detector

Silicon vertex detector

244 fb-1 recorded, more data on its way (e+ e- + - ) = 0.89 nb Almost 220 millions + - events! Results shown today use 90 fb-1
LF V tau decay s Olga Igonkina DPF 2004 4


Search for
1-prong

decay



One prong on the tag side ( , , ee , ÅÅ , ..) 85% Three well identified leptons on the signal side No missing momentum on the signal side MC: - Å- e+ e-
E, GeV
0.4 0.2 0 -0.2 -0.4 -0.6 -0.6 -0.4 -0.2 0

Neutrinoless Decay M = Mrec - M 0 CM E = Erec - EC M /2 0 Smeared by resolution and radiative effects

M, GeV

0.2

0.4

LF V tau decay s Olga Igonkina

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Backgrounds for

decay

Require 1-3 topology, reject conversions; apply lepton identification and selection cuts. Selection cuts and signal box borders are optimized for each channel separately.
E, GeV

qq
0.2 0 -0.2 -0.4 -0.6 -0.6 -0.4 -0.2 0 0.2 0.4

E, GeV

0.4

0.4

QED
0.2 0 -0.2 -0.4 -0.6 -0.6 -0.4 -0.2 0 0.2 0.4

M, GeV E, GeV E, GeV
0.4 0.4

M, GeV


0.2 0 -0.2 -0.4 -0.6 -0.6 -0.4 -0.2 0 0.2 0.4

data
0.2 0 -0.2 -0.4 -0.6 -0.6 -0.4 -0.2 0 0.2 0.4

M, GeV

M, GeV

LF V tau decay s Olga Igonkina

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Selection criteria for
Different channels are contaminated with - e - e+ e- , - e - Å + Å - - e- Å + Å - , - Å - Å + Å - - Å + e- e- , - e + Å - Å -
N events/0.1 GeV N events/0.1 GeV

different backgrounds: - Bhabha, q q , - Di-muon, q q , - qq,
N events

10

120

7 6 5 4

8

100

- -> Å-Å+Å-

- -> Å-e+e-

6

- -> Å-e+e-

80

60 3 2 2 1 0 0 0 0 20 4 40

p oints - data red - signal blue - Background

1

2

3

p

cms 1-prong mometum, , (GeV) 1

4

5

6 GeV

0.5

1

1.5

2

2.5

3

net pT in CMS, (GeV)

3.5 4 4.5 5 total pt in CMS, GeV

0

0

1

2

3

4

5

6

7

8

9

N photons on 3 prong N photons on 3-prong side side

All cuts except the one shown are applied

Suppress Bhabha and di-muon events with pcms > 100 MeV/c; T cms < 4.8 GeV/c and lepton veto for Bhabha and On 1-prong side - p1 di-muon contaminated channels; On 3-prong side - kaon veto and no photon candidates with E > 100 MeV
LF V tau decay s Olga Igonkina DPF 2004 7


Backgrounds estimates
Each background shape (Pi ) is fitted on MC or data control sample. Data are fitted with sum of 3 PDFs. The contribution of backgrounds in signal box is estimated using sideband region.
data, Blinded Events / ( 0.025 ) 25 Events / ( 0.0275 )

P

M data

30

P

E data

25

P f f
qq

data

=
QE D

20

20

QE D

ÇP

+

15 15 10

Ç (1 - f

QE D

) Ç Pq q +

10 5

5

(1 - fQE D - fqq Ç (1 - fQE D )) Ç P
E

0 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 -0

0.1 0.2 0.3 0.4 x: delta M

0

-0.6

-0.4

-0.2

0

M

0.2 0.4 y: delta E

E

data, Blinded 0.4

E y: delta

y: delta E

data

Histogram of hfit___x__y 4 3.5 3 0.4

P

data

0.0025

N

bkg r

=N

GS

SB GS

P P

data

dM dE dM dE

0.2

0.2 0.002 0 0.0015

data

0

2.5 2

-0.2 1.5 -0.4 1 0.5 0.1 0.2 0.3 0.4 x: delta M M 0

-0.2 0.001 -0.4 0.0005

-0.6

-0.6 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4 x: delta M M

LF V tau decay s Olga Igonkina

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 -0

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BABAR
E (GeV)

LFV limits
PRL 92, 121801 (2004)
- e-Å+Å-

- e-e+e0.2 -0

- Å+e-e-

BABAR

-0.2 -0.4 -0.6

- Å-e+e0.2 -0 -0.2 -0.4 -0.6 -0.6 -0.4 -0.2 0 0.2

- e+Å-Å-

- Å-Å+Å-

-0.4

-0.2

0

0.2

-0.4

-0.2

0

0.2

0.4
2

M (GeV/c )

[%] Nbgd Nobs B
90 UL

e- e+ e- 7.3 Á 0.2 1.51 Á 0.11 1 2.0 ç 10
-7

Å + e- e- 11.6 Á 0.2 0.37 Á 0.08 0 1.1 ç 10
-7

Å - e+ e- 7.7 Á 0.1 0.62 Á 0.10 1 2.7 ç 10
-7

e+ Å - Å - 9.8 Á 0.1 0.21 Á 0.07 0 1.3 ç 10
-7

e- Å + Å - 6.8 Á 0.1 0.39 Á 0.08 1 3.3 ç 10
-7

Å- Å+ Å- 6.7 Á 0.1 0.31 Á 0.09 0 1.9 ç 10
-7

No excess found in L = 91.6 fb-1 ; B (
LF V tau decay s Olga Igonkina DPF 2004

) < (1 - 3) ç 10

-7

at 90% CL
9


Old Preliminary BABAR Å Limit
1-prong Å



Select 1-1 topology; e or tag; only Å and on signal side; cuts on missing momentum and net pT in the event. Significant background from ? e+ e- Å+ Å- and ÅÅ
B
A

B

AR

Preliminary

m
1.8 1.6 -1

L = 63 fb-1 = (5.2 Á 0.5)% Expected Nbkgr = 7.8 Á 1.4 Data observed : 13 B ( Å ) < 2.0 ç 10 Update with 220 fb Tau2004
LF V tau decay s Olga Igonkina

(GeV/c )

2

2.0

-6

at 90% CL

-1

is expected for

EC

-0.5

0

0.5

E (GeV)

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BABAR/Belle era
Limits (ç10-7 ) at 90% CL based on 90 fb Mode B ( - B ( - B ( - B ( - B ( - B ( - Å- Å+ Å- ) e- Å + Å - ) e+ Å - Å - ) Å - e+ e- ) Å + e- e- ) e - e+ e- )
-7 -1

per experiment Belle
PLB 589, 103 (2004)

BABAR
PRL 92, 121801 (2004)

1.9 3.3 1.3 2.7 1.1 2.0

2.0 2.0 2.0 1.9 2.0 3.5
-7

Also B ( Å ) < 3.1 ç 10

and B ( Å ) < 3.4 ç 10

(Belle)
-7

B-factories have lowered limits on LFV in tau decays to 10

Over 500 fb-1 expected in next few years; limits should approach O(10-8 ) soon : many predictions are above this level! Many more channels to study hh, Å , p , etc.
LF V tau decay s Olga Igonkina DPF 2004 11


Conclusions

New BABAR results on search of are presented for L = 91.6 ) < (1 - 3) ç 10-7 at 90% CL, where = e or Å. fb-1 . B (

BABAR preliminary result for Å on L = 63 fb-1 is B ( Å ) < 2.0 ç 10-6 at 90% CL - will be updated soon

Current experimental sensitivity to LFV tau decays approaches theoretical predictions

Run 4 was very successful for BABAR, the total integrated luminosity is over 220 fb-1 expect many more new results on tau physics and LFV processes soon!

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