1. Measurement of CP Violation in B Decays with the BaBar detector
Shahram Rahatlou
University of California, San Diego
UCSD HEP Seminar
La Jolla, 25 September 2001

2. B mesons and CP violation in the Standard Model
Outline
What is CP symmetry?
B mesons and CP violation in the Standard Model
How can we measure the CP Violation?
Brief introduction to the BaBar detector
Overview of the measurement technique
B reconstruction
Lifetime measurement
Measurement of B0B0 Mixing Frequency
Extraction of CP Violating Asymmetry
Results and Prospects
25 September 2001
Sh. Rahatlou

3. Discrete Symmetries in Nature
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  -t all time dependent quantities, e.g. momentum, change sign + 
25 September 2001
Sh. Rahatlou

4. 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
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
25 September 2001
Sh. Rahatlou

5. 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 the CKM Matrix b a g
Vtd Vtb *
Vud Vub
Vcd Vcb
Unitarity Triangle
25 September 2001
Sh. Rahatlou

6. Unitarity of the CKM Matrix
The sides and the angles of this triangle can be determined experimentally in the B decays
25 September 2001
Sh. Rahatlou

7. 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 uncertainties => difficult measure CKM matrix elements
Relatively small asymmetries expected in B decays
Indirect CP Violation
Only in neutral B decays
Charge asymmetry in semileptonic decays
Expected to be small in Standard Model B0 b d
u,c,t W-
Mixing Diagram d b W- u s p+ K- B0
Decay Diagram
25 September 2001
Sh. Rahatlou

8. CP Violation in interference between Mixing and Decay
Time evolution of initial pure B0/B0 states: Mixing
Mass eigenstates:
fCP is a CP eigenstate
B0(t)
fCP B0
Initial state
Flavor eigenstate
B0(t)
fCP B0
Initial state
Flavor eigenstate
25 September 2001
Sh. Rahatlou

9. Time evolution of B0 mesons into a final CP eigenstate
Amplitude ratio
Weak Phase
In order to have CP Violation
25 September 2001
Sh. Rahatlou

10. Time-dependent CP Asymmetry
From the time evolution of the B0 and B0 states we can define the time-dependent asymmetry to be
Probe of direct CP violation since it requires
Sensitive to the phase of l even without direct
CP Violation
25 September 2001
Sh. Rahatlou

11. Golden Decay Mode: B0 ® J/y K0S
K0 mixing
u,c,t W-
Theoretically clean way to measure the phase of l (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
25 September 2001
Sh. Rahatlou

12. What does one expect to see?
Dt spectrum and the observed asymmetry for a perfect detector (assuming sin2b = 0.6)
Visible difference between B0 and B0 decay rates
sin 2b
In this ideal case, the amplitude of the oscillation is the CP Asymmetry
time-integrated asymmetry is 0 t
25 September 2001
Sh. Rahatlou

13. Required Experimental Ingredients
Produce many B’s because of the small branching fraction
Need a B-Factory
Determine the flavor of the initial B meson to separate B0 from B0 (Tagging)
Production: B-B pairs in a coherent state
Reconstruction: Good particle identification for Kaons and leptons
Define and measure a ‘time’ in order to study the time-dependent asymmetry
Production: Coherent B-B pair produced with an initial boost in the LAB
Reconstruction: A high precision tracking system to measure the distance between the B decay points
BaBar Asymmetric B-Factory
25 September 2001
Sh. Rahatlou

14. PEP-II Asymmetric B-Factory at SLAC
B B~ production threshold
9 GeV e- on 3.1 GeV e+
U(4S) boost in lab frame
bg = 0.56
25 September 2001
Sh. Rahatlou

15. B-Factory Performance as Today
Records from Yesterday!
PEP-II top luminosity: x 1033cm-2s (design 3.0 x 1033)
Top recorded Lumi/24h: 246 pb-1
Top recorded Lumi/8h: 84 pb-1
BABAR logging efficiency: > 96%
30/fb analysed
for CP
September 25, 2001
October 99
PEP-II delivered: fb-1
BABAR recorded: fb (includes 5.1 fb-1 off peak)
87 million B’s available !!
25 September 2001
Sh. Rahatlou

16. The BaBar Detector e+ (3.1 GeV) e- (9 GeV)
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: % efficiency, 15 mm z hit resolution (inner layers, perp. tracks)
SVT+DCH:(pT)/pT = 0.13 %  pT %
DIRC: K- separation GeV/c  GeV/c
EMC: E/E = 2.3 %E-1/4  1.9 %
25 September 2001
Sh. Rahatlou

17. B Decay Topology in the Boosted (4S) Environmentz
Tag vertex reconstruction
Flavor Tagging
(bg)U(4S) = 0.56
Start the Clock
Coherent BB pair
Exclusive B Meson and Vertex Reconstruction
25 September 2001
Sh. Rahatlou

18. Increasing complexity
Analysis Strategy
Factorize the analysis in building blocks
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
Increasing complexity
Higher precision
25 September 2001
Sh. Rahatlou

19. Measurement of the B0 and B+ Lifetime
U(4s)
bg = 0.56
Tag B
sz ~ 110 mm
Reco B
sz ~ 65 mm p+ Dz
Dz/gbc K0 g D- p- K+
3. Reconstruct Inclusively the vertex of the “other” B meson (BTAG)
Fully reconstruct one B meson in flavor eigenstate (BREC)
Reconstruct the decay vertex
4. compute the proper time difference Dt
5. Fit the Dt spectra
25 September 2001
Sh. Rahatlou

20. Fully-Reconstructed B sample
Flavor eigenstates Bflav : for lifetime and mixing measurements
Cabibbo-favored hadronic decays
“Open Charm” decays
30 fb-1
Neutral B Mesons
Hadronic decays into final states
with Charmonium
Charged B Mesons
[GeV]
25 September 2001
Sh. Rahatlou

21. Vertex and Dt Reconstruction
Beam spot
Interaction Point
BREC Vertex
BREC daughters
BREC direction
BTAG direction
TAG Vertex
TAG tracks, V0s z
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 (<|Dz|> ~ bgct = 260 mm)
Dt resolution function measured from data
25 September 2001
Sh. Rahatlou

22. tB Measurement in BaBar
e-t/t
true Dt
B production point known eg. from beam spot
LEP/SLD
Dt resolution
measured Dt
Resolution
function
lifetime  =
e-|Dt|/t
Either Brec or Btag can decay first (this analysis)
BaBar
Resolution Function Lifetime  =
Need to disentangle resolution function from physics !
25 September 2001
Sh. Rahatlou

23. Dt Resolution Function
sDz
event-by-event s(Dt) from vertex errors
Lifetime-like bias to
Small correlation between the lifetime and the RF parameters
~0.6 ps
Signal MC (B0)
tracks from long-lived D’s in tag vertex asymmetric Resolution Function
Dt (meas-true)/sDt
25 September 2001
Sh. Rahatlou

24. Lifetime Likelihood Fit
Simultaneous unbinned maximum likelihood fit to B0/B+ samples
Use data to extract the properties of background events
Mass distribution provides the signal probability
Use the events in the sideband (mES < 5.27) to determine the Dt structure of the background events 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
25 September 2001
Sh. Rahatlou

25. B Lifetime Fit Results B0/ B0 B
Precision measurements
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
Submitted to PRL
t0 =   ps PDG:  ps
t =   ps
PDG:  ps
t/t0 =   0.011
PDG:  0.029
background
Dt (ps)
25 September 2001
Sh. Rahatlou

26. 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
25 September 2001
Sh. Rahatlou

27. Measurement of B0B0 Mixing
U(4s)
bg = 0.56
Tag B
sz ~ 110 mm
Reco B
sz ~ 65 mm p+ Dz
Dz/gbc K0 g D- p- K+
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
25 September 2001
Sh. Rahatlou

28. Dt Spectrum of Mixed and Unmixed Events
perfect
flavor tagging & time resolution
realistic
mis-tagging & finite time resolution + _
w: the fraction of wrongly tagged events
Dmd: oscillation frequency
25 September 2001
Sh. Rahatlou

29. Extraction of Dmd and mistag fraction
Fraction of Mixed Events
Sensitive to mistag fraction measurement because the mixing has not started yet At t=0 the observed ‘mixed’ events are only due to wrongly tagged events B0 B+
Sensitive to Dmd measurement when the amplitude of the oscillation is at its maximum
25 September 2001
Sh. Rahatlou

30. B Flavor Tagging Methods
Hierarchical Tagging Categories
For electrons, muons and Kaons use the charge correlation b c d l- n B0
D, D* W-
Lepton Tag b d B0 W- W+ c s
K*0
Kaon Tag
NN output
Not Used
Multivariate analysis exploiting the other kinematic information of the event, e.g.,
Momentum spectrum of the charged particles
Information from non-identified leptons and kaons
Soft p from D* decay
Neural Network
Each category is characterized by the probability of giving the wrong answer (mistag fraction w)
25 September 2001
Sh. Rahatlou

31. Flavor 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
The error on sin2b the quality factor Q
Highest “efficiency”
Smallest mistag fraction
25 September 2001
Sh. Rahatlou

32. Dt Resolution Function
Core
Tail
Outlier
Use the event-by-event
uncertainty on Dt
Dt Residual (ps)
R(dDt)
B0 flavour sample
CP sample
sDt (ps)
Different bias
For each tagging
category
25 September 2001
Sh. Rahatlou

33. Fit Parameters Mixing Likelihood Fit
Unbinned maximum likelihood fit to flavor-tagged neutral B sample
Fit Parameters
Dmd
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 = ps
34 total free parameters
All Dt parameters extracted from data
25 September 2001
Sh. Rahatlou

34. B0B0 Mixing Fit Result 20 fb-1
C.L. 28 %
Dmd = ± (stat) ± (syst) h ps-1
Preliminary
25 September 2001
Sh. Rahatlou

35. Dmd Measurement in Comparison
preliminary
Precision Dmd measurement
4% statistical error
3% systematic error dominated by MC correction
25 September 2001
Sh. Rahatlou

36. 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
25 September 2001
Sh. Rahatlou

37. Measurement of CP Asymmetry
U(4s)
bg = 0.56
Tag B
sz ~ 110 mm
CP B
sz ~ 65 mm m+ Dz
Dz/gbc K0 g p+ p-
Ks0 m-
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
25 September 2001
Sh. Rahatlou

38. The fully Reconstructed CP Sample
J/y KS
KS  p0 p0
J/y KS KSp+ p-
Before tagging requirement
data 32 x 106 BB pairs 29 fb-1 on peak
cc1 KS
y(2S) KS
Sample
tagged events
Purity CP
[J/, (2S), cc1] KS
480
96% -1
J/ KL
273
51% +1
J/ K*0(KSp0) 50
74%
mixed
Full CP sample
803
80%
J/y KL
J/y K*
After tagging
25 September 2001
Sh. Rahatlou

39. Dt Spectrum of CP Events
perfect
flavor tagging & time resolution
realistic
mis-tagging & finite time resolution
Mistag fractions w
And resolution function R
CP PDF
Mixing PDF
determined by the flavor sample
25 September 2001
Sh. Rahatlou

40. Fit Parameters Sin2b Likelihood Fit
Combined unbinned maximum likelihood fit to Dt spectra of flavor and CP sample
Fit Parameters
sin2b
Mistag fractions for B0 and B0 tags 8
Signal resolution function 16
Empirical description of background Dt 20
B lifetime fixed to the PDG value tB = ps
Mixing Frequency fixed to the PDG value Dmd = ps-1
Global correlation coefficient for sin2b: 13%
Different Dt resolution function parameters for Run1 and Run2
tagged flavor sample
tagged CP samples
45 total free parameters
All Dt parameters extracted from data
Correct estimate of the error and correlations
25 September 2001
Sh. Rahatlou

41. sin2b = 0.59 ± 0.14 Sin2b Fit Results Combined fit to all modes
Phys. Rev. Lett. 87
(2001)
Cross-checks:
Null result in flavor samples
Consistency of CP channels P(c2) = 8%
Goodness of fit(CP Sample): P(Lmax>Lobs) > 27%
Combined fit to all modes
sin2b = 0.59 ± 0.14
25 September 2001
Sh. Rahatlou

42. Raw CP Asymmetry Raw ACP f = -1 events sin2b=0.56 ± 0.15
All tags
Kaon tags
f = -1 events
sin2b=0.56 ± 0.15
Raw ACP
sin2b=0.59 ± 0.20
25 September 2001
Sh. Rahatlou

43. Consistency Checks sin2b measured in several Dt bins
Combined CP=-1
sin2b measured in
several Dt bins
sin2b vs. J/ decay mode and
tagging category and flavor for
 = -1 events
25 September 2001
Sh. Rahatlou

44. Sources of Systematic Error
Error/Sample KS KL
K*0
Total
Statistical
0.15
0.34
1.01
0.14
Systematic
0.05
0.10
0.16
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/Y KL channel; 0.11 for J/Y K*0
Total = 0.05 for total sample
25 September 2001
Sh. Rahatlou

45. The Unitarity Triangle
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
Method as in Höcker et al, hepex/
(also other recent global CKM matrix analyses)
25 September 2001
Sh. Rahatlou

46. Summary and Outlook sin2b = 0.59 ± 0.14 ± 0.05
BaBar observes CP violation in the B0 system at 4s level
Probability to observe an equal or larger value if no CP violation exists is < 3 x 10-5
Corresponding probability for the hCP = -1 modes only is 2 x 10-4
sin2b = 0.59 ± 0.14 ± 0.05
t0 =   ps
t =   ps
t0 /t =   0.011
Dmd = ± ± h ps-1
New precision measurements of B0/B+ lifetimes and B0B0 mixing frequency Dmd
With anticipated 100 fb-1 by next summer, we expect the error in sin2b to be 0.08
25 September 2001
Sh. Rahatlou

47. Backup Slides Why cc is OK Run1 vs. Run2 World Average J/psi KL plot
Direct CP
B->p-p+
25 September 2001
Sh. Rahatlou

48. Is it possible to measure a large asymmetry ?
The answer is… yes! Suppose at a given time t’ you have
Nevents < 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
25 September 2001
Sh. Rahatlou

49. Large sin2b in B0  C1KS Kaon tags Lepton tags B0 tags B0 tags
fit for B0/B0 Dt PDFs, not for ACP
Large sin2b possible , because
No events where PDF<0 (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 1~2%
Kaon tags
Lepton tags
B0 tags
B0 tags
B0 tags
Dt [ps]
Dt [ps]
25 September 2001
Sh. Rahatlou

50. 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/K*0
Improved vertex resolution for reconstructed and tag B
sin(2b) = 0.34 ± 0.20 (stat) ± 0.05 (syst) PRL 86 (2001) 2515
25 September 2001
Sh. Rahatlou

51. Consistency Check: Run1 vs. Run2
Difference for modes used in the old PRL: 1.8 s
Run 1
Run 2
25 September 2001
Sh. Rahatlou

52. New sin2b world average is 8s significant
The New World Average
New sin2b world average is 8s significant
Measurements assumed
to be uncorrelated
25 September 2001
Sh. Rahatlou

53. Raw CP Asymmetry for J/y KL
sin2b=0.70±0.34
Background contribution
25 September 2001
Sh. Rahatlou

54. Search for Direct CP Violation
Contribution from at least 2 amplitudes with a weak phase difference
|l| might be different from 1
(tree amplitude and leading penguin amplitude for B J/y KS have same weak phase in SM)
Probing 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)
25 September 2001
Sh. Rahatlou

55. B0  p+p- Asymmetry Result
Decay distributions f+(f-) when tag = B0(B0)
Preliminary
A = N(K-p+)-N(K+p-)/N(K-p+)+N(K+p-)
Measurement compatible with no CP in B0  p+p-
Statistically limited due to small branching fraction
Need ~500/fb for s(Spp) ~
25 September 2001
Sh. Rahatlou