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Advanced Particle Identification Techniques in High-Energy Physics

This presentation by Tony Weidberg explores various particle identification techniques utilized in high-energy particle physics, focusing on electrons, muons, and heavy flavors such as beauty and charm. It covers essential concepts such as energy and momentum measurement, muon background estimation, and the use of micro-vertex detectors. The presentation highlights the importance of precise tracking and the use of advanced detectors for particle identification, detailing methods like dE/dx measurements, Cerenkov radiation, and time-of-flight analysis.

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Advanced Particle Identification Techniques in High-Energy Physics

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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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