Feynman diagrams are used to graphically show particle interactions in a simple way. There are some basic rules which can be followed in order to learn how they are drawn.
- Most Feynman diagrams will be drawn with time on the y-axis and space on the x-axis. Therefore time progresses as you move from down to up.
- Solid lines represent matter particles (fermions), a line with a forward arrow depicts a matter particle. A backwards arrow shows an antimatter particle.
- A wiggly line shows one of the force carriers, a boson.
- Vertices represent places where a particle interaction takes place.
For conservation rules for particle interactions go to this article
Emitting a boson:
If a particle experiences one of the four fundamental forces, they will release a boson. For all forces other than the weak interaction, the identity of the particle stays the same.
For example, an electron releasing a photon can be drawn as:
The weak interaction is different to the other fundamental forces in that it has 3 gauge bosons. When a particle releases a Z boson, which has neutral charge, the identity of the particle remains the same. This means the diagram will be very similar to the one shown above.
W bosons have either a positive or negative charge. Due to conservation laws, the charge before and after an interaction has to stay the same. This means the identity of the particle releasing the boson can change.
When a neutron become a proton this occurs due to a W– boson being released by a down quark in order for it to change identity to an up quark:
This can then be simplified to:
Bosons to Fermions
A boson can split into two fermions (as shown in the above diagrams). They can only do so by producing a fermion ans an antifermion because lepton and quark numbers must be conserved and therefore the two particles released must cancel out.
Another example of this would be a photon decaying into an electron and a positron.
The decay produces an electron and its antiparticle. The charges must cancel out as a photon has no charge.
Particle Physics: Dark Matter and Dark Energy by David Chapple