Measurement of CP Violation in B Decays with the BaBar detector

1
Measurement of CP Violation in B Decayswith the
BaBar detector
Gerhard Raven University of California, San Diego

• Nikhef Colloquium
• December 7th 2001

2
Outline

• What is CP (a)symmetry?
• B mesons and CP violation in the Standard Model
• How can we measure CP Violation?
• Brief introduction to PEP-II and the BaBar
  detector
• Overview of the measurement technique
• B reconstruction
• B0, B Lifetime measurement
• Measurement of B0B0 Mixing Frequency
• Time Dependent CP Asymmetries
• sin(2b)
• sin(2aeff)
• Summary and Outlook

3
Discrete Symmetries

• In general if a physical law is symmetric under
  a transformation, then there is a conserved
  quantity
• 3 important discrete symmetries in Particle
  Physics
• Parity, P
• Parity reflects a system through the origin.
  Converts right-handed coordinate systems to
  left-handed ones.
• Vectors (momentum) change sign but axial vectors
  (spin) remain unchanged x ? -x L ? L
• Charge Conjugation, C
• turns a particle into its anti-particle e ? e-
  g ? g
• Time Reversal, T
• Changes, the sign of the time t ? -tall
  time dependent quantities,e.g. momentum, change
  sign

4
Why is CP Violation interesting?

• Universe is matter dominated
• Where has the anti-matter gone?
• In 1967, Sakharov showed that the generation of a
  net baryon number requires
• Baryon number violating processes (e.g. proton
  decay)
• Non-equilibrium state during the expansion,
  therefore unequal number of particles and
  anti-particles
• C and CP symmetry Violation
• Standard Model CP-violation is unlikely to be
  sufficient to explain matter asymmetry in the
  universe
• It means there is something beyond SM in CP
  violation somewhere, so a good place to work

5
Weak Interactions and Symmetry Violation

• In 1957 violation of parity was observed
• Asymmetry in b decays of 60Co ? 60Ni e- n
• Electrons produced mostly in one hemisphere
• C is violated too!
• only left-handed neutrinos and right-handed
  anti-neutrinos
• (assuming massless neutrinos ?)
• In 1964 CP violation was observed in the weak
  decay of neutral K mesons
• Ks ? p p- (CP 1)
• Kl ? p0 p0 p0 (CP -1)
• Observed Kl ? p p- (0.2) ? CP violation!
• Theoretically difficult to precisely interpret CP
  violation results in neutral K systems
• B Mesons expected to show CP violation
• good testing ground for possible sources of CP
  violation

6
The Weak Interactions of Quarks

• The coupling strength at the vertex is given by
  gVij
• g is the universal Fermi weak coupling
• Vij depends on which quarks are involved
• For leptons, the coupling is just g
• For 3 generations, the Vij can be written as a
  3x3 matrix
• This matrix is referred to as the
  CKM matrix
• We can view this matrix as rotating the quark
  states from a basis in which they are Mass
  eigenstates to one in which they are Weak
  eigenstates

W-
b
gVcb
c
7
CP Violation via the CKM matrix

• The CKM matrix is a 3?3 complex unitary matrix
• Requires 4 independent, physical parameters to
  describe it
• 3 real numbers 1 complex non-trivial phase
• The existence of the complex coupling (phase)
  gives rise to CP violation
• All CP violating observables are possible due to
  interference between different decay amplitudes
  involving a weak phase

• If there were only 2 quark generations, the
  corresponding 2?2 matrix would be all real ? No
  CP violation
• CP violation is possible in the Standard Model
  with at least 3 generations

8
The CKM Matrix Wolfenstein parameterization

Complex phase
Unitarity
Wolfenstein parameterization uses the observed
hierarchy of the CKM elements and pushes the
complex phase to the smallest elements
? Vus sin(qcabbibo) 0.2205 0.0018 A Vcb/
?2 0.830.06

• Out of 6 triangles, this one (together with the
  tu one) is special
• It has all sides O(l3)
• Large phases ?potentially large CP asymmetries

9
Unitarity of the CKM Matrix

• The sides and the angles of this triangle can be
  determined experimentally in B decays

Also see Peter Kluits colloquium last month for
measurements of the magnitude of the sides
10
CP violating observables for B mesons

• As mentioned, need at least
• two amplitudes with different phases
• In B decays, we can consider two different types
  of amplitudes
• Those responsible for decay
• Those responsible for mixing
• This gives rise to three possiblemanifestations
  of CP violation
• Direct CP violation
• (interference between two decay amplitudes)
• Indirect CP violation
• (interference between two mixing amplitudes)
• CP violation in the interferencebetween mixed
  and unmixed decays

11
CP violation in decay

• Requires two decay amplitudes
• Eg. Kp-
• easy to measure
• ACP N(Kp-) N(K-p) / N(Kp-) N(K-p)
• Asymmetry expected to be small
• Large asymmetry requires equal amplitudes
• But difficult to interpret
• How large is the penguin contribution?
• What is the relative phase?
• Difficult to disentangle contributions
• To get a feeling for the relative weight, compare
  pp-and Kp-
• Br(Kp-) gtgt Br(pp-)!

12
B0 B0 mixing ARGUS, 1987

• Fully reconstructed mixed event and dilepton
  studies demonstrate mixing
• Integrated luminosity 1983-87
• 103 pb-1

13
CP violation in mixing
Mixing between B0 and B0 can be described can by
effective Hamiltonian G12 describes B0 ?f ?
B0 via on-shell states This is rare the
branching ratios of CP
states is very
small M12 describes B0 ? f ? B0 via off-shell
states CP violation can occur in the
interference between the on-shell and off-shell
amplitudes, and leads to However, for
B0 mesons, G12 is very small mixing is dominated
by Dm2M12 Little CP sensitivity ?
In the SM
14
CP violation in the inference between mixing and
decay
Time evolution of initial B0 (or B0) mesons into
a final CP eigenstate
In order to have CP Violation

• A single decay amplitude is sufficient
• Mixed decay has taken the role of the 2nd
  amplitude
• Thus interfering amplitudes are comparable by
  construction
• and large CP asymmetries are possible!!!

15
Time Dependent CP Asymmetry
From the time evolution of the B0 and B0 states
we can define the time-dependent asymmetry to be
Im l 0.75 l1
16
Golden Decay Mode B0 J/y K0S
K0 mixing

• Theoretically clean way to measure the phase of l
  (i.e. sin2b)
• Clean experimental signature
• Branching fraction O(10-4)
• Large compared to other CP modes!

Golden Modes

• hCP -1
• B0 ? J/? K0S
• B0 ? ?(2s) K0S
• B0 ? cc1 K0S

• hCP 1
• B0 ? J/? K0L

17
B meson production

• Electron-Positron collider ee- ? ?(4s) ? B0B0
• Only 4s resonance can produce B meson pair
• Low B0 production cross-section 1 nb
• Clean environment, coherent B0B0 production

B-Factory approach
BB threshold
B0B0 threshold
18
?(4S) Coherent B0B0 production
Incoherent

• B0B0 system evolves coherentlyuntil one of them
  decays
• CP/Mixing oscillation clock only starts ticking
  at the time of the first decay, relevant time
  parameter Dt
• B mesons have opposite flavour at time Dt0
• Half of the time CP B decays first (Dtlt0)
• Integrated CP asymmetry is 0
• Coherent production requires time dependent
  analysis

At tcp0
B0
B0
Dt tCP - tOtherB
t(ps)
At Dt0
B0
B0
Dt(ps)
Coherent
19
A Symmetric Collider wont work

• CP asymmetry is a time-dependent process
• ACP ? ?t between two B decays, ?t ps
• In reality one measures decay distance between
  two B decays
• In symmetric energy ee- collider, where ?(4S)
  produced at rest, daughter Bs travel 20mm
• Too small a distance to discern with todays
  detector technology

20
Solution Boost the CMS!
This can be measured using a silicon vertex
detector!
21
Asymmetric B Factories
HER LER
Energy (GeV) 9.0 3.1
Number of bunches 1658 1658
Beam Current (A) 1.0 2.1
bg 0.56, ?s M?(4S)
Collisions every 4.2 ns
Large currents!
22
PEP-II

• PEP-II top luminosity 4.3 x 1033cm-2s-1
  (design 3.0 x 1033)
• Best shift 102 pb-1
• Best day 282 pb-1
• Best month 6 fb-1
• Average logging efficiency gt 96

20/fb used for lifetime
30/fb used for CP and mixing
off-peak
December 5, 2001
October 99
PEP-II delivered 63 fb-1 BABAR recorded 60
fb-1 (incl. 6.5 fb-1 off peak) 60 Million B
meson pairs on tape!
23
KEK-B performance
KEK-B has reached 5.5 1033cm-2s-1! (design
1034) Extrapolation suggest both
machines will have delivered 100 fb-1 by the
time of ICHEP 2002 we live in interesting times!
peak luminosity 5.447 1033 /cm2/sec
integrated luminosity shift 101.9 /pb day
280.8 /pb 24h 287.7 /pb 7days 1801. /pb
month 4760. /pb
24
The BaBar Detector
Electromagnetic Calorimeter 6580 CsI(Tl) crystals
1.5 T solenoid
e (3.1 GeV)
Cerenkov Detector (DIRC) 144 quartz bars 11000 PMs
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

25
Silicon Vertex Detector
e- beam
e beam

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

26
Silicon Vertex Detector
Readout chips
Beam bending magnets
Beam pipe
Layer 1,2
Layer 3
Layer 4
Layer 5
27
Drift Chamber

• 40 layers of wires inside 1.5 Tesla magnetic
  field
• Measurement of charged particle momentum
• Limited particle identification from ionization
  loss

28
Cerenkov Particle Identification System

• Cerenkov light in quartz
• Transmitted by internal reflection
• Rings projected in standoff box
• Detected by PMTs
• Essential for Kaon ID gt2 GeV

29
ElectroMagnetic Calorimeter

• 6580 CsI(Tl) crystals with photodiode readout
• About 18 X0, inside solenoid
• Excellent energy resolution, essential for p0 ? gg

?0
s 5.0
30
Instrumented Flux Return

• Up to 21 layers of RPCs sandwiched between iron
  plates
• Muons identified above 500 MeV
• Neutral Hadrons (KL) detected

31
Event Topology and Analysis Strategy
32
Analysis Strategy
Factorize the analysis in building blocks

• Measurements
• B/B0 Lifetimes
• B0 B0-Mixing
• CP-Asymmetries
• sin(2b)
• sin(2aeff)

• 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

33
Blind Analysis

• All analysis were done blind to eliminate
  possible experimenters bias
• In general, measurements of a quantity X are
  done with likelihood fits blinding done by
  replacing X with XR in likelihood fits
• R is draw from a Gaussian with a width a few
  times the expected error
• Random number sequence is seeded with a
  blinding string
• The reported statistical error is unaffected
• It allows all systematic studies to be done while
  still blind
• The sin(2b) result was unblinded 1 week before
  public announcement this summer!

34
Measurement of 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
35
Fully-Reconstucted B sample
Flavor eigenstates Bflav for lifetime and
mixing measurements
Cabibbo-favored hadronic decays
Open Charm decays
30 fb-1
Neutral B Mesons
Charged B Mesons
Hadronic decays into final states with Charmonium
GeV
36
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 ?(4S) momentum
• High efficiency (97)
• Average Dz resolution is 180 mm (ltDzgt bgct
  260 mm)
• Conversion of Dz to Dt takes into account the
  (small) B momentum in ?(4S) frame
• Dt resolution function measured directly from
  data

37
Vertex and Dt reconstruction Belle
38
tB Measurements in BaBar
Need to disentangle resolution function from
physics !
39
Dt Signal Resolution

• event-by-event s(Dt) from vertex errors
• Resolution Function (RF) 2 models
• Sum of 3 Gaussians (mixing CP analyses)
• Lifetime-like bias (lifetime analysis)

sDz
0.6 ps
tracks from long-lived Ds in tag vertex?
asymmetric RF
Signal MC (B0)
high flexibility
small correlation with t(B)
Dt (meas-true)/sDt
40
Lifetime Likelihood Fit

• Simultaneous unbinned maximum likelihood fit to
  B0/B samples
• 19 free parameters
• t(B) and t(B0) 2
• Dt signal resolution 5
• empirical background 12 description
• Background parameters determined from mES sideband

B0 mES
B0 Bkg Dt mESlt5.27 GeV/c2
Dt characteristics determined from data
41
Neutral and Charged B meson Lifetimes

• Precision measurements

20 fb-1

• 2 statistical error
• 1.5 systematic error

Dt RF parameterization, Dt outlier description
PRL 87 (2001)
signal bkgd
t0 1.546 ? 0.032 ? 0.022 ps t? 1.673 ?
0.032 ? 0.022 ps t? /t0 1.082 ? 0.026 ? 0.011
bkgd
outliers
Common resolution function for B and B0
Dt (ps)
Dt distribution well described!
42
Comparison of Lifetime Ratio Measurements
Single most precise measurement
Systematic error 1 in B/B0 lifetime ratio
43
Belle result from 5th KEK conference (end Nov)
44
Belle result from 5th KEK conference (end Nov)
45
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
ü
46
Measurement of B0B0 Mixing
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
47
Dt distribution of mixed and unmixed events
Dmd oscillation frequency
w the fraction of wrongly tagged events
48
Extraction of Dmd and Flavour Mistag Fractions
Fraction of Mixed Events as Function of time
Sensitive to mistag fraction measurement because
the mixing has not started yetAt t0 the
observed mixed events are only due to wrongly
tagged events
Sensitive to Dmd when the rate of change of the
amplitude is at its maximum
49
B Flavour 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)
50
Flavour Tagging Performance
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 () 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
Smallest mistag fraction
Highest efficiency
51
Belle Flavour Tagging
52
Belle Flavour Tagging
53
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) 8816 Empir
ical description of background Dt 19 B lifetime
fixed to the PDG value tB 1.548 ps
54
Beware of Correlations!

• Difficult part of the Dmd analysis are
  correlations
• For this result, 2 correlation are not modeled in
  the likelihood function
• Between mES and Dt
• For mES close to mB, more background due to
  (incorrectly reconstructed) real B mesons
• For smaller mES, more continuum background
• Leads to a 0.002 ps-1 correction determined from
  data
• Between mistag rate and resolution
• Eg. wrong sign K are mainly produced by
  D()D() decays
• Higher charged multiplicity, no (or only low
  momentum) tracks from B decay vertex ? different
  Dt resolution
• Leads to a 0.007 ps-1 correction determined from
  MC
• Next generation of this measurement should / will
  have to model this in the likelihood

55
Mixing Likelihood Fit Result
CL44

• BaBar internal review passed
• currently in final circulation
• Numbers are final
• To be submitted to PRL in the very near future
  (please dont tell your friends on Belle just
  yet!)

56
Cross Checks and Systematic Errors
57
Dmd Measurement in Comparison

• Precision Dmd measurement (3) with Bflav sample
  is still statistically limited
• Systematic error under control (2)
• Dominated by uncertainty on tB
• Followed by resolution fcn and tagging-vertexing
  correlations.
• Theoretical hadronic uncertainties limit
  extraction of Vtd

(PDG 2000)
My Average, using COMBOS
58
Recent Belle Result (5th KEK topical conference)
59
Recent Belle Results (5th KEK topical conference)
60
Analysis Strategy (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
61
Measurement of sin(2b)
p
K0
p-
Tag B sz 110 mm
g
Reco B sz 65 mm
KS0
?(4s)
m
Dz
bg 0.56
m-
Dt _at_ Dz/gbc
3. Reconstruct Inclusively the vertex of the
other B meson (BTAG) ? 4. Determine the
flavor of BTAG to separate B0 and B0 ?
1. Fully reconstruct one B meson in CP
eigenstate (BREC) 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
62
The CP Sample
Before tagging requirement
J/?Ks Ks(pp-)
?(2S)Ks
After tagging
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
mES(GeV/c2)
J/?K0
J/yKL
mES(GeV/c2)
DEEB-?s/2 (GeV)
63
Example of a Fully Reconstructed Event

• ?(2S) Ks
• ? mm- ? pp-
• D p-
• ? D p
• ?K-p
• Exercise for the viewer/reader/listener how many
  ways are there to flavour tag this event?
• Bonus points which tag was actually used?

64
A few words about J/?K0(KSp0)

• J/? K0(KSp0) angular components
• A CP 1
• A0 CP 1
• A? CP -1 (define R? A?2 )
• ? CP asymmetry diluted by D? (1 - 2R?)
• ? R? (16.0 3.2 1.4) (BABAR, to appear in
  PRL)
• gt Effective hf 0.65 ? 0.07 (includes
  acceptance corrections)

Sample used in R? measurement (20.7fb-1)
and the angular fit
65
Dt Spectrum of CP events
CP PDF
Mistag fractions w And resolution function R
66
Sin(2b) likelihood fit
Combined unbinned maximum likelihood fit to Dt
spectra of flavor and CP sample
Driven by
67
Sin(2b) Fit Results
Phys. Rev. Lett. 87 091801 (2001)
Cross-checks Null result in flavor samples
Consistency of CP channels P(c2) 8
Goodness of fit(CP Sample) P(LmaxgtLobs) gt 27
Combined fit to all modes
sin2b 0.59 0.14
68
Raw CP Asymmetry
All tags
Kaon tags
?f -1 events
sin2b0.56 0.15
sin2b0.59 0.20
Raw ACP
69
Raw CP Asymmetry for J/y KL
Backgroundcontribution
70
Check null control sample

• Treat Bflav sample as CP
• No asymmetry seen
• Analysis doesnt create artificial
  asymmetries

71
Consistency checks
sin2b measured in several Dt bins
Combined CP-1
sin2b vs. J/? decay mode and tagging category
and flavor for ? -1 events
72
Is it possible to measure a very large asymmetry?

• The answer is yes! Suppose at a given time t
  you have
• Nevents lt 0 is possible in the 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

73
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
74
Systematic Errors
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
• Tagging 0.03
• possible differences between BCP and Bflavor
  samples
• Backgrounds 0.02 (overall)
• Signal probability, fraction of B background in
  the signal region, CP content of background
• Total 0.09 for J/? KL channel 0.11 for J/y K0
• Total 0.05 for total sample

75
Belle sin(2f1) result
76
Belle sin(2f1) result
77
Belle sin(2f1) result
78
The New World Average
New sin2b world average is 8s significant!
Measurements assumed to be uncorrelated
79
Interpretation of the result
One solution for b is consistent with
measurements of sides of the unitarity triangle
Error on sin2b is dominated by statistics and
will decrease 1/ for the forseeable future
Method as in Höcker et al, hepex/0104062 (see
also many other recent global CKM analyses)
80
Search for Direct CP
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)
81
CP Violation in B0?pp- decays
penguin diagram
tree diagram
Additional phase from penguin diagram
Weak phase (only tree diagram)
? ? 1 ? must fit for direct CP Im (?) ? sin2?
? need to relate asymmetry to ?
Cpp ? 0, Spp sin2aeff
Cpp 0, Spp sin2a
Decay distributions f(f-) when tag B0(B0)
82
B?pp-,Kp-,KK- Data Sample
Likelihood Analyis with high reconstruction
efficiency Loose selection criteria yield 9741
two-prong candidates in 30.4 fb-1 (includes 97
background, almost entirely from continuum)
Lepton
Kaon

• sum of pp-/Kp- mES distributions by tagging
  category
• particle ID used in likelihood fit for pp/Kp
  separation

NT1
NT2
83
B ? pp-/Kp- Likelihood Fit
Simultaneous extended unbinned ML fit to the
yields and CP asymmetries

• 8 event types
• Sig and Bkg pp- , Kp- , K-p , KK-
  ? measure also direct CP violation
  in charge asymmetry
• Discriminating variables
• mES, DE , Fisher (Event shapes), Cerenkov
  angles, Dt
• Mistag rates and Dt signal resolution function
  same as in sin2b fit (uses also untagged events
  to improve BR measurements)
• Empirical background parameters determined from
  mES sidebands
• Dmd, B0 lifetime fixed to PDG values

A N(K-p)-N(Kp-) / N(K-p)N(Kp-)
84
CP Sample pp-/Kp- Candidates
Events after likelihood ratio cuts
L 30.4 fb-1
pp-
pp-
Kp- K-p
Kp- K-p
Measured Branching Ratios (using 20 fb-1)
pp- ( 4.11.00.7 )10-6 Kp-
(16.71.61.6)10-6 KK- lt2.5 10-6 (90CL)
Tagged events
Background (incl. crossfeed)
85
B0 ? pp- Asymmetry Result
To appear in PRD Rapid Communications

• Measurement compatible with no CP in B0 ? pp-
• Statistically limited due to small branching
  fraction
• Need 500fb-1 for s(Spp) 0.10-0.15

86
Summary and Outlook

• New precision measurements of B0/B lifetimes and
  B0B0 mixing frequency Dmd
• Measurement of flavor-tagged, time-dependent B
  decays at asymmetric B factory has become
  established technique
• BaBar observes CP violation in the B0 system at
  4s level
• Probability is lt 3 x 10-5 to observe an equal or
  larger value if no CP violation exists
• Corresponding probability for only the hCP -1
  modes is 2 x 10-4

t0 1.546 ? 0.032 ? 0.022 ps t? 1.673 ?
0.032 ? 0.022 ps t0 /t? 1.082 ? 0.026 ? 0.011
Dmd 0.516 0.016 0.010 ps-1
sin(2b) 0.59 0.14 0.05
87
Summary and Outlook (II)

• First measurement of time-dependent CP asymmetry
  in rare B decay mode B ? pp-
• The study of CP violation in the B system has
  started
• sin(2b) will very soon become precision
  measurement (? unitarity triangle constraints
  will be limited by other CKM parameters)
• Need to compare sin(2b) from different decay
  modes to test standard model
• With anticipated 100 fb-1 by summer, error in
  sin(2b) will be 0.08 and for the asymmetry in B
  ? pp- error will be 0.3
  

88
Summary and Outlook (III)

• 37 years after the discovery of CP violation in
  Kaon decays, a 2nd system with CP violation is
  found and its study is just beginning
• The Standard Model prediction of a single phase
  as the source of CP violation seems right (sofar
  -- given the current experimental data)
• New physics and its contribution to CP violation
  in B decays are possible, but remain to be
  discovered
• Current experimental measurements of CP violation
  in weak interactions are very unlikely to explain
  the CP asymmetry observed in the universe

89
Luminosity Outlook of PEP-II BaBar
Expect gt500 fb-1 by 2007
90
Changes between Run1 and Run2

• First publication in March 2001
• Changes since then
• More data (run 2) 23 ?32 BB pairs
• Improved reconstruction efficiency
• Optimized selection criteria takes into account
  CP asymmetry of background in J/?KL
• Additional decay modes ?C1KS and J/?K0
• Improved vertex resolution for reconstructed and
  tag B

sin(2b) 0.34 0.20 (stat) 0.05 (syst)PRL 86
(2001) 2515