Feynman diagrams
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    Feynman diagrams Feynman diagrams Presentation Transcript

    • Particles Feynman’s Diagrams Annihilation and Pair ProductionThursday, 24 November 2011
    • Feynman’s DiagramsRichard 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
    • Electromagnetic InteractionsThe 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
    • - DecayIn 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 eNote 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.
    • Feynman’s Diagram for - DecayThe 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 BeforeThe neutron decays into a proton releasing a W particle which very quickly decays into an -particle and an anti-neutrino.
    • + DecayIn 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
    • Feynman’s Diagram for + DecayThe 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 BeforeThe proton decays into a neutron releasing a W particle which very quickly decays into an -particle and a neutrino.
    • Electron CaptureIt 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
    • Feynman’s Diagram for e- captureThe 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- BeforeThe proton turns into a neutron by trapping an e-. The exchange particle W leaves a neutrino after the reaction.
    • Electron – Proton CollisionWhen an electron and a proton collide the proton turns into a neutron releasing a neutrino e p e n e
    • Feynman’s Diagram for e- - p collisionsThe 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- BeforeThe proton turns into a neutron by colliding with an e-. The exchange particle W leaves a neutrino after the reaction.
    • Neutrino – Neutron CollisionsWhen 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
    • Feynman’s Diagram e– n collisionThe 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 BeforeThe neutron turns into a proton by colliding against a neutrino. The exchange particle W leaves an electron after the reaction.
    • Anti-neutrino – Proton CollisionsWhen 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
    • Feynman’s Diagram e– p collisionThe 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 eThe proton turns into a neutron by colliding against an anti-neutrino. The exchange particle W leaves a positron after the reaction.
    • AnnihilationWhen 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 energyThis process is called ANNIHILATION.
    • AnnihilationLook at the annihilation of an electron and its anti-particle (positron) e- e+
    • AnnihilationWhy 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
    • Pair ProductionA 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-
    • Pair ProductionIn 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