jochen dingfelder, slac semileptonic decay studies with b a b ar annual doe hep program review, june...
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Jochen Dingfelder, SLAC
Semileptonic Decay Studies
with BABAR
Annual DOE HEP Program Review, June 5-8, 2006, SLAC
BDXc ,Xu
2
We study semileptonic decays B Xc,uℓ Leptonic and hadronic currents factorize!
Experimental approaches: inclusive or exclusive (B → ℓ )
Semileptonic B-Meson Decays
Exclusive Decays Lower signal rate, better bkg
reduction
Need Form Factors to describe hadronization process
Measurement as function of q2, angles
Inclusive Decays Large signal rate, high bkg
Total rate calculated with HQE
Need Shape Function (b-quark motion in B meson) smears kin. spectra. SF param. from b s or b cl.
Eℓ q2 = pℓ
2+p2
MX
D,
electroweak interaction coupling |Vcb| , |
Vub|
strong interaction meson structure
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HQE fit theo. error band
|Vcb| from Ee and MX Moments
1st to 4th MX moments , 0th to 3rd Ee moments (as function of min. cut on Ee)
Enough d.o.f. to fit 8 parameters:Global fit to Ee , MX , E (bs) moments:
|Vcb| = (41.58 + 0.45 +0.58) x 10-3
mb (kin) = 4.591 ± 0.040 GeV
2 (kin) = 0.406 ± 0.042 GeV
Shape function par. needed
for |Vub|
Measure moments of lepton momentum and hadronic mass spectra in B Xcℓ
(|Vcb|) = 1.7%
(mb) = 1%
(mc) = 5%
high mass
charm states
Mx2[GeV2/c4]
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The decay rate is:
f (w=1) = 1 in heavy quark limit; lattice QCD says: f (1) = 0.919
Shape of f (w) expressed in terms of 2 (slope at w=1) and form-factor ratios R1, R2.
Measure decay angles ℓ , V , . Fit3Ddistribution in bins of w to extact 2, R1, R2 .
|Vcb| from B D* ℓ
Phase spaceForm factor
Here we use w = D* boost in B rest frame
Hashimoto et al, PRD 66 (2002) 014503
Caprini, Lellouch, Neubert Nucl. Phys. B 530, 153 (1998)
+0.030–0.035
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B D* ℓ Form Factors & |Vcb|
stat. MC stat. syst.
Factor 5 improvement on FF uncertainty from previous CLEO measurement (1996!)
1D projections of fit result:
88M BB
w cosVcosl
Total systematic error on |Vcb| : 4.5% 3.5%
BABAR
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Inclusive |Vub| from Lepton Endpoint Study inclusive decays B Xu ℓ
Large charm bkg from B Xc ℓ need accurate bkg subtraction Restrict phase space (problematic for
HQE !) shape function becomes important
Select events with high-E lepton: 2.0 < Eℓ < 2.6 GeV
Lower Eℓ cut below charm threshold Larger acceptance, smaller theoretical
error
|Vub|=(4.44±0.25exp ±0.22theo)10-3+0.42– 0.38SF
fully corrected
spectrum
BABAR
MC bkgb cl
Data
Data – bkg
MC signalb ul
88M BB
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MX<1.67 GeV: |Vub| = (4.43 ± 0.45exp± 0.29theo)10-3
Hadronic B tag: Fully reconstruct one Btag in hadronic decay mode
known momentum, flavor, charge of other Bsignal
MX < 1.7 GeV and q2 > 7 GeV2
New “shape function-independent” approach: Combine b ul rate with b s spectrum Or measure full MX spectrum
Inclusive |Vub| with Hadronic B tag
|Vub|=(4.65±0.34exp ±0.23theo)10-3+0.46– 0.38SF
BBtagtag
BBsignalsignalXXuu ll
ee--
DD**
ee++
Leibovich, Low, Rothstein hep-ph/0005124,0105066
theory uncertainties
signal
MXmax
88M BB
88M BB
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|Vub| from B ℓ
Experimentally: Good S/B ratio, untagged & tagged
analyses Measure q2 dependence of form factor
Theoretically: Complementary to inclusive approach Form-factor norm. dominates error on |
Vub|
Untagged analysis: Reconstruct neutrino from missing 4-momentum of event
83 x 106 BB
disfavors ISGW IIquark model
|Vub| = (3.82 ±0.26exp ) 10-3+0.89– 0.53FF
Form Factor LQCD2
Data
Signal MC
Comb. Sig.
Crossfeed
bcl
mES (GeV)
82M BB
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Form factor f (q2 = M2Ds) decay constant fDs
Leptonic decay proceeding through weak annihilation of quarks
DS+
and Decay Constant fDs
Ds → signal
N = 489±55Preliminary
248M BB
stat syst Ds→
Aubin et al. PRL 95 122002 (2005)
(stat) 016.0135.0)(
)(
s
s
D
D
LQCD:
Validates LQCD predictions at 10% level
W , ,e
c
s
++=
+ + +|fDs|2
|Vcs|2
+ Ds(*) mesons from e+ e- cc
Tag one D(*), reconstruct signal side via D*S DS
Normalize to Ds :
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Conclusions BABAR has broad program of semileptonic/leptonic B decays. SLAC plays a leading role here.
Much progress in the last 1-2 years: |Vcb| 2%, |Vub| 7-8% , form factors, …
BABAR is now also exploiting their large D-meson samples. Charm semileptonic decays provide high-quality “lattice calibration”. Techniques validated by charm decays can be applied to beauty decays! These measurements have reached precision to allow critical comparisons with theory calculations: Form Factors & HQE Parameters
fDs results: CKM constraints (“tree” quantities) 2004 2006
status 2004
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SLAC’s Contribution
B Xclincl. Mass spectra, moments, |Vcb| Buchmüller, Lüth
B D*l : Form factors, |Vcb| Snyder
B Xs : Branching Fraction, moments Ozcan, Libby, Jessop
B Xul incl. : Momentum endpoint spectrum, |Vub| Lüth
B Xul incl. : Shape function independent, |Vub| Langenegger
B l & l : Branching fraction, form factor, |Vub| Weinstein, Dingfelder, Kelsey, Lüth
Ds : Branching fraction, decay constant Stelzer, Schindler
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Testing Theory with D K/l
|Vcs| from unitarity determine |
f+(q2)|
|f+(q2)| from LQCD determine |
Vcs|
Goal: measure ratio B(D l) / B(D Kl)
compare with lattice |f+(q2)| / |
f+K(q2)|
mpole = 1.854 ± 0.016 ± 0.020 pole = 0.43 ± 0.03 ± 0.04
D
Study D-meson decays from e+ e- cc with high statistics Tag one c quark via full reconstruction of D(*) meson Dramatically reduces bkg from u, d, s and B mesons
Charm SL decays provide high-quality lattice calibration.Techniques validated by charm decays can be applied to beauty decays!