Beyond the Quasi-Particle picture in Nuclear Matter calculations using   Green's function techniques
  Widths of low-lying states in nuclei are of the order of 30 MeV. These large widths are a consequence of the strong interactions leading to a strongly correlated many body system at the typical densities of nuclear matter. Nevertheless "traditional" Brueckner calculations treat these states as quasiparticles i.e. with spectral functions of zero widths. The width is related to the imaginary part of the selfenergy and is included selfconsistently in an extension of the Brueckner theory using T-matrix and Green's function techniques. A more general formulation applicable also to non-equilibrium systems is contained in the Kadanoff-Baym (KB) equations while still maintaining the basic many-body techniques of Bruecknet theory. In the present work the two-time KB-equations are time-stepped along the imaginary time-axis to calculate the binding energy of nuclear matter as a function of density, including the spectral widths self-consistently. These zero temperature calculations are compared with quasi-particle calculations. The inclusion of the self-consistent widths are found to add several MeV to the binding. The spectral widths are due to the long-ranged correlations. Short-ranged correlations decrease rather than increase the binding. The metod is easily extended to non-zero temperatures where the importance of the widths are expected to increase. 
H. S. Kohler