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Particle Detectors. Iain Bertram Lancaster University June 2002. Outline. Basic Principles Bubble Chamber Pictures Modern Detectors What to Measure? How to identify Particles? Detector Types? Different Experiments. Basic Principles. How and what do we see? - PowerPoint PPT Presentation
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Outline
Basic Principles Bubble Chamber Pictures Modern Detectors
What to Measure? How to identify Particles? Detector Types? Different Experiments
Basic Principles
How and what do we see?
All perception of the material world is via Electromagnetic Interactions
Detectors To first order can only see charged objects. Energy deposition in the detector via
ionization processes. (Exception is photons which “see” electric
charge)
Bubble Chamber Pictures
Birmingham Package
Bubble Chamber – super heated liquid. Small addition of energy boils the liquid creating small bubble
Easy to visualize, too slow for modern experiments
Interpretation
Vertex – neutral particle decaying
Scatter off of an electron – light particle loops
Scatter off of a proton.
Bubble Chamber Information
Bubble Chamber in Magnetic Field Momentum of Particle Charge of Particle Energy Loss as function of distance (dE/dx)
Type of interaction V – Neutral decay Tree – Charged Decay Kink – decay with a neutral Momentum Balance on Tree gives presence of
neutral ?
What do we want to Measure?
Particle Properties Particle Type, electron, pion, kaon, proton,
muon, etc. Charge of the particle Location of Particles Momentum of Particles Lifetime
Need to combine all the above information into an event. For example XttppZ o , e e -
Detection – Rough Particle ID
EM hadronicB
InteractionPoint
Scintillating FiberSilicon Tracking Calorimeter (dense)
Wire Chambers
Abs
orbe
r M
ater
ial
electron
photon
jet
muon
neutrino -- or any non-interacting particle missing transverse momentum
Charged Particle Tracks Energy Muon Tracks
We know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverse
Tracking Detectors
Measure x-y-z location of all charged particles as the pass through predetermined parts of the detector Series of dots Get position of tracks Connect lines to find decay vertices
How do we get momenta Detector in magnetic field Tracks bend in a magnetic field
How to Measure Momentum?
Most Particles are Charged. If a charged particle moves through in a magnetic field it experiences a field
So the acceleration is proportional to the magnetic field and perpendicular to the direction of motion.
Momentum given by:
How can we measure Momentum
BvqF
mavqBF
Magnetic Field into Page
Velocity in direction of arrow.
Dashed Arrow: Force
Charged Particle in a Magnetic Field
In the Program the momentum is given by the following equation: P = momentum (in GeV/c) B strength of magnetic field in Tesla R = radius of curve in metres
So we can calculate the momentum if we can measure the radius of the curve made by the particle in the magnetic field
Simple Mathematics: x2 + y2 = r2
Pick three points on a circle and you get a radius
Brp 3.0
Particle Lifetimes
Most Particles are unstable – I.e they decay. Important property is the lifetime of the particle Quantum Mechanical Effect – Decay is random We have to measure many decays and take the
average to determine a real lifetime (in fact we need to fit the data)
Relativistic Effects are important: Need to take account time dilation etc.
12
2
c
L
cm
E
K
beam
Particle Identification
Want to identify what type of particle in many cases
Simple Classification Muon, electron, jet, photon, …… Straight forward differentiation in most
detectors
Exact type of particle Pion, kaon, proton, etc. Energy Deposition as function of momentum
Detection – Rough Particle ID
EM hadronicB
InteractionPoint
Scintillating FiberSilicon Tracking Calorimeter (dense)
Wire Chambers
Abs
orbe
r M
ater
ial
electron
photon
jet
muon
neutrino -- or any non-interacting particle missing transverse momentum
Charged Particle Tracks Energy Muon Tracks
We know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverse
Energy Deposition
Example of energy deposition for different particles
Note: measure momentum and dE/dx and get particle ID.
Only works for fixed momentum range: Not good at higher momenta
How else can we ID particles?
Masses -
The kaon decays to two pions:
By measuring the momentum and the angle between the two pions we can calculate the mass of the kaon if we know the mass of the pion:
Hence we can identify a kaon via calculating its invariant mass
2121422 cos22 EEppcmcmK
0K
Energy Measurement
1. Measure Momentum – id particle – finished
2. Measure energy without magnetic field Sample energy deposition many times in
dense material Calorimeter Calibrate with known energy depositions Depends on particle type
DØ Detector
CalorimeterMuons
Tracking Goals:
Searches –higgs, SUSY particles, etc.
Top physics,W physics,QCD,
General Purpose detector
“Typical DØ Dijet Event”
ET,1 = 475 GeV, 1 = -0.69, x1=0.66ET,2 = 472 GeV, 2 = 0.69, x2=0.66
MJJ = 1.18 TeVQ2 = ET,1×ET,2=2.2x105 GeV2
Opal Basic Muon Event
Track
Calorimeter
MuonMuons penetrate: Therefore go all the way through the detector
Opal Jets
Jets
Jets are evidence of quarks or gluons.
Collection of hadrons, penetrate further than electrons and usually > 1 particle
http://teachers.web.cern.ch/teachers/archiv/HST2000/teaching/resource/bubble/bubble.htm#An%20Introduction.
Bubble Chambers – Birmingham http://www.ep.ph.bham.ac.uk/user/watkins/seeweb/BubbleChamber.htm