ABSTRACT
The density fluctuation excitation spectrum is continuous, since all particle transitions allowed by the Pauli principle will contribute. This chapter shows that the quasi-particle, as a bare particle moving together with the surrounding distortion brought about by particle interactions. The importance, or indeed the existence, of the mechanism, depends on the quasi-particle energies and the phase space available for the decay process. Higher order collision processes can also limit the lifetime of a quasi-particle. In contrast to the single quasi-particle excitations, the multi-particle excitations will be spread over a continuous range of energies, as one allows the momentum of any single quasi-particle component to vary. For an arbitrary particle interaction, there is no a priori region of momenta for which quasi-particles exist as well-defined elementary excitations of the system. In contrast to the single quasi-particle excitations, the multi-particle excitations will be spread over a continuous range of energies, as one allows the momentum of any single quasi-particle component to vary.
