2.1.9

# Particle Interactions

Test yourself

## The Four Fundamental Forces

Nature is governed by four fundamental forces:

### Gravity

• Gravity is the weakest of the four forces.
• It is so weak that its effects are only noticeable for huge masses like stars and planets.
• Gravity is a purely attractive force.
• Gravity is experienced by all matter.

### Strong nuclear force

• The strong nuclear force is the strongest of the four fundamental forces.
• But, it can only be felt over a very short range (a few fm).
• The strong nuclear force is only experienced by hadrons (e.g. protons and neutrons).
• The strong nuclear force is attractive for separations above about 0.5 fm, but strongly repulsive for separations less than about 0.5 fm.

### Weak nuclear force

• The weak nuclear force affects all types of particles.
• The weak nuclear force is responsible for beta-plus and beta-minus decay, as well as electron capture interactions.
• The weak nuclear force is a very weak, very short range force.

### Electromagnetic force

• The electromagnetic force is very strong and has a very large range.
• The electromagnetic force is responsible for interaction between charged objects like electrons and protons.
• Most everyday forces we experience are because of the electromagnetic force.
• When you touch an object, the force you feel is because of the repulsion between the outer electrons on the object and your hand.

## Exchange Particles

Two objects cannot interact instantaneously over a finite distance. For the objects to feel a force, an exchange particle must travel from one to the other.

### Types of exchange particle

• Each fundamental force has its own exchange particle.
• The gluon/pion is the exchange particle for the strong nuclear force.
• The exchange particle of the electromagnetic force is called a 'virtual photon' (virtual because they only exist for a very short time).
• The weak nuclear force has three exchange particles: the W+, W- and Z0 bosons.
• Exchange particles are sometimes called gauge bosons.

### Repulsion

• Think of two particles as skaters on an ice rink (with no friction).
• If one skater throws a ball straight at the other, both skaters will be pushed further apart as momentum is exchanged.

### Attraction

• Think again about our two skaters.
• Imagine this time that the boomerang is thrown away from himself.
• The boomerang circles round to the other skater and pushes him towards the first skater, bringing them closer.

## Feynman Diagrams

We can represent particle interactions using Feynman diagrams.

### General rules

• Particles start at the bottom and move upwards.
• Particles have straight lines. Exchange particles have wiggly lines.
• Hadrons must stay on the left, leptons on the right.
• Particles cannot cross paths, they can only interact via an exchange particle.
• The charge entering a node must equal the charge leaving the node.
• A W+ boson travelling from left to right is the same as a W- boson travelling from right to left.

### Electromagnetic repulsion

• Two electrons repel each other because of the electromagnetic force.
• The exchange particle is a virtual photon.

### Beta-minus decay

• n → p + e- + νe
• A neutron decays into a proton and W- boson which then decays into an electron and an electron antineutrino.

### Beta-plus decay

• p → n + e+ + νe
• A proton decays into a neutron and W+ boson, which then decays into a positron and an electron neutrino.

### Electron capture

• p + e- → n + νe
• A proton interacts with an electron via a W+ boson, producing a neutron and an electron neutrino.

### Electron-proton collision

• p + e- → n + νe
• The equation is identical to electron capture but the diagram is different.
• The diagram shows an electron colliding with a proton via the W- boson (travelling the other way to a W+ boson), producing a neutron and an electron neutrino.