Semileptonic B decays in BaBar Cristiano Borean University and I.N.F.N. of Trieste on behalf of the BaBar Collaboration CIPANP 2003, New York

Semileptonic B decays in BaBar

Cristiano Borean

University and I.N.F.N. of Trieste

on behalf of the BaBar Collaboration

CIPANP 2003, New York

May 22 2003 C. Borean CIPANP 2003 2

Semileptonic B decays

Semileptonic decays are relatively simple theoretically at the parton

level accessible experimentally sensitive to quark couplings to W±,

i.e. CKM elements |Vcb| and |Vub| probe the dynamics of decays probe impact of strong interactions

Two approaches Inclusive measurements:

need OPE and b-quark mass to extract |Vxb| from the total rate

Exclusive measurements: need form-factors to describe the transition of

the B meson to a lighter one


Inclusive Electrons with Electron Tags

Analysis based on small sample:4/fb on Y(4s), 1/fb off Y(4s)

Data Selection Electron tag: pcms >1.4

GeV/c Electron signal: pcms > 0.6

GeV/c Electron ID:

Combined likelihood based on: Electromagnetic Calorimeter

(E/p, shower shape) DIRC

(Cherenkov angle, # of photons) Driftchamber

(dE/dx) Two signal samples:

Right sign: + - Wrong sign: ++ or - -

e (92.5 1.3) %


Electron Tagged Sample: Background

Principal e Backgrounds Conversion/Dalitz pairs Hadron mis-ID Continuum background

Physics background Secondary e from other B:

b c e+

b - e- , b c cs e-

Secondary e from same B charmonium

B e+e-

B – B Mixing Secondary tag electrons

+ - pairs from same B

Cut: cos +p*e >1.0

cos >- 0.2

background

P*e=0.75 GeV/ccos >0.25


Right- and Wrong Sign Spectra

Mixing both spectra contain prompt and secondary e±

with oa = Efficiency of cut on opening angle :

00*00**

*)1()( f

dp

dNf

dp

dNp

dp

dN secondarypromptoa

signright

)1( 00*00**

fdp

dNf

dp

dN

dp

dN secondarypromptsignwrong

right-sign wrong-sign

Nprompt=25,070

(after correction for electron and tracking efficiency)

right

wrong


Fully Corrected Electron Spectrum

Corrections of prompt spectrum: external bremsstrahlung

2.20 ± 0.35% geom. acceptance

84% single/di-lepton eff. 98%

Total electron signal25,070 ± 413

Total electron tags:304,000 ± 880 ± 2100

Fit to sum of spectra from: B D e , D* e ,

D** e , D(*) e

BABAR

Extrapolation to p = 0: correction: 6.1± 0.9%


Semileptonic Branching Ratio and |Vcb|

Integration over spectrum & extrapolation to pl*=0 :

ARGUS : (9.7 0.5 0.4)% OPAL: (10.78 0.08 0.5)%

CLEO2: (10.49 0.17 0.43)% L3: (10.89 0.20 0.51)%

BELLE : (10.90 0.12 0.49)% ALEPH: (10.70 0.10 0.35)%

Relation to |Vcb| (A. Hoang, Z. Ligeti, A.V.Monahar) [1]

:

WithB=(1.6010.022)ps, and BR(BXu e ) = (1.70.5) 10-3

N.B. the theoretical errors have been added linearly

BR(BX e ) = (10.87 0.18stat 0.30sys)%BR(BX e ) = (10.87 0.18stat 0.30sys)%

|Vcb| = 0.0423 0.0017exp 0.0020theory

|Vcb| = 0.0423 0.0017exp 0.0020theory [1]

Phys. Rev. Lett. 82 (1999) 591.

)012.0017.0019.00.1(6.1

105.0

(0419.0V

1cb bmpertB

c psXB

B


Exclusive mode: B0->D*l Branching Fraction

e/

D0s

B

Reconstruction of a charged D* meson and a lepton (either electron or muon) of opposite charge. Neutrino is NOT reconstructed.

D0 is reconstructed in 4 decay modes

Constrained vertex fit on D0 decay products, soft pion and leptonWhen possible study background from data control

samples Used (85.9+/-0.9) x 106 Y(4s) decays


Classification of backgrounds

D*lnuX bkg


Background Characterization: fit to M [= M(D*)-M(D0)]

fake D*fake lepton

continuum uncorrelated

from data control samples


Fit to the D*lX background

B0 ->D*lX events

All shapes are extracted from MC

e

D0Kchannel

Once other backgrounds are

estimated from the global fit,

B0(+) D*Xl events are separated using the angle between the B0 and the D*l direction.

Can be written as:

D** shape different from the D* onefitted in 4 D0 modes and

e/m subsamples


Results and comparisons

fit: 2/d.o.f. = 21.3/23

BR(BB00->D*->D*--++)=()=(4.82±0.03stat±0.02statMC±0.24exp±0.17BR)%

preliminary

Measurement limited by systematic uncertaintiesMain contributions to the experimental systematic error come from reconstruction efficiency and from the poor knowledge of the BD*Xlnu background composition.

Correlated measurements

prel.

BRaver=5.27 ± 0.29%C.L.= 6.4%

Results rescaled to common inputs (f+/f0).


Exclusive B0 -> e Analysis Used 50.5 1/fb on-resonance and 8.7 1/fb 40 MeV below

Y(4s) resonance Study 5 modes B -> Hue where Hu

= 0, ,,0, in 2 lepton energy ranges: LOLEP : 2.0 < Ee < 2.3 GeV (large b->c e backgrounds) HILEP : 2.3 < Ee < 2.7 GeV (large continuum backgrounds)

Binned Likelihood fit with 2 variables: E=Ehad + Elept + |p| - Ebeam , where |p| ~ |pmiss|, and Mhad = ()

mass Take signal and backgrounds shapes from MC simulations,

and continuum shape from off-peak data Extrapolate partial branching fraction to entire lepton-

energy spectrum using 5 different form-factor calculations Determine |Vub| for each form-factor:


Fit projections for B0-> - e+

Continuum subtracted

Isospin-constrained average of and 0

2 = 91 (93 d.o.f.)


Branching Fraction and |Vub| results

Theoretical error on the combined result: half of the full spread seen between the different form-factors

Combined result: weighted average of 5 form-factor results. Theoretical error on combined result: half of the full spread seen between the FFs (including theoretical errors)


|Vub| from Inclusive Semil. Decays with Recoil Techinque

Brecoil

BrecoD* Y(4S) l

Xu

Reco side

Y(4s)

D*

Recoil side

Y(4s)

Xu

•Subtract combinatorial bkg•Breco (recoil) kinematics know with small uncertainties•Breco (recoil) flavor •Breco (recoil) charge (B0- B+ separation)•~10-3 efficiency: “small” efficiency

l

•MX reconstruction•Lepton charge – Breco (recoil) flavor correlation•Kinematic constraints to improve MX resolution•Partially reconstructed missing mass (soft pion)

B B

missing mass

squaredenergy substituted mass


Analysis Method

generated

generated

reconstructed

reconstructed

Fully reconstruct one B

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 on K± and KS

used to perform the measurement

signal depleted : one or more K± or KS in the event

used as control sample

Systematics reduced by measuring Ru/sl=B(B Xu l )/B(B X l )


Extraction of B(b->ul) Fit on the signal enhanced sample Three components to fit the MX distribution: bbulul, bbclcl,

backgroundbackground Signal efficiency (sel

u Mxu), Breco efficiency ratio (t

u/tsl) and

lepton efficiency ratio (lu/l

slfrom MCThen multiply by B(BXl) measured by BaBar


Result (82 fb-1 at the Y(4s) mass )

XM XM


|Vub| result

(using )

We measured the charmless semileptonic branching ratio

and we extracted

|Vub|(*10-3)

BaBar Excl. B e

3.29 0.42stat 0.47syst

0.60theo

BaBar Incl. electron spectrum endpoint**

4.43 0.29stat 0.25exp 0.50fu 035s

BaBar Incl. Mx spectrum on the recoil

4.52 0.31exp 0.27syst 0.40theo

0.24pert 0.091/mb3

Inclusive |Vub| measurements

(**) see hep-ex/0207081


Summary

Result presented on Br(B->Xe) and |Vcb| with inclusive technique Physical Review D - Rapid Communications Year

2003 Volumn 67 Article/Page 031101 New measurement of Br(B0->D*l)

Exclusive |Vcb| mesurement with HQET zero-recoil is underway

Exclusive |Vub| from B0-> e :

submitted to PRL New Inclusive |Vub| measurement:

submitted to PRL


Backup Slides


The BaBar detector

Cerenkov Detector (DIRC)

144 quartz bars11000 PMTs

1.5 T Solenoid Electromagnetic Calorimeter

6580 CsI(Tl) crystals

Drift Chamber40 stereo layers

Instrumented Flux Returniron / RPCs (muon / neutral hadrons)

Silicon Vertex Tracker5 layers, double sided strips

e+ (3.1 GeV)

e- (9 GeV)

SVT: 97% efficiency, 15 m z hit resolution (inner layers, perp. tracks)

SVT+DCH: (pT)/pT = 0.13 % pT + 0.45 %

DIRC: K- separation 4.2 @ 3.0 GeV/c >3.0 @ 4.0 GeV/c EMC: E/E = 2.3 %E-1/4 1.9 %


Inclusive |Vcb|: systematics


B0->D*l Branching Fraction: Systematics


Br(B0->-e+): systematics


Inclusive |Vub|: systematics

Statistical error (data+MC) 13.7%

Detector simulation errors+

Fit systematics 9.8%

bcl and D decays modeling+

bul decays modeling 6.0%

Fermi motion 17.5%

Documents

Semileptonic B decays in BaBar Cristiano Borean University and I.N.F.N. of Trieste on behalf of the BaBar Collaboration CIPANP 2003, New York