8.  Feynman vertices

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 above, 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.

Typical Feynman vertices look like:

Important points to note about Feynman vertices:

1. 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
   
2. 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. (Note: it is also common to find Feynman diagrams using the convention that time flows from the bottom of the diagram to the top. This is just a matter of taste but the left to right convention is more commonly used)
   
3. 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. Some people find this convention difficult to get used to at first, but after a bit of practice it comes naturally. It is a useful convention as you will see when you construct Feynman diagrams to show interactions.
   

The arms of each vertex can be rotated about the vertex point to generate the full set of possible interactions that the vertex can represent. There will be three vertices  in which a single particle enters and two leave, and three vertices in which two particles enter and one leaves.

The next bit is an important bit.....and it is the key to understanding and interpreting the diagrams: