The Weak Interaction

There are four fundamental interactions: gravitational, electromagnetic, strong and weak. The weak interaction (the weak nuclear force) is responsible for beta decay.Key features of the weak interaction:• It acts on both quarks and leptons. The strong interaction acts only on quarks (hadrons), not on leptons.
• It is the only interaction that can change one type (flavour) of quark into another, for example d → u.
• It has a very short range, about 10⁻¹⁸ m, much shorter than the strong interaction (about 3 fm).
• It is much weaker than the strong and electromagnetic interactions, which is why beta decays can have long half-lives.Examples of weak interactions:β⁻ decay: d → u + e⁻ + ν_e
β⁺ decay: u → d + e⁺ + ν_e
electron capture: p + e⁻ → n + ν_eIn electron capture a proton-rich nucleus absorbs one of its own inner electrons, and an up quark turns into a down quark.In beta decay and electron capture, a quark changes type and leptons such as neutrinos are produced or absorbed. Neutrinos have no charge and are not hadrons, so they feel only the weak interaction and gravity. This is why most neutrinos pass straight through the Earth without interacting.How to spot it: if a quark changes flavour, or a neutrino or antineutrino appears in the equation, the weak interaction is responsible.