Feynman Diagrams

1. Feynman Diagrams • Richard Feynman invented diagrams that represent the particles interactions that are possible due to the standard model • Rules have to be followed when constructing a Feynman diagram to correctly show the interaction • Page 537

2. Feynman Diagrams • Good examples for practice! • Beta Minus Decay • Beta Plus Decay • Electron Capture • Electron-electron interaction

3. Annihilation and Pair Production • Energy and mass equivalence (E= mc squared) • Energy in the form of a photon can, during an interaction turn its energy into a particle and its corresponding anti particle • Called PAIR PRODUCTION • If a particle and its corresponding anti-particle meet they will ANNIHILATE each other (neither exists any more) but their rest mass will get converted into energy.

4. Predicting energies and masses

5. Feynman diagrams and predictions • Looking at the Feynman diagrams for annihilation, pair- production and an electron emitting a photon. • This is all the same particles interacting just in a different way… basically we are swinging the arrows around! • This helps to predict interactions that haven’t been seen by swinging round the Feynman diagrams of those that have!

6. Quark properties • Quark confinement • Quarks do not exist as single entities, they are always confined inside other particles. This is due to the strong nuclear force. As you try and pull the quarks apart they energy transfer increases. If you keep increasing this energy you get enough to produce another quark (or antiquark) so you’ll end up with a meson.

7. Quark properties • The Gluon or the Pion?! • Remember from earlier in the course the difference in the strong nuclear forces’ exchange particle. • Two protons interacting will exchange a pion • Two quarks interacting will exchange a GLUON.

8. Quark Properties • Colour Charge • Two particles interacting via the EM force would be assigned negative or positive numbers to indicate their respective charge quantities • We say the charge is the property causing the electromagnetic force. • So we assign quarks a property to explain how the strong nuclear force is caused.

9. Colour Charge • This property is called COLOUR CHARGE • Less simple than the EM force as there are now three different properties exhibited by the quarks. • As there as three primary colours we assign the quarks a red, blue or green colour (Anti –red etc exist as well) • All particles must be white so a baryon must contain a red, blue and green quark and mesons must be red, anti-red etc… • This also allows the quarks not to violate the Pauli Exclusion principle.

10. The Pauli Exclusion Principle • “No two identical fermions may occupy the same quantum state at the same time” • FERMIONS – particles with a half integer spin value • (This includes all quarks, leptons, protons and neutrons and some atoms) • BOSONS – particles with an integer spin value • (Photons, mesons, gauge bosons, Higgs boson and some atoms)

11. Particles

12. Spin Content of Hadrons • Baryons can have ½ or 1 ½ • Mesons can have 0 or 1 • This makes Baryons fall into the fermion category and mesons are bosons