Feynman Diagrams

Each of the three basic interactions can be described using a symbol called a Feynman vertex. To the particle physicist, each Feynman vertex represents a component of some sophisticated mathematics that is used to calculate various aspects of particle interactions. But we can use the vertices in a non-mathematical way to illustrate how quarks and leptons interact with each other. There are three basic vertices, each one associated with each of the fundamental interactions. There is an electromagnetic interaction vertex, a weak interaction vertex and a strong interaction vertex. The basic structure of a vertex is like this:

In the basic vertex shown to the left, the interaction propagator symbol has been drawn vertical. When drawing interactions it is usual to incline the propagator symbol to suggest that it is moving towards or away from the interaction point.

Important points to note about Feynman Diagrams

- It is important to recognise that a vertex is simply a symbol, it does not represent tracks of particles in space and it is not a space-time diagram
- The symbol is read from left to right. The left hand side of the symbol shows the nature of the particle before the interaction and the right hand side shows the nature of the particle after the interaction.
- We use a forward facing arrow to represent a particle travelling forwards in time and a backward facing arrow to represent an antiparticle also travelling forwards in time.