Open Systems - Unstable Particle Decay in Medium

The problem of unstable particle decay is discussed to show how elementarity of a subsystem immersed in an infinitely larger environment is lost. The decay law, when the same kind of particles as decay product make up a thermal medium, is worked out in detail. The relic abundance of unstable particles does not suffer from the Boltzmann suppression crucial at very low temperatures, because the off shell contribution not considered in the Boltzmann approach, becomes dominant at low temperatures. The short-time behavior of the non-decay probability is also clarified, which is important to discuss physical relevance of the non-observation of nucleon decay. Two powerful methods in this respect are the operator and the path integral approach, both of which are reviewed.