Feynman diagrams
These were developed by the American physicist Richard Feynman as a way of showing the
interaction between two sub-nuclear particles. They are a way of representing what is
happening between the two particles during an interaction. (In the following Feynman
diagrams time goes from bottom to top). Each point where lines come together is called a
vortex and at each vortex charge, baryon number and lepton number must be conserved.
Electromagnetic. An example of a Feynman
after interaction
diagram for an electromagnetic interaction is
-
shown in Figure 1. It shows the interaction e -
e
between two electrons. In classical physics the
electrons, both with a negative charge would repel virtual photon
each other. The Feynman diagram shows that this
repulsion occurs because of the interchange of
photons. Each electron emits a photon which is -
- e
then absorbed by the other electron. The photons e
in the interaction are known as virtual photons before interaction
because they are emitted and absorbed in a time
so short that the uncertainty principle is not
Figure 1
violated. (To simplify the diagram only one of the
virtual photons is shown).
A line which begins and ends in the diagram (as after interaction
with the wavy line in Figure 1) represents a virtual u d
particle.
gluon
Strong. Figure 2 shows the interaction between an d
up quark and a down quark due to the strong force u
between them. Here the interaction is due to before interaction
exchange of gluons.
(As before to simplify the diagram only one of the
virtual gluons is shown) Figure 2
after interaction
p n
Strong. Figure 3 shows the interaction between a Pion (-)
proton and a neutron. Here the interaction is
mediated by the exchange of pions.
p
n
before interaction
Figure 3
1
Weak. The weak force interaction of Figure 4 shows the Feynman diagram for beta decay.
p after decay
e-
W- e
n
before decay
Figure 4
2