UIUC paper review

1
UIUC paper review
-a paper written simply and neatly with full
physics concepts associated to the analysis

• Introduction to Bc
• Event selection
• The blind analysis
• The final result
• The systematic error
• A further check

2
Short introduction to Bc

• Meson(bc) The highest combination of two
  different quark flavors
• Reco. event has not been observed in any
  fully reconstructed decay modes.
• (CDF RunI, D0 RunII measurement of mass
  using semileptonic decay.)
• Mass 6.40?0.39(stat) ? 0.13(syst) GeV/c2
  (PDG-2004)
• The measurement in this analysis results
• 19 fully reconstructed Bc?J/?? events, J/? ?
  ??.
• Mass 6287.0?4.8(stat) ? 1.2(syst) MeV/c2

3
Online event selection

• 360 pb-1 data sample
• Di-muon triggered sample
• Level 1 CMU, CMX stubs are matched to COT axial
  tracks. A muon pair (CMU-CMU, CMU-CMX) is
  required with pT gt 1.5 and 2.0 GeV/c for the
  CMU and CMX muons respectively.
• Level 2 require the opposite charge and impose
  requirement on the opening angle.
• Level 3 three dimensional tracking is done for
  the muon pair require the invariant mass from
  2.7 to 4.0 GeV/c2 .

4
Reconstruct Bc?J/?? events offline

• Ensure good vertex resolution
• Three axial SVX hits.
• Good matching between the fitted muon tracks and
  the muon stubs.
• Di-muon invariant mass window 3.042, 3.152
  GeV/c2 .
• pion candidate each charged particle track that
  is not part of the reconstructed J/?.
• The J/? candidate and pion candidate are
  constrained to a common 3-D vertex with a vertex
  fit cut.
• Bc search mass window is defined from 5.6 to 7.2
  GeV/c2 .

5
The blind analysis

• The search mass window 5.6, 7.2 GeV/c2 .
• 2 standard deviation region around the CDF Run
  I measurement of the Bc mass.
• The mass values of the events in the search
  window are temporarily hidden
• Binning the invariant mass in two intervals above
  or below 6.4 GeV/c2.
• ? function
• Optimize the ? function with MC samples
• A quality requirement on the J/?? three-track
  3-D vertex fit.
• A requirement on the pion track contribution to
  the 3-D ?2 vertex fit.
• The impact parameter of the J/?? pair with
  respect to the beam line.
• The maximum proper decay time of the J/?? pair.
• The transverse momentum of the pion.
• The 3-D angle between the momentum and decay
  direction of the J/?? pair.
• The significance of the transverse decay length
  of the J/?? pair
• Find 390 candidate events in the search window in
  data sample.

?
J/?
?
?
6
Control sample Bu?J/?k

• Bu?J/?k vs Bc?J/??
• Same decay topology
• Different masses
• Check the understanding of the reconstruction of
  the relevant variables in the simulation.
• Mass resolution
• Estimate the expected Bc yield.
• 10 50 Bc?J/?? events in data sample

Three Gaussians and a linear function 2378
signal events 11.5?0.3MeV/c2
Agree with the mass resolution from MC simulation.
7
A sliding fit window

• The sliding search window
• extends from 100 to 200 MeV/c2 in mass
• around each nominal peak position, m.
• seeach mass region 5.7,7.0
• In each search window, do a binned likelihood fit
• Signal Gaussian with fixed width (13-19 GeV/c2)
  from MC.
• Background linear function including two
  components
• Partially reconstructed Bc.
• Combinatorial background.
• Calculate ? in each search window

8
A sliding fit window to search for maximum ?

• Establish a signal or set an upper limit for the
  production?
• Toy MC determines the expected distribution of ?
  for pure bdg samples.
• The overall distribution is consistent with the
  two mass bins of the search window
• Two types of backgrounds are statistically varied
  in each toy MC experiment.
• ? gt3.5 establishes a signal.
• ? distribution from the sliding fit window scans
• Maximum value 3.6 at a mass of 6.29 GeV/c2
• The background is consistent to that in the toy
  MC scans.
• The chance for a pure bgd distribution to yield ?
  gt3.6 is 0.27.

9
The final fit

• Use unbinned likelihood fit to the mass region
  containing the peak.
• Gaussuan linear function
• Gaussian width is fixed to 15.5 MeV/c2, mean is
  allowed to float.
• Results (within a region of ? 2 standard
  deviation of the central value)
• Mass 6287.0 ? 4.8 MeV/c2
• Signal events 18.9?5.7
• Background 10.0?1.4

Bc?J/?? exclusive decay signal Established.
10
Systematic error

• The systematic errors are evaluated from the
  corresponding uncertainties on the Bu mass
  analysis.
• The uncertainties on the track parameters.
• The uncertainty on the momentum scale.
• Differences in the pT spectra of the Bu and Bc
  mesons.
• Limited knowledge of the background shape used in
  the final mass fit.
• The total systematic uncertainty 1.1 MeV/c2

11
A further check to this Bc enriched sample

• For data sample, lower mass window include both
  partially reconstructed Bc and the linear
  combinatorial bgd. The upper mass window contains
  mostly the linear combinatorial bgd.
• Lose cuts to get more events in the data sample
• The lower mass window should be enriched with
  events having small impact parameter of the pion
  wrt the jpsi vertex.

Data sample