Semileptonic B physics at CLEO

Ron PolingUniversity of Minnesota

CLEO CollaborationCLEO-c Collaboration

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CESR and CLEO: 23 years of B Physics

Three generations of detectors, ~170 Ph.D.’s, ~360 papers

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The Global Program of CKM Tests

• Goal: redundant determinations of sides and angles of the Unitarity Triangle(s)

• CLEO’s Role:– Measure UT sides through |Vcb| and

|Vub| in semileptonic B decays.– Detailed studies to guide

interpretation and help clear the “fog” of nonperturbative effects.

• Challenging experimentally and theoretically.

Vcb or Vub

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B Semileptonic Branching Fraction

• CLEO II: 10 fb-1 at (4S)– Mature, well-understood detector,

data, Monte Carlo, generators, etc.• Lepton-Tagged Analysis

– ptag>1.4 GeV/c plus accompanying electron with pe>0.6 GeV/c.

– Charge, angular correlations to separate primary (BXe) from secondary (BD Ye).

• Refined electron ID, background and efficiency determinations.

– Maximize understanding and minimize momentum dependence.

CLEO - Preliminary

Preliminary: B(BXe)=(10.88 0.08 0.33)%

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BSL – Still small-ish, but consistent

• Good agreement among different techniques, experiments.

• Measurements at (4S) have come up and LEP Z0 average has come down.– Most recent LEP fit

result is (10.590.22)%

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The Reality of Semileptonic B Decays

• We idealize them as…

• The real picture is…

• View of b decay obscured by… – Hadronic effects describable by

both perturbative and nonperturbative QCD.

– Uncertainty about our assumption of quark-hadron duality.

• Available tools include Heavy Quark Effective Theory and Lattice QCD. Must be validated by experiment.

Vcb or Vub

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HQET+OPE• Expresses observables of inclusive b decay as expansions in

inverse powers of MB (Voloshin +Shifman, …).• One parameter at order QCD/MB:

~ MB – mb is the energy of light-quark and gluon degrees of freedom in the B meson.

• Two parameters at order 2QCD/M2

B :

1 – negative of the average b-quark momentum squared. 2 – energy of the hyperfine interaction of the b spin with the light degrees of

freedom (from B*-B splitting: 2 = (0.128 0.010) GeV).• Six more parameters at order 3

QCD/M3B

(Gremm and Kapustin):

1, 2, T1, T2, T3, T4 (not all independent).• Meaning of parameters depends on choices: order of perturbative

(0(s/)2 ) and nonperturbative (1/M3B) expansions and mass scheme.

Consistency required. |Vcb| is what counts!

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MOMENTS, INC. – CLEO Branch

• BXs Photon-Energy Moments• BXl Lepton-Energy Moments• BXl Hadronic Recoil Mass Moments

First and second moments, different lepton-energy ranges…

Lots of complementary information.

Data from different processes will tell us if this approach provides a coherent picture of inclusive B decays.

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Measuring BXs

• Use all available tools!– Traditional shape variables.– Pseudo-reconstruction: demand

consistency with BK(n)– Use neural net to compute signal

probability and measure spectrum of “weights.”

– Subtract continuum with below-(4S) data.

– Subtract backgrounds with Monte Carlo, informed by data wherever possible.

Photon-energy spectrum at (4S)

Continuum must be suppressed.

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Measuring BXs• CLEO II – 10 fb-1

• PRL 87, 251807 (2001)• Hard measurement, simple

interpretation:– Almost two-body, with

broadening by Fermi motion, gluon bremsstrahlung, boost of B’s at (4S)

– <E> gives nearly direct access to .

Next, measure the moments of the E spectrum by fitting to model of Ali-Greub or HQT calculation of Kagan-Neubert:

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Hadronic Mass Moments in BXcl

• Select events with leptons with– 1.5 < pl < 2.5 GeV/c

• “ reconstruction” to measure p.• Calculate approximate hadronic

recoil mass using l (take advantage of pB small):

• Construct continuum-subtracted spectrum and fit to mixture:– BDl (CLEO FF’s)– BD*l (CLEO FF’s)– High-mass, nonresonant by models

(ISGW2, Goity/Roberts)

• Use fit components to compute moments:

= 0.2510.023 0.062 GeV2

= 0.6390.056 0.178 GeV2

PRL 88, 251808 (2001)

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Interpreting the Moments

• First moments E and MX2-

MD2 fitted to extract and 1.

• Combined solution is = 0.350.0700.10 GeV1 = -0.2360.0710.078 GeV2

• Precise meaning of parameters is scheme- and order-dependent; these are to order 1/MB

3, order 0s

2 in MS scheme.

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Extracting |Vcb| from (BXcl)

• Ingredients: (BXcl) = (10.80.3)% (CLEO) B0 and B (PDG), f+/ f00 (CLEO) SL = (0.44 0.02) 10-10 MeV

• Result:|Vcb| = 0.041 0.0005 0.0007 0.0009

Overall precision: ~3% + quark-hadron duality.

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|Vcb| from BD*l d/dw

0.0388 0.0011

|Vcb|=(42.6±1.2±2.3)10-3

PRL 88, 251808 (2001)

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New and Improved Measurement of the Hadronic Mass Moments in BXcl

hep-ex/0307081 - Contributed to Lepton-Photon 2003

• Compute recoiling hadronic mass from charged lepton and neutrino kinematics - neutrino “detection.”

• Near hermeticity of CLEO II Neutrino=“What’s missing”

Fit 3-dimensional differential decay rate, extract hadronic mass squared as a function of lepton-energy cut (pl > 1 GeV/c).

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Fit Projections• Selection criteria:

– Cuts to enhance reconstruction– Continuum suppression– Efficiency ~2% for BXcl

• Sample to fit: 122K events• Components of fit:

– BDl– BD*l– BD**l ISGW2

– B(Xc)NRl Goity/Roberts

– BXul ISGW2+NR– Secondaries CLEO MC– Fake Leptons, Continuum fixed

with data

HQET+measured FFs

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Fits Mode-by-Mode BRs

+Generator-Level Info

(Fraction above pl cut, moment value for that cut.)

Moment Value for BXcl

Results

• Consistent with previous CLEO measurements, BaBar summer ’03

• Interpretation continues

PRELIMINARY

DELPHI: energy and hadronic mass moments for full lepton-energy spectrum. hep-ph/0210319 PRELIMINARY

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BXcl Energy Moments PRD 67, 072001 (2003)

Pending…El moments down to 0.6 GeV

e

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Improved Inclusive Measurement of |Vub|

• End-point technique:– Isolate BXul by looking above

(and just below) the BXcl kinematic limit.

– Used for bu discovery (CLEO, ARGUS) and CLEO 93: |Vub|/|Vcb|= 0.080.02

Significant model dependence.

• Can now do better:– Better knowledge of BXcl.

– BXs helps determine fraction of spectrum in end point.

BXs spectrum shape function fu

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Measuring BXul

2.0 < pl <2.6 GeV/c Suppress continuum with neural

net (energy flow w.r.t. lepton). Less q2- and M(Xu)-dependent.

Estimate BXcl by fitting inclusive spectrum. Components reflect state-of-the-art on form factors, b.r.’s, etc.

Trade-off: BXcl uncertainty vs. spectral-fraction uncertainty.

bul excess

ON

OFF

b cl

PRL 88, 231803 (2002)

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• Hoang, Ligeti, Manohar; Uraltsev:

• Determine from B Xs shape-function fits (Kagan-Neubert).

Extracting |Vub|

In preparation: Paper with details of fits for shape-function parameters and fu values for phase-space regions other than just end point.

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Branching Fractions and q2 Dependenceof Bl and B()l

and Extraction of |Vub|

• Exclusive reconstruction of B(///)l through neutrino “detection.”

• Near hermeticity of CLEO II Neutrino=“What’s missing”

hep-ex/0304019 to be published

in PRD

Signal defined by…

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Bl (pl > 1.0 GeV/c) Bl (pl > 1.5 GeV/c)

• CLEO II 10 fb-1

• Event selection to enhance reconstruction, suppress continuum

• Fit Mml and E in three bins in q2 to find branching fractions

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• Extracting |Vub|

– Fit d/dq2

– Discriminates among FFs• Bl

– FF dependence is small– Disfavors ISGW2

• Bl– Larger FF

dependencegreater model uncertainty in |Vub|

Bl

Bl

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RESULTS

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Comparisons and CKM Impact

|Vub|/10-3

90% c. l. contours

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• CLEO is still contributing on |Vub| and |Vcb|.– Sample smaller than B factories, but well understood with

mature tools.– We’re all limited by theoretical uncertainties.

• Continuing and forthcoming CLEO CKM-related measurements:– Completion of moment measurements, coordinated fits of all

available inputs.– Inclusive bul for all CLEO II and CLEO III via

multidimensional fit and bigger phase-space acceptance.• CESR-c/CLEO-c will have significant impact on CKM.

See the upcoming of Karl Ecklund.

Perspectives and Prospects