Reactions, Exchange Particles and Feynman Diagrams

Reactions, Exchange Particles and Feynman Diagrams

The weak interaction is responsible for beta decay. The exchange particles for the weak interaction are the W+, the W- and the Z0.

Consider β decay. n → p

Reactions, Exchange Particles and Feynman Diagrams

We can describe the process as follows. A neutron changes into a proton by emitting a W-, which quickly decays into an electron and an antineutrino.

Note: that charge is conserved at each vertex in the diagram above.

However, the decay is actually the result of a down quark in the neutron changing into an up quark. So a more accurate picture would be:

Reactions, Exchange Particles and Feynman Diagrams

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Reactions, Exchange Particles and Feynman Diagrams

This time a proton changes into a neutron. To conserve charge the W carries a positive charge which it transfers to the positron upon its decay.

As before we can show the quark change as follows:

Reactions, Exchange Particles and Feynman Diagrams

When a reaction is mediated by the weak interaction, one type of quark is always changed into another as in the processes described above. The flavour of the quarks, before and after, changes. Note: that in some reactions quarks can be created in quark-antiquark pairs, but the net flavour of quarks before and after is unchanged as they created pairs cancel.

Another example of the weak interaction is the interaction between a proton and an electron called electron capture. This occurs when a nuclear proton captures an electron from the surrounding cloud and becomes a neutron.

Reactions, Exchange Particles and Feynman Diagrams

Whereas the annihilation of a neutrino-antineutrino pair involves no change of flavour of quark so we know the weak interaction is not responsible.

Reactions, Exchange Particles and Feynman Diagrams

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