Sequential decay in physical representation

Working in a physical representation, we discuss the problem of sequential decay of a discrete 3-level system interacting with boson fields. In such a representation, energy levels are sharp and collision processes conserve the energy. Nevertheless, 2-boson processes induce a broadening of the lines around (Ω2-Ω1), (Ω1-Ω0); both lines have the same width and are truncated at (Ω2-Ω0). In contradistinction, Weisskopf-Wigner's idea of broad energy lebels leads, in general, to lines with different widths and no upper limit. At t=0, the distributions of bare and physical bosons are different; they become identical at t=infinity. The main difference betwen the two representations lies in the dynamics. Any bare boson can be emitted. In a 2-level model, only resonant physical bosons can be emitted; in a 3-level model, non resonant physical bosons can be emitted only with energy lower than (Ω2-Ω0); all other bosons must have been brought through the excitation mechanism. (orig.)