1. How Particles Interact
Particle Physics Lesson 5

2. How Particles Interact Lesson Objectives
Describe how forces are caused by particle exchange.
State that for the electromagnetic force, virtual photons are the exchange particles
Draw Feynman Diagrams for the following interactions:
Neutron-neutrino interaction
Proton-antineutrino interaction
β− decay
β+ decay
Electron capture

3. Homework Know what exchange particles do  PPQ
Learn the four fundamental forces of nature and their exchange particles.
Learn the Feynman diagrams for the following interactions:
Neutron-neutrino interaction
Proton-antineutrino interaction
β− decay
β+ decay
Electron capture

4. Video
In Search of Giants (12 of 15) QED - The Jewel of Physics.wmv

5. Forces
What is Newton’s 2nd Law?
What is momentum?
What causes forces?

6. What causes forces?
Newton’s 2nd Law states that force is equal to the rate of change of momentum.
Momentum is calculated by multiplying the mass and the velocity of a particle.
Richard Feynman came up with the idea that the forces are transmitted by exchange particles.

7. Meet Richard Feynman

8. Meet Richard Feynman

9. Exchange Particles

10. Feynman Diagram Is a pictorial representation of what is going on.
First devised by American particle physicist Richard Feynman (1918 – 1988).
The forces between electrically charged particles are thought to be transmitted by photons, which are emitted and absorbed by the particles. 

11. Weak Nuclear Force
Which force is involved when a neutron changes to a proton or proton  neutron?
Can’t be electromagnetic because neutrons have no charge.
Must be weaker than the strong force otherwise it would affect stable nuclei.
Therefore it is called the weak nuclear force.

12. Weak Nuclear Force II
Due to the exchange of particles called W bosons.
They are unlike photons in that they have:-
Non zero rest mass
A very short range (less than fm!)
Are positively or negatively charged.

13. Four Fundamental Forces
Type of Interaction
Exchange Particle/ Gauge Boson
Particles Affected
Strong
Gluon
Hadrons only
Electromagnetic
Photon
Charged particles only
Weak
W+,W−,Z0
All types
Gravity
Graviton?

14. Feynman diagrams
They were developed by Richard Feynman to decribe the interactions in quantum electrodynamics (QED).
The diagrams are used to describe a variety of particle interactions.
Virtual particles are represented by wavy or broken lines and have no arrows.
Graphical ways to represent exchange forces, time is upward but the lines don’t represent the paths the particles take.

15. Neutron-Neutrino Interaction

16. Proton-Antineutrino Interaction

17. β− decay

18. β+ decay

19. Electron Capture

21. Exchange Particles Exchange particles do the following:-
Transfer momentum
Transfer Energy
Transfer Charge (in the case of W+ and W- bosons.)

22. Summary
The are four fundamental forces of nature: gravity, electromagnetic, strong & weak.
Each force is thought to have a corresponding exchange particle that transfer the momentum.
Feynman diagrams are a way of summarising the interactions.