Rare B decays
The Royal Society of Edinburgh
4th February 2004
Philip J. Clark
University of Edinburgh
Talk overview
Introduction to rare decays
Ways to measure them experimentally
Theoretical methods for calculating them
Various interesting results
Summary
Philip J. Clark
Rare B Decays
Page 2
What are rare B decays? (part 1)
Small CKM matrix element
u
 exclusive b  u charmless hadronic
B0  p+p-,K+p-, ...
B  ppp, Kpp, rr, K*r, …
 exclusive b
B+

...
BR~10
BR~10
exclusive b  u, purely leptonic
B+  l+n
Philip J. Clark
Vub
d, u
d
*
u
|Vub/Vcb|~l
u
-4~-5
fB|Vub|
BR~10
Rare B Decays
d, u
-5 ~ -6
 c with Vus (what is rare?)
`D0K+,
b
-5~-12
Vus
s
b
c
u
u
b
l+
u

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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)
B  fK* … (pure gluonic loop)
B  Kp, Kh’ (gluonic + small tree)

exclusive b
BR~10
W+
s
q
–5 ~ -6
q
d, u
d, u
radiative loop: b  (s,d) g
g
b
B  K*g,rg,wg exclusive (b  s,d g)
inclusive (b  s g)
B  sg
BR~10 -5 ~ -7 d, u
BR ~10 -4

u, c, t
d, u
electroweak loop
B  K`nn, K`l l
BR~10
-6
Philip J. Clark
Rare B Decays
g /Z
b
W
d, u
s, d
+
l+
ls
d, u
Page 4
Charmless hadronic decays
How can we organise them?
JPC classification of light mesons
B  hK, pp, pK, h’p…
PP B  rr, fK*, fw… VV
B  hK*, rp, fp, wK, … PV B  a0p, f0p …
Philip J. Clark
Rare B Decays
SP
Page 5
Success of the quark model
Philip J. Clark
Rare B Decays
Page 6
Theoretical approaches

Diagrammatic
 Methodology
Two main methodologies
 Effective Hamiltonian
 Isospin & SU(3)
 Advantages:
 Very intuitive
 Provides powerful
approximate relations
between decay channels
 Disadvantages:
 Methodology
 QCD operator product
expansion
 Advantages:
 Rigorous computation using
Wilson coefficients
 Disadvantages
 Huge uncertainties in operator
matrix elements
 Solution for B decays:
o (QCD) factorisation
 non exact results
Chiang, Gronau, Luo, Rosner & Suprin
eg. Beneke & Neubert
BPV hep-ph 0307395
BPP hep-ph 0306021
BPV, PP hep-ph 0308039
Philip J. Clark
Rare B Decays
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Example: rare p0 modes
Colour suppressed trees
Gluonic penguins
Electroweak penguins
Singlet penguins
Philip J. Clark
Rare B Decays
Page 8
Theoretical predictions
Experimental data
Heavy Flavor Averaging group (Lepton Photon 2003)
Philip J. Clark
Rare B Decays
Page 9
PEP II/BABAR at SLAC
PEP II
Asymmetric
B Factory
Started construction in1994
Completed in 1999
Reached design luminosity in 2000
Luminosity records
PEP-II/BABAR at SLAC
design peak: 3.0 x 1033 cm-2s-1
best peak: ~7.0 x 1033 cm-2s-1
total recorded:
162 fb-1
9 GeV e- on 3.1 GeV e+
Philip J. Clark
Rare B Decays
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The BABAR detector
DCH
DIRC
EMC
IFR
SVT
Philip J. Clark
Rare B Decays
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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:

e+
qq
e+
Signal
B
e-
eOther
B
Fisher discriminants (linear weighting)
And in some cases neural networks taking
advantage of hidden layers.
Philip J. Clark
Rare B Decays
Page 12
Time dependent meaurements
y
Identify
B or anti-B
z
x
0
 4S
Coherent BB
production
e-
K-
Btag
e+
K+
K- f
B0rec
Dz
p+
0
K
p S
-
D t ~ D z/cg b g
Philip J. Clark
Rare B Decays
Full reconstruction of
B  f Ks 0
Page 13
Example of a fKs event
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Rare B Decays
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Maximum Likelihood fits
B  hp0 h  p+p-p0 
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Rare B Decays
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Results: some branching fractions
Rare decays are not rare anymore!
Philip J. Clark
Rare B Decays
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Why do we want to study rare decays?
Main experimental constraints
on the apex of the UT
CP violation in the kaon system
Measurements of |Vub|
(b → u transitions)
B0 and Bs mixing frequencies
b
BABAR
Phys. Rev. Lett. 89 (2002) 201802
World average (BABAR+Belle+…)
Heavy Flavor Averaging Group 2003
Philip J. Clark
Rare B Decays
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World-wide status of sin2b
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Rare B Decays
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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
Philip J. Clark
Rare B Decays
Page 19
Latest interesting result: B  p+p-
Latest results from BELLE:
S=-1.00  0.21  0.07
C= 0.58  0.15  0.07 hep-ex/0401029
Philip J. Clark
Rare B Decays
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What we have covered:

Summary
What are rare B decays
Various tree diagrams
Several penguin diagrams
Calculating
them
Categorisation of light mesons
The two main theoretical approaches
Where
How
to measure them
Example: The BaBar experiment and PEPII
to measure them
Mulitivariate discriminants, Particle ID, Maximum Likelihood
Results
Lots
Branching fractions
Status of Sin2b
Direct CP violation
Latest News
of new results still to come
Need more data!
Philip J. Clark
Rare B Decays
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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
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Rare B Decays
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