The weak interaction was first recognized in cataloging the types of nuclear radioactive decay chains, as alpha, beta, and gamma. The W boson then decays to produce an electron and an anti-electron type neutrino. Any quark type can convert to any other quark type with a different electric charge by emitting or absorbing a W boson. The relative probabilities of the different possible transitions are shown schematically in the diagram below. Quarks and their weak decays Decay processes always proceed from a more massive quark to a less massive quark, as indicated in the diagram, because the reverse process would violate conservation of energy.
Scattering processes can involve the reverse transitions, provided sufficient energy is available.
The set of observed lepton processes is much simpler. Each charged lepton is converted only to its own neutrino type by emitting or absorbing a W boson. This leads to the three lepton number conservation laws .