W/Z boson exchange, the CKM matrix, and neutrino oscillations
The weak force is the only one that can turn one kind of particle into another. It is how neutrons decay, and why neutrinos change flavor in flight.
Switch between W and Z exchange. Notice which one changes the particles' flavor and which leaves them as they were.
The weak force is unique: it is the only force that can change a particle's flavor (type). A down quark can become an up quark by emitting a W⁻ boson — this is how neutrons decay into protons. The weak force also violates parity (left-right symmetry) and CP symmetry, making it essential for understanding the matter-antimatter asymmetry of the universe.
The W⁺ and W⁻ bosons carry electric charge and change quark/lepton flavors. The Z⁰ boson is neutral and mediates flavor-conserving weak interactions. Both are very massive (~80-91 GeV), which is why the weak force is short-ranged.
Click a cell to see how strongly two quarks couple, and move the λ, ρ̄, and η̄ sliders to reshape the matrix. Notice how bright the diagonal stays.
Quark mixing is described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Its elements |V_ij|² give the probability that an up-type quark i couples to a down-type quark j via the W boson. The matrix is nearly diagonal — quarks prefer to couple within their generation.
Step through the decay with Next and watch a down quark emit a W⁻ and turn into an up quark.
Neutron beta decay (n → p + e⁻ + ν̄ₑ) is the archetype of all weak processes. At the quark level, a down quark emits a W⁻ boson and becomes an up quark. The W⁻ then decays into an electron and an anti-electron-neutrino.
Change the energy and the mixing angle and watch the flavor probabilities swing with distance from the source.
Neutrinos have a remarkable property: they oscillate between flavors as they travel. An electron neutrino produced in the Sun can arrive at Earth as a muon neutrino. This proves neutrinos have mass — the first evidence of physics beyond the Standard Model.
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