Measurement of the Bc Meson Lifetime with the Collider
Detector at Fermilab
Measurement of the Bc Meson Lifetime with the Collider
Detector at Fermilab
Masato Aoki
2
The Bc MesonThe Bc Meson
• Ground state of differently flavored heavy quarks (bottom quark + charm quark)
• Similar binding interaction to the case of heavy quarkonium(cc,bb) but different dynamics
• Only weak decays are possible
• Currently only Tevatron can produce the Bc
B0(b+d) , B+(b+u), Bs0(b+s) Upsilon(b+b), Psi(c+c)
Bc+ (b+c)
3
Decays of the Bc MesonDecays of the Bc Meson
4
Theoretical PredictionTheoretical Prediction
B+ meson : ~1.7 psD0 meson : ~0.4 ps
5
MotivationMotivation
• Contributions from the three major decay diagrams affect the Bc meson lifetime
• Precise measurements of the Bc meson will provide insight into the strong dynamics of heavy quarks
• We measure the Bc lifetime with high statistics data collected by the CDF in Tevatron Run2
6
HistoryHistory
• ~20 signal events• Mass:
6.40.39(stat.)0.13(syst.) GeV/c2
• Lifetime:0.46+0.18/-0.16(stat.)0.03(syst.) ps
CDF Run-I(1998) observed BcJ/l signal
7
Bc Meson ReconstructionBc Meson Reconstruction
BcJ/ee channel– J/ di-muon trigger dataset– Large branching ratio– Unable to fully reconstruct due to neutrino…
• Cannot make a sharp peak• Need to understand all background
Search window M(J/e) : 4~6 GeV
M(J/) M(Bc)
8
Tevatron Run2 (2001~)Tevatron Run2 (2001~)• New main injector
(150 GeV proton storage ring)• New recycler storage ring for p
accumulation• Higher energy pp collisions at 1.96 TeV
(was 1.8 TeV)• Increased number of p and p bunches
from 6x6 to 36x36
(396 ns beam crossing)
• The record peak luminosity at CDF exceeded 1.8x1032 cm-2s-1 (Jan. 06, 2006)
• CDF has recorded >1 fb-1 on tape• Total expected int. luminosity 4.4-8.6 fb-1
in 2009
This analysis
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Collider Detector at FermilabCollider Detector at Fermilab
• Muon system J/ di-muon trigger
• Calorimeter Electron ID
• Central Outer Tracker High efficiency tracking dE/dx for electron ID
• Silicon detector Good vertex resolution
Muon system
Silicon detector
Calorimeter
Central Outer Tracker
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Analysis OverviewAnalysis Overview
• This is the first measurement of BcJ/e decay at CDF Run2
• Need to establish the Bc signal at first
– Need to estimate backgrounds precisely– Signal counting in signal mass window
• Then, try to measure the Bc lifetime
– Fit the J/+electron decay length
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Dataset : J/Dataset : J/
• pT()>1.5 GeV (was 2 GeV in Run1)– Factor ~5 J/ yield (factor ~2 B yield)
• ~2.7M J/ events are used in this analysis (360 pb-1)
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Electron ReconstructionElectron Reconstruction• pT(e)>2 GeV, |(e)|<1.0• Track based electron reconstruction
– higher reconstruction efficiency in low pT region
• Calorimeter fiducial requirement – acceptance ~80%
CEM
COT
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Electron Identification using Calorimeter Information
Electron Identification using Calorimeter Information
• 10 variables from the Calorimeter• Form a Joint Likelihood Function
• L distribution depends on – Isolation– Transverse momentum– Track charge
Change L cut value as functions of them
Constant eID efficiency
Choose ~70% efficiency
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Electron Identification using dE/dx Information
Electron Identification using dE/dx Information
dE/dx :
Energy deposit in COTZe/Z–1.3
Ze=Log((dE/dx)measured/(dE/dx)predicted)
~90% efficiency
2GeV
e
p K
e
p
K
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BackgroundsBackgrounds
• Fake electron– Control sample : J/+track
• Residual photon conversion– Control sample : J/+tagged conversion
• bb– PYTHIA Monte Carlo simulation
• Fake J/– J/ mass sideband events sideband subtraction
• Prompt J/– Decay length cut (Lxy/>3) negligible – This cut is to be released in the lifetime measurement
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Fake RateFake Rate1. Mix fake rates for /K/p with proper fraction
Fraction from PYTHIA Monte Carlo
2. Apply the averaged fake rate to J/+track sample (after dE/dx cut)
(dE/dx)
< ~0.8%
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Fake Electron BackgroundFake Electron Background
Uncertainties in 4-6 GeV
Isolation 2.24 events
Trigger bias 1.11
/K/p fraction 0.29
Fake rate stat.
0.14
J/+track stat.
0.31
15.43 events
*J/ mass sideband subtraction is performed
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Conversion Finding EfficiencyConversion Finding Efficiency
• Remove photon conversion electrons by finding a partner track
100% efficiency
residual conversion events
Residual photon conversions :
J/+tagged conversion
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Residual Photon ConversionResidual Photon Conversion
Uncertainties in 4-6 GeV
pT spectrum 6.36 events
Dalitz decay 0.15
Lifetime 0.29
conv stat. 0.55
J/+conv. stat.
4.38
14.54 events
*J/ mass sideband subtraction is performed
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bb (bJ/, be) Eventsbb (bJ/, be) Events
PYTHIA Monte Carlo
Flavor Creation
Flavor Excitation
Gluon Splitting
<90deg. cut
Bc signal
bb background
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bb Backgroundbb Background
*Normalization : N(B+J/K+)
Uncertainties in 4-6 GeV
MC setting 10.55 events
(dE/dx) stat. 0.32
eID stat. 0.46
(eID) isolation 0.96
BR(B+J/K+) 0.31
N(B+) stat. in data 0.62
N(B+) stat. in MC 0.64
Fiducial coverage 0.31
Monte Carlo stat. 2.20
33.63 events
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Signal CountingSignal Counting
• BKG : 63.64.913.6 events in Bc signal region(4~6 GeV)
• Signal excess : 114.915.513.6 events• Significance : 5.9
*J/ mass sideband subtraction is performed
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Production Cross SectionProduction Cross Section• Normalization mode : B+J/K+
– Topologically similar
: reconstruction efficiency ratio between B+ and Bc
KR
R
…
)(
)(
cBA
BAKR
)(
)(
cB
BR
: kinematic acceptance ratio between B+ and Bc
B+
J/ +
-
K+
Bc+
J/ +
-
e+
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Kinematic LimitsKinematic Limits
• Choose pT(B) > 4 GeV, |y(B)| < 1 as our cross section definition
4GeV -1 < y(Bc) < 1
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Acceptance RatioAcceptance Ratio
Largest uncertainty :
Bc pT spectrum
Central value : M(Bc)=6.271 GeV(Bc)=0.55 ps
hep-ph/0309120
hep-ph/0412071
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Reconstruction Efficiency RatioReconstruction Efficiency Ratio
Most of the efficiencies are expected to be same for Bc and B+
Dominant efficiency eID and dE/dx
)/()()()/()()(
)()/()()(
dXdEeIDeJvertextrigger
KJvertextriggerR
trkcccc
trkuuuu
%90%70
1
)/()(
1
dXdEeIDR
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Production Cross Section : ResultProduction Cross Section : Result
• N(Bc) : 114.915.513.6 events
• N(B+) : 287259 events• RK : 4.421.02• R : ~1/0.63
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Extract Bc Meson LifetimeExtract Bc Meson Lifetime Un-binned likelihood fit
Input : “pseudo-proper decay length” and its “error” Release decay length cut
Need to consider prompt background Decay length shape is assumed to be a Gaussian resolution function Float the number of this background events in the fitting
Estimate the number of expected events for each background again Constrain the fraction
Determine background shapes from each control sample Constrain* Fake J/ background
Use higher statistics J/+track sideband events
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Pseudo-Proper Decay LengthPseudo-Proper Decay Length
• Unable to obtain the proper decay length ( ct ) from data directly due to missing neutrino
• Only “pseudo” proper decay length ( X ) is available • Need a correction factor : K
KX
Bp
BM
eJM
eJp
eJp
eJML
)(Bp
)M(BL)ct(B
cT
cT
Txy
cT
cxyc
)(
)(
)/(
)/(
)/(
)/(
K-distributions for 4 M(J/e) bins
ct : Proper Decay Length
X : Pseudo-Proper Decay Length
K : Correction Factor Monte Carlo simulation
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Background FractionBackground Fraction
fake J/ : 0.2090.012fake e : 0.1410.022 res. conv : 0.0860.041 bb : 0.0800.022
Constrain this fraction during J/+e data fitting
fraction
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Background DistributionsBackground Distributions
Fake J/
bb Fake electron
Photon conversion
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PDF for the Lifetime FitPDF for the Lifetime Fit
• Probability density function for the Bc signal:
• Event probability density function:
• Log likelihood:
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Bc Meson Lifetime ResultBc Meson Lifetime Result
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Systematic UncertaintiesSystematic Uncertainties
Total systematic uncertainty is order of ~7%
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ResultResult
CDF Run2 (360pb-1,J/+e) 0.474+0.073/-0.066 0.033 ps
CDF Run1 (110pb-1, J/+e,) 0.46 +0.18/-0.16 0.03 ps
D0 Run2 (210pb-1, J/+) 0.45 +0.12/-0.10 0.12 ps
Operator Product Expansion 0.55 0.15 ps
Bethe-Salpeter Model 0.46~0.47 ps
Light-Front Constituent Quark Model
0.59 0.06 ps
Light-Front ISGW Model 0.63 0.02 ps
Hard-Soft Factorization 0.55 0.1 ps
QCD Sum Rules 0.48 0.05 ps
Bc meson lifetime
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SummarySummary
• We have established the Bc meson using J/+electron channel with 360 pb-1 of data collected by the CDF2
• Measured Bc Meson Lifetime