Philip J. ClarkUniversity of Edinburgh

Rare B decaysRare B decays

The Royal Society of Edinburgh4th February 2004

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Talk overview

Introduction to rare decaysIntroduction to rare decays

Ways to measure them experimentallyWays to measure them experimentally

Theoretical methods for calculating them Theoretical methods for calculating them

Various interesting resultsVarious interesting results

SummarySummary

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Small CKM matrix element

|Vub/Vcb|~

exclusive b u charmless hadronic

B0 K+...B KK*…

b

ud ,

ud ,

u

u

d*

ubV

b

uu

c

u

susV

BR~10 -5 ~ -6

B+ D0K+, ...

exclusive b c with Vus (what is rare?)

BR~10 -4~-5

exclusive b u, purely leptonic

B+ l+ BR~10 -5~-12

fB|Vub| b

u

l

What are rare B decays? (part 1)

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Leading diagram involves a quantum loop (“penguin” loop)

What are rare B decays? (part 2)

gluonic loop: b s gluon (qq)

radiative loop: b (s,d)

electroweak loop

exclusiveb

ud , ud ,

sq

q

W

ud ,b

ud ,

ds ,

tcu ,,

b

ud ,ud ,

sW

Z/ll

B K* … (pure gluonic loop)B K, K’ (gluonic + small tree)

BR~10 –5 ~ -6

exclusive (b s,d )inclusive (b s )

B K*,,B s

B K, Kl lBR~10 -6

BR~10 -5 ~ -

7

BR ~10 -4

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Charmless hadronic decays

JPC classification of light mesons

How can we organise them?

B K, , K, ’…

B K*, , , K, …

B , K*, …PP

PV

VV

SPB a0, f0 …

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Success of the quark model

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Theoretical approaches

Diagrammatic Methodology

Isospin & SU(3) Advantages:

Very intuitiveProvides 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: o (QCD) factorisation

Two main methodologies

Chiang, Gronau, Luo, Rosner & SuprinBPV hep-ph 0307395BPP hep-ph 0306021

eg. Beneke & NeubertBPV, PP hep-ph 0308039

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Example: rare 0 modes

Colour suppressed trees

Gluonic penguins

Singlet penguins

Electroweak penguins

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Theoretical predictions

Experimental dataHeavy Flavor Averaging group (Lepton Photon 2003)

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PEP II/BABAR at SLACPEP II

Asymmetric B Factory

design peak: best peak: total recorded:

x cmsx cms

fb

PEP-II/BABAR at SLAC

Luminosity records

Started construction in1994 Completed in 1999 Reached design luminosity in 2000

9 GeV e on 3.1 GeV e+

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SVTSVT

-T Solenoid

The BABAR detector

SVTSVT

EMCEMC

IFRIFR

DCHDCH

DIRCDIRC

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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.

e+

e-

e+

e-

qq

Signal B

Other B

How do we find the rare decays?

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

tagB

e

S4

0recB

eCoherent BB

production

-K

IdentifyB or anti-B

IdentifyB or anti-B

zx

y

t z/c t z/c

z

Full reconstruction of

B Ks0

Full reconstruction of

B Ks0

0SK

K+

K-

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Example of a Ks event

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Maximum Likelihood fits

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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?

Phys. Rev. Lett. 89 (2002) 201802

BABAR

World average (BABAR+Belle+…)

Heavy Flavor Averaging Group 2003

Main experimental constraints on the apex of the UT

CP violation in the kaon system

Measurements of |Vub| (b → u transitions)B and Bs mixing frequencies

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World-wide status of sin2

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• 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

Direct CP violation

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Latest interesting result: B +-

Latest results from BELLE:S=-1.00 0.21 0.07C= 0.58 0.15 0.07 hep-ex/0401029

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Summary

What are rare B decaysVarious tree diagramsSeveral penguin diagrams

Calculating themCategorisation of light mesonsThe two main theoretical approaches

Where to measure them Example: The BaBar experiment and PEPII

How to measure themMulitivariate discriminants, Particle ID, Maximum Likelihood

ResultsBranching fractions Status of Sin2Direct CP violationLatest News

Lots of new results still to comeNeed more data!

What we have covered:What we have covered:

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• Need to differentiate kaons from pions. • Crucial to the analysis of many charmless decays

– Especially important at high momenta

K,

Particle Identification