3.2.1.4
Feynman diagrams represent particle interactions visually
Particles, Antiparticles & Photons — AQA A-Level Physics
- Feynman diagrams show particle equations and the exchange particles involved.
Rules for drawing Feynman diagrams
- $y$-axis = time (upwards), $x$-axis = space.
- Incoming particles enter at the bottom; outgoing particles leave at the top.
- Particles: straight lines with arrows pointing forward in time.
- Exchange particles: wavy lines with no arrows (charged W bosons may have an arrow above showing direction of transfer).
- $Vertices = points$ of interaction. Hadrons/quarks on the left, leptons on the right — they must not meet at a vertex.
- ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)., baryon numberA quantum number conserved in all interactions. Baryons (protons, neutrons) have baryon number +1, antibaryons have -1, and mesons/leptons have 0. and lepton numberA quantum number conserved in all interactions. Leptons (electrons, neutrinos) have lepton number +1, antileptons have -1, and hadrons have 0. must be conserved at each vertex.
- Lines must not cross.
Key Feynman diagrams to know
- Electromagnetic repulsion: two electrons exchange a virtual photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. ($\gamma$) and move apart.
- $\beta^{-}$ decay: neutron → proton via $W^{-}$ boson → $W^{-}$ decays into $e^{-}$ and $\bar{\nu}_e$.
- $\beta^{+}$ decay: proton → neutron via $W^{+}$ boson → $W^{+}$ decays into $e^{+}$ and $\nu_e$.
- Electron capture: proton absorbs electron via $W^{+}$ boson → produces neutron and $\nu_e$.
- Electron-proton collision: electron collides with proton via $W^{-}$ boson → produces neutron and $\nu_e$.
Common Mistake
MEDIUM
Students often: Forgetting arrows on the W boson or on particles in Feynman diagrams.
Instead: All particle lines need arrows showing the direction in time. Charged W bosons should have an arrow above showing the direction of exchange. Label every line.
Instead: All particle lines need arrows showing the direction in time. Charged W bosons should have an arrow above showing the direction of exchange. Label every line.