why charmless sl. decays? analysis techniques overview of babar results (3 new results in 2005)

Exclusive Charmless Semileptonic DecaysExclusive Charmless Semileptonic Decaysand Determination of Vand Determination of Vubub

with BaBarwith BaBar

Why charmless sl. Decays? Analysis Techniques Overview of BaBar results (3 new results in 2005) Form Factors Vub Prospects for 500 fb-1

Jochen Dingfelder , SLACSLAC Experimental Seminar, June 9th 2005

06/09/006/09/055

22Jochen Dingfelder – SLAC Experimental Seminar

Study b u transitions Goal: Precise determination of |Vub|

Test CKM description of quark coupling and CP violation

Complementary constraint of UT to angle measurements;

try to reach similar precision as sin2.

Why Charmless Semileptonic B Decays ?


|Vub|/|Vcb| band (±2σ) σ=12%

(|Vcb|) ≈ 2%

(|Vub|) ≈ 12%

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Study b u transitions Goal: Precise determination of |Vub|

Test CKM description of quark coupling and CP violation

Complementary constraint of UT to angle measurements;

try to reach similar precision as sin2.




(|Vcb|) ≈ 2%

(|Vub|) ≈ 12%

Constraintsw/o angles

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Vub is best measured in charmless semileptonic decays B Xul

Factorization of leptonic and hadronic currents

Learn about electroweak interaction (coupling)

Learn about strong interaction study structure of B meson

allows tests of e.g. Lattice QCD



ll

u/du/d u/du/d

bb uu

BB0,0,±±

Vub

XXuu

So far B l and l have been measured (CLEO, Belle, BaBar (l) ). Also l (Belle) and l (CLEO) have been seen. Will present preliminary BABAR results from 2004-05 (82 fb-1 or 211 fb-1)

06/09/006/09/055


Inclusive Decays Large signal rate, high b

cl bkg

“Easy” to calculate (OPE/HQE)

Need Shape Function (b-quark motion inside B meson) . Constrain SF param. mb, 2 with b s or b cl

Exclusive Decays Low signal rate, better bkg

reduction and kinematic constraints

Need Form Factors F(q2) to describe the hadronization process u , …

Measurement as function of q2

Semileptonic B decays : The “Big Picture”

Semileptonic B decays : The “Big Picture”

|Vub|2 / |Vcb|2 ≈ 1 / 50

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How Do We Detect These Decays ?


DIRC

DCH IFRSVT

CsI (Tl)

e- (9 GeV)

e+ (3.1 GeV)

BaBarBaBar

B Xu l+ l = e Good e, ID (p*l>1GeV) and hadron ID (e.g. /K separation reject kaon tracks) Reliable track and reco. (DCH,EMC) Angular coverage ≈ 92% of 4π in CMS (challenge for ν reco)

’s , ’s

• 5-layer SVT tracker

• 40-layer Drift CHamber dE/dx

• Novel RICH based on total internal reflection (DIRC)

• CsI(Tl) crystal calorimeter (e±, γ)

• RPC and LST chambers in flux return for muon ID

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DIRC

DCH IFRSVT

CsI (Tl)

e- (9 GeV)

e+ (3.1 GeV)

BaBarBaBar


’s , ’s






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DIRC

DCH IFRSVT

CsI (Tl)

e- (9 GeV)

e+ (3.1 GeV)

BaBarBaBar


’s , ’s






DIRC /K separation

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DIRC

DCH IFRSVT

CsI (Tl)

e- (9 GeV)

e+ (3.1 GeV)

BaBarBaBar


’s , ’s






EMC

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No Tag: High statistics

High backgrounds and cross-feed Fully reconstruct signal side ( reco.)

Semileptonic Tag: Reconstruct B D(*) l and study recoil

- Full reconstruction of D(*)

- Partial reconstruction of D* (only l, soft) Two tag-B kinematics incomplete

Hadronic Tag: Fully reconstruct hadronic decay of one B:

B D(*) + (+,0,K+,K0) ≈ 1000 modes know kinematics of other B

Tagging MethodsTagging Methods

l-

Y(4S)Y(4S)

l-

Y(4S)Y(4S)

D

l-

l-

Y(4S)Y(4S)

D

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Tagging Methods: Event Yields

Tagging Methods: Event Yields

0.5130.1l+

12.410.440.22 l+

1530.5100.13 l+

S/Bevts fbS/Bevts fbS/Bevts fbDecay

0.5130.1l+

12.410.440.22 l+

1530.5100.13 l+

S/Bevts fbS/Bevts fbS/Bevts fbDecay

1,100,000 evt fb10,000 evt fb4,000 evt fb

No tagD(*) l tagBreco tag

1,100,000 evt fb10,000 evt fb4,000 evt fb

No tagD(*) l tagBreco tag

Advantages of Tagging: Determine tag-B charge, flavor, momentum constraints for signal B Separating the two B decays reduce combinatorics / cross-feed Reduce non-BB (continuum) background looser cuts full phase space

BUT: Significantly lower signal rates !

× 4 ×10

×1/3 ×1/3for l

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Hadro

Hadro

nic

nic

TagsTags

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Tag side: fully reco. B PB sig = -PB tag

Signal side: Plep > 1 GeV

fully constrained : M2miss < 0.5

GeV2

Pro:Pro: Clean sample, almost bkg free

Loose cuts keep full phase space

Con:Con: Very low signal rate !

Hadronic Tags: B (, ) l


Decay modes

l -

l

l

l

l

l

l

a0 l (≈100%

)

a00l (≈100%

)

Variety of signal modes studied !

+lb cl

M2miss

82 fb-

1

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B± l

l

l

’l

±→ωlν

Mhad [GeV/c2]

2.7 ± 1.2 ± 0.0513.9± 4.2’ l-

1.25 ± 0.55 ± 0.249.3 ± 3.30 l-

1.04 ± 0.39 ± 0.1615.7 ± 4.30 l-

3.5 ± 1.1 ± 0.0720.1 ± 5.1+l-

0.91 ± 0.28 ± 0.1415.5 ± 4.10 l-

0.89 ± 0.34 ± 0.1211.0 ± 3.9+ l-

BF x 104No. eventsDecay

2.7 ± 1.2 ± 0.0513.9± 4.2’ l-

1.25 ± 0.55 ± 0.249.3 ± 3.30 l-

1.04 ± 0.39 ± 0.1615.7 ± 4.30 l-

3.5 ± 1.1 ± 0.0720.1 ± 5.1+l-

0.91 ± 0.28 ± 0.1415.5 ± 4.10 l-

0.89 ± 0.34 ± 0.1211.0 ± 3.9+ l-

BF x 104No. eventsDecay

Systematics dominated by MC

statistics

Plus competitive limits for , a0,a +

82 fb-

1

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Semilepto

Semilepto

nic Tag

nic Tag

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Tagging efficiency measured with “Double Tags” (two D(*)l)

Semileptonic Tag MethodSemileptonic Tag Method Reconstruct B D(*)l and study

semileptonic recoil

Correct sl. decay |cosBY| < 1, bkg broader

( tag side: cosBY, signal side: cosB,l )

cos2B < 1 for signal, bkg flat

D(*)ldeca

ys

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Cut-and-count analysis in cosB,l and mD

Signal region: -1.1 < cosB,l < 1.0

Subtract mD sidebands remove cominatoric background

Subtract other background using MC normalized in -20 < cosBl < -1.5

B+ 0lwith SL Tag : Signal Extraction

B+ 0lwith SL Tag : Signal Extraction

82 fb-

1

45 0l

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Extract signal yields by binned fit to cos2B in 3 bins of q2

:

q2 = (pB – p)2 , (q2) ≈ 0.7 GeV2

Fit parameters = signal and background normalizations

B0 +l with SL Tag : Signal Extraction

B0 +l with SL Tag : Signal Extraction

Mainly B0B0

bkg

211 fb-1

21 +l

26 +l

14 +l

≈ 30% l X-feed

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Tag only l+ and p-soft from D*-

D0-soft

D* momentum pD* = f (soft)

Tag side mass: M2 = (pB - pD* -

pl)2

Partial SL (D*l Tag: B l Partial SL (D*l Tag: B l

Signal side: Look for l- +hard

Check consistency of remaining particles (X) with D0 decay (multivariate discriminator)

Signal yield from fit to signal side m

2B(B0->-l+) = (1.46 ± 0.27 ± 0.34 ± 0.03FF) × 10-4

BB bkgl

82 fb-

1

q2>16 q2<8 8<q2<16

MX

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No No Tag

Tag

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Harsh suppression of b cl background e+e- qq background

Untagged B l, B lAnalysis Strategy


Fit all 4 signal modes simultaneously (+, 0, +, 0)

Max-LH fit of signal and background in E, mES, and q2

Neutrino Reconstruction: Reconstruct from full event & ensure

good reco. quality

Select signal decay candidates: Y = hadron () + lepton (e/)

ll

u/du/d u/du/d

bb uuBB0,0,±± XXuu

Y = / + l

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good reco. quality


ll

u/du/d u/du/d


Y = / + l

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Neutrino “Quality Cuts”: Net charge: |Q| ≤ 1, miss > 0.6

rad

limit losses due to acceptance

Missing mass: |M2miss/2Emiss| < 0.4

GeV2

Neutrino ReconstructionNeutrino Reconstruction

B0 lMC

no addKL

add.L

add.

Mmiss/2Emiss (GeV)

Scale neutrino momentum, so that E=0 Improves q2 resolution

w/o E=0

= 70 MeV

Landau: 280 MeV

pmiss resolution

|pmiss| - |p| (GeV)

Tail due to losses

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good reco. quality


ll

u/du/d u/du/d


Y = / + l

06/09/006/09/055


p*h

Signal ()

b cl

p*l p*l

Background SuppressionBackground Suppression

Continuum Suppression:Off-res. data statistics

lowNeed to rely on MCMinimize impact of

continuum bkg L2 = pi*cos2i * < 1.5

GeV Suppress 80% b c l

b c l Suppression:Need to suppress largecharm bkg in lchannel kinem. cuts on p*l ,p*h

(for l we can keep nearly whole phase

space)

L2

costhr costhr

Signal ()

qq

Selection efficiencies: ≈ 3% for l, ≈ 1% for l

b c l

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good reco. quality


ll

u/du/d u/du/d


Y = / + l

Before we fit the signal, we

should check that selection / reconstructionreally work!

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Select high statistics and high purity control sample Perform same selection as for b ul signal (except b cl suppression)

Cross-check MC modeling of signal decays Study modeling of dominant charm background component (D*)

Two modes for B D*-l+ D0-l+ : D0 K+- , D0 K+- 0

.

“Proof of Principle”: B D*l Control Sample

“Proof of Principle”: B D*l Control Sample

K+-0 K+-0

M2miss/2Emiss

(GeV)Ge

V

K+-0

Signal

other D*D/

D**

q2

(GeV2)

Efficiencies of cuts agree between data and MC within a few %.

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good reco. quality


ll

u/du/d u/du/d


Y = / + l

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Fit all signal modes simultaneously

Float , ±, 0 in each q2 bin

5 (3) q2 bins for l (l)

Use isospin relations: (B0 -l+) = 2(B+ 0l+)

(B0 -l+) = 2 (B+ 0l+)

One free parameter for b cl

Total: 9 free parameters

Fit in E-mES and q2Fit in E-mES and q2

Coarse binning

in Sideband Region

Fine binning

in Signal Region

Binned max.-likelihood fit, including MC statistics (Barlow & Beeston)

X-feedXu

fixed

l

qq fixed

cl

l

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Results forl: Fitted E and mES


76 fb-

1

Sum of 396 l, 137 0l

5 bins for l3 bins for l

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5 bins for l3 bins for lSum of 95 l, 98

0l

76 fb-

1

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Light-Cone Sum Rules :

Valid for q2 < 14 GeV2

Ball/Zwicky quote 10-13% error at q2=0

Lattice QCD :

Unquenched calculations by HPQCD, FNAL

Valid for q2 > 15 GeV2

11-13% error at high q2

Use parametrizations (e.g. Becirevic-Kaidalov) to extrapolate to full q2 range

Quark models: ISGW II (no error quoted)

B l Form FactorsB l Form Factors

Theo. FF uncertainties enter twice:

(1) FF shape acceptance

(2) FF normalization extraction of Vub

222

02 3

2 3

( )

24( )F

ub

Gd BV f qp

dq

2

220

2 32 3

( )

24( )F

ub

Gd BV f qp

dq

06/09/006/09/055


Fitting the Form-Factor ShapeFitting the Form-Factor Shape

HPQCD’04 : = 0.42 ± 0.07 , FNAL’04 : = 0.62 ± 0.05

Fit Becirevic-Kaidalov (BK) Parametrization to data:

f(0) = norm. factor, = shape parameter

Result of BK-Fit consistent with unquenched LQCD:

l

ISGW II

LCSR

LQCD

= 0.60 ± 0.15BK-Fit to BABAR data

06/09/006/09/055


Recent LQCD and LCSR calculations agree

well with BABAR B l data

ISGW II shows marginal agreement

Measured q2 DistributionMeasured q2 Distribution

l

ISGW II BK Fit

Improved Data-MC

agreement in kinematic

distributions, e.g.,

hadron momentum.

P*

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Current Measurements of B l as Function of q2

Current Measurements of B l as Function of q2

More data will help!

M. Moriihep-ex/0505070

06/09/006/09/055


Constraining Form-Factor Parametrizations

Constraining Form-Factor Parametrizations Form factor contains soft-overlap () and hard-scattering (H) contributions:

BK param. is approximation that neglects H study more general parametrization: with

Richard Hill, hep-ph/0505129 :

< 1 limits hard scattering contribution Simple Pole Model (only B* pole) ruled out with 99.99% CL Single Pole Model not ruled out ( ∞, 1, (1-) fixed )

68% CL Limits

HPQCD

FNAL

Experiment

=1

=0

bb uu

BB Hard gluon

06/09/006/09/055


04

B l Form FactorsB l Form Factors

l

Good agreement with all FF calculations

Statistical and systematic errors still too large to measure three FFs

For vector mesons we need 3 FFs, e.g. A1,A2,V much more difficult !

Ball & Zwicky ‘04

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Theory Uncertainty: Dependence on FF shape

Theory Uncertainty: Dependence on FF shape

Acceptance effect due to harsh kinem.

cuts for l

Effect of l FF on BF small.

Some effect of X-feed from l

“Do we have a model-independent BF measurement?”

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Experimental Systematic Uncertainties


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Detector effects: Track, Photon, KL efficiencies and resolutions (losses, bkg, …)

06/09/006/09/055




Relative contributionsto charm bkg have large uncertainties:

B D, D*, D**, … BF


DD*

*

D*

All

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Relative contributionsto charm bkg have large uncertainties:

B D, D*, D**, … BF

B ul Hybrid

M(Xu)


DD*

*

D*

All

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Comparison off-resonance data with continuum MC.

Mainly low q2 (secondary leptons)

Relative contributionsto charm bkg have large uncertainties:B -> D, D*, D**, … BFs

B ul Hybrid

M(Xu)


DD*

*

D*

All

06/09/006/09/055


Branching Fraction Measurements

Branching Fraction Measurements

BABAR

CLEO

B l B l

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Consistency Check : Isospin Symmetry

Consistency Check : Isospin Symmetry

Measure B0, B+ separately:

Test isospin constraint

Consistent with 2 within stat. error

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uubb

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Extraction of VubExtraction of Vub

No FF norm.uncertainty

available

BABAR’s choice

LCSR q2 <15 GeV2, LQCD q2 > 15 GeV2 or Extrapolation

to whole q2 range

388.0

52.0 10)11.0

24.014.082.3(||

LQCDFF

syststatubV4%

6%

7% exp.

Extraction of |Vub| relies on FF norm. in distinct q2 regions

Theory error dominates:11-13% in restricted q2 ranges

15-17% for whole q2 range

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“Historical” discrepancy between incl. and excl. Vub results:

Inclusive Vub or Exclusive Vub ?Inclusive Vub or Exclusive Vub ?

Incl. and excl. |Vub| have moved closer together!

It’s important to do exclusive analyses because theoretical uncertainties are complementary.

Incl. BABAR average, BLNP, SF from b clC’

Excl. BABAR untagged B lFNAL’04 FF (CKM’05)

CLEO l

New

DFN

, S

F B

elle b

s

06/09/006/09/055


Need help from theory: Uncertainty on |Vub| dominated by theory error!

Reliable error estimate for l FF needed for and BF and |Vub|

Progress in LQCD for l : theo. error 5-6% (how long will it take?)

We could reach total error of ≈ 8% in exclusive Vub in the near future …

ProspectsProspects Error on BF will decrease through:

more precise measurements of b cland b ul (res & non-res) BF, FF

improved track and neutral reconstruction ( reco.)

more data prospects for 500 fb-1 :

40% 27%

16%

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BABAR has three new, competitive BF and Vub measurements for B l

and one for B l

Untagged analysiswith recocurrently has best precision.

Tagged analyses (semilep. tag, Breco tag) will become very powerful with

more data (“crossover point” >≈ 500 fb-1).

Results of untagged analysis are:

Consistent with previous measurements

Statistically more accurate

Less dependent on theoretical predictions

- First “real” measurement of the FF shape for B l

- Uncertainties for B l still too large

Current uncertainty of exclusive Vub : ≈ 15% (dominated by theory error)

could improve to 8% in the near future !?

ConclusionsConclusions

06/09/006/09/055


ρ


Probing New Physics (?)

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Backup Slides

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Belle SL Tag: B π () l νBelle SL Tag: B π () l ν

140/fb

2-dimensional fit to (Mx,xB2) distribution:

extract normalization for signal land l, Xu l and BB background PDF from MC, constraint on total B (BXu l ) Extract yield from Mx distribution for 3 different q2 bins

4

4

10)30.085.078.054.2()(

10)03.020.028.075.1()(

FFsyststat

FFsyststat

B

B

xB2 = 1-cos2B

why charmless sl. decays? analysis techniques overview of babar results (3 new results in 2005)

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