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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
  • Electrons
  • Muons
  • Beauty/charm/tau
  • Pi/K/p

Particle ID Tony Weidberg

  • 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

  • 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

muon backgrounds
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

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

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

cdf svx
  • Silicon microstrips
  • Wire bonded to hybrid with FE ASICs
  • Barrel layers built up of many ladders.

Particle ID Tony Weidberg

transverse flight path
Transverse flight Path
  • J/y sample. Plot fight path projected onto transverse plane.

Particle ID Tony Weidberg

atlas vertexing
ATLAS Vertexing
  • Impact parameter resolution improves with pt why?
  • Why does it saturate at high pt?

Particle ID Tony Weidberg

  • Significance = d/s(d)
  • Compare significance for b jets and u/d jets.

b jets

u jets

Particle ID Tony Weidberg

jet weights
Jet Weights

u jets

  • Combine significance from all tracks in jet.

B jets

Particle ID Tony Weidberg

efficiency b vs rejection power
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

another way to tag b c
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

pi k p
  • Why do we need this?
  • More difficult…
  • dE/dx
  • TOF

Particle ID Tony Weidberg

pi k separation
Pi/K Separation

Particle ID Tony Weidberg





Particle ID Tony Weidberg

  • 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

particle id by ionisation
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

ionization bethe bloch formula
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

12 2 charged particles in matter ionisation and the bethe bloch formula variation with bg

m+ can

capture e-



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:


  • 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

ionisation variation with particle type

in drift



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”


Particle ID Tony Weidberg