Feynman Diagrams in Particle Interactions

1. Physics 4 – April 23, 2019 Get out #28-38 for Hmk check • P3 Challenge – • Consider the reaction • e- + p+ n + e • If this a valid reaction, demonstrate how each of the 4 conservation laws are validated. If not, state which conservation law or laws it violates and validate all others. • Objective: • 7.3 Structure of Matter • Assignment: • p307 #39-46 • Agenda: • Homework Review • Feynman Diagrams • Examples • Feynman Diagrams Activity

2. Feynman diagrams Richard Feynman developed a graphic representation of particle interactions that could be used to predict the probabilities of the outcomes of particle collisions. A typical Feynman diagram consists of two axes: Space and Time: Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter SPACE FYI Some books switch the space and time axis. The IB presentation is as shown above. TIME

3. e- e- Feynman diagrams Consider two electrons approaching one-another from the top and the bottom of the page… A purely spatial sketch of this interaction would look like this: But if we also apply a time axis, the sketch would look like this: The Time axis allows us to draw the reaction in a spread-out way to make it clearer. Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter The bubble of ignorance e- e- SPACE e- e- FYI The “bubble of ignorance” is the actual place in the plot that exchange particles do their thing. Ingoing and outgoing particles are labeled. TIME

4. Feynman diagrams Particles are represented with straight arrows, as were the two electrons in the previous electron-electron interaction. Exchange (force) particles are represented with either wavy lines (photons, W+, W- and Z0), or curly lines (gluons). Particle Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter Electromagnetic and weak exchange Strong exchange FYI You may have noticed that the electromagnetic exchange particle and the weak exchange particles all have the same wavy symbol. Indeed, it has been found that the two forces are manifestations of a single ELECTRO-WEAK force.

5. e- EXAMPLE: The complete Feynman diagram showing the repulsion of two electrons looks like this: EXAMPLE: Here is a diagram for one electron emitting a photon: e- Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter SPACE  e- e- TIME e- e- SPACE  TIME

6. EXAMPLE: In a Feynman diagram, antimatter points backward in time. This diagram represents two positrons repelling each other: EXAMPLE: Here is a diagram for one positron emitting a photon: e+ e+ Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter SPACE  e+ e+ TIME e+ e+ SPACE  TIME

7. EXAMPLE: Here is a photon producing an electron-positron pair. EXAMPLE: Here is an electron-positron pair annihilating to become a photon: e- SPACE Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter  e+ TIME e+ SPACE  e- TIME

8. EXAMPLE: Here is a diagram of a down quark emitting a W- particle that decays into an electron and an antineutrino: d u e Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter W- SPACE e- TIME FYI One can use Feynman diagrams to map out complete processes – including the bubble of ignorance. Using the conservation rules and the exchange particles, you can predict what kind of processes can occur.

9. p n EXAMPLE: Explain what has happened in this Feynman diagram. SOLUTION: It is a diagram of a down quark emitting a W- particle that decays into an electron and an antineutrino: Recall that a neutron consists of an up-down-down quark combo. Recall that a proton consists of an up-up-down quark combo. This is non other than the beta decay (- ) we talked about a long time ago. u u d d u d Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter e W- SPACE e- TIME n  p + e- + e

10. EXAMPLE: Write the reaction (including the neutrino) for beta (+) decay. SOLUTION: Just know it! EXAMPLE: Now draw the Feynman diagram for the above + decay: p n u u d Topic 7: Atomic, nuclear and particle physics7.3 – The structure of matter d u d p  n + e+ + e e W+ SPACE e+ TIME FYI Why is the neutrino not an anti-neutrino as in the - decay? To conserve lepton number.

11. Feynman Diagrams • Trick to reading them: Time flows from left to right. • Symbols. • Segment with arrow in direction of time to right, matter (labeled) often e- • Segment with arrow opposite direction of time, antimatter (labeled) often e+ • Wavy line, photon • Helix line, gluon, • Wavy line, W+, W-, or Z0labeled

12. Practice matching Matter - antimatter annihilation Absorption of a photon by electron Particle Pair production Emission of a photon by electron Absorption of a photon by antielectron Now do the Feynman Diagrams Activity

13. Exit slip and homework • Exit Slip – Draw the Feynman Diagram for a up quark that emits a W+ exchange particle becoming a down quark. The W+ then splits into a positron and an electron neutrino. • What’s due? (homework for a homework check next class) • p307 #39-46 • Start Studying for U14 Particle Physics Test on Apr 30 (next Tues) • What’s next? (What to read to prepare for the next class) • Read 7.3