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Measurement of the B c Meson Lifetime with the Collider Detector at Fermilab

Measurement of the B c Meson Lifetime with the Collider Detector at Fermilab. Masato Aoki. The B c 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

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Measurement of the B c Meson Lifetime with the Collider Detector at Fermilab

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  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) • 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 Bc+ (b+c) B0(b+d) , B+(b+u), Bs0(b+s) Upsilon(b+b), Psi(c+c)

  3. Decays of the Bc Meson

  4. Theoretical Prediction B+ meson : ~1.7 ps D0 meson : ~0.4 ps

  5. Motivation • 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. History • ~20 signal events • Mass: 6.40.39(stat.)0.13(syst.) GeV/c2 • Lifetime: 0.46+0.18/-0.16(stat.)0.03(syst.) ps CDF Run-I(1998) observed BcJ/yln signal

  7. Bc Meson Reconstruction BcJ/yene channel • J/ydi-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) : 4~6 GeV M(J/y) M(Bc)

  8. 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

  9. Collider Detector at Fermilab • Muon system  J/y di-muon 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

  10. 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/y+electron decay length

  11. Dataset : J/ymm • pT(m)>1.5 GeV (was 2 GeV in Run1) • Factor ~5 J/y yield (factor ~2 B yield) • ~2.7M J/y events are used in this analysis (360 pb-1)

  12. Electron Reconstruction • pT(e)>2 GeV, |h(e)|<1.0 • Track based electron reconstruction • higher reconstruction efficiency in low pT region • Calorimeter fiducial requirement • acceptance ~80% CEM COT

  13. 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 dE/dx : Energy deposit in COT Ze/sZ–1.3 p 2GeV p e m K ~90% efficiency e m p p Ze=Log((dE/dx)measured/(dE/dx)predicted) K

  15. Backgrounds • Fake electron • Control sample : J/y+track • Residual photon conversion • Control sample : J/y+tagged conversion • bb • PYTHIA Monte Carlo simulation • Fake J/y • J/y mass sideband events  sideband subtraction • Prompt J/y • Decay length cut (Lxy/s>3)  negligible • This cut is to be released in the lifetime measurement

  16. Fake Rate • Mix fake rates for p/K/p with proper fraction • Fraction from PYTHIA Monte Carlo • Apply the averaged fake rate to J/y+track sample (after dE/dx cut)  e(dE/dx) < ~0.8%

  17. Fake Electron Background 15.43 events *J/y mass sideband subtraction is performed

  18. Conversion Finding Efficiency • Remove photon conversion electrons by finding a partner track • 100% efficiency  residual conversion events Residual photon conversions :  J/y+tagged conversion

  19. Residual Photon Conversion 14.54 events *J/y mass sideband subtraction is performed

  20. bb (bJ/y, be) Events Bc signal PYTHIA Monte Carlo Flavor Creation Df<90deg. cut bb background Flavor Excitation Gluon Splitting

  21. bb Background *Normalization : N(B+J/yK+) 33.63 events

  22. Signal Counting • BKG : 63.64.913.6 events in Bc signal region(4~6 GeV) • Signal excess : 114.915.513.6 events • Significance : 5.9s *J/y mass sideband subtraction is performed

  23. … Production Cross Section • Normalization mode : B+J/yK+ • Topologically similar m+ m+ J/y J/y m- m- B+ Bc+ n K+ e+ : kinematic acceptance ratio between B+ and Bc : reconstruction efficiency ratio between B+ and Bc

  24. Kinematic Limits -1 < y(Bc) < 1 4GeV • Choose pT(B) > 4 GeV, |y(B)| < 1 as our cross section definition

  25. Acceptance Ratio Largest uncertainty : Bc pT spectrum Central value : M(Bc)=6.271 GeV t(Bc)=0.55 ps hep-ph/0412071 hep-ph/0309120

  26. Reconstruction Efficiency Ratio Most of the efficiencies are expected to be same for Bc and B+ Dominant efficiency  eID and dE/dx

  27. 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

  28. 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 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/y background  Use higher statistics J/y+track sideband events

  29. Pseudo-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 ct : Proper Decay Length X : Pseudo-Proper Decay Length K : Correction Factor  Monte Carlo simulation K-distributions for 4 M(J/ye) bins

  30. Background Fraction fake J/y : 0.2090.012 fake e : 0.1410.022 res. conv : 0.0860.041 bb : 0.0800.022 fraction  Constrain this fraction during J/y+e data fitting

  31. Background Distributions bb Fake electron Fake J/y Photon conversion

  32. PDF for the Lifetime Fit • Probability density function for the Bc signal: • Event probability density function: • Log likelihood:

  33. Bc Meson Lifetime Result

  34. Systematic Uncertainties Total systematic uncertainty is order of ~7%

  35. Result Bc meson lifetime

  36. Summary • We have established the Bc meson using J/y+electron channel with 360 pb-1 of data collected by the CDF2 • Measured Bc Meson Lifetime

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