How Particles Interact Particle Physics Lesson 5

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

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

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

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

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.

Meet Richard Feynman

Meet Richard Feynman

Exchange Particles

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. 

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.

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.

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?

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.

Neutron-Neutrino Interaction

Proton-Antineutrino Interaction

β− decay

β+ decay

Electron Capture

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

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.