How Particles Interact

1. How ParticlesInteract Particle Physics Lesson 5

2. Today’s 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. How Hard Am I Trying?

5. What does the Evidence show? I have about half the homework – which I have marked – it was done pretty well  target grade performance. Does this mean half the class is on track to get a Target Grade -3 or -4?

6. Solve the Following charge flowing into a junction = charge flowing out

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

8. Exchange Particles

9. 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 (1918 – 1988) came up with the idea that the forces are transmitted by 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 0.001 fm!) • Are positively or negatively charged.

13. Four Fundamental Forces

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

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

21. 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.