Bc lifetime measurement in BcJy e X channel

1
Bc lifetime measurement in Bc?J/y e X channel

• Masato Aoki, Ilsung Cho, Ting Miao

• Introduction
• What we did for x-section analysis
• Background shape
• Fake electron background
• Residual conversion
• b-bbar background
• fake J/y
• Signal fitting
• Systematic uncertainties
• Summary

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Introduction

• We had measured the cross section of Bc in J/ye
  X channel (note7518)
• Electron ID using SoftElectronModule, dE/dx
• Lxygt3sigma to kill prompt background
• Background
• Fake electron estimate fake rate, J/ytrack as
  a control sample
• Residual conversion estimate conversion finding
  efficiency using B0?J/y p0, p0?gg or gee MC. Use
  J/ytagged conversion
• b-bbar use Pythia MC, B?J/yK is used for the
  normalization
• Fake J/y J/y mass sideband subtraction
• We release the lifetime cut and measure the Bc
  lifetime
• New background from prompt events

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J/ye selection cuts
4
Summary of x-section measurement
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Event selection

• Use the same cuts as x-section analysis
• see note7518
• Release lifetime cut
• Select events in M(J/ye) 46GeV region
• Apply sLxylt70mm

sLxy B?J/yK
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Summary after releasing lifetime cut
Excess contains prompt BKG and Bc signal
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Background fraction

• Background fraction (the denominator includes
  prompt bkg and Bc signal)
• fake e 0.136241 /- 0.020973
• res. conv 0.095285 /- 0.043344
• bbbar 0.078440 /- 0.021805
• fake J/psi 0.214123 /- 0.011541
• Statistical and systematic errors are included
• Constrain the fractions for the final fitting
  using Gaussian

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Fitter check using B?J/yK

• Simply check our fitter using B?J/yK
• Result
• ct504.1 ? 9.3mm
• Agree with blessed result from CDF

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Fitting procedure

• Background fractions from sources of fake
  electron, conversion, fake J/y, b-bbar are
  Gaussian constrained using background estimation
  procedure
• Their shapes are determined from background
  samples
• Fake electron Use J/ytrack sample
• Res. conversion Use J/ytagged conversion
• Fake J/y See next slide
• b-bbar Use Pythia MC
• Resolution function is assumed to be single
  Gaussian
• Use different resolution functions for syst.
  study
• Prompt background is assumed to have a shape as
  resolution function and the normalization is from
  fitting directly

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Issue on fake J/y shape

• J/ytrack, conversion sample have fake J/y
  component as well as J/yelectron
• Looking at fake J/ytrack, conversion, electron
  events, we found their shapes are similar
• Use common fake J/y shape
• Use J/ytrack sample for every fake J/y shapes
• Limited stat. for conversion, electron samples
• Other fake J/y samples (conversion, electron) are
  used for systematics study

PDF for fake J/y
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Fake electron

• PDF for fake electron BKG

e fake rate N normalization factor
can be expected from J/y mass distribution
Use same error scaling factor for both real J/y
and fake J/y here
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Fit result for fake electron fake J/y
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Residual conversion

• PDF for residual conversion BKG

Constrain parameters of fake J/y and scale factor
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Fit result for conversion BKG
Constrained using J/ytrack sample
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b-bbar background

• PDF for b-bbar BKG
• Background events passing selection cut from each
  production process
• Gluon splitting 70
• Flavor excitation 25
• Flavor creation 5

(scaling factor is not constrained)
?Syst. study GS only and FE only fitting
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Fit result for b-bbar BKG
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Prompt background

• It is difficult to estimate the size of prompt
  background from either MC or data
• ? Float prompt BKG fraction for the final fitting
  
• We use resolution function as prompt background
  shape (Gaussian)
• Maybe there are different resolution functions
• Try different resolution functions for the syst.
  study
• Double Gaussians
• GaussianSymmetric exponential

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K-factor

• Assume M(Bc) 6.271GeV
• Use 4 K-factor distributions for 4 J/ye mass
  bins ( 44.5GeV, 4.55GeV, 55.5GeV, 5.56GeV )

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Likelihood definition for the signal fitting

• PDF for signal
• Likelihood

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Signal fitting

• ct(Bc) 141.8 22.3/-19.8 mm

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Systematic uncertainties

• K-factor
• M(Bc), pT(Bc),lifetime(Bc),decay channel,
• Background shapes
• parameterization, fake J/y, w/o efficiency
  weighting,
• Resolution function
• Double Gaussians, Gaussiansymmetric exponential,
  
• silicon alignment ? borrow result of B exclusive
  analysis

22
Fake J/yelectron

• Comparison
• fake J/yelectron v.s. fit result of fake
  J/ytrack

• Fake J/y shape in J/ytrack sample has
  consistent shape with J/yelectron
• Each sample (fake J/yconversion, electron) for
  syst. study

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Systematic uncertainties
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Summary

• We fit the J/yelectron data to measure Bc
  lifetime
• ct(Bc) 141.8 22.0/-19.8 mm
• We are finishing systematic error studies
• Need to finish all of systematic study ASAP
• CDFNote will be ready in a week or so

25
Backup
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Summary after releasing lifetime cut
Excess contains prompt BKG and Bc signal