Selected Results and Prognostications on Vcb & Vub :

A B Factory Perspective

Vivek Sharma

University of California San Diego

vsharma@ucsd.edu

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

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 | measureme

V |

nt

|

V |

s

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

• Experimentally favored (S/N) w.r.t BDl 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 measurement)

*( ,0)cb(1) and |V |B D

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

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

Seeking Consistency

?

Photon energy spectrum in B Xs

Hadronic mass spectrum in BXc

Lepton energy spectrum in BXc

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

Lepton energy spectrum in BXc

Hadronic mass spectrum in BXc

Photon energy spectrum in B Xs

1 Theoretical

Ellipse

Cleo Moment Analyses: Consistent picture

= 0.39+0.03stat+0.06sys+0.12thGeV

1=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

M

3% measure

Remarkable !

Comparing Exclusive & Inclusive Vcb Measurements

*

Averaging many measurements is when the measurements

are done in different environments/prescriptions

I prefer to look for consistency within ea

CLEO Exclusive :

ch experiment

| | [46.9 1

B

t

.

D

ricky

4cbV

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

ts

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

For pFor p**minmin=1.5 GeV/c and=1.5 GeV/c and

=0.35 ± 0.13 GeV [1]

(reliance on b s spectrum)

1= - 0.17 ± 0.06 ±0.07 GeV2

CLEO [1]

1= - 0.226 ± 0.07 0.08 GeV2

Babar Hadronic Moment Analysis: ICHEP Preliminary

<M

X2 -

MD

spin

2 >

p*min [GeV/c]

OPE (Falk,Luke) = 0.35 GeV

But these parameters do not describe P* dependence of the moments!(0.9 GeV/c) – (1.5 GeV/c)

= 0.22±0.04±0.05 GeV2

OPE (Falk, Luke), 1 free param.

NB: Data points highly NB: Data points highly correlatedcorrelated

BABAR Preliminary

CLEO

[1] CLEO PRL 87, 251808 (2001)

No non-resonant states (MC)

?

Can Hadronic B Decays Help Understand Nature of High Mass Dn states in

SL Decay?

B D** B D** l nu

D**D**

factorization ?

D+ D*+

Belle

Non-reso

What can oneLearn from such

results ?

Desperately Seeking Vub !!

An important measurement in shaping - Real estate

Constraint from Vub

Desperately Seeking Vub ! : By Exclusive Reconstruction

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

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=

New results comparing data q2 with models

Wins the most improved Vubmeasurement award

Measurement statistics limited

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 Measurements

CKM Workshop 2003CKM Workshop 2003

Note : superficially all measurement look too consistent !

Probably because of (large) common Theory systematic error ?

Desperately Seeking Vub : From Inclusive Measurements

Luke@CKM ?vp

?vp

?vp

interpretation

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

(4S)B1 B2

Need ASpherical Detector !

This was ananimation

(4S) Detectors: Characteristic Features

Machine optics

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

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

Beginning of an Experimental Paradigm Shift in Vxb

Measurements 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 (4S) Event: Example of Recoil Side Analysis

In this (rare) case all particles were

sprayed within fiducial volume of detector

Replace this with your Favourite Vxb mode

Advantages in Recoil Side Measurements

• Full reconstruction of B1 “perfect” knowledge of B p• 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 (bclnu, b cs 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”

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 BXclbackground

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

theoretical uncertainties

ii ui

l

l

ub

dual

itydu

ality

Example: Babar’s Inclusive bu l nu Measurement

Brecoil

BrecoD* Y(4S) l

Xu

Reco side Recoil side

•MX reconstruction•Kinematic constraints to improve MX resolution

Y(4s)

Xu

l

BY(4s)

D*

B missing mass

squared

B Candidate Mass

Recoil Side Study : Technique

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-Neubert model

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)

• Brecoil selection and reconstruction of the

X system B Xu l – 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 events– kinematic fit (2-C): improve hadronic mass

resolution

• Separate BXul 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 the

event• used as control sample

• Systematic error in measurement reduced by

measuring ratio

Ru/sl=B(B Xu l )/B(B X l )

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 Recoil

Pe-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 + E - EPEPII = 0

Pbreco + PX + Pl + P - PPEPII = 0

4 Constraints

+ equal mass constraint+ equal mass constraint

M(Breco)=M(X,l,)

1 Constraint

5 Constraints – 3 unknown quantities =

Over constrained system (x 2)

Kinematic Fit To Entire (4S) BBbar Event

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 MX~300 MeV.

lepton requirementquality cutskinematic constraintKaon rejection

Rec

oil a

nal

ysis

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)

• Fit on the signal enhanced sample• Three components to fit the MX distribution: bbulul, bbclcl, & Hadronic

backgroundbackground• Signal efficiency (sel

u Mxu), Breco efficiency ratio (t

u/tsl) and lepton

efficiency ratio (lu/l

slfrom MC

Then multiply by B(BXl) measured by BaBar

Extraction of B(bul

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 (bu l nu) Selection & Background Rejection

statistical errortotal error

BR syst. errordetector syst. error

theory syst. error

Resulting MX Spectrum

Fit to the MX distribution Background Subtracted spectrum

(MC)

Ru/sl

All events, MX < 1.55 GeV 0.0197 0.0025

All events, MX < 1.4 GeV 0.0177 0.0025

All events, MX < 1.7 GeV 0.0211 0.0029

B0 decays, MX < 1.55 GeV 0.0246 0.0043

B+ decays, MX < 1.55 GeV 0.0168 0.0030

Electron sample, MX < 1.55 GeV 0.0226 0.0035

Muon sample, MX < 1.55 GeV 0.0166 0.0036

Breakdown By Category & Stability In Event Selection

• 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 and (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

a

mX(GeV)

Theoretical Uncertainty on bu 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:

(theory) = 17.5% M

xu

mX(GeV)

mX(GeV)

Statistical error (data+MC) 13.7%

Detector simulation errors+

Fit systematics 9.8%

bcland D decays modeling+

buldecays 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/

slq2 (GeV2)

|Vub| Result : Preliminary

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 scale as:

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?]

Future Direction with More Data: q2 vs MX analysis

A combination of cuts on q2 and MX reduces 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 buland bs in not dependent on shape function. Since resonances in bs have to be removed, efficiency will go down by >50%. In 500fb-1 theo(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) (mb)~120MeV

in 500fb-1 (mb)~50MeV theo(Vub) ~ 6% ??

Too aggressive expectation ?

Future: other possible checks & approaches (?)

| | W WE p

Reconstructing Exclusive B0 - l+ , B+ 0 l+ On 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

B0l B+l

Sensitivity With 500 fb-1 ?

B0l

B+l

B+l

B+l

reconstructed500 fb-1

q2(GeV2)

q2 Spectrum: Distinguishing Between Models

q2 spectrum for B0l-

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

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

decays• B • Measurement of the SL BR (b0, b+) And it will be used by other analysis:• B s • B D• Ratio of production N(B0)/N(B+)• B lB lB l• And many others

BaBar analyses using the Recoil

Data-MC Comparisons

Vub inclusive (CKM workshop)

Recoil of B D* l in Belle

Sample Purity : Flexible

10 20 30 30 40 50 60

yield (*103)

S/sqrt(S+B)purity

Yield10 20 30 30 40 50 60

yield (*103)

(4S) Detectors: Belle

Belle

Two ’s from J/

Two ’s from KS We reconstruct B

mesons from detector hit signals

B Factory Detectors: Babar & Belle

• Excellent tracking and calorimetry E > 60 MeV

• Excellent charged Kaon identification

• Excellent KS + - identification

• Due to encroachment of accelerator optics near interaction point

– Holes in the front and back • Hermiticity limited (CLEO much better)• B mesons decay at rest remnants thrown in ALL directions

– Not collimated as at LEP/SLD

Exclusive Decay Rate Measurements

-

-

Errors : Stat, syst, FormFactorTheory error = 50% of entire spread

Experiments have to agree to make Similar variation in their analysis so that their results may be compared.

Need a better concept of “theory” error !

B1 B2

Constraint from Vub

SM and Unitary Triangle

Aim is to collect as many as possible fully reconstructed Bs in order to study the property of the recoil.

• Reconstruct B D(*) n mK pKs q0 but the intermediate resonances are not requested

• This is the so-called SemiExclusive Reconstruction. For instance in B D* if you don’t request the invariant mass in the a1 window you do SemiExclusive reconstruction

SemiExclusive Reconstruction

Vub From Measurement of B / l

B B

Wins the most improved Vub measurement award

Two steps:• Reconstruction of the D meson in hadrons• Reconstruction of the B meson in hadrons

the signal box is defined using two variables:

Uncorrelated variables (just the beam energy but small uncertainty)

Resolution from beam energy

Sensitive to E measurement

SemiExclusive Reconstruction II

Recoil Side Physics : Targeting Exclusive bu l nu modes

BlBlBlBal

• 170 BXul events on data after all cuts for MX<1.55GeV• exclusive channels can be studied with ~same technique• large potential to perform exclusive Vub analyses

The Story So Far & Plan of This Talk

• Semileptonic B decays have been studied since discovery of (4S)

– This audience knows it all ! – See recent CKM workshop page for complete results

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

• In this talk

– Give a short review of current status– What B factory detectors can (not) do – Pick a few interesting new results and project them in

future (generate some discussion)• Future is 2007 or ~500 fb-1 at Babar/Belle each

• Warning : I am an interested observer in this activity, not an active participant + my optic is primarily (4S) based