Radiative B Decays (an Experimental Overview)

1
Radiative B Decays (an Experimental Overview)

• E.H. Thorndike
• University of Rochester
• CLEO Collaboration

FPCP May 18, 2002
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The Observables

• Rates for exclusive decays. eg, B g K(892) g
• Rate for inclusive decay b g s g (actually B g Xs
  g )
• CP asymmetry, inclusive decays
• CP asymmetry, exclusive decays
• Photon energy spectrum in inclusive decays B g
  Xs g
• Same observables for b g d g

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What do you learn?

• Rate for exclusive decays
• Experimentally easiest.
• B gK(892) g first penguin seen(1993).
• Form factors not known, so not good for New
  Physics.
• Rate for inclusive decays
• Loops, w t, so sensitive to other heavy things
  in loop (i.e. New Physics)
• Reliably calculated with SM and with New
  Physics
• excellent for revealing or limiting New
  Physics.
• CP asymmetry
• Expected to be very small in SM.
• 10-20 in some New Physics.
• Inclusive more reliably calculated than
  exclusives,
  but if big in either, New Physics.

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What do you learn? contd

• Photon energy spectrum in b g s g
• Insensitive to New Physics (b g s g is 2-body, a
  line)
• Depends on quark mass and Fermi momentum
• Can give B light cone shape function
  (useful for obtaining Vub from
  b g u l n inclusive).
• Can help determine HQET OPE expansion parameters
  (needed for obtaining Vcb from b g c l n
  inclusive).
• b g d g
• Initial interest will be in determing Vtd
• (but watch out for long distance effects,
  and for additional CKM
  factors from c - and u - quark loops).

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The Experimental Problems

• MUST suppress continuum.
• MUST subtract continuum.
• To push spectrum down below 2.2 GeV, must handle
  backgrounds from other B decay processes.

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Outline for Rest of Talk

• Branching Fractions for Exclusive Decays
• Branching Fraction for Inclusive Decays
• CP Asymmetries
• Photon Energy Spectrum
• b g d g

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Discovery of PenguinsCLEO -1993
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B g K g (BaBar)

• Run I (22.7 M BB)
• H Tanaka
• Moriond 2002

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B g K g (Belle)
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BgK g Branching Fractions
BgKo g B-gK- g
CLEO 93 4.01.70.8 5.73.11.1
CLEO 00 4.550.700.34 3.760.860.28
BaBar 02 4.230.400.22 3.830.620.22
Belle (prelim) 4.080.340.26 4.920.570.38
average 4.210.250.26 4.320.380.30
(All numbers, X10 -5)
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BgK2(1430) g Branching Fractions
CLEO 00 1.660.560.13 x 10-5
Belle (prelim) 1.500.560.12 x 10-5

• Other Exclusives (Belle)
  
• B g Kp-pg 2.4 0.5 0.3 x
  10-5
• Kopg 2.0 0.65
  0.2 x 10-5
• Krog 1.0 0.5
  0.25 x 10-5
• Kp-pg (NR) lt 0.9 x 10-5

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Continuum Suppression forInclusives -CLEO

• Leptons If event has lepton (e or m), use qgl ,
  El for additional continuum suppression.
• Weight For each event with a high energy g,
  determine probability that it is b gs g, rather
  than continuum background. Weight each such
  event, according to probability.

• Event shape variables R2, S , R2,
  cosq, cone energies within
  20o, 30o of g direction and -g
  direction. Into neural net, 8 inputs, 1
  output.
• Pseudoreconstruction Search events for
  combinations of particles that look like B-gtXsg.
  For Xs use K or Kos, and 1-4 p (at most 1 po).
• Calculate
• If event has c2Blt20, use c2B , cosqtt for
  additional suppression.

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CLEO, PRL 87, 251807 (2001)

• Photon energy spectra (weights per 100 MeV)
• Top shows the On Y(4S) and the scaled
  Off-resonance spectra.
• Bottom shows the difference and the spectrum
  estimated from B decay processes other than b g
  sg and b g dg.

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B( b gs g)
Theory Buras,Misiak, et al Hep-ph/0203135
2.2 GeV
CLEO 95
ALEPH 98
?? GeV
2.2 GeV
Belle 01
2.0 GeV
CLEO 01
x10-4
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CP Asymmetry

• NOTE sign convention
• FOLLOW sign convention
• (so far, everyone seems to have.)

BgK(892)g
CLEO, 00 0.080.130.03
BaBar, 02 -0.0440.0760.012
Belle, new 0.0320.0690.020
Average 0.0090.0480.018
CLEO, 01 Inclusive -0.0790.1080.022 (0.965A
(bgs g)0.02A(bgd g))
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Photon Energy Spectrum- the B Backgrounds

• gs from po g gg, h g gg, that have escaped the
  po/h veto.
• The big one (90 of total).
• Measure po, h yields, treating po (h) as if it
  were a g, all cuts as for b g s g
  analysis. Use Monte Carlo to determine po/h veto
  efficiency.
• gs from other sources
• w g pog, h g rog, radiative y decay, r g pg, a1
  g pg, final state radiation. b g u processes, b
  g s g processes.
• Theyre small, and with modest effort to have
  Monte Carlo event generator ok, one can trust the
  Monte Carlo.
• Klong, interactions in calorimeter.
• Determine contribution from lateral distribution
  in calorimeter (E9/E25).

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CLEO (PRL 87, 251807 (2001))

• Observed laboratory frame photon energy
  spectrum (weights/100 MeV) for ON minus scaled
  OFF minus B backgrounds, the putative b g sg plus
  b g dg signal.

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Moments of the Spectrum

• CLEO obtains moments in the B rest frame, for
  Eg(rest frame) gt 2.0 GeV

• HQET plus OPE allows inclusive observables to
  be written as double expansions in powers of as
• and 1/MB to order boa2s and 1/M3B

• C2 and C7 are Wilson coefficients and bo is the
  one-loop QCD b function. The 1/M3B parameters are
  
• estimated from dimensional considerations to be
  (0.5GeV)3.
• Using the first CLEO obtains
• The expression for the second moment converges
  slowly in 1/MB, and so CLEO made no attempt to
  extract parameters from it.

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B g lightquark shape function, SAME (to lowest
order in LQCD/mb) for b g s g a B g Xs g and b g
u ln a B g Xu ln.
B g Xs g (hadron level)
b g s g (parton level)
Convolute with light cone shape function.
b g u l n (parton level)
B g Xu l n (hadron level)
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b g d g

• So far nothing on inclusive. Only upper limits on
  exclusives.
• Expect B(B g r g) 2 x B(B g ro g) 2 x B(B g
  w g)

B Pairs
(Million) B(B g r g) 2 x B(B g ro g) 2 x B(B g w g)
CLEO 00 9.7 13 34 18
Belle 01 11 10 21 ---
BaBar prelim 63 2.8 3.0 ---
Branching Fraction Upper Limits (10-6)
BaBar limit by far the best c (1-r)2h21/2 lt
1.6 (Tanaka, Moriond 02) Still, not an
improvement in limit on Vtd over that from Bs-
s mixing.
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Summary and Conclusions I

• b g s g Exclusive branching fractions.
• Not of great fundamental interest, but by
  identifying a larger fraction of the makeup of B
  g Xs g decays, one will reduce some systematic
  errors on the branching fraction for the
  inclusive process b g s g. Belle progress on this
  front.
• b g s g inclusive branching fraction.
• Experiment agrees with SM theory, places strong
  restrictions on New Physics.
• But really only one good measurement. Babar and
  Belle should get to work! They will need to
• Accept photons down to 2.0 GeV, or lower. (2.2
  GeV is no longer good enough)
• Take a reasonable amount of data below the Y(4S)
  resonance. (continuum subtraction MUST be done
  with DATA.)

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Summary and Conclusions II

• CP asymmetry
• No hint of a non-zero value.
• Limits place weak restrictions on New Physics.
• Plenty of room for improvement.
• Asymmetry for inclusive wanted (Babar, Belle??)
• b g s g photon energy spectrum
• Has helped provide precise determination of Vcb
  from the inclusive semileptonic decay branching
  fractions, and (more important) a good
  determination of Vub from the lepton endpoint
  yield in b g u ln, with DEFENSIBLE ERRORS. Will
  be key for future determinations of Vub from
  inclusive b g u ln.
• Improvements in spectrum very desirable.
• b g d g
• So far, nothing on inclusive, only upper limits
  on exclusives.
• Not yet an improvement in limit on Vtd over
  that from mixing.
• Stay tuned.