experimental particle physics phys6011 joel goldstein, ral
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Experimental Particle Physics PHYS6011 Joel Goldstein, RAL. Introduction & Accelerators Particle Interactions and Detectors (2/2) Collider Experiments Data Analysis. Last week: Ionisation losses and detection This week: Radiation Losses Photon Absorption Electromagnetic Showers - PowerPoint PPT PresentationTRANSCRIPT
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Experimental Particle PhysicsPHYS6011
Joel Goldstein, RAL
1. Introduction & Accelerators
2. Particle Interactions and Detectors (2/2)
3. Collider Experiments
4. Data Analysis
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Joel Goldstein, RAL PHYS6011, Southampton 2
Interactions and Detectors
Last week:
Ionisation losses and detectionThis week:
1. Radiation Losses
2. Photon Absorption
3. Electromagnetic Showers
4. Hadronic Showers
5. Multiple Scattering
6. Detector Categories
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Joel Goldstein, RAL PHYS6011, Southampton 3
Muon Energy Loss
Ionisation/Bethe-Bloch
2
2
222
22 2
ln I
cm
A
ZKq
dx
dE e
Low energy region
Radiation dominated
Critical Energy 800 MeV/(Z+1.2)
for electrons
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Joel Goldstein, RAL PHYS6011, Southampton 4
Radiation Losses
• Bremsstrahlung
• Charged particle in nuclear electric field
• Photon can be very energetic
Energy loss for electrons:
0X
E
dx
dE
0/0
XxeEE
Radiation Length
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Joel Goldstein, RAL PHYS6011, Southampton 5
Radiation Length
• Exponential drop in electron energy
)/287ln()1(
g.cm4.716 -2
0ZZZ
AX
E0
X0
E
x
E0/e
X0 (g cm-2) X0 (cm)
Air 37 30,000
Silicon 22 9.4
Lead 6.4 0.56
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Joel Goldstein, RAL PHYS6011, Southampton 6
Photon Absorption
• Electron-positron pair production
• Exponential absorption
• Length scale 9/7×X0
09
7
X
n
dx
dn
e
e-
e+
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Joel Goldstein, RAL PHYS6011, Southampton 7
Simple Electromagnetic Shower
e+
x 0 X0 2X0 3X0 4X0
N 1 2 4 8 16 0
<E> E0 E0/2 E0/4 E0/8 E0/16 <Ec
• Start with electron or photon
• Depth ~ ln(E0)
• Most energy deposited as ionisation
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Joel Goldstein, RAL PHYS6011, Southampton 8
Real EM Shower
• Shape dominated by fluctuations
Maximum close to naïve depth expectation
Tail
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Joel Goldstein, RAL PHYS6011, Southampton 9
EM Calorimetry 1
• Crystal, glass, liquid
• Acts as absorber and scintillator
• Light detected by photodetector
• E.g. PbWO4
(X0 ≈ 0.9 cm)
Use shower to measure the energy of an electron or photon:
1. Contain shower within a homogeneous calorimeter
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Joel Goldstein, RAL PHYS6011, Southampton 10
EM Calorimetry 2
2. Sampling Calorimeter• Insert detectors into larger absorber:
• Absorber normally dense metal:
• lead, tungsten etc
• Detectors can be scintillator, MWPC...
- Cheaper but less accurate
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Joel Goldstein, RAL PHYS6011, Southampton 11
Hadronic Showers
• Nuclear interaction length >> radiation length
e.g. Lead: X0 = 0.56 cm, λ = 17 cm
• Hadron showers wider, deeper, less well understood• Need much larger calorimeter to contain hadron shower– Always sampling– Dense metals still good as absorbers– Mechanical/economic considerations often important– Uranium, steel, brass…
3/1-2g.cm35 A
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Joel Goldstein, RAL PHYS6011, Southampton 12
Hadronic Calorimeter
Alternating layers of steel and streamer chambers
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Joel Goldstein, RAL PHYS6011, Southampton 13
Multiple Scattering
• Elastic scattering from nuclei causes angular deviations:
θ0/
MeV6.13Xxq
cpRMS
• Approximately Gaussian
• Can disrupt measurements in subsequent detectors
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Joel Goldstein, RAL PHYS6011, Southampton 14
A Brief Pause
So far...
• Particle interactions:– Ionisation losses
– Cerenkov radiation
– Transition radiation
– EM showers
– Hadronic showers
– Multiple scattering
• Electronic detectors
Let’s put order into chaos!
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Joel Goldstein, RAL PHYS6011, Southampton 15
Tracking Detectors
• Low mass– Reduce multiple scattering
– Reduce shower formation
• High precision
• Multiple 2D or 3D points
• Drift chamber, TPC, silicon...
• Can measure momentum in magnetic field (p = 0.3qBR)
Measure trajectories of charged particles
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Joel Goldstein, RAL PHYS6011, Southampton 16
Vertex Detectors
• Spatial resolution a few microns
• Low mass
• A few layers of silicon
Ultra-high precision trackers close to interaction point
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Joel Goldstein, RAL PHYS6011, Southampton 17
EM Calorimeter
Identify and measure energy of electrons and photons
• Need ~ 10 X0
– 10 cm of lead
• Will see some energy from muons and hadrons
• Homogenous
– Crystal
– Doped glass
• Sampling
– Absorber + scintillator/MWPC/…
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Joel Goldstein, RAL PHYS6011, Southampton 18
Hadron Calorimeter
• Need ~ 10 λ– 2 m of lead
• Both charged and neutral
• Will see some energy from muons
• Sampling
– Heavy, structural metal absorber
– Scintillator, MWPC detector
Identify and measure energy of all hadrons
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Joel Goldstein, RAL PHYS6011, Southampton 19
Particle ID
• Muon, pion, kaon, proton
• Measured momentum and energy: m2 = E2 – p2
– Difficult at high energy E ~ p
• Different dE/dx in tracking detectors– Only for low energy 1/2 region, no good for MIPs
Distinguish different charged “stable” particles
• Measure time-of-flight – Fast scintillator
• Measure directly– Cerenkov radiation
• Measure directly– Transition radiation
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Joel Goldstein, RAL PHYS6011, Southampton 20
Muon Detectors
• Muons go where other particles cannot reach:– No nuclear interactions
– Critical energies >> 100 GeVAlways a MIP
– Stable (τ = 2.2 μs)
• A shielded detector can identify muons– “shielding” often
calorimeters
– Scintillator, MWPC, drift chambers…
Identify muons
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Joel Goldstein, RAL PHYS6011, Southampton 21
Next Time...
Putting it all together
- building a particle physics experiment