Examples of Interactions - Feynman Diagrams


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The exchange of particles in an interaction can be visualised using Feynman diagrams. In a Feynman diagram the particles and the exchange particle are shown. The paths (arrows) do not represent the actual direction through space-time of the particles, it is just a visualisation


1. Electrostatic repulsion between two electrons.

The electrons exchange a virtual photon which transfers momentum and causes a repulsive force. Note that no charge is transferred by the virtual photon.


2. Nuclear strong force binding a proton and neutron

Because a positive pi-meson is the exchange particle then charge is transferred and the proton and neutron swap places. If a neutral pi-meson had been the exchange particle then obviously this wouldn't have happened!

What is actually happening is that the proton (uud) looses an up quark and a down / antidown pair (dd) are temporarily created. The down quark stays to form the neutron (udd). The remaining up and antidown quarks form the pi-meson (ud) which then joins with the neutron. Now the reverse happens ... the antidown annihilates with a down in the neutron and the up fills the empty place.



3. Consider a weak interaction: beta decay where a neutron decays to form a proton and an electron

n --> p+ + e- + νe

This is a weak interaction mediated by a boson.

The neutron becomes a proton.

A down quark has transformed to become an up quark and the W- boson which has to be negative to conserve charge.

The boson then decays to form the electron and antineutrino

Consider the conservation laws:


Before exchange

After exchange


Neutron

Total

Proton

Electron

Antineutrino

Total

Charge

0

0

1

-1

0

0

Baryon Number

1

1

1

0

0

1

Lepton number

0

0

0

1

-1

0

In all respects the conservation laws are obeyed



4. A weak interaction for delta particle decay

This is a weak interaction as the strange quark is changed to another flavour


5. A weak interaction between lepton and hadron

The consevation of lepton number dictates the creation of the positron, it would not have been possible to create a muon and still conserve lepton number. Baryon number is conserved.


6. Strong Interaction between Baryons and Mesons

From the properties of the strong interaction it is possible to predict exactly what the unidentified particle will be - this is not possible with the weak interaction where flavour is not conserved.


7. Multiple decay process of an excited psi meson

The excited psi meson (cc) decays via the strong interaction to form the unexcited psi meson and a pair of pi-mesons. The charm quarks then annhilate and a pair of leptons are created.




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