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B d Mixing Measurements with the BaBar Detector. Cecilia Voena Università “La Sapienza” & INFN Roma. on behalf of the BABAR Collaboration. ICHEP2002. B 0 B 0 Mixing. V * td. b. t. d. -Proceeds through box diagram. B 0. B 0. W. W. B 0. d. t. b. - PowerPoint PPT Presentation
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1
Bd Mixing Measurements with the BaBar Detector
Cecilia VoenaUniversità “La Sapienza”
& INFN Roma
ICHEP2002
on behalf of theBABAR Collaboration
2
B0B0 Mixing-Proceeds through box diagram
22 2 2 2 2
02( / ) | |
6 d d d
Fd w B t W B td B B
Gm m e S m m m V B f
2 2(210 40MeV)d dB BB f
(D. Becirevic and L. Lellouch, at this conference)
- Extraction of Vtd limited by theoretical hadronic uncertainties
B0
- md sensitive to Vtd CKM matrix element
if q/p 1 => CP violationin mixing (assuming CPT invariance )
B0 B0
b
b
d
d
W W
t
t
V*td
H H11H12
H21H11
q
pH21
H12
1
2
md 2Re H12H21 1
2
23533+00-23 MeV
Mass difference betweenHamiltonian eigenstates
md constraint in(,) plane
F.Parodi
3
B0B0 Mixing at Asymmetric B Factories
23 x 106 BB (B0D*-l+ and dilepton)32 x 106 BB (hadronic)
-Mixing and lifetime with B0D*-l+l decays-Mixing with hadronic decays (PRL 88:221802[2002])-Mixing with dilepton events (PRL 88:221803[2002])
new result
Before B factories:
At BaBar, asymmetric B factory, we measure:
-can perform time dependent measurements-high statistics sample of fully reconstructed Bd,no Bs
md = 0.472 ± 0.017 ps-1
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B0B0 Mixing with Fully Reconstructed B0
(4s)
= 0.55
Tag B
z ~ 160 m Reco Bz ~ 80 m
l+z|z|> 250 m
K
D0
-K+
-
K+
+
-
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 t z/c 6. Fit the t spectra of mixed (B0B0 or B0B0) and unmixed (B0B0)events
e- e+
5
t Distribution of Mixed andUnmixed Events
Decay Time Difference (reco-tag) (ps)
UnMixedMixed
0
10
20
30
40
50
60
-8 -6 -4 -2 0 2 4 6 8
perfect flavor tagging & time resolution
Decay Time Difference (reco-tag) (ps)
UnMixedMixed
0
10
20
30
40
50
60
-8 -6 -4 -2 0 2 4 6 8
realistic mis-tagging & finite time
resolution
Unmix
xMi
f (Δ t) 1 1 2 cos( ) ResolutionFunction4
Bd
d
d
| Δt |/τ
B
e tτ
mw Δ Δ
Decay time distribution for signal events
oscillation frequencyB0 lifetime Mistag probability
B0B0
B0B0 ,B0B0
-+
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B0D*-l+ Selection
• Cannot reconstruct B0 mass and energy because of the missing => use m(D*)-m(D0) distribution
• Reconstruct candidate D* using full decay tree
D*+ D0+ , D0 K-+,K-+0,K-+-+,Ks+-
• Combine with lepton candidate (p* > 1.2 GeV)
• Require consistency with D*-l kinematics (angles, missing )
signal
background
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Sample Composition
signal sample
Peaking background: real D*non from semileptonicdecays-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
For electrons, muons and Kaons use the charge correlation
b c
d d
l-
B0 D, D*
W-
0
0
l
l
B
B
Lepton Tag
b
d
B0
W- W+c s
d
0
0
0
0
kaons
kaons
Q
Q
B
B
Kaon Tag
Hierarchical, Mutually Exclusive, Tagging CategoriesHierarchical, Mutually Exclusive, Tagging Categories
Neural network exploits information carried by non-identified leptons and kaons, soft pions from D* decays
K* 0 K- +
Tagging Category
Efficiency(%)
Mistag Fractionw(%)
Q=(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 samplesfloat m, B0 , mistag probabilities, signal andbackground t resolution parameters, fraction of charged B
md=0.492±0.018±0.013ps-1
Mixing asymmetry
( ) ( )( )
( ) ( )
(1 2 )cos( )
unmixed mixedmix
unmixed mixed
N t N tA t
N t N t
w m t
B0 =1.523+0.024-0.023±0.022ps
Preliminary
+
-
Floating parameters:Innermost m, B0
m, B0, mistag m, B0, fB+
m, B0, fB+,mistag All signal t par Default fit
errorellipse
correlation coefficient (m, B0) = -0.22
6B0-6B0
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Mixing with Hadronic Events
• Fully reconstructed B0D(*)-+/+/a1+,J/K*0
md=0.516±0.016±0.010 ps-1
mixA
PRL 88[2002]
• All t and mistag parameters simultaneously extracted from data
used for sin2 measurement
• lifetime measurements with hadronic events
0 = 1.546 0.032 0.022 ps = 1.673 0.032 0.023 ps /0 = 1.082 0.026 0.012
PRL 87 [2001]1-2w
/2md
6347 B0
(purity ~86%)• Largest syst. is B0 lifetime
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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
md= 0.493±0.012±0.009 ps-1
PRL 88[2002]
with the same sample:T and CP violation in mixing
|q/p|=0.998±0.006±0.007
PRL 88[2002]
99010 dileptons(B0B0 purity ~40%)
12
B0 Lifetime with Partially Reconstructed B0D*-l+
Kinematic constraints at the (4S):
-pD* = p+
-M2= (pB0- pD* - pl)20
B0 = 1.529 ±0.012 ±0.029 psPRL 89[2002]
reconstructlepton and only
in a small cone around D*direction, and from simulation
missing neutrino
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Summary• New simultaneous measurement
of md and B0 with the exclusive
B0D*-l+l sample
md= 0.492±0.018±0.013 ps-1
B0 = 1.523+0.024-0.023±0.022 ps
• Combined BaBar result for
Bd mixing frequency:
Preliminary
md= 0.500±0.008±0.006 ps-1
2% error
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Backup slideSignal t Model
15
Backup slide
Background t Model
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Backup slideSystematic error
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Backup slideSystematic error
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Backup slide
Tagging Category
Efficiency(%)
Mistag Fractionw(%)
B0/B0 diff.w(%)
Q=(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
Mistag fraction determined from simultaneous fit to Bflav sample
19
Backup slide
Signal Sample
Combinatoric Enriched : Sideband
Peaking Bkgs “l not from Brec” Enriched : D* with
flipped l
Continuum D* Enriched : Off-resonance Data
Fake l Enriched :l candidate fails e, ID
20
Backup slide
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