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 !
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

5. CP Violation in the Standard Model
CKM Matrix
Complex matrix described by 4 independent parameters
Wolfenstein parametrization:
phase
CP Violation:
9/19/2018
Vivek Sharma

6. g Unitarity Triangle Rt Ru b Choice of parameters: 9/19/2018
Vivek Sharma

7. ü 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 << 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
9/19/2018
Vivek Sharma

8. CP Violation in interference between Mixing and Decay
B0(t)
fCP B0
Initial state
Flavor eigenstate
B0(t)
fCP B0
Initial state
Flavor eigenstate
Time evolution of initial pure B0/B0 states: Mixing
fCP is a CP eigenstate
Mass eigenstates:
9/19/2018
Vivek Sharma

9. CP from Interference of Mixing and Decay
amplitude
ratio
CP eigenvalue
Define Time-dependent CP Observable:
9/19/2018
Vivek Sharma

10. The “Golden” Decay Mode: B0 ® J/y K0S
K0 mixing
u,c,t W-
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
9/19/2018
Vivek Sharma

11. Decay Time Distribution in B fCP
9/19/2018
Vivek Sharma

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
sin 2b
In this ideal case, the amplitude of the oscillation is the CP Asymmetry
time-integrated asymmetry is 0 t
9/19/2018
Vivek Sharma

13. Exptal Requirements For CPV Measurement
BR (B fCP) ~  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 PEP-II B Factory as example
9/19/2018
Vivek Sharma

14. The Asymmetric Energy Collider @ U(4S) : PEP-II
Cleanest source of B0 mesons:
BB threshold
evolves coherently until one of the B0 mesons decays, so:
Dt : proper time difference between the two B decays
PEP-II BABAR
Off
ACP(Dt) integrates to zero over all Dt On
measure of Dt
Study of CPV
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

16. PEP-II Performance Has Been Spectacular !
Records from this week!
PEP-II top luminosity
4.21 x 1033cm-2s (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: > 96%
30/fb analyzed
for CP
October 3, 2001
October 99
PEP-II delivered: fb-1
BABAR recorded: fb (includes 5.15 fb-1 off peak)
90 million B’s recorded, being analysed !!
9/19/2018
Vivek Sharma

17. 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 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: % 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 %
9/19/2018
Vivek Sharma

18. B Event Topology at the Boosted (4S)z
Tag vertex reconstruction
Flavor Tagging
(bg)U(4S) = 0.56
Start the Clock
Coherent BB pair
Exclusive B Meson and Vertex Reconstruction
9/19/2018
Vivek Sharma

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
Higher precision
Increasing complexity
9/19/2018
Vivek Sharma

20. 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 self tagging (BREC)
Reconstruct the decay vertex
4. compute the proper time difference Dt
5. Fit the Dt spectra
9/19/2018
Vivek Sharma

21. Fully-Reconstructed Hadronic B Decay sample
Flavor Eigenstates Bflav : for lifetime and mixing measurements
Self-tagging hadronic decays
“Open Charm” decays
30 fb-1
Neutral B Mesons
Hadronic decays into final states
with Charmonium
Charged B Mesons
[GeV]
9/19/2018
Vivek Sharma

22. Recoil (Tag) side Vertex and Dz 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 characterized from data
9/19/2018
Vivek Sharma

23. tB Measurement at Boosted (4S): Unique
e-t/t
true Dt
B production point known eg. from beam spot
LEP/CDF
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
9/19/2018
Vivek Sharma

24. Dt Resolution Function
sDz
event-by-event s(Dt) from vertex errors
Charm Lifetime induced bias leads to
Small correlation between the lifetime and the Resolution Function parameters
~0.6 ps
Signal MC (B0)
tracks from long-lived D’s in tag vertex asymmetric Resolution Function
Dt (meas-true)/sDt
9/19/2018
Vivek Sharma

25. B 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
9/19/2018
Vivek Sharma

26. B Lifetime Results:Calibrating The BaBar Clock
t0 =   ps PDG:  ps
t =   ps
PDG:  ps
t/t0 =   0.011
PDG:  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
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

28. B0B0 Mixing with Fully Reconstructed B Mesons
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
9/19/2018
Vivek Sharma

29. Dt Spectrum of Mixed and Unmixed B Events
perfect
flavor tagging & time resolution
realistic
mis-tagging & finite time resolution + _
w: the fraction of wrongly tagged events
Dmd: oscillation frequency
9/19/2018
Vivek Sharma

30. 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
9/19/2018
Vivek Sharma

31. 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)
9/19/2018
Vivek Sharma

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
Highest “efficiency”
Smallest mistag fraction
9/19/2018
Vivek Sharma

33. 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
9/19/2018
Vivek Sharma

34. 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
9/19/2018
Vivek Sharma

35. B0B0 Mixing Fit Result 20 fb-1
C.L. 28 %
Dmd = ± (stat) ± (syst) h ps-1
Preliminary
9/19/2018
Vivek Sharma

36. Dmd Measurement in Comparison
preliminary
Precision Dmd measurement
4% statistical error
3% systematic error dominated by MC correction
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

38. Measurement of CP Asymmetry : Sin2
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
9/19/2018
Vivek Sharma

39. 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 flavor tagging
9/19/2018
Vivek Sharma

40. Dt Spectrum of CP Events
perfect
flavor tagging & time resolution
realistic
mis-tagging & finite time resolution
CP PDF
Mistag fractions w
And
Resolution function R
Mixing PDF
determined from the flavor sample
9/19/2018
Vivek Sharma

41. 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 in each Cat. 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 CP samples
tagged flavor sample
45 total free parameters
All Dt parameters extracted from data
Correct estimate of the error and correlations
9/19/2018
Vivek Sharma

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 of tagging, 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!
9/19/2018
Vivek Sharma

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/K*0
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
9/19/2018
Vivek Sharma

44. Raw CP Asymmetry in Clean Charmonium Modes
All tags
Kaon tags In
f = -1 events
sin2b=0.56 ± 0.15
sin2b=0.59 ± 0.20
Raw ACP
Raw ACP
9/19/2018
Vivek Sharma

45. Raw CP Asymmetry for J/y KL
sin2b=0.70±0.34
Background contribution
9/19/2018
Vivek Sharma

46. Sin2b = 0.59 ± 0.14 Sin2b Results : July 5th, 2001
Phys. Rev. Lett (2001)
Calibration:Null 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(Lmax>Lobs) > 27%
9/19/2018
Vivek Sharma

47. Run1-Run2 change for PRL modes: 1.8s
Run1 – Run2 Comparison
Run1-Run2 change for PRL modes: 1.8s
Run1
Run2
9/19/2018
Vivek Sharma

48. 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
9/19/2018
Vivek Sharma

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)
9/19/2018
Vivek Sharma

50. Major Sources of Systematic Error in Sin2b
Measurement is Statistics Dominated
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
Resolution model, outliers, residual misalignment of the Silicon Vertex Detector
Flavor Tagging
possible differences between BCP and Bflavor samples
Background Characterization: (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 Systematic Uncertainty: for total sample
9/19/2018
Vivek Sharma

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)
9/19/2018
Vivek Sharma

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)
9/19/2018
Vivek Sharma

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/
(also other recent global CKM matrix analyses)
9/19/2018
Vivek Sharma

54. Summary Of Time-Dependent Measurements
BaBar has observed CP violation in the B0 system at 4.1s 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 Oscillation frequency Dmd
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

56. Luminosity Plans for BABAR & PEP II
Expect 550 fb-1 By 2006
9/19/2018
Vivek Sharma

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
D*D* mode requires angular analysis (in progress)
9/19/2018
Vivek Sharma

58. New Modes for “Sin2b”: 20 fb-1
Go Back
~ 32 events
~ 11 events
B   p0
9/19/2018
Vivek Sharma

59. CP violation in B0  p+p- decays
Decay distributions f+(f-) when tag = B0(B0) u d b
tree diagram
penguin diagram d u b
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
9/19/2018
Vivek Sharma

60. CP Sample B0  p+p- L= 30.4 fb-1 Total Yields (fit): 23 pp 2 Kp
For Illustration purposes:
Events after likelihood ratio cuts
23 pp 2 Kp
20 pp 1 Kp
L= 30.4 fb-1
Total Yields (fit):
126 Kp 3 pp
139 Kp 3 pp
9/19/2018
Vivek Sharma
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 !
9/19/2018
Vivek Sharma

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 !
9/19/2018
Vivek Sharma

63. Summary Of Time-Dependent Measurements
BaBar has observed CP violation in the B0 system at 4.1s 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
With anticipated 100 fb-1 by next summer, we expect the precision in sin2b to be ~ 0.08
Searches for CP violation in B0  p+p- decays started
sin2b = 0.59 ± 0.14 ± 0.05
9/19/2018
Vivek Sharma

64. Backup Slides
9/19/2018
Vivek Sharma

65. Consistency Check: Run1 vs. Run2
Difference for modes used in the old PRL: 1.8 s
Run 1
Run 2
9/19/2018
Vivek Sharma

66. Improved Particle Reconstruction
Y(2S) Ks(p+p-)
ccKs(p+p-)
J/Y K*0(K+p-)
J/Y Ks(p0p0)
J/Y Ks(p+p-)
cc K+
Y(2S) K+
J/Y K+
Run2/Run1
J/Y K* non CP
KS Golden modes ~30% larger than run 1: efficiency improved
9/19/2018
Vivek Sharma

67. Additional Channels: J/YK*0(KSp0)
Improved understanding of the background and its effective CP
(Angular analysis paper about to be submitted)
55 signal events before tagging; 37 after
9/19/2018
Vivek Sharma

68. Improved KL Selection Original analysis was optimized for S2/(S+B)
Fine for BF measurements, but not for CP
Need to optimize accounting for the background asymmetry (S+AB/ASB)2/(S+B)
Re-optimized with Monte Carlo
Expect 10% improvement on the error
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

70. CP Sample: Non-KL Modes
Present Sample: 725
PRL Sample: 425
Before tagging and vertexing requirements
NNOW=672
9/19/2018
Vivek Sharma

71. CP Sample: J/Y KL Run1+Run2 N/p(%) EMC IFR Run1 77/52 96/68 Run2 49/59
32/55
Run1+Run2
128/56
129/65
9/19/2018
Vivek Sharma

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)
9/19/2018
Vivek Sharma

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 %
9/19/2018
Vivek Sharma

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 = ps and Dmd = ps-1 fixed
9/19/2018
Vivek Sharma

75. New sin2b world average is 8s significant
The New World Average
New sin2b world average is 8s significant
Measurements assumed
to be uncorrelated
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

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
9/19/2018
Vivek Sharma

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 
9/19/2018
Vivek Sharma

79. Drift Chamber (DCH)
40 axial and stereo layers inside
1.5 Tesla magnetic field
80:20 helium:isobutane
Measurement of charged particle
momentum and ionization
loss dE/dx for PID
Track reconstruction efficiency 98% for p>200 MeV/c,
>500 mrad, and nominal DCH voltage.
PID up to p=0.7 GeV/c: (dE/dx) =7.5 % (Bhabha)
SVT + DCH: impact parameter resolution
65 µm in z
55 µm in transverse plane
at p=1.0 GeV/c
Reconstruction of the decay J/+- in
selected BB events.
Mass resolution: 11.4 MeV/c2.
9/19/2018
Vivek Sharma

80. PID performance, p0 reconstruction
EMC: e±,g,p0 ID
Muons
Electrons
Egg>300MeV
Mgg (GeV/c2)
9/19/2018
Vivek Sharma

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 GeV/c e- e+
Barbox
Standoff box
Compensating coil
Support tube (Al)
Assembly flange
Typical performance:
number of detected photons:
average track qc resolution: 2.4mrad (in e+e– +– events, 3–9 GeV/c)
9/19/2018
Vivek Sharma

82. Tight Kaon ID
9/19/2018
Vivek Sharma

83. 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) ~
9/19/2018
Vivek Sharma

84. B Lifetime : Systematic uncertainties
Systematic effect
0 (ps)
 (ps)
(/0)
Comment
MC statistics
0.009
0.006
stat. limitations of MC validation studies
Resolution parameterization
0.011
0.003
some included in stat. error (free parameters), study different parameterizations,
SVT alignment algorithm
Common resolution parameters
0.004
0.005
different resol. parameters for charged and neutral B (different D0/D+ mix)
Beam spot
0.002
cancels
propagate errors on BS position and size
t outliers
vary mean and width of outlier PDF
Absolute z scale
0.008
absolute z scale estimated to better than 0.5% using secondary interactions in beam pipe
Boost
propagate errors on PEP-II boost
Signal probability
propagate errors from mES lineshape fit
Background modeling
compare t distribution of background events in signal region and in sideband;
wrong-charge contamination of signal
Total in quadrature
0.022
9/19/2018
Vivek Sharma

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.
<r> (mm)
The distance between these
two points is known from
an independent measurement.
z (mm)
9/19/2018
Vivek Sharma

86. B Lifetime: t Resolution Parameters
Parameterization of resolution function:
f * G(s) + (1- f) * G(s)  E(k) (3 parameters)
Data
Monte Carlo
1.21  0.07
1.057  0.003
1.063  0.005 s
1.04  0.24
0.881  0.015
1.027  0.021 k
0.69  0.07
0.685  0.007
0.709  0.008 f
B0/B0 and B B
B0/B0
Parameter
9/19/2018
Vivek Sharma

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
3.8
3.3
fb-1/month
1.03
0.8
fb-1/week
184
135
pb-1/day
3.28
3.0
nb-1/s
=1033 cm-2 s-1
Achieved
Goal
9/19/2018
Vivek Sharma

88. 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
9/19/2018
Vivek Sharma

89. 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]
9/19/2018
Vivek Sharma

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 p+p-/K+p-:
No particle ID used
until the fit is performed
mES distributions for the different tagging categories
NT1
NT2
9/19/2018
Vivek Sharma

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: p+p- , K+p- , K-p+ , K+K-)
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
9/19/2018
Vivek Sharma

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 the CKM Matrix b a g
Vtd Vtb *
Vud Vub
Vcd Vcb
Unitarity Triangle
9/19/2018
Vivek Sharma

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 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 B 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
9/19/2018
Vivek Sharma

94. Time evolution of B0 mesons into a final CP eigenstate
The decay distribution for events with a B0 (f+) and B0bar tags (f-)
Amplitude ratio
Weak Phase
In order to have CP Violation
9/19/2018
Vivek Sharma

95. Time-dependent CP Asymmetry
From the time evolution of the B0 and B0 states we can define the time-dependent asymmetry :
Probe of direct CP violation since it requires
Sensitive to the phase of l even without direct
CP Violation
9/19/2018
Vivek Sharma

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 d b W- u p+ p- B0
Decay Diagram
u,c,t
Mixing Diagram
9/19/2018
Vivek Sharma