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Measurement of the Bc Meson Lifetime at Fermilab

This study focuses on the ground state of differently flavored heavy quarks (bottom quark and charm quark), particularly the Bc meson, which undergoes only weak decays. By analyzing data from CDF at Fermilab, the Bc meson's mass and lifetime are determined with high precision, shedding light on the strong dynamics of heavy quarks. The measurement involves reconstructing Bc decays to J/y and electron channels, overcoming challenges like background estimation and fake electron identification. The analysis leverages sophisticated detectors and techniques to extract the Bc signal and accurately measure its properties, providing valuable insights into heavy quark interactions.

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Measurement of the Bc Meson Lifetime 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

  37. Run-II DØ result • Bc+ J/ym+X • 210 pb-1 of data • 231 J/ymcandidates • Mass and lifetime combined fit • Mass: • Lifetime:

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