Measurement of the Bc Meson Lifetime with the Collider Detector at Fermilab

1
Measurement of the Bc Meson Lifetime with the
Collider Detector at Fermilab

• Masato Aoki

2
The Bc Meson

• Ground state of differently flavored heavy quarks
  (bottom quark charm quark)
• Only weak decays are possible
• Similar binding interaction to the case of heavy
  quarkonium(cc,bb) but different dynamics
• Currently only Tevatron can produce the Bc

Bc (bc)
B0(bd) , B(bu), Bs0(bs)
Upsilon(bb), Psi(cc)
3
Decays of the Bc Meson

• tBcOPE,PM 0.55 ? 0.15 ps (Other B mesons
  1.5 ps)
• Most significant uncertainty related to the
  choice of quark masses (especially for the charm
  quark)

4
Motivation

• Contributions from the three major decay diagrams
  affect the Bc meson lifetime
• We measure the Bc lifetime with high statistics
  data collected by the CDF in Tevatron Run2
• Precise measurements of the Bc meson will also
  provide insight into the strong dynamics of heavy
  quarks

5
History
CDF Run-I(1998) observed Bc?J/yln signal

• 20 signal events
• Mass
• 6.4?0.39(stat.)?0.13(syst.) GeV/c2
• Lifetime
• 0.460.18/-0.16(stat.)?0.03(syst.) ps

6
Tevatron Run2 (2001)

• New main injector
• (150GeV proton storage ring)
• New recycler storage ring for p accumulation
• Higher energy pp collisions at 1.96TeV (was
  1.8TeV)
• Increased number of p and p bunches from 6x6 to
  36x36
• (396ns beam crossing)
• The record peak luminosity at CDF exceeded
  1.8x1032cm-2s-1 (Jan. 06, 2006)
• CDF has recorded gt1fb-1 on tape
• (this analysis uses 360pb-1 of data)
• Total expected int. luminosity 4.4-8.6fb-1 in 2009

7
Bc Meson Reconstruction

• Use Bc?J/yene channel
• J/y 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/ye) 46GeV
8
Collider Detector at Fermilab

• Muon system
• ? J/y dimuon trigger
• Calorimeter
• ? Electron ID
• Central Outer Tracker
• ? High efficiency tracking
• ? dE/dx for electron ID
• Silicon detector
• ? Good vertex resolution

Muon system
Calorimeter
Silicon detector
Central Outer Tracker
9
Analysis Overview

• This is the first measurement of Bc?J/yen 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/yelectron decay length

10
Dataset J/y?mm

• pT(m)gt1.5GeV (was 2 GeV in Run1)
• Factor 5 J/y yield (factor 2 B yield)
• 2.7M J/y events are used in this analysis
  (360pb-1)

11
Electron Reconstruction

• pT(e)gt2GeV, h(e)lt1.0
• Track based electron reconstruction
• higher reconstruction efficiency in low pT region
• Calorimeter fiducial requirement
• acceptance 80

CEM
COT
12
Electron Identification using the CDF Calorimeter

• 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
13
Electron Identification using dE/dx

• dE/dx
• Energy deposit in COT

Ze/sZgt-1.3
90 efficiency
ZeLog((dE/dx)measured/(dE/dx)predicted)
14
Backgrounds

• Fake electron
• Control sample J/ytrack
• Residual photon conversion
• Control sample J/ytagged conversion
• bb
• Pythia Monte Carlo
• Fake J/y
• J/y mass sideband events ? sideband subtraction
• Prompt (cc,)
• Apply decay length cut ? negligible

15
Fake Rate

• Mix fake rates for p/K/p with proper fraction
• Fraction from PYTHIA Monte Carlo
• Apply the averaged fake rate to J/ytrack sample
  (after dE/dx cut)

dE/dx cut efficiencies
lt 0.8
16
Fake Electron Background
15.43 events
Uncertainties in 4-6GeV Uncertainties in 4-6GeV
Isolation 2.24 events
Trigger bias 1.11
p/K/p fraction 0.29
Fake rate stat. 0.14
J/ytrack stat. 0.31
J/y mass sideband subtraction is performed
17
Conversion Finding Efficiency

• Remove photon conversion electrons by finding a
  partner track
• ?100 efficiency
• ? residual conversion events

Residual photon conversions
? J/ytagged conversion
18
Residual Photon Conversion
14.54 events
Uncertainties in 4-6GeV Uncertainties in 4-6GeV
pT spectrum 6.36 events
Dalitz decay 0.15
Lifetime 0.29
econv stat. 0.55
J/yconv. stat. 4.38
J/y mass sideband subtraction is performed
19
bb (b?J/y, b?e) Events

• Use PYTHIA Monte Carlo

Flavor Creation
Dflt90deg. cut
Flavor Excitation
Gluon Splitting
20
bb Background

• Normalization N(B?J/yK)

Uncertainties in 4-6GeV Uncertainties in 4-6GeV
PDF/ISR 10.55 events
e(dE/dx) stat. 0.32
e(eID) stat. 0.46
e(eID) isolation 0.96
BR(B?J/yK) 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
21
Signal Counting

• BKG 63.6?4.9?13.6 in Bc signal region(46GeV)
• Signal excess 114.9?15.5?13.6
• Significance 5.9s

J/y mass sideband subtraction is performed
22
Production Cross Section

• Normalization mode B?J/yK
• Topologically similar

KA(Bc) , KA(B) kinematic acceptance( from
MC) e(Bc) , e(B) trigger and reconstruction
efficiency kinematic acceptance ratio between
Bc and B trigger and reconstruction ratio
between Bc and B
23
Kinematic Limits
-1 lt y(Bc) lt 1
4GeV

• Choose pT(B) gt 4GeV, y(B) lt 1 as our cross
  section definition

24
Reconstruction Efficiency Ratio
Most of the efficiencies are expected to be same
for Bc and B Dominant efficiency ? e ID and dE/dX
25
Acceptance Ratio
Central value M(Bc)6.271GeV t(Bc)0.55ps

• Largest uncertainty
• Bc pT spectrum

hep-ph/0412071
hep-ph/0309120
26
Production Cross Section Result

• N(Bc) 114.9?15.5?13.6 events
• N(B) 2872?59 events
• RK 4.42?1.02
• Re 1/0.63

27
Extract 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
• Estimate the number of expected events for each
  background again ? Constrain
• Determine background shapes from each control
  sample ? Constrain
• Fake J/y background
• ? Use higher statistics J/ytrack sideband events

28
Correction Factor K

• Unable to obtain the proper decay length ( ct )
  from data directly
• Only pseudo proper decay length ( X ) is
  available
• Need a correction factor K

Use different K distributions for 4 mass bins
29
Background Fraction
fraction

• fake J/y 0.209?0.012
• fake e 0.141?0.022
• res. conv 0.086?0.041
• bbbar 0.080?0.022

? Constrain this fraction during J/ye data
fitting
30
Background Distributions
bb
Fake electron
Fake J/y
Photon conversion
31
PDF for the Lifetime Fit

• Probability density function for the Bc signal
• Event probability density function
• Log likelihood

32
Bc Meson Lifetime Result
33
Systematic Uncertainties
Total systematic uncertainty is order of 7
34
Summary

• We have established the Bc meson using
  J/yelectron channel with 360pb-1 of data
  collected by the CDF2
• Bc Meson Lifetime

• Theoretical prediction(hep-ph/0509211)
• 0.55 ?0.15 ps
• Run1 CDF
• 0.46 0.18/-0.16 ?0.03 ps
• Run2 DZero
• 0.45 0.12/-0.10 ?0.12 ps

35
Backup slides
36

• Form a Cumulated Likelihood Function
• We choose Pelt0.7 ? 70 efficiency