Bd Mixing Measurements with the BaBar Detector

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Bd Mixing Measurements with the BaBar Detector

• Cecilia Voena
• Università La Sapienza
• INFN Roma

on behalf of the BABAR Collaboration
ICHEP2002
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B0B0 Mixing
Vtd
b
d
t
-Proceeds through box diagram
B0
B0
B0
W
W
b
d
t
if q/p ? 1 gt CP violation in mixing (assuming
CPT invariance )
?md constraint in (?,?) plane
Mass difference between Hamiltonian eigenstates
F.Parodi
- ?md sensitive to Vtd CKM matrix element
- Extraction of Vtd limited by theoretical
hadronic uncertainties
235?3300-23 MeV
(D. Becirevic and L. Lellouch, at this conference)
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B0B0 Mixing at Asymmetric B Factories
Before B factories
?md 0.472 0.017 ps-1
-can perform time dependent measurements
At BaBar, asymmetric B factory, we measure
-high statistics sample of fully reconstructed
Bd,no Bs
new result
-Mixing and lifetime with B0?D-l?l
decays -Mixing with hadronic decays (PRL
882218022002) -Mixing with dilepton events
(PRL 882218032002)
23 x 106 BB (B0?D-l? and dilepton) 32 x 106 BB
(hadronic)
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B0B0 Mixing with Fully Reconstructed B0
p-
K
K
K
Tag B sz 160 mm
?
Reco B sz 80 mm
?-
e- e
D0
?-
?(4s)
l
Dz
bg 0.55
ltDzgt _at_ 250 ?m
1. Reconstruct one B meson in the flavor
eigenstates D-l?l or hadronic
(e.g.D-? ) 2. Reconstruct the vertex position
3. Reconstruct inclusively the vertex of the
other B meson (BTAG) 4. Determine the
flavor of BTAG
5. Compute the proper time difference Dt _at_ Dz/gbc
6. Fit the Dt spectra of mixed (B0B0 or B0B0)
and unmixed (B0B0)events
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?t Distribution of Mixed andUnmixed Events
realistic mis-tagging finite time resolution
perfect flavor tagging time resolution
B0B0
B0B0
,B0B0
Decay time distribution for signal events

-
oscillation frequency
B0 lifetime
Mistag probability
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B0?D-l? Selection

• Reconstruct candidate D using full decay tree

D ? D0? , D0 ?K-?,K-??0,K-??-?,Ks??-

• Combine with lepton candidate (p gt 1.2 GeV)

• Require consistency with D-l kinematics
• (angles, missing ?)

signal

• Cannot reconstruct B0 mass and
• energy because of the missing ?
• gt use m(D)-m(D0) distribution

background
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Sample Composition
Peaking background real Dnon from
semileptonic decays
signal ? sample
-uncorrelated lepton from other B -fake
lepton -continuum events
Combinatoric fake D
m(D)-m(D0) (MeV)

• Select several control samples to characterize
  (fraction,?t shape)
• the main sources of backgrounds

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Flavor Tagging
Hierarchical, Mutually Exclusive, Tagging
Categories
For electrons, muons and Kaons use the charge
correlation
W
W-
b
c
s
K 0 ? K- ?
Kaon Tag
Neural network exploits information carried by
non-identified leptons and kaons, soft pions from
D decays
Tagging Category Efficiency e() Mistag Fraction w() Qe(1-2w)2 ()
Lepton 11.1 8.6 7.6
Kaon 34.7 18.1 14.1
Neural Net 54.2 37.7 3.3
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B0? D-l? Mixing Likelihood Fit Results

• Simultaneous fit to all signal and background
  samples

float ?m, ?B0 , mistag probabilities, signal
and background ?t resolution parameters,
fraction of charged B
Preliminary
-
Dmd0.4920.0180.013ps-1

?B0 1.5230.024-0.0230.022ps
correlation coefficient ?(?m, ?B0) -0.22
Floating parameters Innermost ?m, ?B0 ?m,
?B0, mistag ?m, ?B0, fB ?m, ?B0,
fB,mistag All signal ?t par Default fit
error ellipse
Mixing asymmetry
6?B0
-6?B0
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Mixing with Hadronic Events

• Fully reconstructed B0?D()-?/?/a1,J/?K0

• All Dt and mistag parameters simultaneously
• extracted from data

6347 B0 (purity 86)
used for sin2? measurement

• Largest syst. is B0 lifetime

• lifetime measurements
• with hadronic events

1-2w
PRL 87 2001
t0 1.546 ? 0.032 ? 0.022 ps t? 1.673 ?
0.032 ? 0.023 ps t? /t0 1.082 ? 0.026 ?
0.012
??/2?md
PRL 882002
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Mixing with Inclusive Dilepton Events

• Very precise mixing measurement
• Select events with 2 high momentum leptons
  (sample contains 50 B)
• Largest syst. is B0 lifetime and resolution
  function parameterization

Dmd 0.4930.0120.009 ps-1
99010 dileptons (B0B0 purity 40)
PRL 882002
with the same sample T and CP violation in mixing
q/p0.9980.0060.007
PRL 882002
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B0 Lifetime with Partially Reconstructed
B0?D-l?
reconstruct lepton and ? only
Kinematic constraints at the ?(4S)
? in a small cone around D direction, ? and ?
from simulation
-pD ?p??
missing neutrino
-M2? (pB0- pD - pl)2?0
?B0 1.529 0.012 0.029 ps
PRL 892002
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Summary

• New simultaneous measurement
• of Dmd and ?B0 with the exclusive
• B0?D-l?l sample

Preliminary
Dmd 0.4920.0180.013 ps-1
?B0 1.5230.024-0.0230.022 ps

• Combined BaBar result for
• Bd mixing frequency

Dmd 0.5000.0080.006 ps-1
2 error
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Backup slide
Signal ?t Model
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Backup slide
Background ?t Model
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Backup slide
Systematic error
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Backup slide
Systematic error
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Backup slide
Mistag fraction determined from simultaneous fit
to Bflav sample
Tagging Category Efficiency e() Mistag Fraction w() B0/B0 diff. Dw() Qe(1-2w)2 ()
Lepton 11.1 ? 0.2 8.6 ? 0.9 0.6 ? 1.5 7.6 ? 0.4
Kaon 34.7 ? 0.4 18.1 ? 0.7 -0.9 ? 1.1 14.1 ? 0.6
NT1 7.7 ? 0.2 22.0 ? 1.5 1.4 ? 2.3 2.4 ? 0.3
NT2 14.0 ? 0.3 37.3 ? 1.3 -4.7 ? 1.9 0.9 ? 0.2
All 67.5 ? 0.5 25.1 ? 0.8
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Backup slide
Signal m Sample
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Backup slide