Post on 17-Jan-2016
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
May 22 2003 C. Borean CIPANP 2003 3
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) %
May 22 2003 C. Borean CIPANP 2003 4
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
May 22 2003 C. Borean CIPANP 2003 5
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
May 22 2003 C. Borean CIPANP 2003 6
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%
May 22 2003 C. Borean CIPANP 2003 7
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
May 22 2003 C. Borean CIPANP 2003 8
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
May 22 2003 C. Borean CIPANP 2003 9
Classification of backgrounds
D*lnuX bkg
May 22 2003 C. Borean CIPANP 2003 10
Background Characterization: fit to M [= M(D*)-M(D0)]
fake D*fake lepton
continuum uncorrelated
from data control samples
May 22 2003 C. Borean CIPANP 2003 11
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
May 22 2003 C. Borean CIPANP 2003 12
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).
May 22 2003 C. Borean CIPANP 2003 13
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:
May 22 2003 C. Borean CIPANP 2003 14
Fit projections for B0-> - e+
Continuum subtracted
Isospin-constrained average of and 0
2 = 91 (93 d.o.f.)
May 22 2003 C. Borean CIPANP 2003 15
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)
May 22 2003 C. Borean CIPANP 2003 16
|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
May 22 2003 C. Borean CIPANP 2003 17
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 )
May 22 2003 C. Borean CIPANP 2003 18
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
May 22 2003 C. Borean CIPANP 2003 19
Result (82 fb-1 at the Y(4s) mass )
XM XM
May 22 2003 C. Borean CIPANP 2003 20
|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
May 22 2003 C. Borean CIPANP 2003 21
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
May 22 2003 C. Borean CIPANP 2003 22
Backup Slides
May 22 2003 C. Borean CIPANP 2003 23
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 %
May 22 2003 C. Borean CIPANP 2003 24
Inclusive |Vcb|: systematics
May 22 2003 C. Borean CIPANP 2003 25
B0->D*l Branching Fraction: Systematics
May 22 2003 C. Borean CIPANP 2003 26
Br(B0->-e+): systematics
May 22 2003 C. Borean CIPANP 2003 27
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%