1. Feynman Diagrams
Topic 7.3

2. 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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3. 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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4. 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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5. What is happening here?
a.) Electron absorbs a photon and accelerates (changes direction)
b.) Electron emits a photon and changes direction

6. Electron and Positron Vertices

7. Examples of Feynman Diagrams

8. Examples of Feynman Diagrams

9. Rotate the vertex slightly to show a real interaction
Space
time
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10. 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
The amplitude of the diagram is the product of the interaction strength for each vertex i.e.
Probability of taking place process = (amplitude)2
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11. EM vertex
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12. 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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13. Weak Vertices
Ws, Z and gluons video
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15. 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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16. Strong interactions Annotate to show colour and flavour
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17. Feynman Diagram Examples
You should be able to draw Feynman diagrams for the following interactions;
Electron scattering
Beta decay
Pion decay
Electron – positron annhilation
Pair production
Muon decay
Quark interactions
Photon – photon scattering
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18. Electron Scattering

19. Beta Decay

20. Pion - Decay
Pion 1 Decay
Pion + Decary

21. Electron Positron Annihilation

22. Pair Production

23. Muon Decay

24. Quark Interactions

25. Photon Photon Scattering

26. Learn these ones Draw the Feynman diagram for beta (-) decay
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27. 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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28. 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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29. Gluon colour Quarks change colour through gluon exchange.
There are 6 coloured quarks and 2 colour neutral gluons
Note: time should be horizontal
Click diagram for animation
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30. Example
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.
A blue u quark absorbs this gluon. What is its final colour and flavour?
Draw a labeled Feynman diagram for this process.
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31. 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
W boson
Using quarks, draw a Feynman diagram for:
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32. Websites www.particleadventure.com
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