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Particle ID

Particle ID. Electrons Muons Beauty/charm/tau Pi/K/p. Electrons. See calorimeter lectures Different lateral and longitudinal shower profiles. E/p for electrons. E measured by calorimeter. P measured by momentum in tracker.

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Particle ID

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  1. ParticleID • Electrons • Muons • Beauty/charm/tau • Pi/K/p Particle ID Tony Weidberg

  2. Electrons • See calorimeter lectures • Different lateral and longitudinal shower profiles. • E/p for electrons. • E measured by calorimeter. • P measured by momentum in tracker. • Should peak at 1 for genuine electrons and be > 1 for backgrounds. Why? • Cerenkov & Transition radiation (see Guy Wilkinson’s lectures). Particle ID Tony Weidberg

  3. Muons • Use hadron absorber. • Muons only lose energy through ionization  penetrate absorber. • Electrons and hadrons shower  absorbed. • Need > 5 interaction lengths, why ??? • Absorber could be hadron calorimeter and/or passive steel. • Muon signature: • Track segment in muon chambers after absorber. • Matching track in tracker before calorimeter. Particle ID Tony Weidberg

  4. Muon Backgrounds • Hadron punch trhough. • How can we estimate this? • Pi/K decays • Generates real muons? • How can we reduce this background? • How can we estimate residual background? Particle ID Tony Weidberg

  5. Beauty/Charm/Tau • Why is this important? • Detect “long” lifetime with micro-vertex detector • life t~ 1ps  ct ~ 300 mm but remember time dilation can help! • Collider geometry: • Decay happens inside beam pipe. • Measure primary & secondary tracks. • Reconstruct primary & secondary vertices or • Use impact parameter (2D or 3D) wrt primary vertex. Particle ID Tony Weidberg

  6. Micro-vertex • Impact parameter resolution • Low pt dominated by multiple scattering. • High pt dominated by measurement error. • Need infinitely thin and infinitely accurate tracking detector. • Best compromise is silicon (pixels, micro-strips or CCDs). Particle ID Tony Weidberg

  7. CDF SVX • Silicon microstrips • Wire bonded to hybrid with FE ASICs • Barrel layers built up of many ladders. Particle ID Tony Weidberg

  8. Particle ID Tony Weidberg

  9. Transverse flight Path • J/y sample. Plot fight path projected onto transverse plane. Particle ID Tony Weidberg

  10. ATLAS Vertexing • Impact parameter resolution improves with pt why? • Why does it saturate at high pt? Particle ID Tony Weidberg

  11. ATLAS • Significance = d/s(d) • Compare significance for b jets and u/d jets. b jets u jets Particle ID Tony Weidberg

  12. Jet Weights u jets • Combine significance from all tracks in jet. B jets Particle ID Tony Weidberg

  13. Efficiency b Vs Rejection Power • Plot R (rejection power for u/g/c jets versus eb (b jet efficiency) • Why is c more difficult to reject than u? • Why is g more difficult to reject than u??? Particle ID Tony Weidberg

  14. Another way to tag b/c • Use semi-leptonic deays: • b c l n Detect charged l in jet at some pt wrt jet axis. • l could be electrons or muons (which do you think would be easier?). Particle ID Tony Weidberg

  15. Pi/k/p • Why do we need this? • More difficult… • dE/dx • TOF Particle ID Tony Weidberg

  16. Pi/K Separation Particle ID Tony Weidberg

  17. TOF L t2 t1 Particle ID Tony Weidberg

  18. TOF • Scintillation Counter time resolution • Time spread from light paths through scintillator. • Time spread from PMT. • Best resolution s~200 ps. • Spark chambers • Can achieve s~60 ps Particle ID Tony Weidberg

  19. Particle ID by Ionisation • Measure ionisation dE/dx and momentum identify particle type. • Requires very precise measurement of dE/dx  difficult. • Multiple measurements in a wire chamber  truncated mean. Particle ID Tony Weidberg

  20. Ionization: Bethe-Bloch Formula • d=density correction: dielectric properties of medium shield growing range of Lorenz-compacted E-field that would reach more atoms laterally. Without this the stopping power would logarithmically diverge at large projectile velocities. Only relevant at very large bg • BBF as a Function of bg is nearly independent of M of projectile except for nmax and very weak log dependence in d  if you know p and measure b  get M (particle ID via dE/dx): See slide 21 • Nearly independent of medium. Dominant dependence is Z’/A ≈½ for most elements. Particle ID Tony Weidberg

  21. m+ can capture e- Bethe Bloch 12.2 Charged particles in matter(Ionisation and the Bethe-Bloch Formula, variation with bg) • Broad minimum @ bg≈3.0(3.5) for Z=100(7) • At minimum, stopping power is nearly independent of particle type and material Emc = critical energy defined via: dE/dxion.=dE/dxBrem. • Stopping Power at minimum varies from 1.1 to 1.8 MeV g-1 cm2) • Particle is called minimum ionising (MIP) when at minimum Particle ID Tony Weidberg

  22. in drift chamber gas Ionisation variation with particle type • P=mgv=mgbc • variation in dE/dx is useful for particle ID • variation is most pronounced in low energy falling part of curve • if you measured P and dE/dx you can determine the particle mass and thus its “name” e Particle ID Tony Weidberg

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