Experimental Aspects of CP Violation in B Decays : Lecture III

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Experimental Aspects of CP Violation in B Decays
Lecture III

• Vivek Sharma
• University of California, San Diego
• http//vsharma.ucsd.edu/prague/cpv.pdf

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Outline of Lecture II Yesterday

• PEP-II and KEK-B Colliders Notable features
• Detectors at the Asymmetric energy collider
• General requirements for CPV measurements
• Implementation in BaBar Belle (similar but
  different)
• General Data analysis methods
• B Meson Reconstruction Continuum background
  rejection
• B meson flavor determination B or a B ??
• Blind analysis !

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Outline of Lectures 3 4

• Lecture 3
• Three types of CP violation SM expectations in
  B Decays
• Decay amplitude Weak phase structure
• Decay asymmetry prediction in SM
• General strategy for time-dependent CP asymmetry
  measurement
• Observables that probe angle ?
• Time dependent CP asymmetry in B -gt Charmonium KS
  modes Step-by-Step
• Other modes with subdominant or dominant Penguin
• Lecture 4
• Observables that probe angle??
• Observables that probe angle ?
• Summary of current measurements
• Future prospects

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CP Violation In B Decays SM Expectations
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Decay Amplitude Weak Phase Structure in CPV

• Most B decay final states have contributions from
  both Tree and 3 Penguin (Pt,Pc,Pu) diagrams.
• All Tree diagrams (Spectator, W-exchange,
  W-Annihilation, rescattering) have same weak
  phase
• The three Pi can have different Weak and Strong
  phases
• EW penguins suppressed due to EW coupling

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B Decay Amplitude Weak Phase Structure
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Decay Amplitude Weak Phase Structure in CPV
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Decay Amplitude Weak Phase Structure in CPV
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Five Classes of B Decays For CPV
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Five Classes of B Decays For CPV
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Some Examples of Class I (b? c c s) B0 ???KS
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Another Example of Class I (b? u u d) B0 ????-
Neglecting Penguin diagram
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An Example of Class II (b? c c d) B0 ??D D-
Ignoring Penguin Diagram (?)
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CPV in Decay aka Direct CP Violation
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Observation of Direct CPV in B0?K- ?
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BaBar First Observation of Direct CPV in B decay
!
background subtracted
signal enhanced
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Confirmation of Direct CPV by Belle at ICHEP04
ACP -0.101 ? 0.025 ? 0.005
3.9s significance
_ B0? K-p
B0? Kp-
Signal2139 ?53
Combined BaBar Belle significance
5.7? Establishes CPV not just due to phase of B
Mixing (M12) Theoretical (npQCD) uncertainties
insufficient to prove or rule out NP
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Direct CPV in B- ?K- ?0
Belle
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CPV in B0 Mixing

• Occurs when Mass eigenstates?? CP eigenstates
• (q/p?1 andltBHBLgt ?0)
• The Box diagrams provide the required 2 phases
• Strong phases depend on quark masses and
• non-perturbative physics.
• Asymmetries are small and hard to calculate
  precisely

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CPV in B0 Mixing
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CPV in B0 Mixing
Time-dependent CP Asymmetry
Babar ? Search for asymmetry in same-sign
dilepton sample containing 20381 events
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CPV in B0 Mixing
BABAR 20.7 fb-1
Measurement region gt 200mm
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CPV in B0 Mixing
BABAR 20.7 fb-1
BABAR PRL 88, 231801 (2002)
So far, no experimental evidence of large CP
violation in B0 mixing
To a good approximation
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CPV In Interference Between Mixing and Decay
CP asymm. can be very large and can be cleanly
related to CKM angles
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CPV In Interference Between Mixing and Decay
Requires measurement of proper time difference
t?t between the decay of Btag and BCP. Time
dependent rates for a
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Time-Dependent CP Asymmetry with a Perfect
Detector
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Time Dependent CPV Measurement Technique

• Since the techniques of time-dependent analysis
  is common to many modes, I will now describe this
  in detail using the golden mode B0? (cc) K0
  from which CP violation in B0 decays was first
  established.
• The analysis (from 2002) based on 88 fb-1 is
  old but forms basis for all other new (2004)
  analysis results that I will present later

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CP Violation in Picture
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Sin2?? Analysis Strategy
Factorize the Time Dependent analysis into
building blocks Obtain All analysis ingredients
from DATA

• Measurements
• B/B0 Lifetimes
• B0 B0-Mixing
• CP-Asymmetries

• Analysis Ingredient
• Reconstruction of B mesons in flavor eigenstates
• B vertex reconstruction
• Flavor Tagging a b
• Reconstruction of neutralB mesons in CP
  eigenstates a b c

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Calibrating The BaBar Clock With B Meson Lifetime
Measurement
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Measurement of the B0 and B Lifetime
3. Reconstruct Inclusively the vertex of the
other B meson (BTAG)

1. Fully reconstruct one B mesonin flavor
   eigenstate (BREC)
2. Reconstruct the decay vertex

4. compute the proper time difference Dt 5. Fit
the Dt spectra
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Fully-Reconstructed B sample
Flavor eigenstates Bflav for lifetime and
mixing measurements
Cabibbo-favored hadronic decays
Open Charm decays
Neutral B Mesons
21000 signalPurity 85
Charged B Mesons
Hadronic decays into final states with Charmonium
20000 signalPurity 85
GeV
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Vertex and Dt Reconstruction

• Reconstruct Brec vertex from
• charged Brec daughters
• Determine BTag vertex from
• charged tracks not belonging to Brec
• Brec vertex and momentum
• beam spot and U(4S) momentum
• High efficiency (97)
• Average Dz resolution is 180 mm (ltDzgt bgct
  260 mm)
• Dt resolution function measured from data

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tB Measurement in BaBar
Need to disentangle resolution function from
physics !
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Dt Resolution Function
sDz

• event-by-event s(Dt) from vertex errors
• Lifetime-like bias to
• Small correlation between lifetimeand Resolution
  Function parameters

0.6 ps
Signal MC (B0)
tracks from long-lived Ds in tag
vertexasymmetric Resolution Function
Dt (meas-true)/sDt
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Lifetime Likelihood Fit

• Simultaneous unbinned maximum likelihood fit to
  B0/B samples
• Use data to extract the properties ofbackground
  events
• Mass distribution provides thesignal probability
  
• Use the events in the sideband(mES lt 5.27) to
  determine theDt structure of the
  backgroundevents under the signal peak
• 19 free parameters
• t(B) and t(B0) 2
• Dt signal resolution 5
• empirical background 12 description

B0 mES
B0 Bkg Dt
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B Lifetime Fit Results

• Worlds best measurement
• 2 statistical error
• 1.5 systematic error
• Main source of systematic error
• Parameterization of the Dt resolution function
• Description of events with large measured Dt
  (outliers)

20 fb-1
B0/ B0
B?
signal bkg
PRL 87, 201803 (2001)
t0 1.546 ? 0.032 ? 0.022 ps PDG 1.548 ?
0.032 ps t? 1.673 ? 0.032 ? 0.022 ps PDG
1.653 ? 0.028 ps t?/t0 1.082 ? 0.026 ?
0.011 PDG 1.062 ? 0.029
background
Dt (ps)
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B Flavor Mistag Knowledge From Data
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sin2b results from charmonium modes
Start with a B0 beam, slowly (compared to the
lifetime) a B0 component builds up But no Mixed
events at t0. If the detector measures some
mixed events, it must be because it has
measured the flavor of the B incorrectly (?
mistag)
B Lifetime
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Analysis Strategy (II)

• Measurements
• B/B0 Lifetimes
• B0 B0-Mixing
• CP-Asymmetries

• Analysis Ingredient
• Reconstruction of B mesons in flavor eigenstates
• B vertex reconstruction
• Flavor Tagging a b
• Reconstruction of neutral B mesons in CP
  eigenstates a b c

ü
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Measurement of B0B0 Mixing rate Vs ?t
3. Reconstruct Inclusively the vertex of the
other B meson (BTAG) ü 4. Determine flavor
of BTAG to separate Mixed and Unmixed
events
1. Fully reconstruct one B meson in flavor
eigenstate (BREC) ü 2. Reconstruct the decay
vertex ü
5. compute the proper time difference Dt ü 6.
Fit the Dt spectra of mixed and unmixed events
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Dt Spectrum of Mixed and Unmixed Events
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B Flavor Tagging Methods
Hierarchical Tagging Categories
For electrons, muons and Kaons use the charge
correlation
Each category is characterized by the probability
of giving the wrong answer (mistag fraction w)
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Flavor Tagging Performance in Data
The large sample of fully reconstructed events
provides the precise determination of the tagging
parameters required in the CP fit
Tagging category Fraction of tagged events e () Wrong tag fraction w () Mistag fraction difference Dw () Q e (1-2w)2 ()
Lepton 10.9 ? 0.3 9.0 ? 1.4 0.9 ? 2.2 7.4 ? 0.5
Kaon 35.8 ? 1.0 17.6 ? 1.0 -1.9 ? 1.5 15.0 ? 0.9
NT1 7.7 ? 0.2 22.0 ? 2.1 5.6 ? 3.2 2.5 ? 0.4
NT2 13.8 ? 0.3 35.1 ? 1.9 -5.9 ? 2.7 1.2 ? 0.3
ALL 68.4 ? 0.7 26.1 ? 1.2
Highest efficiency
Smallest mistag fraction
BABAR 29.7 fb-1
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Flavor Tagged B Meson Sample For Mixing Studies
Lepton
Lepton
Kaon
NT2
NT1
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Dt Resolution Function
Use the event-by-event uncertainty on Dt
Different bias scale factor For each tagging
category
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Mixing Likelihood Fit on Reconstructed B0 Sample
Unbinned maximum likelihood fit to flavor-tagged
neutral B sample
Fit Parameters Dmd 1 Mistag fractions for
B0 and B0 tags 8 Signal resolution
function 2 x 8 Empirical description of
background Dt 163 B lifetime fixed to the PDG
value tB 1.548 ps
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Mixing with Hadronic Sample
Bgnd mESlt5.27
Signal mESgt5.27
BABAR 29.7 fb-1
Precision measurement consistent with world
average
BABAR PRL 88, 221802 (2002)
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Dmd Measurement in Comparison With World

• Precision Dmd measurement
• 3 statistical error
• 2 systematic error dominated by MC
  correction

BaBar Measurements
World Average 0.496 0.007 ps-1
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B0 B0 Mixing Asymmetry with Hadronic Sample
Unfolded raw asymmetry
Dt ps
BABAR 29.7 fb-1
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Mixing Measurement at Belle (Hadronic Modes)
BELLE 29.1 fb-1
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CP Analysis Analysis Strategy (Step III)

• Measurements
• B/B0 Lifetimes
• B0 B0-Mixing
• CP-Asymmetries

• Analysis Ingredient
• Reconstruction of B mesons in flavor eigenstates
• B vertex reconstruction
• Flavor Tagging a b
• Reconstruction of neutral B mesons in CP
  eigenstates a b c

ü
ü
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Measurement of CP Asymmetry
3. Reconstruct Inclusively the vertex of the
other B meson (BTAG) ü 4. Determine the
flavor of BTAG to separate Mixed and
Unmixed events ü
1. Fully reconstruct one B meson in CP
eigenstate (BCP) 2. Reconstruct the decay
vertex ü
5. compute the proper time difference Dt ü 6.
Fit the Dt spectra of B0 and B0 tagged events
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CharmoniumK0 CP Sample for BABAR (02)
1506 signal candidates, purity 94
988 signal candidates, purity 55
BABAR 81.3 fb-1
(after tagging vertexing)
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Dt Spectrum of CP Events
Mistag fractions w And resolution function R
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Sin2b Likelihood Fit
Combined unbinned maximum likelihood fit to Dt
spectraof flavor and CP sample
Fit Parameters sin2b 1 Mistag fractions for
B0 and B0 tags 8 Signal resolution function
8 Empirical description of background Dt 17 B
lifetime fixed (PDG value) tB 1.548 ps Mixing
Frequency fixed (PDG value) Dmd 0.472 ps-1
tagged CP samples
tagged flavor sample
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sin2b Likelihood Fit Description
Combined unbinned maximum likelihood fit to Dt
spectra of Bflav and CP samples
Fit Parameters Main Sample
Sin2b 1 Tagged CP sample
Mistag fractions for B0 and B0 tags 8 Tagged flavor sample
Signal resolution function 8 Tagged flavor sample
Empirical description of background Dt 17 Sidebands
B lifetime from PDG 2002 0 tB 1.542 ps
Mixing frequency from PDG 2002 0 Dmd 0.489 ps-1
Total parameters 34
Global correlation coefficient for sin2b 13

• All Dt parameters extracted from data
• Correct estimate of the error and correlations

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Check null Control Sample at BABAR
Input Bflav sample to CP fit
No asymmetry expected
Sample sin2b
Bflav 0.0210.022
B 0.0170.025
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BABAR Result for sin2b (July 2002)
hCP -1
hCP 1
sin2b 0.755 ? 0.074
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Pure Gold Lepton Tags Alone
BABAR 81.3 fb-1
220 lepton-tagged hf -1 events
98 purity 3.3 mistag rate 20 better Dt
resolution
CP asymmetry is obvious !
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Systematic Errors on sin2b from BABAR
ssin2b
Description of background events 0.017
CP content of background components
Background shape uncertainties, peaking component
Composition and CP content of J/yKL background 0.015
Dt resolution and detector effects 0.017
Silicon detector residual misalignment
Dt resolution model (Gexp vs 3G, Bflav vs BCP)
Mistag differences between BCP and Bflav samples (MC) 0.012
Fit bias correction and MC statistics 0.010
Fixed lifetime and oscillation frequency 0.005
Total 0.033
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Updated (ICHEP04) sin2b results from Charmonium
Modes
BABAR
BABAR
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Belle Results on sin2b from Charmonium Modes
Belle 2003
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Lessons From sin2? Measurement With B0??K0

• In 2001, CP Violation in B system was discovered
  in this mode by BaBar and Belle. It was the first
  instance of CPV outside the Kaon system.
• It was also the first instance of a CPV effect
  which was O(1) in contrast with the Kaon system
  and confirms the conjecture of Kobayashi
  Maskawa made in 1972 for CPV phenomenon. It
  excludes models with approximate CP symmetry
  (small CPV).
• In 2004 sin2? is a precision measurement (5) and
  agrees well with the constraints in the ??-?
  plane from measurements of the CKM magnitudes.
• Now it appears unlikely that one will find
  another O(1) source of CPV and the enterprise now
  moves towards looking for corrections rather than
  alternatives to the SM/CKM picture
• Focus now shifts to measurements of
  time-dependent asymmetries in rare B decays which
  are dominated by Penguin diagrams in the SM and
  where New Physics could contribute to the
  asymmetries

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sin2? From Penguin Modes B0??K0
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CP Asymmetry In Penguin Modes B0??K0
Analysis based on 227 Million BB pairs
Sample orthogonal to the non-resonant B?KKK0 data
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CP Asymmetry In Penguin Modes B0??K0
B0??KS
B0??KL
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CP Asymmetry In Penguin Modes B0??K0
fKS
Nsig139 ?14
fKL
purity 0.63
Nsig 36 ?15
purity 0.17
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CP Asymmetry In Penguin Modes B0??K0
f K0
Good tags
Poor tags
Good tags
fKS fKL S (fK0) 0.06 0.33 0.09
C (fK0) -0.08 0.22 0.09
2.2s away from SM
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CP Asymmetry In Penguin Modes B0??/ K0
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CP Asymmetry In Penguin Modes B0??/ K0
Belle 274M BB
S 0.65 ? 0.18 ? 0.04 C 0.19 ? 0.11 ?
0.05
sin2? cc _at_ 3.0?
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Results on sin2b from s-penguin modes
All new!
All new!
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Summary of sin2?eff
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World Averages for sin2b and s-penguin modes
No sign of Direct CP in averages
Beginning to look suspicious but must wait for
5?/expt to get exciting
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Projections for Penguin Modes
f0KS KSp0 jKS hKS KKKS
Luminosity expectations
2004240 fb-1 20091.5 ab-1
Kg
Similar projections for Belle as well
5s discovery region if non-SM physics is 30
effect
2009
2004
Projections are statistical errors only but
systematic errors at few percent level
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PEP II Luminosity Projections
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CP Asymmetries in b?c cd Modes
Statistics limited, may get interesting in about
2 years !