Royal society seminar

1
Rare B decays
The Royal Society of Edinburgh 4th February 2004
Philip J. ClarkUniversity of Edinburgh
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Talk overview
Introduction to rare decays
Ways to measure them experimentally
Theoretical methods for calculating them
Various interesting results
Summary
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What are rare B decays? (part 1)

• Small CKM matrix element

• exclusive b ? u charmless hadronic

Vub/Vcbl
B0 ? pp-,Kp-, ... B ? ppp, Kpp, rr, Kr,
BR10 -5 -6

• exclusive b ? c with Vus (what is rare?)

BR10 -4-5
B ?D0K, ...

• exclusive b ? u, purely leptonic

fBVub
B ? ln
BR10 -5-12
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What are rare B decays? (part 2)
Leading diagram involves a quantum loop
(penguin loop)

• gluonic loop b ? s gluon (qq)

exclusive
B ? fK (pure gluonic loop) B ? Kp, Kh
(gluonic small tree)
BR10 5 -6

• radiative loop b ? (s,d) g

exclusive (b ? s,d g) inclusive (b ? s g)
B ? Kg,rg,wg B ? sg
BR10 -5 -7 BR 10 -4

• electroweak loop

B ? Knn, Kl l
BR10 -6
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Charmless hadronic decays
How can we organise them?
JPC classification of light mesons
B ? ?K, ??, ?K, ??
B ? ??, ?K, ??
PP
VV
B ? ?K, ??, ??, ?K,
PV
SP
B ? a0?, f0?
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Success of the quark model
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Theoretical approaches
Two main methodologies

• Diagrammatic
• Methodology
• Isospin SU(3)
• Advantages
• Very intuitive
• Provides powerful approximate relations between
  decay channels
• Disadvantages
• non exact results

• Effective Hamiltonian
• Methodology
• QCD operator product expansion
• Advantages
• Rigorous computation using Wilson coefficients
• Disadvantages
• Huge uncertainties in operator matrix elements
• Solution for B decays
• (QCD) factorisation

Chiang, Gronau, Luo, Rosner Suprin B?PV hep-ph
0307395 B?PP hep-ph 0306021
eg. Beneke Neubert B?PV, PP hep-ph 0308039
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Example rare ?0 modes
Colour suppressed trees
Gluonic penguins
Electroweak penguins
Singlet penguins
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Theoretical predictions
Experimental dataHeavy Flavor Averaging group
(Lepton Photon 2003)
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PEP II/BABAR at SLAC
Started construction in1994 Completed in 1999
Reached design luminosity in 2000
PEP II Asymmetric B Factory
Luminosity records
PEP-II/BABAR at SLAC
design peak best peak total recorded
3.0 x 1033 cm-2s-1
7.0 x 1033 cm-2s-1
162 fb-1
9 GeV e- on 3.1 GeV e
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The BABAR detector
DCH
DIRC
EMC
IFR
SVT
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How do we find the rare decays?

• One method we use is the event shape
• The continuum is light quark pair production, so
  there is lots of extra energy. All the decay
  products bunch into jets
• B mesons are produced almost at rest in our case
  ? The decay products of the B are distributed
  roughly spherically.

• There any many such event shape variables which
  are all correlated
• Fisher discriminants (linear weighting)
• And in some cases neural networks taking
  advantage of hidden layers.

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Time dependent meaurements
0
B
tag
Coherent BB production
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Example of a ?Ks event
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Maximum Likelihood fits

• B ? ??0 (? ? ??-?0 )

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Results some branching fractions
Rare decays are not rare anymore!
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Why do we want to study rare decays?
Main experimental constraints on the
apex of the UT
BABAR
Phys. Rev. Lett. 89 (2002) 201802
World average (BABARBelle)
Heavy Flavor Averaging Group 2003
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World-wide status of sin2b
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Direct CP violation

• Measurements in many charmless hadronic B decays
• These modes are all sensitive to gluonic penguin
  amplitudes which may interfere to produce an
  asymmetry
• Non-SM effects could cause potentially large
  asymmetries in some decays

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Latest interesting result B ? ??-
Latest results from BELLE S-1.00 ? 0.21 ?
0.07 C 0.58 ? 0.15 ? 0.07 hep-ex/0401029
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Summary
What we have covered

• What are rare B decays
• Various tree diagrams
• Several penguin diagrams
• Calculating them
• Categorisation of light mesons
• The two main theoretical approaches
• Where to measure them
• Example The BaBar experiment and PEPII
• How to measure them
• Mulitivariate discriminants, Particle ID, Maximum
  Likelihood
• Results
• Branching fractions
• Status of Sin2b
• Direct CP violation
• Latest News
• Lots of new results still to come
• Need more data!

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Particle Identification

• Need to differentiate kaons from pions.
• Crucial to the analysis of many charmless decays
• Especially important at high momenta

K, p