Summary: | The problem of extending the Quantum Theory of Fields to include a description of decay processes has provoked a rapid growth of interest in recent years. This problem was temporarily by-passed in the early attempts to formulate a relativistic Quantum Field Theory for the obvious reason that it was simpler initially to ignore decay phenomena and to consider only the collision processes of stable particles. The inadequacy of a field theory of stable particles is evident from the fact that among the sixteen experimentally established particles, and of course their sixteen antiparticles — although not all particles are distinct from their anti-particles — only four; the proton, electron, photon and neutrino, are stable.
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