Observation Of CP Violation in B Decays with the BaBar Detector

1
Observation Of CP Violation in B Decays with the
BaBar Detector

• Vivek Sharma
• University of California, San Diego
• (On Behalf of the BaBar Collaboration)

FNAL Joint Theory Experiment Seminar
2
FNAL b quark Physics started here !
3
Outline Of This Talk

• CP Violation, CKM Matrix and the Unitarity
  Triangle
• CP Violation in B Decays The Three
  possibilities
• Observation of CP Violation in the interference
  of Decay and Mixing ? Sin2b
• The PEP-II B Factory The BaBar Detector
• The three linked steps towards the sin2b
  measurement
• B Lifetime
• B Mixing
• CP Asymmetry
• The Way forward Summary and Outlook

4
CP Violation Inner Space, Outer Space

• Search for CP Violation mechanism has been a
  concern of particle physics since its discovery
  in the KL system
• 3 Generation CKM mixing matrix (via the phase)
  provides an elegant explanation for this effect
  which needs to be probed critically and cleanly
• The B meson system is an excellent new laboratory
  for understanding testing the mechanism behind
  CP Violation

SM via CKM phase does incorporate enough CPV to
explain cosmic matter-antimatter asymmetry
So, if there are beyond the SM phenomena, their
effect may be measurable in B system
5
CP Violation in the Standard Model
CKM Matrix
Complex matrix described by 4 independent
parameters
Wolfenstein parametrization
phase
CP Violation
6
Unitarity Triangle
Choice of parameters
Rt
g
Ru
b
7
Three Forms of CP Violation in B Decays

• Direct CP Violation
• Total amplitude for a decay and its CP conjugate
  have different magnitudes
• Difficult to relate measurements to CKM matrix
  elements due to hadronic uncertainties
• Relatively small asymmetries expected in B decays
• CP Violation in Mixing
• Would give rise to a charge asymmetry in
  semi-leptonic decays
• Expected to be small in Standard Model (DG ltlt
  DM)
• CP Violation in the interference of mixed and
  unmixed decays
• Typically use a final state that is a CP
  eigenstate (fCP)
• Large time dependent asymmetries expected in
  Standard Model
• Asymmetries can be directly related to CKM
  parameters in many cases, without hadronic
  uncertainties

ü
8
CP Violation in interference between Mixing and
Decay
9
CP from Interference of Mixing and Decay
Define Time-dependent CP Observable
10
The Golden Decay Mode B0 J/y K0S
K0 mixing

• Theoretically clean mode to measure sin2b
• Clean experimental signature
• Large branching fraction compared to other CP
  eigenstates

Golden Modes

• hCP -1
• B0 ? J/? K0S
• B0 ? ?(2s) K0S

Time-dependent CP asymmetry

• hCP 1
• B0 ? J/? K0L

11
Decay Time Distribution in B ?fCP
12
Decay Time Evolution ACP for B0 J/y K0S

• t spectrum and the observed asymmetry for a
  perfect detector(assuming sin2b 0.6)
• Visible difference between B0 and B0 decay rates

• In this ideal case, the amplitude of the
  oscillation is the CP Asymmetry
• time-integrated asymmetry is 0

t
13
Exptal Requirements For CPV Measurement

• BR (B? fCP) 10-4 ? Need to record and
  reconstruct a large of B Mesons
• Determine the flavor of the initial B meson to
  separate B0 from B0 ( B Flavor Tagging)
• Define and measure a time in order to study the
  time-dependent asymmetry
• B Mesons must travel a measurable distance before
  decaying
• Vertex Reconstruction A high precision tracking
  system to measure the distance between the B
  decay points

BaBar Detector _at_ PEP-II B Factory as example
14
The Asymmetric Energy Collider _at_ U(4S) PEP-II
Cleanest source of B0 mesons
BB threshold
Dt proper time difference between the two B
decays
PEP-II BABAR
Off
ACP(Dt) integrates to zero over all Dt
On
15
PEP-II Asymmetric Energy B-Factory at SLAC
Collides 9 GeV e- on 3.1 GeV e U(4S) boost in
lab frame bg 0.56
16
PEP-II Performance Has Been Spectacular !
Records from this week!

• PEP-II top luminosity
• 4.21 x 1033cm-2s-1
  (design 3.0 x 1033)
• Top recorded Lumi/week 1.4 fb-1
• Top recorded Lumi/24h 282 pb-1
• Top recorded Lumi/8h 96 pb-1
• BABAR logging efficiency gt 96

30/fb analyzed for CP
October 3, 2001
October 99
PEP-II delivered 50.6 fb-1 BABAR recorded
48.0 fb (includes 5.15 fb-1 off peak) 90 million
Bs recorded, being analysed !!
17
The BaBar Detector
Electromagnetic Calorimeter 6580 CsI(Tl) crystals
1.5 T solenoid
e (3.1 GeV)
Cerenkov Detector (DIRC) 144 quartz bars 11000
PMTs
e- (9 GeV)
Drift Chamber 40 stereo layers
Instrumented Flux Return iron / RPCs (muon /
neutral hadrons)
Silicon Vertex Tracker 5 layers, double sided
strips

• SVT 97 efficiency, 15 mm z hit
  resolution (inner layers, perp. tracks)
• SVTDCH?(pT)/pT 0.13 ? pT 0.45
• DIRC K-? separation 4.2 ? _at_ 3.0 GeV/c ?
  2.5 ? _at_ 4.0 GeV/c
• EMC ?E/E 2.3 ?E-1/4 ? 1.9

18
B Event Topology at the Boosted ?(4S)
19
Sin2? Analysis Strategy
Factorize the time-dependent analysis in 3
building blocks Obtain All analysis ingredients
from DATA (not MC)

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

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

20
Measurement of the B0 and B Lifetime
3. Reconstruct Inclusively the vertex of the
other B meson (BTAG)

1. Fully reconstruct one B mesonin self tagging
   (BREC)
2. Reconstruct the decay vertex

4. compute the proper time difference Dt 5. Fit
the Dt spectra
21
Fully-Reconstructed Hadronic B Decay sample
Flavor Eigenstates Bflav for lifetime and
mixing measurements
Self-tagging hadronic decays Open
Charm decays
Neutral B Mesons
30 fb-1
Charged B Mesons
Hadronic decays into final states with Charmonium
GeV
22
Recoil (Tag) side Vertex and Dz 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 characterized from data

23
tB Measurement at Boosted ?(4S) Unique
Need to disentangle resolution function from
physics
24
Dt Resolution Function
sDz

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

0.6 ps
Signal MC (B0)
tracks from long-lived Ds in tag
vertexasymmetric Resolution Function
Dt (meas-true)/sDt
25
B 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
26
B Lifetime ResultsCalibrating The BaBar Clock
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
20 fb-1
B0/ B0

• Precision 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)

B?
signal bkg
background
Dt (ps)
To Appear in PRL
27
Sin2? Analysis Strategy (Part II)

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

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

ü
28
B0B0 Mixing with Fully Reconstructed B Mesons
3. Reconstruct Inclusively the vertex of the
other B meson (BTAG) ü 4. Determine the
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
29
Dt Spectrum of Mixed and Unmixed B Events

_
w the fraction of wrongly tagged
events Dmd oscillation frequency
30
Extraction of Dmd and mistag fraction
Fraction of Mixed Events
Sensitive to Dmd measurement when the amplitude
of the oscillation is at its maximum
31
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)
32
B Flavor Tagging Performance Using B Mixing
The large sample of fully reconstructed hadronic
B decays provides the precise determination of
the tagging parameters required in the CP fit
Tagging category Fraction of tagged events e () Wrong tag fraction w () Q e (1-2w)2 ()
Lepton 10.9 ?0.3 8.9 ? 1.3 7.4 ? 0.5
Kaon 35.8 ?0.5 17.6 ? 1.0 15.0 ? 0.9
NT1 7.8 ?0.3 22.0 ? 2.1 2.5 ? 0.4
NT2 13.8 ?0.3 35.1 ? 1.9 1.2 ? 0.3
ALL 68.4 ?0.7 26.1 ? 1.2
The error on sin2b the quality factor Q
Smallest mistag fraction
Highest efficiency
33
Dt Resolution Function
Use the event-by-event uncertainty on Dt
Different bias For each tagging category
34
Mixing Likelihood Fit
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(scale factor,bias,fractions) 9 Empirical
description of background Dt 16 B lifetime
fixed to the PDG value tB 1.548 ps
35
B0B0 Mixing Fit Result
20 fb-1
C.L. 28
Dmd 0.519 0.020 (stat) 0.016 (syst) h ps-1
Preliminary
36
Dmd Measurement in Comparison
preliminary

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

37
Sin2? Analysis Using (I) and (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
ü
38
Measurement of CP Asymmetry Sin2?
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
39
The fully Reconstructed CP Sample
Before tagging requirement
Sample tagged events Purity CP
J/?, ?(2S), cc1 KS 480 96 -1
J/? KL 273 51 1
J/? K0(KSp0) 50 74 mixed
Full CP sample 803 80
After flavor tagging
40
Dt Spectrum of CP Events
CP PDF
Mistag fractions w And Resolution function R
41
Sin2b Likelihood Fit
Combined unbinned maximum likelihood fit to Dt
spectra of flavor and CP sample
Fit Parameters Sin2b 1 Mistag fractions for
B0 and B0 tags in each Cat. 8 Signal resolution
function 16 Empirical description of background
Dt 20 B lifetime fixed to the PDG value tB
1.548 ps Mixing Frequency fixed to the PDG
value Dmd 0.472 ps-1 Global correlation
coefficient for sin2b 13 Different Dt
resolution function parameters for Run1 and Run2
tagged CP samples
tagged flavor sample
42
Blind analysis !

• The sin2b analysis was done blind to
  eliminate possible experimenters bias
• The amplitude in the asymmetry ACP(Dt) was
  hidden by arbitrarily flipping its sign and by
  adding an arbitrary offset
• The CP asymmetry in the Dt distribution was
  hidden by multiplying Dt by the sign of the tag
  and by adding an arbitrary offset
• The blinded aproach allows systematic studies
  oftagging, vertex resolution and their
  correlations to be done while keeping the value
  of sin2b hidden
• The result was unblinded 1 week before public
  announcement this summer!

43
Improvements Between Run1 and Run2

• First publication in March 2001
• Changes since then
• More data (run 2) 23 ?32 BB pairs
• Significantly (30) improved reconstruction
  efficiency in Run 2
• Optimized selection criteria takes into account
  CP asymmetry of background in J/?KL
• Additional decay modes ?C1KS and J/?K0
• Better alignment of Tracking system ( Kalman )
• Improved vertex resolution for reconstructed and
  tag B
• Statistical Power of the analysis almost doubled
  w.r.t March Publication

sin(2b) 0.34 0.20 (stat) 0.05 (syst)PRL 86
(2001) 2515
44
Raw CP Asymmetry in Clean Charmonium Modes
All tags
Kaon tags
In ?f -1 events
sin2b0.56 0.15
sin2b0.59 0.20
Raw ACP
Raw ACP
45
Raw CP Asymmetry for J/y KL
Backgroundcontribution
46
Sin2b Results July 5th, 2001
Phys. Rev. Lett. 87 091801 (2001)
CalibrationNull result in flavor samples
Combined fit to all modes
Sin2b 0.59 0.14
Consistency of CP channels P(c2) 8
Goodness of fit (CP Sample) P(LmaxgtLobs) gt 27
47
Run1 Run2 Comparison
Run1-Run2 change for PRL modes 1.8s
Run1
Run2
48
Consistency Checks
sin2b measured in several Dt bins
Combined CP-1
sin2b vs. J/? decay mode and tagging category
and flavor for ? -1 events
49
CP Asymmetry Corrected For B Oscillation
Sin 2b value, fitted in bins of Dt
sin 2b, fitted in bins of Dt and multiplied by
sine(Dm Dt)
50
Major Sources of Systematic Error in Sin2b
Measurement is Statistics Dominated
Error/Sample KS KL K0 Total
Statistical 0.15 0.34 1.01 0.14
Systematic 0.05 0.10 0.16 0.05

• Signal resolution and vertex reconstruction
  0.03
• Resolution model, outliers, residual misalignment
  of the Silicon Vertex Detector
• Flavor Tagging 0.03
• possible differences between BCP and Bflavor
  samples
• Background Characterization 0.02
  (overall)
• Signal probability, fraction of B background in
  the signal region, CP content of background
• Total 0.09 for J/Y KL channel 0.11 for J/Y K0
• Total Systematic Uncertainty 0.05
  for total sample

51
Search for Direct CP Violation
Without SM Prejudice
If more than one amplitude present then l might
be different from 1
To probe new physics (only use hCP-1 sample
that contains no CP background)
l 0.93 0.09 (stat) 0.03 (syst)
No evidence of direct CP violation due to decay
amplitude interference (SCP unchanged in Value)
52
Observation of CP Violation In B Meson System
Probability of obtaining observed result if
CP is an exact symmetry (? No CPV)
Full Sample
No evidence for direct CPV (Sine term unchanged
in the fit)
53
The Unitarity Triangle and This Measurement
One solution for b is consistent with
measurements of sides of Unitarity Triangle
BaBar sin2b (with 30/fb)
Error on sin2b is dominated by statistics ? will
decrease as
Example Höcker et al, hep-ph/0104062 (also
other recent global CKM matrix analyses)
54
Summary Of Time-Dependent Measurements
55
Luminosity Projection to Summer 2002
Project 100 fb-1 by Jun 2002
Hope to analyze Data very Quickly As
demonstrated Already
We are Here
56
Luminosity Plans for BABAR PEP II
Expect 550 fb-1 By 2006
57
Prognostications on Future Sin2b Precision

• In the Charmonium Modes
• Add more sub-modes drops in the bucket
• Select ?? hadrons, not just ?? e e- or m m- ,
• smarter event selection (bremstrahlung recovery)
• Expect for charmonium modes
• Add new CP modes
• b? sss ? B ? fKS
• Compare with sin2b from b? c c s
• Cabibbo Suppressed B ? ? p0
• Look for difference in sin2b measured from b?
  ccs
• bound u-quark penguin pollution
• Cabibbo suppressed b? ccd ? B ? D() D(-)
• May contain (small but unknown) penguin pollution
  
• DD mode requires angular analysis (in progress)
  

58
New Modes for Sin2b 20 fb-1
Go Back
32 events
11 events
B ? ? p0
59
CP violation in B0 ? pp- decays
Decay distributions f(f-) when tag B0(B0)
penguin diagram
tree diagram
For single weak phase
For additional weak phase
? ? 1 ? must fit for direct CP Im (?) ? sin2?
? need to relate asymmetry to ?
Cpp 0, Spp sin2a
Cpp ? 0, Spp sin2aeff
60
CP Sample B0 ? pp-
For Illustration purposes Events after
likelihood ratio cuts
23 pp2 Kp
20 pp1 Kp
L 30.4 fb-1
Total Yields (fit)
126 Kp3 pp
139 Kp3 pp
Lepton Photon 2001
61
CP Asymmetry Fit and Results
Preliminary Results (PRD Bound)
Observation of CP Asymmetry (time Dependent or
in Decay) Will be a Major Achievement !
62
BaBar Aim Multiple Measurements and Tests to
Overconstrain the Unitarity Triangle
Sin2b is just one focus of BaBar Work in
progress on Many Fronts An
Exciting era of B physics in Progress !

63
Summary Of Time-Dependent Measurements
64
Backup Slides
65
Consistency Check Run1 vs. Run2
Difference for modes used in the old PRL 1.8 s
Run 1
Run 2
66
Improved Particle Reconstruction
Y(2S) Ks(pp-)
ccKs(pp-)
J/Y K0(Kp-)
J/Y Ks(p0p0)
J/Y Ks(pp-)
cc K
Y(2S) K
J/Y K
J/Y K non CP
Run2/Run1
KS Golden modes 30 larger than run 1
efficiency improved
67
Additional Channels J/YK0(KSp0)

• Improved understanding of the background and its
  effective CP

(Angular analysis paper about to be submitted)
55 signal events before tagging 37 after
68
Improved KL Selection

• Original analysis was optimized for S2/(SB)
• Fine for BF measurements, but not for CP
• Need to optimize accounting for the background
  asymmetry (SAB/ASB)2/(SB)
• Re-optimized with Monte Carlo
• Expect 10 improvement on the error

69
Resulting KL Yields Run1

• For data the improvement is better than expected

Old
New
In the IFR selection the signal yield has not
changed while the background is halved
Run1
70
CP Sample Non-KL Modes
Present Sample 725 PRL Sample 425
Before tagging and vertexing requirements
NNOW672
71
CP Sample J/Y KL
Run1Run2
N/p() EMC IFR
Run1 77/52 96/68
Run2 49/59 32/55
Run1Run2 128/56 129/65
72
Improved Vertex Performance

• We expect some vertex improvements in Run2 from
• Better use of layer 1 SVT hits in the Kalman fit
• Better SVT internal alignment

Improvement in resolution leads to
3-4 on sin2b error
run2
run1
NEW
OLD
s(sin2b)
73
Tagging Performance from Data

• Obtained from mixing fit to data samples

NT2
K
L
L
NT1
K
NT1
NT2
Q eD2 26.1 ?1.2
74
Likelihood Fit Method

• Global unbinned maximum likelihood fit to data
• Mistag rates, Dt resolutions tagged flavour
  sample
• sin2? tagged CP samples
• 45 parameters for mistag rates, ?t resolution
  backgrounds floated to obtain an empirical
  description from data

Separate Dt resolutions for run1 and run2
Largest correlation with sin2?? 13
tB 1.548 ps and Dmd 0.472 ps-1 fixed
75
The New World Average
New sin2b world average is 8s significant
Measurements assumed to be uncorrelated
76
Run1 Run2 Comparison

• Change in central value 1.8s in uncorrelated
  error
• 30 efficiency improvement for all KS modes
• 15 improvement due to vertexing/alignment

77
Silicon Vertex Detector (SVT)
Dipole permanent magnet (?21 cm from I.P.)
Readout chips
Beam pipe (Beryllium) 1 R.L.
Layer 1,2
Layer 3
Layer 4
Layer 5
78
SVT precise B vertex, Dz measurement
200 mm

• 5 Layer AC-coupled double sided silicon detector
• SVT Located in high radiation area
• Radiation hard readout electronics (2Mrad)
• Up to 98 hit reconstruction efficiency
• Hit resolution 15 µm at 00

?
79
Drift Chamber (DCH)

• 40 axial and stereo layers inside
• 1.5 Tesla magnetic field
• 8020 heliumisobutane
• Measurement of charged particle
• momentum and ionization
• loss dE/dx for PID

• Track reconstruction efficiency 98 for pgt200
  MeV/c,
• ?gt500 mrad, and nominal
  DCH voltage.
• PID up to p0.7 GeV/c ?(dE/dx) 7.5 (Bhabha)
• SVT DCH impact parameter resolution
• 65 µm in z
• 55 µm in transverse plane
• at p1.0 GeV/c

Reconstruction of the decay J/????- in selected
BB events. Mass resolution 11.4 MeV/c2.
80
PID performance, p0 reconstruction
EMC e,g,p0 ID
Muons
Electrons
Egggt300MeV
Mgg (GeV/c2)
81
Ring imaging Cherenkov detector (DIRC)

• DETECTION OF INTERNALLY REFLECTED CHERENKOV
  LIGHT
• 144 synthetic fused silica radiator bars
• Photons transmitted by internal reflection
• Rings expand in standoff region (1.2m distance,
  6000 l purified water)
• Detected by 11000 conventional PMTs
• Essential for PID 0.7-4.3 GeV/c

• Typical performance
• number of detected photons 20-50
• average track qc resolution 2.4mrad (in
  ee? ?? events, 39 GeV/c)

82
Tight Kaon ID
83
B0 ? pp- Asymmetry Result
Decay distributions f(f-) when tag B0(B0)
Preliminary
A N(K-p)-N(Kp-)/N(K-p)N(Kp-)

• Measurement compatible with no CP in B0 ? pp-
• Statistically limited due to small branching
  fraction
• Need 500/fb for s(Spp) 0.10-0.15

84
B Lifetime Systematic uncertainties
Systematic effect ??0 (ps) ??? (ps) ?(??/?0) Comment
MC statistics 0.009 0.006 0.006 stat. limitations of MC validation studies
Resolution parameterization 0.011 0.009 0.003 some included in stat. error (free parameters), study different parameterizations, SVT alignment algorithm
Common resolution parameters 0.004 0.005 0.006 different resol. parameters for charged and neutral B (different D0/D mix)
Beam spot 0.002 0.002 cancels propagate errors on BS position and size
?t outliers 0.011 0.011 0.005 vary mean and width of outlier PDF
Absolute z scale 0.008 0.008 cancels absolute z scale estimated to better than 0.5 using secondary interactions in beam pipe
Boost 0.006 0.006 cancels propagate errors on PEP-II boost
Signal probability 0.003 0.003 0.003 propagate errors from mES lineshape fit
Background modeling 0.005 0.009 0.003 compare ?t distribution of background events in signal region and in sideband wrong-charge contamination of signal
Total in quadrature 0.022 0.022 0.011
85
Systematic Error Absolute Z Length Scale
Estimation of the absolute z scale from
measurement of length of Be beam pipe (
Tantalum foil wrapped around it ) using off-beam
electroproduction reactions therein.
Use the beam pipe as a ruler. The beam pipe
radius increases at two points in z close to its
extremities.
ltrgt (mm)
The distance between these two points is known
from an independent measurement.
z (mm)
86
B Lifetime ?t Resolution Parameters
Parameterization of resolution function f G(s)
(1- f) G(s) ? E(k) (3 parameters)
87
PEP-II asymmetric collider
E(e-) 9.0 GeV, E(e) 3.1 GeV

• Asymmetric collider operating at the ?(4S)
  resonance

bg 0.56
88
Is it possible to measure a large asymmetry ?

• The answer is yes! Suppose at a given time t
  you have
• Nevents lt 0 is possible in a likelihood fit
• The signal PDF can be negative in some regions
• Requires having NO OBSERVED event in those
  regions
• The only constraint on the PDF is the
  normalization

89
Large sin2b in B0 ? ?C1KS

• fit for B0/B0 Dt PDFs, not for ACP
• Large sin2b possible , because
• No events where PDFlt0 (eg. lepton tags)
• Sum of signal background PDFs positive (eg.
  Kaon tags)
• Note a single lepton B0-tag at Dt -p/2Dm
  would bring sin2b from 2.6 to 1/(1-2wlep) ? 1.1
• Measure sin2b unbiased for low stat. samples and
  probability to obtain sin2b?2.6 when true value
  0.7 is 12

Kaon tags
Lepton tags
B0 tags
Dt ps
Dt ps
90
CP Data Sample Likelihood Fit
9741 two-prong candidates in 30.4 fb-1 (97
background, almost entirely from continuum)
lepton
kaon
Sum of pp-/Kp- No particle ID used until the
fit is performed
mES distributions for the different tagging
categories
NT2
NT1
91
CP Asymmetry Fit and Results

• Extended ML fit to the BRs and CP done
  simultaneously
• 5 tagging categories (leptons, K, NT1, NT2,
  untagged)
• 8 event species (Sig and Bkg pp- , Kp- , K-p
  , KK-)
• Discriminating variables (mES, DE , F, qc1 , qc2
  , Dt)
• Dilutions, R(Dt) for the signal taken from sin2b
  analysis
• Dmd, B0 lifetime fixed as in sin2b analysis
• R(Dt) for the background taken from sidebands in
  mES distribution

Preliminary Results
92
CP Violation in the Standard Model

• The weak interaction between quarks regulated by
  the Cabibbo-Kobayashi-Maskawa matrix
• With 3 generations of quarks, the SM can
  accommodate CP violation through complex coupling
  constants 3 angles and a complex phase

Unitarity of theCKM Matrix
93
Direct and Indirect CP Violation Mechanisms

• Direct CP Violation Interference of two decay
  amplitudes
• Can occur in both neutral and charged B decays
• Total amplitude for a decay and its CP conjugate
  have different magnitudes
• Large hadronic uncertaintiesgt difficult
  measure CKM matrix elements
• Relatively small asymmetries expected in B decays

• Indirect CP Violation
• Only in neutral B decays
• Charge asymmetry in semileptonic B decays
• Expected to be small in Standard Model

94
Time evolution of B0 mesons into a final CP
eigenstate
The decay distribution for events with a B0 (f)
and B0bar tags (f-)
In order to have CP Violation
95
Time-dependent CP Asymmetry
From the time evolution of the B0 and B0 states
we can define the time-dependent asymmetry
96
CP Violation in B Decays

• To generate a CP violating observable, we must
  have
• Interference between at least two different
  amplitudes
• All 3 quark generations involved
• In B decays, can consider two different types of
  amplitudes
• Those responsible for decay
• Those responsible for mixing
• This gives rise to three possible manifestations
  of CP violation
• Direct CP violation
• (interference between two decay amplitudes)
• Indirect CP violation
• (interference between two mixing amplitudes)
• CP violation in the interference between mixed
  and unmixed decays