ABSTRACT
The single-particle behavior, which is exact for pure real self-energies, breaks down if the self-energy becomes complex. The single-particle description carries over as long as the many-particle system is treated for approximation schemes for which the self-energy does not become complex, as is the case for the relativistic Hartree, Hartree-Fock, and some versions of the T-matrix approximation. This chapter considers the explicit mathematical structure of the baryon two-point function in the interacting particle case. The relativistic Hartree approximation is chosen as the underlying many-body approach. In many-body treatments it is customary and useful to measure energies relative to the chemical potential, μ. The chapter outlines how the number density of baryons is obtained from the baryon two-point Green function. It shows how the single-particle distribution changes with temperature relative to the zero-temperature distribution, whereby the chemical potential plays a most intuitive role.
