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

10

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

11

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)

12

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

14

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

15

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

16

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%

17

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

18

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

19

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

20

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

22

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

23

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+

24

Kinematic LimitsKinematic Limits

• Choose pT(B) > 4 GeV, |y(B)| < 1 as our cross section definition

4GeV -1 < y(Bc) < 1

25

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

26

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

27

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%

35

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

36

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