Exchange Particles and Reading a Feynman Diagram
Every interaction between particles is carried by an exchanged boson, and which boson it is decides the range and the timescale. A Feynman diagram is the bookkeeping for that exchange, with the vertices doing the conserving and the internal line naming the force.
What a learner can do afterwards
- Names the exchange particle for each of the four interactions and links its mass to the range of the force
- Draws the diagram for beta-minus decay, beta-plus decay and electron capture, with the W labelled and the arrows right
- Checks charge, baryon number and lepton number at each vertex of a diagram they did not draw
1 · Read
Every interaction is carried by an exchanged boson, and which boson it is decides the range. The photon carries electromagnetism, the gluon carries the strong force, the W and Z carry the weak force, and the graviton for gravity stays hypothetical. Heavy carriers can borrow their energy for only a tiny moment, so they travel almost no distance, while the massless photon has unlimited range.
Beta-minus decay shows the bookkeeping in action. Time runs upward and a W boson leaves the vertex, turning a down quark into an up quark, while the W becomes an electron plus an antineutrino. Arrows carry meaning too: an arrow pointing backward in time is an antiparticle moving forward, with its charges flipped.
At every vertex you audit three small account books: charge, baryon number, and lepton number. Each total going in must equal the total coming out, and that rule alone kills most impossible diagrams. Electron capture uses the same exchange: an atomic electron joins a proton to make a neutron plus a neutrino, with a W doing the carrying.
To tell electron capture from beta-plus decay, look at the start: capture pulls an atomic electron into a proton, while beta-plus emits from the nucleus. Both balance the same three books through a W.
Bosons carry each force, vertices balance three books, and the W runs beta decay.
More to read
33.2 The Four Basic Forces
Nature runs on four interactions: gravity, electromagnetism, the strong force and the weak force. Each has its own strength and its own reach, from the endless pull of gravity to the weak force, which barely escapes the nucleus. Physicists rank them by comparing what they do to the same particles.
33.4 Particles, Patterns, and Conservation Laws
Every particle carries a set of labels: charge, baryon number, lepton number and more. In any reaction each label must add up the same before and after, and that rule alone kills most impossible diagrams. The patterns in these labels are what led physicists to quarks in the first place. When you check a vertex, you are really auditing three small account books.
11.5 The Standard Model
The Standard Model sorts everything into matter particles and force carriers. Quarks build protons and neutrons, leptons include electrons and neutrinos, and bosons like the photon, gluon, W and Z carry the forces between them. A Feynman diagram is simply a cartoon of these players meeting at vertices. Read the lines, check the arrows, and the story of any decay falls out.
33.5 Quarks: Is That All There Is?
Protons and neutrons turned out to have smaller parts inside: quarks. Six flavours exist, but everyday matter uses only up and down quarks, with lookalike heavier copies appearing in cosmic rays and colliders. Quarks never sit alone, so we only ever see them in pairs and triples. Beta decay is just one down quark flipping to up by emitting a W boson.
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2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.