Reading the diagram
Feynman diagrams represent particle reactions and the exchange particles responsible for them. In the convention used here, the vertical direction represents time and the horizontal direction represents space.
- Lines representing particles before the interaction begin towards the lower part of the diagram.
- Lines for the products extend towards later times at the top.
- Particle paths are drawn as straight lines carrying arrows, with the arrows directed forwards in time.
- An exchange particle is shown using a wavy line.
- A vertex is a meeting point of particle and exchange-particle lines and marks an interaction.
- Charge, baryon number and lepton number must balance at every vertex.
- Particle and exchange-particle lines must not cross.
In weak-interaction diagrams, hadron or quark lines are kept on one side and lepton lines on the other. They connect through the exchanged boson rather than meeting one another directly at a vertex.
The wavy exchange line itself is distinct from a particle-line arrow. When the direction of charged exchange matters, a separate arrow can be shown above the wavy line.
Exam Tip: Label every particle and exchange particle clearly and include the required arrows on straight particle lines. You do not normally need to add space and time axes, but the diagram must still follow the correct orientation.