Reading a Feynman diagram. Time runs upwards. Particles going in start at the bottom and particles coming out leave at the top. The wavy line is the exchange particle. The diagram shows which particles go in and which come out; it is not a picture of the paths the particles take.

The vertices. A vertex is a point where lines meet. Charge, baryon number and lepton number are each conserved at every vertex, so adding up each quantity going into a vertex gives the same total as coming out.

Which W? Charge conservation at the vertex the W leaves sets its charge. In electron capture a proton (+1) becomes a neutron (0), so the W leaving the proton carries +1: a W⁺. The same interaction can also be drawn with a W⁻ travelling from the electron to the proton. Both versions conserve charge at both vertices. Tick “Draw the W the other way” to compare them.

Electron capture and electron–proton collisions. Both have the same particles going in and coming out. In electron capture the proton is in a nucleus and the electron comes from an inner shell of the same atom. In an electron–proton collision a free electron with a high kinetic energy collides with a proton.

Quark level. The weak interaction is the only interaction that can change one type of quark into another. In β⁻ decay a d quark changes into a u quark, so the neutron (udd) becomes a proton (uud). The other two quarks do not take part, so they are not drawn.

Electromagnetic repulsion. Two particles with the same charge repel by exchanging a virtual photon. The photon is called virtual because it exists only during the exchange and cannot be detected.