Feynman diagrams
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Richard Feynman developed diagrams to illustrate particle interactions through the exchange of other particles. His diagrams use straight lines for particles and curved lines for the exchanged particles. The diagrams must conserve charge at interaction points. Electromagnetic interactions exchange photons, while weak interactions exchange W bosons. Annihilation occurs when a particle collides with its antiparticle and they destroy each other, producing gamma rays. Pair production is the reverse, where a high energy photon transforms into a particle-antiparticle pair.
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Feynman diagrams
- 1. Particles Feynman’s Diagrams Annihilation and Pair Production Thursday, 24 November 2011
- 2. Feynman’s Diagrams Richard Feynman designed a way to illustrate interactions between particles through exchange particles. His idea is simple: • Straight lines represent particles before and after the interaction • Wavy lines connect the straight lines and represent the particle exchange • The charge must be conserved at each junction • Particle lines point in the same direction for both attraction and repulsion • The direction of the lines does not show the direction of the particles
- 3. Electromagnetic Interactions The exchange particle responsible for electromagnetic interactions is a photon. So, Feynman’s diagrams for e- - e- and e- - p interactions are: After After e- e- e- p e- e- e- p Before Before electron – electron repulsion electron – proton attraction
- 4. - Decay In Beta decay a neutron in the nucleus decays (turns) into a proton, a fast moving electron ( -particle) and an anti-neutrino e n p e e Note that e- in this case is a fast moving electron ( -particle) emitted from within the nucleus through the decay of a neutron into a proton, and not an atomic electron that orbits around the nucleus.
- 5. Feynman’s Diagram for - Decay The force responsible for - decay is the weak force. So, the exchange particle is the W-. Draw Feynman’s diagram for this reaction. After p e- e W n Before The neutron decays into a proton releasing a W particle which very quickly decays into an -particle and an anti-neutrino.
- 6. + Decay In anti-Beta decay a proton in the nucleus decays (turns) into a neutron, a fast moving positron ( -particle) and a neutrino e p n e e
- 7. Feynman’s Diagram for + Decay The force responsible for + decay is the weak force. So, the exchange particle is the W+. Draw Feynman’s diagram for this reaction. After n e+ e W p Before The proton decays into a neutron releasing a W particle which very quickly decays into an -particle and a neutrino.
- 8. Electron Capture It is possible for a proton in the nucleus to “capture” an electron and turn into a neutron releasing a neutrino e p e n e
- 9. Feynman’s Diagram for e- capture The force responsible for electron capture is the weak force. So, the exchange particle is the W+. Draw Feynman’s diagram for this reaction. After n e W p e- Before The proton turns into a neutron by trapping an e-. The exchange particle W leaves a neutrino after the reaction.
- 10. Electron – Proton Collision When an electron and a proton collide the proton turns into a neutron releasing a neutrino e p e n e
- 11. Feynman’s Diagram for e- - p collisions The force responsible for this collision is the weak force. So, the exchange particle is the W-. Draw Feynman’s diagram for this reaction. After n e W p e- Before The proton turns into a neutron by colliding with an e-. The exchange particle W leaves a neutrino after the reaction.
- 12. Neutrino – Neutron Collisions When two particles collide they can give rise to new matter or cause the particles involved to change. If a neutrino e hits a neutron with sufficient Ek, the neutron turns into a proton and releases an electron. n e p e
- 13. Feynman’s Diagram e– n collision The force responsible for e – n collisions is the weak force. So, the exchange particle is the W+. Draw Feynman’s diagram for this reaction. After p e- W n e Before The neutron turns into a proton by colliding against a neutrino. The exchange particle W leaves an electron after the reaction.
- 14. Anti-neutrino – Proton Collisions When an anti-neutrino e hits a proton with sufficient Ek, the proton turns into a neutron and releases a positron (e+). p e n e
- 15. Feynman’s Diagram e– p collision The force responsible for e – p collisions is the weak force. So, the exchange particle is the W+. Draw Feynman’s diagram for this reaction. After n e+ W p Before e The proton turns into a neutron by colliding against an anti-neutrino. The exchange particle W leaves a positron after the reaction.
- 16. Annihilation When an anti-particle is created it can be observed, but only for a very short time. This is because: • It will soon collide against its particle • The two destroy each other • Their mass is converted in energy This process is called ANNIHILATION.
- 17. Annihilation Look at the annihilation of an electron and its anti-particle (positron) e- e+
- 18. Annihilation Why are two photons of energy produced and not just one? (Hint: any collision must obey all conservation laws) 0 0 0 1 e 1 e 2 0 • One photon only could conserve charge and mass/energy • But to conserve momentum two photons moving in opposite directions must exist
- 19. Pair Production A high energy photon like a -ray can vanish to form a pair particle – anti-particle. This is the opposite of annihilation and we call it PAIR PRODUCTION. e+ e-
- 20. Pair Production In what way would a third particle, e.g. nucleus or electron, get involved in this reaction? (Hint: again all conservation laws must apply) 0 0 0 0 1 e 1 e • The third particle recoils and carries away some of the energy of the photon • The recoil ensures that the momentum is also conserved