A-level Physics (Advancing Physics)/Bosons

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Bosons are particles with an integer spin, such as 1, 2 etc and mediate a specific force. All interactions can be described by one of the four forces, gravity, electromagnetic, weak and strong which are caused by the release of a corresponding boson.

Feynman Diagrams[edit]

One way of representing these interactions is the Feynman diagram. This is a graph with time on the vertical axis, and space on the horizontal axis showing the paths of particles through space and time as lines. So, a stationary electron looks like this:

Feynman stationary electron.svg

It is often useful to define our units of space and time in such a way that, if something is travelling at the speed of light, it makes a 45° angle. Bosons are virtual particles, so they are given wavy lines. So, a photon travelling at the speed of light from A to B looks like the following:

Feynman photon c.svg

Different particles can, of course, interact with each other. These interactions must take place at a definite point in space-time. They can be represented by a certain point on a Feynman diagram, with lines coming in and out of the point representing the velocities of particles which take part in the interaction.

Photons[edit]

Photons are massless, have a spin of +1, neutral charge and carry the electromagnetic force. They are 'given off' by one particle, causing it to change its velocity. They are then 'received' by another particle, causing it too to change its velocity. This can be represented on a Feynman diagram in the following way:

Feynmandiagram.svg

W and Z Bosons[edit]

W and Z bosons carry the weak nuclear force between particles. This occurs, for example, in β decay, which actually takes place in two stages. First, a proton turns into a neutron (or vice versa), emitting a W boson. Then, the W boson 'turns into' an electron / positron and an (anti-) neutrino. This is shown in the following Feynman diagram:

Beta Negative Decay.svg

Gluons[edit]

Gluons carry the colour force between quarks, holding them together. Quarks have a property known as 'colour', as do gluons. The gluons carry colour between the quarks, mediating the colour force. The strong force is the residual colour force that holds hadrons together. You probably won't be asked about gluons in the exam.

Questions[edit]

1. A stationary light source emits single photons at regular intervals. Draw a Feynman diagram to represent this.

2. Write two equations (including a W+ boson) which describe positron emission.

3. What is the charge on a W- boson?

4. Read Richard Feynman's excellent book, "QED - the Strange Theory of Light and Matter", ISBN 978-0-140-12505-4.

Worked Solutions