Download PowerPoint Presentation - Particle Physics Group

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?
 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
Scatter off of an
electron – light
particle loops
Vertex – neutral
particle decaying
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
 e   e-  Z o       , p  p  t  t  X
Detection – Rough Particle ID
Charged Particle Tracks
B
Calorimeter (dense)
EM
Absorber Material
Interaction
Point
Scintillating Fiber
Silicon Tracking
Muon Tracks
Energy
hadronic
electron
photon
Wire Chambers
jet
muon
neutrino -- or any non-interacting
particle missing transverse momentum
We know x,y starting momenta is zero, but
along 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 
F  vqB  ma

 
F  qv  B
So the acceleration is proportional to the
magnetic field and perpendicular to the direction
of motion.
Momentum given by:
Magnetic Field into Page
Velocity in direction of
arrow.
 How can we measure Momentum
Dashed Arrow: Force
Charged Particle in a Magnetic Field
 In the Program the momentum is given by the
following equation: p  0.3Br
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
Tracking
Curved track –
charge and
momentum
Vertex – decaying
particle
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)
Ebeam
 
Relativistic Effects are important:
2
m
c
K
Need to take account time dilation etc.
L

c  2 1
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
Charged Particle Tracks
B
Calorimeter (dense)
EM
Absorber Material
Interaction
Point
Scintillating Fiber
Silicon Tracking
Muon Tracks
Energy
hadronic
electron
photon
Wire Chambers
jet
muon
neutrino -- or any non-interacting
particle missing transverse momentum
We know x,y starting momenta is zero, but
along 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:
K 0    
 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:
mK c 2  2m2 c 4  2 p1 p2 cos  E1E2
 Hence we can identify a kaon via calculating
its invariant mass
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
Tracking
Goals:
Searches –
higgs, SUSY
particles, etc.
Top physics,
W physics,
QCD,
General
Purpose
detector
Calorimeter
Muons
DØ Detector
Silicon Microstrip Tracker
Fibre Tracker
Forward
Preshower
Solenoid
Central Preshower
“Typical DØ Dijet Event”
ET,1 = 475 GeV, h1 = -0.69, x1=0.66
ET,2 = 472 GeV, h2 = 0.69, x2=0.66
MJJ = 1.18 TeV
Q2 = ET,1×ET,2=2.2x105 GeV2
DØ Event: W e
DØ top event: t  W  e b t  W  qq b


BaBar Detector
Opal Detector
Opal Basic Muon Event
Calorimeter
Track
Muon
Muons
penetrate:
Therefore go
all the way
through the
detector
Opal Electron
Track
Calorimeter
Electrons
dump energy
quickly
Photon: No
track
Opal Jets
Jets
Jets are evidence of
quarks or gluons.
Collection of
hadrons, penetrate
further than electrons
and usually > 1
particle
Belle Event: B → +– :




 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
B0  J/y KS