Feynman Diagrams
Topic 7.3
Feynman Vertices
• Each of the three basic interactions can be described using a symbol
called a Feynman vertex.
• We can use the vertices in a non-mathematical way to illustrate how
quarks and leptons interact with each other.
• There is an electromagnetic interaction vertex, a weak interaction
vertex and a strong interaction vertex.
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Drawing Feynman Diagrams
 Each vertex has an arrow going in and one going out. These
represent a lepton – lepton or quark-quark transition.
 Quarks or leptons are solid straight lines
 Exchange particles are either wavy (Photons, W, Z) or curly
(gluons).
 Time flows from left to right
 Arrows from left to right represent particles moving forward in
time.
 Arrows from right to left represent antiparticles moving forward
in time. (think of them as moving left to right).
 Vertices are linked by a line representing an exchange particle
 Charge and colour are conserved at each vertex.
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Forces in particle physics
 Forces are explained by Emission/absorption of particles
 A particle is emitted “ spontaneously”
 Where does the energy to create this particle come from?
(Uncertainty video)
 New law called the Heisenberg Uncertainty Principle
 The particle is known as a virtual particle.
QED Video
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What is happening here?
a.) Electron absorbs a photon and accelerates (changes direction)
b.) Electron emits a photon and changes direction
Electron and Positron Vertices
Examples of Feynman Diagrams
Examples of Feynman Diagrams
Rotate the vertex
slightly to show a
real interaction
Space
time
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Use of Feynman diagrams
Feynman diagrams may be used to calculate
probabilities for fundamental processes.
The picture represents a mathematical
process called the amplitude. For the em
interaction
amplitude   EM
 EM
1

137
The amplitude of the diagram is the product of the interaction strength for
each vertex i.e.
amplitude 
 EM   EM
Probability of taking place process = (amplitude)2
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EM vertex
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Basic em interactions
By rotating the arms of the vertices, the following interaction possibilities are
generated. Note that the time still flows from left to right and a backwards facing
arrow represents an antiparticle travelling forwards in time.
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Weak Vertices
Ws, Z and gluons video
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Strong vertices
The left hand side represents BEFORE and the right
hand side represents AFTER
The gluon can be regarded as a pathway through which colour charge is
exchanged between quarks and antiquarks.
The quark gluon vertices could also show colour flow as quarks interact.
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Strong interactions
Annotate to show colour and flavour
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Feynman Diagram Examples
http://teachers.web.cern.ch/teachers/archiv/HST2002/feynman/e
xamples.htm
http://www.departments.bucknell.edu/physics/animations/Feyn
man_diagrams/
http://www2.slac.stanford.edu/vvc/theory/feynman.html
You should be able to draw Feynman diagrams for the following
interactions;
1. Electron scattering
2. Beta decay
3. Pion decay
4. Electron – positron annhilation
5. Pair production
6. Muon decay
7. Quark interactions
8. Photon – photon scattering
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Electron Scattering
Beta Decay
Pion - Decay
Pion 1 Decay
Pion + Decary
Electron Positron Annihilation
Pair Production
Muon Decay
Quark Interactions
Photon Photon Scattering
Learn these ones
Draw the Feynman diagram for beta (-)
decay
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Strong force and gluon exchange
 Color force and strong force are essentially the same thing
 Colour force binds quarks together in hadrons by exchange
of gluons
 Strong force binds colour-neutral particles together e.g.
protons and neutrons in the nucleus.
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





Gluons
Bosons with spin = 1 and zero mass
Gluons are themselves coloured
Gluons bind quarks together
Force between quarks increases as quarks are separated.
Therefore isolated quarks and quarks cannot be observed.
This is quark confinement
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Gluon colour
 Quarks change colour through gluon exchange.
 There are 6 coloured quarks and 2 colour neutral gluons
GR B , GR G , GG R , GG B , GB R , GB G

G0 , G0
Note: time should be horizontal
Click diagram for
animation
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Example
1. A green s quark emits a gluon and becomes a blue quark.
State the flavour of the new quark and the colours of the
emitted gluon.
2. A blue u quark absorbs this gluon. What is its final colour
and flavour?
3. Draw a labeled Feynman diagram for this process.
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Feynman diagram practice
Using the basic weak interaction vertex involving a W boson and two
fermions (below) draw Feynman diagrams to represent the following
processes
Fermion out
Fermion in
   e    
e

e   e      
    


K      
Using quarks, draw a Feynman diagram for:
p p nn
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W boson
Websites
 www.particleadventure.com
 http://teachers.web.cern.ch/teachers/archiv/HST2002/
feynman/index.html
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