Branching fraction measurements of the decays Ds K 0 and Ds 0 at BABAR

1
Branching fraction measurements of the decays
Ds?K?0 and Ds???0 at BABAR

• Thesis Review
• 14 July, 2005

2
Overview

• Analysis
• Data/MC samples
• Event selection
• Likelihood fit
• Potential problems
• Summary
• Thesis
• Structure
• Chapter outlines
• Summary
• TimeLine

3
Data/MC Samples

• The full BaBar dataset and generic MC sample is
  skimmed for interesting events.
• 99 complete.
• Currently processing this skim to refine the
  selection further.
• Began on 27 June expected to take 3 weeks in
  total to process and copy to disk at Imperial.
• All data/MC will be available from week
  beginning 18 July.
• Analysis development is on-going with a
  sub-sample representing 10 of the full sample.
• Background rejection.
• Maximum likelihood fit.

4
Analysis Development

• Analysis examines 4 decay modes of the Ds meson.
• Ds ? K ?0 , ? ?0 , Ks K , Ks ?.
• Variables for enhancing the signalbackground
  ratio are identified.
• Fitting variables are identified.
• These are valid for all modes.
• The Ds? KsK mode is a reference mode for
  calculating the branching fractions of the signal
  modes.
• Program to reduce the large data/MC skim
  samples into small, best candidate fit samples is
  written, tested and working.

5
Likelihood Fit

• Using a well tested, ROOT based fitting package
  for the maximum likelihood fit.
• 2D fit using a product of two 1D PDFs assuming
  no correlation between the fitting variables.
• Strategy
• Fix PDF forms and parameters using high
  statistic, generic and signal MC samples.
• Verify there are no biases in the fit through toy
  MC studies.
• When PDFs have been thoroughly checked, perform
  an extended likelihood fit to the data to
  determine number of signal and background.
• No plan to float PDF parameters in the fit to
  data at present this may change.

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Optimisation

• Likelihood fit will be optimised to yield the
  best result,
• Current cut values on discriminating variables
  will be varied over the whole space and a full
  fit performed to determine the best sample for
  the fit.
• Various strategies can be tried here
• Minimise error or maximise statistical
  significance of Ns.
• Fit for Ns, then fit again setting Ns0 and
  observe likelihood change this would be
  maximised.
• Fit for Ns for both signal and reference mode,
  take the ratio and minimise the error on this
  ratio.

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Analysis Summary

• Completed Steps
• Event selection for likelihood fit.
• Likelihood fit mechanism.
• In Progress
• Full Data/MC sample skimming.
• Finalising fit.
• Soon to Begin
• Toy MC studies.
• Optimisation of fit sample.
• Potential Problems
• Handling correlations in the fit.
• Removing/accounting for biases in the fit.
• Level of discrimination achievable between signal
  channel and charm background.
• Ready to fit to data by the end of August.

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Thesis Outline

• Current plan 5 chapters
• Theoretical Background
• PEP-II and the BABAR Detector
• Physics Analysis
• Physics Results
• Conclusions
• The first 4 major chapters will be outlined in
  the following slides.

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1. Theoretical Background

• Hadronic charm decays.
• Physics Motivation.
• Ds production from ee- annihilation.
• From B decays.
• From cc fragmentation.
• Reconstructed Modes.
• Feynman diagrams.
• Theoretical/measured branching fractions.
• Justification of reference mode.

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2. PEP-II and the BABAR Detector

• Stanford Linear Accelerator.
• PEP-II ee- Storage Rings.
• BABAR detector overview.
• Silicon Vertex Tracker.
• Drift Chamber.
• Detector of Internally Reflected Cherenkov light.
• Electromagnetic Calorimeter.
• Superconducting Solenoid.
• Instrumented Flux Return.
• Trigger System.
• Data Acquisition System.

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3. Physics Analysis

• Event reconstruction.
• Signal mode.
• Reference mode.
• Background Rejection.
• Discriminating variables for Signal MC vs.
  generic MC.
• Fitting Variables.
• Correlations in background and signal.
• Likelihood Fit.
• 2D fit.
• Non-peaking backgrounds.
• Charm background.
• Peaking backgrounds.
• Toy MC Studies.
• Fit Optimisation.

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4. Physics Results

• Systematic errors.
• BR(Ds ? KsK) is the major systematic.
• Detector effects.
• Imperfect MC detector response.
• Uncertainty on ratio of Ks/?0 reconstruction
  efficiencies.
• Particle ID efficiencies for K/?.
• Physics effects.
• Efficiency over Ds momentum spectrum.
• Measurements/Limits.

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Thesis Summary

• In Progress
• Chapter 2 PEP-II and the BABAR Detector.
• Soon to begin
• Chapter 1 Theoretical Background.
• Allow 1 month for writing each major chapter.
• Planned submission date is 31 October.

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Timeline
JUL
AUG
SEP
OCT
Likelihood Fit
Toy Studies
Optimisation
Data Fit
Analysis
Systematics
Detector
Theory
Conclusion
Thesis
Thesis Submission