Particle decay

Particle decay is the spontaneous transformation of an unstable particle into two or more lighter particles. It occurs through fundamental interactions and obeys conservation laws.

Feynman diagram: A diagrammatic representation of particle interactions and the terms used to calculate their amplitudes. Decay diagrams represent the initial particle and the interaction producing its outgoing particles.
Particle physics: The study of the fundamental particles of matter and the forces governing their interactions. Particle decay is one way particle physics probes fundamental interactions.
Muon decay: The weak decay of a muon into an electron or positron and two neutrinos. It is a clean example of a three-particle final state mediated by the weak interaction.
Scattering: A process in which particles interact and emerge with changed momenta or identities. Scattering involves incoming particles, whereas decay begins with an unstable state.
Decay width: A measure of the probability per unit time for an unstable state to decay. The total width sums contributions from all allowed decay channels.
Invariant mass: The mass assigned to a system that is unchanged by changes of inertial frame. A parent particle's invariant mass sets the energy available to its decay products.
Beta decay: A nuclear process in which a neutron or proton changes type while emitting a beta particle and a neutrino or antineutrino. Its underlying weak interaction changes quark flavor and produces leptons.
Particle annihilation: A process in which particles and antiparticles transform into other particles. Annihilation requires multiple incoming particles rather than a lone unstable parent.
Branching fraction: The fraction of a particle's decays that proceed through a particular channel. Competing channels divide the total decay probability among distinct final states.
Quantum number: A numerical label specifying a conserved or measurable property of a quantum state. Quantum numbers such as charge and lepton number constrain possible decay products.

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