1. CP Violation lectures
Measurement of the time dependence of B0B0bar oscillation using inclusive dilepton events
[BABAR-CONF-00/10]
G. Cerminara

2. Outline The Goal of the measurement How to carry on the measurement
What they wanted to measure
Why to measure B0B0bar oscillation
How to carry on the measurement
B flavor tagging through lepton sign
Time evolution measurement
Experimental challenges
Lepton identification and dileptons event selection
Background rejection
Dz measurement
Results and systematic uncertainties
The fit to data
Results

3. Outline The Goal of the measurement How to carry on the measurement
What they wanted to measure
Why to measure B0B0bar oscillation
How to carry on the measurement
B flavor tagging through lepton sign
Time evolution measurement
Experimental challenges
Lepton identification and dileptons event selection
Background rejection
Dz measurement
Results and systematic uncertainties
The fit to data
Results

4. Why to measure B0B0bar oscillations
B0B0bar oscillation frequency is sensitive to CKM matrix element |Vtd|
Gives stringent constraint on Unitarity Triangle (together with BsBsbar oscillation frequency) B0
B0bar b
bbar
dbar d
(u, c), t
~ VtbVtd mt2/mW2 Dm

5. What they Measured
In order to measure DmB0 between the two mass eigenstates we must estimate:
Therefore we want to measure the time dependent asymmetry
Dt = difference between two decay times of the B mesons
Rate( BB  Same b flavor )
Rate( BB  Opposite b flavor )
DmB0
(t)
(1)
Opposite b flavor
Same b flavor
(2)

6. Outline The Goal of the measurement How to carry on the measurement
What they wanted to measure
Why to measure B0B0bar oscillation
How to carry on the measurement
B flavor tagging through lepton sign
Time evolution measurement
Experimental challenges
Lepton identification and dileptons event selection
Background rejection
Dz measurement
Results and systematic uncertainties
The fit to data
Results

7. Basic Idea (I) To carry on this measurement we must:
Tag the B meson flavor
Measure the flavor of the BB mesons as a function of time
1) Use lepton to tag the B meson flavor
The sign of the lepton determines the flavor of the B meson
BR ( B0  ℓ+n + anything) ~ 10 %
BR ( B0bar  ℓ-n + anything) ~ 10 %
With ℓ = e and m
The time dependent asymmetry becomes:
Inclusive reconstruction in order to have high reconstruction efficiency
(3)

8. Basic Idea (II)
2) Use PEP-II asymmetric B factory for time-dependent measurement
PEP-II asymmetric e+e- collider:
e+: p = 3.1 GeV
e-: p = 9.0 GeV
CM of (4s) has a boost along the beam axis
BBbar almost at rest in (4s) CM but boosted along the beam axis (z)
(4s)  BBbar <bg> = 0.554
It is possible to measure the proper decay time of the B mesons measuring their decay length
We can therefore compute Dt measuring Dz !!! e- e+
(4s)
(4s)
(4s) Dz { B0
B0bar

9. Sketch diagram of the measurement
Bbbar produced by (4s) decay:
e+e-  (4s)  BBbar e- e+
(4s)
Z axis (beam)

10. Sketch diagram of the measurement
Bbbar produced by (4s) decay:
e+e-  (4s)  BBbar e- e+
(4s) B0
Z axis (beam)
B0bar

11. Sketch diagram of the measurement
Bbbar produced by (4s) decay:
e+e-  (4s)  BBbar
B decay semileptonically
B  ℓ + n + X ℓ+ e- e+
(4s) B0
Z axis (beam)
B0bar ℓ-

12. Sketch diagram of the measurement
Bbbar produced by (4s) decay:
e+e-  (4s)  BBbar
B decay semileptonically
B  ℓ + n + X
Leptons are inclusively reconstructed
The lepton sign used for flavor tagging
Track extrapolated to point of closest approach to the beam axis
Dz measured as z difference of positions of closest approach  compute Dt ℓ+ e- e+
(4s) B0 Dz
Z axis (beam)
B0bar ℓ-

13. Outline The Goal of the measurement How to carry on the measurement
What they wanted to measure
Why to measure B0B0bar oscillation
How to carry on the measurement
B flavor tagging through lepton sign
Time evolution measurement
Experimental challenges
Lepton identification and dileptons event selection
Background rejection
Dz measurement
Results and systematic uncertainties
The fit to data
Results

14. The BaBAR Detector
Quick overview of main Babar subdetectors (from in to out):
SVT: Silicon Vertex Detector
DCH: Drift Chamber
Charged particle detection
Momentum measurement
DIRC: Cherenkov Radiation Detector
Particle identification
EMC: Electromagnetic Calorimeter
Photon detection
Pion/electron separation
IFR: Instrumented Flux Return
Return Yoke instrumented with RPCs
Muon identification (rejection of long life particles)

15. Lepton Identification
Selection of dilepton events, basic ingredients are:
Lepton (e, m) identification
Electrons:
Specific requirement on ratio: Energy in the EMC/Momentum in DCH
Lateral shape of energy deposition in the Calorimeter (EMC)
Ionization density in the DCH
Muons:
Energy released in the calorimeter
Compatible with Minimum Ionizing Particle
Reconstruction in the IFR
Track continuity and penetration depth
Efficiencies are monitored with reference samples:
Main sources of contamination are p0 and p+/- (~0.3% and ~3% respectively)
Electrons
Muons
Efficiency
92 %
75 %

16. Background Rejection (I)
Non BB background rejection (Continuum Rejection)
Fox-Wolfram ratio (R2) requirement:
The Fox-Wolfram momentum of order lth is defined as:
Momentum-weighted sum of Legendre polynomial of the lth order computed from the cosine of the angle between all pairs of tracks:
[G.C. Fox and S. Wolfram, Nucl. Phys. B149 (1979) 413]
The Ratio R2 is defined as:
Directional events (Continuum like) have higher R2 than spherical BBbar events:
Requirement: R2 < 0.4
(4)
(5)

17. Background Rejection (II)
Rejection of two-photon events (gg  ℓ + ℓ -)
Squared Invariant mass of the event > 20 (GeV/c2)2
Number of charged tracks > 4
Rejection of lepton pairs from J/y decay (J/y  ℓ + ℓ -):
M(ℓ + ℓ - ) < M(J/y ) – 40 MeV and M(ℓ + ℓ - ) > M(J/y ) + 40 MeV

18. Selection of Dilepton Events
Selection of direct dilepton events
We want to distinguish between cascade leptons (b  c  l) and prompt leptons from B decay
The selection is performed using a Neural Network combining 5 discrimination variables (in (4s) reference frame):
Momenta of two leptons with highest momentum (p*1, p*2)
Total visible energy and missing momentum (Etot, pmiss)
Opening angle between the leptons q12

19. Selection of Dilepton Events
More detail on some input variables:
Powerful discrimination of cascade leptons
Lepton momentum
BBbar direct leptons (signal)
Leptons from cascade

20. Selection of Dilepton Events
More detail on some input variables:
Angle between leptons
Powerful discrimination of cascade leptons
BBbar direct leptons (signal)
Leptons from cascade

21. Selection of Dilepton Events
More detail on some input variables:
Difficult to discriminate between signal and background
Missing momentum
BBbar direct leptons (signal)
Leptons from cascade

22. Selection of Dilepton Events
The NN is trained with 40k events
Dileptons from B0 and B+/-
Output variable used to discriminate:
1  direct events (signal)
0  cascade events
Require NN out > 0.8
Efficiency 9%
Signal purity
78%
NN Output
Cascade
Signal
Cascade
Signal

23. Dz Measurement Determination of z coordinate of B decay vertex:
1st approximation: the z of the point of closest approach between lepton track and the beam spot in the transverse plane
Further refinement: Use both lepton tracks to estimate the beam spot position in the transverse plane using beam spot constraint and a vertex fit. (Not really clear how…)
Resolution on Dz:
Estimated from MC data:
Double gaussian fit:
sn = 87 mm
sw = 195 mm
Cross check with real data (J/y  ℓ + ℓ -) :
5% - 10% uncertainties on these values
(one of major contribution to systematic
uncertainties)

24. From Dz measurement to Dt
The difference between the two decay times is:
With <bg> = 0.554
No correction for B meson motion in (4s) reference system:
In inclusive approach it is not possible to determine the B boost
MC study shows that this effect is negligible compared to the level of accuracy of this analysis
(6)

25. Outline The Goal of the measurement How to carry on the measurement
What they wanted to measure
Why to measure B0B0bar oscillation
How to carry on the measurement
B flavor tagging through lepton sign
Time evolution measurement
Experimental challenges
Lepton identification and dileptons event selection
Background rejection
Dz measurement
Results and systematic uncertainties
The fit to data
Results

26. The Final Fit
Value and error of DmB0 is extracted with c2 fit to dilepton asymmetry
The fit function is:
ƒOS and ƒSS contain contributions from mistagged events and depend on:
DmB0, h0(mistag fraction), R (charged B fraction) and a (time dependence of h0).
(Used as free parameters in the fit)
Lifetimes fo charged and neutral B: G0, G+ (fixed to the world average values)
(7)
ƒreso = resolution function
ƒother = time distribution for background events
(cascade leptons and non-BBbar events)
Estimated from off-resonance data
OS: opposite sign
SS: same sign

27. The Final Fit Four parameter fit to data: Results:
Data collected after an integrated luminosity:
L = 7.73 fb-1 (July 2000)

28. Final result The result for DmB0 measurement is therefore:
Main sources for systematic uncertainties are:
Lepton misidentification
Time-dependence of the cascade events
Resolution function of Dz

29. Backup slides (I) Functions used in the fit:
Time distributions of mixed and unmixed events (explicit form)
Time distributions of Opposite and Same sign events:
Mistagged events