1. BaBar Physics Highlights and UCSD Contributions
Vivek Sharma
University of California, San Diego
October 2nd, 2001

2. BaBar Physics Highlights : A Short summary
Outline of This Talk
BaBar Physics Highlights : A Short summary
Time-dependent B physics measurements
Sin2
B Mixing and Lifetimes
UCSD contributions to BaBar Physics
Leadership roles in Time-dependent B physics measurements
Leadership roles in BaBar Physics Infrastructure
Prolific B meson reconstruction (if time permits)
Physics plans for coming year(s)
9/18/2018
V.Sharma DOE Review 2001

3. Over-Constraining the Unitarity Triangle
Major goal of BaBar experiment : Precisely determine the sides and the angles of the Unitarity Triangle : A terrific start already !
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V.Sharma DOE Review 2001

4. BaBar Physics Highlights : 2001
Observation of CP Violation in B Meson System (PRL, 7/5/2001)
Preceded by “Measurement of CP-violating asymmetries in B0 decays to CP eigenstates” ( PRL, Feb 2001)
Precise measurements of time-dependent B0 Oscillations with
Fully reconstructed B Samples & Dilepton samples
World’s best measurement of B0 and B- Meson lifetimes and ratio
Measurement of B0  + - and K+ - Rates, searches for time-dependent (Sin2) and “ Direct” CP Violating rate asymmetries
Studies of Electroweak Penguins and precise searches for CP asymmetries in B meson Decays
Rare Hadronic B Decays => Future CPV studies
World’s largest sample of fully reconstructed B Mesons and the beginning of a new paradigm: “Recoil Side B physics”
Diverse Program in B Physics : See DM’s talk for publications so far
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V.Sharma DOE Review 2001

5. Major Emphasis of These Past Years : Sin2 and associated Time-dependent B Decay Measurements Focus of UCSD Group’s Physics Effort in BaBar
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V.Sharma DOE Review 2001

6. Data Sample : Available and Analyzed
30/fb analyzed
for CPV related Studies (summer)
October 99
September 25, 2001
PEP-II & BaBar establishing world records in
delivering and recording High Luminosity data
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V.Sharma DOE Review 2001

7. CPV in Interference Between B 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
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V.Sharma DOE Review 2001

8. 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
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V.Sharma DOE Review 2001

9. Time-dependent CP Asymmetry In Mixing and Decay : Sin2
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
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V.Sharma DOE Review 2001

10. “Golden” Decay Mode: B0 ® J/y K0
K0 mixing
u,c,t W-
Theoretically clean mode to measure the phase of l (sin2b)
Clean experimental signature
Large branching fraction compared to other CP eigenstates
“Golden Modes”
Time-dependent CP asymmetry Is
hCP = -1
B0  J/ K0S
B0  (2s) K0S
hCP = +1
B0  J/ K0L
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V.Sharma DOE Review 2001

11. CP Asymmetry: 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
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V.Sharma DOE Review 2001

12. B Decay Topology in the Boosted (4S) Environment
All the action is along the beam (Z) axis
Tag vertex reconstruction
Flavor Tagging
(bg)U(4S) = 0.56
The Clock Starts z
Coherent BB pair
Exclusive B Meson and Vertex Reconstruction
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V.Sharma DOE Review 2001

13. Sin2 Analysis Approach
Factorize the analysis into three conceptual building blocks
Analysis Ingredient
(a) Reconstruction of B mesons in flavor eigenstates
(b) B decay time 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
Increasing complexity
Higher precision
Walk thru each analysis step
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V.Sharma DOE Review 2001

14. Calibrating the t Clock : B Lifetime Measurement
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 “recoil” B meson (BTAG)
Fully reconstruct one B meson in flavor eigenstate (BREC)
Reconstruct the B decay vertex
4. compute the proper time difference Dt
5. Fit the Dt spectra
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V.Sharma DOE Review 2001

15. Fully-Reconstructed Hadronic B Decay Sample
Flavor Eigenstates Bflav : for lifetime and B 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]
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V.Sharma DOE Review 2001

16. B Lifetime 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
To be published in PRL
t0 =   ps PDG:  ps
t =   ps
PDG:  ps
t/t0 =   0.011
PDG:  0.029
background
Dt (ps)
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V.Sharma DOE Review 2001

17. 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
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V.Sharma DOE Review 2001

18. 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
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V.Sharma DOE Review 2001

19. B Meson Flavor Tagging Hierarchical Tagging Categories
For electrons, muons and Kaons use the charge correlation with decay daughters 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)
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V.Sharma DOE Review 2001

20. B Flavor Tagging Performance
The large sample of fully reconstructed B decays provides the precise determination of the B flavor tagging parameters required in the Mixing/ CP Asymmetry analysis
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 related to
the quality factor Q
Highest tag “efficiency”
Smallest mistag fraction
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V.Sharma DOE Review 2001

21. B0B0 Mixing Measurement 20 fb-1
C.L. 28 %
Dmd = ± (stat) ± (syst) h ps-1
Preliminary
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V.Sharma DOE Review 2001

22. Dmd Measurement Precision
preliminary
Precision Dmd measurement
4% statistical error
3% systematic error dominated by MC correction
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V.Sharma DOE Review 2001

23. 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
9/18/2018
V.Sharma DOE Review 2001

24. Back to Sin2 : 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
9/18/2018
V.Sharma DOE Review 2001

25. The fully Reconstructed CP Eigenstate 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
9/18/2018
V.Sharma DOE Review 2001

26. Raw Time-Dependent CP Asymmetry
All tags
Kaon tags
f = -1 events
sin2b=0.56 ± 0.15
Raw ACP
sin2b=0.59 ± 0.20
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V.Sharma DOE Review 2001

27. Observation of CP Violation in B Decays !
Phys. Rev. Lett (2001)
Crosscheck: Null result in
flavor eigenstate samples
Consistency of CP channels P(c2) = 8%
Goodness of fit (CP Sample): P(Lmax>Lobs) > 27%
Combined fit to all CP modes
sin2b = 0.59 ± 0.14 ±0.05
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V.Sharma DOE Review 2001

28. 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
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V.Sharma DOE Review 2001

29. The Unitarity Triangle Now
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, hep-ph/
(also other recent global CKM matrix analyses)
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V.Sharma DOE Review 2001

30. UCSD Contributions to Sin2b And Related Time Dependent Measurements
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V.Sharma DOE Review 2001

31. Riccardo Faccini : Gerhard Raven Soeren Prell Shahram Rahatlou
Sin2b Technical Coordinator
Responsible for integrating all aspects of physics necessary for CP Asymmetry measurement
Co-Convener, Charmonium Group : Reconstruction of CP sample
Gerhard Raven
Co-Convener B Mixing and Lifetime group,
Principal analysis on B Mixing ( with Prell, Rahatlou)
Soeren Prell
Fitting algorithm (tFit) for Sin2b, time-dependent mixing and Lifetime (BaBar Standard)
Shahram Rahatlou
Author of analysis packages for prolific B meson Reconstruction
Ph.D Thesis on Sin2b Measurement
David MacFarlane
B Mixing and Sin2b,
Vivek Sharma
Co- Convener, B Reconstruction (Breco) Group
(B Mixing) and Sin2b : Editor of July CPV “Observation” paper
9/18/2018
V.Sharma DOE Review 2001

32. UCSD Role In Physics Analysis Infrastructure
Critical Role in facilitating BaBar physics, e.g :
Developed and maintained Micro-DST for physics analysis used in all BaBar analyses
CompositionTools : Infrastructure for construction of decay chains , vertex reco., kinematic fitting
Authors of copious B meson reconstruction and analysis packages: CharmUser, BrecoUser and DstarlnuUser
heavily used in BaBar for a variety of “Recoil Side Physics”
Copied for other analyses with our technical assistance
tFIT: Framework for time-dependent CP asymmetry, B mixing and lifetime analysis : a BaBar Standard
Author of CPFramework and C++ based “Post” CPFramework :
Software standard for Data-mining and delivery of Fully Recoed B Mesons to a generic BaBar physicist interested in “Recoil Side B Physics”
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V.Sharma DOE Review 2001

33. Recoil Side B Physics With Fully Reconstructed B Sample
A new B-factory era idea (whose time has come): to alleviate model dependence in key measurements
Fully reconstructed, flavor specific sample of B mesons provide a very clean sample of B tags
Full B reconstruction of 1 B in (4S) provides:
Information about flavor of the other ( Recoil side) B => charge correlation
Provides precise P vector of the recoiling B ( e.g. B -> t n)
=> daughter particles can be transformed to the B rest frame, not (4S) CMS
=> Any two body decay produces sharp peaks in momentum spectrum of daughter particles. (think b -> s g or s g)
No udsc (continuum background to deal with ..or subtract)
Half of the event is accounted for => drastically reduced combinatorial background in studying recoil side B properties
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V.Sharma DOE Review 2001

34. A Completely Reconstructed (4S) Event
Example of Recoil Side Physics
All particles accounted for
Nothing Missing !
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V.Sharma DOE Review 2001

35. Possible Recoil Side Measurements
Bread and Butter Physics examples
Inclusive Semileptonic decay rate for B0 and B- (LP01)
Particle spectra (K, ,  …..)
Right and Wrong Sign (upper vertex ) Charm production
Charm species, Rate and spectrum trivially obtained
More Serious Stuff
B -> s g, and b -> s Glue (exclusive and inclusive with reduced modelling)
B -> t n and B -> s n n (The ONLY way to get to them!)
Inclusive b -> u l n => Minimal Model dependence
Add your favorite physics
9/18/2018
V.Sharma DOE Review 2001

36. Exclusive B Semileptonic Samples: 2000 DataKp
6.1k
~ 1K reconstructed per fb-1
Very High Purity ~80%
=> 500K fully recoed B in 500 fb-1
Kpp0
5.0k
Kppp
KSpp
5.0k
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V.Sharma DOE Review 2001

37. Fully Reconstructed Hadronic B Decay Sample
Run I with D0 p-, D*0 p-, D*0 r-, D*0 a1-, J/y K-, y(2S) K-, cc K- and J/y K*-.
B- Sample: 20K in 20 fb-1
500K Clean in 500 fb-1
This is just a proof of principle!
Expect to increase the reconstruction rate by ~2-3
Expect M Hadronic B of each species by 2005
Sample with the largest Set of Constraints for Recoil side studies
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V.Sharma DOE Review 2001

38. Example of Recoil Side B Physics : Precision Vub Measurement
To distinguish bu from bc theoretically:
better better
q2 spectrum > mhad spectrum > Elepton spectrum
But experimental difficulty is in opposite order
To make major experimental progress in Vub
need powerful suppression of b cl
provided by full reconstruction of companion B
Study this
Reco this
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V.Sharma DOE Review 2001

39. Inclusive Hadronic Mass Spectrum
Jik
select b u with mx< mD (~90% acceptance for b u (model ???)
require: Q(event) =0, 1 lepton/event, missing mass consistent with neutrino
just look at mhad< 1.7 , cut with largest acceptance and hence least theoretical uncertainty, keep bkgd small with p(lepton)>1.4 GeV
firts q^2 plot shows end pointregions, at high q^2 the range of lepton energies is onbly about twice as great as
the endpoint cut the second ploit
is after data have been extrapolated to full q2 range
this means that allowance is amde for the fraction of leptons in each q^2 bin that fail the endpoint lepton cut
the error bar ijs larger at lower q^2 becasue more of the
leptons failed the endpoint cut there
could add the plot from lep tha shows the benefits of working in the m_x, mD
TRKSIM CLEO
III FAST MC
9/18/2018
V.Sharma DOE Review 2001

40. Snowmass Study: Inclusive Vub
~100 b ulv events/30 fb-1 : Method attractive with large data samples
Systematic error dominated by charm leakage into signal region.
Depends on S/B ratio & B. Assume B = fb-1.
S/B can be improved by vertexing.
B can be reduced as Br(B [D*/D**/D/D ] l) and the form factors in these decays become better measured. B can also be reduced through better knowledge of D branching ratios.
Assume these improvements lead to B = fb-1 or higher Ldt.
Recall theoretical error is alleast ~ 10%
year Ldt # bul #b cl Vub Vub Vub
(stat) (sys) (expt)
fb % 2.2% 3.9%
fb % 1.5% 2.1%
fb % 1.5% 1.7%
firts q^2 plot shows end pointregions, at high q^2 the range of lepton energies is onbly about twice as great as
the endpoint cut the second ploit
is after data have been extrapolated to full q2 range
this means that allowance is amde for the fraction of leptons in each q^2 bin that fail the endpoint lepton cut
the error bar ijs larger at lower q^2 becasue more of the
leptons failed the endpoint cut there
could add the plot from lep tha shows the benefits of working in the m_x, mD
9/18/2018
V.Sharma DOE Review 2001

41. UCSD Physics Goals in the Short Term
Continue with successful & singular focus on Sin2b and B0 mixing measurements
Expect a precision of by next summer
Continue to develop tFIT to include more decay modes with their specific physics issues
Transversity (angular) analysis for PsiK*, D*D*
Sin2a
Continue to help build large samples of fully reconstructed B Mesons for
B-   
Model independent B  Xu l  final states : Vub Measurements
In 2002 data and beyond
9/18/2018
V.Sharma DOE Review 2001