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Particle ID Tony Weidberg 1
Particle ID
• 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~ 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 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/ 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/(d)
• Compare significance for b jets and u/d jets.
b jets
u jets
Particle ID Tony Weidberg 12
Jet Weights
• Combine significance from all tracks in jet.
( ) / ( )
logi b i u i
ii
r f S f S
W r
B jets
u 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 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
})p/m1()p/m1{(Lt 2/1221
2/1222
1212 v
L
v
Lttt
2
22
21
p2
L)mm(t
L
t1t2
Particle ID Tony Weidberg 18
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 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
=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.
2 2 22 2 22max
2 2
21 4 1ln
2 2eA
e
m cN ZdT Zdx A m I
h
1ln ln and 28.816 (
2 2p
p Z AI
Particle ID Tony Weidberg 21
12.2 Charged particles in matter(Ionisation and the Bethe-Bloch Formula, variation with )
+ can capture e-
Ec = critical energydefined via:dE/dxion.=dE/dxBrem.
BetheBloch
• Broad minimum @ ≈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 Weidberg 22
in drift chambergas
Ionisation variation with particle type
• P=mv=mc • 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