SCIPP%20High%20Energy%20Physics%20Seminar%20The%20Rarest%20B%20Decay:%20The%20Electroweak%20Penguin%20Process%20b%20?%20s%20l %20l-%20at%20the%20BaBar%20Experiment

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SCIPP High Energy Physics SeminarThe Rarest B
Decay The Electroweak Penguin Process b ? s l
l- at the BaBar Experiment

Jeffrey Berryhill University of California,
Santa Barbara February 27, 2006
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Quarks and the problems of mass and flavor

• The different fermion generations, their masses,
  their flavor violation and CP violation are all
  arbitrary parameters of the Yukawa sector.
• Many mass and flavor-violating parameters
  mysteriously spanning many orders of
  magnitude
• Encodes many rules to be (null) tested
  experimentally
• suppressed flavor-changing neutral currents
• Single phase for CP violation
• CKM unitarity
• New physical degrees of freedom generically
  violate these rules! ?
• Quark flavor transitions could exhibit
  spectacular failures for SM
• Could flavor physics be the Standard Model killer?

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Three Paths to Quark Flavor Violation

• Tree diagram decay down ? up (b?c, b?u)
• Not generally sensitive to new physics
• 2. Box diagram neutral meson mixing
  (K0,Bd0,Bs0)
• down-type ? anti-down type (b?b)
• via double W exchange
• Penguin diagram (b?s, b?d)
• down-type ? down-type
• via emission reabsorption of W
• top-quark couplings Vtd, Vts dominate

g,Z,g
SM penguins are suppressed new physics can
compete directly!
4th gen. quarks, technicolor, LED, etc., etc.,
with possibly enhanced flavor couplings
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PEP-II performance

• PEP-II top luminosity 10.0 x 1033cm-2s-1
  (more than 3x
  design goal 3.0 x 1033)
• 1 day record 711 pb-1
• About 2-3 Amps of current per beam, injected
  continuously
• Run1-4 data 1999-2004
• On peak 211 fb-1
• Today 295 fb-1
• s(ee- ?Y(4S)) 1.1 nb ?
• 229M Y(4S) events produced

650 million b quarks produced!
Also 108 each of u, d, s, c, and t
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The BaBar detector
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Penguin Portrait B? Kg Candidate
Muon from other B decay
High energy photon
Ks ? pp- and p

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Interesting Hint Sin 2b in Trees vs. Penguins
Naively averaging over many penguin decays World
discrepancy with tree decays -2.8 s


Is a new CP violating phase present in gluonic
penguins?
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The BaBar Radiative Penguin Group

• Radiative Penguin Group
• 20 active analysis topics
• 15 BaBar publications, 10-15 planned for 2006.
• Scope Measurements of radiative FCNC decays of
  Bs
• b ?s g b ?d g b ?s l l- b ?d l l-
• Related rare decays
• Branching fractions, spectral distributions,
  asymmetries
• Order one sensitivity to new physics with
  precision SM predictions
• Highlights
• b ?s g decays precision rate, mb, Vub, null
  tests of CPV
• b ?d g rate constraints sensitive to Vtd/Vts
• B? K()ll, first look at complete set of
  electroweak penguins

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b? s g Total Branching Fraction (Eg gt 1.6
GeV)
b? s g Total Branching Fraction (Eg gt 1.6 GeV)

10 measurements agree with 10 NLO SM
predictions Improvements of both to 5 feasible
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b ?s g New Physics Sensitivity

• Constraints on Two Higgs doublet models (Type
  II) charged Higgs
• M(H) lower limit far above LEP2/Tevatron
  limits!
• SUSY conspiracy can
• degrade limit
• Also generally constrains top quark EW couplings

current BABAR data
90 C.L. allowed (above lines)
combined
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b? sg Asymmetries
Direct CP asymmetry generally constrained to lt
5 Isospin asymmetry precision 5 All
measurements statistics limited for forseeable
future
Little room for large new weak phase in photon
penguins!

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The Physics of b ? s l l
Three separate FCNC diagrams contribute? multiple
ways for new physics to interfere Three body
decay? non-trivial kinematic and angular
distributions sensitive to magnitude and phase of
different amplitudes Rare decay? 10-6
branching fractions approach limits of B-factory
sensitivity

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The Physics of b ? s l l
b? s ll rate computed from operator product
expansion with as and L/mb corrections Wi
lson coefficients Ci encode short distance
physics (order as) C7 photon penguin from b ?
s g C9 vector EW C10 axial vector EW unique
to b? s ll
general Hamiltonian of b?s transitions
Also sensitive to Ci, CS, CP
Axial vector
BF(b?smm) 4.2 0.7 10-6
15 uncertainty in NLO inclusive rate

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The Physics of B ? K l l, Kl l
Dominant exclusive decays are B ? Kll, Kll K
ll, Kll rates computed from (perturbative) b? s
ll amplitude convolved with (non-perturbative)
B? K, K form factors for each Oi

30 form factor uncertainty
(Light-cone QCD sum rules)
et al.
et al.
7 B ? K form factors
3 B ? K form factors
BF(B?Kmm) 0.35 0.12 10-6
BF(B?Kmm) 1.2 0.4 10-6
Exclusive branching fractions not a precision
test of the SM ? Asymmetries and distributions
are higher precision tests (form factor
uncertainty cancels) EX Direct CP asymmetry
ACP 10-4 in SM, could be order 1 beyond SM

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Dilepton Mass Distribution
J/Y K()
Dilepton mass distribution probes Wilson
coeffcients Pole at q2 0 for Kee (nearly
on-shell B?Kg) Huge long-distance
contribution from B ?charmonium decays
Y(2S)K()
Dilepton q2

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B ? K l l, Kl l Measurement (208 fb-1)
Find 50 needles in a haystack of 500 million Bs
2 billion light quarks Full B decay
reconstruction to charged tracks (brem photons
added for ee modes) B? Kll B?Kll B
?K e e- B0?Ks e e- B?K e e- B0?K0
e e- B ?K m m- B0 ?Ks m m- B?K m
m- B0?K0 m m- Ks?p
p- K?Ksp K0?Kp- Strict particle ID
requirements Veto peaking backgrounds of B
decays similar to signal Construct multivariate
discriminants to suppress combinatorial
backgrounds Signal yield extraction via
multi-dimensional unbinned maximum likelihood
fit
BLIND ANALYSIS

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Killer App of the Y(4S) precision kinematic
constraints

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Particle Identification
0.3 GeV
0.7 GeV


m, K fake rates lt 2 e fake rate lt 0.1 Huge
control samples for efficiency and misid
studies (eeg, mmg, L? pp, KS? pp, D? K)
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Charmonium Background
Huge (100 times signal) J/y K() and y(2S) K()
background eliminated with
2D veto on dilepton mass
and DE
Mismeasured mass correlated with DE Bigger veto
for electron modes (Bremsstrahlung tail) Energy
loss and tracking response to leptons
well-calibrated in MC (checked with huge
radiative Bhabha and mm samples) Reduced to lt 1
event expected per mode
Veto sample is huge control sample for
validating signal efficiency!

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Continuum Background Suppression
linear combination of variables for which
multi-dimensional gaussian dist. of signal and
background are maximally separated Signal shape
from MC background shape from off-resonance data

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Continuum Suppression
Kaon-lepton mass in Fisher reduces large
background from semileptonic D decays
Background cuts off at D mass
Large charmonium samples validate signal shape
sidebands validate background shape

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Semileptonic B Decay Background
Large fraction (10) of Bs decay to leptons?
large combinatorial B background Separation of
B background from signal using likelihood
function (product of shapes) Missing energy B
production angle Vertex probability Likelihood
shapes from MC
Signal vs. Background missing energy
Cut values scanned For maximal S2/(SB)
Sideband data
Charmonium data
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Hadronic B Decay Background
B decays to hadrons misidentified as muons will
have same kinematics as signal misid 1-2 for
pions and kaons K mm, Kmm events with Km mass
consistent with D decay vetoed Non-resonant
B?K()pp background estimated from
data Convolve misid rates with B ?K()mp events
in data Extract background from a binned fit to
weighted mES distribution

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Maximum Likelihood Fit
Unbinned maximum likelihood fit of (mES, DE,
m(Kp)) maximizes LH function Components
signal, peaking backgrounds, combinatorial
background Pi Pi(mES)Pi(DE)Pi(m(Kp))
(negligible correlation) Signal shape
parameters fixed from charmonium data Signal
yield, background yield and background shape
parameters are floating in fit
K0ee MC
Signal shapes (narrow peaks)
Kem data

Background shapes (not peaked)
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Fit Validation Charmonium
KsJ/y(ee) data
Feed-down from J/y K
Test fit procedure on charmonium J/y K()
branching fractions agree with PDG ACP consistent
with 0 (bounds detector bias) Signal and
background well-modeled by PDFs Also Y(2S) K()
branching fractions K()em Lower-dimensional
and/or smaller range fits Sideband yields agree
with MC
K0J/y(ee) data

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Kll Fits

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Kee- Candidate

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Kll Fits
Kee
Kmm
Ksee
Ksmm
Eff. Syst.
Fit Syst.

Kll modes consistent
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Kll Combined BF
Simultaneous fit to four individual decay
modes Partial rates constrained to same value
Kll signal
Kll feed-down

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Kll Sidebands
Fit describes background shape well

31
slide
Kll Fits

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Kll Fits
K0ee
Kee

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Kll Fits
Kmm
K0mm

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Kll Combined Branching Fraction
Simultaneous fit with constraint G(B?Kmm)/G(B?K
ee)0.752 (Kll BF K0mm BF) Alternatively
, with pole region removed and equal partial
rates

35
Kll Sidebands

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SUSY Higgs physics at a B Factory
b? smm Bs ? mm turned sideways Sensitive to
neutral Higgs penguin for SUSY with large tan
b Ratio R(K) BF(B? Kmm)/BF(B?Kee) isolates
Yukawa enhancement in muon mode In SM, equal to
unity with very high precision Also contributes
to R(K) (1 above the photon pole)
Complementary to Tevatron Bs?mm limit
RK limit of 1.2 Bs?mm limit of lt 10-6

Hiller Kruger hep-ph/0310219
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Ratios of BFs and ACP
ACP measurements consistent with 0
Ratio of Kmm/Kee BFs consistent with unity
Ratio of Kmm/Kee BFs consistent with SM
(0.752)
Ratio of Kmm/Kee BFs (excluding pole)
consistent with unity
Dominant systematic from unknown asymmetry of
peaking background

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B ? K()ll Total Rates
BaBar and Belle branching fractions agree with SM
predictions Experimental uncertainty already
better than theory Difference between BaBar and
Belle becoming significant?
difference 2.6s
difference 1.9s
RAREST B DECAYS EVER OBSERVED

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New! B? Kll Angular Distributions
B? Kll decay kinematics uniquely described by 3
angles dilepton mass (Q2) Most relevant for
probing underlying penguin amplitudes Cos q
lepton- angle in dilepton rest frame.
Forward-backward asymmetric! Cos qK kaon angle
in K rest frame. Gives K polarization
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B? Kll Angular Distributions
Low Q2
High Q2
Asymmetry in cos q vs. dilepton q2 probes
interference between 3 b?sll penguin amplitudes.
Large distortions possible from new physics
amplitudes! In SM LOW Q2 averages to small
positive asymmetry HIGH Q2
uniformly large positive asymmetry
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FT 1 F0
Kll Angular Fits

• Kll AFB measured in two bins of q2 with 2 Kll
  modes
• Null test of fit procedure for Kll

Kll Angular Fits

• F0 measured in q2 bins with 4d (mES, DE, m(Kp),
  cos qK) fit
• Fix F0, measure AFB in q2 bins with 4d (mES, DE,
  m(Kp), cos q) fit
• All procedures validated by control samples and
  toy MC studies

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Kll AFB Projected Uncertainties
If high Q2 measurement agrees with SM, should
exclude wrong sign C9C10 If low Q2 measurement is
maximally asymmetric, would exclude SM at close
to 3 sigma
Stay Tuned for Moriond EW
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Summary

• b? s g penguins beginning to crack the standard
  model?
• Decay rate measurements of b? sg penguins are
  well into the precision era.
• Decay rate measurements of b? d g penguins are
  well into the precision era.
• Ultimate b ?sll penguin will ultimately
  disentangle all electroweak
• penguin effects, SM or not.
• B?Kll ratios and Kll angular asymmetries are
  high precision tests of the
• electroweak scale and beyond

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• Penguin decays are strongly challenging
• the Standard Model!