Radiative and Electroweak Penguin Decays of B Mesons

1
Radiative and Electroweak Penguin Decays of B
Mesons

• Jeffrey D. Richman
• University of California, Santa Barbara
• BABAR Collaboration

11th International Conference on B Physics at
Hadron Machines Oxford, Sept. 28, 2006
2
Outline

• Overview a little history, physics goals, and
  challenges.
• B?rg, B0?r0g, B0?wg and measurement of
  Vtd/Vts
• B?K ll- and B?K ll- search for new physics
  using the lepton forward-backward asymmetry
• Inclusive B?Xs g branching fraction
  measurements and extraction of heavy-quark
  expansion parameters from the Eg spectrum.
• Conclusions

My apologies for not covering all results on
radiative/electroweak penguin decays in this talk!
3
Radiative penguin decays of B mesons
Now its a physics program!
Observation of B?K g CLEO II (1993) Loops in B
decays!
PRL 71, 674 (1993) cited gt500 times!
Rare, but not all that rare!
4
What can we learn from b?s, d transitions?

• Flavor-changing neutral currents probe SM at
  1-loop level.
• New physics can affect the amplitudes at leading
  order!
• As for b?c or b?u semileptonic decays, the
  amplitude in EM/EW penguins is factorizable (only
  one hadronic current).

( WW- box diagram)
(dominated by t quark)
5
What can we learn from b?s,d transitions?

• Presence of only single hadronic current allows
  us to isolate non-perturbative QCD parameters in
  well-defined way. Can be related to same
  parameters for other decays.
• Exclusive decays decay form factors fi(q2). b?s
  transition is similar to b?u (heavy to light)
• Inclusive decays parameters of heavy-quark
  expansion (mb, mp2,)
• Can extract information on CKM elements if info
  on hadronic parameters is available from data,
  theory, or both.

6
Observation of b?d g and Measurement of Vtd/Vts
W annihilation diagram (small)
Ali, Lunghi, Parkhomenko, PLB 595, 323 (2004)
Ball and Zwicky, JHEP 0604, 046 (2006)
I-spin (r), quark model (w). Expect small I-spin
violation (1.1/-3.9).
7
Measurement of b?d g Decays (Belle)
Belle, PRL 96, 221601 (2006) 386 M BB.
Signal
continuum background
B?Kg
Good particle ID is critical in this measurement
to suppress B?K g feed-down.
8
Measurement of b?d g Decays (BABAR)
BABAR, hep-ex/0607099, 347 M BB
projections of 4-D fit
signal bkgnd
bkgnd
B?rg
B?rg
signal
B0?r0g
B0?r0g
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Comparison of b?d g Branching Fractions
CKM fitter includes CDF Bs mixing result. Error
on CKM Fitter prediction includes uncert. on B?Vg
form-factor ratio.
I-spin consistency?
Mode BABAR (10-6) (6.3 s signif.) Belle (10-6) (5.1s signif.)




preliminary hep-ex/0607099
PRL 96, 221601 (2006).
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Extracting Vtd /Vts from b?d g Decays
Belle, PRL 96, 221601 (2006).
courtesy M. Bona (UTfit collab.)
BABAR, hep-ex/0607099 (preliminary)
CDF, hep-ex/0606027 (preliminary)
Consistent within errors.
Theoretical uncertainties limiting both
approaches.
11
B?Kll- and B?Kll- in the SM and Beyond
Photon penguin
Z penguin
WW- box

• Dependence on kinematic variables in 3-body
  decays can be used to study the different
  amplitudes and their interference effects.
• The mode B?Kll- is allowed as well as B?Kll-
  (B?Kg forbidden by conservation of angular
  momentum).

12
Amplitude for B?Kll-
photon penguin dom. at v. low q2
mix of Z-penguin, WW- box
Kruger and Matias PRD 71, 094009 (2005)
Short-distance physics encoded in Cis (Wilson
coefficients) calculated at NNLO in SM
C9, C10 generate asymm. in lepton
angular distribution over most of q2. Cis can be
affected by new physics, which enters at same
order as SM
Ali et al., PRD 61, 074024 (2000)
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Form Factors and Observables

• Long distance QCD physics is mainly described in
  terms of form
• factors, which are functions of
• 4 semileptonic form factors A1, A2, V, A0
  (similar to B?Dln, B?rln)
• 3 penguin form factors T1, T2, T3
• Form factor uncertainies ? 35 uncertainty in
  rate predictions.

Precise SM prediction due to ff cancellation.
14
Predictions for AFB in B?Kll- SM and beyond
Standard Model
15
B?Kll- and B?Kll- q2 distributions
J/yK
Pole from Kg, even in mm-
SUSY models
y(2S)K
SM nonres
SM nonres
q2
q2
constructive interf.
destructive
16
B?Kll- and B?Kll- the J/y veto

• The decays B?J/y K and B?J/y K are huge
  backgrounds and must be carefully removed (also
  B?y(2S)K, y(2S)K).
• These backgrounds are restricted in q2, but there
  is a tail due to bremsstrahlung in the electron
  modes.
• But B?J/y K and B?J/y K are valuable control
  samples use them to study efficiency of almost
  any analysis cut.
• Ali, Kramer, Zhu

m(ee-) projection MC B?Kee-
J/y and y(2S) veto MC B?Kee-
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B?Kll- Signal from BABAR
BABAR, PRD 73, 092001 (2006)

• summed over all K ll- modes (Kee-, Kmm- KS
  ee-, KS mm-)
• significance 6.6 s rarest observed B decay

(averaged)
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B?Kll- Signal from BABAR
BABAR, PRD 73, 092001 (2006)
229 M BB
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B?K()ll- Signals from Belle
Belle, PRL 96, 251801 (2006)
386 M BB
(data sample used for study of Wilson
coefficients)
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B?Kll- and B?Kll- branching fractions
Mode BABAR (10-6) Belle (10-6)


preliminary
PRD 73, 092001 (2006)
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B?Kll- BABAR results on K polarization and AFB
use in 2 bins of q2
BABAR, PRD 73, 092001 (2006)
K polarization
Data
SM
Polarization consistent with SM, but doesnt
discriminate against new physics scenarios with
current data sample.
Theory predictions in graphs Ali et al., PRD 66,
034002 (2002) Ball and Zwicky, PRD 71, 014029
(2005).
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B?Kll- BABAR results on AFB and GL
use in 2 bins of q2
Data
SM
excluded at 3.6s!
Any AFBlt0 excluded at gt2.7s
q2 range (GeV2) AFB FL


(AFB0 in SM and many BSM)
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Belle results on AFB for B?K()ll-
Standard Model
Belle, PRL 96, 251801 (2006)
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Belle results on Wilson coefficients for
B?K()ll-

• fix A7 to SM (B?Xs g)
• fit for A9/A7 and A10/A7
• data consistent with SM
• quadrants II, IV allowed

SM
fit
A10gt0
? A9 A10 lt 0 excludes quadrants I,III at 98.2
C.L.
25
Inclusive B? Xs g

• Canonical process for studying b?s transition.
  Theory uncertainties currently at 10 level
  (NLO) pushing toward 5 (NNLO).
• Huge theoretical effort to predict branching
  fractions photon energy spectrum.
• Branching fraction measures C7 spectrum is
  insensitive to new physics but is sensitive to mb
  and Fermi motion of b-quark (shape function).

T. Hurth, E. Lunghi, W. Porod, Nucl. Phys. B 704,
56 (2005).
M. Neubert, Eur. Phys. J. C 40, 165 (2005).
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Inclusive B? Xs g some history
CLEO, PRL 74, 2885 (1995) 2.01 fb-1 on Y(4S),
0.96 fb-1 below Y(4S)
Backgrounds B decays, continuum, ee-?qqg (ISR),
ee-?qq?p0X
Event-shape analysis
B-reconstruction analysis
total background (points w/error bars)
scaled off resonance
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Challenges of inclusive B? Xs g

• Weak experimental signature single high-energy
  photon event-shape cuts. Lots of background
  from p0s and hs! Fully inclusive analysis is
  not able to exploit the kinematic constraints
  (mB, DE).
• Difficult to carry analysis down to Eg lt 2.0 GeV.
• Want to push toward 5 precision to match the
  expected precision of NNLO calculations. (Its
  amazing that you can do this analysis at all!)
• Two methods have evolved from initial CLEO
  approaches.

Method Advantages Disadvantages
Fully inclusive dont reconstruct Xs Closest correspondence to inclusive B(B?Xs g). Large background limited sensitivity at low Eg .
Sum of exclusive B?K n(p) g Less background due to additional kinematic constraints. Better Eg resolution. More model dependence due to finite set of explicitly reconstructed B?Xs g decays.
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Fully inclusive B? Xs g pushing down the energy
threshold
Belle, PRL 87, 061803 (2004), 140 fb-1 Belle,
hep-ex/0508005
CLEO, PRL 87, 215807 (2001), 9.1 fb-1
Measure for Eggt2.0 extrap. to Eggt0.25 GeV
Measure for Eggt1.8 GeV extrap. to full
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Fully inclusive, lepton-tagged B? Xs g (BABAR)

• Want to suppress large continuum background.
• Strengthen signature for signal by using decay of
  2nd B in event.
• Require high energy lepton pegt1.25 GeV, pmgt1.9
  GeV in addition to event-shape cuts.
• Tag does not compromise inclusiveness of Xs
  selection.

BB dom.
(blind)
contin. dom.
lepton tag from 2nd B meson
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BABAR Fully Inclusive B? Xs g, w/lepton tag
hep-ex/0607071 (preliminary, submitted to PRL)
Spectrum from best fit to kinetic scheme.
Spectrum from best fit to shape function scheme.
not efficiency corrected
(measured)
(extrapolated, kinetic scheme)
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BABAR B? Xs g with Sum of Exclusive Final States
Reconstruct 38 exclusive modes

• DElt40 MeV
• Fit mES distrib. in bins of m(Xs)
• Correct for efficiency of each mode and missing
  modes fraction (?model dependence)

summed over all m(Xs)
comb. BB
signal
continuum
peaking bknd
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BABAR B? Xs g with Sum of Exclusive Final States
BABAR, PRD 72, 052004 (2005)
Energy Range Branching Fraction (10-4)
Eg gt1.9 GeV
Eg gt1.6 GeV (extrapolated)
K(890)

• averages over two shape-function schemes
• errors stat, sys, variation of shape fcn params

K(890)
Eg Moments Value (GeV or GeV2)

• Eg (min) 1.897 GeV

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Summary of B? Xs g Branching Fraction Measurements
HFAG average
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Extraction of heavy-quark expansion parameters
from B?Xs g
Using heavy-quark expansion (HQE), moments of
inclusive B decay distributions can be expressed
in terms of non-perturbative QCD parameters and
quark masses.

• B?Xs g inclusive Eg spectrum
• B?Xc l n inclusive El spectrum and M(Xc)
  hadron mass distrib.
• mb now determined to about 1 and Vcb is
  determined to lt2.

(kinetic energy squared of b-quark)
35
Fits to moments of inclusive B?Xc l n and B?Xs g
distributions
Buchmüller and Flächer, PRD 73, 073008 (2006)
Data from BaBar, Belle, CDF, CLEO, DELPHI
all moments
all moments
kinetic mass scheme
mb used for Vub (7.5 error!)
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Conclusions

• Studies of radiative/electroweak penguins have
  moved far beyond B?Kg.
• Observation of exclusive b?d g decays B?(r0,
  r, w) g
• Use to extract Vtd/Vts consistent with value
  from Bs mixing. Precision limited by
  theoretical uncertainties.
• Electroweak penguins decays B?K l l-, B?K l
  l-, and B?Xs l l- have been
  measured. First studies of decay
  distributions have been performed and exclude
  some non-SM scenarios. Much more data needed
  to exploit full potential.
• Inclusive B?Xs g measurements provide
  information on mb and non-pert. QCD
  parameters and help improve precision on
  Vcb and Vub. Difficult issues with
  systematic errors, but goal si to achieve 5
  uncertainty on branching fraction.
• Much more to learn about penguins we will
  study them for many years to come at BaBar,
  Belle, and LHC-b!

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Backup slides
38
B?Kll- K polarization vs. q2
SM
39
M(ll-) distributions from B?J/y K control
samples data vs. Monte Carlo
BABAR
points data
histogram MC
Bremsstrahlung tails well described by MC.
absolute normalization
40
Lepton angular distribution in l l- rest frame
use l- if B use l if B
Ali, Kramer, Zhu, hep-ph/0601034
41
B?Kll- Dalitz plot
Can see AFB behavior and q2 dependence from the
Dalitz plot
effect of g pole
Note B?Kll- is expected to have very small AFB,
even in presence of new physics effectively
provides a crosscheck.
42
Extracting AFB and FL in bins of q2
BABAR, PRD 73, 092001 (2006)
43
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