6. The Standard Model page 24
Feynman diagrams
We know that the forces are carried by the exchange of virtual particles. As part of his theory of quantum electrodynamics, Richard Feynman developed a method of illustrating the electromagnetic force using Feynman diagrams.
Feynman diagram of force between electrons - interactive
Picture 6.4 We can show the force between two electrons using a Feynman diagram.

Basic Feynman diagrams
Feynman diagrams are useful for showing interactions between two (or more) particles. They are graphs with time on the y-axis and position on the x-axis (though sometimes you will see the axes swapped around). Different particles are represented by different lines in this domain. For example, an electron is represented by a straight line with an arrow (picture 6.4). The arrow is pointing upwards to show it is moving forwards in time.

We can show an electron giving out a virtual photon as a wavy line moving to the right. Notice that the virtual photon has a shallower gradient (this is because it is travelling faster – at the speed of light). Also, notice that the electron recoils when it gives out the virtual photon – this is in line with the conservation of momentum.

Now let's imagine that the virtual photon is absorbed by another electron. The second electron also recoils. So this diagram shows the repulsive force between two electrons. They are pushed apart as they move forwards through time.

Feynman diagram of annihilation - interactive
Picture 6.5 An electron-positron annihilation after which the photon decays as well.
Feynman suggested that a positron (or any other antiparticle) is like a particle travelling backwards in time. So, on a Feynman diagram, this is represented as a line with an arrow pointing downards.

We can see what happens when an electron and positron annihilate. The two come together at a vertex and release energy in the form of electromagnetic radiation.

Sometimes the photons might decay to produce a new particle/antiparticle pair – such as a muon and anti-muon. This process is called pair production. Notice that the positron and the anti-muon are both travelling backwards in time.

Feynman diagram of force between quarks - interactive
Picture 6.6 Exchange of gluon carrying the force between two quarks. Notice that the quarks change colour in the exchange.
Strong force
Feynman diagrams can also be used to describe the strong nuclear force. This is the force between quarks and is carried by the gluon – which is shown as a looping line. The force between quarks depends on a property called colour (of course the quarks do not have colour – it is just a property given to them - sometimes called colour charge – see page 23). The theory that describes this is called quantum chromodynamics. Notice that the gluon carries colour charge itself (unlike the virtual photon that doesn't carry electric charge in the electromagnetic force). This means that the quarks' colour charges change when they emit and absorb the gluon.
Feynman diagram see q 23
Picture 6.7 See question 23.
Question 23
a) What is the interaction shown in picture 6.7?

b) What are the products of this decay?

c) What is the force that is carried by the W particle and causes this decay?

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