Particle decay

In particle physics, particle decay is the spontaneous process of one unstable subatomic particle transforming into multiple other particles. The particles created in this process (the final state) must each be less massive than the original, although the total invariant mass of the system must be conserved. A particle is unstable if there is at least one allowed final state that it can decay into. Unstable particles will often have multiple ways of decaying, each with its own associated probability. Decays are mediated by one or several fundamental forces. The particles in the final state may themselves be unstable and subject to further decay. The term is typically distinct from radioactive decay, in which an unstable atomic nucleus is transformed into a lighter nucleus accompanied by the emission of particles or radiation, although the two are conceptually similar and are often described using the same terminology. Particle decay is a Poisson process, and hence the probability that a particle survives for time t before decaying (the survival function) is given by an exponential distribution whose time constant depends on the particle's velocity: where is the mean lifetime of the particle (when at rest), and is the Lorentz factor of the particle. All data are from the Particle Data Group. {| class=wikitable style="text-align: center;" !Type !Name !Symbol !Mass (MeV) !Mean lifetime |- |rowspan="3" | Lepton |Electron / Positron | |0.511 | |- |Muon / Antimuon | |105.7 | |- |Tau lepton / Antitau | |1777 | |- |rowspan="2" | Meson |Neutral Pion | |135 | |- |Charged Pion | |139.6 | |- |rowspan="2" | Baryon |Proton / Antiproton | |938.2 | |- |Neutron / Antineutron | |939.6 | |- |rowspan="2" | Boson |W boson | |80400 | |- |Z boson | |91000 | |} This section uses natural units, where The lifetime of a particle is given by the inverse of its decay rate, , the probability per unit time that the particle will decay.