Leptonic B-decays at the B factories · Leptonic B-decays at the B factories Lattice QCD Meets...

Leptonic B-decays at the B factoriesLattice QCD Meets Experiment Workshop

FNAL

M. Bellis1 & C.Cartaro2

for the BaBar collaboration

1Department of PhysicsStanford University

2SLAC National Accelerator Laboratory

April. 26th, 2010

M. Bellis April. 2010 FNAL 1 / 26

Outline

1 Overview

2 The B-factories

3 The measurementsLeptonic decaysRadiative leptonic decaysLepton flavour violation

4 Summary


B leptonic decays

A great variety of decays and rare searches.

Leptonic decays

B → τ ν̄B → µν̄B → eν̄

Radiative leptonic decays

B → γ`ν̄

Lepton flavour violating decays

B → ``′

B → K``′

Not a comprehensive list...but some of the most recent/relevant results.

Motivation?


B leptonic decays


Leptonic decays



B → γ`ν̄


B → ``′

B → K``′


Motivation?


B leptonic decays


Leptonic decays



B → γ`ν̄


B → ``′

B → K``′


Motivation?


B leptonic decays


Leptonic decays



B → γ`ν̄


B → ``′

B → K``′


Motivation?


Pure leptonic decays

In the Standard Model

Tree level mediated by only Wboson.Helicity suppressed

B → τ ν̄ ≈ 10−4

B → µν̄ ≈ 10−7

B → eν̄ ≈ 10−12

Sensitive to fB , given Vub

Vub and fB dominate SMuncertainty.

B(B → `ν) =G2FmB

8πm2`(1−

m2`

m2B

)2f 2B |Vub|2τB


Pure leptonic decays beyond SM

Decay mediated by a Higgs

Charged Higgs contribution is not helicitysuppressed.

Model dependent prediction.

B(B → `ν)2HDM = B(B → `ν)× (1− tan2 βm2

B

m2H

)2

B(B → `ν)SUSY = B(B → `ν)× (1−tan2

1 + η0 tanβ

m2B

m2H

)2

W.S. Hou, Phys.Rev.D., 48 (1993) 2342

Akeroyd,Recksiegel J.Phys.G29:2311-2317, 2003



Complex interplay amongst the“unknowns”.

fBQCD

Vub

CKM matrix

New physics

2HDM,SUSY,MSSM,etc.

B(B → `ν̄)

Experimental measurement




fBQCD

Vub

CKM matrix

New physics

2HDM,SUSY,MSSM,etc.

B(B → `ν̄)





fBQCD

Vub

CKM matrix

New physics

2HDM,SUSY,MSSM,etc.

B(B → `ν̄)



Fit at the SM and beyond

The name of the game is to interpret measurements in a consistent framework.

Tensions...

Vub and sin(2β)fB and B(B → τν)

“Freedom” to choose what values to use.




Tensions...






Tensions...





dm∆

τν +τ → +

B

dm∆ and τν +τ →

+Constraint from B

α

βγ

ρ

−1.0 −0.5 0.0 0.5 1.0 1.5 2.0

η

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

excluded area has CL > 0.95

Summer 08

CKMf i t t e r



UTFit Collaboration arXiv:0908.3470

]−4)[10ντ→BR(B

0 0.5 1 1.5 2 2.5 3

]−4

))[1

0ντ

→(B

R(B

σ

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0

1

2

3

4

5

6 σ

arXiv:0908.3480


Lepton flavour universality test

Potentially large violations of LF universality canappear in helicity-suppressed charged-currentmodes within the MSSM.

Large tanβ

Γ(B → µν̄)exp = Γ(B → µν̄µ)+Γ(B → µν̄e)+Γ(B → µν̄τ )

Γ(B → µν̄µ) = SM

Γ(B → µν̄e) ≈ 0

Γ(B → µν̄τ ) ∝ scalar LFV amplitude

Experimental probe

Out of reach of current B-factories.

Rτµ = ΓB→µν̄ΓB→τν̄ Rτe = ΓB→eν̄

ΓB→τν̄

Prediction in non-minimal LFV

Rτµ ∼ 10%Rτµ,SM

Rτe ∼ 103Rτe,SM

G.Isidori, P.Paradisi, Phys.Lett.B 639,499

bR

sL

µR

ν̄τ

H+




Large tanβ



Γ(B → µν̄e) ≈ 0


Experimental probe



ΓB→τν̄



Rτe ∼ 103Rτe,SM


bR

sL

µR

ν̄τ

H+




Large tanβ



Γ(B → µν̄e) ≈ 0


Experimental probe



ΓB→τν̄



Rτe ∼ 103Rτe,SM


bR

sL

µR

ν̄τ

H+


The B-factories

The B-factories

Belle (KEK)BaBar (SLAC)

Ran on Υ(4S) resonance.

Wealth of rich physics under theresonance (cc̄, τ+τ−, ...)

Wealth of backgrounds under theresonance.

High luminosity

On the order of 1.5 billion BB̄ pairs inthe world’s dataset!


The B-factories

The B-factories





High luminosity



The B-factories

The B-factories





High luminosity



The B-factories

The B-factories





High luminosity



The B-factories

The B-factories





High luminosity



B-factory physics

Colliding e+e− beams

Running on the Υ(4S)resonance.

Decays to BB̄

One B will decay toour signal of interest.The other B decaysin any number ofways.

e− e+


B-factory physics



Decays to BB̄


e− e+

B0

B0


B-factory physics



Decays to BB̄


e− e+

B0

B0


Beam constraints

B mass

Beam energy resolution is better than Benergy (combined track ~p) resolution.

mES =√

14s − (p∗B)2

∆E = E∗B −12

√s

Discriminating power in 2D plane.

Signal process (Monte Carlo)

2B mass GeV/c5.2 5.22 5.24 5.26 5.28 5.3

even

ts/M

eV

0

100

200

300

Entries: 11922

All background processes (Monte Carlo)

2B mass GeV/c5.2 5.22 5.24 5.26 5.28 5.3

even

ts/M

eV

0

20

40

60

80

100

120

140 Entries: 11248


Beam constraints

B mass


mES =√

14s − (p∗B)2

∆E = E∗B −12

√s



2 GeV/cESM5.2 5.22 5.24 5.26 5.28 5.3

even

ts/M

eV

0

500

1000

1500

2000 Entries: 14328


2 GeV/cESM5.2 5.22 5.24 5.26 5.28 5.3

even

ts/M

eV

0

200

400

600 Entries: 47036


Beam constraints

B mass


mES =√

14s − (p∗B)2

∆E = E∗B −12

√s



E GeV∆-0.2 -0.1 0 0.1 0.2

even

ts/M

eV

0

200

400

600

800

1000

1200

1400 Entries: 14328


E GeV∆-0.2 -0.1 0 0.1 0.2

even

ts/M

eV

0

200

400

600

Entries: 47036


Beam constraints

B mass


mES =√

14s − (p∗B)2

∆E = E∗B −12

√s



2 GeV/c

ESM5.2 5.22 5.24 5.26 5.28 5.3 E GeV∆

-0.2-0.1

00.1

0.2020406080

100120140160180200220

Entries: 14328


2 GeV/c

ESM5.2 5.22 5.24 5.26 5.28 5.3 E GeV∆

-0.2-0.1

00.1

0.202468

101214161820

Entries: 47036


Tagging methods

Completely reconstruct one B.

Breco or Btag

Search the other B for decays of interest.

Brecoil or Bsig

Two tagging methods:

Hadronic tag

More pure.Less efficient. (∼ 0.2%)∼ 1000 different decay modes!B → D(∗) + π

B → D(∗) + ππ

B → D(∗) + πK

.

.

.

Semileptonic tag

Less pure.More efficient. (∼ 1.5%)ν leads to “missing” energyHandful of different decay modes.B → D(∗)`ν̄


Tagging methods


Breco or Btag


Brecoil or Bsig


Hadronic tag


B → D(∗) + ππ

B → D(∗) + πK

.

.

.

Semileptonic tag



Tagging methods


Breco or Btag


Brecoil or Bsig


Hadronic tag


B → D(∗) + ππ

B → D(∗) + πK

.

.

.

Semileptonic tag



Tagging methods


Breco or Btag


Brecoil or Bsig


Hadronic tag


B → D(∗) + ππ

B → D(∗) + πK

.

.

.

Semileptonic tag



B → `ν with semileptonic tag

Topology, kinematics and particle ID are used to suppress background.

Eextra strongest discriminating variable.

Extra neutral energy in the reaction.ν’ will not deposit extra energy.Peaks at 0 for reaction of interest.

Look at multiple decay modes of the τ !

τ → eν̄ντ → µν̄ντ → πντ → ρν

Phys.Rev.D80:051101,2010 (arXiv:0809.4027) 459× 106 BB̄ pairs


















B → `ν results

B → τ ν̄


B → τ ν̄ (Total)

SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4


B → `ν results

B → τ ν̄



SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4

(GeV)extraE

0 0.2 0.4 0.6 0.8 1 1.2

Ev

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ts/0

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5 G

eV

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Generic MC

On Peak Data

Signal MC


B → `ν results

B → τ ν̄



SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4

(GeV)extraE

0 0.2 0.4 0.6 0.8 1 1.2

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Generic MC

On Peak Data

Signal MC


B → `ν results

B → τ ν̄



SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4

(GeV)extraE

0 0.2 0.4 0.6 0.8 1 1.2

Ev

en

ts/0

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Generic MC

On Peak Data

Signal MC


B → `ν results

B → τ ν̄



SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4

(GeV)extraE

0 0.2 0.4 0.6 0.8 1 1.2

Ev

en

ts/0

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5 G

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(GeV)extraE

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1.5

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2.5

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3.5

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Generic MC

On Peak Data

Signal MC


B → `ν results

B → τ ν̄



SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4

(GeV)extraE

0 0.2 0.4 0.6 0.8 1 1.2

Ev

en

ts/0

.12

5 G

eV

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(GeV)extraE

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eV

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20

25

30 Generic MC

On Peak Data

Signal MC


B → `ν results

B → τ ν̄



SL:B(B → τ ν̄) = 1.8±0.8±0.1×10−4

Had:B(B → τ ν̄) = 1.8±0.4±0.2×10−4

PRL 97,251802 (2006)

arXiv:0809.3834

SL:B(B → τ ν̄) = 1.65±0.4±0.4×10−4

Had:B(B → τ ν̄) = 1.79±0.5±0.5×10−4


B → `ν results

B → µν̄

11 observed15±10 expected bkg.B(B → µν̄)UL90% = 11× 10−6

B → eν̄

17 observed24±11 expected bkg.B(B → eν̄)UL90% = 7.7× 10−6

(GeV)extraE

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On Peak Data

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(GeV)extraE

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Generic MC

On Peak Data

Signal MC


B → e/µν̄ inclusive

Very rare (BR∼ 10−7, 10−12 respectively)

Other experimental methods?

Inclusive measurement.

Very strong signal-side signature...

A single, monochromatic lepton in the Brest frame.

Momentum smeared distribution inthe CM frame.Apply tight particle ID criteria andreject events with more leptons.

Inclusive approach for the rest of theevent.

Highly efficient...but highbackground too.Build an inclusive 4-momentumwith everything else in the event.Discriminate with mES and ∆EBackground suppression withkinematic and topological variablescombined with a Fisherdiscriminant.

PRD 79,091101 (2009) 468× 106 BB̄ pairsM. Bellis April. 2010 FNAL 18 / 26





















B → e/µν̄ inclusive results

Simultaneous fit to:

mES of the inclusive B.p∗` : transformed lepton momentum(CM and B rest frame)

B → eν̄

B at 90% CL < 1.9× 10−6

B at 90% CL < 0.98× 10−6

B → µν̄

B at 90% CL < 1.0× 10−6

B at 90% CL < 1.7× 10−6]2 [GeV/cESm

5.2 5.22 5.24 5.26 5.28 5.3

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( 0

.00

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B → eν̄

B at 90% CL < 1.9× 10−6

B at 90% CL < 0.98× 10−6

B → µν̄

B at 90% CL < 1.0× 10−6

B at 90% CL < 1.7× 10−6]2 [GeV/cESm

5.2 5.22 5.24 5.26 5.28 5.3

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B → eν̄

B at 90% CL < 1.9× 10−6

B at 90% CL < 0.98× 10−6

B → µν̄

B at 90% CL < 1.0× 10−6

B at 90% CL < 1.7× 10−6]2 [GeV/cESm

5.22 5.24 5.26 5.28 5.3

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B → eν̄

B at 90% CL < 1.9× 10−6

B at 90% CL < 0.98× 10−6

B → µν̄

B at 90% CL < 1.0× 10−6

B at 90% CL < 1.7× 10−6]2 [GeV/cESm

5.22 5.24 5.26 5.28 5.3

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B → `νγ with hadronic tags

No helicity suppression.

Dependence on αEM and form factors.

Model dependent

HQET LO: fA = fVOthers: fA = 0

Experimentally, making no requirementson lepton or photon momenta reduces themodel dependence.

B(B+ → `+νγ) =αG2

FmB

288π|Vub|2f 2

Bm5BτB(

Qu

λB−Qu

λb)2

PRD 80,111105 (2009) arXiv:0907.1681 465× 106BB̄ pairs


B → `νγ with hadronic tags results

Model independent estimation of B.

Cut and count analysis with ULdetermined with frequentistapproach.

B → eν̄γ

FC CL band: < 17× 10−6

B → µν̄γ

FC CL band: < 26× 10−6

B → `ν̄γ

FC CL band: < 15× 10−6

B = 6.47+7.6+2.8−4.7−0.8 × 10−6 at 2.1σ

)4/c2 (GeVeν

2m−1 −0.5 0 0.5 1 1.5 2 2.5 3 3.5

)4

/c2

En

trie

s/(

0.2

Ge

V

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1

2

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ν 2m

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B → eν̄γ

FC CL band: < 17× 10−6

B → µν̄γ

FC CL band: < 26× 10−6

B → `ν̄γ

FC CL band: < 15× 10−6

B = 6.47+7.6+2.8−4.7−0.8 × 10−6 at 2.1σ

)4/c2 (GeVeν

2m−1 −0.5 0 0.5 1 1.5 2 2.5 3 3.5

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/c2

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B → eν̄γ

FC CL band: < 17× 10−6

B → µν̄γ

FC CL band: < 26× 10−6

B → `ν̄γ

FC CL band: < 15× 10−6

B = 6.47+7.6+2.8−4.7−0.8 × 10−6 at 2.1σ

)4/c2 (GeVeν

2m−1 −0.5 0 0.5 1 1.5 2 2.5 3 3.5

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Model dependent estimation of B.

Cut on `− γ angle and ν − γ anglein hypothesis of fA = fV or fA = 0

fA = fVeν̄γ < 8.4× 10−6

µν̄γ < 6.7× 10−6

`ν̄γ < 3.0× 10−6

fA = fVeν̄γ < 8.4× 10−6

µν̄γ < 6.7× 10−6

`ν̄γ < 3.0× 10−6

)4/c2 (GeVeν

2m−1 −0.5 0 0.5 1 1.5 2 2.5 3 3.5

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/c2

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)4/c2 (GeVµ

ν 2m

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

)4

/c2

En

trie

s/(

0.2

Ge

V

0

1

2

3

4

5

6

7





fA = fVeν̄γ < 8.4× 10−6

µν̄γ < 6.7× 10−6

`ν̄γ < 3.0× 10−6

fA = fVeν̄γ < 8.4× 10−6

µν̄γ < 6.7× 10−6

`ν̄γ < 3.0× 10−6

)4/c2 (GeVeν

2m−1 −0.5 0 0.5 1 1.5 2 2.5 3 3.5

)4

/c2

En

trie

s/(

0.2

Ge

V

0

1

2

3

4

5

6

7

)4/c2 (GeVµ

ν 2m

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

)4

/c2

En

trie

s/(

0.2

Ge

V

0

1

2

3

4

5

6

7





fA = fVeν̄γ < 8.4× 10−6

µν̄γ < 6.7× 10−6

`ν̄γ < 3.0× 10−6

fA = fVeν̄γ < 8.4× 10−6

µν̄γ < 6.7× 10−6

`ν̄γ < 3.0× 10−6

)4/c2 (GeVeν

2m−1 −0.5 0 0.5 1 1.5 2 2.5 3 3.5

)4

/c2

En

trie

s/(

0.2

Ge

V

0

1

2

3

4

5

6

7

)4/c2 (GeVµ

ν 2m

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

)4

/c2

En

trie

s/(

0.2

Ge

V

0

1

2

3

4

5

6

7


Lepton flavour violating modes

First two generations less challenging than τ .

But the third generation is most sensitive to NPUsually published along with LFC analyses of B → `+`− and B → K`+`− (where` = e, µ)

Both analyses use very similar methodology based on hadronic tag reconstruction.

PRD 77,091104 (2008) arXiv:0801.0697 378× 106 BB̄ pairs

PRL 99,201801 (2007) arXiv:0708.1303 383× 106 BB̄ pairs


B → `τ(` = e, µ) with hadronic tags results

Well established methodology.

Same as B → `ν

Hadronic B tag

Lepton monochromatic in the B restframe.

Very clear signature.

Straightforward reconstruction of the τ

Full 4-momenta

90% CL upper limits.

B(B0 → eτ) < 2.8× 10−5

B(B0 → µτ) < 2.2× 10−5

B(B0 → K+µτ) < 7.7× 10−5

Lepton Momentum (GeV/c)1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7E

ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40

-τ

+ e→0

B

BABAR

Lepton momentum (GeV/c)1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7E

ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40 BABAR

-τ+µ →

0B




Same as B → `ν

Hadronic B tag




Full 4-momenta


B(B0 → eτ) < 2.8× 10−5

B(B0 → µτ) < 2.2× 10−5

B(B0 → K+µτ) < 7.7× 10−5


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40

-τ

+ e→0

B

BABAR


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40 BABAR

-τ+µ →

0B




Same as B → `ν

Hadronic B tag




Full 4-momenta


B(B0 → eτ) < 2.8× 10−5

B(B0 → µτ) < 2.2× 10−5

B(B0 → K+µτ) < 7.7× 10−5


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40

-τ

+ e→0

B

BABAR


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40 BABAR

-τ+µ →

0B




Same as B → `ν

Hadronic B tag




Full 4-momenta


B(B0 → eτ) < 2.8× 10−5

B(B0 → µτ) < 2.2× 10−5

B(B0 → K+µτ) < 7.7× 10−5


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40

-τ

+ e→0

B

BABAR


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40


ntr

ies

/ 0

.02

5 G

eV/c

0

10

20

30

40 BABAR

-τ+µ →

0B


B → K+µτ

Potentially the most sensitive LFVchannel to NP.

Hadronic B tag

Signal side completely reconstructed.

τ mass defines the signal.

Main background (and control sample)from b → c`ν

M.Sher and Y.Yuan,PRD44,1461 (1991) T.P.Cheng and

M.Sher,PRD35,3484 (1987)

)2 (GeV/cτm0 0.5 1 1.5 2 2.5 3

2E

ven

ts /

12

5 M

eV/c

0

2

4

Arb

. U

nit

s2

per

25

MeV

/c

)2

(GeV/cτ

m1.6 1.8 2


Summary

B-factories have had much success with B-leptonic decays

B → τ ν̄ MEASURED!

Tension in SM

B → µν̄ Upper limit...but but at edge of SM!B → eν̄ Upper limit...B → γ`ν̄ Upper limit...but at edge of SM!


B → ``′ Upper limit...B → K``′ Upper limit...

Belle is still running and BaBar is still analyzing.

Next generation B-factories could be very exciting!

Thanks for your time!


Summary



Tension in SM








Summary



Tension in SM








Summary



Tension in SM








Summary



Tension in SM








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