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

7

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

8

|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

qq

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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Backup Slides

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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!