Selected Results and Prognostications on V cb & V ub : A B Factory Perspective
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Transcript of Selected Results and Prognostications on V cb & V ub : A B Factory Perspective
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Selected Results and Prognostications on Vcb & Vub :
A B Factory Perspective
Vivek SharmaUniversity of California San Diego
Ringberg Phenomenology Workshop on Heavy Flavors : Rottach-Egern, Germany
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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 |
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
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Inclusive Semileptonic Decay Rate
Babar (also CLEO)
Basic foundation of all semileptonic studies. Modern measurements agree
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• 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
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CLEO
A complete set of measurements from CLEO:
3| | [46.9 1.4 2.0 1.8 ] 10cb stat syst thV
~6.5% Measurement
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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”
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|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
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Seeking Consistency
?
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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
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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 !
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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 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
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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
<MX
2 -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)
?
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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 ?
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Desperately Seeking Vub !!
An important measurement in shaping - Real estate
Constraint from Vub
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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=
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New results comparing data q2 with models
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Wins the most improved Vubmeasurement award
Measurement statistics limited
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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)
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(4) |Vub| From Inclusive MeasurementsCKM Workshop 2003CKM Workshop 2003
Note : superficially all measurement look too consistent !
Probably because of (large) common Theory systematic error ?
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Desperately Seeking Vub : From Inclusive Measurements
Luke@CKM ?vp
?vp
?vp
interpretation
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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
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(4S) Detectors: Characteristic Features
Machine optics
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Babar Not Hermetic Due to Intrusion of accelerator optics near Interaction Region (dipoles)
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Belle is Perhaps Better but not like CLEO
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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
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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
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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
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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”
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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 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
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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
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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
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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)
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• 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)
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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 + 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
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Linear CorrelationUnbiased Mass Reconstruction
Mass Resolution ~ 300 MeV
MX Correlation: Generated Vs Reconstructed
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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
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: 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)
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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. errortheory syst. error
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Resulting MX Spectrum
Fit to the MX distribution Background Subtracted spectrum
(MC)
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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
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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 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)
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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)
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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
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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
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Precision in this measurement alone is better than the LEP average
Inclusive |Vub| measurements
This Result on |Vub|
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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?]
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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 % ?
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• 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
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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
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Sensitivity With 500 fb-1 ?
B0l
B+l
B+l
B+l
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reconstructed500 fb-1
q2(GeV2)
q2 Spectrum: Distinguishing Between Models
q2 spectrum for B0l-
models
In 500fb-1 enough statistics to discriminate among models ?
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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
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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
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Data-MC Comparisons
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Vub inclusive (CKM workshop)
Recoil of B D* l in Belle
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Sample Purity : Flexible
10 20 30 30 40 50 60yield (*103)
S/sqrt(S+B)purity
Yield10 20 30 30 40 50 60
yield (*103)
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(4S) Detectors: Belle
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Belle
Two ’s from J/
Two ’s from KS We reconstruct B
mesons from detector hit signals
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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
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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 !
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B1 B2
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Constraint from Vub
SM and Unitary Triangle
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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
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Vub From Measurement of B / l
B B
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Wins the most improved Vub measurement award
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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
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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
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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