Post on 03-Feb-2022
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Semileptonic B Decays at BABAR
Vladimir GolubevBudker Institute of Nuclear Physics,
Novosibirsk, Russia
(for the BaBar Collaboration)
Beauty 2005Assisi (Perugia), June 20–24, 2005
V.Golubev - BABAR Beauty2005, Assisi 2
Semileptonic B Decays – Why are they Interesting
Semileptonic decaystheoretically simple at parton level
rate depends on CKM elements |Vcb| and|Vub|, the quark masses mb and mc
the leptonic current factors out cleanly,
thus one can probe strong interactions in B mesons
sensitive to QCD corrections, OPE
A precise determination of |Vcb| and |Vub| with reliable errors
important for
understanding B decay rates
testing of the unitarity of the CKM matrix and predictions of CP violation in B mesons
b
+
uc,
−W
ν
−l
3222
2
)(||192
)( cbbcbF mmmVGcb −=→Γπ
νl 522
2
||192
)( bubF mVGubπ
ν =→Γ l
ubcb VV ,
V.Golubev - BABAR Beauty2005, Assisi 3
Semileptonic B decay studies at BABAR
Inclusive decays Exclusive decay modes
b → c l ν B → Xc l ν|Vcb|, total decay rate, HQE parameters
B → D* l ν, B → D l ν,...Branching fractions, decay form factors, |Vcb|
b → u l ν B → Xu l ν|Vub|, total decay rate, lepton momentum and hadron mass spectra
B → π l ν, B → ρ l ν,...Branching fractions, decay form factors, |Vub|
2-nd B tagging: untagged, lepton tag, hadronic tag
V.Golubev - BABAR Beauty2005, Assisi 4
Inclusive B → Xc lν decays: HQE ExpansionsHeavy Quark Expansions, tool to correct for QCD effects
Expansion in terms of 1/mb and αs(mb)Separate short- and long-distance effects at µ ~ 1 GeV
Perturbative corrections calculable from mb mc αs(mb)Non-perturbative parameters cannot be calculated
We choose calculation by Gambino & Uraltsev hep-ph/0401063 & 0403166
Kinetic mass scheme to Eℓ moments mX moments
8 parameters to be fitted
Measure 8 moments, each as a function of minimum lepton energy El
cbV bm cm 2πµ 2
Gµ 3Dρ 3
LSρ( )cB X ν→B l
kinetic
chromomagnetic
Darwin
spin-orbit
2(1/ )bmO
3(1/ )bmO
2( )sαO( )sαO
3(1/ )bmO
V.Golubev - BABAR Beauty2005, Assisi 5
Measurement of Electron Energy Moments
Inclusive e± spectrum in 3x106 electron-tagged (1.4<p*<2.3GeV/c) BB events
Corrected for detector effectsCorrected for non-prompt electrons – lepton charge correlation and MCCorrected for B0 – B0 mixing
Moments for Ecut = 0.6 … 1.5 GeVCorrected for the final state radiationTranslated to B rest frameSubtracted B → Xuℓν decays
0B
dM
Γ=
Γ∫l
BABAR
Partial BF
Lepton Energy Moments
47 + 9 /fb
156,000 e±
Albert et al. Phys.Rev. D 69, 111104 (2004)
V.Golubev - BABAR Beauty2005, Assisi 6
Measurement of Hadron Mass Moments
2/350)( cMeVM x ≈σ
high mass charm states
Mx2[GeV2/c4]
BB events tagged by a fully reconstructed hadronic B decays
e± or µ± with El > Ecut (0.9…1.6 GeV/c)
Lepton charge – B flavor correlationImprove mx measurement by kinematic fit to whole event, resolution ~350 MeV
To eliminate dependence of moments on uncertain BF and unknown masses of high mass charm mesons we calibrate mx measurementCalibrate mx based on MC simulation
Linear relation between mxmeas and mX
true
Validate calibration with excl. B→ D(*) l ν
Moments corrected for detector effects and bkg
BABAR
Hadron mass moments
7114 Events
D, D*
mxm
eas(G
ev)
mXtrue (GeV)
Even
ts/0
.5 G
eV2
mX2 (GeV2)
Aubert et al. Phys.Rev. D 69, 111103 (2004)
V.Golubev - BABAR Beauty2005, Assisi 7
HQE Fits to Hadron Mass and Lepton Energy Moments
MX moments
Eℓ moments
● = used, ○ = unusedin the nominal fit
Red line: HQE fitYellow band: theory errors
BABAR
χ 2/ndf = 20/15
M1x M2
x M3x M4
x
M0l M1
l M2l M3
l
Aubert et al. Phys.Rev.Lett. 93, 011803 (2004)
V.Golubev - BABAR Beauty2005, Assisi 8
HQE Fit Results (kinetic mass scheme, scale µ=1GeV)
s
s
s
3exp HQE th
exp HQE
exp HQE
exp HQE
2 2exp HQE
2exp HQE
(41.4 0.4 0.4 0.6 ) 10
(10.61 0.16 0.06 )%
(4.61 0.05 0.04 0.02 )GeV
(1.18 0.07 0.06 0.02 )GeV
(0.45 0.04 0.04 0.01 )GeV
(0.27 0.06 0.03 0.0
cb
c
b
c
G
V
m
m
ν
α
α
π αµ
µ
−= ± ± ± ×
= ± ±
= ± ± ±
= ± ± ±
= ± ± ±
= ± ± ±
B l
s
s
s
2
3 3exp HQE
3 3exp HQE
2 )GeV
(0.20 0.02 0.02 0.00 )GeV
( 0.09 0.04 0.07 0.01 )GeVD
LS
α
α
α
ρ
ρ
= ± ± ±
= − ± ± ±
BABAR
Separate fits to hadron and lepton moments give consistent resultsµG2 and ρLS
3 are consistent with B-B* mass splitting and HQ sum rules
Considerable improvement in precision for |Vcb| (±2%) and Bclν (1.6%)and quark masses, as well as HQE parameters
V.Golubev - BABAR Beauty2005, Assisi 9
|Vub| from total B→ Xu l ν decay rate
Total b → u ℓ ν decay rate from OPE:
|Vub| from inclusive BF(B → Xu l ν) (theory error ∼5%):
Experimentally challenging due to charmed semileptonic background:
2
2( ) 1( ) 50
ub
cb
Vb ub c V
νν
Γ →≈ ≈
Γ →l
l
V.Golubev - BABAR Beauty2005, Assisi 10
|Vub| from partial B→ Xu l ν decay rates
Charmed semileptonic background can be controlledin limited regions of phase space
Non-perturbative Shape Functions are used to extract |Vub| from partial branching fractions. The SF parameters are extracted from b → sg photon spectrum
New approach to |Vub| extraction:
• Differential decay rate parametrization from BLNPBosch, Lange, Neubert, and Paz, Nucl. Phys. B 699, 335 (2004);Lange, Neubert, and Paz, hep-ph/0504071
• New source of information on SF parameters – moments of theb → c ℓ ν lepton spectrum Neubert, Phys.Lett. B 612, 13 (2005)
V.Golubev - BABAR Beauty2005, Assisi 11
|Vub| from partial B→ Xu l ν decay rates
Different types of partial decay rate measurements:- electron endpoint
- Ee and q2 regions with neutrino reconstruction
- Mx and q2 regions
hep-ex/0408075
hep-ex/0506036
hep-ex/0408068
q2(Gev/c)2 vs. Mx(GeV)q2(Gev/c)2 vs. Ee(GeV)
Kinematic regionsof B → Xclν decays
q2=(pl+pν)2
V.Golubev - BABAR Beauty2005, Assisi 12
|Vub| from B→ Xu l ν endpoint spectrum
0
0.025
0.05
0.075
0.1
2 2.1 2.2 2.3 2.4 2.5 2.6 2.7
B A B A R
preliminary
Electron Momentum (GeV/c)
∆B(1
0-3)
/ (50
MeV
/c)
10 5
10 3
B A B A R
preliminary
(a)
10
10 3
10 5 (b)
0
5000
1.1 1.5 1.9 2.3 2.7 3.1 3.5
(c)
Electron Momentum (GeV/c)
N
umbe
r of
Ele
ctro
ns /
(50
MeV
/c)
(On-peak)-(Off-peak) data
Histogram – B → Xulν MC
On-peak dataOff-peak data
(On-peak)-(Off-peak) data
Histogram – B → Xclν MC
Differential Β→ Xu e ν branching fraction as a function of electron
momentum, corrected for detector effects and final-state radiation
(On-peak)−(Off-peak) dataHistogram – B → Xulν MCSignal extraction: subtraction of BB
and continuum background
V.Golubev - BABAR Beauty2005, Assisi 13
Endpoint spectrum analysis: |Vub| extraction
Determination of ∆Br for pe>pmin
Extrapolation to total charmless semileptonic decay Br using shape function parameters from B → Xsγ spectrum and calculations of DeFazio-Neubert(*) and Kagan-Neubert(**)
Extraction of |Vub| using the OPE relation.BABAR preliminary results, based on BELLE shape function parameters and DFN approach (hep-ex/0408075):∆Br(pe>2.0GeV/c)=(0.531±0.032stat±0.049syst)×10-3
Br=(2.16 ±0.24exp ±0.27SF)×10-3
|Vub|=(4.40 ±0.24exp ±0.28SF ±0.21OPE+mb)×10−3
Analysis based on BLNP calculation is being completed
(*) F.De Fazio and M.Neubert, JHEP 9906, 017 (1999)(**) A.L.Kagan and M.Neubert, Eur.Phys.J. C 7, 5 (1999)
V.Golubev - BABAR Beauty2005, Assisi 14
q2 − Ee analysis with neutrino reconstruction
Event selection:High energy electron Ee>2GeVMissing momentum used for neutrino parameters estimationCuts on missing momentum magnitude and direction, and event shape
Suppression of b→ceν by shmax=m2
D:
(slightly modified for B -meson motion in the Υ(4S) rest frame)
),4
(22
22max
eeBBh E
qEmqms +−+=
Acceptance ~ 14%, signal/bkg.~0.6
V.Golubev - BABAR Beauty2005, Assisi 15
BABARpreliminaryq2 − Ee analysis results
Test of neutrino energy resolution using pure control sample of B→D(*)eν events
Signal extraction for Ee>2.0 GeV and sh<3.5 GeV2,BB background normalization for sh>4.25 GeV2,
∆Br(Ee>2.0GeV, sh<3.5GeV2)=(3.54±0.33 ±0.34) ×10-4
ps023.0604.1 , rate partial normalized- ,)/(|| (*) ±=∆×∆∆= BBub BV τζτζ
(*) B.O.Lange, M.Neubert and G.Paz, hep-ph/0504071
358.042.0 10)25.019.018.095.3(|| −+
− ×±±±=ubV
V.Golubev - BABAR Beauty2005, Assisi 16
Mx−q2 analysis
Select events with a fully reconstructed B → D(*)X decay (Breco)
Study charmless semileptonic decay of B recoiling against Breco
- require lepton with p*lep > 1 GeV
2D fit to measure partial BFin [Mx<1.7 GeV, q2>8 GeV2]
Signaldownfeed
b→clν + other
Mx < 1.7 GeV 1.7 < Mx < 2.2 GeV 2.2 < Mx < 2.8 GeV
82 fb-1
hep-ex/0408068
V.Golubev - BABAR Beauty2005, Assisi 17
BABARpreliminary
(CKM 2005)
Mx−q2 analysis: the results
Mx spectrum unfolding and fitting:
Mx−q2 2-dimentional fit:
)(DFN 10)25.021.031.030.022.5(|| *3OPEfuexp.syststat
−×±±±±=ubV
4syststat
22 10)44.143.196.8()GeV8 GeV,7.1( −×±±=><∆ qMBr X
)(BLL 10)47.039.040.098.4(||
)(DFN 10)25.022.040.041.018.5(||**3
OPEfuexp.syststat
*3OPEfuexp.syststat
−+
−
×±±±=
×±±±±=
ub
ub
V
V
(*) F.De Fazio and M.Neubert, JHEP 9906, 017 (1999)(**) Bauer, Ligeti and Luke, hep-ph/0111387
V.Golubev - BABAR Beauty2005, Assisi 18
BABARpreliminaryInclusive B→ Xu l ν decay summary
Technique |Vub|×103
El>2GeV(ICHEP2004)
El vs. q2
(NEW)
Mx vs. q2
(CKM 2005)
Average(using the procedure of HFAG)
OPEfuexp.syststat 21.028.025.013.040.4 ±±±±
theoSF58.042.0exp.syststat 25.0)(19.018.095.3 ±+±± +
−
OPEfuexp.syststat 25.022.040.041.018.5 ±±±±
45.0CL 57.022.4 =±
V.Golubev - BABAR Beauty2005, Assisi 19
|Vub| from exclusive B→ Xu l ν decays
Strong nonperturbative effects included in form factors
HPQCD’04
Ball’04
FNAL’04
ISGW2
q2 = (plep + pν )2
22233
2
|)(||48
dqqfGx F+∫=
ππk|
Bub x
BBVτ
νπ )(|| l→=
22233
2
2 |)(|||||24
)( qfVGdq
Bdub
F+=
→Γππ
νπ kl
CalculationsLight-Cone Sum Rules: Ball,Zwicky (hep-ph/0406232)Unquenched Lattice QCD: HPQCD (hep-lat/0408019) and FNAL (hep-lat/0409116)ISGW2: quark model
V.Golubev - BABAR Beauty2005, Assisi 20
Exclusive decays, experimental techniques
Different experimental approaches to measure BF:
PURITY EFFICIENCYNeutrino reconstruction
B →π ℓ ν, B →ρ ℓ νUntagged analysis
Recoil of B → D(*) ℓ νB →π ℓ νSemileptonic Tags
Recoil of B → D(*)X9 channels (π,ρ,ω,η,η’,a0) Hadronic Tags
V.Golubev - BABAR Beauty2005, Assisi 21
Exclusive B→ π l ν decays, tagged analysis
l
l D(*)v
vπ
signal Btag B
Use B→D(*)lν decays of opposite B to “tag” eventUse events with B→D(*)lνdecays on both sides to check tagging efficiencyMeasure ∆B(B0→π-l+ν) in bins of q2
BF for B+→π0l+ν
Discriminating VariablesB0 → π-l+ν: angle between the B momentum and the plane defined by the D(*)l and πl momenta: cos2φB
B+ → π0l+ν: angle between B and π0l system: cos θB-πl
V.Golubev - BABAR Beauty2005, Assisi 22
BABARpreliminary
(CKM 2005)
B→ π l ν tagged analysis: the results
on-peak datasignal MCB ρlv MCb ¨ ulv MCB0B0 MCB+B− MCoff-peak data
232 million BB pairs
B0 π-l+ν
B+ π0l+ν
Branching Fraction Results
88 million BB pairs
40 10)13.025.003.1()( −+− ×±±=→ syststatBB νπ l
in 3 bins of q2
40 10)23.037.080.1()( −++ ×±±=→ syststatBB νπ l
V.Golubev - BABAR Beauty2005, Assisi 23
Exclusive B→ π(ρ) l ν decays, untagged analysis
• Reconstruct neutrino from full event + ensure good reconstruction quality
• Signal yield extracted from common fit to ∆E and mES in 5 q2
bins for π+lν, and 3 bins for ρ−lν and ρ0lν each
2* ||4/ BES psm r−=
2/* sEE B −=∆
22 )( νppq l +=
neutrino momentum scaled so ∆E=0 → improved q2 resolution
q2 resolution
22
21
GeV0.495σGeV0.163σ
=
=
V.Golubev - BABAR Beauty2005, Assisi 24
BABARpreliminaryUntagged B→ π(ρ) l ν, signal extraction
DataSignal MCComb. Sig.Crossfeedb clνqq
B→π−l+ν
B→ρlν
4
0
10)08.018.010.038.1()(
−
+−
×±±±
=→
FFsyststat
BB νπ l
4
0
10)28.053.021.014.2()(
−
+−
×±±±
=→
FFsyststat
BB νρ l
Branching Fraction Results
83 million BB pairs
V.Golubev - BABAR Beauty2005, Assisi 25
BABARpreliminary
(CKM 2005)Form factor fits and |Vub| extraction
The πlν untagged sample begins to distinguish among FF calculations: ISGW II is disfavored: Prob(χ2) = 3%Other FF calculations tested are compatible with data
Untagged πlν ρlν
Extraction of |Vub|
V.Golubev - BABAR Beauty2005, Assisi 26
Summary
HQE fits to moments hadronic mass and lepton energy moments:Based on 80 fb-1, ~30% of the current BABAR data, we have made significant advances in the analysis of semileptonic B decays.
Br(B→ Xc l ν)= (10.61 ±0.16exp ±0.06 HQE)%High precision measurement of mb and mc, and exp. determination of HQE parameters.
Inclusive B→ Xu l ν decays, three different approaches:The most precise determination of |Vub|Measurement of partial semileptonic branching fractions for different kinematic regions
Exclusive B→ π(ρ) l ν decay modes in tagged and untagged samples:Determination of |Vub|Begin to test decay form factor calculations
The analyses will be repeated for larger data samples and more preciseshape function parameters derived from from B→ Xsγ photon spectrum
3exp 10)6.04.04.04.41(|| −×±±±= theoryHQEcbV