Presentation on theme: "Particle ID Tony Weidberg1 Particle ID Electrons Muons Beauty/charm/tau Pi/K/p."— Presentation transcript:
Particle ID Tony Weidberg1 Particle ID Electrons Muons Beauty/charm/tau Pi/K/p
Particle ID Tony Weidberg2 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 Weidberg3 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 Weidberg4 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 Weidberg5 Beauty/Charm/Tau Why is this important? Detect “long” lifetime with micro-vertex detector life ~ 1ps c ~ 300 m 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 Weidberg6 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 Weidberg7 CDF SVX Silicon microstrips Wire bonded to hybrid with FE ASICs Barrel layers built up of many ladders.
Particle ID Tony Weidberg10 ATLAS Vertexing Impact parameter resolution improves with pt why? Why does it saturate at high pt?
Particle ID Tony Weidberg11 ATLAS Significance = d/ (d) Compare significance for b jets and u/d jets. b jets u jets
Particle ID Tony Weidberg12 Jet Weights Combine significance from all tracks in jet. B jets u jets
Particle ID Tony Weidberg13 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 Weidberg14 Another way to tag b/c Use semi-leptonic deays: –b c l 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 Weidberg15 Pi/k/p Why do we need this? More difficult… dE/dx TOF
Particle ID Tony Weidberg16 Pi/K Separation
Particle ID Tony Weidberg17 TOF L t1t1 t2t2
Particle ID Tony Weidberg18 TOF Scintillation Counter time resolution –Time spread from light paths through scintillator. –Time spread from PMT. –Best resolution ~200 ps. Spark chambers –Can achieve ~60 ps
Particle ID Tony Weidberg19 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 Weidberg20 Ionization: Bethe-Bloch Formula =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 BBF as a Function of is nearly independent of M of projectile except for max and very weak log dependence in if you know p and measure 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 Charged particles in matter (Ionisation and the Bethe-Bloch Formula, variation with ) + can capture e - E c = critical energy defined via: dE/dx ion. =dE/dx Brem. Bethe Bloch Broad ≈3.0(3.5) for Z=100(7) At minimum, stopping power is nearly independent of particle type and material 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 Weidberg22 in drift chamber gas Ionisation variation with particle type P=m v=m c 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