1

Selected Results and Prognostications on Vcb & Vub :

A B Factory Perspective

Vivek SharmaUniversity of California San Diego

vsharma@ucsd.edu

Ringberg Phenomenology Workshop on Heavy Flavors : Rottach-Egern, Germany

Special Thanks!

• CLEO– David Cassel– Karl Ecklund

• BaBar– Daniele delRe– Oliver Buchmuller

• HFAG Working group

I have heavily borrowed from their recent presentations at CKM’03. APS’03 & Moriond’03

2

The Two Approaches in Vxb

*cb

cb

xb

u

u ub

b

4. First steps towards Inclus

Moments

ive

in B & |V

Topics

B X & |

today:1. B D & |V |2. 3. B [ , ] & |

5. Future Directions in clean |V | measuremeV |nt

|V |

sν

ν

π ρ ω ν→

→

→

This talk is an “appetizer” not a review. See recent CKM workshop page for complete results

http://ckm-workshop.web.cern.ch/ckm-workshop/ckm-workshops/Default2003.htm

Inclusive Semileptonic Decay Rate

Babar (also CLEO)

Basic foundation of all semileptonic studies. Modern measurements agree

3

• Experimentally favored (S/N) w.r.t B Dl nu• Experimental Challenges:

1. Slow pion tracking ( helical path of decreasing radius)

2. Charm branching fractions3. Knowledge of higher mass Dnpi states:

• excited D**, Non-res. D(*) npi l nu ??4. Form Factors (mostly for ρ measurement)

*( ,0)cb(1) and |V |B D ν+ −→

CLEO has been the most experienced playeron this topic new results with ¼ total data

Silicon vertex tracker

CLEO

A complete set of measurements from CLEO:

3| | [46.9 1.4 2.0 1.8 ] 10cb stat syst thV −= ± ± ± ×

~6.5% Measurement

4

Branching Ratio of B D* l nu

disagreement between CLEO & Babar/Belle in value of

the branching ratio

Each experiment have much more dataleft to analyze

Remove stat. fluctuation as source of disagreement, focus on finding

systematic biases

“Repeat n>1 more time and very carefully”

|Vcb |World Average (CKM’03 preliminary)

3% Stat

Updated prelim average using F(1) = 0.91 ± 0.04

|Vcb|=[ 42.6 ± 0.6(stat) ±1.0(syst) ± 2.1(theory) ] 10-3

5

Seeking Consistency

?

Photon energy spectrum in B→ Xs γ

Hadronic mass spectrum in B→Xc ν

Lepton energy spectrum in B→Xc ν

Cleo Moment Analyses(II) Moments in B Decay: Elegant Measurements from CLEO

Use HQE/OPE to predict SL rate &Moments of inclusive B decay spectra

6

Lepton energy spectrum in B→Xc ν

Hadronic mass spectrum in B→Xc ν

Photon energy spectrum in B→ Xsγ

1σ Theoretical Ellipse

Cleo Moment Analyses:Consistent picture

Λ = 0.39+0.03stat+0.06sys+0.12thGeV

λ1=−0.25+0.02stat+0.05sys+0.14th GeV2

31

exp 3cb , ,

|V | [40.8 0.5 0.4 0.9 ] 10sl s B

PDG theo

Mλ α −−

Γ Λ= ± ± ± × 3% measure

Remarkable !

Comparing Exclusive & Inclusive Vcb Measurements

*

Averaging many measurements is when the measurementsare done in different environments/prescriptionsI prefer to look for consistency within ea

CLEO Exclusive :

ch experiment

| | [46.9 1B

t

.D

ricky

4cbVυ→

= ±

31

3

exp 3cb , ,

Seem a bit different

CLEO I

?? Ve

2.0 1.8

ry int

] 10

|V | [40.8 0.5

eresting to watch what Belle & BaBar get from similar set of

nclus

0.4 0.

measuremen

ive

9

] 10sl s B

stat syst th

PDG theo

M

and

λ α −

−

−Γ Λ

± ± ×

= ± ± ± ×

ts

7

Strong dependence of moments Strong dependence of moments on p*on p*minmin

For For pp**minmin=1.5 =1.5 GeV/cGeV/c andand

Λ=0.35 ± 0.13 GeV [1]

(reliance on b → sγ spectrum)

λ1= - 0.17 ± 0.06 ±0.07 GeV2

CLEO [1]

λ1= - 0.226 ± 0.07 ± 0.08 GeV2

Babar Hadronic Moment Analysis: ICHEP Preliminary

<MX

2 -M

Dsp

in2 >

p*min [GeV/c]

OPE (Falk,Luke) Λ = 0.35 GeV

But these parameters do not describe P* dependence of the moments!

λ1(0.9 GeV/c) – λ1(1.5 GeV/c)= 0.22±0.04±0.05 GeV2

OPE (Falk, Luke)Λ, λ1 free param.

NB: Data points highly correlatedNB: Data points highly correlated

BABAR PreliminaryCLEO

[1] CLEO PRL 87, 251808 (2001)

No non-resonant states (MC)

?

Can Hadronic B Decays Help Understand Nature of High Mass Dnπ states in SL Decay?

B D** π B D** l nu

D**D**

factorization ?

D+ πD*+ π

Belle

Non-reso

What can oneLearn from such

results ?

8

Desperately Seeking Vub !!

An important measurement in shaping ρ-η Real estate

-1

0

1

-1 0 1 2

sin 2βBABAR

∆md

∆ms & ∆md

εK

εK

|Vub/Vcb|

ρ

η

CK Mf i t t e r

Constraint from Vub

Desperately Seeking Vub ! : By Exclusive Reconstruction

• Recent measurements from Babar, Belle, Cleo talk about this• SL decays missing ν need to measure 4-momentum in absence of ν

signature in detector very demanding and very important for Vub !– Exploit Hermiticity of detector (if you have few holes and have excellent

particle reconstruction capability• CLEO is best for this purpose (95% hermetic) , then Belle, then Babar

MB=

9

New results comparing data q2 with models

Wins the most improved Vubmeasurement award

Measurement statistics limited

10

Vub From Measurement of Exclusive Final States

Difficult to combine results from several experiments due to diff ranges of modeling, FF variations…dust needs to settle here (HFAG)

(4) |Vub| From Inclusive MeasurementsCKM Workshop 2003CKM Workshop 2003

Note : superficially all measurement look too consistent !

Probably because of (large) common Theory systematic error ?

11

Desperately Seeking Vub : From Inclusive Measurements

Luke@CKM ?vpµ

?vpµ

?vpµ

interpretation

The Perfect Detector for B Semileptonic Measurements Has No Holes ! (B Decay “bomb” goes off in all directions)

ϒ(4S)B1 B2

Need ASpherical Detector !

This was ananimation

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ϒ(4S) Detectors: Characteristic Features

Machine optics

Babar Not Hermetic Due to Intrusion of accelerator optics near Interaction Region (dipoles)

13

Belle is Perhaps Better but not like CLEO

Missing Momentum Resolution : Hermiticity Issues

85 MeV >> 160 MeV could be very costly in SL measurements

Need an alternate solution that pays in the long run

14

Beginning of an Experimental Paradigm Shift in VxbMeasurements at B Factories

• Hermiticity, so vital for SL measurements is not the best feature of Belle/ Babar detectors due to intrusion of machine into detector– Necessary holes !

• But B-factory detectors recording ever increasing samples of B-Bbar pairs (> 100 Million BBbar recorded already 1000 Million)– At the price of modest efficiency (4%), can fully

reconstruct one B decay into all hadronic final states (Breco) and examine the other(recoiling) B decay

• This “Recoil side” studies perfectly suited for many Vxb studies • Much “cleaner” and more powerful than neutrino reconstruction a

la CLEO • I will show you (with example) that this is the most promising way

for the future Vub and other Semileptonic measurements

The Perfect ϒ(4S) Event: Example of Recoil Side Analysis

In this (rare) case all particles were

sprayed within fiducialvolume of detector

Replace this with your Favourite Vxb mode

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Advantages in Recoil Side Measurements

• Full reconstruction of B1 “perfect” knowledge of B pµ

• Turn around and examine the recoiling B2 with this info– Pmissing knowledge much better than in “neutrino

reconstruction)– Most backgrounds in Vxb measurements “disappear”

• Udsc background (continnum)• Leptons from other B (b clnu, b c s l nu)• Combinatorial (1/2 event accounted for)

• The Y(4s) decay so much better understood, – Various charge correlation (D not a Dbar) allows

background rejection– Can fit entire event for event hypothesis in question

• Price to pay: Efficiency (but have ever growing data)

• Sometimes one can “have the cake and eat it too”

This reconstruction has been used by:• Measurement of the hadronic moments in SL

decays• B→ τ ν• Measurement of the SL BR (b0, b+) And it will be used by other analysis:• B→ s γ• B→ D∗ τ ν• Ratio of production N(B0)/N(B+)• B→ π0(+)lν, B→ ρ0(+)lν, B→ ω lν, ...• And many others

BaBar analyses using the Recoil

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Theoretically relatively “simple” in principle but– parton level calculation has to be

extended to account for hadronization effects and Fermi motion (b quark mass)

– calculation of decay rate relies on OPE for which the convergence depends on full acceptance and is impacted by non-perturbative effects

Exptal measurement challenging because • Rejection of large B→Xclν background

– (~60 times higher BR)• Extrapolation to full phase space introduces

theoretical uncertainties

ΣΣii Χui

lν

≡≡

lν

ub

dual

itydu

ality

Β

Example: Babar’s Inclusive b u l nu Measurement

Brecoil

ν

BrecoD*π Y(4S) l

Xu

Reco side Recoil side

•MX reconstruction•Kinematic constraints to improve MX resolution

Y(4s)

Xu

lν

BY(4s)

D*π

B missing mass

squared

B Candidate Mass

Recoil Side Study : Technique

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Require Lepton (p*>1.0GeV)

S/B~0.3

S/B~2.5

Fully Reconstructed B Sample

•Initial B Reco efficiency is 0.4%

•About 4000 B/fb-1

1300 B0

2700 B-

Analysis optimized to provide maximum Number of reconstructed B without consideration of “recoil side” physics

Use basic requirements of “recoil side “ Physics to clean up signal, e.g. additional

•Lepton•High energy photon

Purely non-resonantmodel based on theDe Fazio-Neubertmodel

Hybrid model: Babar MCResonant (PDG +ISWG2) + non-resonant

Integral of the hybrid model has to be compatible with the non-resonant one (dualityhypothesis)

Steps in Measurement: B -> Xu l nu Signal Generation

mX(GeV) mX(GeV) mX(GeV)

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• Brecoil selection and reconstruction of theX system B →Xu l ν:

– One and only one lepton with p*> 1 GeV/c – Correlation between lepton charge and Breco

flavor(B0 mixing is corrected)

– Cut on the missing mass: Mmiss2 < 0.5GeV2,

– charge conservation: Qtot=0– Partially reconstructed neutrino to reject B0

→D* l ν events– kinematic fit (2-C): improve hadronic mass

resolution

• Separate B→Xulν in signal enriched and depleted:– signal enriched: veto events with K± and KS

• used to perform the measurement

– signal depleted : one or more K± or KS in theevent

• used as control sample

• Systematic error in measurement reduced by measuring ratio

Ru/sl=B(B →Xu l ν )/B(B →X l ν )

generated

generated

reconstructed

reconstructed

Analysis Method: If it walks like a duck, talks like a duck …it is a duck !...(well, most of the time)

mX(GeV)

mX(GeV)

Reco. B:4 measured quantities

Lepton:3 measured quantities

Neutrino:3 unknown quantities

Breco RecoilPe-Pe+

Well known energy andmomentum of the incoming

Electron and positron

(EPEPII , PPEPII) well known!

Energy and Momentum ConservationEnergy and Momentum Conservation

Ebreco + EX + El + Eν - EPEPII = 0

Pbreco + PX + Pl + Pν - PPEPII = 0

⇒ 4 Constraints

+ equal mass constraint+ equal mass constraint

M(Breco)=M(X,l,ν)

⇒ 1 Constraint

5 Constraints – 3 unknown quantities =

Over constrained system (x 2)

Kinematic Fit To Entire ϒ(4S) BBbar Event

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Linear CorrelationUnbiased Mass Reconstruction

Mass Resolution ~ 300 MeV

MX Correlation: Generated Vs Reconstructed

S/B when only lepton required S/B after recoil side selection

Unbiased MX reconstruction and σ(MX)~300 MeV.

lepton requirementquality cutskinematic constraintKaon rejection

Rec

oil a

naly

sis

S/B ~ 0.05S/B ~ 1.7

S/B

S/B

Effect of Tight Recoil Side (Vub) Selection

mX(GeV) mX(GeV)

mX(GeV)mX(GeV)

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• Fit on the signal enhanced sample• Three components to fit the MX distribution: bb→→ululνν , bb→→clclνν , &

Hadronic backgroundbackground• Signal efficiency (εsel

u εMxu), Breco efficiency ratio (εt

u/εtsl) and lepton

efficiency ratio (εlu/εl

sl) from MC

Then multiply by B(B→Xlν) measured by BaBar

Extraction of B(b→ulν)

mX(GeV)

Mx cut optimized by minizing the total error:

Statistical+Branching ratio uncertainty+Other experimental systematics+Systematics from theory (mb & a)

→ Optimal point is 1.63 GeV→ Cut lowered to a safer 1.55 GeV cut

(negligible change in the total error)

MX (b u l nu) Selection & Background Rejection

statistical errortotal error

BR syst. errordetector syst. errortheory syst. error

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Resulting MX Spectrum

Fit to the MX distribution Background Subtracted spectrum

(MC)

0.0168 ± 0.0030B+ decays, MX < 1.55 GeV

0.0226 ± 0.0035Electron sample, MX < 1.55 GeV

0.0166 ± 0.0036Muon sample, MX < 1.55 GeV

0.0246 ± 0.0043B0 decays, MX < 1.55 GeV

0.0211 ± 0.0029All events, MX < 1.7 GeV

0.0177 ± 0.0025All events, MX < 1.4 GeV

0.0197 ± 0.0025All events, MX < 1.55 GeV

Ru/sl

Breakdown By Category & Stability In Event Selection

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• b quark is not at rest in the B meson (Fermi motion)

• Fermi motion depends on non-perturbative parameters (mb and a)

• Uncertainties on mb and a affect the shape of MX spectrum

• MX spectrum reweighted– (De Fazio et al JHEP 9906,017) taking into

account uncertainties on λ1 and Λ(from CLEO moments analysis PRL87:251808,2001)

Theoretical Uncertainty Due to MX Cut

(1 )

2

1

( ) (1 ) ; ( )

3 1

a a x

b B

f x N x e x f

m m

aλ

+= − = Λ

= −Λ Λ = − +

mX(GeV)

Theoretical Uncertainty on b u l nu rate

Two effects:• Systematic error due to efficiency

of the MX cut(MX <1.55 GeV) changes since the MX spectrum changes

• Systematic error due to selection efficiency (since the efficiency depends mostly on MX itself)

The combination of these two effects (of the same order of magnitude, ~9% each) gives:

σ (theory) = 17.5%

ε Mxu

mX(GeV)

mX(GeV)

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Statistical error (data+MC) 13.7%

Detector simulation errors+

Fit systematics 9.8%

b→clν and D decays modeling+

b→ulν decays modeling 6.0%

Fermi motion 17.5%

Total Systematic Uncertainty in Rate Measurement

Measure the charmless semileptonic branching ratio

And extract Vub

Interesting check of theory uncertainty:result very stable if apply a cut on the invariant mass of the lepton-neutrino system (q2)

(Bauer et at. hep-ph/0111387)

Ru/

sl

q2 (GeV2)

|Vub| Result : Preliminary

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Precision in this measurement alone is better than the LEP average

Inclusive |Vub| measurements

This Result on |Vub|

Future Prognostications (conservative)

Redoing the same analysis (no improvement) in 500fb-1 data , the errors should scaleas:

stat err. exp. syst theo. syst total

NOW 6.8% 6% 10.5% 13.5%

500fb-1 < 2.7% < 3% 10.5%?? 11.2%

Measurement will be dominated by theoretical uncertainty if nothing improves But… errors on mb and a should go down in future

Expect the total error can go well below 10% ?Expected systematic error due to shape function? [decrease it with info from Radiative Penguin measurements?]

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Future Direction with More Data: q2 vs MX analysis

A combination of cuts on q2 and MXreduces theoretical error (Bauer et al. hep-ph/0111387)

With 80fb-1 this 2D technique is notsuited (additional 40% efficiency due to

q2 cut)With 500 fb-1 can lead to better

precision

With a combination of MX <1.7GeVq2> 8.0 GeV2

Theory error < 9 % ?

• Try combination of variables. Ciuchini et al. ph/0204140

This approach (uses b→ulν and b→sγ) in not dependent on shape function. Since resonances in b→sγ have to be removed, efficiency will go down by >50%. In 500fb-1 σtheo(Vub) ~ 5% ??

• Can we measure mb directly on our data-sample?Kowalewski et al. (ex/0205038) claim one can, using With the current data-sample (80fb-1) σ(mb)~120MeV

⇒ in 500fb-1 σ(mb)~50MeV ⇒ σtheo(Vub) ~ 6% ??

⇒ Too aggressive expectation ?

Future: other possible checks & approaches (?)

| |< − >W WE p

26

Reconstructing Exclusive B0→ π- l+ ν , B+→ ρ0 l+ νOn the Recoil Side

Fitted MX

18

16

14

12

10

8

66

4

2

~500fb-1

Fitted MX

mX(GeV)mX(GeV)

(*)cuts not yet optimized

B0→π−lν(∗) B+→ρ0lν (∗)

Sensitivity With 500 fb-1 ?

B0→π−lν

B+→π0lν

B+→ρ0lν

B+→ωlν

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reconstructed500 fb-1

q2(GeV2)

q2 Spectrum: Distinguishing Between Models

q2 spectrum for B0→π+l-ν

models

In 500fb-1 enough statistics to discriminate among models ?

Conclusions

• Many improvements in – Exclusive & inclusive measurements of Vxb

– Interplay between theory and experiment crucial• Already shows nice synergy (precise results)

• Gathering more focus and attention at BaBar & Belle (now that Sin2beta is not the primary focus)

• Battle for precision Vub developing