semileptonic b decays in babar cristiano borean university and i.n.f.n. of trieste on behalf of the...
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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
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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
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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) %
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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
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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
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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%
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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
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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
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May 22 2003 C. Borean CIPANP 2003 9
Classification of backgrounds
D*lnuX bkg
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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
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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
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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).
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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:
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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.)
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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)
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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
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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 )
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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
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May 22 2003 C. Borean CIPANP 2003 19
Result (82 fb-1 at the Y(4s) mass )
XM XM
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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
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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
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May 22 2003 C. Borean CIPANP 2003 22
Backup Slides
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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 %
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May 22 2003 C. Borean CIPANP 2003 24
Inclusive |Vcb|: systematics
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May 22 2003 C. Borean CIPANP 2003 25
B0->D*l Branching Fraction: Systematics
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May 22 2003 C. Borean CIPANP 2003 26
Br(B0->-e+): systematics
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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%