Getting the Exchange Particle

The aspect of Feynman diagrams that students often struggle with is labelling the exchange particle and the direction to draw it. Look at what you start with:

If it is positive and becomes neutral you can think of it as throwing away its positive charge so the boson will be positive. This is the case in electron capture.

If it is positive and becomes neutral you can think of it as gaining negative to neutralise it so the boson will be negative. This is the case in electron-proton collisions.

If it is neutral and becomes positive we can think of it either as gaining positive (W+ boson) or losing negative (W– boson in the opposite direction).

Work out where the charge is going and label it.