Non-instant collisions and two concepts of quasiparticle
  The kinetic theory recently implemented in heavy ion reactions combines a non-local and non-instant picture of binary collisions with quasiparticle features. We show that the non-instant description is compatible with the spectral concept of quasiparticles while the commonly used variational concept is consistent only with instant collisions. The rearrangement energy, by which the variational concept surpasses the spectral one, is shown to be covered by a medium effect on non-instant collisions. 
Pavel Lipavsk\'y and V\'aclav \v{S}pi\v{c}ka and Klaus Morawetz