Decays

Decays refer to processes in which an unstable system transforms into more stable products. In physics, decays are common for atomic nuclei and elementary particles. A decay is probabilistic with a characteristic lifetime, and the number of undecayed entities declines exponentially over time. The decay constant λ relates to lifetime τ by τ = 1/λ, and the half-life T1/2 = τ ln 2. Decays conserve energy, momentum, angular momentum, and quantum numbers such as charge, baryon number, and lepton number, subject to allowed transitions. They proceed through specific decay modes or channels, such as alpha decay (emission of a helium nucleus), beta decay (emission of an electron or positron with a neutrino or antineutrino), and gamma decay (emission of a photon). Electron capture and internal conversion are related processes in which a nucleus interacts with orbital electrons.

In particle physics, unstable particles decay into lighter particles through weak, electromagnetic, or strong interactions. Examples include muon decay to an electron and two neutrinos; pion decay to a muon and a neutrino; and neutron beta decay to a proton, electron, and antineutrino. Decay rates depend on coupling strengths, available phase space, and conservation laws. The terms decay width and lifetime describe the same property from quantum-mechanical and experimental perspectives; a broader width corresponds to a shorter lifetime.

Decay chains describe sequences where initial decay products themselves decay further, often progressing toward stable end products. Observations of decay processes provide insights into fundamental interactions and the properties of matter, and have applications ranging from radiometric dating to medical diagnostics and treatment.