W/Z boson exchange, the CKM matrix, and neutrino oscillations
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.
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.
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.
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.