Beta-minus (β⁻) decay: a neutron turns into a proton
Here the weak force does something that looks like alchemy: it changes one type of quark into another, and it creates an electron and an antineutrino that didn't exist a moment before.
The quark diagram (a "Feynman diagram")
Physicists draw this as a diagram where time runs left → right and each line is a particle moving forward in time. Points where lines meet are where something happens.
The W boson puzzle, and why the "weak" force is weak
The W⁻ weighs about 80,000 MeV, roughly 86 protons' worth. Carbon-14 has only 0.156 MeV to spend, half a million times too little. So how can it make a W? It can't, as a normal particle. Quantum mechanics allows a brief "virtual" particle: a short-lived ripple in the W field that links the quark to the electron and antineutrino. The heavier the go-between, the less likely this is. That huge mismatch is exactly why weak-force processes are so rare and slow, and why β⁻ half-lives run from milliseconds to billions of years, compared with the 10⁻²³ s timescale of the strong force.
Why β⁻ happens, and why the antineutrino has to exist
Two energy ladders
Inside a nucleus, protons and neutrons sit on separate energy ladders. A quantum rule (the Pauli exclusion principle) allows only 2 of the same kind per rung, so extra neutrons have to stack onto higher and higher rungs.
If a nucleus has too many neutrons, its top neutron sits high up while lower proton rungs are empty. Turning that neutron into a proton lets it drop down, and the drop is the energy that creates the electron and antineutrino and launches them.
A neutron that's alone also decays (half-life ≈ 10 min) because it's slightly heavier than a proton + electron. Inside a stable nucleus it can't, because no lower proton rung is free.
Why there must be an antineutrino
Early experimenters saw only the electron, and it came out with a different energy every time. If only two things (electron + nucleus) came out, momentum and energy conservation would force the electron to carry the same energy every time, like the alpha does. In 1930 Wolfgang Pauli proposed an invisible third particle taking the rest. That's the antineutrino, finally detected in 1956. Hover over the chart:
Carbon-14 electron energies (shape simplified). Total available is always 0.156 MeV. Whatever the electron doesn't get, the antineutrino does.
Checking the books
Nature enforces several strict accounting rules. Each one is why a particular particle has to show up:
| Rule | Before (d) | After (u + e⁻ + ν̄) | So… |
|---|---|---|---|
| Charge | −⅓ | +⅔ − 1 + 0 = −⅓ ✓ | the created particle must be negative → electron |
| Quark count | 1 | 1 ✓ | quarks can change type but not appear or vanish |
| Lepton count (electron & neutrino = +1, their antiparticles = −1) | 0 | 0 + 1 − 1 = 0 ✓ | making an electron (+1) forces an antineutrino (−1) alongside it |
| Energy | C-14 atom outweighs N-14 atom by 0.156 MeV | that's the energy shared by e⁻ and ν̄ | |